Pyroxenite from Diavik, Canada: metasomatic origin from eclogite

Major and trace element contents, including H₂O, of garnet and clinopyroxene were determined in eclogite and pyroxenite xenoliths from the Diavik diamond mine, Slave Craton, Canada. Three eclogite (A, B, C) and two pyroxenite types (B, C) are distinguished on the basis of garnet composition (A: high Ca/low Mg; B: high Ca/high Mg; C: low Ca/high Mg).

The 20 xenoliths were derived from a 100 km depth range (c. 120–220 km; one sample: 230-240 km). The data show that pyroxenite and eclogite of C-type invariably come from lower depth (<175 km) than B-type pyroxenite and eclogite (>200 km). Type-A eclogite is present in both intervals. The equilibrium conditions of orthopyroxene-bearing samples (exclusively C-types) point to a geotherm equivalent to 37 mW/m² surface heat flow.

The contents of structural H₂O in clinopyroxene are variable in the sample set (123–1509 wt.ppm) with an average of 440 wt.ppm. Excluding three samples with exceptionally high contents, the range is reduced to 123–522 ppm (17 samples). Clinopyroxene in B-type eclogite and pyroxenite (i.e., 360–1149 and 225–1509 ppm) has considerably higher and much more heterogeneous H₂O contents than C-type samples (eclogite: 123–165 ppm; pyroxenite 321–393 ppm). Structural H₂O of clinopyroxene is positively correlated to some trace (Cu, Ni, and K) but not to major elements and also depends on the rock type. Eclogitic clinopyroxene has lower H₂O contents relative to pyroxenite. While the contents of B-type eclogite and pyroxenite overlap, there is a distinct gap between C-type eclogite and pyroxenite.

In garnet, the contents of structural H₂O are low (0– 41 ppm) and correlate neither with rock type nor with mineral composition. Most garnet grains additionally contain molecular H₂O (in contrast to pyroxene), which is correlated to compositional parameters of both garnet (positive: Mg; negative: Ca, Sr, Be, Na) and clinopyroxene (positive: MREE, Ca, Th; negative: Al, K, Na, Li).

The observation that structural H₂O in both minerals is unrelated to major elements, the highly variable contents of structural H₂O in clinopyroxene, and the lack of an H₂O-Ca correlation in garnet are unusual and point to disequilibrium. This, and the correlation of the mineral composition with molecular H₂O – being of secondary origin – indicate that structural H₂O does not reflect equilibrium at PT peak conditions. All these characteristics imply that structural H₂O was affected by secondary processes related to metasomatism due to reaction with a hydrous fluid or melt. Metasomatic changes that led to lower clinopyroxene Na and Zn contents along with higher contents of Mg#, Cr, Sr, REE, Pb, Th, U, and Cu. The data indicate that Diavik pyroxenite formed from an eclogitic precursor: pyroxenite B from eclogite B at greater (200-240 km) and pyroxenite C from eclogite C at lower depth (120-175 km).