Sediment Melts at Sub-arc Depths: an Experimental Study

The phase and melting relations in subducted pelites have been investigated experimentally at conditions relevant for slabs at sub-arc depths (T = 600-1050°C, P = 2·5-4·5 GPa). The fluid-present experiments produced a dominant paragenesis consisting of garnet-phengite-clinopyroxene-coesite-kyanite that coexists with a fluid phase at run conditions. Garnet contains detectable amounts of Na2O (up to 0·5 wt%), P2O5 (up to 0·56 wt%), and TiO2 (up to 0·9 wt%) in all experiments. Phengite is stable up to 1000°C at 4·5 GPa and is characterized by high TiO2 contents of up to 2 wt%. The solidus has been determined at 700°C, 2·5 GPa and is situated between 700 and 750°C at 3·5 GPa. At 800°C, 4·5 GPa glass was present in the experiments, indicating that at such conditions a hydrous melt is stable. In contrast, at 700°C, 3·5 and 4·5 GPa, a solute-rich, non-quenchable aqueous fluid was present. This indicates that the solidus is steeply sloping in P-T space. Fluid-present (vapour undersaturated) partial melting of the pelites occurs according to a generalized reaction phengite + omphacite + coesite + fluid = melt + garnet. The H2O content of the produced melt decreases with increasing temperature. The K2O content of the melt is buffered by phengite and increases with increasing temperature from 2·5 to 10 wt%, whereas Na2O decreases from 7 to 2·3 wt%. Hence, the melt compositions change from trondhjemitic to granitic with increasing temperature. The K2O/H2O increases strongly as a function of temperature and nature of the fluid phase. It is 0·0004-0·002 in the aqueous fluid, and then increases gradually from about 0·1 at 750-800°C to about 1 at 1000°C in the hydrous melt. This provides evidence that hydrous melts are needed for efficient extraction of K and other large ion lithophile elements from subducted sediments. Primitive subduction-related magmas typically have K2O/H2O of ∼0·1-0·4, indicating that hydrous melts rather than aqueous fluids are responsible for large ion lithophile element transfer in subduction zones and that top-slab temperatures at sub-arc depths are likely to be 700-900°

The age of homo naledi and associated sediments in the rising star cave, South Africa

New ages for flowstone, sediments and fossil bones from the Dinaledi Chamber are presented. We combined optically stimulated luminescence dating of sediments with U-Th and palaeomagnetic analyses of flowstones to establish that all sediments containing Homo naledi fossils can be allocated to a single stratigraphic entity (sub-unit 3b), interpreted to be deposited between 236 ka and 414 ka. This result has been confirmed independently by dating three H. naledi teeth with combined U-series and electron spin resonance (US-ESR) dating. Two dating scenarios for the fossils were tested by varying the assumed levels of222Rn loss in the encasing sediments: a maximum age scenario provides an average age for the two least altered fossil teeth of 253 +82/-70 ka, whilst a minimum age scenario yields an average age of 200 +70/-61 ka. We consider the maximum age scenario to more closely reflect conditions in the cave, and therefore, the true age of the fossils. By combining the US-ESR maximum age estimate obtained from the teeth, with the U-Th age for the oldest flowstone overlying Homo naledi fossils, we have constrained the depositional age of Homo naledi to a period between 236 ka and 335 ka. These age results demonstrate that a morphologically primitive hominin, Homo naledi, survived into the later parts of the Pleistocene in Africa, and indicate a much younger age for the Homo naledi fossils than have previously been hypothesized based on their morphologyWe would also like to thank the many funding agencies that supported various aspects of this work. In particular we would like to thank the National Geographic Society, the National Research Foundation and the Lyda Hill Foundation for significant funding of the discovery, recovery and initial analysis of this material. Further support was provided by ARC (DP140104282: PHGMD, ER, JK, HHW; FT 120100399: AH). The ESR dosimetry study undertaken by CENIEH and Griffith University has been supported by a Marie Curie International Outgoing Fellowship (under REA Grant Agreement n˚ PIOF-GA-2013–626474) of the European Union’s Seventh Framework Programme (FP7/2007-2013) and an Australian Research Council Future Fellowship (FT150100215). ESR and U-series dating undertaken at SCU were supported by ARC (DP140100919: RJB)

Melanesian arc far-field response to collision of the Ontong Java Plateau: geochronology and petrogenesis of the Simuku Igneous Complex, New Britain, Papua New Guinea

Understanding the evolution of the mid-Cenozoic Melanesian arc is critical for our knowledge of the regional tectonic development of the Australian-Pacific plate margin, yet there have been no recent studies to constrain the nature and timing of magmatic activity in this arc segment. In particular, there are currently no robust absolute age constraints at the plate margin related to either the initiation or cessation of subduction and arc magmatism. We present the first combined U–Pb zircon geochronology and geochemical investigation into the evolution of the Melanesian arc utilizing a comprehensive sample suite from the Simuku Igneous Complex of West New Britain, Papua New Guinea. Development of the embryonic island arc from at least 40 Ma and progressive arc growth was punctuated by distant collision of the Ontong Java Plateau and subduction cessation from26 Ma. This change in subduction dynamics is represented in the Melanesian arc magmatic record by emplacement of the Simuku Porphyry Complex between 24 and 20 Ma. Petrological and geochemical affinities highlight genetic differences between 'normal' arc volcanics and adakite-like signatures of Cu–Mo mineralized porphyritic intrusives. The contemporaneous emplacement of both 'normal' arc volcanics and adakite-like porphyry intrusives may provide avenues for future research into the origin of diverse styles of arc volcanism. Not only is this one of few studies into the geology of the Melanesian arc, it is also among the first to address the distant tectono-magmatic effects ofmajor arc/forearc collision events and subduction cessation on magmatic arcs, and also offers insight into the tectonic context of porphyry formation in island arc settings

Continental collision, orogenesis and arc magmatism of the Miocene Maramuni arc, Papua New Guinea

The Maramuni arc represents the only continuous record of the tectonic evolution of Papua New Guinea during the Miocene, and hence provides an opportunity to gain insight into subduction dynamics, orogenesis and crustal processes that operated throughout this dynamic period. We present an integrated U-Pb geochronology, Hf isotope and geochemical investigation of the Maramuni arc utilizing a suite of intrusive rocks from the Kainantu region of the eastern Papuan Highlands that span the Late Miocene from ca. 12 Ma to 6 Ma. The magmatic rocks formed from ca. 12–9 Ma have compositional affinities of subduction-zone magmas, but record increasing incompatible trace element contents and decreasing εHf with time, which we interpret to reflect a progressive increase in the crustal component of the magmas. Porphyry suites emplaced at 7.5-6 Ma are distinct from the older magmatic rocks by their marked HREE-depletion, which reflects a dramatic shift in arc-mantle dynamics. Based on these results we propose a revised geodynamic model for the tectonic evolution of Papua New Guinea involving arrival of the Australian continent at a north-dipping Pocklingon trough from ca. 12 Ma. Continent collision then led to growth of the New Guinea Orogen from 12 Ma aided by underthrusting of the leading continental margin, which contributed crustal material to magma-genesis at ca. 9 Ma. From ca. 7 Ma slab break-off and lithospheric delamination are reflected in a second phase of orogenesis that produced the HREE-depleted geochemical signatures of the contemporaneous magmatic rocks

Redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies; implications for deep subduction-zone processes

The transfer of fluid and elements from subducting\ud crust to the overlying mantle wedge is a fundamental\ud process affecting arc magmatism and the chemical differentiation of the Earth. While the production\ud of fluid by breakdown of hydrous minerals is well understood, the liberation of trace elements remains\ud generally unconstrained. In this paper, we evaluate the\ud behaviour of trace elements during prograde metamorphism\ud and dehydration using samples of high-pressure,\ud low-temperature metamorphic rocks from New Caledonia.\ud Samples examined include mafic and pelitic rocktypes\ud that range in grade from lawsonite blueschist to eclogite facies, and represent typical lithologies of subducting\ud crust. Under lawsonite blueschist facies conditions,\ud the low temperatures of metamorphism inhibit equilibrium partitioning between metamorphic minerals and allow for the persistence of igneous and detrital minerals. Despite this, the most important hosts for trace-elements include lawsonite, (REE, Pb, Sr), titanite (REE, Nb, Ta), allanite (LREE, U, Th), phengite (LILE) and zircon (Zr, Hf). At epidote blueschist to eclogite facies conditions, trace-element equilibrium may be attained and epidote (REE, Sr, Th, U, Pb), garnet (HREE), rutile (Nb, Ta), phengite (LILE) and zircon (Zr, Hf) are the major trace-element hosts. Chlorite, albite, amphibole and omphacite contain very low concentrations of the investigated trace elements. The\ud comparison of mineral trace-element data and bulk-rock\ud data at different metamorphic grades indicates that trace\ud elements are not liberated in significant quantities by\ud prograde metamorphism up to eclogite facies. Combining\ud our mineral trace-element data with established phase equilibria, we show that the trace elements considered\ud are retained by newly-formed major and accessory minerals during mineral breakdown reactions to depths of up to 150 km. In contrast, significant volumes of fluid are released by dehydration reactions. Therefore, there is a decoupling of fluid release and trace element release in subducting slabs. We suggest that the flux of trace elements from the slab is not simply linked to mineral breakdown, but results from complex fluid-rock interactions and fluid-assisted partial melting in the slab

Remnants of ancient Australia in Vanuatu: Implications for crustal evolution in island arcs and tectonic development of the southwest Pacific

We report new geochemical and geochronological data on igneous rocks of the little-studied Western Belt of the Vanuatu intra-oceanic arc. Ar-Ar dating of igneous hornblende from hornblende andesites and U-Pb dating of zircon from a tonalite place formation of these rocks in the late Eocene to Miocene; hence, they represent part of the earliest arc development at Vanuatu. The petrological and geochemical characteristics of these rocks are typical of island arc magmas, except they contain inherited zircon grains with significant age populations at ca. 2.8–2.5 Ga, 2.0–1.8 Ga, 1.75–1.5 Ga, 850–700 Ma, 530–430 Ma, and 330–220 Ma. This inheritance signature is unlike anything recognized from the oceanic realm of the southwest Pacific, but in general does match the age of major crustal blocks of the Australian continent. An exception is the significant proportion of zircons of Rodinia breakup age (ca. 800 Ma) that previously have not be found in such amounts in eastern Australia or the southwest Pacific. We propose that part of the Vanuatu arc basement comprises continental material that was rifted and transported thousands of kilometers from northeastern Australia prior to the Cenozoic. The presence of hitherto-unrecognized ancient continental material within an intra-oceanic arc provides an alternative source for the crustal trace element and isotopic signature of island-arc magmas, and may help reconcile the relatively large thickness and low density of the crust of Vanuatu and possible other intra-oceanic arcs

The importance of talc and chlorite ‘‘hybrid’’ rocks for volatile recycling through subduction zones; evidence from the high-pressure subduction melange of New Caledonia

The transfer of fluid and trace elements from the slab to the mantle wedge cannot be adequately explained by simple models of slab devolatilization. The eclogite-facies melange belt of northern New Caledonia represents previously subducted oceanic crust and contains a significant proportion of talc and chlorite schists associated with serpentinite. These rocks host large quantities of H2O and CO2 and may transport volatiles to deep levels in subduction zones. The bulk-rock and stable isotope compositions of talc and chlorite schist and serpentinite indicate that the serpentinite was formed by seawater alteration of oceanic lithosphere prior to subduction, whereas the talc and chlorite schists were formed by fluid-induced metasomatism of a melange of mafic, ultramafic and metasedimentary rocks during subduction. In subduction zones, dehydration of talc and chlorite schists should occur at subarc depths and at significantly higher temperatures (* 800C) than other lithologies (400–650C). Fluids released under these conditions could carry high trace element contents and may trigger partial melting of adjacent pelitic and mafic rocks, and hence may be vital for\ud transferring volatile and trace elements to the source\ud regions of arc magmas. In contrast, these hybrid rocks are\ud unlikely to undergo significant decarbonation during subduction and so may be important for recycling carbon into\ud the deep mantle

Petrogenesis of the Greenhills Complex, Southland, New Zealand: magmatic differentiation and cumulate formation at the roots of a Permian island-arc volcano

A Permian (∼265 Ma) intrusive complex which formed as a magmatic feeder reservoir to an immature island-arc volcano is fortuitously exposed in southern New Zealand. Known as the Greenhills Complex, this intrusion was emplaced at shallow crustal levels