Diamonds from the Machado River alluvial deposit, Rondônia, Brazil, derived from both lithospheric and sublithospheric mantle

Abstract

Diamonds from the Machado River alluvial deposit have been characterised on the basis of external morphology, internal textures, carbon isotopic composition, nitrogen concentration and aggregation state and mineral inclusion chemistry. Variations in morphology and features of abrasion suggest some diamonds have been derived directly from local kimberlites, whereas others have been through extensive sedimentary recycling. On the basis of mineral inclusion compositions, both lithospheric and sublithospheric diamonds are present at the deposit. The lithospheric diamonds have clear layer-by-layer octahedral and/or cuboid internal growth zonation, contain measurable nitrogen and indicate a heterogeneous lithospheric mantle beneath the region. The sublithospheric diamonds show a lack of regular sharp zonation, do not contain detectable nitrogen, are isotopically heavy (δ 13 CPDB predominantly-0.7-5.5) and contain inclusions of ferropericlase, former bridgmanite, majoritic garnet and former CaSiO3-perovskite. This suggests source lithologies that are Mg-and Ca-rich, probably including carbonates and serpentinites, subducted to lower mantle depths. The studied suite of sublithospheric diamonds has many similarities to the alluvial diamonds from Kankan, Guinea, but has more extreme variations in mineral inclusion chemistry. Of all superdeep diamond suites yet discovered, Machado River represents an end-member in terms of either the compositional range of materials being subducted to Transition Zone and lower mantle or the process by which materials are transferred from the subducted slab to the diamond-forming region. Keywords bridgmanite; majorite; subduction; carbon; superdeep; diamond 1. Introduction Although the vast majority of the world's diamonds are characterised by being inclusion-free (96-99 %), and ~ 28 % of inclusions are trace-element poor olivines and chrome spinels (Stachel and Harris, 2008), they have continued to draw keen interest from petrologists and geochemists. While diamonds undoubtedly offer a selective view of the deep Earth, in particular of the highly depleted Archaean cratonic mantle (Stachel and Harris, 2008), the fidelity with which the chemistry of inclusions is preserved makes them a valuable scientific resource. Around 5-10 % of diamonds are derived from beneath the base of the lithosphere (i.e. sublithospheric, or "superdeep") and are direct evidence of the mixing and recycling processes that lead to mantlederived lavas with diverse isotopic and elemental characteristics (e.g. Pietruszka et al., 2013). The minerals that are included in these superdeep diamonds may only represent a single fragment of a single geological event in a rock's life cycle, but this information is essential for reconstructing the geological history of subducted material. Variations in the characteristics of diamonds and their inclusions within and between localities testify about the spatial and/or temporal variability of the source region: they reveal, for example, the presence of multiple

New High‐Pressure Phase of CaCO3 at the Topmost Lower Mantle: Implication for the Deep‐Mantle Carbon Transportation

“…The CO 2 released by the CaCO 3 ‐VII‐Stv reaction, together with the possible presence of water, will lower the melting temperature likely leading to the formation of the C‐O‐H‐bearing fluid or melt needed for the growth of diamonds (Dasgupta & Hirschmann, ; Litasov & Ohtani, ). The local abundance of Ca‐Pv produced by the CaCO 3 ‐VII‐Stv reaction will be higher than that of the normal mantle (Burnham et al, ). This observation could explain why the abundance of Ca‐Pv in some deep diamonds is higher than what is expected from typical mantle mineralogy (Burnham et al, ; Walter et al, ) and why ferropericlase is absent from recently found deep diamonds (Smith et al, ).…”

“…The local abundance of Ca‐Pv produced by the CaCO 3 ‐VII‐Stv reaction will be higher than that of the normal mantle (Burnham et al, ). This observation could explain why the abundance of Ca‐Pv in some deep diamonds is higher than what is expected from typical mantle mineralogy (Burnham et al, ; Walter et al, ) and why ferropericlase is absent from recently found deep diamonds (Smith et al, ). Meanwhile, the residual CaCO 3 from the CaCO 3 ‐VII‐Stv reaction can be captured by diamonds as inclusions or transformed into post‐aragonite, thereby becoming the carbon‐bearing phase responsible for the transport of carbon to even deeper depths (Brenker et al, ; Kaminsky et al, ; Wirth et al, ).…”

“…Petrologists have recently found more evidence for the formation of deep diamonds at the topmost lower mantle, suggesting the transportation of surface carbon to the Earth's deep interior (Brenker et al, 2007;Burnham et al, 2016;Kaminsky et al, 2009Kaminsky et al, , 2016Shirey et al, 2013;Smith et al, 2016;Wirth et al, 2009). Carbonates, including calcite (CaCO 3 ), magnesite (MgCO 3 ), and dolomite (CaMg(CO 3 ) 2 , are believed to be one of the most important carriers to transport surface carbon to the Earth's mantle.…”

FTIR thermochronometry of natural diamonds: A closer look

“…A comprehensive study of diamonds from Machado River, Brazil is included in the present volume (Burnham et al, 2016), and one diamond, P16, has exceptionally interesting N zoning features. This diamond is peridotitic, with an olivine inclusion in the core, but no inclusions in the outer zones.…”

Breyite inclusions in diamond: experimental evidence for possible dual origin

“…Since the discovery by Joswig et al (1999), CaSiO 3 inclusions in diamond have been reported by a number of researchers. Monomineralic CaSiO 3 inclusions as well as CaSiO 3 coexisting with larnite, titanite-structured CaSi 2 O 5 , calcite (aragonite), perovskite, silica polymorphs, orthopyroxene (possibly converted from bridgmanite), clinopyroxene, ringwoodite, ferropericlase, monticellite and cuspidine have been described in diamonds from Kankan, Guinea (Joswig et al, 1999;Stachel et al, 2000;Nasdala et al, 2003); Juína, Brazil (Hayman et al, 2005;Brenker et al, 2007;Walter et al, 2008;Wirth et al, 2009;Bulanova et al, 2010;Pearson et al, 2014;Anzolini et al, 2016;Kaminsky et al, 2016); Machado River, Brazil (Burnham et al, 2016); and the Slave province, Canada (Davies et al, 2004;Tappert et al, 2005). CaSiO 3 was also documented as a daughter mineral of multiphase inclusions in Juína diamond, which presumably formed through the crystallization of carbonatitic melts ).…”