Heterogeneity of the Caribbean plateau mantle source: Sr, O and He isotopic compositions of olivine and clinopyroxene from Gorgona Island

Revillon, S., Chauvel, C., Arndt, N.T., Pik, R., Martineau, F., Fourcade, S. and Marty, B. (2002) Heterogeneity of the Caribbean plateau mantle source: Sr, O and He isotopic compositions of olivine and clinopyroxene from Gorgona Island. Earth and Planetary Science Letters, 205 (1-2), 91-106. (doi:10.1016/S0012-821X(02)01003-8).

Abstract

The composition of the mantle plumes that created large oceanic plateaus such as Ontong Java or the Caribbean is still poorly known. Geochemical and isotopic studies on accreted portions of the Caribbean plateau have shown that the plume source was heterogeneous and contained isotopically depleted and relatively enriched portions. A distinctive feature of samples from the Caribbean plateau is their unusual Sr isotopic compositions, which, at a given Nd isotopic ratio, are far higher than in samples from other oceanic plateaus. Sr, O and He isotopic compositions of whole rocks and magmatic minerals (clinopyroxene or olivine) separated from komatiites, gabbros and peridotites from Gorgona Island in Colombia were determined to investigate the origin of these anomalously radiogenic compositions. Sequentially leached clinopyroxenes have Sr isotopic compositions in the range ⁸⁷Sr/⁸⁶Sr = 0.70271–0.70352, systematically lower than those of leached and unleached whole rocks. Oxygen isotopic ratios of clinopyroxene vary within the range δ¹⁸O = 5.18–5.35 ‰, similar to that recorded in oceanic island basalts. He isotopic ratios are high (R/R_a = 8–19). The lower ⁸⁷Sr/⁸⁶Sr ratios of most of the clinopyroxenes shift the field of the Caribbean plateau in Nd–Sr isotope diagrams toward more 'normal' values, i.e. a position closer to the field defined by mid-ocean ridge basalts and oceanic-island basalts. Three clinopyroxenes have slightly higher ⁸⁷Sr/⁸⁶Sr ratios that cannot be explained by an assimilation model. The high ⁸⁷Sr/⁸⁶Sr and variations of ¹⁴³Nd/¹⁴⁴Nd are interpreted as a source characteristic. Trace-element ratios, however, are controlled mainly by fractionation during partial melting. We combine these isotopic data in a heterogeneous plume source model that accounts for the diversity of isotopic signatures recorded on Gorgona Island and throughout the Caribbean plateau. The heterogeneities are related to old recycled oceanic lithosphere in the plume source; the high ³He/⁴He ratios may indicate that the source material once resided in the lower mantle.

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