Chemical Evolution of Intercumulus Liquid, as Recorded in Plagioclase Overgrowth Rims from the Skaergaard Intrusion

Humphreys, M. C. S. (2009) Chemical Evolution of Intercumulus Liquid, as Recorded in Plagioclase Overgrowth Rims from the Skaergaard Intrusion. Journal of Petrology, 50 (1). pp. 127-145. ISSN Print ISSN 0022-3530 - Online ISSN 1460-2415 DOI 10.1093/petrology/egn076

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Abstract

The intercumulus liquid of a crystal mush fills pore spaces, and typically solidifies to form overgrowths on cumulus grains and poikilitic post-cumulus minerals. If the liquid is immobile, solidification produces zoned intercumulus minerals, as a result of progressive fractionation of the residual liquid. Convection within the mush results in buffering of the liquid composition, and thus limits mineral zonation. For fully solidified cumulates, fossil' changes in liquid composition or porosity are difficult to identify. However, detailed study of immobile minor components of plagioclase overgrowth rims can provide information about the progressive solidification of intercumulus material. Ti contents of plagioclase overgrowths, in samples from the lowermost parts of the Skaergaard Intrusion, show strong variations with anorthite content. With decreasing XAn, Ti concentrations first rise and then fall, consistent with changing TiO2 contents of the intercumulus liquid during solidification. TiO2 in plagioclase decreases sharply at An55, reflecting local saturation of Fe-Ti oxides. Ti in clinopyroxene oikocrysts also falls rimward, but zoning in faster diffusing species (Fe, Mg) is limited. Other than slight reverse zones that may occur on the plagioclase margins, XAn falls continuously during crystallization. The reverse zoning is interpreted as the result of compaction-driven dissolution and reprecipitation of plagioclase. The continual decrease in XAn is exploited, together with back-scattered electron images of the cumulates, to produce calibrated images showing regions of progressive crystallization. This allows the regions crystallizing at each stage of solidification to be visualized. These images show that the final remnants of interstitial melt were present in triangular pockets and as thin grain-boundary melt films. This approach can provide information about the progressive reduction of porosity during cumulate solidification.

Item Type: Article
Uncontrolled Keywords: 08AREP;
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title: Journal of Petrology
Volume: 50
Page Range: pp. 127-145
Identification Number: 10.1093/petrology/egn076
Depositing User: Sarah Humbert
Date Deposited: 28 May 2009 15:00
Last Modified: 23 Jul 2013 09:55
URI: http://eprints.esc.cam.ac.uk/id/eprint/1026

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