Geochemistry of ultramafic xenoliths from Kapfenstein, Austria: evidence for a variety of upper mantle processes

Details Geochemistry of ultramafic xenoliths from Kapfenstein, Austria: evidence for a variety of upper mantle processes Geochemistry of ultramafic xenoliths from Kapfenstein, Austria: evidence for a variety of upper mantle processes

Jan 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980gecoa..44...45k&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 44, Issue 1, pp.45-51

Computer Science

26

Scientific paper

Major, minor, and trace element contents have been determined in seven ultramafic xenoliths, the host basanite, and some mineral separates from xenoliths from Kapfenstein, Austria. Most of the xenoliths represent residues after extraction of different amounts of basaltic liquid. Within the sequence Iherzolite to harzburgite contents of Al, Ca, Ti, Na, Sc, V, Cr and the HREE decrease systematically with increasing Mg/Fe and decreasing Yb/Sc. Although all samples are depleted in highly incompatible elements, the less depleted end of our suite very closely approaches the chondritic Yb/Sc ratio and consequently the primitive upper mantle composition. Chromium behaved as a non-refractory element. Consequently it should have higher abundances in basalts than observed, suggesting that most basalts experienced Cr fractionation by chromite separation during ascent. Several processes have been active in addition to partial melting within the upper mantle beneath Kapfenstein: 1. (1) a hornblendite has been identified as wet alkali-basaltic mobilisate; 2. (2) an amphibole Iherzolite is the product of alkali-basalt metasomatism of a common depleted Iherzolite; 3. (3) two amphibole Iherzolites contain evidence for rather pure water metasomatism of normal depleted Iherzolites; 4. (4) a garnet-spinel websterite was a tholeiitic liquid trapped within the upper mantle and which suffered a subsequent partial melting event (partial remobilization of a mobilisate). 5. (5) Abundances of highly incompatible elements are generally very irregular, indicating contamination of upper mantle rocks by percolating liquids (in the mantle). Weathering is an important source of contamination: e.g. U mobilization by percolating groundwater. Contamination of the xenoliths by the host basanite liquid can only amount to approximately 5.5 × 10 -4 parts. Distributions of minor and trace elements between different minerals apparently reflect equilibrium and vary with equilibration temperature.

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