Fluid-induced transition from banded kyanite- to bimineralic eclogite and implications for the evolution of cratons

Sommer, Holger and Jacob, D. E. and Stern, R. A. and Petts, D. and Mattey, D. P. and Pearson, D. G. (2017) Fluid-induced transition from banded kyanite- to bimineralic eclogite and implications for the evolution of cratons. Geochimica et Cosmochimica Acta, 207 . pp. 19-42. ISSN 0016-7037

PDF - Published Version Restricted to Repository staff only Download (2MB) | Request a copy

Abstract

Heterogeneous, modally banded kyanite-bearing and bimineralic eclogites from the lithospheric mantle, collected at the
Roberts Victor Diamond mine (South Africa), show a reaction texture in which kyanite is consumed. Geothermobarometric
calculations using measured mineral compositions in Perple_X allowed the construction of a P-T path showing a steep, cool
prograde metamorphic gradient of 2 �C/km to reach peak conditions of 5.8 GPa and 890 �C for the kyanite eclogite. The
kyanite-out reaction formed bimineralic eclogite and is probably an integral part of the mineralogical evolution of most
archetypal bimineralic eclogites at Roberts Victor and potentially elsewhere. The kyanite-out reaction occured at close to peak
pressure (5.3 GPa) and was associated with a rise in temperature to 1380 �C. Mass balance calculations show that upon breakdown,
the kyanite component is fully accommodated in garnet and omphacite via a reaction system with low water fugacity
that required restricted fluid influx from metasomatic sources. The d18O values of garnets are consistently higher than normal
mantle values. Each sample has its characteristic trend of d18O variance between garnets in the kyanite-bearing sections and
those in the bimineralic parts covering a range between 5.1‰ and 6.8‰. No systematic change in O-isotope signature exists
across the sample population. Differences in garnet trace element signatures between differing lithologies in the eclogites are
significant. Grossular-rich garnets coexisting with kyanite have strong positive Eu-anomalies and low Gd/Yb ratios, while
more pyrope-rich garnets in the bimineralic sections have lost their positive Eu-anomaly, have higher Gd/Yb ratios and generally
higher heavy rare earth element contents. Garnets in the original kyanite-bearing portions thus reflect the provenance of
the rocks as metamorphosed gabbros/troctolites. The kyanite-out reaction was most likely triggered by a heating event in the
subcratonic lithosphere. As kyanite contains around 100 ppm of H2O it is suggested that the kyanite-out reaction, once initiated
by heating and restricted metasomatic influx, was promoted by the release of water contained in the kyanite. The steep
(high-P low-T) prograde P-T path defining rapid compression at low heating rates is atypical for subduction transport of
eclogites into the lithospheric mantle. Such a trajectory is best explained in a model where strong lateral compression forces
eclogites downward to higher pressures, supporting models of cratonic lithosphere formation by lateral collision and
compression

Item Type:	Journal Article
Uncontrolled Keywords:	Kaapvaal craton; Cratonic lithosphere; Perple_X; Kyanite; Oxygen isotopes; Eclogite
Subjects:	G Geography. Anthropology. Recreation > GB Physical geography G Geography. Anthropology. Recreation > GE Environmental Sciences
Divisions:	Faculty of Science, Technology and Environment (FSTE) > School of Geography, Earth Science and Environment
Depositing User:	Fulori Nainoca - Waqairagata
Date Deposited:	04 Jul 2017 00:15
Last Modified:	04 Jul 2017 00:17
URI:	https://repository.usp.ac.fj/id/eprint/9995

Actions (login required)

View Item