ORCID Profile
0000-0002-7527-1230
Current Organisation
University of Cambridge
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Publisher: American Geophysical Union (AGU)
Date: 09-2023
DOI: 10.1029/2023GC011040
Publisher: Springer Science and Business Media LLC
Date: 28-01-2021
DOI: 10.1007/S00410-020-01768-Z
Abstract: 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 $$\\left( {a_{{{\\text{CaCrTs}}}}^{{{\\text{cpx}}}} } \\right)$$ a CaCrTs cpx , and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2 n polynomial using multiple linear regression found the following expression for pressure: $$P\\left( {{\\text{GPa}}} \\right) = 11.03 + \\left( { - T{ }\\left( {\\text{K}} \\right){\\text{ ln}}(a_{{{\\text{CaCrTs}}}}^{{{\\text{cpx}}}} ) \\times 0.001088{ }} \\right) + \\left( {1.526 \\times {\\text{ln}}\\left( {\\frac{{{\\text{Cr}}\\#^{{{\\text{cpx}}}} }}{{T{ }\\left( {\\text{K}} \\right)}}} \\right)} \\right){ }$$ P GPa = 11.03 + - T K ln ( a CaCrTs cpx ) × 0.001088 + 1.526 × ln Cr # cpx T K where $${\\text{Cr}}\\#^{{{\\text{cpx}}}} = \\left( {\\frac{{{\\text{Cr}}}}{{{\\text{Cr}} + {\\text{Al}}}}} \\right)$$ Cr # cpx = Cr Cr + Al , $$a_{{{\\text{CaCrTs}}}}^{{{\\text{cpx}}}} = {\\text{Cr}} - 0.81 \\cdot {\\text{Cr}}\\#^{{{\\text{cpx}}}} \\cdot \\left( {{\\text{Na}} + {\\text{K}}} \\right),$$ a CaCrTs cpx = 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 l roite 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.
Publisher: American Geophysical Union (AGU)
Date: 10-2023
DOI: 10.1029/2023GC011120
Publisher: Springer Science and Business Media LLC
Date: 07-04-2021
DOI: 10.1007/S00410-021-01791-8
Abstract: 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 olv ≅ 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 $$\\left( {{\\text{lnD}}_{{{\\text{grt}}/{\\text{olv}}}}^{{{\\text{Ni}}}} \\frac{{{\\text{Ni}}_{{{\\text{grt}}}} }}{{{\\text{Ni}}_{{{\\text{olv}}}} }}} \\right)$$ lnD grt / olv Ni ; Ni grt Ni olv , the Ca mole fraction in garnet ( $${\\mathrm{X}}_{\\mathrm{grt}}^{\\mathrm{Ca}} $$ X grt Ca ; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet ( $${\\mathrm{X}}_{\\mathrm{grt}}^{\\mathrm{Cr}} $$ X grt Cr ; 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: $$T \\left(^\\circ{\\rm C} \\right)=\\frac{-8254.568}{\\left(\\left( {\\mathrm{X}}_{\\mathrm{grt}}^{\\mathrm{Ca}} \\times 3.023 \\right)+\\left({\\mathrm{X}}_{\\mathrm{grt}}^{\\mathrm{Cr}} \\times 2.307 \\right)+\\left({\\mathrm{lnD}}_{\\frac{\\mathrm{grt}}{\\mathrm{olv}}}^{\\mathrm{Ni}} - 2.639 \\right)\\right)}-273\\pm 55$$ T ∘ C = - 8254.568 X grt Ca × 3.023 + X grt Cr × 2.307 + lnD grt olv Ni - 2.639 - 273 ± 55 where $${\\mathrm{D}}_{\\mathrm{grt}/\\mathrm{olv}}^{\\mathrm{Ni}}= \\frac{{\\mathrm{Ni}}_{\\mathrm{grt}}}{{\\mathrm{Ni}}_{\\mathrm{olv}}},$$ D grt / olv Ni = Ni grt Ni olv , Ni is in ppm, $${\\mathrm{X}}_{\\mathrm{grt}}^{\\mathrm{Ca}}$$ X grt Ca = Ca/(Ca + Fe + Mg) in garnet, and $${\\mathrm{X}}_{\\mathrm{grt}}^{\\mathrm{Cr}}$$ X grt Cr = 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.
Publisher: Springer Science and Business Media LLC
Date: 27-07-2022
DOI: 10.1007/S00410-022-01944-3
Abstract: 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 $$^\\circ{\\rm C}$$ ∘ 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) $$\\left( {\\Delta {\\text{T}} = {\\text{T}}_{{\\exp - { }}} {\\text{T}}_{{{\\text{calc}}}} } \\right)$$ Δ T = T exp - T calc of less than $$\\pm$$ ± 50 °C with a standard deviation of $$\\Delta {\\text{T}}$$ Δ T between $$\\pm$$ ± 50 to 150 °C. Most geothermometers performed well across a narrow range in ln $${\\text{Kd}}_{{{\\text{Fe}} - {\\text{Mg}}}}^{{{\\text{A}} - {\\text{B}}}}$$ Kd Fe - Mg A - B (where $${\\text{Kd}}_{{{\\text{Fe}} - {\\text{Mg}}}}^{{{\\text{A}} - {\\text{B}}}} =$$ Kd Fe - Mg A - B = $$\\frac{{\\left( {{\\text{Fe}}_{{\\text{A}}} \\times {\\text{Mg}}_{{\\text{B}}} } \\right)}}{{({\\text{Fe}}_{{\\text{B}}} \\times {\\text{Mg}}_{{\\text{A}}} )}}$$ Fe A × Mg B ( Fe B × Mg A ) ), however, systematic overestimation and underestimation of T were observed outside of the optimal range of $${\\text{lnKd}}_{{{\\text{Fe}} - {\\text{Mg}}}}^{{{\\text{A}} - {\\text{B}}}}$$ lnKd Fe - Mg A - B . Increases in experimental pressure (P) adversely affected several geothermometers, particularly those calibrated empirically using natural s les. 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 yroxenitic 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: $${\\text{T}}_{{{\\text{Fe}} - {\\text{Mg}}}}^{{{\\text{grt}} - {\\text{cpx}}}} { (}^{ \\circ } {\\text{C)}} = { }\\frac{3356.34}{{\\left( {\\left( { - 0.008 \\times {\\text{P }}\\left( {{\\text{kbar}}} \\right)} \\right) + \\left( {0.259 \\times {\\text{X}}_{{{\\text{Ca}}}}^{{{\\text{grt}}}} } \\right) + \\left( {0.914 \\times {\\text{X}}_{{{\\text{Mg}}}}^{{{\\text{grt}}}} } \\right) + \\left( { - 0.159 \\times {\\text{Jd}}^{{{\\text{cpx}}}} } \\right) + \\left( {{\\text{ln}}\\left( {{\\text{ Kd}}_{{{\\text{Fe}} - {\\text{Mg}}}}^{{{\\text{grt}} - {\\text{cpx}}}} } \\right) + 1.265} \\right)} \\right)}} - 273{ }$$ T Fe - Mg grt - cpx ( ∘ C) = 3356.34 - 0.008 × P kbar + 0.259 × X Ca grt + 0.914 × X Mg grt + - 0.159 × Jd cpx + ln Kd Fe - Mg grt - cpx + 1.265 - 273 $${\\text{T}}_{{{\\text{Fe}} - {\\text{Mg}}}}^{{{\\text{grt}} - {\\text{opx}}}} { (}^{ \\circ } {\\text{C)}} = { }\\frac{1851.85}{{\\left( {\\left( { - 0.007 \\times {\\text{P }}\\left( {{\\text{kbar}}} \\right)} \\right) + \\left( { - 1.83 \\times {\\text{X}}_{{{\\text{Ca}}}}^{{{\\text{grt}}}} } \\right) + \\left( {{\\text{ln}}\\left( {{\\text{ Kd}}_{{{\\text{Fe}} - {\\text{Mg}}}}^{{{\\text{grt}} - {\\text{cpx}}}} } \\right) + 1.08} \\right)} \\right)}} - 273$$ T Fe - Mg grt - opx ( ∘ C) = 1851.85 - 0.007 × P kbar + - 1.83 × X Ca grt + ln Kd Fe - Mg grt - cpx + 1.08 - 273 . where, $${\\text{X}}_{{{\\text{Ca}}}}^{{{\\text{grt}}}} =$$ X Ca grt = $$\\frac{{{\\text{Ca}}}}{{\\left( {{\\text{Ca}} + {\\text{Fe}} + {\\text{Mg}}} \\right)}}$$ Ca Ca + Fe + Mg , $${\\text{Kd}}_{{{\\text{Fe}} - {\\text{Mg}}}}^{{{\\text{grt}} - {\\text{opx}}}} =$$ Kd Fe - Mg grt - opx = $$\\frac{{\\left( {{\\text{Fe}}_{{{\\text{grt}}}} \\times {\\text{Mg}}_{{{\\text{opx}}}} } \\right)}}{{({\\text{Fe}}_{{{\\text{opx}}}} \\times {\\text{Mg}}_{{{\\text{grt}}}} )}} , {\\text{X}}_{{{\\text{Mg}}}}^{{{\\text{grt}}}} =$$ Fe grt × Mg opx ( Fe opx × Mg grt ) , X Mg grt = $$\\frac{{{\\text{Mg}}}}{{\\left( {{\\text{Ca}} + {\\text{Fe}} + {\\text{Mg}}} \\right)}}$$ Mg Ca + Fe + Mg , $${\\text{Jd}}^{{{\\text{cpx}}}} = {\\text{Na}} - {\\text{Cr}} - 2 \\times {\\text{Ti}}$$ Jd cpx = Na - Cr - 2 × Ti , $${\\text{Kd}}_{{{\\text{Fe}} - {\\text{Mg}}}}^{{{\\text{grt}} - {\\text{cpx}}}} =$$ Kd Fe - Mg grt - cpx = $$\\frac{{\\left( {{\\text{Fe}}_{{{\\text{grt}}}} \\times {\\text{Mg}}_{{{\\text{cpx}}}} } \\right)}}{{({\\text{Fe}}_{{{\\text{cpx}}}} \\times {\\text{Mg}}_{{{\\text{grt}}}} )}},$$ Fe grt × Mg cpx ( Fe cpx × Mg grt ) , with all elements calculated on the basis of 12 oxygen anions in garnet and 6 oxygen anions in clino- and orthopyroxene. Fe 2+ = 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.
Publisher: American Geophysical Union (AGU)
Date: 11-2022
DOI: 10.1029/2022GC010558
Abstract: 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 ,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 Cr 2 O 3 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 ( km) comprises high Cr 2 O 3 lherzolite with elevated TiO 2 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.
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Zachary Sudholz.