Geocheminar - fall-2019

Sept. 26, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853

Presented By:

• Quinn Shollenberger - Univ. Münster

The oldest dated solids in our Solar System, CAIs, contain isotopic anomalies in a whole suite of elements relative to later formed Solar System materials. Previous work has reported differences in the proportions of nucleosynthetic components between CAIs and terrestrial rocks as a function of mass. However, the nucleosynthetic fingerprint of the CAI-forming region is still lacking significant data in the heavier mass range (A > 154). Erbium (Er) and ytterbium (Yb) isotopic data along with hafnium (Hf) isotopic compositions yield insights into in a wide variety of CAIs derived from a variety of CV and CK chondrites. Relative to terrestrial rock standards, CAIs - regardless of host rock, petrologic or chemical classification - have uniform and resolvable Er, Yb, and Hf isotopic compositions. The CAI isotopic patterns correspond to r-process deficits (or s-process excesses) relative to terrestrial values of 9 ppm for Er, 18 ppm for Yb, and 17 ppm for Hf. This new Er, Yb, and Hf data help complete the nucleosynthetic fingerprint of the CAI-forming region, further highlighting the systematic difference between the CAIs and later formed bulk planetary bodies. Such a systematic difference between CAIs and terrestrial rocks cannot be caused by different amounts of any known single presolar phase but is likely the result of a well-mixed reservoir made of diverse stellar sources.

Oct. 3, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853

Presented By:

• Elizabeth Bell - UCLA

Igneous zircon is not only a robust geochronometer but also reflects the trace element content of its source magma and incorporates mineral inclusions inherited from the magma during growth. All of these clues help constrain the origins of detrital and other out-of-context zircon, with applications ranging from tectonic reconstructions to studies of Earth’s earliest crust. The mineral inclusion assemblage of igneous zircons is underdeveloped as a tracer for zircon origins, despite potentially promising applications of oxide and phosphate inclusions for determining the composition of detrital zircon source granitoids. Although apatite-poor zircons appear to mainly derive from highly silicic melts, apatite-rich zircons appear to derive from a variety of sources. Relative mineral inclusion abundances in igneous accessory minerals should be affected by 1) crystallization sequence, 2) proximity to other crystallizing phases, and 3) overall magmatic composition. We have selected several plutonic units in the Peninsular Ranges Batholith (PRB) of southern California with zircons rich in apatite (>20% of inclusions) in which whole rock petrology, zircon trace element chemistry, and several robust accessory minerals in a clear crystallization sequence elucidate the relative importance of these three factors. In these PRB granitoids, earlier crystallizing phases have proportionally larger contents of apatite inclusions and lower contents of late-crystallizing phases (i.e., quartz, K-feldspar, muscovite). Ilmenite inclusion assemblages are shifted toward higher contents of mafic minerals relative to zircon and hornblende. Contents of apatite and late-crystallizing phases in inclusion assemblages appear to be more correlated with the first two factors than more directly with melt composition. Within one zoned pluton (La Posta pluton), melt compositional evolution as shown by zircon trace element chemistry correlates more clearly with apatite and late phase inclusion contents than whole rock SiO2 and other compositional factors such as A/CNK.

Oct. 10, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853

Presented By:

• Ed Young - UCLA

Differentiated planetary bodies exhibit small but discernible elevations in heavy/light stable isotope ratios among rock forming elements. Preferential evaporation of light isotopes from melt exposed to space is a possible mechanism. A theoretical framework has been developed for evaluating the isotopic effects of evaporation as a function of size of the body and the nature of any enveloping gas. We are testing this framework in the laboratory by laser heating aerodynamically levitated samples in gases of controlled compositions. Our data bear on the effects of variable saturation on evaporative isotope fractionation, providing tests of the theoretical predictions. Results show the 56Fe/54Fe the vapor/melt isotope fractionation factors defined by the experimental products are closer to unity than evaporation to vacuum and define Fe gas saturation values of ~0.7, corresponding to effective total pressures of 0.1 to 0.2 bar in the boundary layer of the flowing gas. Results like these constrain models for evaporative fractionation under ranges in relevant conditions.

Oct. 17, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853

Presented By:

• Xiao-Ming Liu - UNC

Chemical weathering plays an important role on continental crust evolution, as it preferentially removes solutable elements such as Li and Mg and can shift the crust composition towards more felsic compositions, thus helping to solve the crustal composition paradox. In addition, chemical weathering is critical in making Earth’s surface habitable by regulating the global carbon and oxygen cycles and by transporting nutrients into the oceans. Lithium isotopes have been developed to trace continental weathering, especially continental silicate weathering due to its low content in carbonate rocks. Using Li isotopic compositions in planktonic foraminifera as a seawater proxy, Misra and Froelich (2012) demonstrated that the δ7Li of seawater increased by 9‰ during the Cenozoic. They suggested this change was due to an increase in incongruent weathering, resulting in increased clay formation, which could elevate δ7Li in the riverine input into the ocean associated with uplift of the Himalaya around 40 Ma. However, the interpretation of this δ7Li curve is complicated by the many variables that influence δ7Li in seawater. Although substantial changes in seawater δ7Li over the Cenozoic are probably influenced by inputs from riverine Li derived from continental weathering, the observed increase can be influenced by changing hydrothermal flux through time. In addition, many different parameters have been explored to explain this Cenozoic δ7Li increase in seawater, including but not limited to increasing Li riverine flux without increasing δ7Li, significant change in denudation rates, increasing fluid-rock interactions between river water and its suspended loads, or floodplains associated with mountain formation caused clay formation. In summary, the causes of the observed increase in seawater δ7Li through the Cenozoic is still very difficult to explain, complicating the usage of Li isotopes as tracers of continental silicate weathering.

Oct. 24, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853

Presented By:

• Xiyuan Bao - UCLA

Are hotspots hotter than ridges? Answering this question is fundamental to understanding the possible source region of hotspots vs mid-ocean ridges. Are hotspots sourced from compositionally distinct active (and therefore hotter) upwellings deep in the mantle, such as the large show shear wave velocity provinces (LLSVPs)? The geochemical signals of ocean island basalts (OIBs) found at hotspots seem to indicate that is the case. The 3He/4He ratios of OIBs is as high as 50 times the atmospheric ratio (Ra), while the typical signal for MORB is about 8 Ra. OIBs with Low ε -Neodymium, which characterize enriched sources seem to be correlated with proximity to LLSVPs. Previous studies of the excess potential temperature of hotspot lava based on the olivine geothermometer suggest that indeed hotspots are hotter than ridges by as much as ~300K. Lower excess temperatures for OIBs seem to indicate proximity to the ridge, and MORBs uninfluenced by hotspots have no excess temperature with respect to the ambient mantle. I re-examine the differences in temperature and geochemical signals between hotspots and ridges by converting seismic velocities to temperature using a state-of-the-art thermodynamic model. My preliminary results suggest that: 1) not all hotspots are hotter than ridges, indeed the two are virtually indistinguishable. Hotspots have a global mean only 5K higher than ridges (1660 vs 1655K); 2) hotspots with higher 3He/4He, buoyancy flux or lower 133Nd/134Nd tend to be hotter; 3) the range of hotspot temperatures is independent of their proximity to ridges, while the mean temperature of ridges close to hotspots are slightly higher than that of those far away; 4) the mean temperature values of both hotspots and ridges are hotter than reference adiabat of 1600K. At face value, the results imply larger compositional than temperature variations for both hotspots and ridges, an ambient mantle that melts at temperatures at the upper range of what is usually assumed, as much variability in ridges as in hotspots, and a tight degree of correlation with proximity to LLSVPs. There are however many caveats related to the inherent uncertainty in seismic tomography and the uncertain dynamical nature of plumes.

Oct. 31, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853

Presented By:

• Francois Tissot - Caltech

Zircon is zirconium-rich (~48 wt%) silicate whose extreme chemical resistance and high uranium content (100s to 1000s of ppm) has elevated to the status of gold standard for the study of Earth’s early history and chronology. As a results numerous chemical and isotopic proxies have been investigated in zircons and their inclusions, including oxygen, silicon and hafnium isotopes, bringing a wealth of information on sedimentary recycling in magmatic systems or the timescale of crustal differentiation. Somewhat surprisingly, the isotopic compositions of zirconium and uranium in zircons, remain unexplored. This is surprising because (1) Zr is the main constituent of zircon and stable isotope variations are expected given the difference in bounding environment of Zr in silicate melt and in zircon, and (2) U isotope variations would impact the precision and accuracy of U-Pb and Pb-Pb dates. In this talk I will present the first Zr and U isotope data obtained on single zircon grains. Contrary to previous claims, I will show that it is not only possible to measure at sufficient precision the U isotope composition of single zircon but that resolvable variations exists between single crystals. In this framework, I will discuss the burgeoning possibilities of using U isotopes in single zircon to improve geochronological constraints and study redox shifts in Earth’s history. For Zr, our investigations reveal extreme isotope fractionation during magmatic differentiation, highlighting the potential of Zr isotopes as a very powerful tracer of magma evolution.

Nov. 7, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853

Presented By:

• Kevin McKeegan - UCLA

In the early morning of February 8, 1969, the Allende meteorite, the largest carbonaceous chondrite presently in captivity, fell over the Mexican state of Chihuahua marking the beginning of a new era in cosmochemistry. The early recognition of Allende’s large, white Calcium-Aluminum-rich Inclusions (CAIs) as compositionally similar to the first mineral phases expected to condense from a cooling solar nebula and the subsequent discovery that they are enriched in 16O relative to terrestrial materials along a non-mass dependent trajectory opened Pandora’s box. Intensive study of CAIs by an evolving suite of mass spectrometric techniques has revealed anomalies in both stable and short-lived radioactive isotopes that clearly demonstrate that the solar nebula was, in fact, not fully homogenized by passage of all incoming presolar materials through processes of evaporation, mixing, and recondensation. By virtue of their very heterogeneous distributions among different classes of primitive meteorites, and their isotopic differences from chondrules and other chondritic components, CAIs are generally thought to be xenoliths, perhaps formed near the Sun and then later introduced into the accretion regions of chondrites. Among issues that are not clear is the relationship(s) between anomalies at small spatial scales and large, nebula-scale, distributions of isotopic signatures, some of which have been interpreted as reflecting a fundamental dichotomy in accretion regions and/or timing in the nebula. Seven months after the fall of Allende, an even more exotic gift arrived above the skies of south-eastern Australia in the form of the Murchison CM chondrite. CM chondrites also contain refractory inclusions that, while distinctive in petrology, mineral chemistry, and some isotopic compositions from CV CAIs, are likewise generally thought to have formed in the solar nebula. These inclusions are also presumably xenoliths that, for unknown reasons, are very rare in other classes of chondrites, including in particular, the CV chondrites (like Allende). Fifty years of study of the Murchison and Allende meteorites presents an opportunity to reconsider the relationships of the refractory phases present as xenoliths in different classes of chondrites in light of new perspectives gained regarding the nature and distributions of isotopically anomalous material in the solar nebula, including oxy-gen isotopes, short-lived radioactivity, and distinctive nucleosynthetic components.

Nov. 14, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853

Presented By:

• Kaitlyn McCain - UCLA

Carbonaceous chondrites are useful analogues for many water-rich solar system bodies, as they show evidence of water-rock interaction early in the solar system's history. To investigate the chemistry and timescales of these aqueous environments, it is possible to measure the stable isotopic compositions and relative ages of carbonate minerals which precipitated directly from the water. Though diverse carbonates (including calcite, dolomite, and magnesite) are present in carbonaceous chondrites, most previous work has focused on calcite due to analytical difficulties associated with analyzing carbonates with substantial Fe contents. We show results of in-situ measurements by SIMS of carbon and oxygen stable isotopes in calcite and dolomite in the highly altered CM chondrite ALH 84034. We find that calcite and dolomite formed under substantially different alteration regimes, and that matrix-matched standards are essential for interpretation of the oxygen isotopic results. We also give an update on the development of dolomite standards for relative age dating using the Mn-Cr system, which will be applied to the same carbonates to place the aqueous environments of these bodies into the timeline of solar system formation.

Nov. 21, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853

Presented By:

• Abby Kavner - UCLA

A series of recent stable isotope electrodeposition experiments has demonstrated a rate-dependent kinetic isotope effect in a variety of different stable isotope systems, including Fe, Zn, and Li. In this presentation, I present a simplified model for kinetic isotope fractionation resulting from electrochemical reactions, based on a Marcus-type model for electron transfer. The model predicts a rate-dependent kinetic isotope effect, which depends on a small group of dimensionless parameters that define the activation energy trajectory. The model allows predictions of kinetic isotope fractionations as a function of temperature, rate, mass, and thermodynamic properties of the transition state. Finally, I present a “wish-list” of experiments designed to test the models’ hypotheses.

Dec. 5, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853

Presented By:

• Heather Kirkpatrick - UCLA

Seminar Description coming soon.