Kankan diamonds (Guinea) II: lower mantle inclusion parageneses

Stachel, Thomas; Harris, Jeff W.; Brey, Gerhard P.; Joswig, W.

doi:10.1007/s004100000174

Cited by 252 publications

(136 citation statements)

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“…In fact, there has been a growing body of evidence for the ultradeep subduction of carbonates and their participation in the formation of diamonds. This evidence involves the discovery of inclusions in diamonds, consisting of carbonates in association with the superdeep phases CaSiO 3 or MgO + MgSiO 3 , as well as some experimental and geochemical observations (51)(52)(53)(54). Our results suggest that the Ca-rich carbonate melt, which forms through the carbonate-iron interaction and can be generated even in the absence of alkalis and H 2 O, can be considered as a transmantle interstitial melt.…”

Section: Significancesupporting

confidence: 51%

Mantle–slab interaction and redox mechanism of diamond formation

Palyanov

Bataleva

Sokol

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

174

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Subduction tectonics imposes an important role in the evolution of the interior of the Earth and its global carbon cycle; however, the mechanism of the mantle-slab interaction remains unclear. Here, we demonstrate the results of high-pressure redox-gradient experiments on the interactions between Mg-Ca-carbonate and metallic iron, modeling the processes at the mantle-slab boundary; thereby, we present mechanisms of diamond formation both ahead of and behind the redox front. It is determined that, at oxidized conditions, a low-temperature Ca-rich carbonate melt is generated. This melt acts as both the carbon source and crystallization medium for diamond, whereas at reduced conditions, diamond crystallizes only from the Fe-C melt. The redox mechanism revealed in this study is used to explain the contrasting heterogeneity of natural diamonds, as seen in the composition of inclusions, carbon isotopic composition, and nitrogen impurity content.carbonate-iron interaction | high-pressure experiment | mantle mineralogy | deep carbon cycle S ubduction of crustal material plays an important role in the global carbon cycle (1-6). Depending on oxygen fugacity and pressure-temperature (P-T) conditions, carbon exists in the Earth's interior in the form of carbides, diamond, graphite, hydrocarbons, carbonates, and CO 2 (7-11). In the upper mantle, the oxygen fugacity (fO 2 ) varies from one to five log units below the fayalitemagnetite-quartz (FMQ) buffer, with a trend of a decrease with depth (6,(12)(13)(14)(15). At a depth of ∼250 km, mantle is reported to become metal saturated (16, 17), which holds true for all mantle regions below, including the transition zone and lower mantle. The subduction of the oxidized crustal material occurs to depths greater than 600 km (4-6). The main carbon-bearing minerals of the subducted materials are carbonates, which are thermodynamically stable up to P-T conditions of the lower mantle (10,11,18). As evidenced by the compositions of inclusions in diamond, which vary from strongly reduced, e.g., metallic iron and carbides (19-23), to oxidized, e.g., carbonates and CO 2 (6,20,(24)(25)(26)(27)(28), carbonates may be involved in the reactions with reduced deep-seated rocks, including Fe 0 -bearing species (29-31). A scale of these reactions is determined mainly by the capacity of subducted carbonate-bearing domains. An important consequence of such an interaction is that it can produce diamond. However, studies on diamond synthesis via the reactions between oxidized and reduced phases are limited (32-35).To understand the mechanisms of the interaction of carbonbearing oxidized-and reduced-mineral assemblages, we performed high-pressure experiments with an iron-carbonate system; an approach was used that enabled the creation of an oxygen fugacity gradient in the capsules (Materials and Methods and SI Materials and Methods). Results and DiscussionThe experimental results and the phase compositions are given in Table 1 and Table S1, respectively. At temperatures of 1,000 and 1,100°C, the iron-car...

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Section: Significancesupporting

confidence: 51%

Mantle–slab interaction and redox mechanism of diamond formation

Palyanov

Bataleva

Sokol

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

174

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“…However, as observed in the upper mantle, heterogeneities of the lower mantle redox state probably exist: For instance, subduction zones are made of more oxidized materials (10) that could survive on long time scales. Coexistence of both reduced and oxidized species in the deep Earth is also suggested by examples such as CO 2 -rich kimberlitic magmas that have transported diamonds to the surface (11,12), and some carbonate inclusions that were found in diamonds from the lower mantle (13,14). Results of high-pressure experiments on carbonate stability suggest that the rhombohedral structure of MgCO 3 magnesite is stable up to 115 GPa at 2,000-3,000 K, but that it adopts a new structure at higher pressures (8).…”

mentioning

confidence: 81%

New host for carbon in the deep Earth

Boulard

Gloter

Corgne

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

124

140

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The global geochemical carbon cycle involves exchanges between the Earth's interior and the surface. Carbon is recycled into the mantle via subduction mainly as carbonates and is released to the atmosphere via volcanism mostly as CO 2 . The stability of carbonates versus decarbonation and melting is therefore of great interest for understanding the global carbon cycle. For all these reasons, the thermodynamic properties and phase diagrams of these minerals are needed up to core mantle boundary conditions. However, the nature of C-bearing minerals at these conditions remains unclear. Here we show the existence of a new Mg-Fe carbon-bearing compound at depths greater than 1,800 km. Its structure, based on three-membered rings of corner-sharing ðCO 4 Þ 4− tetrahedra, is in close agreement with predictions by first principles quantum calculations [Oganov AR, et al. (2008)

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“…It was the discovery of diamonds containing coexisting, but nontouching, inclusions of ferropericlase, MgSiO 3 and CaSiO 3 (Harte et al 1999) that provided convincing evidence that some diamonds are exhumed from the lower mantle. 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).…”

Section: Introductionmentioning

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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Kankan diamonds (Guinea) II: lower mantle inclusion parageneses

Cited by 252 publications

References 0 publications

Mantle–slab interaction and redox mechanism of diamond formation

Mantle–slab interaction and redox mechanism of diamond formation

New host for carbon in the deep Earth

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

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