Kankan diamonds (Guinea) I: from the lithosphere down to the transition zone |
| |
Authors: | Thomas Stachel Gerhard P Brey Jeff W Harris |
| |
Institution: | Institut für Mineralogie, Universit?t Frankfurt, Senckenberganlage 28, 60054 Frankfurt, Germany, DE Division of Earth Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK, GB
|
| |
Abstract: | Peridotitic inclusions in alluvial diamonds from the Kankan region of Guinea in West Africa are mainly of lherzolitic paragenesis.
Nevertheless, extreme Cr2O3 contents (max. 17 wt%) in some of the exclusively lherzolitic garnets document that the diamond source experienced a previous
stage of melt extraction in the spinel stability field. This initial depletion was followed by at least two metasomatic stages:
(1) enrichment of LREE and Sr and (2) introduction mainly of MREE–HREE and other HFSE (Ti, Y, Zr, Hf). The Ti- and HFSE-poor
character of stage (1) points towards a CHO-rich fluid or carbonatitic melt, the high HFSE in stage (2) favour silicate melts
as enriching agent. Eclogitic inclusions are derived from a large depth interval ranging from the lithosphere through the
asthenosphere into the transition zone. The occurrence of negative Eu anomalies in garnet and clinopyroxene from both lithosphere
and transition zone suggests a possible relationship to subducted oceanic crust. Lithospheric eclogitic inclusions are derived
from heterogeneous sources, that may broadly be divided into a low-Ca group with LREE depleted trace element patterns and
a high-Ca group representing a source with negative LREE–HREE slope that is moderately enriched in incompatible elements relative
to primitive mantle. High-Ca inclusions of majoritic paragenesis are significantly more enriched in incompatible elements,
such as in Sr and LREE. Calculated whole rock compositions require metasomatic enrichment even if a derivation from MORB is
assumed.
Received: 26 January 2000 / Accepted: 18 May 2000 |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|