Symplectites in upper mantle peridotites: Development and implications for the growth of subsolidus garnet, pyroxene and spinel Article

Field, SW, Haggerty, SE. (1994). Symplectites in upper mantle peridotites: Development and implications for the growth of subsolidus garnet, pyroxene and spinel . CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 118(2), 138-156. 10.1007/BF01052865

cited authors

  • Field, SW; Haggerty, SE

authors

abstract

  • Silicate-oxide symplectites in complex mineral intergrowths are relatively common in upper mantle xenoliths and in xenoliths in the Jagersfontein Kimberlite, South Africa. Harzburgites of olivine and high-Al (1.9-3.6 wt%), Ca (0.6-0.9 wt%) and Cr (0.3-0.9 wt%) enstatite contain symplectites of spinel and diopside, or spinel, diopside and lower-Al (0.8-2.2 wt%), Ca (0.1-0.4 wt%) and Cr (0.02-0.8 wt%) enstatite. From textures and mineral chemistries these symplectites are interpreted to have formed by mineral unmixing and migration from Al-Ca-Cr discrete enstatite to adjoining mineral interfaces. Garnet harzburgites are composed of large (0.5-1 cm) olivine, equally large discrete low-Al (0.6-1.1 wt%), Ca (0.1-0.5 wt%), and Cr (0.1-0.3 wt%) enstatite and smaller interstitial garnet, diopside, and high-Cr and low-Al spinel. Symplectites are composed of either spinel+diopside+garnet, or garnet+spinel. Spinel diopside garnet symplectites have cores of spinel+diopside, resembling symplectites in harzburgites, but surrounded by rims of garnet or garnet+undigested globular spinel. From textures and chemistries we suggest that the spinel+diopside cores formed from Ca-Al-Cr-rich orthopyroxene initially as a nonstoichiometric homogeneous single phase clinopyroxene enriched in Fe, Cr and Al. This was followed by decomposition of the clinopyroxene to diopside+spinel, and subsequent garnet formation in a prograde reaction with olivine or enstatite. In both harzburgites and garnet harzburgites the metastable cellular structures may also have formed by the simultaneous precipitation of pyroxene and spinel. In all cases there is a strongly preferred embayment of symplectite bodies into olivine. Olivine appears to have activated adjacent © 1994 Springer-Verlag.

publication date

  • June 1, 1994

published in

Digital Object Identifier (DOI)

start page

  • 138

end page

  • 156

volume

  • 118

issue

  • 2