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Unformatted text preview: low fragment weakly devitriﬁed glass rim (0·5 cm), slightly vesicular interior, aphyric 110DS-3 lava fragment with ropy surface structure, thin glass crust, interior slightly vesicular, aphyric 110DS-4 see 110DS-3 110DS-5 similar to 110DS-3, but more channelled surface—almost ropy 110DS-6 piece of lava tube with glassy rim (up to 0·3 cm thick), contains rare feldspar phenocrysts 110DS-7 see 110DS-6 1448 FRETZDORFF et al. PETROGENESIS OF EAST SCOTIA RIDGE Olivine phenocrysts are mostly rounded or highly
skeletal crystals. Subordinate olivine is present in the
matrix and glass rims. Generally, individual olivine
phenocrysts show a comparable range of Fo89 (cores) to
Fo83 (rims). There are no distinct chemical diﬀerences
between phenocrysts and matrix minerals. Calculations
show that most olivines are in equilibrium with the matrix
glass at atmospheric pressure for a Fe/Mg KD of 0·3
(Roeder & Emslie, 1970; Ulmer, 1989).
Plagioclase phenocrysts are up to 5 mm long in volcanic
rocks from segments E5–E7 and with smaller sizes of up
to 2 mm in the lavas of segments E3 and E4. Some
phenocrysts show zonation from calcic cores to more
sodic rims. Core compositions fall in the range An92–85
and rims, as well as unzoned phenocrysts, have An
contents of 78–87. Matrix plagioclase crystals are the
least calcic with the range An70–82.
Generally, opaque minerals are scarce in East Scotia
Ridge lavas. Titanomagnetite, magnetite and chrome
spinels mostly form inclusions in olivine phenocrysts in
the lavas of segments E3 and E4. The cr-number in the
spinels ranges from 62 to 66. Magmatic geochemistry
Major element analyses of the dredged and wax core
samples are presented in Table 1. Figure 3 shows all the
East Scotia Ridge lavas as well as the chemical trends of
the subduction-related magmas from the South Sandwich
Islands in the K2O–SiO2 classiﬁcation diagram of Peccerillo & Taylor (1976). The majority of the back-arc
samples range from basalt to basaltic andesite and are
primarily low-K tholeiites, although some show mediumK compositions. For a given SiO2 abundance, samples
from segments E2 and E4 are signiﬁcantly enriched in
K2O relative to the other segments. MgO contents for
the majority of segments lie in the restricted range of
6–9 wt % and have mg-number between 60 and 70.
However, segments E2 and E8 have yielded samples with
MgO contents as low as 3 wt % and with mg-number as
low as 40. These two segments also show systematic
variations of CaO with MgO (Fig. 4a). In the plot of
Al2O3 against MgO, each segment forms a distinct trend,
parallel to those from the other segments and to the
Mid-Atlantic Ridge (Fig. 4b).
Water contents of selected East Scotia Ridge magmas
cover the range of 0·2–2·3 wt % (Table 1). Water contents
of MORB vary between 0·05 and 1·3 wt % (Michael,
1995), although most N-MORB have contents between
0·1 and 0·4 wt % (Michael & Chase, 1987; Dixon et al.,
1988; J. E. Dixon, personal communication, 2001). All
samples from East Scotia Ridge segments E6 and E7
show H2O contents within this N-MORB range. Some
samples from segments E2, E3, E4 and E9, however, contain up to 2·2 wt % H2O...
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This document was uploaded on 02/01/2014.
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