Geocheminar - winter-2016

Jan. 5, 2016
noon - 1 p.m.
Slichter 3853

Presented By:

• Rosario Esposito - UCLA

Degassing of H2O-bearing fluids from basaltic melts, recorded by liquid H2O in melt inclusion bubbles

Fluids exsolved from mafic melts are thought to be prevalently CO2-H2O ± S fluids. The concentration of H2O exsolved in these fluids controls the explosivity of volcanic eruptions and ore formation. Curiously, although CO2 and S have been reported and quantified in bubbles of mafic melt inclusions (MI) hosted in olivine, direct evidence of H2O exsolution into vapor bubbles has not been reported (although sometimes assumed). In this study, we reheated olivine-hosted MI from Mt. Somma-Vesuvius, and quenched the MI to a bubble-bearing glassy state. We report evidence that volatiles exsolved from melts in MI may consist predominantly of H2O. These results allow direct characterization of magmatic fluids exsolved from melts at depth beneath volcanoes, and corroborate solubility-model predictions that fluids exsolved from mafic melts may consist predominantly of H2O, with subordinate CO2.

Jan. 12, 2016
noon - 1 p.m.
Slichter 3853

Presented By:

• Maryjo Brounce - Caltech

Variations in magmatic redox caused by source and process

There is considerable interest in the variability of the oxygen fugacity (fO2) of the mantle sources of erupted magmas in both space and time. This is because fO2 controls the speciation of multi-valent elements and thus material transfer from the interior to the exterior of Earth. Spatial variations in the fO2 of the mantle are debated, in part because Fe in arc basalts is more oxidized than in mid-ocean ridge basalts, but trace element proxies for fO2 suggest that the mantle is uniform across tectonic regimes. Temporal variations in the fO2 of the mantle may have been transmitted to Earth’s atmosphere and oceans by volcanic degassing. However, this is also unclear because it is not certain how the redox states of volatiles relate to their source magmas because degassing and assimilation can impact magmatic fO2 before or during eruption. Here, I present measurements of the oxidation states of Fe and S in a variety of natural basaltic glasses from the global mid-ocean ridge, Mariana arc, Mariana back-arc, and Hawaiian volcanism in order to elucidate the relative influence of mantle sources and volcanic processes on the redox state of basaltic magmas erupted to the surface. I offer a dynamic explanation for the oxidized nature of Mariana arc basalts, where oxidized fluids and melts from the subducting slab generate melt production in the mantle wedge and produce oxidized primary arc basalts. An imbalance between oxidized inputs to the Mariana subduction system and the output of oxidized basalts along the arc and back-arc suggests that there may be a reservoir of oxidized materials that can be stored and transported in the deep mantle on geologic timescales. Undegassed Hawaiian magmas are more oxidized than mid-ocean ridge magmas, which may reflect the incorporation of ancient, oxidized surface materials in the Hawaiian plume. Hawaiian magmas become reduced as the result of volcanic degassing, which serves to limit the effect of depressurization in emitting oxidizing volcanic gases to Earth’s atmosphere.

Jan. 19, 2016
noon - 1 p.m.
Slichter 3853

Presented By:

• Shuning Li - Rice University

Process fingerprinting with triple oxygen and clumped isotopes: new approaches to kinetic isotope effects

Mass-dependent fractionation of stable isotopes can be divided into two categories: equilibrium and kinetic. The studies of novel stable isotopic systems including triple oxygen and clumped isotopes have progressed significantly in the last ten years as new tools to quantity kinetic fractionation in hydrological and biochemical processes.

Recent advances in the ability to measure small differences in ?17O, the deviation from an expected relationship between 18O/16O and 17O/16O ratios, in both waters and low-temperature minerals and rocks (e.g., carbonates, bioapatites, silicates, oxides) present the opportunity to use triple oxygen isotope measurements in hydrological and paleoclimate studies. In particular, the sensitivity of ?17O to kinetic fractionation means that it can be used to constrain the influence of kinetic effects on variations in d18O. I review recently generated datasets on the triple oxygen isotope composition of the hydrosphere and show that there is considerably more variation in ?17O of continental waters than initially proposed, which is due to various processes including evaporation, evapotranspiration and supersaturation during snow formation. I also report the D17O data from an experimental study on synthetic carbonates to evaluate triple oxygen isotope fractionation during the precipitation of carbonate from water, and kinetic effect associated with acid digestion process of the carbonate. This makes it possible to facilitate the use of D17O of carbonate records, in combination with ?18O and ?47 data to permit more accurate reconstruction of d18O in meteoric waters from which the carbonate precipitated.

Finally, I present the first measurements of ?30 (the abundance of 15N15N relative to that predicted by chance alone) using the high-resolution Panorama mass spectrometer, and discuss the potential utility of ?30 as an independent proxy to trace the nitrogen cycle. The parameter ?30 is insensitive to the bulk 15N/14N isotopic ratio of the reservoir; instead, it reflects isotopic ordering in N2, which is altered when N-N bonds are made or broken. Our preliminary measurements of N2 from denitrifying soils and pure cultures of denitrifiers indicate large kinetic isotopic effects during N-N bond formation during denitrification. We also observed a nonstochastic excess in 15N15N was measured in tropospheric N2 [?30 = +19.05 ± 0.12 ‰ (1?)]. This excess likely comes from fixed-nitrogen loss processes in the biosphere. Variations in ?30 of N2 from pure culture experiments (+16.9 to +18.9‰) probably reflect the different isotopic signatures of distinct enzymes that catalyze N-N bond formation. Overall, our results suggest that the degree of isotopic ordering of tropospheric N2 may be applied to reflect the relative contributions of global natural N2 sources.

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

Presented By:

• Jeana Drake - UCLA

A NanoSIMS study on the mechanisms of calcification in coral cells

Feb. 2, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Ming-Chang Liu - UCLA

The CAMECA ims-1290 ion microprobe: What can it do, what has it done and what do we plan to do with it?

The new ion microprobe ims-1290 was delivered to UCLA late August last year. After three months of installation and testing, it came online early December 2015 as the latest addition to the UCLA-NSF National Ion Microprobe Facility. I will introduce the improvements of this instrument over the ims-1270, and show some “publishable” data acquired from this instrument in the last two months. This talk will end with potential scientific applications that can be done with the ims-1290.

Feb. 9, 2016
noon - 1 p.m.
Slichter 3853

Presented By:

• Adam Makhluf - UCLA

Water in granitic magmas: critical phenomena, activity measurements, and evidence for 5 wt% water during batholith formation

Water plays a fundamental role in how we view processes that take place inside the deep Earth. In this talk, I will discuss recent developments on the role of water in granitic systems. First, I will discuss the complications that arise when granitic systems reach critical conditions and will show some field examples where it is inferred supercritical fluids have existed. Secondly, I will address the controversy of dehydration melting versus fluid present melting as models for genesis of A and I type granites at deep crustal conditions. Reversed liquidus measurements on a rock of haplogranite composition at deep crustal pressures of 1 GPa ( ~30km depth) indicate that it takes approximately 5 wt% H2O to completely melt a granite at 850 oC. This is much more water than is present in a tonalite, which contains only ~0.8 wt% H2O in the form of hydrous minerals biotite and amphibole and as a result, would only produce ~16 wt% granitic magma. We suggest that there must be an external influx of water (as well as heat) into the lower crust, likely from a crystalizing basalt that contains ~4wt% H2O (Plank, 2013). Lastly, I consider new H2O activity measurements on albitie and granitic melts and show how that for a granitic liquid to form from a granulite assemblage in equilibrium, it must contain ~5 wt% H2O at an H2O activity of ~0.4 at deep crustal pressures of 1.0 GPa.

Feb. 16, 2016
noon - 1 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

Feb. 23, 2016
noon - 1 p.m.
Slichter 3853

Presented By:

• Doug LaRowe - USC

Biogeochemical limits to subsurface habitability

Microorganisms have been found in just about every subsurface environment where scientists have looked. Little is known about what reactions these organisms are catalyzing or at what rate they are active. As a result, the influence that microorganisms have driving global element cycles in a significant fraction of Earth’s habitable volume is poorly understood. In this presentation, I will describe recent experimental and computational work that is designed to quantify what microorganisms are doing and how fast they’re doing it in energy-limited, subsurface habitats. In particular, I’ll present some modeling work that is geared towards quantifying what constitutes a habitable environment in marine sediments, and show the results of some nanocalorimetric microbial growth experiments.

March 1, 2016
noon - 12:50 p.m.
Slichter 3853

Presented By:

• Will Defliese - UCLA

Can we determine temperatures associated with critical transitions during the evolution of metazoan life? Application of ‘Clumped’ Isotope Thermometry to the Neoproterozoic and Paleozoic

March 8, 2016
noon - 1 p.m.
Slichter 3853

Presented By:

• Matthew Steele-MacInnis - University of Arizona

The Secret Life of Salts

Numerous geologic environments, such as continental geothermal systems and submarine hydrothermal vents, involve circulation of aqueous fluids at elevated temperature (T) and pressure (P). At such conditions, aqueous fluids exhibit physical and chemical properties that differ dramatically from those at ambient T and P, leading to systematic variation in the complexing and, thus, solubility of electrolytes (salts). Our current understanding of the contrasting behavior of different salts in hydrothermal solutions at varying T and P is largely based on empirical observations, whereas underlying molecular-scale controls on hydrothermal salt behavior is largely inferred ex post. To identify strategies for predicting and modeling chemical properties and phase equilibria, we target molecular-scale interactions and their link to macroscopic thermodynamic properties using combined spectroscopy and molecular simulations. This presentation will focus on approaches and results pertaining to sodium ± potassium sulfate solutions. Spectroscopic analyses and simulation results provide a consistent picture of ion-ion interactions and pre-nucleation clustering, which heralds the onset of precipitation with increasing temperature and/or decreasing pressure. Moreover, the contrasting properties of sodium versus potassium ions in solution lead to solubility enhancement in ternary solutions. These results yield insights into the underlying controls on phase equilibria and properties of saline hydrothermal fluids, and provide a basis for predictive modeling of fluid-rock reactions in extreme environments.