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Li, Be, B concentrations and δLi values in plagioclase phenocrysts of dacites from Nea Kameni (Santorini, Greece).

Li, Be, B concentrations and δLi values in plagioclase phenocrysts of dacites from Nea Kameni (Santorini, Greece).

Authors :
Cabato, Joan
Altherr, Rainer
Ludwig, Thomas
Meyer, Hans-Peter
Source :
Contributions to Mineralogy & Petrology; Jun2013, Vol. 165 Issue 6, p1135-1154, 20p, 12 Graphs, 1 Map
Publication Year :
2013

Abstract

Li, Be, B and δLi SIMS analyses of plagioclase phenocrysts from the 1040-1941 Niki dacite lava (Nea Kameni, Santorini, Greece) exhibit varied processes. From their anorthite contents alone, the crystals may be segregated into four main types: type-N shows the normal decline in An during crystallisation (An); type-O has only oscillatory zoning accompanied by resorption surfaces (An); type-C is complex with high-An cores (subtype C1: An, subtype C2: An) and normal rims (An). Type-A plagioclase with high An content (An) is found within mafic enclaves. On the basis of their Li concentrations, type-O crystals may be subdivided into subtype O1 with flat Li concentration profiles and subtype O2 with decreasing Li concentration from core to rim. The concentrations of Be and B of all four types show a negative correlation with anorthite content (An), but Li concentration profiles differ amongst the different plagioclase types. Types N and O1, and the cores of type-C, are equilibrated in Li concentration. Types O2 and A, and the mantles of type-C display an initial enrichment in Li, probably from volatile influx into the melt. Consistent with the propensity towards equilibrium with the melt, these crystals display dramatic rim-ward declines in Li concentration. All analysed plagioclase crystals, except for the xenocrystic type-A, have nearly the same Li, Be and B concentrations at their rims. These coincide with the composition of plagioclase microlites in the groundmass, thereby affording estimates of plagioclase-melt partitioning for the light elements: K = 0.19-0.28, K = 0.24-0.38 and K = 0.007-0.009. δLi profiles in type-O2 and type-A phenocrysts manifest an unmistakable inverse relation to Li concentration, with variations of up to ~39 ‰, revealing preferential kinetic diffusion. This may have been driven by Li loss from the melt, most likely through degassing during decompression, perhaps in the course of magma ascent to subsequent eruption. Considering the rapid diffusion of Li in plagioclase, in situ phenocryst analyses may yield useful information about processes leading up to, or even causing, eruptions. [ABSTRACT FROM AUTHOR]

Details

Language :
English
ISSN :
00107999
Volume :
165
Issue :
6
Database :
Complementary Index
Journal :
Contributions to Mineralogy & Petrology
Publication Type :
Academic Journal
Accession number :
87583731
Full Text :
https://doi.org/10.1007/s00410-013-0851-z