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Geocheminar - winter-2015

Comparison of the O-isotopic compositions of the Earth and the Moon

Jan. 13, 2015
noon - 1 p.m.
Slichter 3853

Presented By:

  • Issaku Kohl - UCLA
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Oxygen isotopes are used to fingerprint planetary materials. Recently, significant improvement in the precision of ?17O measurements has resulted in a deeper understanding of how mass fractionation can produce small (<50 ppm, 0.05‰) ?17O values depending on the slope or exponent chosen for the (lab) internal reference line. Here we present a new terrestrial rock-based reference frame (San Carlos Olivine reference frame) with which we compare the bulk silicate Earth (BSE) to bulk silicate Moon (BSM) to re-evaluate potential moon-forming scenarios. We will discuss the shergottite Mars meteorites and the potential for a Martian-water-oxygen contribution to impact melt glass. Finally, the (soon to arrive) Nu Instruments Panorama, a large radius gas source mass spectrometer, will allow us to eliminate certain ambiguities concerning our current abilities to make these measurements accurately.

Seconds after impact: Insights from tektites and experiments

Jan. 27, 2015
noon - 1 p.m.
Slichter 3853

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Tektites are natural glasses formed as a result of melting and quenching of distally ejected terrestrial material upon hypervelocity (>11 km/s) impact on Earth. Some tektites contain inclusions of lechatelierite (nearly pure SiO2 glass; 99-100 wt. % SiO2), generally thought to be the amorphous relicts of partially digested quartz grains. This study exploits the presence of these local heterogeneities to extract information about tektite thermal histories by investigating chemical diffusion between molten silica inclusions and surrounding peraluminous felsic melt in natural tektites and experimental analogues.

An overview on the use of the fluid/melt inclusion technique to answer geological questions

Feb. 3, 2015
noon - 1 p.m.
3853 Slichter

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Fluid and melt inclusions are small aliquots of fluid trapped by minerals during growth in a variety of geological settings from ocean basin to the asteroid belt out into the space. Ideally, the host mineral acts as a protecting capsule from the external chemical/physical variations, and thus, fluid and melt inclusions can track the geochemical evolution of a rock prior to its formation. Since the first description of fluid and melt inclusions by Sorby (1858), many researchers have applied the fluid and melt inclusion technique to answer geological relevant questions. For this Geocheminar, I will review the basis of the petrography and the microthermometry analyses based on fluid and melt inclusions.

Accreting the Earth and Forming its Core: Experimental and Theoretical Constraints

Feb. 10, 2015
noon - 1 p.m.
Slichter 3853

Presented By:

  • James Badro -
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Popigai Crater Probably Formed in the Late Eocene

Feb. 17, 2015
noon - 1 p.m.
Slichter 3853

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The 90 km diameter Popigai crater in Siberia is one of the five largest impact craters known on Earth. For at least 20 years it has been generally assumed that this large impact event was the source of the global impact deposit containing clinopyroxene-bearing (cpx) spherules. This ejecta deposit clearly formed in the late Eocene in magnetochron 16N. 40Ar-39Ar ages of ~35 Ma for Popigai are consistent with this date, but Wielicki et al. presented a 33.9 Ma age at the 2014 Goldschmidt, based on (U-Th)/He analyses of zircons from the Popigai impact melt. They suggested that the Popigai impact could be related to the Eocene/Oligocene boundary mass extinctions. An obvious implication of this age is that the cpx-spherule layer must be from some other large but unknown impact event.

The cpx-spherule ejecta layer is in many ways comparable to, if a bit smaller than, the ejecta layer from the larger 150 km Chicxulub crater at 65 Ma. Additionally, isotopic and chemical data are all consistent with the cpx deposit being derived from Popigai. Since there is clear evidence of an exceptionally large continental impact in the late Eocene, and no such event at a later date, it’s reasonable to suggest that Popigai formed in the late Eocene. Further, the Popigai melt sheet was extensive, up to 600 m thick, and has experienced significant hydrothermal alteration. It’s entirely possible that these zircons took up to 1 m.y. to cool below the ~100?C closure temperature for the He system; a model of hydrothermal activity in Chicxulub infers a duration of 1.5 to 2.3 Ma depending on permeability.

MinKin: A new computational tool for modeling the kinetics of multi-component geochemical systems

Feb. 24, 2015
noon - 1 p.m.
Slichter 3853

Presented By:

  • Daniel Hummer -
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Noble gases in micrometeorites in Antarctica

March 3, 2015
noon - 1 p.m.
Slichter 3853

Presented By:

  • Bastian Baecker -
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Changes in thermocline oxygen concentration occurring over various space and time scales

March 10, 2015
noon - 1 p.m.
Slichter 3853

Presented By:

  • Will Berelson -
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