Guano-derived rare earth-rich phosphatic amygdales in gabbroic inclusions from Ascension Island

Abstract

Four analysed gabbros from a suite of augite-plagioclase (An 60–70)-olivine cumulates found as fragments in redistributed pyroclastic sediment on Middleton's Ridge, Ascension Island, have large negative Ce anomalies (Ce */Ce up to 5). Along grain boundaries and in the interstices between the cumulus minerals, quenched intercumulus liquid is preserved. Contraction vesicles are filled with, and in places the quenched liquid is replaced by, a REE-rich phosphate mineral ((Ca, Sr, Ba, REE, Y) Al 3H[OH 6(PO 4) 2]) of composition intermediate between 4 end members: crandallite (Ca), goyazite (Sr), gorciexite (Ba) and florencite (REE + Y). Comparison of REE and Y concentrations with “immobile” elements (Nb, Zr) show that the REE and Y are enriched by a factor > 10 in the whole rocks. The REE-rich phosphate has ΣREE up to 13–14 wt.% and it too has a negative Ce anomaly. The phosphate appears to have been derived from guano, the only feasible source of phosphorus on Ascension. The REE content of guano is very small and similarity between the REE profiles of the gabbros and unaltered Ascension lavas suggest that the latter are the source of the REE. The most probable mode of emplacement of the amygdales is via a low-temperature aqueous fluid.