The geochemistry of potassic lavas from Vulsini,central Italy and implications for mantle enrichment processes beneath the Roman region |
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| Authors: | N W Rogers C J Hawkesworth R J Parker J S Marsh |
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| Institution: | (1) Department of Earth Sciences, Open University, Walton Hall, MK7 6AA Milton Keynes, UK;(2) Department of Geology, University of Auckland, Auckland, New Zealand;(3) Department of Geology, Rhodes University, P.O. Box 94, 6140 Grahamstown, South Africa |
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| Abstract: | Major and trace element and 143Nd/144Nd (0.51209–0.51216) and 87Sr/86Sr (0.70879–0.71105) isotope analyses are presented on a representative group of lavas from the Vulsini district of the Roman
magmatic province. Three distinct series are identified; the high-K and low-K series are similar to those described from other
Italian volcanoes, while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought
to be near-primary mantle melts. Major and trace element variations within the high-K series are consistent with fractional
crystallisation from a parental magma similar to the most magnesian leucitites. Crustal contamination resulted in an increase
in 87Sr/86Sr with increasing fractionation, but it was superimposed on magmas which had already inherited a range of incompatible element
and isotope ratios from enrichment processes in the sub-continental mantle. These are reviewed using the available results
from Vulsini, Roccamonfina and Ernici. Transition element abundances and Ta/Yb ratios indicate that the pre-enrichment mantle
was similar to that of E-type MORB, and that these elements were not mobilised by the enrichment process. Mixing calculations
suggest that three components were involved in the enrichment process; mantle comparable with the source of MORB, and two
other components rich in trace elements. One, the low-K component, had high Sr/Nd, Th/Ta and Ba/Nb and no europium anomaly
while the second had lower Sr/Nd, a negative europium anomaly and very high Th/Ta. It was also characterised by low Nb/Ba
and high Rb/Ba ratios, similar to those reported from phlogopite-rich peridotite xenoliths. The trace element enrichment processes
are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics
of the high-K end-member reflecting the subduction of sediments and the stabilisation of mantle phlogopite. |
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