The Re-Os isotope system has been applied to several marine and lacustrine petroleum systems worldwide, showing good potential for dating crude oils and correlating them to their source rocks. Here, for the first time, we explore the applicability of the Re-Os geochronometer and Os isotope fingerprinting to terrestrial oils by comparing the Re and Os systematics of Late Cretaceous terrestrial oils from Taranaki Basin, New Zealand, and their correlated coaly source rocks. Comparison is also made with selected Late Cretaceous–Paleocene marine oils and source rocks with varying levels of terrestrial organic matter input. The asphaltene fractions of nine genetically related terrestrial oils from the Maui, Maari-Manaia and Tui Area fields in offshore Taranaki Basin contain low concentrations of Re (0.18–0.45 ppb) and 192 Os (1.3–12.7 ppt) comparable to their correlated Late Cretaceous coaly source rocks (Rakopi and North Cape formations; Re 0.19–0.37 ppb, 192 Os 5.3–9.6 ppt). The Re and 192 Os concentrations in these terrestrial oils are generally one to two orders of magnitude lower than those in marine-sourced oils from the Kora Field in offshore Taranaki Basin and surface seeps in East Coast Basin.The 187 Re/ 188 Os and 187 Os/ 188 Os ratios of the terrestrial oils failed to yield a precise Re-Os isochron age. We attribute this to: (1) insufficient homogenisation of oils with widely variable initial 187 Os/ 188 Os (Os i ) values inherited from thick, coaly source rock intervals (up to about 2700 m) within the Maui sub-basin and northern Kahurangi Trough kitchens; (2) insufficient spread of 187 Re/ 188 Os values (only 275 units); (3) insufficient time since oil expulsion (modelled to be from approximately 10 Ma to the present day) for the evolution of an isochron; and (4) possible effects of water washing of the oil columns. Although all of the studied oils are water-washed to varying degrees, there is no definitive evidence that water washing has disturbed the Re-Os systematics.The Os i values for the studied terrestrial oils inherited at the modelled time of oil expulsion (approximately 10 Ma) display a wide range (0.47–1.14) and do not provide a unique fingerprint of their Late Cretaceous coaly source rock formations. Osmium isotope compositions therefore appear to have limited potential for broad oil-source rock correlation within the predominantly coal-sourced petroleum systems of Taranaki Basin. The Os i values may, however, provide useful distinction of the terrestrial oils emanating from the Kahurangi Trough (Tui Area oils) from those of the Maui sub-basin (Maui and Maari-Manaia oils) based on the significantly more radiogenic values of the Tui Area oils (0.84–1.14 compared with 0.47–0.65 from Maui and Maari-Manaia oils). Overall, this study has provided useful insights into the potential application of the Re-Os isotope system to terrestrial, coal-sourced petroleum systems.

Phosphate deposits can form in a range of environments, and despite similar P contents, their mineralogy, and hence major and trace element chemistry, can vary substantially. Continental margins with high surface-water productivity are recognised as a major environment for the formation of extensive phosphate deposits. The geochemical characterisation of continental-margin-derived phosphorites from Chatham Rise and south-eastern offshore New Zealand, as well as newly discovered seamount phosphorites from Bollons Seamount (first documented here), offer insights into the different mechanisms of phosphorite formation from these two different environments. Though less studied, seamounts are increasingly recognised for their phosphorite deposits as well as Fe-Mn crusts and Mn nodules. We propose the first model for the formation of nodules at Bollons Seamount, and an updated model for Chatham Rise nodules (and south-eastern offshore NZ phosphorites) is presented. Suboxic-to-anoxic enrichments of U and V are observed in Chatham Rise and south-eastern offshore phosphorites that contrast with oxic enrichments of Mn, Ce, Co, and Cr in Bollons Seamount phosphorites. Differences in uptake of Light Rare Earth Elements (LREE) vs. Heavy Rare Earth Elements (HREE) and variations in mineralogy (e.g., presence of glauconite vs. Fe-Mn crusts), allow phosphate deposits that formed in organic-rich, upwelling continental-margin environments to be distinguished from those that formed in oxic-suboxic organic-poor seamount environments.

Abstract The Marie Byrd Land (MBL) lithospheric mantle xenolith record comprises over 100 samples from a range of localities spanning both major crustal terranes that comprise MBL: Ross and Amundsen provinces. Coarse granular to porphyroclastic in texture, the xenoliths are predominantly Type I spinel-bearing lherzolites to harburgites, but include rare dunite and pyroxenite examples. Garnet is absent and no hydrous phases, such as amphibole or mica, have been reported to date, although traces of apatite may be present. Characterisation of the lithospheric mantle composition and its evolution however, is hampered by patchy and uneven geochemical analyses across the xenolith suite. Nonetheless, a picture emerges of a heterogeneous lithosphere beneath both Ross and Amundsen Provinces. Previously published and new data reported here are consistent with samples ranging from variably cryptically metasomatized residua from variable (10–25%) degrees of partial melt extraction to refertilized compositions. Limited isotopic data point to a complex history, providing evidence for both ancient Proterozoic lithospheric mantle and preservation of Ordovician events. The Sr–Nd–Pb composition of the sampled lithospheric mantle overlaps the common low-µ isotopic endmember identified in Cenozoic magmatism from MBL and the wider West Antarctic Rift System.

In the middle-late Paleocene, a marine, organic-rich sedimentary unit (Waipawa Formation [Fm]) in which the organic matter was derived mainly from terrestrial plants was deposited in many of New Zealand's sedimentary basins. The unique organofacies of this formation has not been identified in any other time interval within the geological history of the Southwest Pacific, indicating that unusual climatic and oceanographic conditions likely prevailed during this time. It has, therefore, attracted wide scientific interest due to its significance for regional and global reconstruction of the early Paleogene transitional climate as well as potential for oil and gas production. Scarcity of age-diagnostic fossils, presence of unconformities and lack of volcanic interbeds have, however, hindered precise dating and correlations of all the known occurrences of the formation. Here, rhenium-osmium (Re-Os) geochronology has yielded the first radiometric age for the formation (57.5 +/- 3.5 Ma), which is consistent with available biostratigraphic age determinations (59.4-58.7 Ma). Further, a comparison of Re-Os, bulk pyrolysis, sulfur and palynofacies data for the Waipawa Fm with those of more typical marine sediments such as the underlying Whangai Fm supports the interpretation that the chelating precursors or fundamental binding sites responsible for uptake of Re and Os are present in all types of organic matter, and that these elements have a greater affinity for organic chelating sites than for sulfides. The results also indicate that sedimentation rate may not play a dominant role in enhanced uptake of Re and Os by organic-rich sedimentary rocks. The initial Os-187/Os-188 values for the Waipawa (similar to 0.28) and Whangai (similar to 0.36) formations are broadly similar to those reported for coeval pelagic sediments from the central Pacific Ocean, further constraining the low-resolution marine Os-187/Os-188 record of the Paleocene. We present a compilation of Os-187/Os-188 values from organic-rich sedimentary rocks spanning the period between 70 and 50 Ma which shows that seawater Os gradually became less radiogenic from the latest Cretaceous, reaching a minimum value in the earliest late Paleocene (similar to 59 Ma) during the deposition of Waipawa Fm, and then increased through the later Paleocene and into the early Eocene. The composite Os isotope record broadly correlates with global temperature (delta O-18 and TEX86) and carbon isotope (delta C-13) records from the middle Paleocene to early Eocene, which is inferred to reflect climate-modulated changes in continental weathering patterns.

The factors controlling Re-Os systematics and potential for geochronology in organic-rich sedimentary rocks deposited in fully terrestrial to paralic environments are not well understood. Here we present Re-Os, bulk pyrolysis and sulfur data for coals and coaly mudstones from the Late Cretaceous Rakopi and North Cape, Paleocene Farewell and Eocene Mangahewa formations, Taranaki Basin, New Zealand, to investigate a range of depositional controls on the behaviour of Re and Os in coaly rocks. These rocks were deposited in various fluvial, estuarine and coastal plain environments, and exhibit varying degrees of marine influence, as indicated by total sulfur content, presence of dinoflagellate cysts, and other parameters. The Taranaki coaly rocks have low Re (0.1-1.3 ppb) and Os (14.2-66.2 ppt) concentrations, even for strongly marine-influenced, high-sulfur samples. These low concentrations are similar to those reported for entirely terrestrial coals, but are up to two orders of magnitude lower than in marine-influenced coals from the Carboniferous Matewan coal seam, USA. Unlike the Taranaki coaly rocks and other coals analysed for Re and Os, the Matewan coal seam is directly overlain by a fully marine shale. This suggests that such juxtaposition of depositional environments may be required for enhanced Re and Os enrichment in coals, for example, through drowning of the precursor peat mires by Re- and Os-rich seawater during the deposition of the overlying marine shale. The initial Os-187/Os-188 (Os-i) compositions of the Taranaki coaly rocks show significant variation. Samples from the Rakopi Formation exhibit radiogenic Os-i values (0.8-1.2), which is expected for coals deposited in fully terrestrial settings and which source Os from weathering of surrounding upper continental crust. In contrast, samples from the progressively younger North Cape, Farewell and Mangahewa formations exhibit significantly less radiogenic Os-i values (0.3-0.5). We attribute this to variable levels of marine influence from moderately radiogenic contemporaneous seawater and a change in sediment source composition following eruption and weathering of nearby subaerial volcanoes between 78 and 72 Ma, after deposition of the Rakopi Formation. The Re-187/Os-188 vs Os-187/Os-188 relationship for coaly rocks from the Farewell Formation exhibits significant scatter without any linear trend, precluding Re-Os geochronology. This scatter appears to have resulted from a combination of heterogeneous Osi (0.1 units) and limited variations in Re-187/Os-188 (145 units). Improved Re-Os isochroneity is noted in strongly marine-influenced coaly rocks from the Mangahewa Formation, with the Re-187/Os-188 vs Os-187/Os-188 relationship yielding an isochron age of 28 +/- 16 Ma. Although this age is imprecise, it is within uncertainty of the estimated biostratigraphic age (37 +/- 1 Ma) of the rocks.

The intra-oceanic Kermadec arc system extends -1300 km between New Zealand and Fiji and comprises at least 30 arc front volcanoes, the Havre Trough back-arc and the remnant Colville and Kermadec Ridges. To date, most research has focussed on the Kermadec arc front volcanoes leaving the Colville and Kermadec Ridges virtually unexplored. Here, we present seven 40Ar/39Ar ages together with a comprehensive major and trace element and Sr-, Nd-, and Pb-isotope dataset from the Colville and Kermadec Ridges to better understand the evolution, petrogenesis and splitting of the former proto-Kermadec (Vitiaz) Arc to form these two remnant arc ridges. Our 40Ar/39Ar ages range from similar to 7.5-2.6 Ma, which suggests that arc volcanism at the Colville Ridge occurred continuously and longer than previously thought. Recovered Colville and Kermadec Ridge lavas range from matic picro-basalts (MgO = similar to 8 wt%) to dacites. The lavas have arc-type normalised incompatible element patterns and Sr and Pb isotopic compositions intermediate between Pacific MORB and subducted lithosphere (including sediments, altered oceanic crust and serpentinised uppermost mantle). Geochemically diverse lavas, including ocean island basalt-like and potassic lavas with high Ce/Yb, Th/Zr, intermediate Pb-206/Pb-204 and low Nd-143/Nd-144 ratios were recovered from the Oligocene South Fiji Basin (and Eocene Three Kings Ridge) located west of the Colville Ridge. If largely trench-perpendicular mantle flow was operating during the Miocene, this geochemical heterogeneity was likely preserved in the Colville and Kermadec sub arc mantle. Between 4.41 +/- 035 and 3.40 +/- 0.24 Ma some Kermadec Ridge lavas record a shift from Colville Ridge- to Kermadec arc front-like, suggesting the proto-Kermadec (Vitiaz-) arc split post 4.41 +/- 0.35 Ma. The Colville and Kermadec Ridge data therefore place new constraints on the regional tectonic evolution and highlight the complex interplay between pre-existing mantle heterogeneities and material fluxes from the subducting Pacific Plate. The new data allow us to present a holistic (yet simplified) picture of the tectonic evolution of the late Vitiaz Arc and northern Zealandia since the Miocene and how this tectonism influences volcanic activity along the Kermadec arc at the present. (C) 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

High-Ti and low-Ti lava series occur simultaneously in many large igneous provinces (LIP); however, their origins remain debated. To address this issue, we performed a detailed geochemical study of olivine and olivine-hosted melt inclusions from the Dali picrites in the Emeishan LIP. Although the Dali picrite lavas have a limited range of whole-rock Ti/Y ratios (~356–404; Wu et al., 2018), olivine-hosted melt inclusions have Ti/Y ratios that range from low-Ti to high-Ti (213–741). The Sr-Pb isotopic compositions and trace element ratios (e.g., Nb/U and Nb/La) of the melt inclusions have a restricted range and display little correlation with Ti/Y ratios, indicating that the low-Ti and high-Ti lavas share compositionally similar source. Trace element mass balance modeling of the Emeishan lavas indicates a mantle source comprising ~84% peridotite from the lower mantle, ~15% recycled MORB, and ~1% pelagic sediment. Modeling results for the Emeishan and Karoo LIPs show that partial melts formed at greater depth in the mantle have higher Ti contents and Ti/Y ratios than those formed at shallower depths. Mixing between melts formed at different depths with different degrees of partial melting can produce the continuous range of Ti/Y ratios observed. The results demonstrate the strong influence of partial melting degree and pressure on the Ti content and Ti/Y ratio of partial melts.

The age and style of opening of the Havre Trough back-arc system is uncertain due to a lack of geochronologic constraints for the region. 40Ar/39Ar dating of 19 volcanic rocks from across the southern Havre Trough and Kermadec Arc was conducted in three laboratories to provide age constraints on the system. The results are integrated and interpreted as suggesting that this subduction system is young (<2 Ma) and coeval with opening of the continental Taupo Volcanic Zone of New Zealand. Arc magmatism was broadly concurrent across the breadth of the Havre Trough.

Picrites are potentially near‐primary melts that offer rare insights into the earliest stages of magmatic evolution. However, without detailed investigation of magmatic processes, robust records of early melt evolution and mantle melting behavior cannot be acquired. Here petrological and geochemical interrogations of the Dali picrites were conducted to investigate the magmatic processes. Compositional variation observed in the Dali picrites is dominated by variable accumulation of a wehrlitic assemblage, transported by a low MgO carrier melt. Groundmass clinopyroxenes were produced by direct crystallization of the carrier melt. In contrast, compositions of olivine‐hosted melt inclusions and clinopyroxene macrocrysts indicate that olivine and clinopyroxene macrocrysts crystallized from diverse melt batches that are genetically related to the carrier melt, which formed as a consequence of magma mixing. The compositional range of melts in equilibrium with clinopyroxene macrocrysts with Mg# < 88 (> 88) is similar to (notably smaller than) the range of melt inclusions hosted in olivine macrocrysts with equivalent forsterite values. This observation can be explained by coupling between major and trace elements during partial melting in the mantle. Overall, a process involving periodic magma replenishment, tapping, and fractional crystallization in deep magma chamber(s) can explain the compositional variations recorded in the Dali macrocryst assemblage. Close genetic relationships between macrocrysts and carrier melts appear to be common to many primitive mafic rocks, which may reflect common deep magma chamber processes whereby diverse mantle‐derived melts are injected and evolved, and the primitive macrocrysts thus formed are subsequently transported by mixed liquids, producing macrocryst‐bearing primitive mafic rocks.

Intraplate volcanism across Zealandia, South Eastern Australia, the Ross Sea Embayment and Marie Byrd Land in Antarctica define a magmatic province characterised by basalts with elevated 206Pb/204Pb (18.9–22.5), 87Sr/86Sr = ∼0.7035, Light Rare Earth enrichment [(Ce/Yb)n > 10], and convex-up mantle normalised incompatible multi-element patterns, peaking at Nb-Ta, with negative K and Pb anomalies. Trace element abundances and ratios (e.g. Zr/Nb, Y/Zr) resemble Ocean Island Basalts (OIB), distinct from Mid-Ocean Ridge Basalt (MORB), suggesting derivation from OIB-like reservoirs. Our preferred scenario envisages partial melting across the garnet-spinel stability fields involving asthenospheric and lithospheric mantle components. Melts accumulate in a column where the deep (asthenospheric) source is PM and the shallower source a melange of PM and subcontinental lithospheric mantle (DMM+1) enriched by carbonatite. Evolution of primary and near-primary magmas is controlled by olivine + clinopyroxene fractionation. Trachybasalts, trachytes and rhyolites show isotopic evidence for interaction with continental crust.

(‰) Abstract Late accretion of chondritic material to differentiated planetary bodies is thought to have been common in the early solar system. However, the timing and scale of admixing this material to terrestrial planets are poorly constrained. Using platinum (Pt) stable isotope data in a range of solar system bodies, we show that Earth’s post-Archean mantle has chondritic 198 Pt/ 194 Pt, consistent with addition of a chondritic late-veneer after core formation. Conversely, terrestrial Archean samples record non-chondritic, heavy, 198 Pt/ 194 Pt, indicating preservation of early mantle components that escaped complete mixing with the late-veneer. These data suggest admixing of ≤50 % of the eventual full late-veneer inventory. Such effective mixing within Earth’s mantle by 3.85 Ga is most consistent with modern-style plate tectonics. and -temperature core-forming conditions. Experimentally determined HSE partition coefficients at

Little is known about the effects that subducting an oceanic large igneous province (LIP) has on the petrogenesis of submarine arc volcanoes and their geochemical composition. The southern Kermadec arc represents a rare example where an LIP-the Hikurangi Plateau-is currently subducting and where its effect on mantle composition, element recycling and arc volcanism can be studied. We present mineral chemistry and whole-rock major and trace element, and Sr-Nd-Pb isotope data from samples recovered from the southern Kermadec arc volcanoes Rumble II East and Rumble II West, together with shipboard gravity and magnetic measurements. The Rumble II volcanoes (including a volcanic cone similar to 10km further west) form an similar to 23km long arc-backarc transect located similar to 250km north of New Zealand above the subducting Hikurangi Plateau. Although only a short distance apart, rocks from the two volcanoes have different mineral and whole-rock geochemical compositions. Lavas from Rumble II East are predominantly basaltic and contain primitive olivine phenocrysts (similar to Fo(91)), high-Mg# clinopyroxene (<= 96) and anorthitic plagioclase (<= An(97)). Geochemically these lavas are very diverse and cover a spectrum from low Th/Yb (< 0.15) at high Ba/Th (> 1014) to higher Th/Yb (> 0.15) at lower Ba/Th (< 844). This spectrum, together with Pb-206/Pb-204 and Nd-143/Nd-144 in the range of 18.74-18.83 and 0.51309-0.51298 respectively (at similar to slightly elevated Sr-87/Sr-86), suggests a mantle wedge that has undergone previous melt extraction and significant fluid addition from the subducting Pacific Plate and that contains sediment and HIMU-type Hikurangi Plateau components. The geochemistry of the sediment-HIMU-type components is exemplified in an olivine pyroxenite (e.g. Pb-206/Pb-204 = 20.02; Sr-87/Sr-86 = 0.70516; Nd-143/Nd-144 = 0.5126). We propose that the olivine pyroxenite formed through melt or fluid-rock metasomatism and represents the first direct evidence of a near Moho arc mantle rock that shows the imprint from a subducting HIMU-type (Hikurangi) seamount. Conversely, lavas from Rumble II West and the cone similar to 10km to the west are generally more silica rich than Rumble II East lavas and mainly contain plagioclase with less ortho- and clinopyroxene+olivine phenocrysts. The low Ba/Th (< 470) and Pb-206/Pb-204 (< 18.74), a range of Nd-143/Nd-144 (0.51297-0.51307) and elevated Th/Yb (0.13-0.39) in these lavas can best be explained by minor sediment input into a less depleted mantle wedge. In addition, the geochemical composition of the Rumble II West lavas does not require involvement of a Hikurangi component, placing a spatial limit on Hikurangi material influencing regional melt generation beneath the backarc. Supported by a gravity model requiring two distinct magma chambers, the different geochemical compositions of Rumble II East and West lavas are inconsistent with a shared magma plumbing system. The different geochemical compositions of lavas from the two Rumble II volcanoes furthermore demonstrate that across-arc geochemical heterogeneities can occur within a few kilometres and may originate from both a geochemically heterogeneous mantle wedge and Moho transition layer, recording inherited geochemical heterogeneities beneath the volcanoes.

The Plutonic Well Greenstone Belt (PWGB) is located in the Marymia Inlier between the Yilgarn and Pilbara cratons in Western Australia, and hosts a series of major Au deposits. The main episode of Au mineralisation in the PWGB was previously interpreted to have either accompanied, or shortly followed, peak metamorphism in the late Archean at ca 2650 Ma with a later, minor, event associated with the Capricorn Orogeny. Here we present new Pb isotope model ages for sulfides and Rb-Sr ages for mica, as well as a new Pb-207-Pb-206 age for titanite for samples from the Plutonic Gold Mine (Plutonic) at the southern end of the PWGB. The majority of the sulfides record Proterozoic Pb isotope model ages (2300-2100 Ma), constraining a significant Au mineralising event at Plutonic that occurred >300 Myr later than previously thought. A Rb-Sr age of 2296 +/- 99 Ma from muscovite in an Au-bearing sample records resetting or closure of the Rb-Sr system in muscovite at about the same time. A younger Rb-Sr age of 1779 +/- 46 Ma from biotite from the same sample may record further cooling, or resetting during a late-stage episode of metasomatism in the PWGB. This could have been associated with the 1820-1770 Ma Capricorn Orogeny, or a late-stage hydrothermal event potentially constrained by a new Pb-207-Pb-206 age of 1725 +/- 26 Ma for titanite in a chlorite-carbonate vein. This titanite age correlates with a pre-existing age for a metasomatic event dated at 1719 +/- 14 Ma by U-Pb ages of zircon overgrowths in a sample from the Marymia Deposit. Based on the Pb-isotope data presented here, Au mineralising events in the PWGB are inferred to have occurred at ca 2630, 2300-2100 Ma, during the Glenburgh and Capricorn orogenies, and 1730-1660 Ma. The 2300-2100 Ma event, which appears to have been significant based on the amount of sulfide of this age, correlates with the inferred age for rifting of the Marymia Inlier from the northern margin of the Yilgarn Craton. The texturally-later visible Au may have been deposited during the Glenburgh and Capricorn orogenies.

Habitat-forming deep-sea scleractinian and alcyonacean corals from around the southwest Pacific were analysed for their calcium carbonate mineralogy. Scleractinian coral species Solenosmilia variabilis, Enallopsammia rostrata, Goniocorella dumosa, Madrepora oculata and Oculina virgosa were all found to be 100% aragonitic, while some members of the alcyonacean taxa Keratoisis spp., Lepidisis spp., and Paragorgia spp. were determined to be high magnesium (Mg) calcite (with 8–11mol% MgCO3) and Primnoa sp. is bimineralic with both aragonite and Mg calcite. The majority of these habitat-forming deep-sea corals are found at intermediate depths (800–1200m) in the Antarctic Intermediate Waters (AAIW) with low salinities (~34.5), temperatures of 4–8°C and high oxygen concentrations (>180μmol/kg) and currently sitting above the aragonite saturation horizon (ASH). However, habitat-forming corals have been recorded from greater depths, in cooler waters (2–4°C) that are undersaturated with respect to aragonite (Ωaragonite<1), but with oxygen levels still >160μmol/kg. To address the sampling depth bias the coral records were normalised by the number of benthic stations (sampling effort) in the same depth range. This shows that the highest number of corals per sampling effort is between 1000 and 1400m with corals present in over 5% of the stations at these depths. The normalised records and Boot Strap analyses suggests that scleractinian corals, especially S. variabilis should be present in >1% of stations down to 1800m water depth, with E. rostrata, M. oculata and G. dumosa slightly shallower. While alcyonacean corals are found in >1% down to 2600m, with Keratoisis spp. the deepest down to 2600m, while Lepidisis spp. and Paragorgia spp. found down to 1800m. This suggests that most species can probably tolerate some undersaturation of aragonite (Ωaragonite=0.8–0.9), with several species/genera (S. variabilis; Keratoisis spp.) even more tolerant of lower carbonate concentrations ([CO32−]), down to Ωaragonite of 0.7. With this tolerance for some carbonate undersaturation it is unclear how deep sea habitat-forming corals might respond to future ocean acidification. It is likely that some species/genera will cope better than others. However, future changes in oxygen concentrations and food availability, are also going to have a strong influence on the depth and spatial distribution of deep-sea corals in the southwest Pacific.

Hydrogenetic ferromanganese (Fe-Mn) crust and nodules are slow-growing chemical sediments that form by direct precipitation from seawater, resulting in a record of changing seawater chemistry. These sediments are the primary sink for platinum in the modern oxic marine environment, hosting well-documented enrichments over other platinum-group elements (PGEs): the Pt anomaly . Platinum is a non-bio-essential, highly siderophile, transition metal with six stable isotopes (Pt, Pt, Pt, Pt, Pt, and Pt) with several oxidation states (Pt, Pt and Pt). Platinum is generally considered to exist in the hydrosphere as Pt although its behaviour in the marine environment is poorly constrained, and Ptmay also be present. Variations in ocean redox state, together with changes in source fluxes to the oceans, may therefore lead to small variations (< ±1% ̧) in the stable isotopic composition of marine platinum, raising the potential of adding platinum to the growing arsenal of paleoceanographic tracers.

We report a method for the chemical purification of Pt from geological materials by ion-exchange chromatography for subsequent Pt stable isotope analysis by multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) using a 196Pt-198Pt double-spike to correct for instrumental mass bias. Double-spiking of samples was carried out prior to digestion and chemical separation to correct for any mass-dependent fractionation that may occur due to incomplete recovery of Pt. Samples were digested using a NiS fire assay method, which pre-concentrates Pt into a metallic bead that is readily dissolved in acid in preparation for anion-exchange chemistry. Pt was recovered from anion-exchange resin in concentrated HNO3 acid after elution of matrix elements, including the other platinum group elements (PGE), in dilute HCl and HNO3 acids. The separation method has been calibrated using a precious metal standard solution doped with a range of synthetic matrices and results in Pt yields of ≥90% with purity of ≥95%. Using this chemical separation technique, we have separated Pt from 11 international geological standard reference materials comprising of PGE ores, mantle rocks, igneous rocks and one sample from the Cretaceous-Paleogene boundary layer. Pt concentrations in these samples range from ca. 5 ng g-1 to 4 μg g-1. This analytical method has been shown to have an external reproducibility on δ198Pt (permil difference in the 198Pt/194Pt ratio from the IRMM-010 standard) of ±0.040 (2 sd) on Pt solution standards (Creech et al., 2013, J. Anal. At. Spectrom. 28, 853-865). The reproducibility in natural samples is evaluated by processing multiple replicates of four standard reference materials, and is conservatively taken to be ca. ±0.088 (2 sd). Pt stable isotope data for the full set of reference materials have a range of δ198Pt values with offsets of up to 0.4‰ from the IRMM-010 standard, which are readily resolved with this technique. These results demonstrate the potential of the Pt isotope system as a tracer in geochemical systems.

Clark volcano of the Kermadec arc, northeast of New Zealand, is a large stratovolcano comprised of two coalescing volcanic cones; an apparently younger, more coherent, twin-peaked edifice to the northwest and a relatively older, more degraded and tectonized cone to the southeast. High-resolution water column surveys show an active hydrothermal system at the summit of the NW cone largely along a ridge spur connecting the two peaks, with activity also noted at the head of scarps related to sector collapse. Clark is the only known cone volcano along the Kermadec arc to host sulfide mineralization.Volcano-scale gravity and magnetic surveys over Clark show that it is highly magnetized, and that a strong gravity gradient exists between the two edifices. Modeling suggests that a crustal-scale fault lies between these two edifices, with thinner crust beneath the NW cone. Locations of regional earthquake epicenters show a southwest-northeast trend bisecting the two Clark cones, striking northeastward into Tangaroa volcano. Detailed mapping of magnetics above the NW cone summit shows a highly magnetized ring structure 350 m below the summit that is not apparent in the bathymetry; we believe this structure represents the top of a caldera. Oblate zones of low (weak) magnetization caused by hydrothermal fluid upflow, here termed burn holes, form a pattern in the regional magnetization resembling Swiss cheese. Presumably older burn holes occupy the inner margin of the ring structure and show no signs of hydrothermal activity, while younger burn holes are coincident with active venting on the summit.A combination of mineralogy, geochemistry, and seafloor mapping of the NW cone shows that hydrothermal activity today is largely manifest by widespread diffuse venting, with temperatures ranging between 56 degrees and 106 degrees C. Numerous, small (<= 30 cm high) chimneys populate the summit area, with one site host to the similar to 7-m-tall "Twin Towers" chimneys with maximum vent fluid temperatures of 221 degrees C (pH 4.9), consistent with delta S-34(anhydrite-pyrite) values indicating formation temperatures of similar to 228 degrees to 249 degrees C. Mineralization is dominated by pyrite-marcasite-barite-anhydrite. Radiometric dating using the Ra-228/Ra-226 and Ra-226/Ba methods shows active chimneys to be <20 with most <2 years old. However, the chimneys at Clark show evidence for mixing with, and remobilizing of, barite as old as 19,000 years. This is consistent with Nd and Sr isotope compositions of Clark chimney and sulfate crust samples that indicate mixing of similar to 40% seawater with a vent fluid derived from low K lavas. Similarly, REE data show the hydrothermal fluids have interacted with a plagioclase-rich source rock.A holistic approach to the study of the Clark hydrothermal system has revealed a two-stage process whereby a caldera-forming volcanic event preceded a later cone-building event. This ensured a protracted (at least 20 ka yrs) history of hydrothermal activity and associated mineral deposition. If we assume at least 200-m-high walls for the postulated (buried) caldera, then hydrothermal fluids would have exited the seafloor 20 ka years ago at least 550 m deeper than they do today, with fluid discharge temperatures potentially much hotter (similar to 350 degrees C). Subsequent to caldera infilling, relatively porous volcaniclastic and other units making up the cone acted as largescale filters, enabling ascending hydrothermal fluids to boil and mix with seawater subseafloor, effectively removing the metals (including remobilized Cu) in solution before they reached the seafloor. This has implications for estimates for the metal inventory of seafloor hydrothermal systems pertaining to arc hydrothermal systems.