Continuous Gradations among Primary Carbonatitic, Kimberlitic, Melilititic, Basaltic, Picritic, and Komatiitic Melts in Equilibrium with Garnet Lherzolite at 3–8 GPa

Abstract

Multianvil melting experiments in the system CaO-MgO-Al 2 O 3-SiO 2-CO 2 (CMAS-CO 2) at 3-8 GPa, 1340-1800 C, involving the garnet lherzolite phase assemblage in equilibrium with CO 2-bearing melts, yield continuous gradations in melt composition between carbonatite, kimberlite, melilitite, komatiite, picrite, and basalt melts. The phase relations encompass a divariant surface in P-T space. Comparison of the carbonatitic melts produced at the low-temperature side of this surface with naturally occurring carbonatites indicates that natural magnesiocarbonatites could be generated over a wide range of pressures >2Á5 GPa. Melts analogous to kimberlites form at higher temperatures along the divariant surface, which suggests that kimberlite genesis requires more elevated geotherms. However, the amount of water found in some kimberlites has the potential to lower temperatures for the generation of kimberlitic melts by up to 150 C, provided no hydrous phases are present. Compositions resembling group IB and IA kimberlites are produced at pressures around 5-6 GPa and 10 GPa, respectively, whereas the compositions of some other kimberlites suggest generation at higher pressures still. At pressures <4 GPa, an elevated geotherm produces melilitite-like melt in the CMAS-CO 2 system rather than kimberlite. Even when a relatively CO 2-rich mantle composition containing 0Á15 wt % CO 2 is assumed, kimberlites and melilitites are produced by <1% melting and carbonatites are generated by even smaller degrees of melting of <0Á5%.

Experimental and Theoretical Investigations on Structural and Vibrational Properties of Melilite-Type Sr2ZnGe2O7 at High Pressure and Delineation of a High-Pressure Monoclinic Phase

“…The same coordination for T 1 and T 2 cations favors the intermixing of the cations of these two sites and hence differences in local coordination around these sites have been observed in natural melilite-type and related structures. 7,10 The local distortion in the tetrahedral net and the A-site coordination plays a crucial role in governing the topology of the tetrahedral units and final symmetry of the structure of such complex compounds. 5,9 Furthermore, local distortions in the cation polyhedra resulting from slightly different compositions are strongly reflected on materials properties; in particular, on the optical properties.…”

Carbonate metasomatism and CO2 lithosphere–asthenosphere degassing beneath the Western Mediterranean: An integrated model arising from petrological and geophysical data

“…Red triangles indicate recent and active volcanoes. Dalton and Presnall (1998), and Gudfinnsson and Presnall (2005), and peridotite -CO 2 (2.5 wt %) from . Dry peridotite solidus in the CMAS system is from .…”

Upper Cretaceous to Pleistocene melilitic volcanic rocks of the Bohemian Massif: petrology and mineral chemistry

“…Similarly Brey (1978) and Keshav & Gudfinnsson (2004) concluded that melilitites and nephelinites are partial melts of carbonated lherzolites at 3 GPa (or higher). In the experiments of Gudfinnsson & Presnall (2005), the melilitite melts resembling natural me- -data for melilite olivine nephelinite. Gudfinnsson & Presnall (2005) our analytical data plot mostly to or close to a field for melilites near the 3 GPa isobar, yet some are located outside the field at considerably lower pressures.…”

“…In the experiments of Gudfinnsson & Presnall (2005), the melilitite melts resembling natural me- -data for melilite olivine nephelinite. Gudfinnsson & Presnall (2005) our analytical data plot mostly to or close to a field for melilites near the 3 GPa isobar, yet some are located outside the field at considerably lower pressures. Falloon & Green (1990) estimated the formation of the olivine melilitite magma in equilibrium with garnet-phlogopite lherzolite within the dolomite stability field as 1020 °C at pressures > 2.5 GPa.…”