The petrogenesis of ultramafic rocks in the > 3.7 Ga Isua supracrustal belt, southern West Greenland: Geochemical evidence for two distinct magmatic cumulate trends

Abstract

Ultramafic rocks with MgO >25wt.% are found as large slivers within the Eoarchaean Isua supracrustal belt. Some of these ultramafic bodies have previously been proposed to represent relicts of residual mantle. In this study we present new bulk-rock major, trace and platinum-group element data, as well as mineral chemistry for these rocks in order to constrain their origin.The ultramafic rocks form two distinct geochemical trends, which project away from local tholeiitic basalts and boninite-like volcanic rocks, respectively. The ultramafic rocks have FeOt contents of up to 16wt.% and Al2O3 up to 11wt.%, with abundant normative orthopyroxene. Their trace element patterns are broadly parallel with the two types of volcanic rocks with which they are associated. All analysed rocks have fractionated chondrite-normalised platinum-group element patterns with relatively low Os and Ir abundances. The few spinel grains that were found to be potentially primary (Fe3+# <10) have Cr# of around 73 and most have Mg# of about 23.None of the above geochemical features are compatible with a residual mantle origin of the ultramafic rocks in the Isua supracrustal belt. Instead, these data suggest an origin of the ultramafic rocks by accumulation of mainly olivine+spinel during fractional crystallisation of the tholeiitic basalts and the boninite-like volcanic series, and possibly continued crystallisation of plagioclase±orthopyroxene to form the more Al-rich cumulates. This interpretation is supported by modelling of the liquid evolution and the corresponding bulk-cumulates, for the two volcanic sequences. We find that the observed and calculated liquid evolutions match reasonably well, and can be explained mainly by olivine and spinel crystallisation. However, depending on the amount of accumulated plagioclase and orthopyroxene, the observed bulk-cumulate requires additional input from the evolving liquids to account for the elevated SiO2, Al2O3, TiO2, normative orthopyroxene and the trace element abundances of the ultramafic rocks.We conclude that igneous crystal fractionation, in combination with cumulate–liquid interaction is capable of explaining all of the geochemical variation observed for these ultramafic rocks, and that there is no evidence for residual mantle rocks in the Isua supracrustal belt.