Experimentally dictated stability of carbonated oceanic crust to moderately great depths in the Earth: Results from the solidus determination in the system CaO‐MgO‐Al₂O₃‐SiO₂‐CO₂

Abstract

[1] Solidus melting phase relations are reported for carbonated eclogite in the system CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 at 12 to 25 GPa. From 12 to 16 GPa, melts are in equilibrium with clinopyroxene, stishovite, garnet, aragonite, and magnesite. At 20 and 25 GPa, melts are in equilibrium with garnet, stishovite, calcium-alumino silicate, calcium perovskite, and magnesite. Melting reactions demonstrate that from 12 to 16 GPa, stishovite is in reaction with the melt. At 20 and 25 GPa, garnet and stishovite together are produced upon melting of model, carbonated eclogite. At 20 and 25 GPa, calcium perovskite is also the phase that contributes the most toward liquid production. Melt compositions at all pressures are carbonatitic, with roughly 37-40 wt% dissolved CO 2 . From 12 to 16 GPa, the liquids are calciocarbonatites with Ca# molar of ∼69-71; liquid compositions become less calcic with Ca# of ∼52-55 at 20 and 25 GPa. Given these melting phase relations, suitable subduction zone adiabats do not intersect the solidus of model carbonated eclogite at depths investigated in the present study. Hence, on this basis, it is fair to say that carbonated eclogite possibly avoids melting in subduction zone settings, thereby delivering carbonate to at least moderate depths in the Earth. However, owing to local heating events, small-degree melting of carbonated eclogite is not completely precluded, and the liquids liberated from this melting can be viewed as agents of chemical mass transfer in the deep Earth. At present, however, geochemical consequences of subduction-related melting of carbonated eclogite are difficult to evaluate.Citation: Keshav, S., and G. H. Gudfinnsson (2010), Experimentally dictated stability of carbonated oceanic crust to moderately great depths in the Earth: Results from the solidus determination in the system CaO-MgO-Al 2 O 3 -SiO 2 -CO 2 ,

High‐Pbehavior of anorthite composition and some phase relations of the CaO‐Al2O3‐SiO2 system to the lower mantle of the Earth, and their geophysical implications

“…The melt in Figure 1b has a very low fraction, and did not quench to clear glass. The melt in Figure 1e shows extensive dendritic melting texture, and special morphology of the CAS phase, which is much comparable to the observation of Keshav and Gudfinnsson [2010].…”

“…With the experimental investigation in the system CaAl 4 Si 2 O 11 ‐NaAl 3 Si 3 O 11 by Akaogi et al [2010], it is clear now that the CAS phase is actually a complex solid solution. The highest contents of Fe pfu , Mg pfu , Na pfu and K pfu observed in high‐ P experiments are 0.308 [ Grassi and Schmidt , 2011], 0.388 [ Keshav and Gudfinnsson , 2010], 0.524 [ Akaogi et al , 2010] and 0.323 [ Grassi and Schmidt , 2011], respectively. On the other hand, the contents of Ti pfu (up to 0.068), Cr pfu (up to 0.005) and Mn pfu (up to 0.002) are generally negligible [ Hirose and Fei , 2002; Hirose et al , 2004; Grassi and Schmidt , 2011].…”

“…The composition data from the system CaO‐Al 2 O 3 ‐SiO 2 (CAS system; 16 data points) demonstrate the substitution between 3Si and 4Al, and those from the system CaO‐Al 2 O 3 ‐SiO 2 ‐Na 2 O (CAS‐Na system; 7 data points) suggest the substitution of Si + Na→Al + Ca. There were only two composition data in the system CaO‐MgO‐Al 2 O 3 ‐SiO 2 (CMAS system) [ Keshav and Gudfinnsson , 2010], and two composition data in the system CaO‐MgO‐Al 2 O 3 ‐SiO 2 ‐Na 2 O (CMAS‐Na system) [ Litasov and Ohtani , 2010]. Note that the investigated systems in Keshav and Gudfinnsson [2010] and Litasov and Ohtani [2010] actually contained CO 2 which might have some minor effects on the compositions of the CAS phase; but since these four data were the only available data in these systems, we chose to ignore the CO 2 effect.…”

Apatite solubility in carbonatitic liquids and trace element partitioning between apatite and carbonatite at high pressure

“…In the case of melting of carbonated eclogites, very calcic melts (CaO up to 46 wt.%) can be produced (Hammouda, 2003;Keshav and Gudfinnsson, 2010). Other recent studies on carbonated eclogites (Litasov and Ohtani, 2010) and sediments (Thomsen and Schmidt, 2008) showed les calcic melts (up to 37 and 32 wt.%, respectively), which may be related to alkali presence in the melts (up to 7 and 6 wt.%, respectively).…”

Melting of carbonated pelites at 8–13 GPa: generating K-rich carbonatites for mantle metasomatism

“…Although there are a large number of studies on carbonated mafic eclogites (Yaxley and Green 1994;Hammouda 2003;Yaxley and Brey 2004;Dasgupta et al 2004Dasgupta et al , 2005Litasov and Ohtani 2009;Keshav and Gudfinnsson 2010), there are only three studies on carbonated pelites at upper mantle conditions (Domanik and Holloway 1998;Thomsen and Schmidt 2008a, b). As has been pointed out (Dasgupta et al 2005;Thomsen and Schmidt 2008b), the melting systematics of peridotites and carbonated K-poor mafic eclogites at 2-7 GPa is similar to the melting relations in the simple CaCO 3 -MgCO 3 ± FeCO 3 system (Irving and Wyllie 1975;Buob et al 2006).…”