Mafic bodies in the Sierra Nevada batholith
Though it is dominantly granodioritic, the Sierra Nevada Batholith preserves numerous upper crustal mafic complexes. These dioritic to gabbroic intrusions have experienced both lower and upper crustal processes in a continental arc, and so are opportune locations to understand arc magmatic fractionation. I have completed field mapping, petrography, geochemical analysis, and U-Pb zircon geochronology from the 18 mafic complexes in the batholith. My fractionation model for these compositions supports a polybaric crystallization hypothesis, with an initial stage of differentiaion in the lower crust (1.1 GPa) and a second stage in the upper crust (0.3 GPa). Download our recent paper in Contributions to Mineralogy and Petrology below, and see my blog post about the geochronology results here.
Effects of caldera-forming eruptions on magma storage systems (field results coming in late-Oct!)
High-silica caldera forming eruptions occur through the evacuation of large magma chambers in the shallow crust, causing collapse of the chamber roof and eruption of large volumes of volcanics. The effects of these types of catastrophic eruptions on the magma storage system- the crystals and melt left behind in the crust after the eruption- are not well understood. Using exposed volcanic through plutonic sections in the Stillwater Range, NV, this work will combine high-precision geochronology, geochemistry, and thermodynamic modeling to explore questions including: how does the volatile content of a shallow magma chamber change following an eruption? How much of the stored magma is eruptible at a given time? Can a magma reservoir erupt multiple times without injection of new melt?
Explosive mid-ocean ridge eruptions
Basaltic glass shards found in sediment cores taken from the flanks of the Pacific Antarctic Ridge and East Pacific Rise are the products of large explosive submarine eruptions. The major and trace element geochemistry of the shards is more evolved than typical MORB, and MELTS modeling shows that the shard compositions can be produced by fractionation of typical MORB from the same ridge axis. Eruptions from the Pacific Antarctic Ridge are approximately 130 ka, placing these eruptions coincident with the penultimate glacial termination, T-II. The melt producing the glass shards stalled and fractionated due to a decrease in melt production during sea level rise (190 ka), and were explosively ejected by rejuvenation of the magmatic system by a melt pulse generated during the sea level drop prior to T-II . This melt pulse takes several thousand years to reach the axial magma chamber after it is generated, leading to large explosive eruptions contemporaneous with T-II.
REE-hosting minerals and the REE budget of BIFs
Whole-rock analyses of rare earth element concentrations and ratios in banded iron formations are used to determine the relative input of iron oxide and silica from different sources (hydrothermal, seawater, detrital), as well as oxidation conditions. But are these whole rock analyses representative of the BIF depositional environment? I have measured REE concentrations in single mineral grains in situ in samples from the Neoproterozoic Wadi Karim BIF. In this case, the iron oxides (magnetite and hematite) and chert host very low concentrations of REEs, so most of the REEs are hosted in an accessory phosphate- apatite. We are using the REE patterns in apatite to investigate its formation process. This will determine if the apatite and whole-rock REE concentrations reflect the depositional environment of the BIF, or if a correction must be made to obtain an accurate record of the BIF source’s REE signature.