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Science 10 May 1991:
Vol. 252. no. 5007, pp. 827 - 830
DOI: 10.1126/science.252.5007.827
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Articles
Ultradeep (>300 Kilometers) Ultramafic Xenoliths: Petrological Evidence from the Transition Zone
VIOLAINE SAUTTER 1,
STEPHEN E. HAGGERTY 2, and
STEPHEN FIELD 3
1 Laboratoire de Géophysique et de Géodynamique Interne, Université de Paris-Sud, 91405 Orsay CEDEX, France
2 Department of Geology, University of Massachusetts, Amherst, MA 01003
3 Department of Geology, Stockton State College, Pomona, NJ 08240
The seismologically delineated transition zone, at depths between 400 and 670 kilometers, is a fundamental discontinuity in the earth that separates the upper mantle from the lower mantle. Xenoliths from within or close to the transition zone are dominated by pyropic garnet and associated pyroxene or mineralogically heterogeneous garnet lherzolite. These xenoliths show evidence for the high-pressure (90 to 120 kilobars) transformation of pyroxene to a solid solution of pyroxene in garnet (majorite) and silicon in octahedral coordination; low-pressure (less than 80 kilobars) exsolution of clinopyroxene or orthopyroxene from the original majorite is preserved. Although mineral modes and rock proportions below the transition zone and the relative amount of eclogite present cannot be accurately assessed from the xenoliths, it is likely that both majorite and  -spinel help produce the observed seismic gradient of the transition zone.
Submitted on January 2, 1991
Accepted on March 5, 1991
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