Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust

Namur, O. and Charlier, B. and Pirard, C. and Hermann, J. and Liégeois, J.-P. and Auwera, J.V. (2011) Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust. Geochimica et Cosmochimica Acta, 75 (17). pp. 4998-5018. DOI 10.1016/j.gca.2011.06.013

[img] PDF
Anorthosite_O_Namur_GCA_.pdf
Restricted to Registered users only

Download (2MB)
Official URL: http://www.sciencedirect.com/science/article/pii/S...

Abstract

The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100-500m-thick) and (2) as cm- to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe-Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An68-An60), trace elements (Sr: 1023-1071ppm; Ba: 132-172ppm) and Sr isotopic ratios (87Sr/86Sri: 0.70356-0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An60-An38). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75g/cm3 during early fractionation of troctolites and then decreased continuously to 2.16g/cm3 with fractionation of Fe-Ti oxide-bearing gabbros. Plagioclase (An69-An60) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (<An60) became negatively buoyant and anorthosite stopped forming. Blocks of anorthosite (autoliths) even fell downward to the basal cumulate pile. The presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and/or minor plagioclase nucleation within the main magma body. Dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid.The processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust, with (gabbro-)noritic rocks at the base and anorthositic rocks at the top. We propose that the lunar crust mainly crystallized bottom-up. This basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian-suite without KREEP contamination, while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust. © 2011 Elsevier Ltd.

Item Type: Article
Uncontrolled Keywords: NILAREP
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title: Geochimica et Cosmochimica Acta
Volume: 75
Page Range: pp. 4998-5018
Identification Number: 10.1016/j.gca.2011.06.013
Depositing User: Sarah Humbert
Date Deposited: 23 Mar 2012 15:00
Last Modified: 23 Jul 2013 10:03
URI: http://eprints.esc.cam.ac.uk/id/eprint/2411

Actions (login required)

View Item View Item

About cookies