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Trace-element partitioning between synthetic potassic-richterites and silicate melts, and contrasts with the partitioning behaviour of pargasites and kaersutites

¹ CNR-Istituto di Geoscienze e Georisorse (CNR-IGG), sezione di Pavia, via Ferrata 1,I-27100 Pavia, Italy
² Mineralogisch-Petrologisches Institut, Universität Göttingen, Goldschmidtstr. 1, D-37077 Göttingen, Germany.
³ Institut für Geologische Wissenschaften, Universität Greifswald, Jahnstraße 17a, D-17487 Greifswald, Germany
⁴ Dipartimento di Scienze della Terra, Università degli Studi di Pavia, via Ferrata 1, I-27100 Pavia, Italy

^* Corresponding author, e-mail: tiepolo{at}crystal.unipv.it

Present address: ICR-Jena, Moritz-von-Rohr Str. la, D-07745 Jena, Germany

Solid/liquid partition coefficients for large ion lithophile elements (Ba, Rb, Sr), high field strength elements (Zr, Hf, Nb, Ta, Ti), rare earth elements (La-Yb), Pb, Th, U and selected transition elements (Sc, V) were determined by means of Secondary Ion Mass Spectrometry on potassic-richterites synthesised at upper mantle conditions (P = 1.4 GPa and T = 850-1020°C) from silica-rich lamproites. Most trace elements display an incompatible behaviour in potassic-richterites; only Sr, Ti, Sc and V show strong positive anomalies in the partitioning pattern. When ^S/LD for potassic-richterites are compared with those for calcic amphiboles (pargasites and kaersutites) several differences become evident. In general, ^S/LD are lower in potassic-richterites; also, different partitioning patterns are apparent for RE and LIL elements. These differences are discussed in terms of the distinct crystal-chemical behaviour of the involved amphibole end-members, with particular emphasis to the available charge-balance mechanisms and to the site dimensional constraints ruling incorporation of trace elements in the various sites.

The distinct partitioning behaviours of trace elements in potassic-richterites and pargasites and kaersutites imply that melts produced from amphibole-bearing sources may differ markedly depending on the type of amphibole crystallised. Therefore, the new partitioning data are used to discuss the role of potassic-richterite in its principal modes of occurrence, namely in lamproites, in peralkaline ultramafic veins in the lithospheric mantle, and in the deeper parts of subduction zones.

Key-words: amphibole, trace elements, partition coefficients, potassic-richterite, experimental petrology, alkaline rocks..

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