Diamond formation in the deep mantle: the record of mineral inclusions and their distribution in relation to mantle dehydration zones

Abstract

Studies of the inclusions contained in natural diamonds have shown the occurrence of minerals which must have formed at depths below the lithosphere and which may be closely matched with the silicate mineral assemblages determined by high pressure and temperature experimental studies for depths of 300 to 800 km in the Earth's mantle. The inclusions come principally from two main depth zones:(1) the lower asthenosphere and upper transition zone; (2) the Upper Mantle/Lower Mantle (UM/LM) boundary region and the uppermost LM. The inclusions from zone 1 are very largely majoritic garnets (with or without clinopyroxene) which indicate bulk compositions of eclogitic/metabasic affinity. The minerals from zone 2 include Ca-Si and Mg-Si perovskites and ferropericlase and are dominantly of metaperidotitic bulk composition, but include some possible metabasite assemblages. In many of these natural assemblages, the tetragonal almandine pyrope phase occurs rather than the garnet found in experiments.As natural diamonds are believed to crystallize in fluids/melts, the hypothesis is developed that the restriction of diamonds and inclusions of particular compositions to the above two depth intervals is because they are controlled by loci of fluid/melt occurrence. Attention is focused on subduction zones because both suites of inclusions show some evidence of subducted protoliths. The lower zone (600À800 km) coincides with the region where dehydration may be expected for hydrous ringwoodite and dense hydrous Mg-silicates formed in subducted peridotites. The dehydration of lawsonite in subducted metabasites provides a particular location for melt formation and the inclusion of the shallower (~300 km) majoritic inclusions. For the deeper majoritic inclusions in the region of the upper transition zone, melt development may occur as a consequence of the hydrous wadsleyite-to-olivine transformation, and such melt may then interact with the upper crustal portion of a subducting slab. These suggestions offer an explanation of the depth restrictions and the compositional restrictions of the inclusions. The differences in d13 C values in the host diamonds for the two suites of inclusions may also be explained on this basis.KEYWORDS: majoritic garnet, Ca-Si perovskite, Mg-Si perovskite, hydrous wadsleyite, hydrous ringwoodite, dense hydrous Mg-silicates, dehydration reactions. Introduction and backgroundTHIS paper considers the provenance of mineral inclusions found in natural diamonds which appear to come from below the Earth's lithosphere. Attention will be focused on the inclusions which appear to come from ultrabasic and basic bulk rock compositions. Comparison of these inclusions with experimental data shows that they have selective distributions with respect to depth: with basic ones coming predominantly from a region in the lower asthenosphere and upper transition zone, and ultrabasic ones from a region spanning the base of the transition zone Revi ew (TZ) and the uppermost lower mantle. The hypothesis will be developed that this d...

Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds

“…Since conversion of metastable graphite to diamond is not expected to produce macrocrystals (Sung, 2000), diamond formation during a fluid/melt aided dissolution -precipitation process is likely. The volatile-rich nature of the protolith is consistent with the recent suggestion by Harte (2010) that ultradeep diamonds are associated with devolatilisation and possibly melting reactions. , the histogram on the X-axis is of cratonic diamonds (Stachel et al, 2009).…”

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

“…As with majorite-bearing samples, the number of possible lower mantle diamond occurrences has grown since the initial discoveries, and examples are now recognised in South Africa (Moore et al 1986), Guinea (Stachel et al 2000b(Stachel et al , 2002, Canada (Davies et al 2004;Tappert et al 2005), Australia (Tappert et al 2009a, b) and Brazil, both from Machado river ) and the Juina region (see below). However, in contrast to majorite-bearing diamonds, the number of definitively lower mantle diamonds remains small and most of the diamonds are associated with peridotitic material (Harte 2010;Kaminsky 2012).…”

“…In fact, more than half of all recognised lower mantle diamonds are from Juina sources (Harte 2010), which makes Juina a unique geological locality. Studies of alluvial Juina diamonds (Harte et al 1999;Kaminsky et al 2001;Hutchison et al 2001;Hayman et al 2005;Brenker et al 2007) have revealed mineral inclusion assemblages associated with peridotitic material whilst the diamonds generally carry a "mantle-like" carbon isotope signature (i.e.…”

“…However, a small proportion (<5 %) of diamonds studied worldwide originated at much greater depths (Stachel 2001;Stachel et al 2005), with some containing inclusions indicating their transport from up to 800 km depth (Harte 2010). These diamonds are of great geological importance, providing the only direct samples of the sub-lithospheric mantle, and they allow us to test models of the processes we believe are occurring in Earth's deep interior.…”

Fe-periclase reactivity at Earth’s lower mantle conditions: Ab-initio geochemical modelling

“…In addition, Fe 3+ is theoretically associated to the appearance of vacancies in non-stoichiometric wü stite and (Mg,Fe)O (Otsuka et al, 2010;Wdowik et al, 2015). Measured iron in Fe-periclase and bridgmanite, both coexisting as inclusions in lowermantle diamonds (Kaminsky and Lin (2017) (Harte, 2010;Kaminsky, 2012), the vacancy generation effect induced by ferric iron can therefore be assumed negligible in the present modelling.…”