TY - JOUR
AU - HARTE,, BEN
AB - Abstract Xenolith JJG41 is from the Roberts Victor kimberlite and is a bimineralic eclogite which is striking for its Al- and Ca-rich clinopyroxene crystals showing garnet exsolution lamellae. The development of the exsolution has been interpreted as a result of a slow cooling at depth from near-solidus conditions (c. 1400°C) towards normal mantle lithosphere temperatures (Harte & Gurney, 1975). The clinopyroxene retains marked compositional gradients adjacent to the garnet lamellae and the present paper is concerned with the generation and preservation of these diffusion gradients within a rock from the Earth's mantle In order to understand the mechanism of exsolution reaction a re-examination has been made of the microtexture and chemistry of the garnet lamellae in relation to the compositional gradients in adjacent clinopyroxene. Three sets of garnet lamellae, which appear to have crystallized in sequence, may be recognised: type A, large lamellae, nucleated first and closest to the transformation temperature; type B of intermediate size and age; and type C, small lamellae, nucleated last and with the greatest overstep of the transformation temperature. The major compositional zoning in JJG41 clinopyroxene, a decrease of Al as Si and Mg increase, is consistent with the garnet growth reaction 2Diop+Al2Si−1Mg−1=2Gros, 1Py.Ca, unlike most of the elements, shows very flat composition profiles, but with a higher concentration than the initial Ca content of the unexsolved clinopyroxene. Garnet lamellae are individually homogeneous, but Ca contents vary between lamellae as a function of lamellae size. In contrast the Fe/Mg distribution coefficients at interfaces between garnet and clinopyroxene are relatively constant irrespective of garnet size. The redistribution of the principal cations—Ca, Fe, Mg, Al, Si—between and within the clinopyrox ene and garnet, during garnet exsolution and cooling, obviously proceeded differently for different elements. Two principal stages in the cooling history may be identified: (1) The growth of the sets of garnet lamellae controlled by Al2Mg−1Si−1 redistribution in clinopyroxene. This redistribution was both part of the net transfer reaction creating garnet, and an exchange reaction in clinopyroxene essential for the diffusional transport of Al to the growing garnet. Al diffusion in clinopyroxene was probably the rate-limiting step, and all other cations, Ca and Fe as well as Mg and Si, were mobile during this stage. (2) The occurrence, after the cessation of garnet growth, of diffusion of Fe, Mg and Ca in garnet and interdiffusion of Fe-Mg in clinopyroxene. This resulted in the setting of the KD Fe-Mg at the Cpx-Gt interfaces to a roughly constant value equivalent to approximately 1000°C, which is taken to be the final (‘freezing-in’) temperature for redistribution of any elements. During this post garnet-growth stage Ca also became homogenized within individual garnet lamellae, but there is no evidence of Ca equilibration with the clinopyroxene. Under the P-T conditions operating, the initial clinopyroxene composition probably resulted in a maximum (M2 site fully occupied) Ca content in clinopyroxene during the stage of garnet growth, and this was maintained during the post-growth stage. This content is only available as a PDF. © Oxford University Press
TI - Diffusion Gradients in an Eclogite Xenolith from the Roberts Victor Kimberlite Pipe: 1. Mechanism and Evolution of Garnet Exsolution in Al2O3-rich Clinopyroxene
JF - Journal of Petrology
DO - 10.1093/petrology/29.6.1325
DA - 1988-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/diffusion-gradients-in-an-eclogite-xenolith-from-the-roberts-victor-k8WVmQuSHf
SP - 1325
EP - 1352
VL - 29
IS - 6
DP - DeepDyve
ER -