The compositional evolution of differentiated liquids from the Skaergaard Layered Series as determined by geochemical thermometry
A. A. Ariskin
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 1085o 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.
Citation: Ariskin, A. A. (2003), The compositional evolution of differentiated liquids from the Skaergaard Layered Series as determined by geochemical thermometry, Russ. J. Earth Sci., 5, No.1, 1-29, doi:10.2205/2003ES000115.
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