Fossilized Melts in Mantle Wedge Peridotites

0491911 - GLÚ 2019 RIV GB eng J - Journal Article
Naemura, K. - Hirajima, T. - Svojtka, Martin - Schimizu, I. - Iizuka, T.
Fossilized Melts in Mantle Wedge Peridotites.
Scientific Reports. Roč. 8, July 4 2018 (2018), č. článku 10116. ISSN 2045-2322. E-ISSN 2045-2322
Institutional support: RVO:67985831
Keywords : trace-element * garnet peridotites * carbonatite melts * fluid inclusions * subduction zonen * Ulten zone * ICP-MS * metasomatism * GPA * origin
OECD category: Geology
Impact factor: 4.011, year: 2018

The shallow oxidized asthenosphere may contain a small fraction of potassic silicate melts that are enriched in incompatible trace elements and volatiles. Here, to determine the chemical composition of such melt, we analysed fossilized melt inclusions, preserved as multiphase solid inclusions, from an orogenic garnet peridotite in the Bohemian Massif. Garnet-poor (2 vol.%) peridotite preserves inclusions of carbonated potassic silicate melt within Zn-poor chromite (<400 ppm) in the clinopyroxene-free harzburgite assemblage that equilibrated within the hot mantle wedge (Stage 1, >1180 degrees C at 3 GPa). The carbonated potassic silicate melt, which has a major element oxide chemical composition of K₂O = 5.2 wt.%, CaO = 17 wt.%, MgO = 18 wt.%, CO₂ = 22 wt.%, and SiO₂ = 20 wt.%, contains extremely high concentrations of large ion lithophile elements, similar to kimberlite melts. Peridotites cooled down to. 800 degrees C during Stage 2, resulted in the growth of garnet relatively poor in pyrope content, molar Mg/(Mg + Fe + Ca + Mn), (ca. 67 mol.%). This garnet displays a sinusoidal REE pattern that formed in equilibrium with carbonatitic fluid. Subsequently, subduction of the peridotite resulted in the formation of garnet with a slightly higher pyrope content (70 mol.%) during the Variscan subduction Stage 3 (950 degrees C, 2.9 GPa). These data suggest the following scenario for the generation of melt in the mantle wedge. Primarily, infiltration of sediment-derived potassic carbonatite melt into the deep mantle wedge resulted in the growth of phlogopite and carbonate/diamond. Formation of volatile-bearing minerals lowered the density and strength of the peridotite. Finally, phlogopite-bearing carbonated peridotite rose as diapirs in the mantle wedge to form carbonated potassic silicate melts at the base of the overriding lithosphere.
Permanent Link: http://hdl.handle.net/11104/0285535