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La, Ce, Nd, and Sr behavior in minette magmas during crystallization of apatite-clinopyroxene-mica paragenesis at upper-mantle conditions

¹ Universidade Federal da Bahia. Universidade Federal da Bahia (UFBA) and Departamento Nacional de ProduçãoMineral (DNPM). Rua Frei Caneca, 210/207, Edf. San Martin, Agronômica, 88025-000, Florianópolis – SC, Brazil
² Universidade Federal do Rio Grande do Sul. Av. Bento Gonçalves, 9500, Instituto de Geociências, 91501-970, C.P. – 15011, Brazil
³ Universidad de Barcelona, Facultad de Geología, C. Marti i Franquès, s/n, 08028, Barcelona, Spain
⁴ Université de Montpellier II, ISTEEM, CC 049, CNRS, pl. E. Bataillon, 34095 Montpellier Cedex 5, France

^* Corresponding author, e-mail: jorge.cid{at}bol.com.br

Mafic microgranular enclaves produced by co-mingling between minette and syenite magmas are found in the Piquiri Syenite, southern Brazil. These enclaves were crystallized under upper mantle conditions. K-clinopyroxene and pyrope are among the early-crystallized phases, suggesting an initial pressure around 5GPa. The mineralogy of minettes comprises phenocrysts of magnesium biotite and diopside, occasionally apatite, in a groundmass formed by these minerals plus alkali feldspar. Two types of apatites are observed: (i) euhedral grains, as inclusions or groundmass constituents, and (ii) aciculate crystals formed during undercooling of mafic magma. Substitutional schemes suggested for these apatites are Si P in the tetrahedral site, and Sr and LREE replacing Ca. La, Ce, Nd, and Sr concentrations reveal that these elements are extremely enriched in apatite. Partition coefficients between early-crystallized apatites and estimated melt compositions are D_La (11.8–30.6), D_Ce (16.1–33), D_Nd (12–31) and D_Sr (5.1–8.2). Apatite concentrates the LREE up to 200 times compared to clinopyroxene, even higher values were determined for ap/mica partitioning. The ap/cpx and ap/mica partition coefficients for undercooled minerals are close to the values determined for the near-liquidus phases, which indicates constant behavior of LREE and Sr between apatite-clinopyroxene-mica parageneses and minette liquids. Furthermore, these partitioning data preclude any chemical diffusion of these elements between minette and syenite magmas during co-mingling.

Key-words: apatite, clinopyroxene, SIMS, EPMA, partition coefficients.