Early mixing and mingling in the evolution of basaltic magmas: evidence from phenocryst assemblages, Slamet Volcano, Java, Indonesia

Slamet Volcano, in Central Java, Indonesia, is an active calc-alkaline stratovolcano composed largely of basalts and basaltic andesites. The phenocryst mineralogy of the most magnesian basalts (MgO > 7 wt%) has been studied in detail to investigate the nature of early magmatic processes in a large arc volcano. On the basis of stratigraphy, mineralogy, petrography and geochemistry, the studied basalts are subdivided in two groups; Old Slamet (OS) and New Slamet (NS). Olivine in the OS basalts is within the range Fo(92-65) and shows a homogenous composition distribution, with a significant proportion of crystal cores near equilibrium with whole-rock compositions. However, distinct high- and low-Ni sub-populations may be distinguished at any given Fo content (e.g. 0.25-0.10% and 0.32-0.27 wt% NiO for the low- and high-NiO sub-populations respectively at Fo(85)). Chromian spinel inclusions within the high-NiO olivines have higher Cr# (75-80) and Fe2+/Fe3+ ratio (2.6-3.2) and lower TiO2 (0.23-0.44 wt%) contents than those within low-NiO olivines (Cr# 58-77, Fe2+/Fe3+ 1.3-2.5 and TiO2 0.73-0.91 wt%). Plagioclase and pyroxene phenocrysts display dominantly oscillatory zoning, with cores close to equilibrium with whole-rock compositions. Olivine in the NS basalts is within the range Fo(90-61) and shows a discontinuous distribution of composition including two principal peaks: the first is similar toFo(78-90), close to expected equilibrium compositions; the second is similar toFo(62-70), clearly too Fe-rich to be in equilibrium with whole-rock compositions and composed mainly of reversely zoned crystals. In terms of NiO content, a single (low-Ni) sub-population is observed. Chromian spinel inclusions with high Cr# and Fe2+/Fe3+ ratio and low TiO2 are not observed within the olivines of these basalts. Plagioclase and pyroxene phenocrysts are typically reversely zoned and display ubiquitous disequilibrium textures. The cores of these crystals are not in equilibrium with host basalt compositions. The mineralogy of these basalts indicates that the OS basalts resulted from mixing between two parental magmas produced from contrasted sources, probably a relatively depleted and H2O-poor harzburgite and a less depleted and more hydrated harzburgite to lherzolite. Subsequent evolution occurred in a magmatic system in which variations in volatile contents and/or temperature played a more important role than magma mixing. The NS basalts were produced from the less depleted source only, but magmas interacted extensively with remnants of earlier crystal mush/magma batches. Open-system processes operated early in the life of all these magmas, and influenced their geochemistry. Magma mixing was ubiquitous process and together with fractional crystallisation controlled the evolution of the basaltic magmas of Slamet Volcano.