Compositional evolution of high-temperature sheared lherzolite PHN 1611

Journal Article · Mon Feb 01 00:00:00 EST 1993 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(93)90371-3· OSTI ID:6716326

Smith, D ^[1]; Griffin, W L; Ryan, C G ^[2]

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1. Univ. of Texas, Austin (United States)
2. CSIRO, North Ryde (Australia)

The evolution of fertile' mantle has been studied by proton microprobe (PIXE) analysis of minerals of a high-temperature sheared xenolith from the Thaba Putsoa kimerlite in Lesotho, southern Africa. Analyzed elements include Ni, Cu, Zn, Ga, Sr, Y, and Zr. Garnets are homogeneous in Ni and Zn but have rims enriched relative to cores in Zr and Y. Compositions of olivine neoblasts define intergranular gradients of Fe, Zn, and Ni; Fe-rich olivine is relatively Zn-rich but Ni-poore. Although individual clinopyroxene grains are nearly homogeneous, clinopyroxene associated with Fe-rich olivine is relatively Fe- and Zn-rich but Sr- and Cr-poor. The trace-element abundances and compositional gradients constrain the processes of periodotite enrichment and the thermal history. Enrichment of Zr, Y, and Fe in garnet rims documents infiltration of a silica-undersaturated melt. The Fe-rich olivine compositions and the Zn and Fe gradients establish that the xenolith was sampled from near a melt conduit. Mechanical mixing of inhomogeneous peridotite and melt infiltration may have been concurrent. Because garnets appear homogeneous in Ni, mantle temperature changes affecting PHN 1611 occurred before or over a longer period than the melt infiltration. Measured and calculated abundances of many incompatible trace elements in the rock are similar to those proposed for primitive mantle. Calculated chondrite-normalized abundances of Sr, Ti, Zr, and Y are like those of appropriate REE. Enrichment processes in PHN 1611 proceeded at unusually high recorded temperature and in the apparent absence of minor phases common in lower-temperature metasomatized rocks, but similar processes may be common. In particular, mechanical mixing near mantle dikes may frequently occur. These enrichment mechanisms may produce xenolith compositions that resemble some proposed for primitive mantle but that have different implications for mantle evolution. 61 refs., 7 figs., 2 tabs.

• OSTI ID: 6716326
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 57:3; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
58 GEOSCIENCES
MINERALS
PIXE ANALYSIS
AFRICA
CHROMIUM
COPPER
DIKES
EARTH MANTLE
ELEMENTS
ENRICHMENT
GALLIUM
GARNETS
IRON
LESOTHO
MIXING
NICKEL
OLIVINE
PLANETARY EVOLUTION
ROCKS
STRONTIUM
TEMPERATURE DEPENDENCE
YTTRIUM
ZINC
ZIRCONIUM
ALKALINE EARTH METALS
CHEMICAL ANALYSIS
DEVELOPING COUNTRIES
GEOLOGIC STRUCTURES
METALS
NONDESTRUCTIVE ANALYSIS
OXYGEN COMPOUNDS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
SOLAR SYSTEM EVOLUTION
TRANSITION ELEMENTS
X-RAY EMISSION ANALYSIS
400102* - Chemical & Spectral Procedures
580000 - Geosciences