Geocheminar - spring-2014

April 1, 2014
noon - 1 p.m.
3853 Slichter

Presented By:

• John Wasson - UCLA

Chondrules formed by melting in the solar nebula. Their spheroidal shapes indicate high (>50%) degrees of melting at temperatures of 1800-1900 K. Almost all chondrules contain phenocrysts indicating that melting was incomplete. Many chondrules contain relict grains originally formed in an earlier generation of chondrules. The origin of chondrules is still not understood. A key problem is the determination of cooling rates. The ones in use are based on attempts to duplicate the structures of porphyritic chondrules in simple experiments involving a single cooling event in a furnace. These have yielded cooling rates in the range 0.001 to 0.1 K s-1. The cooling rates expected in a relatively transparent nebula (with chondrules radiating into cold space) are far higher, of the order of 100 K s-1. Thus the furnace based experiments require chondrule formation in a well-insulated environment. They require the immersion of chondrules in large dust clouds having temperatures much higher than the 1300 K evaporation temperature of mafic silicates in a nebula with pH2 <10-3 atm. The furnace experiments were carried out in a low-tech environment. They do not simulate the environment of the solar nebula in terms of pressure and composition of the gas nor do they attempt to account for the multiple heatings that chondrules have experienced. They have mainly attempted to duplicate the formation of olivine, the most common mafic phase in chondrules. There are several indicators that cooling rates were far higher than those needed to grow a 200 mm phenocryst from a tiny nucleus. Today I will discuss an orthopyroxene phenocryst in a primitive chondrite that shows 8 overgrowth layers (that increase the linear dimension by a factor of 2) while retaining the overall spheroidal shape of the chondrule. These overgrowths are interpreted to have formed by the melting of glassy sodic mesostasis at about 1450 K while leaving the gross structure of the chondrule unchanged. These layers do not show up on olivine, apparently because diffusion is 40´ more rapid in olivine compared to opx. This is one more indication that cooling rates were orders of magnitude higher than the values used in nearly all chondrule formation models.

April 8, 2014
noon - 1 p.m.
3853 Slichter

Presented By:

• Matt Wielicki - UCLA

Seminar Description coming soon.

April 8, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Matt Wielicki - UCLA

We investigate the presence of epitaxial overgrowth rims and 'reset' zircon from highly shocked terrestrial impactites to constrain the occurrence of such phenomena in impact environments and explore the use of (U-Th)/He dating of zircon to evaluate this geochronometer in identifying impact ages. Our results show that (U-Th)/He ages of zircon, isolated from the shocked target of the Morokweng impact structure, can accurately date an impact event and provides another tool with which to determine impact ages when no dateable melt sheet exists, an alternative to problematic interpretations of commonly used apparent 40Ar/39Ar plateau ages. No evidence of epitaxial overgrowth rims and/or zircon 'reset' in U-Pb was observed and suggests that even shocked zircon has remarkably slow Pb diffusion, possibly explaining the lack of 'reset' grains in terrestrial impactites. We also present a zircon (U-Th)/He age for the ~100 km-diameter Popigai crater, Siberia, Russia, of 33.9±1.3 Ma. This is significantly younger than a 35.7±0.2 Ma age previously reported (Bottomley et al., 1997) and is, within error, synchronous with the Eocene-Oligocene boundary mass extinction at 33.7±0.5 Ma (Prothero, 1994). Dramatic cooling observed at this interval is presumably due to gas fluxes and 'impact winter' type conditions consistent with modeled scenarios for the Chicxulub impact (Pope et al., 1994) and is similar to estimates for Popigai (Kring, 2003), providing the first evidence of an impact induced mass extinction in the Cenozoic.

April 15, 2014
noon - 1 p.m.
3853 Slichter

Presented By:

• Michael Webb - Caltech

Seminar Description coming soon.

April 15, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Michael Webb - Caltech

Stable isotope analyses are essential to the study of many atmospheric, environmental, and geochemical processes. In particular, measuring the extent of isotopic enrichment provides a powerful tool to deduce formation temperatures or infer the origin of samples. Such measurements facilitate a number of different applications such as reconstructing ancient climates, assessing methane and other hydrocarbon deposits, and providing constraints on the atmospheric budget of greenhouse gases. We utilize path-integral methods with high-quality potential energy surfaces to rigorously calculate equilibrium isotope effects in a variety of systems relevant to stable isotope analysis, including CO2, N2O, and methane. We leverage these capabilities to illustrate their utility for geochemical applications, identify errors in existing theoretical predictions, and quantify the potential impact on experimental determinations.

April 22, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Ed Young - UCLA

Seminar Description coming soon.

April 29, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Carolyn Crow - UCLA

We conducted a survey of Apollo 14 lunar zircon microstructures to search for and compare shock features common in zircons from terrestrial impact structures. The zircons were separated from two breccia rocks (14305 and 14321) and one soil (14259) sample by crushing and heavy liquid density separation. Pb-Pb crystallization ages range from ~3.9 to 4.4 b.y. for both breccia and soil grains, making them ideal candidates for investigating the early impact history of the Moon. Previous microstructural studies of lunar zircons have found evidence of low grade shock deformation. The presence of crystallographically controlled shock microstructures and associated lattice misorientation was first searched for using low kV secondary electron (SE), low kV backscatter electron (BSE), and color cathodoluminescence (CL). Electron backscatter diffraction (EBSD) maps of potential shock features were then collected using a Hitachi SU6600 variable pressure, analytical field emission gun - scanning electron microscope (FEG-SEM) at University of Western Ontario, Zircon and Accessory Phase Laboratory (ZAP Lab). A wide range of microstructural states and textures were observed. The low-strain end of the spectrum included featureless low-strain grains, with primary oscillatory and sector zoning. Shock microstructures in intermediate shock levels include planar and curviplanar fractures hosting micron scale melt inclusions (now partly devitrified glass). Highest shock pressures show domains of granular texture zircon. The highest-grade shock features (microtwins and granular texture) are seen in zircons from both breccias 14305 and 14321, while the soil zircons only exhibit lower grade shock deformation such as curviplanar fractures and small degrees of misorientation. Some grains have multiple sets of microstructures possibly due to different impacts events on the Moon or different stages of shock loading and unloading not previously documented. Further high special resolution investigations into the relationship between the U-Pb system and lunar microstructures will validate and refine the nature of the early lunar impact history, which likely mirrors that of our own planet.

May 6, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Beth Ann Bell - UCLA

Seminar Description coming soon.

May 20, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Natascha Riedinger - UC Riverside

Seminar Description coming soon.

May 27, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Laurence Yeung - UCLA

Seminar Description coming soon.

June 3, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Mojhgan Haghnegahdar, - UCLA
• Jeanine Ash - UCLA
• John Mering - UCLA

Biological Influences on Isotopic Ordering in O2 Jeanine Ash (UCLA) First-principles Models of Equilibrium Tellurium Isotope Fractionation Mojhgan Haghnegahdar (UCLA)