RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 5, NO. 1, PAGES 1–29, doi:10.2205/2003ES000115, 2003

The compositional evolution of differentiated liquids from the Skaergaard Layered Series as determined by geochemical thermometry

Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia

Abstract

[1]  Based on the COMAGMAT-3.65 crystallization model a set of phase equilibria calculations (called geochemical thermometry) have been conducted at P =1 kbar and closed conditions with respect to oxygen for 65 rocks representing the principle units of the Layered Series of the Skaergaard intrusion. It allowed us to define the range of initial temperatures (1145 to 1085^o C) and oxygen fugacities (1-1.5 log units above QFM to slightly below QFM ) of the original crystal mush from which the rocks from LZa to UZa crystallized. In parallel, average major-element compositions of residual (interstitial) liquid were calculated demonstrating a trend of continual enrichment of FeO (up to ~18 wt.%) and TiO ₂ (up to ~5.5 wt.%) with only minor variations in the SiO ₂ contents (48 to 50 wt.%). Projection of the compositions onto the OLIV - CPX - QTZ diagram provides evidence that most of the Layered Series crystallized on the Ol - Pl - Cpx -oxide cotectic. Systematic differences between the calculated residual liquid compositions for LZa/LZb and LZc to UZa (which are unlikely to reflect fractional crystallization) are within the accuracy of the COMAGMAT model, but may be also indicative of a late-stage process involving migration and re-equilibration of interstitial liquids. Estimated amounts of interstitial melts trapped in the Skaergaard "cumulates" range around 50 wt.%. Wager's compositions inferred from simple mass-balance were found to lie too far from the Ol - Pl - Cpx boundary to represent a realistic approximation of the low-pressure Skaergaard magma evolution. The main problem of genetic interpretations of the Skaergaard intrusion is a strong misbalance between the parental compositions followed from contact rocks and the results of geochemical thermometry and that of the whole differentiated body. It is most apparent for TiO₂ and P₂O₅ which are almost twice as high in the average intrusion composition compared to the proposed parents. Moreover, the intrusion composition has of 2-4 wt.% less SiO₂ and much more iron. One possible explanation is to assume the Skaergaard magma came to the chamber with an amount of crystals ( Ol+Pl ) equilibrated with the calculated parental liquid. However, even if some amount of "hidden" troctolitic material exists, it is unlikely that crystallization in a closed system could produce large volumes of rocks rich in Fe-Ti oxides without complementary more felsic differentiates.

Received 5 January 2003; published 22 January 2003.

Keywords: COMAGMAT, crystallization model, evolution of differentiated liquids, geochemical thermometry.