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  1. Abstract

    Dike swarms are the fossil remains of regions of the crust that have undergone repeated magma injections. Volcanic earthquake swarms and geodetic measurements are, at least in part, a record of active injection of fluids (water, gas, or magma) into fractures. Here, we link these two ways of observing magmatic systems by noting that dike thicknesses and earthquake magnitudes share similar scaling parameters. In the Jurassic Independence dike swarm of eastern California median dike thickness is ∼1 m, similar to other swarms worldwide, but glacially polished exposures reveal that a typical dike comprises a number of dikelets that are lognormally distributed in thickness with a mean of ∼0.1 m. Assuming that dikes fill penny‐shaped cracks of a given aspect ratio, the geodetic moment and earthquake magnitude of a diking event can be estimated. A Monte Carlo simulation of dike‐induced earthquakes based on observed dike thickness variations yields a frequency‐magnitude distribution remarkably like observed volcanic earthquake swarms, with ab‐value of ∼1.7. We suggest that swarms of dikes composed of dikelets, as well as plutons built incrementally by sheet intrusions, are physical complements to volcanic seismic swarms, and that at least some earthquake swarms are a palpable expression of incremental magma emplacement.

     
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  2. Ian Carmichael wrote of an “andesite aqueduct” that conveys vast amounts of water from the magma source region of a subduction zone to the Earth’s surface. Diverse observations indicate that subduction zone magmas contain 5 wt % or more H2O. Most of the water is released from crystallizing intrusions to play a central role in contact metamorphism and the genesis of ore deposits, but it also has important effects on the plutonic rocks themselves. Many plutons were constructed incrementally from the top down over million-year time scales. Early-formed increments are wall rocks to later increments; heat and water released as each increment crystallizes pass through older increments before exiting the pluton. The water ascends via multiple pathways. Hydrothermal veins record ascent via fracture conduits. Pipe-like conduits in Yosemite National Park, California, are located in or near aplite–pegmatite dikes, which themselves are products of hydrous late-stage magmatic liquids. Pervasive grain-boundary infiltration is recorded by fluid-mediated subsolidus modification of mineral compositions and textures. The flood of magmatic water carries a large fraction of the total thermal energy of the magma and transmits that energy much more rapidly than conduction, thus enhancing the fluctuating postemplacement thermal histories that result from incremental pluton growth. The effects of water released by subduction zone magmas are central not only to metamorphism and mineralization of surrounding rocks, but also to the petrology and the thermal history of the plutons themselves. 
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  3. 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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