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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: The Origin of Olivine Bombs and Related Inclusions in Basalts
By
Rice University Ph. D. thesis, 264 p., May, 1965
The generation and differentiation of basaltic magmas and the chemical
nature of the upper mantle is of considerable interest. A geochemical investigation
of ultramafic and mafic inclusions in basaltic rocks was initiated to
ascertain if the chemical compositions of the several minerals present reflect
fractionation or chemical homogeneity of the upper mantle.
Eight hundred and fifty modal analyses of olivine-bearing inclusions and two
hundred and ninety modal analyses of feldspar-bearing inclusions were performed
in the field. Mineral separates from fifty-seven inclusions and their
host rocks from nine continental localities were analyzed for nineteen elements
by atomic absorption spectrophotometry, colorimetry, and gamma spectrometry
to a precision generally better than five per cent of the amount present.
Forty-six
of the inclusions were from Kilbourne Hole, New Mexico. One of these
inclusions contained sufficient phlogopite for a potassium-40/argon-40 determination,
yielding an age of 6.7 million years.
Three inclusion types were recognized in the field: Type-I olivine rich;
Type-II, pyroxene rich; and Type-III, feldspar rich. Frequency diagrams of
mineral percentages indicate a bimodal distribution with a continuous variation
between Types I and II. A plot of the mineral percentages from the forty-six
Kilbourne Hole inclusions against a differentiation index (fayalite content of the
coexisting olivine) indicates (a) considerable dispersion from possible trend lines
for Type-I inclusions, and (b) narrow dispersion from possible trend lines for
Type-II inclusions. Plots of elemental values against the same differentiation
index indicate close correlations in most cases.
These correlations indicate that (a) for major elements no discernible
distinction between the minerals of the two types; however, there are significant
differences from some minor and trace elements, especially nickel, chromium,
and titanium; (b) Type-I olivines display a variation series from seven to twenty-four mole percent fayalite, and
Type-II olivines display a variation series from
about seven to nineteen mole percent fayalite; (c) spinel varies from magnesio-chromite
(40% Cr2O3) to hercynite-rich spinel (60%
Al2O3); (d) in general,
aluminum varies sympathetically with iron; (e) orthopyroxene is present in
inclusions with fayalite contents less than nineteen mole percent; (f) variations
in chromium and nickel are similar to those of the Skaergaard Intrusion; and
(g) euhedral olivines were present in Type-I inclusions with fayalite contents
greater than fourteen mole percent.
These correlations between chemical and mineralogic composition lead to the
probable conclusion that these olivine inclusions were formed by fractional
crystallization from a mafic (pictritic?) magma, which thus trended toward a
basaltic composition. Variations in the generation and differentiation of such
mafic magmas might give rise to local heterogeneities in the earth's upper mantle. End_of_Record - Last_Page 15--------