Formation process of olivine–clinopyroxene cumulates inferred from Takashima xenoliths, Southwest Japan arc

Abstract

Rocks of the dunite-clinopyroxenite series (dunite, wehrlite, olivine clinopyroxenite, and clinopyroxenite) are common as cumulates formed around Moho in arc-related environments, but their formation processes remain unclear. They are common as xenoliths of Group I from Takashima in the Southwest Japan arc, and a description of their formation process is provided here. The rocks vary from dunite to clinopyroxenite via wehrlite and olivine clinopyroxenite, and all showing mosaic equigranular to weakly porphyroclastic textures. The rocks are completely free from plagioclase, and they contain <3 vol% chromian spinel. Some of them contain up to 3 vol% orthopyroxene; these are approximate mixtures of olivine and clinopyroxene. As the dunites change to clinopyroxenites, the Mg# [= Mg/(Mg + total Fe) atomic ratio] varies from 0.93 to 0.84 in olivine, and from 0.92 to 0.87 in clinopyroxene. The Cr# [= Cr/(Cr + Al) atomic ratio] of the chromian spinel varies from 0.8 to 0.2 with decreases in the Mg# of olivine and clinopyroxene. The Mg-Fe distribution relation between olivine and clinopyroxene suggests their subsolidus equilibration is around 800-900°C. Initial Mg#s expected at a magmatic temperature indicate that their formation proceeded from magma in the order of Mg-rich dunites followed by clinopyroxenites and then less Mg-rich dunite-wehrlite-olivine clinopyroxenite. This suggests a zigzag liquid path, starting from a mantle-derived olivine-oversaturated magma, around the olivine-clinopyroxene cotectic boundary. Continuous crystallization of olivine or clinopyroxene due to supersaturation could have enabled the magma to straddle the cotectic boundary to form alternately clinopyroxene-and olivineoversaturated magmas.

Pb and Hf isotope evidence for mantle enrichment processes and melt interactions in the lower crust and lithospheric mantle in Miocene orogenic volcanic rocks from Monte Arcuentu (Sardinia, Italy)

“…However, Hf may not always behave as a conservative element, particularly where partial melting of subducted sediments is involved (Woodhead et al, 2001;Hanyu et al, 2002;Münker et al, 2004;Tollstrup and Gill, 2005;Tollstrup et al, 2010;Handley et al, 2011). Therefore, where sub-arc mantle enrichment processes are dominated by aqueous fluids, Hf is likely to be conservative, but where silicate melts are involved, particularly of subducted sediments, Hf may behave in a non-conservative fashion.…”

Geochemical variability among stratiform chromitites and ultramafic rocks from Western Makran, South Iran

“…For example, the Mg# of clinopyroxene can be slightly increased due to reequilibration with olivine in harzburgites and probably in olivine websterites (e.g., Belousov et al, 2021). The Mg-Fe +2 distribution between olivine and clinopyroxene is supposed to be variable and dependant on the equilibration temperature (Muroi and Arai, 2014). In the Sorkhband ultramafic complex, the Fe-Mg distribution diverges slightly from unity and thus the Fe# is slightly higher in olivine (olivine average Fe# = 12.7 in harzburgites and 13.3 in olivine websterites and clinopyroxenites) than in coexisting clinopyroxene (clinopyroxene average Fe# = 7.1 in harzburgites and 8.2 in olivine websterites and clinopyroxenites).…”

Petrological Features of the Burlakski and Nizhne-Derbinsk Mafic-Ultramafic Plutons (East Sayan Mountains, Siberia, Russia)

“…Moreover, Cu-Ni-platinum-group elements (PGE), Cr, and Ti ore deposits are often associated with mafic-ultramafic intrusions [8][9][10][11], so understanding how these intrusions form helps guide ore deposit exploration and extraction. Plutonic rocks of the dunite-clinopyroxenite series (dunites, wehrlites, olivine clinopyroxenites, and clinopyroxenites) are especially important as they are the main constituents of the Moho transition zone (MTZ) and their petrogenesis still remains unclear [12]. There have been many studies that have focused on dunites and clinopyroxenite xenoliths [13][14][15][16], but comparatively few studies on dunite-clinopyroxenite series rocks [14].…”