| Jan. 9: |
No Seminar - Andrea Ghez Science Colloquium, 1425 Physics
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| Jan. 16: |
Jonathan Hunt (UCLA)
Speciation of aqueous silica using Raman spectroscopy and first principles calculations
Abstract:
Dissolved
silica plays an important role in lithospheric fluid chemistry. Silica concentrations in quartz-saturated aqueous fluids vary over the range of crustal temperatures and
pressures enough to allow for significant mass transport of silica via
fluid-rock interaction. The
polymerization of silica could afford silicate crystal-like or melt-like sites
to otherwise insoluble elements such as titanium, leading to enhanced mobility
for those elements. It would
therefore be useful to understand what the independent effects of concentration,
composition (pH and incorporation of other elements), pressure, and temperature
are on silica polymerization. This
study presents Raman spectra of high-pH silica solutions taken at ambient
conditions with varying silica concentrations. First principles calculations of explicitly
solvated silica monomers and dimers are used to interpret the experimental
spectra. The results are a promising first
step towards a set of experiments to be done in a hydrothermal diamond anvil
cell, independently exploring the effects of temperature, pressure, and
concentration on silica polymerization.
|
| Jan. 23: |
Ming-Chang Liu (UCLA)
What can hibonite tell you about early Solar System irradiations?
Abstract:
Energetic particle irradiation by the hyper-active Sun is an important
early solar system process. Interactions of energetic projectiles with
targets would result in some isotopic effects that might be recorded by
early solar system materials. In this talk, I will introduce some
astronomical evidence for energetic events and discuss how refractory
materials from the early solar system can tell us about these processes.
|
| Jan. 30: |
Mark Harrison (UCLA)
Diffusion of Ar in muscovite
40Ar/39Ar
analysis of muscovite treated at high temperature and water pressure in
a piston cylinder apparatus yield a linear array indicative of 40Ar* transport by volume diffusion. However, the in vacuo 40Ar/39Ar
degassing behaviour of the hydrothermally treated material is
characterised by both convex and concave forms. This
distinctive age spectra shape differs from that expected from a
sample that has been outgassed from a single diffusion domain.
SEM textural characterization of the run products shows that, despite
careful pre- and post-hydrothermal experiment sieving of the
muscovites, the size population of the run material is not
uniform. We investigated the hypothesis that the form of in vacuo gas release is due to loss of 40Ar*
from multiple diffusion domains. A numerical model constructed to
test this hypothesis yields excellent fits between synthetic and actual
degassing spectra, supporting the multiple diffusion domain
hypothesis. However, the presence of multiple diffusion domains
in the hydrothermal run products impact calculated diffusion
parameters. Deconvolution of the contribution of multiple
diffusion domains to the experimentally determined factional loss yield
estimates of D that are an order of magnitude lower than that assuming
bulk loss of a single domain size. Diffusion data generated in
this manner indicate an activation energy of 64±6 kcal/mol and
log Do of 8 cm2/sec and an activation volume for Ar of 2-6 cm3/mol.
These values correspond to a closure temperature (Tc) of 415°C for
a muscovite grain with a 100 µm radius cooling at 10°C/Ma at
10 kbar and ca. 390°C at 5 kbars.
|
| Feb. 6: |
Weifu Guo (Caltech)
Kinetic isotope effects during carbonate
clumped isotope fractionation and quantitative speleothem
thermometry
|
| Feb. 13: |
Dianne Taylor (UCLA)
Chronology of large-scale silicate differentiation on the Moon: The Lu-Hf perspective
A timescale for the cooling and crystallization of the
lunar magma ocean has not been firmly established, and is essential to
the understanding of the silicate differentiation of the terrestrial
planets. Using the long-lived 176Lu-176Hf
isotope system coupled with U-Pb ages of ancient lunar zircons, a
closure age for KREEP (representing the last part of the magma ocean to
crystallize) has been obtained. Our results indicate that the
large-scale differentiation of the lunar crust was complete by 4.50 Ga.
|
| Feb. 20: |
Jeremy Boyce (UCLA)
Laser Microprobe (U-Th)/He Thermochronometry
The (U-Th)/He thermochronometer (sensitive to temperatures below 300°C)
has been recently applied to a diverse set of Earth science problems
including the fields of tectonics, geomorphology, tephrochronology, and
meteorite studies. Unfortunately, (U-Th)/He is a time-consuming
technique with poor reproducibility and stringent sample requirements.
I will present an alternative to conventional (U-Th)/He, employing
in situ techniques including laser ablation. Laser microprobe
(U-Th)/He can generate ages that are as precise and accurate as
conventional (U-Th)/He, but can be applied to crystals that are
otherwise not amenable to (U-Th)/He. This technique also allows tests
of reproducibility within single grains, increases throughput, and
permits characterization of crystals prior to analysis, further
reducing errors. Applications from 450 Ma to less than 1 Ma will be
discussed, as well as the future of laser microprobe (U-Th)/He, which
includes micron-scale depth profiling, and direct measurement of key
physical parameters essential to (U-Th)/He: diffusivity and
alpha-ejection distances. |
| Feb. 27: |
Kathryn Dyl (UCLA)
In-Situ UV-Laser Fluorination
Oxygen Isotopic Analyses of an Efremovka CAI and Matrix: Implications
for Oxygen Isotope Exchange in the Solar Nebula
The oxygen isotope anomaly present in CAIs reveals the earliest
evolution of solar system materials. Of specific concern in this study
is how the oxygen isotopic composition of various mineral phases in an
individual CAI can be related to its thermal and temporal
history. We used the unique UV-laser ablation fluorination
technique to obtain high-precision in situ data for E44, a type B CAI
from the reduced CV Efremovka. Using these results, as well as
diffusive exchange coefficients for the phases in question, we
determined that the oxygen isotopes in anorthite, melilite, and
fassiate in E44 could be explained by solid-state oxygen isotopic
exchange between this object and a nebular gas.
|
| March 5: |
Max Coleman (JPL)
Oxygen isotopes to characterize pyrite oxidation processes: latest score in Rio Tinto field versus lab experiments
For more than 20 years there have been publications on
characterization of the processes of microbial and inorganic oxidation
of pyrite. There has been consensus neither on the mechanism(s) nor on
how well oxygen isotopes might quantify the respective contributions
from atmosphere and water to oxygen in the sulfate. We have made
further lab culture experiments and analyses to add to the confusion
but our new results indicate plausible reaction mechanisms. Geochemical
data from the Rio Tinto area of Southern Spain have been valuable in
identifying different processes in this immense area of acid mine
drainage. There are two main end-members in mixtures of very low pH
(0.9-1.7), iron sulfate waters: those with Fe(III) and lower �18O values (~4.0‰) and the Fe(II) end-member with more positive �18O
(11.2-13.0‰). We are not convinced that these data
quantify the contributions from the oxygen sources but can be used to
identify processes occurring in inaccessible parts of the system. |
| March 12: |
Haibo Zou (UCLA)
Radiogenic Isotope Constraints on the Origins of Continental Basalts From a Diffuse Igneous Province |