The Partitioning of U, Th and Pb between Amphibole and Melt: Crystal-chemical Constraints and Implications

Massimo Tiepolo (tiepolo@crystal.unipv.it)¹, Pierantonio Bottazzi (bottazzi@crystal.unipv.it)², Riccardo Vannucci (vannucci@crystal.unipv.it)¹, Roberta Oberti (oberti@crystal.unipv.it)², Alberto Zanetti (zanetti@crystal.unipv.it)² & Stephen Foley (sfoley@gwdg.de)³

¹ Dipartimento di Scienze della terra, Università di Pavia, via Ferrata 1, 27100 Pavia, Italy

² CNR-CS Cristallochimica e Cristallografia, via Ferrata 1, 27100 Pavia, Italy

³ Mineralogisch-Petrologisches Institut, Universitat Gottingen, Goldschmidtstrasse 1, 37077 Göttingen, Germany

Among trace elements, U, Th and Pb deserve great interest in modern geochemistry both for the interpretation of petrogenetic processes and for isotopic systematics. Most of the available studies were aimed to unravel U, Th and Pb partitioning between the melt and clinopyroxene, and almost nothing is known about their partitioning with amphibole. A series of titanian pargasites and kaersutites in equilibrium with melts ranging in composition from picro basalts to trachyte were crystallised starting from natural and synthetic basanites and alkali basalts with a piston cylinder apparatus. Amphibole/liquid partition coefficients (^Amph/LD) for U, Th and Pb were determined by secondary ion mass spectrometry (SIMS) on amphibole/glass pairs. ^Amph/LD for U and Th are in the range 0.004-0.034 and 0.003-0.033, respectively, and show a strong positive correlation with the SiO₂ content of the melt. ^Amph/LD for Pb are about one order of magnitude higher (0.032-0.173), and show the same correlation with melt composition. ^Amph/LD_U/Th ranges between 0.637 and 1.269. When compared with ^Cpx/LD obtained from the literature and from our experiments, ^Amph/LD for U and Th are within the same order of magnitude, thus confirming that these elements enter analogous sites (M4_Amph, M2_Cpx) according to analogous crystal-chemical mechanisms. Pb is one order of magnitude more compatible in amphibole than in clinopyroxene, and this is in agreement with its incorporation into the A site with a strongly asymmetric coordination. This latter requirement may explain the lower D observed in phlogopite. The major implication for petrogenetic studies is that amphibole has a minor influence on Pb isotopes systematics because of the generally low ^Amph/LD values for U, Th and Pb. However, the higher compatibility for Pb relative to that for U and Th causes an unradiogenic trend in ^208/204Pb, ^207/204Pb and ^206/204Pb.

New Constraints on Rare Earth Elements (REE) and Y Partitioning between Amphibole and Silicic Melts at Upper Mantle Conditions

Massimo Tiepolo (tiepolo@crystal.unipv.it)¹, Riccardo Vannucci (vannucci@crystal.unipv.it)¹, Pierantonio Bottazzi (bottazzi@crystal.unipv.it)², Roberta Oberti (oberti@crystal.unipv.it)², Alberto Zanetti (zanetti@crystal.unipv.it)² & Stephen Foley (sfoley@gwdg.de)³

¹ Dipartimento di Scienze della Terra, Università di Pavia, via Ferrata 1, 27100 Pavia, Italy

² CNR-CS Cristallochimica e Cristallografia, via Ferrata 1, 27100 Pavia, Italy

³ Mineralogisch-Petrologisches Institut, Universität Göttingen, Goldschmidtstrasse 1, 37077 Göttingen, Germany

The Rare Earth Elements (REE) are probably the most important series of trace elements used for the interpretation of petrogenetic processes. Few data are available on REE partitioning between amphibole and melt in the SiO₂ range from 45 to 55 wt%, notwithstanding that amphibole both plays a leading role in the evolution of subduction-related and intra-plate magmatism and is an important and widespread product of mantle metasomatism. A series of partitioning experiments has been carried out with a piston-cylinder apparatus at upper mantle conditions (P: 1.4 GPa; T: 950-1075°C) starting from basanitic and alkali-basaltic compositions. Starting material has been doped with a mixture of trace elements including REE and Y in proportion to minimise statistical errors during SIMS analyses. Run products are amphibole, glass and subordinate Ol and Cpx. Amphiboles are titanian pargasites and kaersutites with significant nearly constant dehydrogenation. Glasses range in composition from picrobasalt to trachyte. The REE pattern of amphibole/liquid partition coefficients (^Amph/LD) is characterised by a nearly flat HREE region (^Amph/LD_Gd/Yb = 1.04-1.84) and a marked depletion in LREE (^Amph/LD_La/Sm = 0.110-0.217). A slight negative Eu anomaly is occasionally observed. ^Amph/LD_REE ranges within about one order of magnitude (^Amph/LD_La = 0.06-0.5; ^Amph/LD_Yb = 0.25-2.01), and multiple regression analysis showed a strong positive correlation with the SiO₂ content of the melt. The crystal structure plays only a minor role because of the nearly constant composition. The major implication for natural systems is that HREE can become compatible in amphibole even in not extremely Si-rich systems, while LREE remain always incompatible. The crystallisation of amphibole may decouple LREE from HREE, thus leading to residual liquids variably LREE enriched and with a nearly flat HREE pattern. The new ^Amph/LD values are applied in modelling the effects produced by amphibole as crystallising phase during fractional crystallisation and mantle metasomatism.