Pyroxenite

Intrusive and Mantle Pyroxenites

The igneous pyroxenites are closely allied to the olivine. This connection is indicated also by their mode of occurrence, for they usually accompany masses of gabbro and peridotite and seldom are found by themselves.

They are often very coarse-grained, containing individual scapolite.

Pyroxenites can be formed as cumulates in chromite cumulates.

Pyroxenites are also found as layers within masses of peridotite. These layers most commonly have been interpreted as products of reaction between ascending magmas and peridotite of the upper mantle. The layers typically are a few centimeters to a meter or so in thickness. Pyroxenites that occur as xenoliths in basalt. It has been proposed that large volumes of pyroxenite form in the upper mantle as a result of reaction between peridotite and magma derived from partial melting of eclogite, and that such pyroxenite volumes are important sources of basalt magma (e.g., Sobolev and others, 2007).

Pyroxenite lavas

Purely pyroxene-bearing volcanic rocks are rare, restricted to spinifex textured sills, lava tubes and thick flows in the Archaean greenstone belts. Here, the pyroxenite lavas are created by in-situ crystallisation and accumulation of pyroxene on the floor of a lava flow, creating the distinctive spinifex texture, but also occasionally mesocumulate and orthocumulate segregations. This is in essence similar to the formation of olivine spinifex textures in komatiite lava flows, the chemistry of the magma differing only to favor crystallisation of pyroxene.

A type locality is the Gullewa Greenstone Belt, in the Murchison region of Western Australia, and the Duketon Belt near Laverton, where pyroxene spinifex lavas are closely associated with gold deposits.

Distribution

They frequently occur in the form of dikes or segregations in gabbro and peridotite: in Shetland, Cortland on the Hudson river, North Carolina (websterite), Baltimore, New Zealand, and in Saxony.

  The pyroxenites are often subject melanite pyroxenites associated with the borolanite variety found in the Loch Borralan igneous complex of Scotland.

References

Sobolev, A. V., and others, 2007, The amount of recycled crust in sources of mantle-derived melts, Science 316, p. 412-417 (abstract) Retrieved on 6 October 2007.