Geocheminar - fall-2017

Oct. 2, 2017
noon - 1 p.m.
Slichter 3853

Presented By:

• Student Organized - UCLA

Seminar Description coming soon.

Oct. 13, 2017
noon - 1 p.m.
Slichter 3853

Presented By:

• Paul Warren - UCLA

Until recently, all of the eucrites appeared to be simple products of anhydrous basaltic magmas, mostly annealed by dry thermal metamorphism. However, we now know that unannealed eucrites commonly feature secondary veining, typically dominated by an anomalous combination of fayalitic olivine and Na-poor plagioclase within pyroxene; from which secondary volatile-rich fluids have been inferred. The unannealed NWA 11040 eucrite shows new varieties of secondary alteration. Late, in-situ reduction is suggested from: a high Fe-metal abundance; pyroxene Mg/Fe and Fe/Mn zoning consistent with late diminution in [FeO]; and rims of many pyroxene grains, especially near Fe-metal, show a distinctive corrosion texture, with elongate silica-dominated inclusions oriented perpendicular to the rim, suggesting FeO reduction caused decomposition of rim pyroxene. Also, deposition from fluid at a far-subigneous T is implied by a peculiar texture where compositionally anomalous pyroxene fills cracks within cristobalite. But, you ask, then why pyroxene and not amphibole? Unearthly low pressures make an asteroidal crust ill-disposed to engender igneous or postigneous hydrous silicates.

Oct. 16, 2017
noon - 1 p.m.
Slichter 3853

Presented By:

• Jabrane Labidi - UCLA

Seminar Description coming soon.

Oct. 20, 2017
noon - 1 p.m.
Slichter 3853

Presented By:

• Melanie Barboni - UCLA

Seminar Description coming soon.

Oct. 27, 2017
noon - 1 p.m.
Slichter 3853

Presented By:

• Beth Ann Bell - UCLA

Owing to both its ability to record U-Pb ages and its mechanical and chemical resilience, zircon is the premier crustal geochronometer in most geologic settings. Its ability to survive multiple sedimentary cycles makes it a key tracer for continental evolution and indeed, zircon is to date the only material known to survive from Earth's first 500 million years. However, zircon's ability to serve as a "time capsule" preserving information about this period is only commensurate with its ability to resist later alteration, and radiation damage can render it susceptible to recrystallization or hydrothermal alteration. By examining zircon from known geologic settings with well-constrained tectonothermal histories, we are able to establish patterns of alteration affecting many of the lines of evidence useful for constraining continental evolution: zircon trace element chemistry and the cargo of mineral inclusions which most zircons contain. Using the Jack Hills detrital zircons, we have developed textural criteria for recognizing altered inclusion assemblages, and using zircon from a suite of recent granites, we can further constrain the frequency of inclusion alteration and telltale signs. We have also used the Jack Hills zircons to develop an index for chemical alteration of zircon. Application to Phanerozoic granite zircon reveals its general applicability. Alteration to zircon trace chemistry and mineral inclusions appears to usually be identifiable either texturally or chemically, respectively. Secondary features in zircon can in some circumstances yield their own useful information -- for example, secondary inclusions which are amenable to U-Pb dating, when examined on a 5-10 micron scale, provide a chronicle of the metamorphic events undergone by their host zircons throughout their residence in the crust.

Nov. 3, 2017
noon - 1 p.m.
Slichter 3853

Presented By:

• Student Organized - UCLA

Seminar Description coming soon.

Nov. 10, 2017
noon - 1 p.m.
Slichter 3853

Presented By:

• Dustin Trail - University of Rochester

Seminar Description coming soon.

Nov. 17, 2017
noon - 1 p.m.
Slichter 3853

Presented By:

• Thomas Kruijer - Lawrence Livermore National Laboratory

We report the first detailed study of Si isotopes in zircon, coupled with simultaneous detection of O isotope ratios, to better understand the relationship between crustal weathering and recycling. Current knowledge about Si isotopes shows their potential to complement and enhance that understanding because weathering of the crust typically results in a residuum with lower ³⁰Si (where δ³⁰Si is the deviation in ‰ of a sample’s ³⁰Si/²⁸Si ratio to that of a standard) when compared to ‘fresh’ igneous material. Samples explored include zircons from S-, I-, and A-type Phanerozoic granitoids, and detrital zircons from the Jack Hills. We also explore the relationship between the δ³⁰Si value of individual minerals and the whole rock by quantifying high temperature Si isotope fractionations by experiment.

Nov. 27, 2017
noon - 1 p.m.
Slichter 3853

Presented By:

• Issaku Kohl - UCLA

Full Title: Doubly substituted isotopologues of methane: Insights into production mechanisms and mixing in natural systems

Dec. 4, 2017
noon - 1 p.m.
Slichter 3853

Presented By:

• Student Organized - UCLA

Seminar Description coming soon.