The phase relations of mafic rocks within the Earths mantle and geobarometers for eclogite and pyroxenite rocks to conditions of the Earths transition zone.
Final Report Abstract
Pressure dependent changes in garnet compositions were calibrated using the results of experiments conducted in a multi-anvil apparatus at pressures between 6 and 16 GPa and temperatures of 1000 to 1400 °C. Using the results of these experiments a barometer was formulated based on an empirical parameterization of the two major majoritic substitutions, referred to as majorite (Maj; Al3+ = Mg2+ + Si4+) and Na majorite (Na-Maj; Mg2+ + Al3+ = Na+ + Si4+). Previously published experimental garnet compositions from basaltic, kimberlite, komatiite and peridotite bulk compositions were included in the calibration, which consequently covers pressures from 6 to 20 GPa and temperatures from 900 to 2100 °C. Experimental pressures are reproduced over these conditions with a standard deviation of 0.86 GPa. The barometer was then used to determine equilibration pressures of approximately 500 reported garnet inclusions in diamonds from a range of localities. As the majority of these inclusions are proposed to be syngenetic this allows a detailed picture of diamond formation depths and associated source rocks to be established using inclusion chemistry. Geographic differences in diamond source rocks are mapped within the sublithospheric mantle to over 500 km. Continuous diamond formation occurs over this depth range within lithologies with eclogitic affinities but also in lithologies that appear transitional between eclogitic and peridotitic bulk compositions, with an affinity to pyroxenites. The geographic differences between eclogitic and pyroxenitic diamond source rocks are rationalised in terms of diamond formation within downwelling and upwelling regimes respectively. Macroscopic diamond formation in rocks with pyroxenite compositions are likely facilitated in the deep mantle by higher average oxidation states and low mineral H2O solubility compared to the surrounding mantle, which aid the mobility of C-O-H volatile species. The apparent lack of inclusions with a peridotite affinity likely results from generally low oxygen fugacities in such lithologies, which reduces carbon mobility, and the lack of a suitable oxidising agent to allow diamonds to form from CH4. This glimpse of deep carbon cycle processes implies that heterogeneities in the carbon content, redox state and chemical composition of the mantle may be strongly coupled. The majorite barometer along with a suite of petrological geobarometer equilibria commonly used to determine the equilibration depths of upper mantle rocks were tested in a series of experiments where pressure was determined simultaneously using x-ray diffraction of pressure standard materials. Barometers were simultaneously compared with pressures determined by in-situ x-ray measurement of the unit cell volumes of MgO, NaCl, Re, and h-BN between 3.6 and 10.4 GPa, and 1250 and 1500 °C. Experiments were conducted using multianvil presses installed at beamlines 13-ID-D of the APS, Argonne, Chicargo and ID06 of the ESRF, Grenoble. Noble metal multi-chamber capsules were loaded with a range of bulk compositions representing peridotitic, eclogitic, and pyroxenitic lithologies. Geobarometers applicable to different mantle lithologies under similar pressure and temperature conditions were compared and evaluated. Discrepancies were identified between the calculated and experimental pressures for which we propose simple linear or constant correction factors to some of the previously published barometric equations. As a result, we establish internally-consistent crosscalibrations for a number of garnet-orthopyroxene, garnet-clinopyroxene, Ca-Tschermaks-inclinopyroxene, and majorite geobarometers.
Publications
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(2017) The depth of sub-lithospheric diamond formation and the redistribution of carbon in the deep mantle. Earth and Planetary Science Letters 461: 30 – 39
Beyer C, Frost DJ
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(2018) An internally consistent pressure calibration of geobarometers applicable to the Earth's upper mantle using in situ XRD. Geochimica et Cosmochimica Acta 222: 421 – 435
Beyer C, Rosenthal A, Myhill R, Crichton WA, Yu T, Wang YB, Frost DJ
