Water content in primary magma compositions and mantle source of chromitites in Mesoarchean Nuggihalli greenstone belt (India)

Abstract

Chromitites, found in Archean greenstone belts as the oldest ultramafic rocks, offer critical insights into the early Earth's evolution. Water plays a fundamental role in aggregation and mineralization of chromite, yet the water content of parental magma of chromitite remains unclear. This study present petrologic and elemental data on chromitites from the 3.3-3.1 Ga Nuggihalli greenstone belt, Western Dharwar craton (India), and address origin of chromitites in Archean greenstone belts particularly water contents in primary magmas. Major and trace element data of chromites and clinopyroxenes from chromitites coupled with thermobarometry, and hygrometry calculation, as well as numerical modeling reveal that the chromites are high-Cr chromites (Cr# 68.1-78.6) with low TiO2 (0.19%-0.29%), Al2O3 (9.10%-13.13%), and Ga (4.24-10.37 ppm), similar to the parental melts of high-Cr podiform chromites and boninites in modern ophiolites. The clinopyroxenes are augites with high Mg# (93-95) which corresponds to equilibrated melts with the Nuggihalli chromitites having high Mg# values (79-86), resembling ancient komatiitic or picritic magmas (75-90), crystallized in a primary magma environment. Thermobarometry and hygrometry calculation suggest that the clinopyroxenes could form at temperatures of 1194-1221℃ and pressures of 0.7-5.8 kbar with high water contents (~2.4-2.6 wt.%), exceeding water contents in mid-ocean basalts (MORB) and oceanic island basalts (OIB) source. Numerical modeling of rare earth elements indicates that moderate degrees of partial melting (1%~20%) of hydrated spinel lherzolite mantle (water content: ~478-5408 ppm). The primary magmas of the 3.3-3.1 Ga Nuggihalli chromitites are characterized by enrichment of water contents (~2.4-2.6 wt.%) and high field strength elements (HFSEs, e.g., Nb and Hf), demonstrating high comparability with boninites and arc magmas. Furthermore, the rock associations including ultramafic rocks with chromitites, high-Mg basalts and sheets of gabbro-anorthosites in the Nuggihalli greenstone belt may represent the relic of oceanic lithosphere. This declares that the 3.3-3.1 Ga Nuggihalli chromitites could have formed at a forearc basin on a subduction system.