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Mafic and Ultramafic Nodules in Basaltic Rocks of Hawaii

By
Hisashi Kuno
Hisashi Kuno
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Published:
January 01, 1969

Mafic and ultramafic nodules of Hawaii are classified into (1) a lherzolite series, (2) a wehrlite series comprising dunite, wehrlite, and pyroxenite, (3) an eclogite series comprising pyroxenite and eclogite, and (4) a group of gabbro. The lherzolite series appears to constitute the major part of the upper mantle. It ranges in Mg0/∑Fe0 from 6.0 to 2.3. With the decrease of this ratio, Al203, total Fe0, Ca0, Na20 + K20, Ti02, and Mn0 increase, whereas Mg0 and Cr203 decrease. This variation may have been produced by crystal accumulation in some mafic basaltic magma in a remote past. It is a little difficult, if not impossible, to explain the whole variation as due to different degrees of subtraction of basaltic magmas erupted from the Hawaiian volcanoes. But the lower Mg0/∑Fe0 members of the lherzolite series may be a potential source of various basaltic magmas, provided the amount of partial melting is relatively small. The hypothetical primordial mantle material “pyrolite” is very close in composition to some members of the lherzolite series. The series can be distinguished from the wehrlite series by the compositions of clinopyroxenes. The latter series was probably formed as crystal accumulates at comparatively shallow depths, possibly representing the lower portions of solidified magma reservoirs of the Hawaiian volcanoes.

The eclogite series is distinct from the lherzolite series in its trend of compositional variation. With the decrease of Mg0/∑Fe0, A1203, total Fe0, Na20 + K20, Ti02, and Mn0 increase, whereas Si02, Mg0, Ca0, and Cr203 decrease. The main sequence of crystallization is (1) pyroxenite, (2) eclogite with clinopyroxene-garnet intergrowth, (3) eclogite, and (4) olivine eclogite. These rocks are supposed to constitute a local pocket or pockets within the mass of the lherzolite series, probably at depths between 40 and 60 km from the surface, where some mafic magma was trapped. It is suggested that the fractional crystallization of the magma which produced the eclogite series gave rise to a magma having a composition close to some of the Hawaiian oceanites. The gabbro nodules are fragments of the crust, some of them being possibly the upper portions of the mass of the wehrlite series.

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GSA Memoirs

Igneous and Metamorphic Geology

Leonard H. Larsen
Leonard H. Larsen
Editor
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;
Martin Prinz
Martin Prinz
Co-editors
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;
Vincent Manson
Vincent Manson
Co-editors
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Geological Society of America
Volume
115
ISBN print:
9780813711157
Publication date:
January 01, 1969

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