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《中国科学院地质与地球物理研究所二○○三学术论文汇编·第一卷(地球动力学)》2003年
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Chromian dissakisite-(Ce) in a garnet lherzolite from the Chinese Su-Lu UHP metamorphic terrane:Implications for Cr incorporation in epidote-group minerals and recycling of REE into the Earth's mantle

MASAKI ENAMI  
【摘要】:正Chromian dissakisite-(Ce),a Cr-enriched Mg analogue of allanite-(Ce),is found in a garnet lherzolite from the Su-Lu ultrahigh-pressure(UHP) metamorphic terrane,eastern China.Major minerals of the lherzolite are olivine[X_(Mg) = Mg/(Mg + Fe) = 0.92],pyrope-rich garnet(Prp_(70.1-73.2)Alm_(11.3- 13.4)Grs_(6.9-8.3)Uvr_(3.6-5.1)Anr_(0-0.4)Sps_(0.7-1.7)),orthopyroxene(X_(Mg) = 0.92-0.93),and clinopyroxene(X_(Mg) = 0.92-0.95).The chromian dissakisite-(Ce) occurs as a single-crystal inclusion in a clinopyroxene neoblast that also contains several irregularly shaped olivine grains.It is partly metamict,and there is intensive radial cracking in the surrounding clinopyroxene.Its composition is represented by the averaged formula:(Ca_(0.86)Na_(0.03))_(0.89)(REE_(0.86)Th_(0.04))_(0.90)(Mg_(0.98)Fe~(2+)_(0.20)Fe~(3+)_(0.20)Fe~(3+)_(0.21)Cr_(0.36)Al_(1.39)Ti_(0.04)Si_(3.03)O_(12)(OH), with Ce_2O_3,MgO and Cr_2O_3 contents up to 13.1,7.3,and 5.4 wt%,respectively.Because the M3 site in the chromian dissakisite-(Ce) is fully occupied by Mg and Fe~(2+),Fe~(3+) and Cr can only occupy the smaller Ml site.Similar site occupancy is noted for a chromian allanite from the Sanbagawa metamorphic belt,Japan,which contains up to 0.547 Cr atoms per formula unit(apfu).Such compositional characteristics of the chromian dissakisite/allanite and existing polarized absorption data on Cr-bearing epidote suggest the existence of possible end-members,"CeCa(Mg,Fe)CrAlSi_3O_(12)(OH)." for the epidote-group minerals,which may form solid solution with Ca_2Al_3Si_3O_(12)(OH) through coupled substitutions of REEMgCa_(-1)Al_(-1).REEFeCa_(-1)Al_(-1),and CrAl_(-1).The rare earth elements(REE),Ca, and Na total 1.79 apfu,implying that Mn and Fe~(2+) are also present on the nine-coordinated Al site. The high REE abundance,the presence of minor Na,and the Mg and Cr-enriched characteristics of the mineral suggest that it was formed as a result of reaction between the host lherzolite and crust-derived melt/fluid at UHP conditions.Textural relations suggest that the chromian dissakisite-(Ce) was unstable in the garnet lherzolite and reacted with olivine to form clinopyroxene.Dissakisite-(REE) in subducted ultramafic rocks can transport rare earth elements into the Earth's mantle.

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【共引文献】
中国期刊全文数据库 前1条
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