The lithospheric-to-lower-mantle carbon cycle recorded in superdeep diamonds

Slab Transport of Fluids to Deep Focus Earthquake Depths—Thermal Modeling Constraints and Evidence From Diamonds

“…This is consistent with trace element patterns (Bulanova et al, 2010;Stachel, 2001;Stachel et al, 2005;Thomson et al, 2014;Walter et al, 2008) and heavy oxygen isotopic compositions exhibited by inclusions (Burnham et al, 2015;Ickert et al, 2013;Regier et al, 2020) that support derivation from altered oceanic crust. Indeed, such very heavy oxygen isotopic compositions (δ 18 O from +6‰ to +10‰) are at the extreme end of the seawater altered basalt compositional range and could be derived directly from carbonate melts released from altered oceanic crust (Regier et al, 2020). These sublithospheric diamonds are characterized by isotopically variable, typically light carbon with a composition (δ 13 C as low as −28.7‰) far below that of the average ambient mantle composition in δ 13 C of around −5‰ (Table 1; Bulanova et al, 2010;Palot et al, 2014Palot et al, , 2017Shirey et al, 2019;Walter et al, 2008Walter et al, , 2011.…”

“…Rather, in this examination of the role of carbonate, we are only considering the role of cooler slabs and carbonate in the deeper, altered igneous oceanic crust (Li et al, 2019), especially since it is only the cooler slabs that produce deep seismicity regardless of mechanism (Figures 5a, 6a, and 6b). This association with oceanic crust is cemented further by oxygen isotope measurements on majorite inclusions in sublithospheric diamonds (Burnham et al, 2015;Ickert et al, 2013;Regier et al, 2020). Just as with water in altered peridotite, carbonate in the deeper igneous oceanic crust of hot slabs will be removed shallower than 300 km depths (Figure 5b; e.g., Kerrick & Connolly, 2001), especially if water is infiltrating from cooler, deeper crustal levels (e.g., Gorman et al, 2006;Martin & Hermann, 2018).…”

“…On the scale of individual diamonds, carbonatitic diamonds are dominated by oceanic crustal mineral assemblages (e.g., Bulanova et al, 2010;Thomson et al, 2014;Walter et al, 2008Walter et al, , 2011 whereas aqueous sublithospheric diamonds are dominated by mantle mineral assemblages (e.g., Smith et al, 2016Smith et al, , 2018Smith et al, , 2021. Neither paragenesis fully excludes the other (Table 1) which gives the impression that the fluids generated have not been fully confined to their mantle or crustal hosts and may have migrated (i.e., Regier et al, 2020). On the diamond locality or slab domain scale, collections of sublithospheric diamonds are usually not mixtures of similar proportions of carbonatitic and aqueous types found together.…”

“…Mineral inclusions in some sublithospheric diamonds have been shown to be compositionally compatible with the high‐pressure phases expected to be present in subducted mid‐ocean ridge basalts (Stachel, 2001a; Thomson et al., 2014; Walter et al., 2011), indicating that these diamonds crystallized within or near the oceanic crust of subducted slabs. Mineral inclusions in other diamonds suggest an origin from the slab mantle (Regier et al., 2020; Smith et al., 2016, 2017, 2018, 2021). In many cases, reconstructed high‐pressure phase assemblages of sublithospheric inclusions indicate that they were encapsulated by the diamond within the mantle transition zone and near the upper/mantle lower mantle boundary (e.g., Harte, 2010; Harte et al., 1999; Hutchison et al., 2001; Stachel et al., 2005).…”

Deep carbon cycle constrained by carbonate solubility

“…Carbon-bearing species were assumed to dissolve in a specific order, starting with the leaching of organic carbon and followed by the dissolution of carbonate minerals (see Supplementary Data 2). This assumption is supported by (1) the organic C-rich nature of subduction zone fluids 70 , and the isotopic signature of asthenospheric to transition zone diamonds that is consistent with carbonated igneous oceanic crust, rather than sediments rich in organic carbon, being the dominant source of their carbon 4,71 . Even if some of the organic carbon does not dissolve and eventually graphitizes as the slab experiences higher pressures and temperatures with depth, the available water will dissolve calcite/aragonite instead.…”