Crustal versus mantle-level aggregation of heterogeneous melts at mid-ocean ridges

Abstract Mid-ocean ridge basalts (MORBs) have long been used to investigate the composition of the upper mantle. The isotopic heterogeneity of MORB correlates inversely with spreading rate, indicating that enhanced magma mixing at magmatically robust fast-spreading ridges mutes the signature of mantle heterogeneity. It has remained unclear, however, whether this mixing occurs during melt extraction from the mantle or in crustal magma reservoirs. To discriminate between mantle aggregation and crustal magma mixing, we measured the Nd isotopic composition of cumulus plagioclase and clinopyroxene cores within lower crustal gabbros from the fast-spreading crustal section exposed at Hess Deep (equatorial Pacific Ocean). Our data reveal that the mantle is heterogeneous at the scale of melt extraction, and the crystal record from the lower crust shows greater 143Nd/144Nd heterogeneity than the overlying MORB. Hence, Pacific MORBs do not reflect the full heterogeneity of their mantle source, and some aggregation of melts occurs within the crust. However, isotopic heterogeneity in the lower crust at Hess Deep is lower than in slower-spreading settings, suggesting that the extent to which melts aggregate in the mantle versus the crust is controlled by spreading rate.