Abstract
The upper mantle is highly heterogenous and represents a mixture of depleted peridotites and recycled materials. Isotopic studies (Sm—Nd, Lu—Hf and Re—Os) on abyssal peridotites and oceanic basalts have revealed the recycling of ancient depleted peridotites that record melt extraction as old as 2 Ga. However, the nature, proportions and melting dynamics of these ancient domains are poorly constrained. Here, we present integrated petrological and geochemical data, particularly high-precision clinopyroxene Sm—Nd and Lu—Hf isotopes, on a suite of magnesium-rich and ultra-depleted cumulates in the plume-related Pengco ophiolite from central Tibet. Petrological and geochemical compositions of Pengco cumulates indicate they were derived from a water-poor, magnesium-rich, and ultra-depleted primary melt similar to the low-Ti basalts from Manihiki oceanic plateau. Clinopyroxene in Pengco cumulates exhibit modestiy radiogenic Nd isotopes [eNd (t) = +6.0 to +7.0] but highly radiogenic Hf isotopes [eHf (t) = +20.2 to +23.3], which deviate from the mantle array by as much as +11.4 epsilon units. Thus, Pengco cumulates represent the most Hf—Nd isotopic decoupling ever documented for oceanic crustal rocks. Based on geochemical models, we argue that the ultra-depleted melt compositions and extremely decoupled Nd—Hf isotopes of Pengco cumulates can be explained by re-melting of a source mainly composed of ancient depleted mantle that were recycled into the upper mantle. This agrees on the idea that ancient depleted peridotites might represent one of the dominant components in Earth's upper mantle. Melts from an ancient depleted mantle contribute prominently to the oceanic basalt compositions, although they have limited leverage on the Nd—Hf isotopic compositions of the fully aggregated melts due to its low incompatible element abundances. The unique Nd—Hf isotopic compositions preserved in tlie Pengco cumulates demonstrate that the melts preserved in the lowermost oceanic crust exhibit greater isotopic variability than the erupted magmas. Our results are consistent with the idea that full melt aggregation occurred at crustal level. Hf—Nd isotopes of the lowermost oceanic crust thus provide a critical re-evaluation of current models of the heterogeneity and melting dynamics in the upper mantle.
NSTL