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. (C) 2002 Elsevier Science B.V. All rights
reserved.