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Trace element constraints on the differentiation and crystal mush solidification in the Skaergaard intrusion, Greenland.

Namur, Olivier and Humphreys, Madeleine C.S. (2018) 'Trace element constraints on the differentiation and crystal mush solidification in the Skaergaard intrusion, Greenland.', Journal of petrology., 59 (3). pp. 387-418.

Abstract

New major and trace element analyses of plagioclase and clinopyroxene in gabbros from the Skaergaard intrusion are used to understand the mechanisms of crystallization in the main magma body and in the crystal mush. Crystal cores show a continuous chemostratigraphic evolution in the Layered Series, with compatible elements (e.g. Cr and Ni in clinopyroxene) being progressively depleted from the bottom up, whereas incompatible elements (e.g. Sr, Ba, REE in plagioclase and clinopyroxene) become progressively enriched. We performed numerical models and showed that these trends can be explained by a simple process of fractional crystallization, except for the upper 20% of the intrusion where plagioclase and clinopyroxene trace element compositions depart from fractional crystallization trends. At this stage of magma chamber solidification, fractional crystallization becomes less efficient and is replaced by a major proportion of in situ, equilibrium crystallization. Trace elements also show significant variations in interstitial overgrowths on plagioclase and clinopyroxene cumulus crystals. They result from crystallization of the interstitial liquid in the liquid + crystal mush. However, incompatible elements, and especially REE, show a degree of enrichment in plagioclase rims (> 20 ppm Ce) that strongly exceeds the highest concentrations observed in plagioclase cores (up to 4 ppm Ce) at the top of the Layered Series. Such a strong enrichment is difficult to reconcile with a simple process of fractional crystallization of the interstitial liquid in the crystal mush, but may be related to the development of silicate liquid immiscibility in the crystal mush or to delayed nucleation of apatite. After the crystallization of plagioclase and clinopyroxene overgrowths, diffusive re-equilibration during a period of 0.1-0.25 Myr significantly changed the original zoning profiles. Ce flux from clinopyroxene into the plagioclase lattice of adjacent crystals could also have contributed to the strong Ce enrichment observed in plagioclase rims.

Item Type:Article
Full text:(AM) Accepted Manuscript
First Live Deposit - 14 March 2018
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1093/petrology/egy032
Publisher statement:This is a pre-copyedited, author-produced version of an article accepted for publication in Journal Of Petrology following peer review. The version of record Namur, Olivier & Humphreys, Madeleine C.S. (2018). Trace element constraints on the differentiation and crystal mush solidification in the Skaergaard intrusion, Greenland. Journal of Petrology is available online at: https://doi.org/10.1093/petrology/egy032.
Record Created:14 Mar 2018 13:28
Last Modified:02 Apr 2019 09:21

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