Coupled major and trace elements as indicators of the extent of melting in mid-ocean-ridge peridotites

Abstract

Rocks in the Earth's uppermost sub-oceanic mantle, known as abyssal peridotites, have lost variable but generally large amounts of basaltic melt, which subsequently forms the oceanic crust. This process preferentially removes from the peridotite some major constituents such as aluminium, as well as trace elements that are incompatible in mantle minerals (that is, prefer to enter the basaltic melt), such as the rare-earth elements. A quantitative understanding of this important differentiation process has been hampered by the lack of correlation generally observed between major- and trace-element depletions in such peridotites. Here we show that the heavy rare-earth elements in abyssal clinopyroxenes that are moderately incompatible are highly correlated with the Cr/(Cr + Al) ratios of coexisting spinels. This correlation deteriorates only for the most highly incompatible elements-probably owing to late metasomatic processes. Using electron- and ion-microprobe data from residual abyssal peridotites collected on the central Indian ridge, along with previously published data, we develop a quantitative melting indicator for mantle residues. This procedure should prove useful for relating partial melting in peridotites to geodynamic variables such as spreading rate and mantle temperature.

Multi-stage melt–rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence

“…The lack of a good correlation between spinel and clinopyroxene chemistry is evident in Fig. 9b, which shows the Cr-number in spinel vs. Yb N in clinopyroxene for both spinel and plagioclase peridotites, together with the melting trend defined by abyssal peridotites (Hellebrand et al 2001). Spinel peridotites lie roughly on the melting trend, whereas plagioclase peridotites display higher Cr-number in spinel, because the equilibration with plagioclase modifies the spinel chemistry towards Cr richer and Al poorer compositions.…”

“…5b). If we also consider the spinel chemistry, and use the Hellebrand et al (2001) equation to calculate the degree of melting recorded by spinel peridotites, we obtain fractional melting degrees in the range 7-11%, slightly higher than those inferred from clinopyroxenes. The lack of a good correlation between spinel and clinopyroxene chemistry is evident in Fig.…”

“…b Cr/ (Cr + Al) in spinel vs. Yb N in clinopyroxene for the Mt.Maggiore spinel and plagioclase peridotites. Also reported is the mantle melting trend, as defined by Hellebrand et al (2001) based on spinel and clinopyroxene compositions in abyssal peridotites melt-rock interaction produces harzburgites and dunites with lower bulk Mg-number (at a given olivine modal proportion) than predicted by melting models (Bedini et al 2003). In the MM spinel peridotites, bulk Mg-numbers define a rather narrow range (0.898-0.905) in spite of large modal olivine variation (69-81 wt%), and spinel harzburgite MM2/11 has the highest modal olivine content, thus plotting on the reactive porous flow trends.…”

On the Metamorphic Modification of Cr-Spinel Compositions From the Ultrabasic Rocks of the Pindos Ophiolite Complex (Nw Greece)

“…Their composition has been used as a petrogenetic indicator due to their sensitivity to melt differentiation, crystallisation pressure and the degree of melting of the mantle source region (Irvine 1965a, b, Dick and Bullen 1984, Hellebrand et al 2001. Spinels are highly resistant minerals and tend to preserve their primary magmatic composition.…”

Petrology of Peridotites and Related Gabbroic Rocks Around the Kairei Hydrothermal Field in the Central Indian Ridge

“…14.9). According to these results, the granular peridotite samples collected from the studied area are residue left after moderate to high degrees of partial melting (13-15 % in the majority and up to 17 % based on the method of Hellebrand et al (2001)). It should be realized that this study, coupled with previous results, confirms a wide exposure of relatively depleted peridotites on the ocean floor in the studied region.…”