GEMOC ARC National Key CentreNatsue Abe
GEMOC Macquarie
Highly serpentinized peridotites derived from the upper mantle beneath a non-volcanic continental margin were sampled from the southern Iberia Abyssal Plain, during ODP Leg 173 (Return to Iberia). To characterize the upper mantle petrologically, the primary mantle minerals were analyzed by electron microprobe for major and by secondary ion microprobe for trace elements.
The Cr#(=Cr/(Cr+Al) atomic ratio) of chromian spinels varies widely from 0.1 to 0.6. The Na2O content in clinopyroxene is rather constant, 0.5 to 0.8 wt%. The chondrite-normalized rare-earth element (REE) patterns have LREE (light REE) depleted convex-upward shapes. The LREE/HREE (heavy REE) ratio in clinopyroxene is constant irrespective of the degree of melt extraction of the sample measured by Cr# 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 peridotite 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 made by "open-system melting", which involves melting simultaneously combined with enrichment of trace elements by the influx agent.
The possible origins of the Iberia abyssal peridotite are as follows,
(1) That it was made by open system melting with flux from the plume causing
the rifting, (2) That the mantle materials experienced the processes of
a
supra-subduction zone before rifting.