Geocheminar - Winter-2018

Jan. 19, 2018
noon - 1 p.m.
Slichter 3853

Presented By:

• Mojhgan Haghnegahdar - UCLA

Seminar Description coming soon.

Jan. 26, 2018
noon - 1 p.m.
Slichter 3853

Presented By:

• Zhou Zhang - UCSD

Geophysical and geochemical observations indicate anomalous mantle composition and structure in the deep mantle, for example in the Large Low Shear Velocity Provinces. Previous hypotheses of the compositional differences mainly include: major minerals mode variation, Fe/Mg enhancement of major minerals, H in major minerals and silicate melt. In this talk, I will discuss three aspects of a petrological model of accessory phases in a bulk silicate rock: (1) their role in creating both geophysical and geochemical anomalies; (2) constraints on the composition and abundance of the accessory phases; (3) dynamic process leading to the origin, enrichment and preservation of the accessory phases in the mantle from the early Earth to the present day.

Feb. 2, 2018
noon - 1 p.m.
Slichter 3853

Presented By:

• Uri Ryb - Caltech

The 18O/16O ratios of calcite fossils increase by ~8‰ between the Cambrian and present. It has long been controversial whether this change reflects evolution in the δ18O of seawater, or a decrease in ocean temperatures, or greater extents of diagenesis of older strata. I will present measurements of the oxygen and ‘clumped’ isotope compositions of Phanerozoic dolomites, and compare these data with published oxygen isotope studies of carbonate rocks. The δ18O records of dolomites and calcite fossils overlap one another, suggesting they are controlled by similar processes. Clumped isotope measurements of Cambrian to Pleistocene dolomites imply formation temperatures of 15 to 158˚C and parent waters having δ18OVSMOW values from -2 to +12‰. These data are consistent with dolomitization through a km-scale circulation of seawater and diagenetically modified seawater, over timescales of 100 Myr, and suggest that like dolomite, temporal variations of the calcite fossils δ18O record are largely driven by diagenetic alteration. We find no evidence that Phanerozoic seawater was significantly lower in δ18O than pre-glacial Cenozoic seawater. Thus, the fluxes of oxygen-isotope exchange associated with weathering and hydrothermal alteration reactions have remained stable throughout the Phanerozoic, despite major tectonic, climatic and biologic perturbations. This stability implies that a long-term feedback exists between the global rates of seafloor spreading and weathering. Massive dolomites have formed in pre-Cenozoic units at temperatures >40°C. Since Cenozoic platform strata generally have not reached such conditions, the paucity of dolomite in Cenozoic strata (also known as the ‘dolomite problem’) could simply reflect the thermal immaturity of recent platform sections.

Feb. 9, 2018
noon - 1 p.m.
Slichter 3853

Presented By:

• Chi Ma - Caltech

Nanomineralogy is the study of Earth and planetary materials at nanoscales, focused on characterizing nanofeatures (such as inclusions, exsolution, zonation, coatings, pores) in minerals and rocks, and revealing nanominerals and nanoparticles. With advanced high-resolution analytical scanning electron microscopy (FE-SEM with EDS and EBSD), we are now capable to characterize solid materials down to nanoscales easier and faster. Nanofeatures are being identified in many common minerals, providing insights into their genesis and physical properties. New minerals and new materials with important geological significance are being discovered at micron to nanoscales. During an ongoing nanomineralogy investigation of meteorites at Caltech since 2007, 36 new minerals have been discovered. 13 are refractory minerals like allendeite Sc4Zr3O12, davisite CaScAlSiO6, tistarite Ti2O3, panguite (Ti4+,Al,Sc,Mg,Zr,Ca,□)2O3, and rubinite Ca3Ti2Si3O12, which are among the first solid materials formed in the solar nebula. To date, ~60 refractory minerals plus ~15 presolar minerals mark the very beginning of the solar mineral evolution at 4.568 billion years ago. 7 new high-pressure minerals found in shocked meteorites are bridgmanite (MgSiO3-perovskite, the most abundant mineral in Earth), ahrensite (Fe2SiO4-spinel), tissintite ((Ca,Na,□)AlSi2O6-clinopyroxene), liebermannite (KAlSi3O8-hollandite), zagamiite (CaAl2Si3.5O11), chenmingite (FeCr2O4-CF), and stöfflerite (CaAl2Si2O8-hollandite). Each and every one of the new extraterrestrial minerals reveal distinctive forming environments, providing new insights into nebula, parent-body processes, or shock conditions and impact processes on Mars or small bodies in the early solar system. Natural high-pressure minerals also help investigations of phase transformation mechanisms in the deep Earth. Presented here are some of our discovery stories, demonstrating how nanomineralogy works and plays a unique role in Earth and planetary science research.

Feb. 15, 2018
2 p.m. - 3 p.m.
Slichter 3853

Presented By:

• Maria Drout - Carnegie Observatory

It has long been realized that approximately half of the elements heavier than iron are formed via a process known as rapid-neutron capture (r-process) nucleosynthesis. However, it has been less clear where the r-process predominantly occurs, with proposed sites including core-collapse supernovae and neutron star mergers. In this talk I will give a broad overview of our current understanding of the origin of r-process elements, with an emphasis on results from the recent discovery of an electromagnetic counterpart to the neutron star merger GW170817. I will review our previous constraints on the origin of the r-process elements from both theory and observations of metal-poor stars. I will then describe the optical and infrared emission associated with the the binary neutron star merger, GW170817, and place these new results in context. I will discuss the implications of these observations on our understanding of the ejecta from neutron star mergers, the synthesis of r-process elements, and the enrichment of the interstellar medium with this material.

Feb. 23, 2018
noon - 1 p.m.
Slichter 3853

Presented By:

• Douglas LaRowe - USC

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 work that is designed to quantify what microorganisms are doing and how fast they're doing it in energy-limited, subsurface habitats.

March 2, 2018
noon - 1 p.m.
Slichter 3853

Presented By:

• Chris McGuire - UCLA

he thermal conductivity of lower mantle minerals affects the heat flux across the core-mantle boundary and the dynamics of mantle convection. The pressure and temperature dependence of lattice thermal conductivity for dielectric materials, such as oxides and silicates in Earth’s mantle, can be well-explained by Debye theory and a thermal equation of state for the material. However, phase transitions in the lower mantle, such as the spin transition in ferropericlase, complicate extrapolation from lower pressures, and necessitate measurements of high pressure phases. As a method test, I present measurements of thermal conductivity of NaCl across the B1/B2 pressure-induced phase transition. I apply this technique to make measurements of ferropericlase, and show a significant decrease in thermal conductivity across the spin transition, in the pressure range of 40 – 60 GPa. Combining these results with previous measurements on iron-bearing bridgmanite, I construct a model of the depth dependence of lattice thermal conductivity for the lower mantle.

March 9, 2018
noon - 1 p.m.
Slichter 3853

Presented By:

• Gen Li - UCLA

Over geological timescales, continental erosional processes exert a primary control on the global carbon cycle. Specifically, river systems transport particulate organic carbon (POC) from continental carbon reservoirs to oceans. The burial of this riverine-exported POC in marine sediments represents a major geological sink of atmospheric CO2, whose magnitude is comparable to silicate weathering. Previous studies gauged short-term (annual to decadal) riverine POC fluxes, but lacked constraints on the fluctuations caused by abrupt, catastrophic events. In this presentation, I will first show how human activities,specifically dam building and the combustion of coals, affect the cycling of organic carbon in the Yangtze river basin, China. Taking the 2008 great Sichuan earthquake as another case study, I will then discuss how a high-magnitude seismic event enhances mountain denudation and promotes erosion of organic carbon from biomass, soils, and bedrocks in the Yangtze headwater regions. Overall, these projects provide key insights into carbon dynamics in areas undergoing significant changes driven by human activities and natural catastrophic events.

March 15, 2018
2 p.m. - 3 p.m.
Slichter 3853

Presented By:

• Thomas Giunta - U. Toronto

Comming soon.

March 16, 2018
noon - 1 p.m.
Slichter 3853

Presented By:

• Haolan Tang - UCLA

Seminar Description coming soon.