Geoscience Reference
In-Depth Information
3.1 Olivine-I
Light-green or light-yellow olivine-I is present as rounded and oval crystals, or more often
as angular fragments with smooth edges Fig. 3a, b). Angular olivine-I is characteristically
transparent and large (0.5 to 7-8 mm), whereas ovoid grains are smaller (0.7-2 mm) and
often 'dusted' with inclusions (Fig. 3a, b). Melt and fluid inclusions occur in angular olivine-
I and some round crystals only in “secondary” trails along healed fractures (Fig. 3a, b).
Olivine-I is characterised by variable forsterite content (Fo) from 85 to 94 mol%, although
most grains are Fo>91 (Fig. 6a). Most grains appear to be homogeneous, at least in terms of
their Fo content, except outermost rims and around healed fractures. Abundances of trace
elements Ca, Ni, Cr and Mn in olivine-I vary strongly with Fo (Fig. 6a). The trace element
composition trends resembling fractionation can be seen for NiO decreasing (0.43-0.13 wt%)
and MnO increasing (0.07-0.17 wt%) as the Fo value decreases (Fig. 6a). However, it should
be noted that NiO in the majority of olivine-I is almost constant (0.35-0.39 wt%).
(a)
(b)
N
N
18
35
14
25
10
15
6
5
2
85
87
89
91
93
95
85
87
89
91
93
95
0.5
0.5
NiO, wt%
NiO, wt%
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
85
87
89
91
93
95
85
87
89
91
93
95
Fo, mol%
Fo, mol%
Fig. 6. Forsterite and trace element compositions of olivine-I (a) and olivine-II, 0.3-0.5mm
size (b). Grey and black circles represent cores and rims of olivine-II, respectively. N,
number of grains. The analytical error (1, equals 0.08% for Fo and 2% for NiO) is smaller
than the size of the symbols.
3.2 Olivine-II: Morphology and zoning
Olivine-II is represented by relatively small (0.05-0.8 mm) euhedral flattened grains (Fig.
3c,d). Crystals display a tabular habit (tablet shape), and crystal growth was preferentially
developed in the {100} and {001} directions. Olivine-II is colourless or slightly greenish or
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