Geocheminar - winter-2024
How do I and Pu partition during core formation?
Jan. 18, 2024
3:30 p.m. - 4:30 p.m.
3853 Slichter Hall
Presented By:
- Weiyi Liu - Caltech
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In silicate portions of our planet, 129Xe* comes from the decay of now extinct volatile 129I (t1/2=15.7 Ma) and part of the 136Xe*Pu has been produced by refractory extinct 244Pu (t1/2=80 Ma). The 129Xe* /136Xe*Pu ratio evolves as a function of time and reservoirs compositions (i.e. I/Pu ratio) early in the Earth history, and thus could be used to investigate the accretion history of the Earth. Recently, high-precision data on ocean-island basalt (OIB) samples (thought to originate from deep plume mantle) revealed a uniformly low 129Xe*/136Xe*Pu ratio compared to mid-ocean-ridge basalt (MORB) samples (originating from shallow upper mantle). This low 129Xe* /136Xe*Pu ratio in the plume mantle is proved to indicate a low I/Pu was established during accretion processes. Two main competing mechanisms have been proposed to explain the I/Pu contrast between MORB and OIB mantle reservoirs: (i) a heterogeneous volatile history (i.e different I/Pu ratios in building block materials) for the Earth, or (ii) a homogeneous volatile accretion with partitioning of iodine into liquid metal during core formation. However, both models suffer from notable shortcomings. The heterogeneous volatile accretion model doesn’t consider the effect of core formation processes, and neither model considers the Pu partitioning at P-T relevant to core formation due to the near absence of experimental data on metal-silicate Pu partitioning. In this talk, I will show our new I and Pu partitioning data, obtained through the two phase first-principle molecular dynamic method. I will also explain how this new data challenges the homogeneous volatile accretion model and instead supports a heterogeneous accretion history in which volatile-poor differentiated planetesimals is the main accretion materials of the Earth.
A multi-geochemical tracer perspective on calcium carbonate biomineralization
Jan. 25, 2024
3:30 p.m. - 4:30 p.m.
3853 Slichter Hall
Presented By:
- Rob Ulrich - EPSS
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When organisms biomineralize, they record information about the environment they grew in through their geochemistry (e.g., elemental and isotopic ratios) and have become powerful tools for reconstructing past environments. Organisms exert varying levels of control over biomineralization, and this control can alter the resulting geochemistry recorded in the biomineral. Research into these vital effects has shown that they are often species-specific, suggesting the plausibility of geochemical identities and the potential to use geochemical proxies to advance our understanding of how organisms biomineralize. In this week's geocheminar, I will talk about my dissertation work that explores broad-scale patterns and biomineralization mechanisms through the geochemistry of biogenic carbonates from a diverse array of cultured species and through the transformation of abiotic amorphous calcium carbonates (ACC), which is a widespread biomineralization strategy.
Exploring the hydride substitution in perovskites and its implications for deep earth hydrogen
Feb. 1, 2024
3:30 p.m. - 4:30 p.m.
3853 Slichter Hall
Presented By:
- William Palfey - Caltech
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Many of the nominally anhydrous silicates and oxides of Earth’s mantle can incorporate structural hydrogen in the form of OH- (hydroxyl). Taken collectively, the OH- in these minerals accounts for the majority of the planet’s hydrogen, likely constituting multiple ocean’s worth of water, and influences planetary scale processes. However, an emerging branch of materials science research has revealed an alternative to the OH- group for hydrogen speciation in oxides: the H- (hydride) ion. Hydride incorporation is especially favorable in perovskites (the most common class of materials in earth’s interior), but hydride isn’t widely considered as a component of the mantle, representing a possible oversight in Earth’s hydrogen budget. In this talk, I present my work to better understand the H- ion in perovskite materials. First, I will discuss a study on the analogue material SrTiO3 that employs a combination of computational and experimental methods, in particular density functional theory (DFT) and infrared spectroscopy. Using insights gained from SrTiO3, I will explore the possibility of H- incorporation in the mantle-abundant perovskite davemaoite (CaSiO3) and discuss the outlook for detecting H- in deep earth materials.
Water and Redox Evolution in Earth’s Mantle
Feb. 8, 2024
3:30 p.m. - 4:30 p.m.
3853 Slichter Hall
Presented By:
- Jun-Jie Dong - Caltech
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This talk explores the interrelated evolution of water and the oxidation state of the Earth's mantle through thermodynamic modeling. I'll begin by showing how the capacity of the Earth's mantle to hold water increases as it cools. This finding suggests that Earth's early, hotter mantle may have held less water, potentially leading to an Archean "water world" with minimal land exposure. I'll then quantify the iron disproportionation reaction in the cooling mantle and discuss the extent to which the precipitated metallic iron in the mantle reacts with subducted water over Earth's history. Geologic evidence for the mantle redox state will be used here to refine our understanding of the subducted water budget through geologic time, linking the evolution of water and the oxidation state of the Earth's mantle through a deep oxygen cycle.
The Potential Role of Marine Biogenic Methane Cycling on the Early Earth Biosphere: Insights from Green Lake in Fayetteville, New York
Feb. 15, 2024
3:30 p.m. - 4:30 p.m.
3853 Slichter Hall
Presented By:
- Emily Klonicki - EPSS
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Earth has experienced significant shifts in its ecosystems over its long history, propelled by microbial metabolic diversification in its ancient oceans. However, unraveling the contribution of the earliest forms of life to planetary evolution poses a persistent challenge because of limited physical and chemical records. Analog sites have deepened our knowledge of early life and its co-evolving environments. Green Lake near Fayetteville, New York, is an exceptional analog site relevant to the Proterozoic oceans (2.5 to 0.541 Ga), given its persistent anoxic/euxinic conditions and productive shallow chemocline. The relevance of this modern analog is elevated because of remaining uncertainties in methane’s early role in the primitive biosphere. Using a combination of sediment and water column analysis across the chemocline, potential electron acceptors and donors (sulfur, nitrogen, iron, and carbon-species) were constrained. Relevant methane cycling metabolic rates were investigated using radiotracer techniques, specifically, incubations with 14C-methane, 14C-mono-methylamine, and 35S-sulfate were conducted ex-situ. The evidence examined from Green Lake supports the notion of productive biogenic methane cycling in early euxinic settings with important implications for climate regulation and biosignatures that are relevant within and beyond our solar system.
Unveiling lunar volcanic gas: A peek through samples
Feb. 22, 2024
3:30 p.m. - 4:30 p.m.
3853 Slichter Hall
Presented By:
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Volatile-rich magma is present on the Moon as evidenced by the pyroclastic deposits and by vesicular basalts. Despite these examples, we only started to recognize the presence of OH in the pre-eruptive lunar magma about 15 years ago. In contrast, sample-based observations of volcanic gas are mostly absent, mainly due to the fact the volcanic gas involved in these eruptions had long gone. However, vapor deposits left by lunar volcanic gas can provide insights on the chemistry and species of lunar volcanic gas. Here, I present our recent observations at nano- to micro-meter scale in lunar samples enabled by microbeam techniques.
Tracing origins of magmas at convergent plate boundaries by potassium stable isotopes
Feb. 29, 2024
3:30 p.m. - 4:30 p.m.
3853 Slichter Hall
Presented By:
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Convergent plate boundaries are where two lithospheric plates collide to form either a subduction zone or a collision orogenic belt. They are the key tectonic units for crustal formation and differentiation as well as crust-mantle interactions. In this talk, I will discuss the recent applications of K stable isotope geochemistry in studying the petrogenesis of island arc basalts in representative subduction zones and leucogranites in the Himalaya-Tibet orogenic belt. Our results suggest that fluids and melts derived from the subducted sediments and altered basalts contribute differently to different types of arc lavas, and extreme fractional crystallization plays a key role in the formation and evolution of leucogranites. Potassium isotope geochemistry emerges as a powerful tracer of magmatic processes and crust-mantle interactions.
Miocene Climate Evolution in the Uplifting Patagonian Andes
March 7, 2024
3:30 p.m. - 4:30 p.m.
3853 Slichter Hall
Presented By:
- Daniel Sepulveda/Hayley Bricker - EPSS, UCLA
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We examine variations in climate associated with the uplift of the southern Patagonian Andes, which creates an orographic rain shadow effect that significantly impacts the southern hemisphere westerlies. We measured the clumped isotope composition of Miocene paleosols from the Santa Cruz Formation to gain insights into the evolution of climate and precipitation in the eastern foreland of the Patagonian Andes. Our preliminary results indicate a gradual increase in mean annual temperatures (MAT) from approximately 22.8°C ± 7.0°C (1 SE) in the early Miocene to 31.9°C ± 4.7°C during the Middle Miocene Climatic Optimum (MMCO) (~17 to 14 million years ago). Reconstructed water δ18O (V-SMOW) shows a slight enrichment from -10.2‰ ± 1.5‰ to -9.1‰ ± 0.9‰ from the early Miocene to the MMCO. At the same time, there are increasing δ13C values, and δ18O values exhibit a slight decrease from -11.95‰ ± 0.16‰ VPDB in the early Miocene to -12.73‰ ± 0.17‰ VPDB in the MMCO. The analysis of paleosol samples utilizing clumped isotope thermometry can provide valuable insights into Earth's climate system and geological processes.
Model-Proxy Comparison of Last Glacial Maximum Climate in the Eastern Canary Islands
March 7, 2024
3:30 p.m. - 4:30 p.m.
3853 Slichter Hall
Presented By:
- Daniel Sepulveda/Hayley Bricker - EPSS
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During the Last Glacial Maximum (LGM), global temperatures are estimated to be ~6 oC cooler than present. Large-scale proxy syntheses and assimilation products largely focus on the mid- and high-latitude northern hemisphere, and marine environments. Here, we present work applying carbonate clumped paleothermometry, a paleotemperature proxy method constrained by thermodynamics, to terrestrial snail shells from the Canary Islands in the subtropical Atlantic off the coast of NW Africa to elucidate environmental conditions during the LGM, including mean annual surface air temperature and δ18O of paleoprecipitation. Proxy data support mean annual air temperatures during the LGM that were ~8 oC cooler than modern. We use these data to evaluate model skill, with a focus on climate models from the Paleoclimate Modeling Intercomparison Project (PMIP3), the PMIP3 model ensemble mean, and the Hadley Center’s isotope-enabled model. We find that all models underestimate the magnitude of cooling in this region. A comparison of reconstructed mean annual temperatures with a LGM paleodata assimilation product (Tierney et al., 2020) shows good agreement, bolstering confidence in this product.
Insights into rock formation and ocean circulation from titanium and zirconium isotopes
March 14, 2024
1:30 p.m. - 2:30 p.m.
3853 Slichter Hall
Presented By:
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New advances in multicollector inductively coupled plasma mass spectrometry have allowed geoscientists to measure extremely small variations in the isotope ratios of elements across the periodic table. Stable titanium (Ti) isotopes have been applied to the rock record to understand crustal differentiation because they are tracers of fractional crystallization, and they are also sensitive tracers of magmas produced in different tectonic settings. In this talk, I will discuss Ti isotope compositions of orthogneisses from the Acasta Gneiss Complex spanning the Hadean to Eoarchean transition, which fall on two distinct magmatic differentiation trends. Hadean tonalitic gneisses how Ti isotopic compositions comparable to modern evolved tholeiitic magmas, formed by differentiation of dry parental magmas in plume settings. Younger Eoarchean granitoid gneisses have Ti isotopic compositions comparable to modern calc-alkaline magmas produced in convergent arcs. Zirconium (Zr) isotopes also recently emerged as tracers of magmatic processes; however, few studies have focused on its behavior in low temperature or aqueous environments. I will describe a new method for analysis of Zr isotopes in water samples, and the application of this method to natural seawater samples collected from a water column profile in the Pacific Ocean. We find large mass-dependent isotope fractionations in our seawater profile which vary with respect to water depth and salinity, which suggests that Zr isotopic compositions may be sensitive to seawater chemical properties and source.