Abstract The first known occurrence of rhyolite along the submarine segments of the mid-ocean ridge (MOR) system was discovered on Alarcon Rise, the northernmost segment of the East Pacific Rise (EPR), by the Monterey Bay Aquarium Research Institute in 2012. Zircon trace element and Hf and O isotope patterns indicate that the rhyolite formed by extreme crystal fractionation of primary mid-ocean ridge basalt (MORB) sourced from normal to enriched MOR mantle with little to no addition of continental lithosphere or hydrated oceanic crust. A large range in zircon ɛHf spanning 11 ɛ units is comparable to the range of whole rock ɛHf from the entire EPR. This variability is comparable to continental granitoids that develop over long periods of time from multiple sources. Zircon geochronology from Alarcon Rise suggests that at least 20 kyr was needed for rhyolite petrogenesis. Grain-scale textural discontinuities and trace element trends from zircon cores and rims are consistent with crystal fractionation from a MORB magma with possible perturbations associated with mixing or replenishment events. Comparison of whole rock and zircon oxygen isotopes with modeled fractionation and zircon-melt patterns suggests that, after they formed, rhyolite magmas entrained hydrated mafic crust from conduit walls during ascent and/or were hydrated by seawater in the vent during eruption. These data do not support a model where rhyolites formed directly from partial melts of hydrated oceanic crust or do they require assimilation of such crust during fractional crystallization, both models being commonly invoked for the formation of oceanic plagiogranites and dacites. A spatial association of highly evolved lavas (rhyolites) with an increased number of fault scarps on the northern Alarcon Rise might suggest that low magma flux for ~20 kyr facilitated extended magma residence necessary to generate rhyolite from MORB.
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Spreading rate-dependent variations in crystallization along the global mid-ocean ridge system: CRYSTALLIZATION AT MID-OCEAN RIDGES
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Abstract We present a new algorithm,
ReversePetrogen (RevPet) , to infer mantle melting conditions (pressure, temperature, source composition) using evolved basalts that have experienced multiphase fractional crystallization.RevPet measures and minimizes the compositional distance between experimentally predicted phase saturation boundaries and an erupted basalt and the more primitive liquids that return it to a primary melt. We useRevPet to investigate mantle melting conditions at mid‐ocean ridges (MORs) using a global data set of 13,589 basaltic glasses. We find that their average apparent mantle potential temperature (T P*) is 1322°C ± 56°C with melting pressures of 13.0 ± 5 kbars. Inferring the true initial (pre‐melted)T PfromT P* requires knowing the style and degree of melting of the input basalts. If MORB glasses were entirely produced by near‐fractional melting of a homogenous source, they would record the cooling of the mantle during melting from an initialT P = ∼1380°C (ΔT P = 0°C) down toT P = ∼1270°C. If, instead, they were all fully pooled near‐fractional melts of the same source, they would record variations in ambient MORT Pfrom ∼1300°C to 1450°C (ΔT P = 150°C). However, because MOR basalts are thought to be both near‐fractional and variably pooled melts of variable sources, MORT Pmust be intermediate between these two extremes. Our best estimate, consistent with MOR crustal thickness, is that ambient MORT Pis homogenous (∼1350°C–1400°C) except near hotspots whereT Preaches ∼1600°C. Some primitive glasses found near slow‐spreading ridges and back‐arcs record very low temperatures (T P* < 1250°C) and pressures of melting (<10 kbar) and reflect mantle cooling during melting and melt equilibration in the mantle lithosphere.
