Apatite compositions from oceanic cumulates with implications for the evolution of mid-ocean ridge magmatic systems

Apatite distributions and compositions from cumulates from the EPR at Hess Deep, the MAR at the Kane Transform, and the SWIR at CDP Hole 735B were determined to assess the variability at each setting and to evaluate the potential utility of apatite in understanding the evolution of the lower ocean crust. Apatite in cumulates with low P2O5 contents are heterogeneously distributed along linear arrays or in tight clusters suggesting they crystallized from planar or pipe-like channels of evolved liquid. Most of the variation in the apatite composition is in the halogen site. The X-Ap(OH) with EPR X-Ap(OH) = 0.25, MAR X-Ap(OH) = 0.45, and SWIR X-Ap(OH) = 0.65. These trends are defined both by sample averages and by the range of individual analyses in samples with large ranges in their F/Cl ratio. These trends are interpreted to reflect the reequilibration of apatite from variably evolved liquids/fluids that moved through the crystal mush. Comparison of the F-Cl-OH contents of apatite with the F-Cl-OH contents of glasses recovered from the same ridges show approximate correspondence: apatite and glasses from the EPR have the highest halogen contents while apatite and glasses from the MAR and SWIR have more OH. Some of the higher Cl contents in the apatite are interpreted to be produced by degassing of the cumulus pile while others reflect the assimilation of a seawater component. We suggest that systematic analysis of apatite from oceanic cumulates might allow cumulates that crystallized at shallow depths and assimilated seawater-derived components to be distinguished from those that crystallized below the level of seawater interaction. (C) 2001 Elsevier Science B.V. All rights reserved.