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  1. Abstract

    Water is key to plate tectonics on Earth, which, in turn, is vital to the production of continental crust. Although arc lavas erupt in a volatile‐rich state and calc‐alkaline arc plutons are distinguished by the presence of hydrous minerals such as hornblende and biotite, the water content of arc magmas earlier in their evolution—in the deep crust—remains poorly constrained. Here, we report H2O contents in nominally anhydrous minerals measured in situ on petrographic thin sections by secondary ion mass spectrometry of Proterozoic deep crustal xenoliths from Colorado, USA. Clinopyroxene, orthopyroxene, and garnet contain average H2O contents ranging from 75–760, 233–410, and 42–139 ppm, respectively. Reconstructed bulk rock H2O contents range from ~60 to ~650 ppm. Intermineral H2O ratios overlap experimental mineral/meltDvalues and are used to calculate H2O of melts last in equilibrium with the xenoliths. We propose that these xenoliths represent cumulates fractionated from a primitive, hydrous (≥1 wt.% H2O) melt at high (~1 GPa) pressures, similar to conditions in modern subduction zones and potentially associated with widespread arc accretion that formed the core of North America in the Precambrian.

     
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  2. Abstract

    The accessory minerals rutile and apatite are rare or absent in the convecting upper mantle but occur in shallow, cooler, metasomatized continental lithospheric mantle (CLM) where they serve as carrier phases for the trace elements Ta (in rutile) and Th (in apatite). Because both minerals crystallize near‐solidus and are eliminated early during partial mantle melting, the relative abundances of rutile and apatite should control the Ta and Th abundances of mantle melts and provide a means of identifying the involvement of rutile‐ and/or apatite‐bearing metasomatized CLM in mafic continental magmatism. As a test, we investigated published Ta and Th abundances data from ~2,000 whole‐rock samples of mafic to intermediate composition, Cenozoic volcanic rocks in southwestern North America. Roughly half of the samples have Ta/Th values similar to those of island arc volcanic rocks (<0.2) or ocean island and mid‐ocean ridge basalts (>0.6). The remaining samples have intermediate and variable Ta/Th values between 0.2 and 0.6, independent of specific indices of crustal interaction (e.g., wt% P2O5/wt% K2O). We interpret the intermediate Ta/Th rocks as the products of direct melting of, or of extensive melt‐rock interaction with, rutile‐ and/or apatite‐bearing CLM. Intermediate Ta/Th rocks also have uniformly high87Sr/86Sr (0.706 to 0.708) compared to oceanic basalts that, unlike their Nd isotopic compositions, do not covary with lithospheric age. These observations are consistent with widespread metasomatism of the CLM by Sr‐rich, Nd‐poor, aqueous fluids generated by dehydration of oceanic lithosphere, and its overlying tectonic mélange during early Cenozoic subduction beneath southwestern North America.

     
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