Thirteen Cr-bearing spinels from major horizons
of magma replenishment in the open-system Rum
Layered Suite have been analysed by X-ray single crystal
diffraction and electron microprobe analyses. On the basis
of the structural parameters and the chemistry of these
spinels the so-called Rum trend, in which Al-content
increases at the expense of Cr and Fe3+, has been easily
recognised. In addition, natural spinels with Fe3+ content
similar to synthetic spinels on the MgCr2O4–MgFe2O4 join
have been analysed for the first time. Layers of chromitite,
anorthosite and peridotite situated within several cm of one
another have yielded different intracrystalline exchange
temperatures using an intercrystalline spinel-olivine thermometer.
The Rum anorthosite Cr-spinels are interpreted
as having crystallised within the cumulus pile following
rejuvenation of the crystal mush. Their low Al-content is a
function of simultaneous plagioclase crystallisation,
reducing the amount of Al3+ present for the Cr-spinel. By
contrast, Cr-spinels in well-known Archean anorthosites
(e.g. Ujaragssuit nunat and Fiskenæsset, western Greenland)
and Sittampundi (southern India) are very aluminous
in composition, attributed to crystallisation of Cr-spinel
from high-alumina basalts in lower crustal magma chambers
and linked to the control exerted by plagioclase
crystallisation on Al content of the melt, in the absence of
clinopyroxene crystallisation. The compositional differences
between the Rum anorthosite Cr-spinels and the
Fiskenæsset and Sittampundi Cr-spinels suggest that postcumulus
reaction of Cr-spinel and melt to low (800–900°C)
temperatures, as invoked for the Rum crystals, may not
have been as important a process in the Archean
anorthosites.
