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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:PGPa=11.03+-TKln(aCaCrTscpx)×0.001088+1.526×lnCr#cpxTKwhere 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.

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 (Niolv ≅ 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=-8254.568XgrtCa×3.023+XgrtCr×2.307+lnDgrtolvNi-2.639-273±55where 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.

The mechanisms governing a commonly observed seismic velocity drop in the cratonic lithosphere, referred to as the mid‐lithospheric discontinuity (MLD), have been widely debated. To identify the composition and seismic structure of MLDs, we have analyzed Sp receiver functions (SRF) and mantle xenocrysts for six regions across Australia. We utilize locations where seismic stations and kimberlite‐hosted mantle xenocrysts are both available, allowing for comparison between seismological and petrological constraints. Our results show negative SRF phases indicative of the MLD coincide with clinopyroxene‐depleted zones at 60–140 km depth. Clinopyroxenes with different chemical compositions across the MLD define a litho‐chemical discontinuity. Modeling and experimental data show that MLDs may be explained by modified lherzolite with 10%–20% modal pargasite. Pargasite MLDs may form when rising H2O‐bearing melts cross the amphibole dehydration curve and react with clinopyroxene in lherzolite. Because the amphibole dehydration curve is isobaric at 80–120 km, pargasite will be precipitated as horizontal channels. Plain Language Summary Seismic imaging of Earth's lithosphere has revealed a seismic velocity drop at 60–120 km depth, termed the mid‐lithosphere discontinuity (MLD). The mechanisms governing the MLD have been extensively debated. To advance the understanding of the MLD we carried out an interdisciplinary study to investigate the seismic structure and composition of MLDs identified in the cratonic lithosphere of Australia. We integrated analysis of seismic converted signals from discontinuities recorded by permanent seismometers with geochemical data for kimberlite‐hosted mantle xenocrysts collected in the vicinity. The method allows direct comparison between seismological and petrological constraints. Our results suggest that the MLD comprises anomalously low abundances of clinopyroxene and separates geochemically distinct layers within the lithospheric mantle. Experimental results and modeling suggests that the observed decrease in seismic velocity and absence of clinopyroxene may relate to the formation of pargasite channels in modified lherzolite. Key Points Sp receiver functions and mantle xenocrysts are used to study mid‐lithosphere discontinuities beneath Australian cratons Mid‐lithosphere discontinuity at 60–120 km depth corresponds with mantle xenocryst populations that are depleted in clinopyroxene Modeling and experimental data suggests the mid‐lithosphere discontinuity is caused by pargasite‐bearing lherzolite

Diamond exploration over the past decade has led to the discovery of a new province of kimberlitic pipes (the Webb Province) in the Gibson Desert of central Australia. The Webb pipes comprise sparse macrocrystic olivine set in a groundmass of olivine, phlogopite, perovskite, spinel, clinopyroxene, titanian‐andradite and carbonate. The pipes resemble ultramafic lamprophyres (notably aillikites) in their mineralogy, major and minor oxide chemistry, and initial 87Sr/86Sr and εNd‐εHf isotopic compositions. Ion probe U‐Pb geochronology on perovskite (806 ± 22 Ma) indicates the eruption of the pipes was co‐eval with plume‐related magmatism within central Australia (Willouran‐Gairdner Volcanic Event) associated with the opening of the Centralian Superbasin and Rodinia supercontinent break‐up. The equilibration pressure and temperature of mantle‐derived garnet and chromian (Cr) diopside xenocrysts range between 17 and 40 kbar and 750–1320°C and define a paleo‐lithospheric thickness of 140 ± 10 km. Chemical variations of xenocrysts define litho‐chemical horizons within the shallow, middle, and deep sub‐continental lithospheric mantle (SCLM). The shallow SCLM (50–70 km), which includes garnet‐spinel and spinel lherzolite, contains Cr diopside with weakly refertilized rare earth element compositions and unenriched compositions. The mid‐lithosphere (70–85 km) has lower modal abundances of Cr diopside. This layer corresponds to a seismic mid‐lithosphere discontinuity interpreted as pargasite‐bearing lherzolite. The deep SCLM (>90 km) comprises refertilized garnet lherzolite that was metasomatized by a silicate‐carbonatite melt. Key Points A new alkaline province in Central Australia is described. Intrusions resemble aillikites based on mineralogy and Sr‐Nd‐Hf isotopes Aillikites were emplaced at 806 Ma along the rifted margin of the Centralian Superbasin in response to Rodinia breakup A seismic and petrological mid‐lithosphere discontinuity at 80–85 km depth may relate to pargasite channels

The lithology, geochemistry, and architecture of the continental lithospheric mantle (CLM) underlying the Kimberley Craton of north‐western Australia has been constrained using pressure‐temperature estimates and mineral compositions for >5,000 newly analyzed and published garnet and chrome (Cr) diopside mantle xenocrysts from 25 kimberlites and lamproites of Mesoproterozoic to Miocene age. Single‐grain Cr diopside paleogeotherms define lithospheric thicknesses of 200–250 km and fall along conductive geotherms corresponding to a surface heat flow of 37–40 mW/m 2 . Similar geotherms derived from Miocene and Mesoproterozoic intrusions indicate that the lithospheric architecture and thermal state of the CLM has remained stable since at least 1,000 Ma. The chemistry of xenocrysts defines a layered lithosphere with lithological and geochemical domains in the shallow (<100 km) and deep (>150 km) CLM, separated by a diopside‐depleted and seismically slow mid‐lithosphere discontinuity (100–150 km). The shallow CLM is comprised of Cr diopsides derived from depleted garnet‐poor and spinel‐bearing lherzolite that has been weakly metasomatized. This layer may represent an early (Meso to Neoarchean?) nucleus of the craton. The deep CLM is comprised of high Cr 2 O 3 garnet lherzolite with lesser harzburgite, and eclogite. The peridotite components are inferred to have formed as residues of polybaric partial mantle melting in the Archean, whereas eclogite likely represents former oceanic crust accreted during Paleoproterozoic subduction. This deep CLM was metasomatized by H 2 O‐rich melts derived from subducted sediments and high‐temperature FeO‐TiO 2 melts from the asthenosphere. The Kimberley Craton has retained a thick (>220 km) thermally stable lithospheric root since the Mesoproterozoic The lithospheric mantle comprises a depleted shallow layer (<100 km) and metasomatized deep layer (>150 km) separated by a diopside‐depleted mid‐lithosphere discontinuity Diamondiferous lamproites at the craton margin are associated with eclogite and high levels of K 2 O metasomatism of the lithospheric mantle

To improve the understanding of the formation and evolution of the sub‐continental lithospheric mantle (SCLM) underlying the South Australian Craton we have conducted a detailed petrological study on >3,000 mantle xenocrysts from 13 kimberlites emplaced across the craton. Pressure (P) and temperature (T) estimates on Cr diopside and garnet have been coupled with their chemical concentrations to constrain lithospheric thickness and chemo‐lithostratigraphy. We show that lithospheric thickness is greatest beneath the Gawler Craton, whereas thinner lithosphere occurs beneath the Adelaide Fold Belt. Mineral compositions highlight two litho‐chemical domains within the shallow and deep SCLM that are separated by a mid‐lithosphere discontinuity (MLD). The shallow SCLM (60–130 km) comprises low Cr2O3 lherzolite and wehrlite. Shallow SCLM xenocrysts record depleted and refertilized compositions enriched in light rare earth elements related to metasomatism by kimberlite or related melts. The mid‐lithosphere (130–160 km) is depleted in garnet and Cr diopside which may relate to a layer of pargasite lherzolite. The deep SCLM (>160 km) comprises high Cr2O3 lherzolite with elevated TiO2 and FeO. We interpret the litho‐chemical stratification of the SCLM to reflect a multi‐stage top‐down growth. The shallow SCLM reflects an amalgamation of Precambrian cratonic nuclei characterized by heterogeneity in geochemical enrichment and depletion. Interaction of the shallow SCLM with mantle plumes accreted melts along the paleo‐lithosphere‐asthenosphere boundary, which now occurs as a MLD. The deep SCLM represents depleted mantle residue formed during mantle plume impingement and thickened during orogenesis. This domain has been metasomatized and refertilized by high‐T melts from the asthenosphere. Key Points The South Australian Craton comprises three litho‐chemical layers within the shallow, middle and deep lithospheric mantle Xenocrysts define a heterogeneously depleted‐kimberlite melt metasomatized shallow sub‐continental lithospheric mantle (SCLM) and fertile‐melt metasomatized deep SCLM The mid‐lithosphere is a seismic and geochemical discontinuity marked by negative Vp and lower modal proportions of garnet and diopside

New data are presented for groundmass chromian spinel, perovskite, ilmenite, and K-Ti-Ba-rich phases from the Miocene olivine and leucite lamproites of the West Kimberley region. The spinels range from early Ti-Al-Mg chromite through Ti-Mg chromite to Ti-chromite and, in Ellendale 4 and 9, Ti-Cr magnetite and Ti-magnetite. Most crystallized at 850–1220 °C and f O 2  ~ MW + 1–2 log units except for Ellendale 4 and 9 spinels which underwent marked late oxidation at ~650–750 °C with fO 2 increasing sharply to ~FMQ + 2–3 log units. Perovskite is ubiquitous in the olivine lamproites and the Walgidee Hills (WH) lamproite. Compositional features of the perovskite are a wide range in Cr, and high Sr, Nb, Th, and LREE contents with highly fractionated REE patterns (La/Yb CN  ~ 750–3000). Perovskite from WH defines an evolutionary trend of enrichment in Na, Sr, Y, Nb, U and REE, and depletion in Cr, Fe, and Th with magma fractionation. Late crystallizing WH perovskite shows a decrease in LREE due to relative depletion of LREE in residual magma by extended crystallization of perovskite (and apatite). Priderite ((K,Ba)(Ti,Fe 3+ ) 8 O 16 ) has low Mg and V, and a range in Cr contents which decrease with magma evolution. Jeppeite ((K,Ba) 2 (Ti,Fe) 6 O 13 ), has higher Sr and Nb content than priderite. Both contain low Y and REEs. Wadeite (K 2 ZrSi 3 O 9 ), a ubiquitous groundmass phase, has high Sc, Rb and Hf contents, and strongly LREE-depleted REE patterns with positive Ce anomalies. Noonkanbahite, a late crystallizing phase in WH, has low Cr and Ni, and high Sr, Nb and Y contents. REE patterns for noonkanbahite display high HREE, depleted MREE, enriched La-Ce-Pr, and a positive Eu anomaly.

The reliability of eight Fe–Mg exchange geothermobarometers for garnet-bearing peridotites, pyroxenites and eclogites has been examined using a database comprised of more than 300 published peridotite, pyroxenite and eclogite experiments conducted from 10 to 70 kbar and 850 to > 1650∘C. We have tested Fe–Mg exchange geothermometers suitable for a range of mantle lithologies, including websterite, harzburgite, wehrlite and eclogite. All geothermometers maintained an average difference in experimental and calculated temperature (T) ΔT=Texp-Tcalc of less than ± 50 °C with a standard deviation of ΔT between ± 50 to 150 °C. Most geothermometers performed well across a narrow range in ln KdFe-MgA-B (where KdFe-MgA-B=FeA×MgB(FeB×MgA)), however, systematic overestimation and underestimation of T were observed outside of the optimal range of lnKdFe-MgA-B. Increases in experimental pressure (P) adversely affected several geothermometers, particularly those calibrated empirically using natural samples. All previously published calibrations of the garnet-clinopyroxene geothermometer were unable to reliably reproduce the experimental T for both peridotite and eclogite experimental compositions, which hinders their confident application to natural datasets. To improve the state of mantle geothermobarometry we have used our experimental database to recalibrate the (1) garnet-clinopyroxene Fe–Mg exchange geothermometer, and (2) garnet-orthopyroxene Fe–Mg exchange geothermometer. Each geothermometer has been recalibrated across an extended P, T, and compositional range. The inclusion of eclogitic experiments in the calibration for the garnet-clinopyroxene geothermometer permits application to both eclogitic and peridotitic/pyroxenitic assemblages equilibrated under a wide range of PT conditions in the upper mantle. Using multiple linear regression to solve for lnKd, we found the following expressions best reproduced the experimental T (℃) of our dataset: TFe-Mggrt-cpx(∘C)=3356.34-0.008×Pkbar+0.259×XCagrt+0.914×XMggrt+-0.159×Jdcpx+lnKdFe-Mggrt-cpx+1.265-273TFe-Mggrt-opx(∘C)=1851.85-0.007×Pkbar+-1.83×XCagrt+lnKdFe-Mggrt-cpx+1.08-273. where, XCagrt=CaCa+Fe+Mg, KdFe-Mggrt-opx=Fegrt×Mgopx(Feopx×Mggrt),XMggrt=MgCa+Fe+Mg, Jdcpx=Na-Cr-2×Ti, KdFe-Mggrt-cpx=Fegrt×Mgcpx(Fecpx×Mggrt), with all elements calculated on the basis of 12 oxygen anions in garnet and 6 oxygen anions in clino- and orthopyroxene. Fe2+  = total Fe. Our updated calibrations resolve several issues with earlier calibrations, including a poor performance at elevated P and compositional limitations. An improvement in precision and accuracy has been demonstrated through application to the experimental calibration dataset, a second independent set of published experimental data, and to natural peridotites, pyroxenites and eclogites from on and off craton settings. Iterative PT estimates on natural datasets calculated using our updated calibrations compare well with estimates from widely used calibrations such as the Taylor (1998) two-pyroxene solvus geothermometer. We anticipate that this contribution will provide an important reference for the reliability of mantle geothermometers and that our updated calibrations will be used in future studies on peridotite, pyroxenite and eclogite inclusions in diamond and mantle-derived xenoliths.

Isotope compositions of basalts provide information about the chemical reservoirs in Earth’s interior and play a critical role in defining models of Earth’s structure. However, the helium isotope signature of the mantle below depths of a few hundred kilometers has been difficult to measure directly. This information is a vital baseline for understanding helium isotopes in erupted basalts. We measured He-Sr-Pb isotope ratios in superdeep diamond fluid inclusions from the transition zone (depth of 410 to 660 kilometers) unaffected by degassing and shallow crustal contamination. We found extreme He-C-Pb-Sr isotope variability, with high ³He/⁴He ratios related to higher helium concentrations. This indicates that a less degassed, high-³He/⁴He deep mantle source infiltrates the transition zone, where it interacts with recycled material, creating the diverse compositions recorded in ocean island basalts.

Issue Title: Kimberlites, lamproites, carbonatites and associated alkaline rocks - a special issue dedicated to Professor Rex Tregilgas Prider, 1910-2005