The original electron microprobe analyses of suite of basaltic glasses recovered in 1968 by W.G. Melson and co-workers from the Blanco Trough have extremely unusual characteristics. Their compositions plot in a region of the normative Ol-Di-Pl-SiO2 tetrahedron which suggests that they represent liquids in equilibrium with an upper mantle assemblage of olivine + orthopyroxene at 10-15 kb. Given the present state of controversy surrounding the composition and depth of origin of primary MORB, natural examples of such liquids would be very important. The major element variations observed in the suite imply that the dominant phases are olivine and an iron-aluminum rich spinel. The crystal/melt partitioning data are consistent with an unusually large Fe2O3 component in the melt, which could help to explain the variation in liquidus phases, and thus the suite's position in Ol-Di-Pl-SiO2 space.
New analyses of the original samples, performed on the microprobe at Rensselaer Polytechnic Institute, have failed to reproduce the original anomalies. The R.P.I. data does show that the glasses are slightly enriched in FeO* giving them unusually low Mg/(Mg + Fe) ratios for otherwise primitive looking mid-ocean ridge tholeiites. The new analyses plot near the Ol-Di join when projected from PI onto the Ol-Di-SiO2 plane, a region not uncommon for primitive MORB. The disagreement between the two data sets appears to be the result of a transposition of the FeO* and CaO abundances in the Smithsonian analyses prior to their publication by Melson et al. (1977), in addition to minor differences in calibration between the two laboratories.
All of the Blanco Trough glasses that were studied in thin section contain olivine, while 61.5% are saturated in chromian spinel. Some of the olivines display deformation bands and possible fluid inclusions, indicating that they are probably cumulate xenocrysts. In contrast with most other MORB suites, plagioclase is a major phenocryst phase in only one sample and is absent completely from 38.5% of the thin sections. Based on petrography, the first phase to appear is olivine, followed by assemblages of olivine + chromian spinel, olivine + chromian spinel + plagioclase and olivine + plagioclase. Although the appearance and disappearance of phenocryst phases agrees very well with the phase equilibria in the system An-Fo-Di, no simple relationship exists between the Mg/(Mg + Fe) ratio of the liquid and the phenocryst assemblage.
Least-squares mixing models show that the primitive and evolved ends of the suite cannot be related simply by the fractional crystallization of an assemblage of olivine + chromian spinel + plagioclase. The best fit is obtained if clinopyroxene is added to the crystallizing assemblage, although it is not found as a phenocryst phase in any of the samples. The problem of needing to crystallize significant amounts of clinopyroxene to explain a suite of MORB which does not contain clinpyroxene is not unique to this study, but has been recognized by other workers for a number of years. The petrogenesis of the suite appears to involve several stages, in addition to fractional crystallization of olivine, chromian spinel and plagioclase in a shallow level magma chamber. Polybaric crystallization of several batches of parental magma, followed by homogenization during episodes of magma-mixing seems likely.
Gaetani, G.A., 1990. Petrology and major element geochemistry of basaltic
glasses from the Blanco Trough, northeast Pacific. Unpublished MSc. thesis,
State University of New York at Albany. 244pp., +x
University at Albany Science Library call number: SCIENCE Oversize (*) QE 40 Z899 1990 G34
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