Geocheminar - spring-2015

March 31, 2015
noon - 1 p.m.
Slichter 3853

Presented By:

• Beth Ann Bell - UCLA

The history of life on Earth is manifestly preserved in the rock record. However, the fossil record only extends to ~3.4 billion years before present (Ga), the chemofossil record arguably to ~3.83 Ga, and the rock record to 4 Ga. Detrital zircons from Jack Hills, Western Australia range to 4.4 Ga and contain an abundance of geochemical evidence likely pointing toward granitic origins, including a mineral inclusion suite dominated by quartz and muscovite. Geochemical alteration to the zircons and their inclusions appears to be mostly limited to cracks and hairline fractures. Although the majority of mineral inclusions are silicate, we have searched the >3.8 Ga Jack Hills zircons for carbonaceous inclusions in order to determine their nature and implications for the Hadean carbon cycle. One U-Pb concordant, 4.10 Ga zircon contains graphite in two inclusions which are isolated from cracks and fully enclosed within the zircon crystal, 300 Myr earlier than the earliest previously investigated carbon. Their ?13CPDB of -24.2±2.3 and -24.4±2.3 is consistent with biogenic origins. This graphite is older than the canonically accepted Late Heavy Bombardment (LHB) spike in impact events at ~3.9 Ga and occurs contemporaneously with more protracted LHB models. Further investigation of similar samples may help to constrain the Hadean carbon cycle.

April 7, 2015
noon - 1 p.m.
3843 Slichter

Presented By:

• Ed Young - UCLA

The concentrations of short- and long-lived radionuclides in the early solar system are clues to its origins. Theories for the provenance of these nuclides can be divided into 1) the radiochemistry of the solar system has no significance beyond purely chance encounters with a variety of nucleosynthesis sources or 2) the birth environment of the solar system was like the self-enriched massive star-forming regions of today, leaving no signature of specific and identifiable encounters with individual supernovae or AGB stars. In an effort to move past qualitative arguments for and against these disparate theories, Bayesian statistical methods are used to assess quantitatively the relative likelihoods of one scenario type relative to the other. Results of analyses of this type should be considered in formulating a comprehensive theory for the formation of the solar system.

April 14, 2015
noon - 1 p.m.
3843 Slichter

Presented By:

• Patrick Boehnke - UCLA

One of the most significant concepts to emerge from the Apollo-era lunar exploration is the hypothesis that a late, heavy bombardment (the LHB) occurred in the inner solar system at ~3.9 Ga, possibly caused by a sudden massive delivery of planetesimals from the outer solar system. The nature of the LHB has profound implications for the emergence of life on Earth, the role of giant planet migration in mitigating habitability, and the calibration of ‘crater counting’ chronologies. Unfortunately, the majority of the evidence for the LHB comes from misinterpretations of apparent plateau ages on thermally disturbed Apollo samples and appears to contradict bombardment histories inferred from lunar meteorites.

In the present study we undertake a thermochronologic investigation of Apollo 16 samples from North Ray Crater to examine their post-crystallization thermal histories. In contrast to practice over the past 40 years, we conducted high-resolution 40Ar/39Ar step-heating analyses using both isothermal duplicate heating steps and temperature cycling to maximize resolution of kinetic information needed to couple with high precision age measurements to reveal accurate thermal histories. Data are modelled using an refined formulation of the multi-diffusion domain model that incorporates multiple phases and activation energies to estimate episodic loss thermal histories. This approach demonstrates the significant limitations inherent in arbitrary selection of plateau ages. Further we hypothesize that 40Ar/39Ar can record evidence of shock heating, allowing for a possible mechanism by which to separate impact from non-impact related heating. The highly variable initial ages recorded in our age spectra demonstrate a more protracted bombardment history than generally believed.

April 21, 2015
noon - 1 p.m.
3843 Slichter

Presented By:

• Tanja Bosak - MIT

Microbial sulfate reduction (MSR) is an anaerobic metabolism that oxidizes organic matter and uses sulfate as an electron acceptor. This metabolism is thought to degrade about one third of organic matter in marine sediments. Because MSR fractionates sulfur isotopes, producing sulfide with a smaller 34S/32S relative to that of the initial sulfate, the isotopic composition of sedimentary sulfides and sulfates are used to detect sulfur based metabolisms throughout geologic history. Fractionations larger than 45‰ were also used to constrain the oxygenation of the environment during the Proterozoic. To understand processes that control the magnitude of sulfur isotope fractionation, we study the impact of carbon and phosphate limitation on isotopic and lipid signatures produced by marine sulfate reducing bacteria. Experiments conducted in the absence of oxygen show that microbes can fractionate sulfur isotopes in the 5-65‰ range. Therefore, the type and the availability of organic substrates exert the foremost influence on the magnitude of sulfur isotope fractionation, showing that sulfur isotope fractionations larger than 45‰ are not good indicators of environmental oxygenation. We also report for the first time that, when limited by phosphate, a number of sulfate reducing bacteria produce phosphorus-lacking glycolipids and aminolipids instead of phospholipids. This demonstrates the ability of non-phosphorus lipids to substitute for phospholipids in obligate anaerobes and reduce the requirement of these organisms for phosphorus. Furthermore, the prevalence of aminolipids and glycolipids in modern anoxic water columns and sediments suggests that non-photosynthetic microbes in these environments experience phosphate limitation. The physiological and ecological implications of these widespread lipid substitutions remain to be explored.

April 28, 2015
noon - 1 p.m.
Slichter 3843

Presented By:

• Chris Snead - UCLA

In 2006, NASA's Stardust mission returned dust samples from comet 81P/Wild2; these samples represented the first known samples from a cometary parent body. Important motivations for collecting material from a Kuiper belt object included constraining models for the origin and evolution of oxygen isotope heterogeneities in the early solar nebula and comparing comet dust to other types of extraterrestrial materials such as Interplanetary Dust Particles (IDPs) and meteorites. I will discuss the development of techniques for measuring the oxygen isotope composition of impact crater residues Al foil targets and report the results of measurements for seven of eight allocated craters from the Stardust spacecraft.

May 5, 2015
noon - 1 p.m.
3843 Slichter

Presented By:

• Hap McSween - U Tennesee

Geochemical investigations of martian rocks have been accomplished through analyses by instruments on Mars rovers (with geologic context provided by orbiter measurements), and by laboratory analyses of martian meteorites. Igneous rocks are primarily lavas and volcaniclastic rocks of basaltic composition, and ultramafic cumulates; alkaline rocks are common in ancient terranes and tholeiitic rocks occur in younger terranes, suggesting global magmatic evolution. Relatively uncommon feldspathic rocks represent the ultimate fractionation products, and calc-alkaline/granitic rocks are unknown. Sedimentary rocks are of both clastic (mudstone, sandstone, conglomerate, all containing significant igneous detritus) and chemical (evaporitic sulfate and less common carbonate) origin, and high-silica sediments formed by hydrothermal activity. Sediments on Mars formed from different protoliths and were weathered under different environmental conditions from terrestrial sediments, and illustrate different chemical weathering trajectories. Metamorphic rocks have only been inferred from orbital remote-sensing measurements. Metabasalts and serpentinites have mineral assemblages consistent with those predicted from low-pressure phase equilibria and likely formed in geothermal systems. The martian rock cycle during early time periods was similar in many respects to that of Earth. However, without plate tectonics Mars did not experience the thermal metamorphism and flux melting associated with subduction, nor deposition in subsided basins and rapid erosion resulting from tectonic uplift. The rock cycle during more recent time has been truncated by desiccation of the planet’s surface and a lower geothermal gradient in its interior. Mars rocks are intriguingly different from Earth, but the tried-and-true methods of geochemistry are clearly translatable to another world.

May 12, 2015
noon - 1 p.m.
Slichter 3843

Presented By:

• Jason Utas - UCLA

Seminar Description coming soon.

May 19, 2015
noon - 1 p.m.
Slichter 3843

Presented By:

• Jeanine Ash - UCLA

Even in the decades prior to Urey's 1947 "birth of stable isotope geochemistry", Earth scientists were interested in the oxygen isotopic composition of the atmosphere and water. In this talk, I'll discuss atmospheric oxygen isotopes and the biosphere from first a historic perspective and then through the lens of photochemical and terrarium experiments we've undertaken. In a closed-system terrarium experiment, we demonstrate that biological oxygen (O2) cycling drives the clumped-isotope composition of O2 away from isotopic equilibrium. Our model of the system suggests that unique biological signatures are present in clumped isotopes of O2—and not formation temperatures. Photosynthetic O2 is depleted in 18O18O and 17O18O relative to a stochastic distribution of isotopes, unlike at equilibrium, where heavy-isotope pairs are enriched. Similar signatures may be widespread in nature, offering new tracers of biological and geochemical cycling.

May 26, 2015
noon - 1 p.m.
Slichter 3843

Presented By:

• Mojhgan Haghnegahdar - UCLA

Methane is an important greenhouse gas with a significant impact on the chemistry of the troposphere and stratosphere. Methane in the atmosphere reacts with Cl•, OH•, and other reactive molecules, to produce the methyl radical (CH3•). The rates and isotopic signatures of these reactions are of interest for understanding the budgets of methane, water vapor, CO and other reactive gases in air. In this study we estimated kinetic isotope effects (KIE) for the doubly substituted isotopologues of methane, CH2D2 and 13CH3D, reacting with OH• and Cl•, using electronic structure modeling and transition state theory. Then, doubly substituted isotopologues of methane were used to predict the different atmospheric methane sources’ isotopic signature in the air. We are developing a model to track methane in atmosphere based on its chemical behavior using CH2D2 and 13CH3D.

June 2, 2015
noon - 1 p.m.
Slichter 3843

Presented By:

• Adam Makhluf - UCLA

Supercritical fluids in rock-H2O systems have been proposed to be important agents of mass transfer in high-pressure environments such as subduction zones. New experimental studies were conducted on the model granite system NaAlSi3O8(Ab)-H2O to investigate phase relations at pressure (P) and temperature (T) approaching those of critical mixing between aqueous fluid and silicate melt.

Our results provide a comprehensive account of the solution properties of subcritical and supercritical fluids in this model granite system at temperatures and pressures corresponding to the deep-crust regions of granite magma generation. Textures of quenched charges indicate that near-critical melts are extremely fluid, which fact may have considerable bearing on element transport, extractability of partial melt, and ultimate level of melt emplacement in the middle and upper crust.

The liquidus water content of a granitic melt at high pressure (P) and temperature (T) is important because it constrains the volume of granite that could be produced by melting of the deep crust. Previous estimates based on melting experiments at low P (?0.5 GPa) show substantial scatter when extrapolated to deep crustal P and T (700-1000?C, 0.6-1.5 GPa). To improve the high-P constraints on water concentration at the granite liquidus, we preformed experiments in piston-cylinder apparatus at 1 GPa using a range of granite compositions.

The reversed liquidus temperatures at 2.97, 4.15, 5.82, 7.92, and 12.00 wt% water are respectively 950-985, 875-910, 800-850, 750-775, and 650-675?C. Our results plot on the extreme end of the extrapolated water contents at the liquidus when compared to all other previous determinations, and, as a result, give significantly higher water contents than used by most dehydration melting models.