Contributions to Mineralogy and Petrology

, Volume 153, Issue 5, pp 607–624

Metasomatism of sub-arc mantle peridotites below southernmost South America: reduction of fO2 by slab-melt

  • Jian Wang
  • Kéiko H. Hattori
  • Rolf Kilian
  • Charles R. Stern
Original Paper

DOI: 10.1007/s00410-006-0166-4

Cite this article as:
Wang, J., Hattori, K.H., Kilian, R. et al. Contrib Mineral Petrol (2007) 153: 607. doi:10.1007/s00410-006-0166-4

Abstract

Quaternary basalts in the Cerro del Fraile area contain two types of mantle xenoliths; coarse-grained (2–5 mm) C-type spinel harzburgites and lherzolites, and fine-grained (0.5–2 mm) intensely metasomatized F-type spinel lherzolites. C-type xenoliths have high Mg in olivine (Fo = 90–91) and a range in Cr# [Cr/ (Cr + Al) = 0.17–0.34] in spinel. Two C-type samples contain websterite veinlets and solidified patches of melt that is now composed of minute quenched grains of plagioclase + Cr-spinel + clinopyroxene + olivine. These patches of quenched melts are formed by decompression melting of pargasitic amphibole. High Ti contents and common occurrence of relic Cr-spinel in the quenched melts indicate that the amphibole is formed from spinel by interaction with the Ti-rich parental magma of the websterite veinlets. The fO2 values of these two C-type xenoliths range from ΔFMQ −0.2 to −0.4, which is consistent with their metasomatism by an asthenospheric mantle-derived melt. The rest of the C-type samples are free of “melt,” but show cryptic metasomatism by slab-derived aqueous fluids, which produced high concentrations of fluid-mobile elements in clinopyroxenes, and higher fO2 ranging from ΔFMQ +0.1 to +0.3. F-type lherzolites are intensely metasomatized to form spinel with low Cr# (∼0.13) and silicate minerals with low MgO, olivine (Fo = ∼84), orthpyroxene [Mg# = Mg/(Mg + ΣFe) = ∼0.86] and clinopyroxene (Mg# = ∼0.88). Patches of “melt” are common in all F-type samples and their compositions are similar to pargasitic amphibole with low TiO2 (<0.56 wt%), Cr2O3 (<0.55 wt%) and MgO (<16.3 wt%). Low Mg# values of silicate minerals, including the amphibole, suggest that the metasomatic agent is most likely a slab melt. This is supported by high ratios of Sr/Y and light rare earth elements (REE)/heavy REE in clinopyroxenes. F-type xenoliths show relatively low fO2 (ΔFMQ −0.9 to −1.1) compared to C-type xenoliths and this is explained by the fusion of organic-rich sediments overlying the slab during the slab melt. Trench-fill sediments in the area are high in organic matter. The fusion of such wet sediments likely produced CH4-rich fluids and reduced melts that mixed with the slab melt. High U and Th in bulk rocks and clinopyroxene in F-type xenoliths support the proposed interpretation.

Keywords

Mantle oxidation state Mantle wedges Subduction zone Adakites 

Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • Jian Wang
    • 1
  • Kéiko H. Hattori
    • 1
  • Rolf Kilian
    • 2
  • Charles R. Stern
    • 3
  1. 1.Department of Earth SciencesUniversity of OttawaOttawaCanada
  2. 2.Universität Trier, Lehrstuhl für GeologieTrierGermany
  3. 3.Department of Geological SciencesUniversity of ColoradoBoulderUSA