GSW: 1994
MEETING MINUTES
MINUTES OF THE GEOLOGICAL SOCIETY OF
1246th Meeting, 12 January 1994
President Rosalind Helz
called the meeting to order at 8:01 p.m. The minutes of the 1245th meeting, prepared
by John Slack, were read and approved. Five new members were announced: Connie
Bertka, Yingwei Fie, Reto Giere, Geophysical Lab, David Bruce, Planet
Productions, Ltd.; Kevin Crowley, National Research Council;.
Four (?) guests were
introduced: Duncan Hurt, Goddard; Mike Walter and Tom Duffy, Geophysical Lab;
Doug Brown.
E-an Zen made two announcements: First,
There were no informal
communications.
The scheduled first
speaker was Karen Prestegaard, scheduled to talk on the 1993
Sorena talked on
"The Tales of Two Meta Tuffs". Her collaborators include Mark Barton,
Brooks Hanson, and Dick Fiske. The two meta tuffs strike approximately NW-SE
through the largest roof pendent in the Sierra Nevada Batholith, the pendent
being a metamorphosed, west- facing section which ranges in age from 210 to 140
million years (Triassic to mid-Cretaceous).
Sorena subjected 30-plus
samples from these tuffs to a battery of technology-intensive analyses aimed at
deducing their geologic history. The δ18O ratio of the tuffs
was about +12. There was pervasive potassium alteration of the tuffs, as indicated
by the lush blues of rather pretty photomicrographs, so that the tuffs are now
K-feldspar quartz rocks with a little plagioclase. Whole rock analysis, X-ray
fluorescence and neutron activation for trace elements indicated stratigraphic
controls within the two tuffs on an assortment of elements, including Ti, Al,
Sc, Si, and Th. Sodium is anomalously low in the whole section.
Rare earth abundance
patterns of the two tuffs are virtually identical, except that near a pluton,
there was evidence of rare earth mobility in the upper tuff. When normalized to
a factor depending on concentrations in the Bishop tuff and Al2O3,
the two tuffs are congruent with each other and with the Bishop tuff. The significance
of the Bishop tuff is that it is unaltered, and being fight next door, it presumably
passed through the same crust as Sorena's meta tuffs did.
Sorena interprets the
tuffs to have formed as part of a volcanic arc; subaerial deposits originally,
later submerged and altered by seawater under relatively low temperature
(<250°C), and then subjected to contact metamorphism of about 500°,
producing recrystallization, with especially significant effects within 50m of
the pluton. In her words, "although the rocks appear hopelessly screwed up
geochemically, they still remember who they once were," although, as a
question brought out, memory of the parent magma chamber is rather dim.
In opening her talk Sorena had pointed out that her title,
"The Tales of Two Meta Tuffs" was loaded with literary allusions, including,
among others, Dickens' A Tale of Two Cities Because she chose her title and
pointed to its analogy with Dickens, one wonders if she meant for us to recall
at the end of her talk, Dickens' sentence at the end of A Tale of Two Cities
Dickens gave his main character the final sentence, which includes these words:
"It is a far, far better thing that I do, than I have ever done."
Questions by John Price,
Charles Meade of the
Geophysical Laboratory gave the second talk on "Ultra High Pressure and
Temperature X-ray Diffraction of Mantle Minerals". Charles continues the
tradition of the Geophysical Laboratory in investigating the state of selected
minerals under conditions appropriate to the depths of the earth. In this case,
the mineral is silicate perovskite (pyroxene composition with perovskite structure),
which has a density and elemental composition that fits seismic constraints on
minerals possible in the lower mantle. If deductions based on these constraints
are correct, some form of silicate perovskite is the most abundant mineral in
the earth.
Charles used the diamond
anvil to achieve high pressures, in this case 38GPa and 65Gpa, and a CO2
laser to obtain the heat, in this case up to 1800°K for sure, and in excess of
2300°K implied. These pressures are typical of those in the upper half of the
lower mantle, and this temperature is that estimated for a cool mantle. The diamond
anvil and laser heating are techniques well known from prior Geophysical Lab
investigations. The principal technological advance in this investigation has
been to connect the pressure-temperature apparatus to a source of very bright,
highly-polarized radiation now available from synchrotrons. In these
experiments, the laser heats a spot approximately 100 microns in diameter, and
the X-ray looks at a spot approximately 20 microns in diameter within that 100
micron heated area.
Charles found that
perovskite at 38GPa and 1800°K preserves its orthorhombic structure. When
raised to 65GPa and higher temperature, he found reversible changes in
diffraction lines, which he concluded were driven by temperature.
To explain them, Charles
looks at three hypotheses:
anisotropic thermal expansion;
texturing of the sample;
transition to a higher-symmetry phase.
The data suggest that (1) is not very likely, and that (2) and (3)
are more probable, but not yet distinguishable. Charles concluded on a general
level that he had demonstrated techniques that could be used for a new class of
structural studies at high pressure and temperature for a wide range of
proposed mantle minerals, and on a more specific level, that silicate
perovskite could undergo important structural changes in the mantle.
Questions by Mike Ryan,
Motoaki Sato, Alperr (?), E-an Zen, and Brooks Hanson.
Lindrith Cordell, USGS,
gave the final talk of the evening, coauthored with Stanislaw Wybraniec of the
Geological Survey of Poland, on "High Resolution Geophysical Mapping of
the Buried Alkalic-Ultramafic Tajno Massif in
Lindrith emphasized the
geophysical potential fields in his presentation, relying on his Polish
colleagues for the geological constraints. The very dense network of observations
supplied by the Poles enabled extreme downward continuation of the surface
data.
The two geophysical
fields of interest are the gravity and magnetic fields. The form of the plotted
magnetic field is counter-intuitive for the person trying to visualize the shape
of the rock mass generating the field, but the form of the plotted
gravitational field resembles the rock mass.
Therefore, it is of interest to transform the observed magnetic field
into an equivalent pseudo-gravity field for better understanding. You can do
this because the magnetic and gravity fields are related to each other by the
ratio of their respective Green's functions, each Green's function describing
how its respective field falls off with distance from the rock mass.
When transformed, mapped,
and projected down to 600m depth, the geophysical fields sharply defined an
annular rock mass that was consistent with geologic expectation. The magnitude
of the resulting Bouguer anomaly was about six milligal and the magnetic
anomaly was about 1500 gammas, both relatively small anomalies on a regional
scale, but for this feature 3 km in diameter at 0.6 km depth, they stand out.
Questions by Dallas
Peck, Mike Ryan, Pat Taylor, Bevin French, Fred Keer, E-an Zen, Carter Hearn, Vasile
Rusu, Gene Robertson, and Pat Taylor (again).
President Helz adjourned
the meeting at 9:36 p.m. Attendance approximately 71 (67 - 75).
Respectfully submitted
(26 Jan 94)
Cyril Galvin
MINUTES OF THE GEOLOGICAL SOCIETY OF
1247th Meeting, 26 January 1994
President Rosalind Helz
called the meeting to order at 8:01 p.m. The minutes of the 1246th meeting were
read and approved. One new member was announced: Richard B. Carfin, USGS.
Seven guests were
introduced: Edna Gallegher, USGS; Elsa McFarland,
John Jens, Public
Service Committee, Science Fairs Division, presented a schedule of science
fairs in the coming months and asked for volunteer judges. President Helz endorsed
this request. There were no informal communications.
Bruce Lipin, USGS, gave
the first talk of the scheduled program: "Rising and Expanding CO2
Bubbles, the Cause of Pressure Increases and Chromite Deposits in the
Stillwater Complex,
In vertical section, the
There are three major
hypotheses to explain the origin of the Stillwater Complex: (1) Magma mixing.
(The most widely held opinion.) (2)
Increase in oxygen fugacity, causing chromium to crystallize out. (3) Increase in total pressure. (The minority
opinion which Bruce favors.)
Bruce began his analysis
by critically examining inconsistencies in the fluid mechanics needed for early
magma mixing. Using data on the liquid properties of magma, he concluded that
As evidence in favor of
his third hypothesis, Bruce examined the system olivine- plagioclase-silica,
where the order of crystallization appears to follow the cyclic unit in the
Questions or comments
by: Motoaki Sato, Mike Ryan, Bevin French, Gene Robertson, Dave Stewart, Roz
Helz, Mike Ryan (again), Mark McBride, Roz Helz (again), Pete Stifel, and Kevin
Crowley.
Carol Simpson of Johns
Hopkins and NSF presented the second talk on "Structural Analysis of Rocks
in Shear Zones that are Neither Pure nor Simple." The Shear Zone of the
title is a mass of rock contained between a pair of rigid parallel rock
surfaces; for example, a mylonite between the opposite wails of a fault. The
mylonite undergoes Pure Shear if the fault walls move perpendicular to the
fault, compressing or stretching the mylonite. The mylonite undergoes Simple Shear
if the fault wails maintain a constant spacing while moving parallel to the
fault, dragging the mylonite in the direction that the nearest fault wail
moves.
It is desired to use the
sense of rotation of objects embedded in the mylonite to determine the
direction of shear. However, when the shear is a composite of Pure Shear and
Simple Shear, it may happen that adjacent objects apparently indicate opposite
directions of shear.
These ambiguities may be
clarified by considering the exact motion of randomly-distributed elongate
objects in the Shear Zone, when subject to a combination of Pure Shear and
Simple Shear. In such cases, a minority of objects having particular orientations
will indicate a retro- grade sense of shear as shear planes are compressed into
near parallelism with the foliation.
For crystals embedded in
the Shear Zone, rotation is often accompanied by recrystallization and the development
of tails. Carol indicates that the tails shed by rotating crystals have a
geometry determined by the ratio of recrystallization rate (R) to strain rate
(gamma). When R/ gamma is high, wedge-shaped tails result (sigma tails) and the
direction of shear is given by looking for the direction in which pairs of
tails shed from the same crystal "step up" across the Shear Zone.
When R/gamma is low, stringy tails result (delta tails) which require more
careful study to determine the shear direction.
Questions by Mike Ryan,
Bruce Hansen, Chris Talbert, Dave Stewart, and Gene Robertson.
Boyce Rensberger of
The Washington Post gave the final paper of the evening on "What Makes
Science News?". In the 1930's, Albert Einstein's opinions on everything,
scientific and non-scientific, were worth reporting. Jonas Salk was given
somewhat similar treatment when Salk began nationwide testing of his polio
vaccine in 1953. Even 20 years ago, science had unquestioned authority, and
"breakthroughs" were common. But now science is treated more realistically.
Boyce attributed the present more realistic consideration of science by the
public to education by science reporters like him. Scientists must read the
science news articles as well as non-scientists, because the citation rate for
a science paper soars after it is mentioned in the Post or New York
Times.
Boyce then fulfilled the
promise in his title by giving us five criteria for determining what makes
science news. These five criteria are:
1.
Inherent
fascination. Dinosaurs, earthquakes and volcanoes have greater star quality
than algae and soil erosion. A complicated story usually lacks sufficient fascination.
2.
The
size of the natural audience. Diseases have a large natural audience because
sickness affects us all. Earthquakes have a large natural audience in southern
California.
3.
Importance
of topic. How much difference will the subject make in the lives of readers?
Boyce said that the spread of AIDS is more important than proof that
significant climate change will take 1000 years.
4.
Reliability
of results. Reliability is indicated by publication in a peer-reviewed journal
(not necessarily the top journal). A controversial story usually has lower
reliability in Boyce's sense. Asked about the reliability of government scientists,
Boyce endorsed USGS geologists, who were the majority in the audience before
him, and scientists at NIH, where his wife works. Scientists with commercial interests have
lower reliability.
5.
Timeliness.
The newer the better.
His leads for stories
come largely from Science and Nature. Ideas for stories also come from his own
mental science citation index, personal curiosity, and a sense of when the time
is ripe. He receives perhaps 40 press releases per day from University public
relations offices touting new findings at the University, but he does not pay
much attention to them. He does not attend many national science meetings.
The science reporter
competes for space and position in the paper by selling the editor on the
story, which involves a temptation to hype a story. The copy editor writes the
headline, and the reporter must go out of his way to have any influence on its
wording. (As if to illustrate this point, the printed GSW announcement of
Boyce's talk miss-spelled "Washington" in describing his affiliation).
There was some discussion about whether a science reporter may over-glamorize
science by leaving out the boring parts. Boyce said he went into journalism to
avoid boredom.
The unusually extended
discussion of this talk included questions by: Moto Sato (who effectively gave
the speaker an open-ended opportunity to go beyond the 20-minute limit to
finish his talk), Margaret Chauncey, Mike Ryan, Jeff Williams, Pete Toulmin,
Louis Pribyl, Lindrith Cordell, Bob Ilchik, Tom Dutro, Murray Hitzman, Hendrick
van Oss, Reginald Spiller, Alan Linde.
President Helz adjourned
the meeting at 10:17 p.m. Attendance approximately 81.
Respectfully submitted
(23 Feb 94)
Cyril Galvin
Meetings Secretary
MINUTES OF THE GEOLOGICAL SOCIETY OF WASHINGTON
1248th Meeting, 23 February 1994
President Rosalind Helz
called the meeting to order at 8:13 p.m. The minutes of the 1247th meeting were
read and approved. There were no new members to announce.
Fourteen guests were
introduced: Jaquidon Gallego and Margaret Lewis from Defense Mapping Agency;
Genyong Peng, Diane Loy, Adrian Abraham, Joe Dyer, Bill Logan, Sean Ben- Safed
-- all from George Washington University; Scott Ator, Scott Daly, Judy Tegler,
Mee Lim, Sarah Leiker-- all from the University of Maryland; and Jeff Eppink of
ICF Resources.
Jeff Williams invited
the listeners to attend the 20th Annual Assateague Shelf and Shore Workshop
which will meet in Ocean City, Maryland, for technical discussion on 15 April
and tour the Delmarva shore on 16 April.
John Jens, Chairman of
the Public Service Committee, introduced Ellen Houseknecht, a high school junior,
who won the 1993 GSW Science Fair Grand Prize for her project on "The Effect
of Sediment Size of Sand and Gravel on Permeability". President Helz
awarded a book and $50.00 check to Ellen. John then lobbied for more Science
Fair judges, the next pressing need being two fairs on 12 March -- the Prince
William - Manassas Fair in Nokesville and the Fairfax County Fair at Robinson
Secondary School.
There were no informal
communications.
Dan Hayba, USGS, gave
the first scheduled talk, "Petroleum Migration in the Los Angeles
Basin". Dan presented the results of a numerical investigation of fluid
flow along a cross-section from Long Beach on the west to Brea Olinda on the
east, a distance of about 30 km. This cross- section is floored by a granitoid
intrusive and includes up to 8 km of sands, silts and clays ranging in age from
mid Miocene to Pleistocene and Recent. The numerical model was a version of
BASIN 2, developed by Craig Bethke, which considers subsidence, sedimentation,
compaction, and thermal evolution to track the migration of groundwater in a basin
accumulating sediment.
For boundary conditions,
Dan selected no water or heat flow across the sides, continental heat flux from
below, constant 1 atm pressure at the surface, surface temperature which
depended on the presence or absence of free-surface water, and a string of
wells to control the section. As brought up in the questions, the boundary
conditions seem to assume no significant faults to interrupt the path of the
water flow.
The outputs were time
slices of water flow at 3.2, 1.8, 0.5, and 0.0 million years. Water was driven
in the earlier times by compaction at velocities of 25 to 50 km/my to the east,
but a slight uplift along the east edge of basin about 0.5my ago abruptly reversed
the direction and speeded up the flow.
BASIN 2 computes water,
but not oil, flow. To estimate oil flow, Dan took the ratio of the equation for
the flow of oil to the same equation for water. This ratio indicates that oil
moved primarily because of its buoyancy in relatively steep strata. The oil now
being pumped at the West Coyote Field on the east edge of the basin probably
left source beds in the center of the basin 2.5 to 0.7 my ago, flowing as a
separate phase and in thin layers, at velocities nearly an order of magnitude
faster than the water.
Questions by Nick
Woodward, George Helz, Lin Cordell, E-an Zen, Moro Sato, and Gene Robertson.
Paul Lowman, NASA
Goddard Space Flight Center, spoke on the "Size and Shape of the Sudbury
Structure, Ontario". The Sudbury Structure includes and surrounds the
Sudbury Igneous Complex. The Complex is a 60 km by 30 km, basin-shaped body of
norites and granophyres, with an age of 1.85 billion years. Paul acknowledged
Dietz and Bevan French as pioneers in this study, and urged us to believe that
he is not mad at Grieve et al (JGR, 1991), whose reasons for enlarging the
Sudbury Structure formed the outline against which Paul's talk argued. Grieve's
primary conclusion is that the Sudbury Structure was originally a much larger
and approximately circular structure that attained its present ENE elliptical
shape from post-impact Proterozoic orogeny.
Paul criticized six
supposed lines of evidence supporting Grieve's primary conclusion: Airborne
radar indicates reasonably conclusively that the supposed Outer Ring Fracture
of Grieve is a misinterpretation of two independent sets of fractures in the bedrock.
Field studies show that there are no downfaulted outliers attributable to
impact. The Levach Gneiss is not impact-related, as per Grieve, but Archean
rock, deformed in early Proterozoic, and definitely pre-impact. Much of the
supposed Sudbury Breccia has a tectonic origin, and not an impact origin.
Shatter cones from an extended area north of the more narrowly defined Sudbury
Structure are poorly developed or not present, based on Paul's field work, and
at any rate, shatter cones are highly dependent on the rock lithology, not well
developed in coarsely crystalline rock, and not a quantitative measure of
impact. Finally, Paul attributes the anomalously large volume of suspected
impact melt to the 1.85by age of the structure, at which time, Paul believes,
heat flow from the earth was considerably higher than at present, so that the
geometric relations developed from Phanerozoic impact craters are not
applicable to Sudbury. Paul concluded (1) that the Sudbury Structure was never
more than 5km wider on the North than it is now, and (2) that the crater was
originally elliptical and that the ellipticity was only increased, not initiated
by, the Penokean Orogeny.
Questions by Mary Hill
French, Bevan French, Doug Rankin, Gene Robertson, Grant Pearson and Bevan
French (again).
David Bell, Geophysical
Laboratory, presented the third talk, "the Global Geochemical Cycle of
Water in the Mantle". Rubey is credited
with the classical idea for the source of earth's water: the degassing of the
mantle, basically a one-way, outward flow. What David calls the Recent View is
the hypothesis implied in the title of his talk, namely that processes analogous
to the hydrologic cycle operate at a number of depth scales, including a cycle
in which water resides part of the time in the mantle. The historical pedigree
of this hypothesis includes studies of water producing hydrolyric weakening in
quartz, development of solid state solutions involving structural OH motivated
by the possibility of hydrogarnet in the mantle, and the discovery of OH peaks
in almost any nominally anhydrous mineral you look closely at.
We now know that there
is hydrogen in the mantle in fluids and melts, in the essential structure of
relatively common crystalline minerals such as amphibole, mica, humite; in
hydrous silicate minerals at high pressure; and, the mechanism David
investigated, as a trace substituent in common anhydrous minerals.
Although the trace
substituents are present as OH, David's study reports concentrations in ppm by
weight of 1-120, consistent with other geochemical work. In these units,
pyroxenes of suspected mantle origin are the most hydrous minerals, ranging
from 100 to 1000 ppm. Garnet and olivine have relatively little OH. In terms of
whole rock, in water units, source regions of N MORBs have 100 to 200 ppm,
sources of E MORBs have 300 to 400 ppm, and sources of Hawaii basalts have 250
to 550 ppm. These results show that there can be water in the mantle, which is
expected to influence rheology and diffusivity in the mantle.
In an attempt to balance
the books on the hydrologic cycle in the mantle, David provided a five-box flow
diagram, identifying sources and sinks. The principle source of water from the
mantle is through the ocean ridge basalts. These basalts interact with the
ocean, increasing their water content. By hypothesis, this water can be stored
as OH in the subducted slabs returning to the mantle, and eventually reside in
the eclogite and peridotite of the mantle.
Questions from Steve
Shirey, J. K. Bohlke, Moto Sato, and Craig Schiffries.
President Helz adjourned
the meeting at 10:13. Attendance approximately 85.
Respectively submitted
(9 Mar 94)
Cyril Galvin
Meetings Secretary
MINUTES OF THE GEOLOGICAL SOCIETY OF WASHINGTON
1249th Meeting, 9 March 1994
President Rosalind Helz
called the meeting to order at 8:05 p.m. The minutes of the 1248th meeting were
read and approved. Three new members were announced: Jaquidon Gallego and
Margaret Lewis, both of Defense Mapping Agency, and Jeff Eppink of ICF
Resources.
Ten guests were
introduced: Cliff Oss (?), USGS; Bob Swinton, EPA; Eleanor Ingraham, future AGI
Intern; Rod Feldmann, Kent State University; Art Snoke, VPI; Sean Kennedy, DOE;
Cathleen Dyer Williams, (?); and three post-docs from DTM: Lanbo Liu, Ingi
Bjarnason, and John Vandecar.
Bill Burton announced
that Randall Orndorff and Jack Epstein will lead the GSW Spring Field Trip on
"A Structural and Stratigraphic Excursion through the Shenandoah Valley of
Virginia", Saturday, 7 May 1994, leaving 8:30 a.m. from USGS Visitor's
Parking Lot.
President Helz requested
volunteers to judge upcoming science fairs. There is an acute need for judges
from GSW.
There were no informal
communications.
Steve Obermeier, USGS,
began the scheduled program with a talk on "Liquefaction Evidence for a
Large Earthquake 300 Years Ago Along the Cascadia Subduction Zone". A
significant earthquake causes loosely consolidated sediments to shake. The
shaking can liquefy sands between depths of 2 and 20 meters. When this happens,
the weight of the overlying muds presses down on the liquefied sand, forcing
sand up cracks in the mud and on to the surface. When the flow stops, a marker sand bed is
left on the surface, and the cracks through which the sand escaped become sand
dikes.
Steve identified sand
sheets and dikes in the eroding banks of marsh islands in the Columbia River.
About half the sand sheets were found to have sand dikes, but the dikes, in
Steve's classification, were dinky (max of 30 cm thick), and their thickness decreased
going inland.
The age of these sand
dikes is constrained by the absence of large earthquakes during historic times,
the ages of trees (up to 230 years) growing on the sand sheets, 14C
ages of 500 to 800 years from material cut by the dikes, and a 1480 AD tephra
layer underlying most dikes.
Based on comparison of
sand dike features and on analysis of blow counts needed to penetrate the
source sands, Steve concluded that his earthquake had a magnitude well below
the 9.2 magnitude of the great 1964 Alaska earthquake, but above magnitude 7.
His best guess is about magnitude 8. Based on the fact that he found only one
significant interval of sand sheets with dikes in a section going back over
1000 years, the area appears to have been relatively quiet, seismically. In
discussion, points were raised on the logic behind the blow-count curve, and on
the possibility of long duration, low frequency earthquakes escaping detection.
Questions by George
Helz, Dallas Peck, Meyer Rubin, John Price, Kurt Dodd, Bob Ilchik, and Alan
Linde.
Walter Hayes, USGS,
Reston, presented the second scheduled talk on "Lessons of the January 17,
1994 Northridge, California Earthquake". The Northridge earthquake, of
revised magnitude 6.8, struck 18 miles from downtown Los Angeles, at 4:31 a.m.
on a holiday, thereby considerably lessoning injuries and fatalities, but it
still managed to cause damage now estimated at $30 billion, possibly the most
expensive disaster to strike the US. The "big one" of magnitude 8.25,
which has, by definition, more than 31 times the energy of the Northridge
earthquake, is still to come. The most significant scientific issue raised by
the Northridge earthquake, in Walter's opinion, concerns missing earthquakes:
why aren't there more magnitude 6 to 7 earthquakes in the historic record?
These are needed to account for the accumulated slip on more than 300 active
faults in the region.
The Northridge
earthquake occurred on a little known "blind" thrust fault, about 20
miles from the epicenter of the 1971 magnitude 6.5 San Fernando earthquake.
After the San Fernando earthquake, building codes were improved, older
structures were retrofitted, and instrumentation installed more widely in
southern California. The Northridge earthquake tests these improvements, with
potential for even greater improvements.
Measured accelerations
exceeded 0.5g in many places, and reached 2g in some places. An unexpected
feature of this earthquake was the relative strength of the vertical
accelerations, possibly a consequence of the earthquake originating on a thrust
fault. (Vertical accelerations were not previously a specific element in
seismic design.) This earthquake had a focal depth of 19 km, slightly deeper
than the norm of 16 km, a factor Walter considers significant in warning of
future large quakes. There was evidence of a large stress drop (150 to 200
bars), but no evidence of aseismic slip, and no fault breaks at the surface.
There have been over 3000 after-shocks, but no evidence of precursor events in
the nodal area. Maximum vertical and horizontal displacements appear to be 30
to 70 cm uplift, 30+ cm shortening. Drought may have reduced liquefaction.
Questions by: Moto Sato,
Dallas Peck (twice), Martitia Tuttle, Gene Robertson, and Margo Kingston(OSTP)
Chris Talbott, Hans
Ramberg Tectonic Laboratory, Uppsala University, gave the final talk on
"Fountains of Salt". Although not evident from his title, Chris
stated that his talk continued the theme of the previous two talks, the first
having dealt with the deformation of unconsolidated sediments, the second with
stick-slip motion in elastic solids, and his with the ductile flow of a crystalline
fluid, namely rock salt.
The density of rock salt
changes little, but the density of fine grained sediments increases
significantly, with depth of burial. As a result, buried salt is usually
buoyant. Whether it rises to the surface depends on the local density contrast,
relative strain rate, and rate of sedimentation. If the salt does reach the surface, its
subsequent history depends on the rate at which it is dissolved.
Chris quoted evidence
that Louanne salt upslope from the Sigsbee Deep in Gulf of Mexico extruded onto
the surface and spread downslope as an overturned nappe. The cross-sections of
the salt from drilling and seismic work matched the shapes obtained from
subaerial extrusion in the laboratory, and in numerical models.
In the Zagros Mountains
of Iran, Precambrian salt extrudes under the weight of the overlying country
rocks and domes upward with a cross-section matching the shape of viscous fountains
obtained in the laboratory. This salt flows downhill under its own weight,
forming large recumbent folds. Field observation indicates that in dry weather,
the salt expands and contracts elastically due to daytime heating and nighttime
cooling. But only a slight rain converts the elastic salt to a plastic fluid
which flows downhill as fast as ice glaciers of the same size.
Chris identified
applications of his findings, including the potential hazard of trying to drill
production wells through mobile salt to reach oil in the Gulf of Mexico; the
poor prospects for storing rad waste in salt; and academic models of orogen
spreading.
Questions by Bill
Burton, George Helz, Moto Sato, Dallas Peck, Dan Milton, Gene Roberts, E-an
Zen, Bill Burton (again), Moto Sato (again).
President Helz adjourned
the meeting at 9:58. Attendance approximately 91.
Respectively submitted
(13 April 1994)
Cyril Galvin
MINUTES OF THE GEOLOGICAL SOCIETY OF WASHINGTON
1250th Meeting, 3 March 1994
President Rosalind Helz
called the meeting to order at 8:04 p.m. The minutes of the 1249th meeting were
read and approved. One new member was announced, Scott Daly, University of
Maryland and Apex Environmental.
Only three guests were
introduced at the start of the meeting, an apparent minimum for the year so
far: Professor Sahil Alsinawi, University of Sana'a, Yemen, and Gene and
Carolyn Shoemaker, USGS.
Jane Hammarstrom
announced that she was helping Andy Tisinger, Centreville High School, to
prepare a geology course for high school juniors and seniors. This is part of
the GSA Partnering in Excellence program. Jane sought input on appropriate text
books and other assistance. President Helz, acting for John Jens who arrived
later, asked for volunteer science fair judges in three remaining fairs. Bill
Burton said that announcements for the GSW Spring Fieldtrip (7 May to Shenandoah
Valley) were on the table in back of the room.
President Helz showed a
proposed revision of the bylaws dealing with approving new members of the GSW.
This proposal was, in itself, a major revision of a proposed revision discussed
last year. She said that actual practice has departed significantly from the
literal meaning of the existing bylaw. The proposed revision is intended to
bring the bylaw more nearly in line with practice, while reserving for the GSW
Council a measure of control. Discussion by Steve Huebner and E-an Zen. In
accordance with the procedure for amending bylaws, the proposed revision will
be brought up again, discussed, and voted on at the next meeting.
There were no informal
communications.
Co-authors Peter Warwick
and Bruce Wardlaw jointly presented the first scheduled talk on
"Paleocene/Eocene Stratigraphy of Pakistan and Tectonic
Implications". Bruce spoke first,
proving to be about 60% more equal than Peter, measured in speaking time.
There are two primary
lithologic units at the surface: the Ghazij (mostly early Eocene), which is a
pile of shales locally containing coal, overlain by the Kirthar (middle
Eocene), which is a less thick limestone. Bruce and Peter had to interpret the
relation between these units and their relations to the underlying Lockhart, a
foraminiferal limestone. Peter indicates that the lower and middle Ghazij is a
coarsening-upward sequence that ends below the upper Ghazij.
Their field work was
motivated by the potential for economic coal deposits within the Ghazij. Bruce
asserted, in answer to anonymous reviewers, that they actually had been to the
field sites under discussion. At the time, the area was subject to political
unrest, so a platoon of armed guards accompanied Bruce and Peter to the field,
and an unknown party fired a rocket at them during their travels.
Bruce proposes the
following geologic history, based on their field work and available literature:
The Paleocene and Eocene strata of northern Pakistan accumulated in a northeast-
trending backarc basin between the Indian plate and offshore volcanic arcs.
Uplift in the north end of this basin allowed red bed deposition by late Paleocene,
and shut off marine access from the north. Bruce believes that this uplift records
the collision and accretion of the Kohistan volcanic arc complex to the Indian
plate. The basin was further fragmented by a large delta that prograded from
the northwest by early Eocene. The delta separated the remaining basin into an
evaporitic northern segment, and a southern segment where normal marine
carbonate and shale deposition continued into the Lower Oligocene. Bruce believes
that the plate-tectonic collision caused the uplift that shed the sediments
that built the delta.
Questions by Moto Sato,
Blair Jones, Hal Gluskoter, Gene Robertson (a planted question), Fred Simon,
and E-an Zen.
Pat Taylor, NASA
Goddard, gave the second scheduled talk on the "Origin of the Bangui
(C.A.R.) Magnetic Anomaly: A Satellite View".
The largest negative magnetic
anomaly in Africa (possibly the largest in the world) includes an area around
Bangui, Central African Republic. Pat believes this anomaly outlines the
remnants of a large impact structure produced early in Earth history. Four lines of evidence suggest the impact
origin. Ranked by Pat in order of importance, these four are the carbonado
deposits, the gravity anomaly, topography, and the magnetic anomaly. Shaded
digital topo data hint at the existence of concentric rings, 810 and 490 km in
diameter. A negative Bouguer anomaly occupies this bullseye, which is
consistent with negative gravity anomalies at all craters.
Carbonados are placers
consisting of polycrystalline diamonds, used as industrial diamonds. Carbonados
have a crustal, not mantle origin, and no kimberlites have been found in the C.
A. R., despite much prospecting. The C.A.R. is the world's principal source of
carbonados, primarily from two mines located on the concentric circles
surrounding the anomaly. Earlier geologic mapping had identified charnockites
with a 2 - 3 billion year age. Subsequent field work by Bruce Marsh, Bob Regan
and A1 Chichester had collected rocks, but the whereabouts of these rocks is currently
unknown. Pat suggests that shattercone striations would look similar to the
foliation striations in granite gneiss, shown on slides of these lost samples.
He mentioned the peculiar fact that, worldwide, many geologists working in the
areas of supposed impact craters do not believe they are impacts.
His new theory: the impact
raised the temperature of lower crustal rocks (the charnockites) above the
Curie Point, so they remagnetized with a TRM without going into a melt phase.
Questions by Lin
Cordell, Doug Rankin, Bob Ilchik, Gene Shoemaker, Moto Sato, Lin Cordell
(again), E-an Zen, Gene Robertson, Tom Armstrong, Gene Shoemaker (again).
Karen Prestegaard,
University of Maryland, gave the final talk of the evening on "Did
Land-use Influence the Magnitude of the 1993 Iowa Floods".
Karen has personal
interest and experience in her subject because her family has a farm in
southern Wisconsin. For the July 1993 floods, her study area was in the lower
Raccoon River above Des Moines. The 1993 summer rainfall patterns cut across
the drainage basins locally, and successive storm tracks moved southward regionally.
As a result, the upper reaches of many basins did not suffer record floods. In
addition, about 50 years ago, the upper reaches of the drainage basins had been
ditched, eliminating meanders, so that water now drains rapidly from these areas.
For a given discharge, the flood flow had significantly less width, more depth
and higher velocities in the channelized streams than in the meandering
streams. Using channel slope and depth, Karen concludes that there was more
shear in the ditched channels. Although questions in the discussion period were
unable to clarify how the computed shears were used.
Local bank erosion
damaged bridges over the straightened channels. The net result of rapid
drainage in the headwaters and southward shifting storm tracks was to bring a
lot of water at one time to the lower reaches of the drainage basins.
Questions by Curt
Linsey, E-an Zen, Bruce Hansen, Raymond Rye, Curt Linsey (again), Dennis Krohn,
Rick Wonderman, Mary Hill French, Bob Schneider, Gene Robertson, and Mar), Hill
French (again). During the discussion,
Karen introduced three of her coworkers and students: Brendan Shane, Nancy
Cato, and Kevin Houghton, thereby eliminating the apparent year-to-date minimum
for guests set at the start of the meeting.
President Helz adjourned
the meeting at 9:52. Attendance approximately 79.
Respectively submitted
(13 April 1994)
Cyril Galvin
MINUTES OF THE GEOLOGICAL SOCIETY OF WASHINGTON
1251st Meeting, 13 April 1994
President Rosalind Helz
called the meeting to order at 8:05 p.m. The minutes of the 1250th meeting were
read and approved with one correction. One new member was announced, Gordon P.
Eaton, USGS.
Three guests were
introduced: James Joyce, University of Maryland; Jorge Aranda, Instituto de
Geologia, UNAM; and Howard Evans, USGS.
President Helz announced
the recent death of Clyde Wahrhaftig. The members stood for a moment of silence
in his memory. Bill Burton said that announcements for the GSW Spring Fieldtrip
(7 May to Shenandoah Valley) were on the table in back of the room. He encouraged
teachers to attend.
President Helz
introduced the proposed revision of the bylaws dealing with approving new
members of the GSW. This revision had been explained and discussed at the
previous meeting. There was further discussion by E-an Zen and Dallas Peck, and
further explanation by the President. The question was moved by Phil Bethke and
seconded by (?). The proposed revision was adopted.
Pete Stifel gave the
first informal communication of the year on problems of slope stability caused
by construction on and near the University of Maryland Campus, College Park.
The University is at the
boundary between the Coastal Plain and the Piedmont where the Coastal Plain
sediments have had a long history of problems. Pete pointed to a dual failure
to study the past: the geologic past which produced the unstable sediments and
the historic past which witnessed man-made problems with the sediment. Pete's
slides showed classic and anomalous landslips, scarps, and hummocky topography
on newly steepened slopes in the Cretaceous Patuxent formation. The steepening
was caused by widening roads and parking lots. The convenient location made for
excellent teaching opportunities.
Questions by Bill
Hauser, Murray Hitzman, Moto Sato, and Brace Doe.
Paula L. Gori, USGS,
gave the first scheduled talk on "The Social Dynamics of a False
Earthquake Prediction and the Response by the Public Sector."
In October 1989, the
late Iben Browning predicted that there existed a 50% chance for a magnitude
6.5 to 7.5 earthquake in the New Madrid area in early December 1990. Browning
held a PhD in zoology, but was a self-identified climatologist. Eventually over
300 news articles in 45 publications dealt with Browning's prediction. David
Stewart of Southeast Missouri State College, a PhD geologist with a past history
of supporting a false prediction, gave early support to Browning's prediction,
but it was not until a year later (October 1990) that the National Earthquake
Prediction Evaluation Council denounced the prediction.
In addition to the lack
of strong government response, Paula identified other factors that enabled this
prediction to be considered seriously: the extremely severe 1811, 1812
earthquakes in the area; the heavy media coverage of the 1989 Loma Prieta
earthquake; an unlucky, new USGS Circular that said a magnitude 8+ earthquake
was within the realm of possibility; the recent death of three earthquake
experts; and other factors.
Paula believes the
prediction was costly both in monetary terms and in government credibility,
because local and state agencies had begun to react to public interest as if
the predicted earthquake was imminent. She recommends that government agencies
prepare in advance to evaluate unofficial predictions, and vigorously denounce
them if they are unjustified.
Questions by Moto Sato
(two), Karen Prestegaard, Dallas Peck, Bill Burton, Sorena Sorensen, and Bob
Ilchik. President Helz deferred further questions till after the formal
meeting.
Jim McDougall,
Department of Geography and Earth Systems Science, George Mason University,
gave the second talk of the evening, switching places on the program with the
scheduled second speaker. He spoke on "Cenozoic Tectonics of West-Central
New Zealand."
Jim's field area was a
zone across the northern part of South Island, the southern part of North
Island, and adjacent marine areas. His field problem was the interpretation of
surface and seismic evidence for major faults in the vicinity of the Alpine
strike-slip fault, but not the Alpine fault itself.
Two major faults, the
Waimea and the Flaxmore, merge northward into the subsea Manaia fault, known
from seismic data. Another group of faults branch westward off the Alpine
fault.
Surface outcrop and two
east-west seismic lines going down 20 km suggest that the faults are reverse
faults. They tend to be en echelon, east over west, with displacements ranging
from 3 to 6 km. The submarine Manaia fault has a 3 to 4 km offset with Miocene
in the footwall. Plio-Pleistocene strata are thinner and relatively complete on
either side of the faults.
The evidence does not
admit of significant strike-slip motion during the Cenozoic on those faults
which branch off the Alpine fault, and seems to restrict the amount of right
lateral motion on the Alpine fault itself.
After cutting off
questions on the first talk, President Helz sought questions on the second
talk. Gene Robertson and Murray Hitzman responded.
Betsy Moll-Stalcup,
USGS, the scheduled second speaker, gave the third talk of the evening on
"Redoubt Volcano, Alaska: A Progress Report on Arc Magma Genesis".
Redoubt is a
stratovolcano that threatens air traffic and a petroleum tank farm in the area,
but Betsy's principal interest in it is as an example of arc magmagenesis. The
cone-building magmas at Redoubt are generally more mafic than the ash. Most of
the lavas are low-Mg, high-A1 basaltic andesite and andesite. Petrologists believe the magmas originate in
a subduction zone, and the accepted cross-section has Redoubt about 100 km
above the Benioff zone. The fundamental question is, What melts? The slab, the
mantle, or both? The magmas have negative Nb and Ta anomalies (less Nb and Ta
than Alkali and Rare Earth abundances in the magma suggest), but none of the proposed
magma sources have this anomaly. Betsy thinks the proper explanation is given
in high-pressure experiments by Tasumi. He found that the alkali elements are
highly soluble in water and that the LREE are at least soluble in water, but
that Nb and Ta are highly insoluble. The implication is that the magma gets the
alkalis and LREE, but the Nb and Ta remain behind in the slab.
These arc magmas are
believed to incorporate some oceanic sediment because they usually have higher
concentrations of beryllium 10 than magmas from other environments, and
beryllium 10 is strongly enriched in ocean sediments. The implied sediment
source is supported by lead isotope studies. Previous plots of 207Pb
against 206Pb have produced separate clusters for MORB and for
pelagic sediments, but the data from Redoubt and the Aleutians scatter along a
trend joining the MORB and pelagic clusters, suggesting a mixture.
Betsy then examined what
happens in the magma chamber, including crystal fractionation, magma mixing,
and the interaction of the mantle magma with crustal melts or country rock. She
interpreted two photomicrographs, one as showing feldspar from two distinct
magmas in the same thin section, and the other as a cumulate cluster of clinopyroxene
breaking up.
Questions by Moto Sato,
Bruce Doe, Dallas Peck, Phil Bethke, Steve Shirey, Eirik Krogstad; R. Helz (comment)
and Carter Hearn.
President Helz adjourned
the meeting at 10:09. Attendance approximately 81.
Respectively submitted (27
April 1994)
Cyril Galvin
MINUTES OF THE GEOLOGICAL SOCIETY OF WASHINGTON
1252nd Meeting, 27 April 1994
President Rosalind Helz
called the meeting to order at 8:02 p.m. The minutes of the 1251st meeting were
read and approved. One new member was announced, Karen Bryant, USGS.
Two guests were
introduced: Morris Aizenman, NSF, and Don Hull, State Geologist, Oregon.
President Helz reminded
members of the GSW Spring Fieldtrip (7 May to Shenandoah Valley). There were no
informal communications.
Tom Wright, USGS, gave
the first scheduled talk on "Understanding Kilauea Volcano: A Historical
Perspective."
Tom's talk summarized
materials to be used in a book on Kilauea by Tom and Dick Fiske. The general impression from the talk is that
a more-or-less constant magma supply from depth produces complex responses near
the surface including earthquakes, magma intrusion, eruptions, caldera
formation, spreading, rifts, and failed rifts. The terms were not only
plate-tectonic, but anthropomorphic.
Early missionaries
recorded that around 1790, the entire caldera was active and filling; there was
a significant decline in eruptive activity during the 19th century; the rift
zone reopened around 1840, a unique event since the arrival of Europeans. In
1969, a continuous rift eruption first occurred; and in 1975, a magnitude 7.2
earthquake marked an abrupt downslope movement of the south flank of the volcano.
Tom interprets these observations to show a progressive shift from summit to
rift eruptions in the past two centuries. The potential exists for runaway
spreading, leading to sector collapse, a once-in-a-volcano lifetime event.
In the general case,
magma intrusion and extrusion eventually construct a mass that is
gravitationally unstable. This instability is perturbed by the continuous
addition of magma from below, producing rifting. Rifting spreads the base and
allows the escape of magma at lower elevations, temporarily increasing the
structural stability of the volcano, whereupon the summit eruptions may resume.
Questions from Doug
Rankin, Glen Brown, Frank Jacoby (?), Dallas Peck, Murray Hitzman, and Gene
Robertson.
Gene Shoemaker, USGS,
Flagstaff and Lowell Observatory, gave the scheduled second talk on "The
Crash of P/Shoemaker-Levy 9 on Jupiter." These minutes are being written
in early May 1994, shortly after Gene's talk on 27 April, so they reflect the
anticipation in Gene's talk, but not whatever will have been learned from the actual
impact by the time these minutes are read.
The observing team at
the Lowell Observatory scans the entire sky with the 18-inch telescope during
one year of observing. It is basically a fishing expedition to see if anything
new is happening in their field area. Something new is found by spotting the motion
of bright objects against the background of more-or-less stationary stars.
Carolyn Shoemaker picks out this motion from the photographs, and she found a
squashed comet, P/Shoemaker-Levy 1993e, on 27 March 1993. This finding was
immediately confirmed by astronomers at other observatories, and shortly afterwards
Harold Marsden computed the orbit which indicated that the comet probably had
begun to break up on 7 July 1992, following capture sometime within the past
century, and that the comet will impact Jupiter in July 1994.
Pieces from the broken
comet are strong out in a line, giving it the squashed appearance noted by
Carolyn. Initial observations identified at least five pieces, which became 10
or 11 nuclei on further inspection. Assuming the same albedo as Halley, these
nuclei have estimated (upper bound) diameters of 2.5 to 4.3 km. The original,
unbroken comet was probably about 10 km in diameter. Empirical observation
suggests a conservation of nuclei law for the comet, with new nuclei being
formed as old ones disintegrate. No OH has yet been found; we see only dust now.
The expected impacts on
Jupiter will be just out of sight, around the rim of Jupiter from earth
observatories, but the impact sites will quickly rotate into view of earth.
Considerable effort is being made to sense the impacts from Galileo, which will
have the best view, and indirectly from earth by measuring the disturbance in
Jupiter's cloud patterns and gravity waves at Jupiter's cloud top.
Gene is happy to have
this July impact to show that comets do indeed collide with planetary bodies.
Questions from Mike
Ryan, Steve Shirey, Pat Taylor, Gene Robertson, Moto Sato, Dallas Peck, An
McBride, and Bill Hauser.
Mike Walter, Geophysical
Laboratory, Carnegie Institution of Washington, gave the third scheduled talk
of the evening on "Experimental Constraints on Segregation of the Earth's
Core."
The earth's metallic
core consists of iron, nickel, and the light elements of your choice,
surrounded by a silicate mantle with much lower concentrations of iron and
nickel. Mike presented experimental results on partitioning of nickel, cobalt,
and tungsten between silicate and metallic phases as a means of understanding
the segregation of the earth's core. Some elements are chemically suited to go
with metal - these are the siderophile elements, including, among others, Ni,
Co, and W. The distribution of elements in the C1 Chondrite is taken as the
standard, against which the distribution of the siderophile elements may be
compared. Moderately and highly siderophile elements are depleted relative to
the C1 Chondrite. Analyses indicate that siderophile elements are included in
upper mantle above the level expected using 1 arm distribution coefficients.
Many explanations have been advanced, but recent work suggests that siderophile
elements show greater lithophile behavior as pressure and temperature
increases.
Mike ran experiments
using apparatus with pressure capability up to 25GPa, temperatures in the range
2100 - 3000°K known to + 50°K, durations of 3 to 10 minutes, liquid
quenched. Many elements show a
significant dependence on oxygen fugacity when treated in isolation, but
two-component distribution coefficients show relative independence of fO2.
Ni, Co, and W show
decreased siderophile tendency (increased lithophile tendency) with increased
pressure, and to a lesser extent with increased temperature. Within the (P, T)
range investigated, the observed partition coefficients still cannot account
for the observed upper mantle abundances of siderophile elements. Within the
magma ocean, pressure is an order of magnitude more important than temperature.
Questions from Moto Sato
and Gene Shoemaker.
President Helz adjourned
the meeting at 9:57. Attendance approximately 90.
Respectively submitted
(14 September 1994)
Cyril Galvin
MINUTES OF THE GEOLOGICAL SOCIETY OF WASHINGTON
1253rd Meeting, September 14, 1994
President Helz opened
the meeting at 8:05 p.m. Eleven new
members were announced: Sean Kennedy, Timothy Merrill, Allison MacFarlane,
Michael Fleming, Harold Williams, Judy Hannah, Daniel Patronik, Jeffrey
Hightower, Judith Tegeler, Cathleen Tenace, and Gary Solar. Seven guests were introduced, including Yasutaka Terakado, Thomas Meisel,
Edward Bolton, Munir Humayun, Cecile Wolf, David Applegate, and Judy
Hannah. President Helz announced the
deaths of five members-- or former members--of the Society: Betsy Pebbles, Cornelia Cameron, Frank Flanagan, James Balsley, and Irving
May. John Price of NSF announced that
the American Chemical society is sponsoring a "kids in chemistry project"
and looking for volunteers to go to area schools to give children hands-on
activity in the classroom. President
Helz also announced that she had
received from AAPG a copy of a color magazine entitled "Kids Discover"
that talks about the uses and benefits of oil, which can be ordered for
distribution to grade-school children. There were no informal communications.
The first formal talk of
the evening was by George Stephens of George Washington University, and
coauthors Francisco Nullo and Paul Baldauf of CONICET in Argentina, who spoke
on "Tertiary Tectonic Evolution of the Central Argentinian
Andes." George described the
results of recent field studies in western Argentina that focus on the uplift
history of a Mesozoic fold-and-thrust belt, through detailed stratigraphic and
structural analysis of an adjacent Tertiary and Quaternary sedimentary sequence
derived from it. Basement in the study
area consists of Carboniferous schists and Permian to Triassic granites. Overlying this basement is the
fold-and-thrust belt, which comprises two fining-upward sequences of Early
Jurassic to Late Cretaceous age, consisting of nonmarine arkosic sandstone and
conglomerate at the base, marine shale and limestone in the middle of the
cycles, and evaporitic gypsum deposits at the top. Andesitic plugs and other intrusions, which
are common within the fold-and-thrust belt, yield Ar-Ar ages of 10.6 to 5.3 Ma,
which are believed to approximate emplacement ages since the andesite bodies
probably cooled relatively quickly.
The Tertiary and
Quaternary basin to the east, which was deposited as a series of coalesced
alluvial fans, formed in part by the erosion of the adjacent fold-and-thrust
belt. Clast lithology and other
features such as deformation permit the basin to be divided into five
subdivisions, which are used to understand the uplift and erosion history of
the fold-and-thrust belt. The nature
and structure of the Tertiary and Quaternary sediments also record the tectonic
history of the fold-and-thrust belt.
Based on these and other relations, George suggested that 1) horizontal
shortening in the fold-and-thrust belt ended prior to the emplacement of the
Molles suite of andesitic plugs, 2) vertical uplift began at the beginning of
the Tertiary, and 3) significant vertical uplift continues to the present.
Questions by Murray
Hitzman, Dave Stewart, Gene Robertson, Nick Woodward, and Charles Druit. (19.5 minutes)
The second talk, by
Elisabeth Widom of the Mass Spectrometry Group at the National Institute of
Standards and Technology, was on "Os Isotopic Systematics of the Azores
and Canary Islands." Elisabeth
reported on osmium and other radiogenic isotope data for Holocene basalts from
the Azores and Canary Islands, as a mechanism for evaluating the degree of
crustal contamination in the basalts. Osmium isotope studies are well suited to
this task, because rhenium is incompatible while osmium is compatible during
partial melting of the mantle, whereas the parent and daughter isotopes of
strontium and lead are all incompatible during partial melting. Osmium isotopes thus can be used to identify
recycled crustal material in the deep mantle.
The osmium contents of
the basalts are extremely low, reaching
a maximum of less than 150 picograms per gram (one picogram per gram is one
part per trillion). Higher 187Os/186Os
ratios and low osmium abundances are interpreted to reflect contamination of
the basalts by manganese oxides, whereas the opposite trend suggests control by
a mantle lithosphere component. Attempts
to decipher the degree of crustal contamination through strontium isotopes were
unsuccessful, because although the strontium isotope range is large, the osmium
isotope range is small, thus precluding a distinction between recycled terrigenous
sediments and subcontinental lithosphere.
However, thorium isotope data were helpful in distinguishing largely
mantle sources (lower 230Th/232Th) from sources
influenced significantly by crustal contamination (higher 230Th/232Th);
lower crustal contamination is indicated for basalt samples from the Canary
Islands that are believed to have come from a mantle plume. Elisabeth concluded by suggesting that 1)
osmium isotope ratios in ocean island basalts are affected by lithospheric
mantle and not by crustal contamination, 2) radiogenic osmium isotope ratios
may be produced by contamination from manganese oxides and perhaps from carbonatites,
and 3) unradiogenic osmium isotope ratios are produced by contamination with
lithospheric mantle.
Questions by Ken Towe,
Craig Schiffries, Murray Mitzman, Roz Helz, and Charles Druit. (19.0 minutes).
The last talk of the
evening was by Marilyn Fogel of the Geophysical Laboratory, who spoke on
"Isotopic Records of Global Change."
Marilyn used data on nitrogen abundance and nitrogen isotopes to
evaluate the role of humans on the global cycling of nitrogen. Major sources of nitrogen to the atmosphere
are industry, agriculture, auto emissions, lightening, and coastal waters. In general, rainfall in oceanic areas has
low nitrogen contents, whereas continental storms produce relatively high nitrogen
rain; the nitrogen in the rain is present as ammonia and as nitrate. Marilyn described her study area in coastal
North Carolina, where rain has been systematically collected for the past 4
years. Measurements have been made of
amount of rainfall, pH, amount of ammonia, and amount of nitrate. Seasonal variations exist, including increased pH during winter and
fall, which is associated with increased ammonia and nitrate contents of the
rain. Stable nitrogen isotope data,
reported as d15N values, decrease with increasing rainfall, although the
observed variation is only from +2 to +5 permil.
Marilyn then described
her "Mesocosm" Experiments in which nitrate and ammonia were
artificially added to rainwater and coastal waters, and monitored for d15N values and parameters such as chlorophyll. Significantly, adding small quantities of
ammonia resulted in a major decrease in phytoplankton productivity. Detailed monitoring by the hour, however,
shows a lag of approximately one day in chlorophyll production, in response to
adding the ammonia. The presence of ammonia
in rainwater thus affects the biosphere, since an estimated 35-80% of the
nitrogen in coastal waters is derived from atmospheric deposition.
Questions by George
Helz, Brooks Hanson, Ken Towe, Bob Ilchik,
and Moto Sato. (22.0 minutes).
President Helz adjourned
the meeting at 9:35 p.m. Attendance was
91.
Respectfully submitted,
MINUTES OF THE GEOLOGICAL SOCIETY OF WASHINGTON
1254th Meeting, September 28, 1994
President Helz opened
the meeting at 8:05 p.m. Minutes of the
1253rd meeting were read and approved following minor corrections. Five new members were announced: Holly
Stein, Gene La Porta, Barbara Fowler, Sharon Givens, and James Joyce. Pour guests were introduced, including
Victor Press, Lori Keith, Jackie Mann, and Mark Frank. President Helz announced that the GSW Council
had lowered student dues to $11, and did not approve Ellis Yochelson as a corresponding
member. There were no informal
communications.
The first talk of the
evening was by Philip Candela of the University of Maryland, who spoke on
"Tertiary Magmatic Phase Permeability in Granites, Theory and Field
Observations.,, Phil described the
results of studies aimed at developing models of magmatic phase evolution,
specifically with regard to the mechanisms of volatile egress from crystallizing
plutons. Recent field studies have
included work on several high-level plutons in the Hodgkinson, New England, and
Lachland fold belts of eastern Australia, where the common presence of
miarolitic cavities in the plutons indicates that the magmas were saturated
with respect to a volatile phase. Phil
showed hand-specimen photographs of granites that contain miarolitic cavities
interconnnected by miarolitic textures, and crudely concentric bands of
miarolitic cavities within an aplitic matrix.
The externally nucleated micropegmatitic textures are considered
evidence of an interconnected magmatic volatile phase during crystallization,
which is enhanced by low pressures and wet magmas. Fractal experiments were shown that may have
relevance to modeling the evolution of volatiles in high-level granites. There are additional implications for trace
element distributions in granites, in terms of batch devolatiltzation versus
finite reservoir fractionation. With
respect to mineralization, high-level granites with textural variability
and miarolitic cavities are commonly associated with magmatic and hydrothermal
deposits of tin, molybdenum, gold, and tungsten.
Questions by Moro Sato,
Bob Ilchik, Sorena Sorensen, Bruce Doe, and Mike Brown. (20.0 minutes).
The second talk, by Erik
Hauri of the Department of Terrestrial Magnetism of the Carnegie Institution of
Washington, was on "Chromatographic Effects of Melt Migration: A Case
Study from Kilauea Volcano, Hawaii." Erik presented various chemical, isotopic,
and phase equilibria data bearing on the nature of volcanism at Kilauea
Volcano. Major unanswered questions are:
1) what is the composition of the primary Kilauea magma?; 2) What is the depth
of generation of primary Kilauea magma?; 3) What is the depth of last equilibration
of the primary Kilauea magma?; and 4) What is the role of the lithosphere in
shield building of Hawaiian volcanoes? A
significant paradox exists between experimentally- and chemically-based models
for the primary Kilauea magma.
Experimental melting of Hawaiian picrites are inconsistent with
equilibration with garnet lherzolite, yet rare earthy element data on these
picrites are similar to data produced by melting of garnet lherzolite, and
residual garnet is apparently required to buffer the heavy REE contents of the
picrites.
Erik presented a model
that separates elements like a chromatographic column, as a function of
different partition coefficients of elements between liquid and solid
phases. Lead, for example, has a very low partition
coefficient of about 0.001, which is thus preferentially fractionated into the
liquid phase; osmium, in contrast, has a
very high coefficient of about 10, and thus stays behind in the solid
phases. Osmium isotope data- shown as 187Os/186Os,
plotted against lead isotope data, shown as 206Pb/204Pb,
suggest that Kilauea magmas have not significantly interacted with the
lithosphere or asthenosphere. The
Kilauea magmas thus must move through the lithosphere quickly to avoid
contamination during ascent.
Questions by George
Helz, Mike Ryan, Bruce Doe, Bevin French, Sorena Sorensen, and Hat Yoder. (18.5 minutes).
The last talk of the
evening was by Brooks Hanson of Science Magazine, who spoke on "Making
Room for Granites Slowly: Bringing Ocean Ridges on Land." Brooks started by reviewing the three principal
means proposed for granite intrusion in the crust, including stoping, diapirism or forceful emplacement,
and extension, all of which have major mechanistic problems. A plot of over a thousand granitoids in the
western United States that are believed to have been emplaced at greater than
or equal to 6 kilometers depth are mainly less than 20 kilometers in
diameter. Examples were shown from the
White Mountains and the southern Sierra Nevada, the latter containing granites
commonly spaced 40-50 kilometers apart.
Are these features consistent with extension as a means for providing
room for granite intrusion in the crust?
The problem with an extensional model, according to Brooks, is that extension
is too slow by several orders of magnitude, based on the recurrence interval of
modern earthquakes in regions undergoing active extension. What extension rate, therefore, is necessary?
Scaling Norm Sleep's theoretical data from ocean-ridges to continents
and use of temperature-time plots of half-extension rates shows that slow
spreading--like that typical of continents--can maintain small magma
chambers. At a minimum extension rate
of 2 centimeters per year, a stable magma chamber can form in relatively cool
crust; only 1 centimeter per year extension is necessary to maintain the
chamber once it has been created.
Brooks also suggested that zoned mafic to felsic magma chambers are very
stable thermally, and that the actual size of magma chambers may be much
smaller than the size of the preserved granitoids.
Questions by Bevin
French, Mike Ryan, E-an Zen, and Phil Candela.
(19.0 minutes).
President Helz adjourned
the meeting at 9:50 p.m. Attendance was
74.
Respectfully submitted,
MINUTES OF THE GEOLOGICAL SOCIETY OF WASHINGTON
1255th Meeting, 12 October 1994
President Rosalind Helz
called the 1255th meeting to order at 8:03 p.m. This was a joint meeting with
the Paleontologic Society of Washington. The minutes of the 1254th meeting were
read and approved with one correction. No new members were announced.
Three guests were
introduced: Brett Lesley, EPA (?); Leonid Popov, VSEGEI; and Dr. Tong Jinnan,
Associate Professor, China University of Geosciences.
President Rosalind Helz
announced that Professor George Helz heads the 1995 slate of GSW officers proposed
by Doug Rumble, Chair of the Nominating Committee.
Ellis Yochelson showed
two slides of a portrait of Charles Doolittle Walcott [1850-1927], in an
informal communication, appropriate for this joint meeting because Walcott had
been both the first president of GSW and a noted paleontologist. Ellis said
that the portrait probably represents Walcott around age 75, but was finished
posthumously in 1928 by Walcott's daughter [Helen Younger], and recently given
to the National Portrait Gallery by Walcott's great granddaughter. These slides were the first public showing of
the painting. Although it is not evident on the portrait, Ellis said that
Walcott once had red hair.
Bruce Molnia, USGS, gave the first scheduled talk on the "1993-1994 Surge in