The origin of abyssal peridotites: a reinterpretation of constraints based on primary bulk compositions

Abstract

We calculated primary bulk compositions for a global suite of abyssal peridotites using primary mineral modes and either analyzed or calculated phase compositions. The latter were obtained through correlations between reported mineral compositions and modal olivine contents. Both the modal data and the mineral compositions were averaged by dredge site, drill hole, or fracture zone (FZ) depending on the amount of available data. Our calculated abyssal peridotite compositions yield major-element oxide-MgO trends that are generally in good agreement with those based on compilations of ultramafic nodules and peridotite massifs. In particular, we find no statistically significant correlation between FeO* (total Fe as FeO) and MgO and, therefore, no evidence for significant olivine accumulation. Previous reports of a positive correlation reflect an artifact of the regressions used to calculate missing phase compositions and result in a relationship between the Mg# of olivine and modal olivine abundance that is inconsistent with observed variations in abyssal peridotites. There is a slight positive correlation between bulk FeO* and MgO if individual thin sections are used to derive the mineral composition versus modal olivine regressions, but the large grain sizes and heterogeneous distributions of phases within abyssal peridotities make it unlikely that individual thin section modes accurately reflect phase proportions in meter-sized dredge-haul samples. The variability of Na and Ti contents in pyroxenes from plagioclase-free abyssal peridotites suggests to us, as it has to other workers, that a majority of these samples interacted to varying degrees with small amounts of melt. On the other hand, lower bounds on Na and Ti contents in the pyroxenes at a given dredge site as a function of modal olivine content are broadly consistent with calculated partial melting residues. Thus, abyssal peridotites may retain information both on the original partial melting process and on concurrent or later interactions with partial melts from other sources.