Garnet Bearing Lherzolites and Discrete Nodule Suites from the Malaita Alnoite, Solomon Islands, S.W. Pacific, and their Bearing on Oceanic Mantle Composition and Geotherm

  1. F.R. Boyd and
  2. Henry O.A. Meyer
  1. Peter H. Nixon1 and
  2. F.R. Boyd2

Published Online: 19 MAR 2013

DOI: 10.1029/SP016p0400

The Mantle Sample: Inclusion in Kimberlites and Other Volcanics

The Mantle Sample: Inclusion in Kimberlites and Other Volcanics

How to Cite

Nixon, P. H. and Boyd, F.R. (1979) Garnet Bearing Lherzolites and Discrete Nodule Suites from the Malaita Alnoite, Solomon Islands, S.W. Pacific, and their Bearing on Oceanic Mantle Composition and Geotherm, in The Mantle Sample: Inclusion in Kimberlites and Other Volcanics (eds F.R. Boyd and H. O.A. Meyer), American Geophysical Union, Washington, D.C.. doi: 10.1029/SP016p0400

Author Information

  1. 1

    Department of Geology, P. O. Box 4820, University, Papua New Guinea

  2. 2

    Geophysical Laboratory, Carnegie Institution, 2801 Upton Street, N.W., Washington, D.C. 20008

Publication History

  1. Published Online: 19 MAR 2013
  2. Published Print: 1 JAN 1979

ISBN Information

Print ISBN: 9780875902135

Online ISBN: 9781118664858

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Keywords:

  • Clinopyroxene;
  • Discrete nodule suite;
  • Geotherm caluclations;
  • Granet-bearing nodules;
  • Malaita Alnoite;
  • Oceanic environment;
  • Orthopyroxene;
  • Ultrabasic nodules

Summary

A suite of xenoliths of deep-seated origin, similar to those found in continental kimberlites, occur in a phlogopite-melilite-olivine-pyroxene (alnoite) breccia, 33.9 m.y. old, in an oceanic environment. The coarse lherzolites have modal proportions of olivine 50–75, orthopyroxene 5–45, clinopyroxene 0–20, garnet 0–15 and spinel 0–10. Some specimens show slight deformation. There are small, but significant variations in mineral composition indicating depletion in the nodules of deepest origin (low Na, Ti, Fe/Mg and A1/Cr). Pyroperich garnet is restricted to nodules of deeper origin, and Al-rich spinel to the nodules of shallower origin. The discrete nodule (megacryst) suite includes pyrope-rich garnet (single specimens up to 8.2 kb), augite, augite-ilmenite lameliar intergrowths, subcalcic diopside and bronzite. A pyroxene geotherm, calculated for these garnet-bearing nodules, has a much higher gradient than those previously estimated for continental kimberlite nodules. A high-temperature inflection in the geotherm can be taken as evidence for a lithosphere thickness of 110 km. At depths less than the inflection there is a lower horizon of lherzolite (source of Alite volcanics ?) overlain by less depleted lherzolites and an upper horizon of harzburgites (source of oceanic crust?). The alnoite appears to have been emplaced near the edge of a thickened portion of the Pacific Plate (the Ontong Java Plateau).