Exhumed crust and mantle
Highly serpentinized peridotites derived from the upper mantle beneath a non – volcanic continental margin were sampled from the southern Iberia Abyssal Plain (Ocean Drilling Program Leg 173), and the primary mantle minerals were analysed for major and trace elements to petrologically characterize the upper mantle. The Cr-number (= Cr/(Cr + Al) atomic ratio) of the peridotite spinels varies widely from 0.1 to 0.6. The Na20 content of clinopyroxene is rather constant, 0.5-0.8 wt %. The chondritenormalized rare earth element (REE) patterns have light REE (LREE) depleted convex – upward shapes. The LREE/HREE (heavy REE) ratio in clinopyroxene is constant irrespective of the degree of melt extraction of the sample as measured by the Cr-number of spinel. The trend of the peridotites’ mineral chemistry is different from both the simple melt extraction and the general mantle metasomatic trends. The geochemical character of the Iberia Abyssal Plain peridotite is intermediate between those of abyssal peridotites and peridotite xenoliths from continental regions. These geochemical features, especially for the trace elements in clinopyroxene, are rather similar to those in peridotite xenoliths from arcs. This chemical trend is probably the result of ‘open – system melting’, which involves melting simultaneously with enrichment of trace elements by the influx agent.