A micro-analytical approach to partition coefficients in plagioclase and clinopyroxene of basaltic sills in Serra Geral Formation, Paraná Basin, Brazil

Uma abordagem microanalítica dos coeficientes de partição em plagioclásio e clinopiroxênio de soleiras basálticas da Formação Serra Geral, Bacia do Paraná, Brasil

Abstract:

In the present study, detailed micro-analytical data (major and trace elements) in minerals from sills of Serra Geral Formation represents the first attempt to evaluate the partition coefficient of Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Cs, Ba, REE, Hf, Ta, Pb, Th and U among coexisting plagioclase, augite and pigeonite from 12 tholeiitic basaltic sills along the eastern border of the Paraná Basin, Brazil. These 12 samples come from Rio Grande do Sul, Paraná, São Paulo and Goiás states. Plagioclase/melt partition coefficients K_D were determined in core, intermediate and rim zones using LA-ICP-MS and the results were compared with the variations of the elements Ca, Al, Na and Si in those areas. Partition coefficients of Sr and Eu decrease with the increase of Ca concentration in plagioclase. A reduction in temperature and consequently a rise in Na and Si levels increase the partition coefficients of Sr and Eu at the rim of mineral The K_D of Ni and V in clinopyroxene has positive correlations with Mg²⁺, Ca²⁺ and Al³⁺ due to higher concentrations of those elements at the core and lower concentration towards the rim and negative correlations of K_D of Sc, Cr, Co, Pb and Lu, due to preferential substitutions by Fe²⁺ in sixfold coordination. The use of LA-ICP-MS and EPMA made possible the chemical quantification of major and trace elements and the determination of partition coefficients in coexisting plagioclase and clinopyroxene in basaltic melts.

Keywords:
Partition coefficient; plagioclase; clinopyroxene; Serra Geral Formation; LA-ICP-MS