Reduced methane-bearing fluids as a source for diamond

Diamond formation in the Earth has been extensively discussed in recent years on the basis of geochemical analysis of natural materials, high-pressure experimental studies, or theoretical aspects. Here, we demonstrate experimentally for the first time, the spontaneous crystallization of diamond from CH4-rich fluids at pressure, temperature and redox conditions approximating those of the deeper parts of the cratonic lithospheric mantle (5-7 GPa) without using diamond seed crystals or carbides. In these experiments the fluid phase is nearly pure methane, even though the oxygen fugacity was significantly above metal saturation. We propose several previously unidentified mechanisms that may promote diamond formation under such conditions and which may also have implications for the origin of sublithospheric diamonds. These include the hydroxylation of silicate minerals like olivine and pyroxene, H2 incorporation into these phases and the "etching" of graphite by H2 and CH4 and reprecipitation as diamond. This study also serves as a demonstration of our new high-pressure experimental technique for obtaining reduced fluids, which is not only relevant for diamond synthesis, but also for investigating the metasomatic origins of diamond in the upper mantle, which has further implications for the deep carbon cycle

Ni-in-garnet geothermometry in mantle rocks: a high pressure experimental recalibration between 1100 and 1325 °C

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Ni  ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule (lnDgrt/olvNi;NigrtNiolv), the Ca mole fraction in garnet (XgrtCa; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet (XgrtCr; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as: T(∘C)=-8254.568((XgrtCa×3.023)+(XgrtCr×2.307)+(lnDgrtolvNi-2.639))-273±55 where Dgrt/olvNi=NigrtNiolv, Ni is in ppm, XgrtCa = Ca/(Ca + Fe + Mg) in garnet, and XgrtCr= Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer. olvZS was the recipient of an Australian government funded RTP domestic PhD scholarship and stipend. Karol Czarnota of Geoscience Australia is thanked for his support and interest in this project. Paul Agnew of Rio Tinto Exploration kindly provided the garnet concentrate from Diavi

Experimental recalibration of the Cr-in-clinopyroxene geobarometer: improved precision and reliability above 4.5 GPa

The pressure dependence of the exchange of Cr between clinopyroxene and garnet in peridotite is applicable as a geobarometer for mantle-derived Cr-diopside xenocrysts and xenoliths. The most widely used calibration (Nimis and Taylor Contrib Miner Petrol 139: 541–554, 2000; herein NT00) performs well at pressures below 4.5 GPa, but has been shown to consistently underestimate pressures above 4.5 GPa. We have experimentally re-examined this exchange reaction over an extended pressure, temperature, and compositional range using multi-anvil, belt, and piston cylinder apparatuses. Twenty-nine experiments were completed between 3–7 GPa, and 1100–1400 °C in a variety of compositionally complex lherzolitic systems. These experiments are used in conjunction with several published experimental datasets to present a modified calibration of the widely-used NT00 Cr-in-clinopyroxene (Cr-in-cpx) single crystal geobarometer. Our updated calibration calculates P (GPa) as a function of T (K), CaCr Tschermak activity in clinopyroxene (aCaCrTscpx), and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2n polynomial using multiple linear regression found the following expression for pressure: P(GPa)=11.03+(-T(K)ln(aCaCrTscpx)×0.001088)+(1.526×ln(Cr#cpxT(K))) where Cr#cpx=(CrCr+Al), aCaCrTscpx=Cr-0.81·Cr#cpx·(Na+K), with all mineral components calculated assuming six oxygen anions per formula unit in clinopyroxene. Temperature (K) may be calculated through a variety of geothermometers, however, we recommend the NT00 single crystal, enstatite-in-clinopyroxene (en-in-cpx) geothermometer. The pressure uncertainty of our updated calibration has been propagated by incorporating all analytical and experimental uncertainties. We have found that pressure estimates below 4 GPa, between 4–6 GPa and above 6 GPa have associated uncertainties of 0.31, 0.35, and 0.41 GPa, respectively. Pressures calculated using our calibration of the Cr-in-cpx geobarometer are in good agreement between 2–7 GPa, and 900–1400 °C with those estimated from widely-used two-phase geobarometers based on the solubility of alumina in orthopyroxene coexisting with garnet. Application of our updated calibration to suites of well-equilibrated garnet lherzolite and garnet pyroxenite xenoliths and xenocrysts from the Diavik-Ekati kimberlite and the Argyle lamproite pipes confirm the accuracy and precision of our modified geobarometer, and show that PT estimates using our revised geobarometer result in systematically steeper paleogeotherms and higher estimates of the lithosphere‒asthenosphere boundary compared with the original NT00 calibration.All EPMA Analyses were completed at the Centre for Advanced Microscopy an advanced imaging precinct of Microscopy Australia, a facility that is funded by the Australian National University, and State and Federal Governments. Jef Chen is thanked for his assistance with the EPMA analyses. ZS was the recipient of an Australian Government funded domestic student RTP PhD scholarship and stipend. We thank Karol Czarnota of Geoscience Australia for his interest and support of this projec

Mantle melting versus mantle metasomatism - The chicken or the egg dilemma

Most eclogitic mantle xenoliths brought to the surface exhibit a certain degree of enrichment with incompatible elements, usually attributed to the effect of mantle metasomatism by a putative metasomatic fluid. The metasomatic overprint is represented mainly by enrichments in Na, K, Ba, Ti and LREE and the original source of this fluid remains unknown. In this paper, we present a detailed petrological study of a typical eclogitic mantle xenolith from the Roberts Victor kimberlite mine in South Africa. We find that its textural and mineralogical features present strong evidence for incipient melting. The melting assemblage we observe did not necessarily require introduction of additional components, that is: in-situ melting alone could produce highly incompatible element enriched melt without involvement of a hypothetical and speculative “metasomatic event”. Due to the higher abundance in incompatible elements and lower solidus temperature than peridotites, mantle eclogites, some of which represent previously subducted oceanic crust, are much more plausible sources of mantle metasomatism, but on the other hand, they can be considered as highly metasomatised themselves. This brings us to the “chicken or egg” dilemma – was the secondary mineral assemblage in mantle lithologies a result or a source of mantle metasomatism?The research in Oxford University was financially supported by NERC grant NE/L010828/1 to ESK and by European Research Council grant 267764 to B. Wood. Research at ANU was supported by ARC Future Fellowship to GM

Methane-bearing fluids in the upper mantle: an experimental approach

The main obstacle to understanding of the geological role of reduced, CH4-bearing fluids is the absence of a reliable experimental technique applicable to solid-media high-pressure apparatuses, allowing their observation and direct characterisation under laboratory conditions. In this study, we describe the main pitfalls of earlier designs and technical aspects related to achievement of strongly reduced oxygen fugacity (fO2) conditions (i.e., Fe–FeO, IW) and maintenance of a constant fluid equilibrium during an experiment. We describe a new triple-capsule design made of an Au outer capsule with an Au-inner capsule containing a metal/metal oxide oxygen buffer and water, as well as an inner olivine container filled with a harzburgitic sample material and Ir powder that serves as a redox sensor. The bottom of the outer capsule is covered with a solid fluid source (e.g., stearic acid). The outer capsule is surrounded by a polycrystalline CaF2 pressure medium to minimise H2-loss from the assembly. Application of this design is limited to temperatures below the melting temperature of Au, which is pressure dependent. Metals other than Au can lead to fluid disequilibrium triggered by a dehydrogenation and carbonation of the methane. Test experiments were carried out at 5 GPa, temperatures < 1300 °C, at Mo–MoO2 and Fe–FeO buffer conditions. IrFe alloy sensors demonstrate successful achievement and maintenance of reduced fluid environment at ∆logfO2 ≈ IW + 0.5. The fluid phase was trapped in numerous inclusions within the olivine sample container. Raman spectra reveal that the fluid consists mainly of CH4, along with small amounts of higher hydrocarbons like C2H6. No water was detected, but H2 was found to be present in fluid and incorporated into the olivine structure. Our results are inconsistent with published fluid speciation models that predict significant H2O contents at these fO2 conditions. It is also apparent that fluids with significant CH4 contents are likely to be stable under the conditions recorded by some mantle samples.The Deutsche Forschungsgemeinschaft is gratefully acknowledged for funding the project WO652/26-1

An experimental study of trace element distribution during partial melting of mantle heterogeneities

Trace elements are widely used to interpret the origin of mantle-derived magmas, yet we lack detailed understanding of how trace elements behave during melting of mantle source components. Here, we present new data on trace element distribution and partitioning between phases from high pressure (3.0 to 5.0 GPa), high-temperature (1230 to 1550 °C) melting experiments on starting compositions that represent altered oceanic crust and metasedimentary protoliths. These compositions are expected to be recycled into the mantle via subduction or delamination to form heterogeneous mantle domains that are implicated in the genesis of intraplate and/or ocean floor magmas. In most of the experiments, the investigated trace elements behave incompatibly, expect for HREE and Y, which are compatible in garnet, and V, Cr and Zn, which partition into both garnet and clinopyroxene. Relative to Nd, P is more compatible in garnet than clinopyroxene, leading to fractionation of P/Nd with melting in some cases. Melt compositions in some experiments with low melt fractions feature distinctive negative anomalies for Nb, and for Sr, Ba and Eu, due to retention of these elements in minor/accessory rutile and feldspar, respectively. We also show that highly incompatible trace element (e.g., Cs, Th, U, LREE) concentrations in melts are strongly controlled by melt fraction, whereas moderately incompatible (M-HREE, Zr) to compatible (Cr, V) element concentrations are controlled by temperature and/or phase composition. Pressure has relatively little influence on trace element behaviour at the investigated conditions. Based on our results, we suggest that partial melting of eclogitic components of mantle domains may ultimately produce magmas with trace element compositions that are unlike peridotite-sourced magmas. Therefore, the trace element systematics of mantle-derived magmas should not only be interpreted in terms of mantle source compositions, but also with consideration to source petrology (e.g., mineral compositions and accessory phase stability) and melting conditions (e.g., melt fraction, pressure, temperature)

Kimberlites from Source to Surface: Insights from Experiments

High-pressure experiments are unconvincing in explaining kimberlites as direct melts of carbonated peridotite because the appropriate minerals do not coexist stably at the kimberlite liquidus. High-pressure melts of peridotite with CO2 and H2O have compositions similar to kimberlites only at pressures where conditions are insufficiently oxidizing to stabilize CO2: they do not replicate the high K2O/Na2O of kimberlites. Kimberlite melts may begin their ascent at ≈300 km depth in reduced conditions as melts rich in MgO and SiO2 and poor in Na2O. These melts interact with modified, oxidized zones at the base of cratons where they gain CO2, CaO, H2O, and K2O and lose SiO2. Decreasing CO2 solubility at low pressures facilitates the incorporation of xenocrystic olivine, resulting in kimberlites' characteristically high MgO/CaO

Experimental phase and melting relations of metapelite in the upper mantle: implications for the petrogenesis of intraplate magmas

We performed a series of piston-cylinder experiments on a synthetic pelite starting material over a pressure and temperature range of 3.0–5.0 GPa and 1,100–1,600°C, respectively, to examine the melting behaviour and phase relations of sedimentary rocks at upper mantle conditions. The anhydrous pelite solidus is between 1,150 and 1,200°C at 3.0 GPa and close to 1,250°C at 5.0 GPa, whereas the liquidus is likely to be at 1,600°C or higher at all investigated pressures, giving a large melting interval of over 400°C. The subsolidus paragenesis consists of quartz/coesite, feldspar, garnet, kyanite, rutile, ±clinopyroxene ±apatite. Feldspar, rutile and apatite are rapidly melted out above the solidus, whereas garnet and kyanite are stable to high melt fractions (>70%). Clinopyroxene stability increases with increasing pressure, and quartz/coesite is the sole liquidus phase at all pressures. Feldspars are relatively Na-rich [K/(K + Na) = 0.4–0.5] at 3.0 GPa, but are nearly pure K-feldspar at 5.0 GPa. Clinopyroxenes are jadeite and Ca-eskolaite rich, with jadeite contents increasing with pressure. All supersolidus experiments produced alkaline dacitic melts with relatively constant SiO2 and Al2O3 contents. At 3.0 GPa, initial melting is controlled almost exclusively by feldspar and quartz, giving melts with K2O/Na2O ~1. At 4.0 and 5.0 GPa, low-fraction melting is controlled by jadeite-rich clinopyroxene and K-rich feldspar, which leads to compatible behaviour of Na and melts with K2O/Na2O ≫ 1. Our results indicate that sedimentary protoliths entrained in upwelling heterogeneous mantle domains may undergo melting at greater depths than mafic lithologies to produce ultrapotassic dacitic melts. Such melts are expected to react with and metasomatise the surrounding peridotite, which may subsequently undergo melting at shallower levels to produce compositionally distinct magma types. This scenario may account for many of the distinctive geochemical characteristics of EM-type ocean island magma suites. Moreover, unmelted or partially melted sedimentary rocks in the mantle may contribute to some seismic discontinuities that have been observed beneath intraplate and island-arc volcanic regions

Carbonatites: Classification, Sources, Evolution, and Emplacement

Carbonatites are igneous rocks formed in the crust by fractional crystallization of carbonate-rich parental melts that are mostly mantle derived. They dominantly consist of carbonate minerals such as calcite, dolomite, and ankerite, as well as minor phosphates, oxides, and silicates. They are emplaced in continental intraplate settings such as cratonic interiors and margins, as well as rift zones, and rarely on oceanic islands. Carbonatites are cumulate rocks, which are formed by physical separation and accumulation of crystals that crystallize from a melt, and their parental melts form by either (a) direct partial melting of carbonate-bearing, metasomatized, lithospheric mantle producing alkali-bearing calciodolomitic melts or (b) silicate-carbonate liquid immiscibility following fractional crystallization of carbonate-bearing, silica-undersaturated magmas such as nephelinites, melilitites, or lamprophyres. Their emplacement into the crust is usually accompanied by fenitization, alkali metasomatism of wallrock caused by fluids expelled from the crystallizing carbonatite. Carbonatites are major hosts of deposits of the rare earth elements and niobium, and the vast majority of the global production of these commodities is from carbonatites. ▪ Carbonatites are igneous rocks formed from carbonate-rich magmas, which ultimately formed in Earth's upper mantle. ▪ Carbonatites are associated with economic deposits of metals such as the rare earth elements and niobium, which are essential in high-tech applications. ▪ There are more than 600 carbonatites in the geological record but only one currently active carbonatite volcano, Oldoinyo Lengai in Tanzania.M. Anenburg was supported by an Australian Research Council Linkage Grant (LP190100635). S. Tappe acknowledges financial support from the National Research Foundation (NRF) of South Africa through IPRR and DSI-NRF CIMERA grants. Jian Sun is thanked for helping to convert Ca isotope ratios from different studies to make them comparable. T. Guzmics’s research was supported by the National Research, Development and Innovation Office of Hungary (K-119535)

Detrital zircon age constraints on the provenance of sandstones on Hatton Bank and Edoras Bank, NE Atlantic

U-Pb dating of detrital zircons shows that the provenance of Cretaceous-Palaeogene sandstones on Hatton and Edoras banks (SW Rockall Plateau) comprises magmatic rocks dated at c. 1800 Ma and c. 1750 Ma respectively. Their depositional setting, first-cycle mineralogy and unimodal nature detrital zircon populations suggest these sandstones are of local origin. The zircon age data are therefore considered to provide constraints on these poorly-understood areas of the Rockall Plateau. The U-Pb dates are directly comparable with U-Pb zircon crystallisation ages from granitoid rocks reported from the Ketilidian Belt of southern Greenland and from the Rhinns Complex of western Britain. Hf isotopic data from the Edoras Bank sample are consistent with derivation from a juvenile Palaeoproterozoic block. In conjunction with previously reported Sm-Nd TDM model ages from the Ketilidian Belt, Rockall Bank and the Rhinns Complex, these data extend the known distribution of a large juvenile Palaeoproterozoic terrane spanning the southern NE Atlantic. By contrast, Hf isotopic data from the Hatton Bank sample imply a large contribution from Archaean crust. The zircon population from Edoras Bank also contains sparse Mesoproterozoic grains, providing evidence for the presence of volumetrically minor Grenville-age intrusions in the southern part of the Rockall Plateau