East Scotia Ridge unsecured

The trends can be modelled as the result of either

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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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