Abstract
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 clinopyroxene 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., +xx
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 1990 G34
thesis (scanned
text) - 6.9MB pdf file [p. 1-122, +xx]
Appendix 1 - Electron
microprobe analyses of the Blanco Trough glasses and VG-2
basaltic glass standard - 0.5MB pdf file [p. 123-142]
Appendix 2 - Electron
microprobe analyses of phenocrysts from Blanco Trough glasses
- 2.0MB pdf file [p. 143-228]
Appendix 3 - Detailed
petrography - 0.3MB pdf file [p. 229-237]
Appendix 4 - Least-squares
mixing models - 0.1MB pdf file [p. 238-244]
link
to Google Earth kmz file showing location of Fig. 2 profile,
containing the site of Blanco Trough dredge samples
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