Browsing by Author "Zellmer G"

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    Crystal entrainment from cool, low-silica rocks into hot, high-silica melts: diverse primary melt compositions at Taranaki volcano, New Zealand
    (The Geological Society of London, 2023-05-19) D'Mello N; Zellmer G; Kereszturi G; Ubide T; Procter J; Stewart R
    The prevalence of antecrysts in arc volcanic rocks is widely accepted, yet the origin of their carrier melts remains debated. Crystal cargo in lava flows from Taranaki volcano, New Zealand, is dominated by plagioclase, clinopyroxene and amphibole. Except for some crystal rims, mineral phases are in disequilibrium with the melt they are entrained in. Major element chemistry reveals an almost complete compositional overlap between the crystals in the lava and those in xenoliths. The large volume fraction of crystals (35–55 vol%) exerts a strong control on whole-rock compositions, reducing silica by 5–11 wt% compared with the carrier melt. Yet there is no clear relationship between mineral proportion and bulk-rock compositions. Our data are inconsistent with extensive fractional crystallization, commonly invoked as a driver of magma evolution towards silica-rich compositions. Instead, high-temperature, aphyric carrier melts with varied compositions (55–68 wt% SiO2) entrain crystal cargo while ascending through colder, low-silica rocks. Thus, some parental melts at Taranaki volcano are significantly more silica-rich than arc basalts commonly invoked as primary magmas. Further, thermometric and hygrometric constraints preclude a deep crustal hot zone for the source of these melts, which we argue are of subcrustal origin.
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    Olivine melt inclusion constraints on some intensive properties of subvolcanic crystal mushes and their evolution through boundary layer fractionation in northern Japan
    (Oxford University Press, 2022-03-02) Brahm R; Zellmer G; Kuritani T; Sakamoto N; Yurimoto H; Nakagawa M; Sato E
    Magma differentiation in arc settings has usually been attributed to an interplay of processes (fractional crystallization, assimilation, and magma mixing). Homogeneous fractional crystallization has been widely used to model the magmatic evolution of volcanic systems in arc settings due to its simplicity, even though boundary layer fractionation (BLF) has been proposed as a preponderant process of differentiation in hydrous magmatic systems. Both models produce distinct compositional paths and the application of the wrong model yields erroneous estimates of parameters like pressure–temperature-H2O conditions and primary melt compositions. Melt inclusion (MI) populations corrected for post-entrapment processes have the potential to help discriminate between these two types of fractional crystallization, as their compositions are not affected by crystal accumulation and should capture the magmatic evolution as crystallization occurs. In this study, olivine-hosted MIs are used to assess the differentiation trends of basic arc magmas in northern Japan. Differentiation trends from five arc volcanic systems in northern Japan show that BLF is ubiquitous. Homogeneous fractionation models are unable to explain the liquid lines of descent of minor elements, like TiO2 and P2O5. To reproduce these differentiation trends, the presence of accessory phases like titanomagnetite or apatite are required, which in many cases are not equilibrated by the melt or need to be fractionated in amounts that are incompatible with homogeneous fractionation. The prevalence of BLF in all studied arc magmas of northern Japan indicates that solidification fronts are key environments in the crustal evolution of some hydrous subduction zone magmas.
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    Olivine-Hosted Melt Inclusions Track Progressive Dehydration Reactions in Subducting Slabs Across Volcanic Arcs
    (Oxford University Press, 2024-04) Brahm R; Coulthard D; Zellmer G; Kuritani T; Sakamoto N; Taniuchi H; Yurimoto H; Nakagawa M; Sato E
    The stability and breakdown of mineral phases in subducting slabs control the cycling of trace elements through subduction zones. Stability of key minerals and the partitioning of trace elements between these minerals and liquid phases of interests have been charted by natural sample analysis and experimental constraints. However, systematic study from arc front to far back arc has rarely shown that the expected geochemical variations of the slab liquid are actually recorded by natural samples. Complexities arise by uncertainties on the nature of the slab component (melts, f luids and supercritical liquids), source heterogeneities and transport processes. Using data from olivine-hosted melt inclusions sampled along and across the NE Japan and southern Kurile arcs, we demonstrate that experimentally and thermodynamically constrained phase stabilities in subducted materials indeed control the trace element signatures as predicted by these models and experiments. The main reactions that can be traced across arc are progressive breakdown of light rare earth element-rich accessory phases (e.g. allanite), enhanced dehydration of the lithospheric mantle (serpentine breakdown) and changes in the nature of the slab component. This work elucidates subduction zone elemental cycling in a well-characterized petrogenetic setting and provides important constraints on the interpretation of trace element ratios in arc magmas in terms of the prograde metamorphic reactions within the subducting slab.