Isotopic fractionation of oxygen and carbon in decomposed lower-mantle inclusions in diamond

Abstract

Two carbonatitic mineral assemblages, calcite + wollastonite and calcite + monticellite, which are encapsulated in two diamond grains from the Rio Soriso basin in the Juina area, Mato Grosso State, Brazil, were studied utilizing the NanoSIMS technique. The assemblages were formed as the result of the decomposition of the lower-mantle assemblage calcite + CaSi-perovskite + volatile during the course of the diamond ascent under pressure conditions from 15 to less than 0.8 GPa. The oxygen and carbon isotopic compositions of the studied minerals are inhomogeneous. They fractionated during the process of the decomposition of primary minerals to very varying values: δ 18 O from -3.3 ‰ to +15.4 ‰ SMOW and δ 13 C from -2.8 ‰ to +9.3 ‰ VPDB. These values significantly extend the mantle values for these elements in both isotopically-light and isotopically-heavy areas.

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

“…In addition, some CaCO 3 in the slab could also be produced through dolomite‐pyroxene and/or dolomite‐olivine reactions (Biellmann et al, ; Hammouda, ; Kushiro et al, ; Martinez et al, ; Shirasaka et al, ). The residual CaCO 3 could thus be present in the form of CaCO 3 ‐VII at the topmost lower mantle, and some of them could be captured as inclusions in diamonds as shown in recent petrological studies (Brenker et al, ; Kaminsky et al, , ; Wirth et al, ). CaCO 3 ‐VII is thus a potential carbon‐bearing phase in this region.…”

“…Some CaCO 3 that transforms from calcite to aragonite will likely react with mantle pyroxene and/or olivine during the subduction process, whereas a large amount of CaCO 3 may become unstable in slab melting and cannot be transported to the deeper depth (Biellmann et al, ; Stagno et al, ; Thomson et al, ). However, some CaCO 3 in cold slabs will likely survive, depending on the local mineral proportions and pressure‐temperature‐compositional environments of the slab (Brenker et al, ; Kaminsky et al, , ; Thomson et al, ; Wirth et al, ). In addition, some CaCO 3 in the slab could also be produced through dolomite‐pyroxene and/or dolomite‐olivine reactions (Biellmann et al, ; Hammouda, ; Kushiro et al, ; Martinez et al, ; Shirasaka 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, ). However, the residual CaCO 3 after the CaCO 3 ‐VII‐Stv reaction could be limited.…”

“…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.…”

“…Through hydrothermal metasomatism, a significant amount of CaCO 3 in the form of calcite enters the oceanic crust to form carbonate-bearing altered oceanic basalt (Alt & Teagle, 1999;Jarrard, 2003;Kelemen & Manning, 2015). Analysis of the inclusions in deep diamonds has revealed evidence of the transportation of CaCO 3 to depths of at least the topmost lower mantle via sinking slabs (Brenker et al, 2007;Kaminsky et al, 2009Kaminsky et al, , 2016Wirth et al, 2009). Previous studies have further shown that CaCO 3 undergoes a series of phase transitions at high pressures (Catalli & Williams, 2005;Gavryushkin et al, 2017;Lobanov et al, 2017;Oganov et al, 2008Oganov et al, , 2006Ono et al, 2005;Pickard & Needs, 2015;Smyth & Ahrens, 1997;Suito et al, 2001).…”