Thermodynamic models for eclogitic mantle lithosphere

摘要

Cratonic mantle eclogite xenoliths occur with diamond-bearing kimberlite. The modes and mineral compositions of eclogite contain important information on their origin, physical-chemical conditions of formation, and their geophysical properties. We have used the pseudosection option of the program PERPLEX to explore how mineral assemblage, abundance, and composition vary with bulk composition, temperature (T) and pressure (P). We considered a range of protolith compositions, including: fresh unaltered basalt, altered seafloor basalt, cumulates resulting from high-pressure crystallization of basaltic melts, and metasomatically altered (SiO_2-depleted) basalt. Stable mineral assemblages and associated geophysical properties for each protolith were calculated at P–T conditions found along the mantle geotherm for the Slave craton. At depths greater than 90 km, the predicted modal mineralogy of eclogite for all protoliths changes little along the geotherm. Within the diamond stability field of cratonic mantle, eclogite has a single mineral assemblage that reflects protolith composition. We recognize three basic classes: (1) a silica-oversaturated assemblage indicated by the presence of coesite, (2) a silica-saturated assemblage distinguished by the absence of coesite and olivine, and (3) a silica-undersaturated assemblage containing olivine. Within the diamond stability field for the Slave geotherm, kyanite is stable only in extremely Al-enriched protoliths. Most kimberlite-hosted eclogite belongs to class 2 and our results show that it cannot derive from either fresh or altered seafloor basalt. We suggest an origin involving mantle metasomatism (SiO2 depletion) operating on subducted basalt although we cannot rule out an origin from cumulates of metaluminous, subalkaline magmas. Predicted seismic velocities of model eclogite are higher than corresponding velocities for peridotite and most crustal rocks and therefore may be distinctive in regional seismic surveys. However, the only class of eclogite that could be distinguished from seismic data is the oversaturated assemblage (class 1). The quartz–coesite transition produces sharp changes in both P and S wave velocities at depths of about 90 km along the ambient mantle geotherm.