Majoritic garnet: A new approach to pressure estimation of shock events in meteorites and the encapsulation of sub-lithospheric inclusions in diamond

Collerson, K. D.; Williams, Quentin; Kamber, Balz S.; Omori, Soichi; Arai, Hiroyoshi; Ohtani, Eiji

doi:10.1016/j.gca.2010.07.005

Cited by 74 publications

(74 citation statements)

References 87 publications

(170 reference statements)

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“…This is consistent with the production of diamond forming melts of oceanic crust in the transition zone as the carbonated MORB solidus intersects slab geotherms between ~ 350 and 600 km depth (Thomson et al, 2016). This depth range is consistent with barometry estimates for the formation pressures of diamond-hosted garnet inclusions (Beyer, 2015;Collerson et al, 2010;Harte, 2010;Stachel, 2001). Although, it is unclear whether majorite barometers can be applied to natural inclusions, given they are believed to have co-crystallised from a melt with calcium perovskite (Thomson et al, 2016) and undergone some reequilibration during uplift (Harte and Cayzer, 2007;Thomson et al, 2014) that may not have been fully accounted for in reported compositions.…”

Section: Is Inclusion Enrichment An Indicator Of a Low-degree Melt Sosupporting

confidence: 84%

Trace element composition of silicate inclusions in sub-lithospheric diamonds from the Juina-5 kimberlite: Evidence for diamond growth from slab melts

Thomson

Kohn

Bulanova

et al. 2016

Lithos

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Trace element composition of silicate inclusions in sublithospheric diamonds from the juina-5 kimberlite: Evidence for diamond growth from slab melts, LITHOS (2016), doi: 10.1016/j.lithos.2016.08.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. A C C E P T E DM A N U S C R I P T ACCEPTED MANUSCRIPT ABSTRACTThe trace element compositions of inclusions in sub-lithospheric diamonds from the Juina-5 kimberlite, Brazil, are presented. Literature data for mineral/melt partition coefficients were collated, refitted and employed to interpret inclusion compositions. As part of this process an updated empirical model for predicting the partitioning behaviour of trivalent cations for garnet-melt equilibrium calibrated using data from 73 garnet-melt pairs is presented. High levels of trace element enrichment in inclusions interpreted as former calcium silicate perovskite and majoritic garnet preclude their origin as fragments of an ambient deep mantle assemblage. Inclusions believed to represent former bridgmanite minerals also display a modest degree of enrichment relative to mantle phases. The trace element composition of 'NAL' and 'CF phase' minerals are also reported. Negative Eu, Ce, and Y/Ho anomalies alongside depletions of Sr, Hf and Zr in many inclusions are suggestive of formation from a low-degree carbonatitic melt of subducted oceanic crust. Observed enrichments in garnet and 'calcium perovskite' inclusions limit depths of melting to less than ~ 600 km, prior to calcium perovskite saturation in subducting assemblages. Less enriched inclusions in sub-lithospheric diamonds from other global localities may represent deeper diamond formation. Modelled source rock compositions that are capable of producing melts in equilibrium with Juina-5 'calcium perovskite' and majorite inclusions are consistent with subducted MORB. Global majorite inclusion compositions suggest a common process is responsible for the formation of many superdeep diamonds, irrespective of geographic locality. Global transition zone inclusion compositions are reproduced by fractional crystallisation from a single parent melt, suggesting that they record the crystallisation sequence and melt evolution during this interaction of slab melts with ambient mantle. All observations are consistent with the previous hypothesis that many superdeep diamonds are created as slab-derived carbonatites interact with peridotitic mantle in the transition zone.

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Section: Is Inclusion Enrichment An Indicator Of a Low-degree Melt Sosupporting

confidence: 84%

Trace element composition of silicate inclusions in sub-lithospheric diamonds from the Juina-5 kimberlite: Evidence for diamond growth from slab melts

Thomson

Kohn

Bulanova

et al. 2016

Lithos

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“…The formation pressures of the two majoritic garnet inclusions have been calculated using the geobarometer of Collerson et al (2010). Pressures for the bulk composition of each inclusion are calculated as 15.2 ± 1.4 and 12.9 ± 1.4 GPa, suggesting original growth at depths equivalent to the shallowest parts of the transition zone.…”

Section: Depth Of Individual Inclusion Formationmentioning

confidence: 99%

Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

Thomson

Kohn²,

Bulanova

et al. 2014

Contrib Mineral Petrol

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interpreted as evidence of diamond growth from abiotic organic carbon added to the oceanic crust during hydrothermal alteration. The bulk isotopic composition of the oceanic crust, carbonates plus organics, is equal to the composition of mantle carbon (−5 ‰), and we suggest that recycling/mixing of subducted material will replenish this reservoir over geological time. Several exposed, syngenetic inclusions have bulk compositions consistent with former eclogitic magnesium silicate perovskite, calcium silicate perovskite and NAL or CF phases that have reequilibrated during their exhumation to the surface. There are multiple occurrences of majoritic garnet with pyroxene exsolution, coesite with and without kyanite exsolution, clinopyroxene, Fe or Fe-carbide and sulphide minerals alongside single occurrences of olivine and ferropericlase. As a group, the inclusions have eclogitic affinity and provide evidence for diamond formation at pressures extending to Earth's deep transition zone and possibly the lower mantle. It is observed that the major element composition of inclusions and isotopic compositions of host Juina-5 diamonds are not correlated. The diamond and inclusion compositions are intimately related to subducted material and record a polybaric growth history across a depth interval stretching from the lower mantle to the base of the lithosphere. It is suggested that the interaction of slabderived melts and mantle material combined with subsequent upward transport in channelised networks or a buoyant diapir explains the formation of Juina-5 diamonds. We conclude that these samples, despite originating at great mantle depths, do not provide direct information about the ambient mantle, instead, providing a snapshot of the Earth's deep carbon cycle. Keywords Diamonds · Sub-lithospheric mantle · Carbon cycle · SubductionAbstract Forty-one diamonds sourced from the Juina-5 kimberlite pipe in Southern Brazil, which contain optically identifiable inclusions, have been studied using an integrated approach. The diamonds contain <20 ppm nitrogen (N) that is fully aggregated as B centres. Internal structures in several diamonds revealed using cathodoluminescence (CL) are unlike those normally observed in lithospheric samples. The majority of the diamonds are composed of isotopically light carbon, and the collection has a unimodal distribution heavily skewed towards δ 13 C ~ −25 ‰. Individual diamonds can display large carbon isotope heterogeneity of up to ~15 ‰ and predominantly have isotopically lighter cores displaying blue CL, and heavier rims with green CL. The light carbon isotopic compositions are Communicated by O. Müntener. Electronic supplementary materialThe online version of this article

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“…Garnets of pyroxenitic affinity are higher in MgO and lower in Al 2 O 3 than those of eclogitic affinity. Using geobarometers based on the Si content of majoritic garnet (Collerson et al, 2010;Beyer, 2015), these inclusions were formed at pressures of 7-18 GPa with values above 15 GPa being rare. At these pressures clinopyroxene should coexist with the garnet in some, but not all, pyroxenitic compositions, an inference which is supported by general compositional differences between majorite garnet and bulk MORB and pyroxenite Pieces of the Earth's mantle occurring either as tectonic fragments or xenoliths in volcanic rocks are dominantly peridotites, assemblages of olivine, ortho-and clinopyroxene with minor garnet and/or spinel.…”

Section: Figurementioning

confidence: 99%

The pyroxenite-diamond connection

Kiseeva¹,

Wood²,

Ghosh³

et al. 2015

Geochem. Persp. Let.

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Majoritic garnet: A new approach to pressure estimation of shock events in meteorites and the encapsulation of sub-lithospheric inclusions in diamond

Cited by 74 publications

References 87 publications

Trace element composition of silicate inclusions in sub-lithospheric diamonds from the Juina-5 kimberlite: Evidence for diamond growth from slab melts

Trace element composition of silicate inclusions in sub-lithospheric diamonds from the Juina-5 kimberlite: Evidence for diamond growth from slab melts

Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

The pyroxenite-diamond connection

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