Geocheminar - fall-2014

Oct. 7, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Sarah Palaich - UCLA

Seminar Description coming soon.

Oct. 14, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Ming-Chang Liu - UCLA

Full title: A lower initial abundance of short-lived 41Ca: Implications for the origins of short-lived radionuclides

Oct. 21, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Camille Risi - UC Berkeley

Climate models exhibit a persistent spread in their projected future changes in tropical precipitation. Can we use paleoclimatic proxies of past precipitation to constrain these projections? If a climate model simulates more realistically past precipitation changes, is it more credible for the future? To address this question, we analyze precipitation changes simulated for past and future climates by climate models from the CMIP5 archive. Over some regions like in South America, we can find encouraging statistical links between model behavior for past and future climates. Are the same physical mechanisms at play for past and future climates? And are the same mechanisms explaining the model spread? I'll present some work in progress on these issues. Then, to use paleoclimatic data to constrain precipitation projections, we need paleo precipitation records. Can we use archives of the past precipitation isotopic composition as proxies of precipitation? Or are they proxies of temperature? An isotopic general circulation model is used to try to resolve this long-standing controversy. Rather than resolving it, we find that the relative importance of precipitation and temperature in controlling the isotopic composition at paleo time scales depends on the model representation of cloud physics. New satellite measurements now allow us to better understand what control the isotopic composition at the daily scale. To what extent improved knowledge of isotopic controls at the daily time scale can help us better understand the controls at the paleo time scale? Finally, we look in more detail at why the relationship between precipitation and isotopic composition is so complex and model-dependent. We find that the relative importance of different cloud types and the shape of the vertical velocity profile affects this relationship. This complicates the interpretation of paleo records of isotopic composition. But in turn, measurements of water vapor isotopic composition could be used to better constrain convective and cloud processes. As an example, we present how measurements of water vapor isotopic composition could help us evaluate the relative timing of different convective and cloud processes simulated by models during intra-seasonal variations of tropical convection.

Oct. 28, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Ernst Zinner - Washington University

In the last 25 years a new way to study the stars has become available: the analysis of stardust in the laboratory. Primitive meteorites contain tiny dust grains that condensed in stellar outflows and explosions. These grains can be isolated from the meteorites and studied in detail in the laboratory. The stellar origin of these grains is evidenced by their isotopic compositions, which are completely different from those of the Solar System. We can measure the isotopic ratios of the major elements, as well as of many minor and trace elements, with secondary ion mass spectrometry (SIMS) in single grains that range in size from a couple of 100 nm to several µm. Such detailed isotopic analysis of individual stardust grains has provided a wealth of information on stellar nucleosynthesis, stellar mixing, and galactic chemical evolution. In addition to isotopic analysis one can study the mineralogy of stardust grains by several microanalytical techniques. Examples are transmission electron microscopy in connection with energy dispersive X-ray analysis and electron diffraction analysis, Auger spectroscopy, Raman spectroscopy, and Synchrotron radiation analysis. Examples of each will be given. Finally, I will present examples of isotopic measurements in silicon carbide grains from core-collapse supernovae that present evidence for short neutron pulses with high neutron densities

Nov. 4, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Jon Bundy - U. of Bristol

Formation of porphyry copper deposits (PCDs), that host 75% of the world's economic copper reserves, requires elevated Cl and H2O to concentrate Cu in magmatic brines, and elevated S to precipitate Cu-rich sulphides. These twin requirements are hard to reconcile with experimental and petrological evidence that voluminous Cl-rich, hydrous silicic magmas lack sufficient S to precipitate directly the requisite quantities of sulphides. These features are, however, consistent with observations of active volcanic arcs whereby PCDs can be viewed as roots of dome volcanoes above shallow reservoirs where silicic magmas accumulate over long time spans. During protracted periods of dormancy metal-enriched brines accumulate in and above the silicic reservoir through slow, low-pressure degassing. Meanwhile cogenetic volatile-rich mafic magmas and their exsolved, sulphur and CO2-rich fluids accumulate in deeper reservoirs. Periodic destabilisation of these reservoirs leads to short-lived bursts of volcanism liberating sulphurous gases, which react with the shallow-stored brines to form copper-rich sulphides and acidic (HCl) gas. We test this hypothesis with a novel set of experiments designed to simulate low-pressure interaction of mafic magma-derived, S-rich gases with brine-saturated, Cu-bearing, but S-free, granite. Our experiments result in direct precipitation of Cu-sulphides within the dacite, at magmatic temperatures, supporting previous suggestions of gas-brine interaction as an ore-forming process. The simultaneous production of HCl during sulphide precipitation drives alteration reactions in granites and their wall-rocks that replicate associations of sulphides and alteration haloes around PCDs.

Nov. 18, 2014
noon - 1 p.m.
Slichter3853

Presented By:

• Paul Warren - UCLA

New gravity data (GRAIL) and recent seismic inferences have important implications for the bulk composition of the Moon. The volume and density of the crust imply that contrary to old prejudices, the bulk Moon is not much enriched in refractory lithophile elements (Al2O3, CaO, etc.). In its present much-cooled state, a remarkably high proportion of the lunar interior has dense garnet and/or spinel, not low-density plagioclase, as the dominant Al-silicate; a circumstance which implies a relatively low density for the mafic silicates (olivine + pyroxene). Density modeling suggests the bulk-Moon composition probably has a mildly lower mg than bulk Earth; mg = 85 mol% implies an [FeO] enrichment of approx. 1.36 times bulk-Earth. The greatest uncertainty in this approach is mantle temperature. Both the refractory-lithophile component of the bulk-Moon composition, and its FeO content, could be constrained by augmenting the current meager heat flow data.

Nov. 25, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Melanie Barboni - UCLA

The potential for cataclysmic volcanic hazards depends on the longevity and volume of shallow-crustal magmatic reservoirs, which are controlled by the crystal content of the reservoir and the flux of magma into the system. Published measurements constraining the timescales of crystal growth and upper crustal liquid residence have been determined exclusively for the snapshots of magma evolution recorded by volcanic rocks. However, to understand why certain magmas erupt and others do not, comprehensive datasets from both volcanic and plutonic rocks are required. Here I determine crystal growth rates in a plutonic system through high-precision U-Pb geochronology on zircon inclusions within cores and rims of 6-8 cm K-feldspar “megacrysts” in a ~7 million year old upper crustal intrusion in Elba, Italy. I determine K-feldspar growth rates of 0.2 to 1.1 µm/yr, which when combined with petrologic observations and phase equilibrium modeling show that the transition from eruptible magma to immobile granitic mush occurred in 10-40 kyr. Such a short post-emplacement eruption window substantiates evidence for brief upper crustal magma residence in systems capable of eruptions similar to those documented in historical records.

Dec. 2, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Whitney Doss - UCLA

Paleoceanographic reconstructions and modern field studies of the response of marine calcifying organisms to their environment using elemental ratios are crucially dependent upon better understanding of biologically mediated fractionation. In particular, the hydrographic parameter calcium carbonate (CaCO3) saturation state (DCO32-) has been shown to affect biomineralization. Therefore robust empirical relationships between measurements and known seawater influences must be exactingly calibrated and supported by inclusive mechanistic explanations in order to understand past environments and also to predict future behavior. To this end, my research focuses on understanding and applying trace and minor element proxies in calcite and aragonite precipitated in tropical deep (by benthic foraminifera) and surface (by scleractinian coral) waters.

Dec. 9, 2014
noon - 1 p.m.
Slichter 3853

Presented By:

• Atreyee Bhattacharya - UCLA

Mineral dust emitted from arid and semi-arid regions of North Africa is an important component of the Earth’s climate and has been causally related to prolonged periods of drought in the Sudano-Sahel region in the later part of the 20th century. We use 4He - a novel proxy of mineral dust- to reconstruct dust activity from the Sahara-Sahel region. The goal is to understand meteorological and climatological controls on the decadal to centennial trends in dust activity from North Africa as well as to evaluate the effect of future climate change on dust mobilization from Sudano-Sahel region.