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Primary spinel + chlorite inclusions in mantle garnet formed at ultrahigh-pressure

M. Campione1,

1Department of Earth and Environmental Sciences, Università degli Studi di Milano Bicocca, Piazza della Scienza 4, I-20126 Milano, Italy

S. Tumiati2,

2Department of Earth Sciences, Università degli Studi di Milano, Via Mangiagalli 34, I-20133 Milano, Italy

N. Malaspina1

1Department of Earth and Environmental Sciences, Università degli Studi di Milano Bicocca, Piazza della Scienza 4, I-20126 Milano, Italy

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Campione, M., Tumiati, S., Malaspina, N. (2017) Primary spinel + chlorite inclusions in mantle garnet formed at ultrahigh-pressure. Geochem. Persp. Let. 4, 19–23.

Italian Ministry of Education, University and Research (MIUR) [PRIN-2012R33ECR] and Deep Carbon Observatory (DCO).

Geochemical Perspectives Letters v4  |  doi: 10.7185/geochemlet.1730
Received 13 April 2017  |  Accepted 25 July 2017  |  Published 23 August 2017




Figure 1 (a) Photomicrograph of a multiphase solid inclusion in metasomatic garnet from Maowu Ultramafic Complex (Dabie Shan, China). Inset represents the relative orientation of the spinel {100} surface lattice (light blue) with respect to the garnet {100} surface lattice (violet) for the coincidence at θ = −45° (from Malaspina et al., 2015

Malaspina, N., Alvaro, M., Campione, M., Wilhelm, H., Nestola, F. (2015) Dynamics of mineral crystallization from precipitated slab-derived fluid phase: first in situ synchrotron X-ray measurements. Contributions to Mineralogy and Petrology 169, 26.

). (b) and (c) Negative-crystal shaped multiphase solid inclusion (plane polarised transmitted light and Secondary Electron image) with evident microstructural relations between spinel, chlorite and amphibole (gedrite).
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Figure 2 Isochemical P-T section in the MASH system showing the predicted mineral assemblages calculated with Perple_X software package (Connolly, 1990

Connolly, A.D. (1990) Algorithm based on generalized thermodynamics. American Journal of Science 290, 666–718.

) for a bulk composition corresponding to pyrope [MgO (3 mol) - SiO2 (3 mol) - Al2O3 (1 mol)] + excess H2O. Mineral abbreviations: chl = chlorite, mctd = Mg-clorithoid, ta = talc, mcar = Mg-carpholite, ky = kyanite, q = quartz, opx = orthopyroxene, sud = sudoite, cor = corundum, sill = sillimanite, crd = cordierite, sapp = sapphirine.
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Figure 3 Chemography of the MgO-Al2O3-SiO2-H2O system at 4 GPa and 800 °C, projected from water, showing the stable assemblages coe-ky-py (purple field), py-en-fo (green), and py-sp-clin (yellow). Experimental equilibrium slab fluid compositions and mantle fluid compositions are indicated by purple and green dots, respectively. Calculated compositions of a fluid in equilibrium with py-sp-clin assemblage is indicated by a yellow dot. The bulk composition of the orthopyrexenite containing multiphase inclusions in garnet is indicated by a grey dot. Mineral abbreviations same as in Figure 2 and coe = coesite, mst = Mg-staurolite, py = pyrope, en = enstatite, clin = clinochlore, sp = spinel, fo = forsterite, chum = clinohumite, br = brucite.
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Figure 4 Schematic cartoon showing aqueous fluid entrapped by growing metasomatic garnet (1) after the interaction of slab-derived supercritical liquid (SCL) and the supra-subduction mantle peridotite forming garnet orthopyroxenite (grey layer and veins). Light blue hexagons represent primary aqueous inclusions in pyrope. Garnet/fluid interaction yields a dissolution and precipitation process that triggers epitaxial nucleation of spinel and chlorite during garnet growing at UHP (2). Subsequent post-entrapment crystallisation of the other hydrous phases such as gedrite, phlogopite, pargasite and talc during the retrograde P-T path (3) leaves an eventual residue of water solution (light blue rim). Modified after Malaspina et al. (2017)

Malaspina, N., Langenhorst, F., Tumiati, S., Campione, M., Frezzotti, M.L., Poli, S. (2017) The redox budget of crust-derived fluid phases at the slab-mantle interface. Geochimica et Cosmochimica Acta 209, 70–84.

.
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Supplementary Figures and Tables


Table S-1 MgO, Al2O3 and SiO2 molalities of aqueous solutions in equilibrium with different assemblages, calculated using the aqueous speciation-solubility code EQ3 adapted to include equilibrium constants calculated with the Deep Earth Water (DEW) model.
Equilibrium slab-fluid
DEW-EQ3 (py-coe-ky assemblage)EXP (Kessel et al., 2005) purple dot in Figure 3
m = 0.017m = 0.11
a = 0.0037a = 0.18
s = 1.77s = 3.7


Equilibrium mantle-fluid
DEW-EQ3 (en-fo-py assemblage)EXP (Dvir et al., 2011) green dot in Figure 3
m = 0.25m = 5.29
a = 0.0026a = 0.31
s = 0.20s = 3.0


Fluid in equilibrium with py-sp-clin assemblage
DEW-EQ3EXP (Fockenberg et al., 2008) yellow dot in Figure 3
m = 0.15m = 0.39
a = 0.0096a = 0.13
s = 0.12s = 0.39
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Figure S-1 Compatibility diagrams of the water-saturated MgO-Al2O3-SiO2-H2O system at 800 °C and 1.5–4 GPa, projected from water, showing that the stable assemblage pyrope-spinel-clinochlore occurs only at UHP conditions. Mineral abbreviations same as Figures 2 and 3 of the manuscript.
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