Jump to ContentJump to Main Navigation
Show Summary Details
More options …

American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2017: 2.645

CiteScore 2017: 2.31

SCImago Journal Rank (SJR) 2017: 1.440
Source Normalized Impact per Paper (SNIP) 2017: 1.059

Online
ISSN
1945-3027
See all formats and pricing
More options …
Volume 84, Issue 4

Issues

Magnetite lamellae in olivine and clinohumite from Dabie UHP ultramafic rocks, central China

Ru Y. Zhang
  • Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, U.S.A.
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Jin F. Shu
  • Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, D.C. 20015-1305, U.S.A.
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Ho K. Mao
  • Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, D.C. 20015-1305, U.S.A.
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Juhn G. Liou
Published Online: 2015-11-13 | DOI: https://doi.org/10.2138/am-1999-0410

Abstract

Ultrahigh-pressure (UHP) minerals of the Maowu mafic-ultramafic complex in the Dabieshan, east-central China exhibit many exsolution textures. Magnetite lamellae are common in olivines and Ti clinohumites from harzburgite and garnet pyroxenite. Monazite-(Ce) lamellae occur in apatites from the garnet pyroxenite. Independent P-T estimates suggest that these ultramafic rocks formed at P > 5 GPa and T = 700 ± 50 °C. The lamellae-bearing minerals are believed to preserve an earlier, higher P-T record prior to exsolution. Compositions and unit-cell parameters of the magnetite and host olivine and intergrowth relations were determined using a newly developed X-ray diffraction microprobe technique employing synchrotron radiation. The host olivine and magnetite lamellae bear a topotaxial relation with [220]Mag | | [200]Ol, [111]Mag | | [33̅1̅]Ol, [111̅] Mag | | [331]Ol, [242]Mag | | [22̅0]Ol. The recalculated composition of primary olivine may contain up to 1.5 wt% Fe2O3. Four hypotheses may explain the observed intergrowths of oriented magnetite lamellae in olivine: (1) oxidation of olivine; (2) decomposition of Fe3+-bearing olivine formed at >6 GPa; (3) exsolution of a spinel (wadsleyite) solid-solution Fe3O4-(Fe,Mg)2SiO4 during decompression; and (4) breakdown of phase A [Mg7Si2(OH)6] + enstatite. The third hypothesis appears to be the most likely inasmuch as no additional silicate phase occurs as associated inclusions in the olivine host. However, the actual mechanism for exsolution of magnetite from olivine remains to be studied experimentally.

About the article

Received: 1997-10-28

Accepted: 1998-11-11

Published Online: 2015-11-13

Published in Print: 1999-04-01


Citation Information: American Mineralogist, Volume 84, Issue 4, Pages 564–569, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-1999-0410.

Export Citation

© 2015 by Walter de Gruyter Berlin/Boston.

Comments (0)

Please log in or register to comment.
Log in