Carbonate melts in the hydrous upper mantle

Weidendorfer, Daniel; Manning, Craig E. und Schmidt, Max W. (2020): Carbonate melts in the hydrous upper mantle. In: Contributions To Mineralogy and Petrology, Bd. 175, Nr. 8, 72

Abstract

Carbonatite compositions resulting from melting of magnesian calcite + olivine + clinopyroxene were experimentally determined in the system CaO-MgO-SiO2-CO2-H2O as a function of temperature and bulk H2O contents at 1.0 and 1.5 GPa. The melting reaction and melt compositions were found to be highly sensitive to H-loss or -gain during experiments. We hence designed a new hydrogen-trap technique, which provided sufficient control to obtain consistent results. The nominally dry solidus temperatures at 1.0 and 1.5 GPa are 1225-1250 degrees C and 1275-1300 degrees C, respectively. At 1.0 GPa, the solidus temperature decreases with H2O increasing to 3.5 wt% (1025-1050 degrees C), then remains approximately constant at higher H2O concentrations. Our nominally dry solidus temperatures are up to 140 degrees C higher than in previous studies that did not take measures to limit hydrogen infiltration and hence suffered from H2O formation in the capsule. The near-solidus anhydrous melts have 7-8 wt% SiO(2)and molar Ca/(Ca + Mg) of 0.78-0.82 (X-Ca). Melting temperatures decrease by as much as 200 degrees C with increasing X-H2O in the coexisting COH-fluid. Concomitantly, near-solidus melt compositions change with increasing bulk H2O from siliceous Ca-rich carbonate melts to Mg-rich silico-carbonatites with up to 27.8 wt% SiO2 and 0.55 X-Ca. The continuous compositional array of Ca-Mg-Si carbonatites demonstrates the efficient suppression of liquid immiscibility in the alkali-free system. Diopside crystallization was found to be sensitive to temperature and bulk water contents, limiting metasomatic transformation of carbonated upper mantle to wehrlite at 1.0-1.5 GPa to < 1175 degrees C and < 7 wt% bulk H2O.

• Dokumententyp: Zeitschriftenartikel
• Fakultät: Geowissenschaften > Department für Geo- und Umweltwissenschaften
• Themengebiete: 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften, Geologie
• ISSN: 0010-7999
• Sprache: Englisch