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

Abstract

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.

Subducting carbon

“…Such a model predicts that inclusion assemblages are chemically linked although they appear to reflect different host rocks, and that there should be straightforward elemental and isotopic patterns consistent with the evolution of conditions from the eclogite source to the ambient mantle. This model better explains the highly enriched character of inclusion minerals, which are otherwise anomalous if they are direct samples of the slab 76 . It also explains C-O stable isotopic systematics 77 .…”

Interphase REE Partitioning at the Boundary between the Earth’s Transition Zone and Lower Mantle: Evidence from Experiments and Atomistic Modeling

“…In most experimental studies, the distribution of REE between mantle perovskites and melt is considered. In particular, very low bridgmanite/melt REE partitioning coefficients calculated for the high-pressure conditions (~10 −3 for La to~1 for Lu) [23] suggest the primary origin of Brd grains with their further trapping by host superdeep diamonds, but not crystallization of bridgmanite from melt, as is suggested in some studies [24]. In contrast, CaSiO 3 -perovskite/melt partition coefficients for REE are much higher [25], which causes the higher concentrations of REE (by up to two to three orders of magnitude) with significant enrichment in LREE in natural CaPrv in relation to Brd.…”

Deep Carbon

“…Wang et al 174 suggested a role for carbonated melt in the origin of Ca-perovskite inclusions, and the distinct trace element patterns in Ca-perovskite and majorite inclusions from Juína, Brazil, indicate that the inclusions crystallized from carbonated melt derived from subducted oceanic crust. 20,21,56,176,189 The involvement of recycled crustal components in the origin of super-deep diamonds is supported by observations that the diamond hosts show a wide range of carbon isotope compositions extending to very light values (e.g.~0% to -25%) 20,21 and that majorite garnet and other silicate inclusions have isotopically heavy oxygen isotope compositions. 104,109 The high and variable ferric iron content of majorite inclusions is also consistent with an oxidized carbonate component in their origin.…”

“…Refs. 18,20,21,49,56,98,100,102,105,118,[174][175][176]. Inclusions interpreted as former majorite garnet or Ca-perovskite often exhibit composite mineralogy that is interpreted to have formed by unmixing from primary precursor minerals exhibiting solid solution (e.g.…”