Importance of the size and distribution of chemical heterogeneities in the mantle source to the variations of isotope ratios and trace element abundances in mid-ocean ridge basalts

Abstract

The size and distribution of chemical heterogeneities in the mantle source affect compositions of basalts erupted on the surface. To understand the effect of size and distribution of heterogeneous mantle source on the variations of isotope ratios and incompatible trace element abundances in mid-ocean ridge basalts (MORB), we develop a time-dependent two-porosity ridge model that features disequilibrium melting and channelized melt migration in a truncated triangular melting region. We run numerical simulations for a two-component mantle that consists of the depleted mantle and enriched blobs spanning a range of enriched compositions and sizes. The volume fraction of enriched mantle varies under 10% of the mantle source and the size of the melt production area that contributes to the pooled melt is also treated as a variable. Results show that higher abundance of incompatible trace element in the enriched mantle, larger size of enriched blobs, and narrower melt pooling area produce larger variations of isotope ratios in the pooled melt. We develop an empirical scaling equation that quantifies the fluctuation of isotope ratios as a function of elemental abundance, size and average spacing of enriched heterogeneities in the source, and the width of melt pooling area. The two-standard deviation of 143Nd/144Nd, 176Hf/177Hf, 87Sr/86Sr, and 206Pb/204Pb in global MORB not affected by hotspots can be produced by melting a two-component mantle with 5.2–8.2% enriched heterogeneities of 5.1–6.7 km mean size and 7.0(±1.6), 5.3(±1.2), 2.0(±0.5), and 9.8(±2.2) times higher abundances of Nd, Hf, Sr, and Pb than the depleted mantle, respectively. Incomplete mixing of melts at the base of the lithosphere is likely important for producing the variation of incompatible trace elements in MORB. The melt erupted at the ridge axis may represent an ensemble of the melt pooled from the center of the melt pooling area and melts pooled from local areas away from the ridge axis. The present study underscores the importance of the size and distribution of mantle heterogeneities to the interpretation of isotope ratios and trace element abundances in basalts.