Geocheminar - spring-2016

March 29, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Chris McGuire - UCLA

Iron silicide thermal equation of state: implications for silicon in Earth’s core

Silicon is a candidate for the major alloying element in Earth’s outer core. From a mineral physics perspective, the thermal equation of state, melting temperature and thermal conductivity are key measurable quantities that can be compared with seismic observations and geophysical models to either support or rule out Si as a major alloying element. We present new measurements of the isothermal equation-of-state, thermal expansion, and the Grüneisen parameter of iron silicide (Fe5Si3) at high pressures and temperatures. We performed X-ray diffraction experiments on iron silicide in the laser-heated diamond anvil cell at the Advanced Photon Source (APS) beamline 13 ID-D. Diffraction patterns were measured in situ at pressures up to 96 GPa and temperatures up to 2500 K. Our high-pressure, high-temperature measurements provide previously unknown thermal parameters, which are essential for addressing questions of core composition. Additionally, our new isothermal equation of state is in contrast with previous measurements, and shows that iron silicide compressibility, KT,0 = 172 (6) GPa, is indistinguishable from the compressibility of pure Fe. Differences between measured compressibility of this material can be explained by non-hydrostaticity in the diamond anvil cell, a problem that is mitigated (though not solved) with noble gas loading of diamond cells (as was done in the present work). Recent work on liquid Fe-Ni-Si alloys (Williams, 2015) has placed limits of 1-2% Si in the outer core. Our iron silicide data show no such limitation and imply that Si is thermoelastically compatible up to ~15% by weight in the solid phase. Implications of the differences between liquid and solid data are discussed in terms of free energy and melting temperature.

April 5, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Josh West - USC

Revisiting 70 million years of solid Earth degassing and its role in planetary carbon cycle and climate

April 12, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Kaitlan McCain - UCLA

First Measurements of the Thermal Properties of Ordinary Chondrites at High Temperature

Here we report measurements of the thermal properties (heat capacity, thermal conductivity, and thermal diffusivity) of ordinary chondrite samples as a function of temperature using the laser flash method. These measurements have been carried out for the first time at temperatures above 500 K, including values near the peak temperatures expected during thermal metamorphism of the meteorite parent bodies. Values are generally in agreement with those reported by previous authors. Our first results suggest that at high temperatures, thermal diffusivity is more affected by metal content and less affected by porosity than at low temperatures. We use fits derived to our measurements to model the thermal evolution of meteorite parent bodies and compare these results to those that are collected using estimates for the thermal properties commonly found in the literature. We note the strong effect that even fairly subtle alterations in the thermal properties of the material have on the evolution of the model parent bodies, and look ahead to upcoming results which will allow us to construct more representative models of parent-body alteration.

April 12, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Michael Jordan - UCLA

Equilibrium Metal-Silicate Fe Isotope Fractionation and Implications for Planetary Differentiation

Magmatic iron meteorites exhibit an enrichment in 57Fe/54Fe relative to chondrites of ~0.2‰. Though not definitive, this is suggestive of heavy Fe partitioning into the cores of differentiated bodies. We aim to determine if core formation is accompanied by an isotopic signature. Understanding equilibrium Fe isotope fractionation between metal and silicate phases is fundamental to assessing the significance of the variation of Fe isotopes in the Solar System. Here we determine the equilibrium Fe isotope fractionation between the metal and silicate phases using the equilibrated aubrite meteorites Norton County and Mount Egerton. The calculated isotopic fractionation ?57Femetal-silicate is 0.08‰ ± 0.039 (2 SE) for Norton County and 0.09‰ ± 0.019 (2 SE) for Mount Egerton, indicating that the heavy isotopes of Fe partition into the metallic phase. Using the stable Fe isotope fractionation and the temperature of equilibration for these meteorites, we explore what we can learn about the core sizes and compositions of asteroid parent bodies. We conclude that the observed difference in isotopic composition between magmatic iron meteorites and chondrites can be explained if the parent bodies of these iron meteorites had relatively small cores.

April 19, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Cayman Unterborn - ASU

How Can Exoplanets Teach Us About the Earth and Planetary Diversity?

The Earth is a habitable, dynamic planet, with plate tectonics creating a deep water and carbon cycle. These cycles regulate surface and atmospheric C and water abundances, and therefore long-term climate, which is vital to Earth's habitability. The driving force behind plate tectonics is the convection of the mantle. The fact that the Earth transports its interior heat via convection instead of conduction is a result of a confluence of factors that include the internal energy budget as well as mantle size and composition. Relative to the Sun stars that host extrasolar planets vary in their refractory rock-building element proportions relative to Si by an order of magnitude. This variation will create terrestrial planets with unique mineralogies and dynamical behavior. How similar these planets are to Earth, chemically and physically, is the focus of this talk. Here, I will present results from my work relating stellar composition to planetary mineralogy and structure with with the end goal being to answer the question: “Is the Earth special?”

April 26, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Haolan Tang - UCLA

36Cl-36S and 26Al-26Mg systematics in the U-depleted Curious Marie CAI: New insights into early aqueous alteration and the co-existence of 26Al and 36Cl in the early solar nebula

Curious Marie is an intensely altered Allende CAI with Group II REE pattern. It was characterized as U-depleted with high d235U produced from 247Cm decay (t1/2=15.6 Myr). Here we analyzed oxygen isotopes, 26Al-26Mg and 36Cl-36S systematics to understand the origin and evolution of this CAI. Oxygen isotopic compositions in the secondary minerals of Curious Marie are consistent with the common CAIs. However, unlike the terrestrial 26Mg isotopic composition in the secondary sodalite observed in previous studies, an elevated, yet uniform 26Mg excess was observed. The model 26Al/27Al ratio calculated using bulk Al/Mg ratio and the uniform 26Mg excess is identical to the canonical value. Since the aqueous alteration involving an external oxidizing fluid can result in chemical exchange and the redistribution of Mg isotopes, the canonical model 26Al/27Al ratio implies not only the early condensation of its precursor but the extremely early aqueous alteration when 26Al was still close to the canonical value. Subsequently, a closed system thermal process is required to homogenize Mg isotopic composition without the contamination from chondritic Mg. In addition, homogeneous 36S excesses attributed to 36Cl decay (t1/2=0.3 Myr) is also detected in the secondary phases of Curious Marie, although the bulk Cl/S ratio is unknown, hindering a reliable estimate on the model ratio of 36Cl/35Cl. However, given that 36Cl can be introduced through Cl-rich fluid in the aqueous alteration related to sodalite formation, we propose the possibility that 26Al and 36Cl co-existed in the early solar nebula, and therefore, 36Cl in the extraterrestrial objects can not only be produced from late local irradiation but also inherited from the stellar source.

May 3, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Will Newman - UCLA

Planetesimal Swarms are not so Nice

Nice-type models have been a controversial element of the prevailing debate over solar system evolution. They incorporate a massive planetesimal swarm in the region between Uranus and Neptune, take many short-cuts in computing the gravitational interactions between the planetesimals themselves, as well as the major planets, and often change the physics that they employ in their computations as the simulations proceed. We shall show that this description for the planetesimals is fundamentally unstable, and that the outcomes presented for such models are, therefore, not valid.

May 10, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Elizabeth Bell - UCLA

When do time capsules leak? Zircon alteration and its implications for the geologic record

Despite the robust nature of zircon in most deep crustal and surface environments, chemical alteration, especially associated with radiation damaged regions, can affect its geochemistry as well as that of its mineral inclusion cargo. This consideration is especially important when drawing inferences from the detrital record where the original rock context is missing. Typically, alteration of zircon is qualitatively diagnosed through inspection of zircon REE patterns and the style of zoning shown by cathodoluminescence imaging, since fluid-mediated alteration often causes a flat, high LREE pattern. Due to its much lower abundance in zircon relative both to other crustal materials and to the other REE, disturbance to the LREE pattern is the most likely first sign of disruption to zircon trace element contents. Using a database of 378 (148 new) trace element and 801 (201 new) oxygen isotope measurements on zircons from Jack Hills, Western Australia, we propose a quantitative framework for assessing chemical contamination and exchange with fluids in this population. The Light Rare Earth Element Index is scaled on the relative abundance of light to middle REE from which to estimate the degree of zircon chemical alteration, or LREE-I = (Dy/Nd) + (Dy/Sm). LREE-I values vary systematically with other known contaminants (e.g., Fe, P) more faithfully than other suggested proxies for zircon alteration (Sm/La, various absolute concentrations of LREEs) and can be used to distinguish primary compositions when textural evidence for alteration is ambiguous. Chemical trends in alteration of Jack Hills zircons are consistent with known phases contaminating cracks in the zircons. An examination of apparent alteration to mineral inclusion records in zircon from a variety of settings reveals common spatial patterns of alteration (largely in cracked and isotopically disturbed regions) and a variety of alteration phases

May 17, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Mojhgan Haghnegahdar - UCLA

Constructing an Atmospheric Methane Budget using 13CH3D and CH2D2

We develop a theoretical model using relative abundances and fractionations of 13CH3D and CH2D2, the doubly substituted mass-18 isotopologues of methane, to quantitatively track the sources and the sinks of atmospheric methane. The goal is a better determination of the methane budget in the atmosphere. Atmospheric methane has been increasing over recent centuries. From 1999 to 2006, however it approached a steady state, with emissions roughly in balance with the sum of sinks (Dlugokenchy et al., 2003; Khalil et al., 2007). Since 2007 a renewed increase in atmospheric CH4 is observed. The reasons for this evolution are not yet known.

Different methane sources have different isotope ratios because of variations in substrates, formation reactions, and temperatures. Isotope ratio measurements will provide useful constraints on source components and sink processes. However, bulk isotope ratios alone are unlikely to be diagnostic because of mixing of sources.

Using recently published budgets (Whiticar and Schaefer 2007) and estimates of equilibration temperatures of various methane sources (Stolper et al., 2014; Wang et al., 2015), along with ?13CH3D, and ?CH2D2 measured in biogenic methane sources in Ed Young’s lab using the new Panorama gas-source mass spectrometer, we estimate the source flux of singly- and doubly-substituted isotopologues to the air, in terms of both bulk ratios and deviations from the stochastic distributions of multiply-substituted species. The composition of atmospheric methane is also influenced by sink reactions. The main sink reactions with OH• and Cl• are modeled with first-principles transition state theory. At steady state conditions, our model predicts that the main sink reactions in the atmosphere generate a distinct signature of higher ?CH2D2 relative to the source composition, while at the same time increasing ?13CH3D and ?CH2D2. Finally in order to mimic recent changes in atmospheric methane concentration with the use of its rare isotopes as the tracker, we construct a one-box model with exercising different scenarios. For this purpose, initially a steady state is considered and then in our dynamic model we apply the non-steady state conditions by inducing changes in sources and sinks.

May 24, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Patrick Boenhnke - UCLA

Apparent Late Heavy Bombardments

The Late Heavy Bombardment is one of the major hypotheses to emerge from Apollo-era scientific work. While it was originally formulated based off a handful of Rb/Sr dates and Pb isotope analyses, the majority of the evidence now marshaled for its existence comes from compilations of 40Ar/39Ar “plateau” ages. However, 40Ar/39Ar analyses of extra-terrestrial samples show pervasive evidence for 40Ar loss due to later reheating events (e.g., subsequent impacts) which when interpreted in a “plateau” age framework leads to misleadingly young ages. In order to test the reliability of impact histories derived from 40Ar/39Ar “plateau” ages, we construct a first principles diffusion model for 40Ar. Our model is constrained to a compilation of ages derived from the early heating steps from 40Ar/39Ar analyses of Apollo samples, as they constrain the timing of the last reheating event, and a lunar crustal age distribution derived from zircon 207Pb-206Pb ages. The model is then used to generate monotonic impact histories that fit the constraints, and we compile model “plateau” age histograms to evaluate the degree to which these histograms accurately reflect the input impact history. We relate our model to 40Ar/39Ar step-heating analyses of Apollo samples and while it is broadly applicable, it does not consider the complications arising from the presence of multiple activation energies for 40Ar diffusion. Our modeling shows that the 40Ar/39Ar system as currently interpreted is prone to showing bombardment episodes that are more apparent than real. Furthermore, the complicated nature of 40Ar diffusion in Apollo samples underscores the inability of the current 40Ar/39Ar interpretative framework (i.e., “plateau” ages) to recover the true impact history.

May 31, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Ellen Alexander - UCLA

Seminar Description coming soon.