The Earth’s Mantle is More Depleted Than we Thought
Abstract
The isotopic composition of the Earth’s mantle has mostly been inferred from analyses of mid-ocean ridge basalts (MORB). Far fewer isotope analyses of oceanic mantle rocks (i.e. abyssal peridotites) exist, owing to their scarcity and often altered state. Here we present new Hf and Nd isotope analyses from abyssal peridotites from the Gakkel ridge in the North Atlantic and the Indian Ocean. The Hf and Nd isotope ratios of the Indian Ocean peridotites mostly overlap those of MORB [1, 2, 3, 4], whereas both Hf and Nd isotope ratios in the samples from Gakkel ridge extend to much higher values. Coupled depletions in Nd and Hf isotope ratios in the Gakkel ridge samples range to ɛHf and ɛNd values of 60.4 and 20.5, respectively, and hence considerably extend the MORB and ocean island basalt (OIB) Hf-Nd isotope array to more depleted values. In addition to such highly depleted Hf and Nd values, some samples from the Gakkel ridge range to extreme Hf isotope values up to ɛHf of 104, but lack the corresponding depletion in Nd isotopes (ɛNd of about 8), similar to what is observed in Hawaiian peridotite xenoliths from Salt Lake Crater, Oahu [4, 5]. Notably, the Hf isotope ratios in the Gakkel ridge peridotites correlate well with the extent of depletion recorded by their major and trace element (e.g. Al2O3 and Yb content, spinel Cr#) and Os isotope compositions [6]. Hence the Hf and Os isotope compositions of these samples preserve a record of ancient mantle depletion, whereas the Nd isotope signatures generally do not. Most, but not all, Nd isotope ratios are at least partially reset by melt-rock interaction during the latest melting event at the ridge and only few Nd isotope ratios in abyssal peridotites actually preserve ancient mantle depletion, similar to the Os [6] and Hf isotope ratios. The oceanic mantle consequently ranges to much more depleted Nd-Hf-(Os) isotope signatures and is isotopically much more variable than indicated by the basalts (MORB) or by Nd isotope analyses of abyssal peridotites to date [1, 2, 3]. The comparatively much larger variability in Hf than in Nd isotope composition observed in the peridotites may explain the greater scatter in the Hf-Nd isotope systematics in MORB relative to OIB [8, 9, 10], but also suggests that estimates of the isotope composition of the depleted mantle derived from MORB may greatly underestimate the actual extent of depletion of the Earth’s mantle. [1] Snow et al., 1994, Nature 371, 57-60; [2] Salters & Dick, 2002. Nature 418, 68-72; [3] Cipriani et al., 2004. Geology 32, 657-660; [4] Salters, V. J. M. & Zindler, A., 1995. EPSL 129, 13-30; [5] Bizimis et al., 2007. EPSL 257, 259-273; [6] Liu et al., 2008. Nature 452, 311-316; [8] Patchett, 1983, Lithos 16, 47-51; [9] Salters, 1996, EPSL 141, 109-123; [10] Chauvel & Blichert-Toft, 2001, EPSL 190, 137-151.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V24A..03S
- Keywords:
-
- 1025 GEOCHEMISTRY / Composition of the mantle;
- 1030 GEOCHEMISTRY / Geochemical cycles;
- 1032 GEOCHEMISTRY / Mid-oceanic ridge processes;
- 1038 GEOCHEMISTRY / Mantle processes