5 results for Price, RC

  • Are arc-type rocks the products of magma crystallisation? Observations from a simple oceanic arc volcano: Raoul Island, Kermadec Arc, SW Pacific.

    Smith, Ian; Stewart, RB; Price, RC; Worthington, TJ (2010)

    Journal article
    The University of Auckland Library

    Raoul Island is the emergent summit of a large intra-oceanic strato-volcano in what is globally one of the simplest of subduction settings. In this simple setting erupted magmas span the compositional range from basalt to dacite but none have the high Mg-numbers and high Ni and Cr expected of primitive mantlederived melts. The lavas range from aphyric to highly porphyritic and are characterised by phenocryst assemblages dominated by plagioclase accompanied by clinopyroxene, olivine, orthopyroxene and spinel. Phenocryst core compositions and zoning profiles are remarkably uniform irrespective of total phenocryst content or geochemical composition, indicating a decoupling of melt and crystal components in the system. A consistent model for the Raoul magmatic system is that primitive high-Mg magma generated in a melt column within the underlying mantle wedge is transformed into a series of derivative low-Mg magmas by fractional crystallisation within the lower crust. Low-Mg magma accumulates variable quantities of crystal cargo as it ascends toward the surface through a crystal mush zone. These processes are essentially those that characterise continental subduction-related magmatic systems but differ only in the absence of an evolved crustal component.

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  • Petrogenesis of Dacitie in an oceanic subduction environment: Raoul Island, Kermadec Arc

    Smith, Ian; Worthington, TJ; Price, RC; Stewart, RB; Maas, R (2006)

    Journal article
    The University of Auckland Library

    Raoul Volcano in the northern Kermadec arc is typical of volcanoes in oceanic subduction systems in that it is composed mainly of low-K high-Al basalts and basaltic andesite. However, during the last 4 ka Raoul Volcano has produced mainly dacite magma in pyroclastic eruptions associated with caldera formation. The rocks produced in these episodes are almost aphyric containing only sparse crystals of plagioclase, clinopyroxene, orthopyroxene and magnetite. These apparent phenocrysts have chemical compositions that suggest that they did not crystallise from melts with the chemical composition of their host rocks. Rather they are xenocrysts and only their rims show evidence for crystallisation from their host melt. Chemical compositions of samples of the dacites show that each eruption has tapped a distinct magma batch. Compositional variations through the analysed suite cannot be accommodated in any reasonable model of fractional crystallisation from likely parental magma compositions. The hypothesis that best fits the petrology of Raoul Island dacites is one of crustal anatexis. This model requires heating of the lower crust by a magma flux to the point where dehydration melting associated with amphibole breakdown produces magma from a preconditioned source. It is suggested that Raoul is passing through an adolescent stage of development in which siliceous melts are part of an open system in which felsic and mafic magmas coexist.

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  • Tonga-Kermadec arc and Havre-Lau back-arc system: Their role in the development of tectonic and magmatic models for the western Pacific

    Smith, Ian; Price, RC (2006)

    Journal article
    The University of Auckland Library

    The convergent plate boundary marked by the Tonga???Kermadec arc system is one of the clearest examples of an intra-oceanic arc on the modern Earth. Arising from ground breaking research in the 1970's and continuing through the present, the arc has become a testing ground for defining and explaining models for the tectonic and magmatic evolution of intra-oceanic subduction systems. Despite its simple oceanic setting the arc shows significant variations along its length in terms of rock types and structure and because of the lack of involvement of continental crust these allow refinement of the details of or models of how arcs develop and evolve. Of particular importance are the variations in convergence rates and vectors between the major plates and the effect these have on the tectonic evolution of the arc and back-arc. The erupted rocks of the arc have provided the means of testing the contribution of subducted sediment, slab, mantle wedge and overlying crust and most recently tests of P???T???time paths for recycled sediments. Although the arc is dominated by basalts and basaltic andesites, there is increasing evidence for significant proportions of silicic magmas erupted in recent times. Silicic volcanism is also evidenced in the presence of caldera structures revealed by detailed multibeam mapping that have been a feature of research in the last decade. As we enter the 21st century, the Tonga??? Kermadec arc continues as an archetypical natural laboratory for the testing of new ideas in subduction zone tectonics and petrology.

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  • An assessment of the mantle and slab components in the magmas of an oceanic arc volcano: Raoul Volcano, Kermadec arc.

    Smith, Ian; Price, RC; Stewart, RB; Worthington, TJ (2009)

    Journal article
    The University of Auckland Library

    Raoul Volcano occupies a simple oceanic subduction setting in the northern part of the Kermadec arc on the Pacific???Australian convergent plate boundary. The primary inputs to the magmatic system that feeds the volcano are a subduction component derived from the subducting old Pacific oceanic lithosphere and its veneer of pelagic sediment, and the overlying peridotitic mantle wedge. Conservative trace elements that are very incompatible during mantle melting are relatively depleted in Raoul lavas indicating a source that has been depleted during an earlier melting event. Major element co-variations indicate magma genesis by 25% near fractional melting of a mantle source that is weakly depleted (2% melt extraction) relative to a fertile MORB source. An important influence on the composition of the mantle component is progressive melt extraction coupled with minimal advection of fresh material into the sub-arc zone followed by melt extraction from a melting column beneath the spreading centre of an adjacent back arc basin. High field strength element and rare earth element systematics indicate involvement of a subduction-related component of constant composition. Two fluid components can be distinguished, one enriched in large ion lithophile elements inferred to be an aqueous fluid that is continuously added to the ascending melt column and the other a less mobile fluid that transfers Th. A homogeneous subduction-related component of constant composition and magnitude arises if the slab-derived flux migrates from the slab???mantle interface to the sub-arc melting column by repeated episodes of amphibole formation and decomposition its composition is then governed by the distribution coefficients of pyroxene and its magnitude by the degree of amphibole saturation of mantle peridotite. The results from Raoul Volcano are comparable to those from other oceanic subduction-related arcs such as South Sandwich and Marianas suggesting that this is a general model for oceanic arcs.

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  • New insights into the evolution of the magmatic system of a composite andesite volcano revealed by clasts from distal mass-flow deposits: Ruapehu volcano, New Zealand

    Tost, Manuela; Price, RC; Cronin, Shane; Smith, Ian (2016-05)

    Journal article
    The University of Auckland Library

    Stratovolcanoes characteristically build large composite edifices over long periods with stacked lavas intercalated with pyroclastic deposits. In most cases, only the most recent volcanic products are exposed on the flanks of the volcano, and consequently the search for deposits recording an older eruptive and magmatic history is typically focussed far from the cone, within distal tephra deposits. Clasts within lahar and debris avalanche deposits may also provide unique insights into the earliest eruptive and magmatic history of long-lived volcanoes, especially when widespread fallout is absent. Careful sampling and subsequent petrological and geochemical analyses of lava and pumice clasts from six distal mass-flow deposit sequences (hyperconcentrated flow, debris flows and debris avalanche deposits) from Mt. Ruapehu (New Zealand), combined with detailed stratigraphic studies and radiometric age dating, give new perspectives on the pre-50 ka magmatic system of this complex volcano. A conglomerate emplaced between 340 and 310 ka contains evidence for the oldest episode of Mt. Ruapehu volcanism, and unusually for the composite cone, pumice clasts from this unit contain amphibole-bearing xenoliths. Chemical and petrological data for these oldest Ruapehu clasts indicate that a deep (???40 km) crustal storage system had already developed under Mt. Ruapehu before ???340 ka. From the very earliest stages, evolution was largely controlled by magma mixing, along with decoupled assimilation and fractional crystallization within numerous isolated small-scale magma batches stored throughout the crust. From around 340 to 160 ka, there was a progressive shift towards more primitive compositions, suggesting that during this period large-scale replenishment events involving mantle-derived basaltic magmas occurred within the mid- to upper crustal storage system. Subsequent magmas became progressively more evolved due to decoupled fractional crystallization and assimilation processes accompanied by magma recharge events, which triggered major phases of eruptive activity

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