Earth's heterogeneous mantle: A product of convection-driven interaction between crust and mantle

Abstract

Ubiquitous heterogeneity in the Earth's mantle has been documented by numerous chemical and isotopic analyses of oceanic basalts. Despite the ever-increasing amount of data, the way in which compositional heterogeneity is manifest in the Earth's mantle, as well as the processes leading to mantle heterogeneity remain fundamental questions. The large amount of available isotope data in oceanic basalts shows that, statistically, only two principal compositional vectors capture the essential features of the data. Care must be taken, however, when estimating the isotopic composition of mantle from basalt samples. This is because partial melting, and melt mixing during melt extraction leads to a biased representation and subdued compositional variability in the basalts relative to their mantle sources. In both ridge and ocean island settings, for example, erupted lavas are expected to be isotopically less depleted than the most depleted source components. Abyssal peridotites indeed range to much more depleted isotope compositions than mid ocean ridge basalts (MORB). The extent of heterogeneity of the MORB mantle source, the depleted mantle, therefore depends on the proportion, as well as differences in composition, age, and sampling of its various depleted and enriched source components. While MORB data thus do not reflect the full extent of mantle heterogeneity, the large amount of trace element and isotope data in ocean island basalts (OIB) suggests that enriched isotope signatures in OIB closely correspond to those of their average enriched mantle components. OIB can therefore be used to trace the geologic reservoirs that exchange mass with the mantle and to identify the geological processes that introduce enriched material into the Earth's mantle. The generation and subduction of oceanic plates into the deeper mantle, together with small amounts of lower and upper continental crust, appears to be the main process for mantle enrichment. Thereby, erosion and subduction of the lower continental crust accounts for a large part of the enriched isotope signatures in oceanic basalts. Recycling of the upper continental crust, on the other hand, is inferred to be only a minor process, but required to explain the entire spectrum of enriched OIB signatures. Hence a first order geologic process – the generation and subduction of oceanic plates – accounts for the first-order heterogeneity of the Earth's mantle. Moreover, one of the main processes for establishing the composition of the continental crust – erosion and recycling of the lower continental crust – is also one of the main processes for the generation of mantle heterogeneity. Overall, large-scale chemical cycling between Earth's two major lithophile element reservoirs, the mantle and the oceanic and continental crust, is responsible for mantle enrichment. Once introduced into the mantle, the heterogeneous materials become stretched, reduced in size and distributed by mantle convection. The isotopic heterogeneity observed in melt inclusions and abyssal peridotites suggests that eventually, the heterogeneity of the mantle sources of oceanic basalts will exist at relatively small scales, certainly on the kilometer scale of the melting region but perhaps even smaller. The way in which mantle heterogeneity is manifest in the source of oceanic basalts is therefore directly related to the fluid dynamics of mantle convection, whereas the timing, nature, and extent of crust–mantle interaction govern the differentiation and compositional evolution of the silicate Earth.