Incomplete mixing of isotopically variable melts in the lower oceanic crust and its implications on mantle heterogeneity: Evidence from oceanic cumulates from 14°45′N, Mid-Atlantic Ridge

Abstract

Abyssal peridotites and mid-ocean ridge basalts (MORB) offer only a partial glimpse into the mantle's composition and the magmatic process beneath the mid-ocean ridges. Recently, lower crustal cumulates have emerged as potential candidates for capturing a broader range of mantle heterogeneity and depicting a detailed process in MORB generating system. However, it remains uncertain to what extent the lower crustal cumulates can represent mantle source variability. In this study, we present detailed in-situ geochemical and isotopic data, including major and trace element compositions of minerals and in-situ Sr isotopic analysis on plagioclase from lower crustal cumulates collected at 14°45′N on the Mid-Atlantic Ridge (MAR). Trace element compositions of melt in equilibrium with clinopyroxenes from the lower crustal cumulates exhibit strong similarities to nearby MORBs. Exception is that the equilibrium melt of the core of a clinopyroxene oikocryst records a transitional MORB composition, different from the more enriched MORB composition at rims. This phenomenon, together with element changing in minerals from cores to rims, suggests a history of melt-rock interactions, during which more enriched melts transport through minerals that had previously crystallized from relatively more depleted melts. The in-situ 87Sr/86Sr ratios, which vary from 0.70249 to 0.70306, span the range of the global MORB Sr isotope values. Thus, both the trace element compositions and 87Sr/86Sr ratios of cumulate minerals confirm the coexistence of depleted and enriched melts. Our findings furtherly suggest that the melts from heterogenous mantle components underwent incomplete mixing in the lower oceanic crust, even in the crystal mush, thus preserving variable heterogeneities not seen in MORB. Through integrated analyses of geochemistry, isotopic data, and geophysical information from existing literature, we identify the most likely mantle components in the study area as depleted peridotites veined with recycled oceanic crust (pyroxenites).