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Open Access Publications from the University of California

Scholarly Works (4 results)

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UC San Diego Electronic Theses and Dissertations (2024)

Titanium (Ti) isotopes are a well-established tracer of magmatic differentiation capable of providing useful insight into the magma series (e.g., calc-alkaline versus tholeiitic) and geodynamic setting of rock formation. Titanium isotope fractionation is sensitive to oxygen fugacity, water content, and the initial geochemical composition of a melt, however the impacts of partial melting during plate subduction upon the Ti isotopic composition of a rock are still not well constrained. In this study, we use a suite of samples from two relatively modern, well-characterized sites from the Aleutian arc thought to have little to no influence from subducted sediment in the source of the arc magmas to probe how formation processes (e.g., partial melting versus fractional crystallization/magma mixing) influences the measured Ti isotope compositions. We present Ti isotopic compositions for 18 volcanic samples from two Aleutian arc sites; Western Aleutian seafloor lavas from the Ingenstrem Depression, thought to have formed through partial melting of mid-ocean ridge basalt (MORB) in eclogite facies, and lavas from Korovin Volcano on Atka Island, which formed from fractional crystallization and/or magma mixing. Samples dredged from the Ingenstrem Depression span a relatively narrow range in Ti isotopic composition (δ49Ti = +0.031 to +0.147‰) with a relatively muted trend in Ti isotopic composition with respect to increasing silica content (50.41 to 66.30 wt.%). The samples from Korovin Volcano span a larger range (δ49Ti = -0.021 to +0.355‰) and more pronounced Ti isotopic compositions with increasing silica content (50.38 to 64.09 wt.%). We use thermodynamic isotopic modeling (using THERMOCALC software 3.53c) to demonstrate that the rocks from Ingenstrem Depression were formed through partial melting of MORB in eclogite facies and rocks from Korovin Volcano are likely formed by mixing of a mafic and dacitic magmas.

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  • Peer Reviewed
UC San Diego Previously Published Works (2020)
Plate subduction greatly influences the physical and chemical characteristics of Earth's surface and deep interior, yet the timing of its initiation is debated because of the paucity of exposed rocks from Earth's early history. We show that the titanium isotopic composition of orthogneisses from the Acasta Gneiss Complex spanning the Hadean to Eoarchean transition falls on two distinct magmatic differentiation trends. Hadean tonalitic gneisses show titanium isotopic compositions comparable to modern evolved tholeiitic magmas, formed by differentiation of dry parental magmas in plume settings. Younger Eoarchean granitoid gneisses have titanium isotopic compositions comparable to modern calc-alkaline magmas produced in convergent arcs. Our data therefore document a shift from tholeiitic- to calc-alkaline-style magmatism between 4.02 and 3.75 billion years (Ga) in the Slave craton.
    Cover page: Titanium isotopes constrain a magmatic transition at the Hadean-Archean boundary in the Acasta Gneiss Complex
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2017)
      Cover page: Geochemical characterization of critical dust source regions in the American West
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2022)
      Dust provides an ecologically significant input of nutrients, especially in slowly eroding ecosystems where chemical weathering intensity limits nutrient inputs from underlying bedrock. In addition to nutrient inputs, incoming dust is a vector for dispersing dust-associated microorganisms. While little is known about dust-microbial dispersal, dust deposits may have transformative effects on ecosystems far from where the dust was emitted. Using molecular analyses, we examined spatiotemporal variation in incoming dust microbiomes along an elevational gradient within the Sierra Nevada of California. We sampled throughout two dry seasons and found that dust microbiomes differed by elevation across two summer dry seasons (2014 and 2015), which corresponded to competing droughts in dust source areas. Dust microbial taxa richness decreased with elevation and was inversely proportional to dust heterogeneity. Likewise, dust phosphorus content increased with elevation. At lower elevations, early season dust microbiomes were more diverse than those found later in the year. The relative abundances of microbial groups shifted during the summer dry season. Furthermore, mutualistic fungal diversity increased with elevation, which may have corresponded with the biogeography of their plant hosts. Although dust fungal pathogen diversity was equivalent across elevations, elevation and sampling month interactions for the relative abundance, diversity, and richness of fungal pathogens suggest that these pathogens differed temporally across elevations, with potential implications for humans and wildlife. This study shows that landscape topography and droughts in source locations may alter the composition and diversity of ecologically relevant dust-associated microorganisms.
        Creative Commons 'BY-NC-ND' version 4.0 license
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