Chemistry Reference
In-Depth Information
iron-rich H chondrites (bronzite), Fo 79-74 iron-poor L chondrites (hyper-
sthene). The olivine compositions of achondrites are much more variable:
Fo 96-66 for the ones with hypersthene, Fo 42-35 for the ones with augite.
Pallasites are lithosiderites (stony-iron) with olivine (Fo 90-88) in an iron-
nickel matrix.
Igneous rocks
It is estimated that the
mantle
olivines have compositions in the range of
Fo 92. Lower forsterite contents suggest partial melting-differentiation
phenomena.
Olivines from peridotites of
ophiolitic complexes
and alpine-type lher-
zolite intrusions have very constant compositions Fo 93-89 (mostly in the
range Fo 92-91). Some rocks, in minor amounts, associated with these peri-
dotites (some cumulative dunites, olivine pyroxenite indicating partial melt-
ing) show compositions ranging from Fo 88 to Fo 78.
Olivines in peridotite
inclusions in basalts and kimberlites
have com-
positions similar to those of alpine peridotites: Fo 90-91. In some cases,
compositions in the range of Fo 88-83 have been found. In some enclaves of
kimberlites compositions in the range of Fo 93 or more have been found.
Cortlandites
are amphibole-olivine rocks, that are generally associated
with mafic rocks (gabbros, diorites) in complex granitic batholiths.
Komatiites
are ultramafic volcanic rocks formed from very magnesian
magma: major olivine (Fo 90 or more), with chromian pyroxene, much sub-
ordinated anorthite and chromian spinel. The lava flows includes a thick
cumulative basis (forsterite-rich olivine, chromian spinel in primary magma)
and, with sharp boundaries, a thinner upper part in which the rapid crystal-
lization causes a characteristic dendritic texture (spinifex).
Olivines are major minerals of
volcanic and hypabyssal rocks
, mostly in
basic rocks and less frequently in the intermediate and acidic rocks.
Generally the compositions of olivine are consistent with the hypothesis
of fractional crystallization with an evolution from forsterite-rich terms to
terms richer in fayalite. This trend is observed in the same rock by the zona-
tion of the phenocrysts and differences between the phenocrysts and micro-
lites, or between the various rocks of the same magmatic suite. However in
some rocks, such as olivine crystals, are xenocrysts that do not follow from
these laws of fractional crystallization.
The picrites and oceanites are cumulative olivine basaltic; they may
belong either to alkaline or tholeiitic series.
In the
subalkaline series
, there is
a priori
only a single generation of oli-
vine: early phenocrysts, more or less zoned, which tend to be resorbed, cor-
roded by the magma and/or react with it, to eventually form hypersthene.
In the tholeiitic series (and the MORB), olivine is common in poorly dif-
ferentiated terms (olivine tholeiites, basaltic picrites, basalts): its composition
