Hydrous cumulates in arc crust, refinements to the amphibole sponge model

<p dir="ltr">The fractional crystallization of amphibole-rich cumulate bodies in the crust of convergent margins have been shown to have a profound effect on ascending melts, acting as a storage area or “sponge” for H2O and incompatible trace elements. These bodies are inferred to be located primarily in the mid to lower crust, as experimental studies indicate that amphibole is not stable at low pressures. In order to infer the presence of these cumulate bodies, rare earth element (REE) ratios are used, as the fractionation of significant quantities of amphibole will cause a depletion in the Dy/Yb ratio of a melt, based on the partition coefficients between amphibole and basaltic melts. In this thesis, the amphibole sponge model is investigated in two areas representative of upper and lower crustal pressures, and with an experimental study of apatite/melt trace element partitioning.</p><p dir="ltr">Two primary styles of petrogenesis have been proposed for amphibole cumulate bodies, fractionation directly from a melt, or formation via melt rock interaction between a typically dry rock and a hydrous melt. New petrographic and geochemical analyses of amphibole-rich cumulate rocks from a series of discontinuous plutons in the Median Batholith of New Zealand provide insights into the dominant method of petrogenesis for the amphibole sponge in the lower crust. Two groups of amphibole can be distinguished on the basis of textural and geochemical patterns, one group of igneous amphibole formed via fractionation directly from a melt, whereas the other group of amphibole formed via interaction between hydrous melts and pre-existing rocks. Direct crystallisation from a melt is the dominant style of formation for the amphibole cumulates in the Median Batholith, with melt-rock interaction amphibole playing a subordinate role. Our geochemical analyses of these amphiboles show that the fractionated amphibole would cause depletions of the Dy/Yb ratio in their associated melts in accordance with the amphibole sponge model, while the amphiboles formed by melt-rock interaction have more complex effects on their associated melts depending on the precursor and co-crystallizing mineral assemblages.</p><p dir="ltr">Some varieties of amphibole are stable under conditions in the upper crust. Amphibole from an upper crustal pluton at Torres del Paine (TPIC), Chile, is characterised here to provide insights into the effects of amphibole fractionation from an upper crustal pluton. The crystallization of amphibole at upper crustal conditions has significant implications for the amphibole sponge model, as the partition coefficient ratios for MREE/HREE increase with decreasing pressure, thus having a greater effect on the Dy/Yb ratios of their melts than amphibole formed lower in the crust. In addition to these effects from amphibole, other minerals in the TPIC pluton also show strong differentiation between the REEs. Titanite and apatite are abundant in the TPIC mafic rocks, and fractionation modelling shows that the fractionation of even small amounts of these minerals could alter the MREE/HREE of a melt significantly. Comparison of amphibole from the TPIC to upper crustal and volcanic amphibole from across the Andes, many of which also contain apatite or titanite, indicates that these upper crustal amphibole may be more significant than previously thought.</p><p dir="ltr">In order to infer the presence of cryptic amphibole fractionation in the crust, MREE/HREE ratios are used. Of the common mafic minerals, only garnet, amphibole and clinopyroxene differentiate between the REEs. Partition coefficients for amphibole show that it preferentially incorporates the MREEs over the HREEs, while garnet does the opposite. Clinopyroxene has a similar effect to amphibole, however it has partition coefficients nearly an order of magnitude lower than amphibole. Thus, amphibole fractionation is interpreted to cause a depletion of Dy/Yb in ascending melts.</p><p dir="ltr">Minor minerals such as apatite or zircon, which also differentiate between the REEs, are inferred to have little effect on the composition of the ascending melt, as they are typically low in abundance. However, apatite is a common mineral in many studies of the amphibole sponge and is present in small amounts in all of our study areas, sometimes reaching nearly 5 modal percent of a sample. New experimentally determined partition coefficients for apatite in andesitic melts show that apatite has significantly higher partition coefficients when compared to amphibole and fractionates Dy/Yb even more than amphibole. Fractionation models indicate that even small amounts (<5%) of apatite fractionation can cause significant changes in Dy/Yb ratios in their melts, amplifying the effect of amphibole fractionation. These new experiments also demonstrate the substantial impact of phosphorus on melt structures, with important implications for apatite saturation and the formation of immiscible phosphorus-iron melts alongside silicate melts.</p>