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

Abstract

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 • C) and oxygen fugacities (1-1.5 log units above QF M to slightly below QF M) 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-CP X-QT Z diagram provides evidence that most of the Layered Series crystallized on the Ol-P l-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-P l-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 TiO 2 and P 2 O 5 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 SiO 2 and much more iron. One possible explanation is to assume the Skaergaard magma came to the chamber with an amount of crystals (Ol + P l) 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.

Petrology of the Skaergaard Layered Series

“…suggested that the Skaergaard magma was filled by several early pulses of plagioclase and olivine-bearing magma eventually leading to a ballooning of the chamber to its final size. This is consistent with the observation by Ariskin (2003) that the initial magma contained plagioclase and olivine phenocrysts.…”

“…Forward modelling results of Toplis & Carroll (1996) and Thy et al (2006Thy et al ( , 2008Thy et al ( , 2009a) largely recorded the abovequoted experimental temperatures for the respective initial melt compositions, but were unable to constrain the terminal liquidus temperature. Ariskin (2002Ariskin ( , 2003) used a crystallisation model based on existing experimental low-pressure information for basaltic systems to constrain the temperature variation in the Skaergaard intrusion. His conclusion was that the initial magma filling the chamber was olivine and plagioclase-phyric, which in the LZa had equilibrated at 1145°C.…”

Pearce Element Ratio Diagrams and Cumulate Rocks

“…The points to be compared, then, to the simulated patterns are the weighted means of the coeval units. Pearce element ratios for the three sets of residual melts: (Chayes 1970 algorithm;McBirney 1996;Ariskin 2003) can be compared on Fig. 43.7.…”

“…McBirney's (1996) estimates for the compositions of the residual melts at the end of LZa, LZc, MZ, UZa, and UZb do not fit the expected pattern in that they plot up-slope from their respective cumulate compositions. All of the points representing the residual melt compositions estimated by Ariskin (2003) plot down-slope from the points McBirney (1996) and Ariskin (2003), and points calculated with Chayes (1970) algorithm representing their respective cumulate compositions as do residual melt compositions calculated with Chayes' (1970) algorithm. Only the latter however, fall in sequential order, a pattern expected for a series of melts formed by fractionation of a single initial magma.…”

Trace element variation in olivine in the Skaergaard intrusion: petrologic implications

“…The Skaergaard, therefore, is one of the best locations to study how liquids and coexisting mineral phases change as the differentiation of a magma proceeds (McBirney, 1975(McBirney, , 1995McBirney and Naslund, 1990;Thy et al, 2006;Thy et al, 2008;Thy et al, 2009). Although the Skaergaard intrusion has been the subject of an extensive amount of publication, there is still debate about some of the most basic conclusions drawn by Wager and his coworkers (Wager and Deer, 1939;Wager and Brown, 1967), including: (1) the compositional trend of differentiation of the Skaergaard magma Sparks, 1987, 1990;Brooks and Nielsen, 1990;McBirney and Naslund, 1990;Morse, 1990;Toplis andCarroll, 1995, 1996;Loucks, 1996;Tegner, 1997Ariskin, 2003;Jakosen et al, 2005;Toplis, 2005;Thy et al, 2006;Tegner et al, 2008;Thy et al, 2008;Thy et al, 2009); (2) whether the intrusion remained as a open or closed system during differentiation Sparks, 1987, 1990;Stewart and DePaolo, 1990;McBirney, 1998McBirney, , 2002Holness et al, 2007a); and (3) whether the magma convected during crystallization or was stagnant once crystallization started (Huppert and Sparks,1984;Marsh, 1988Marsh, , 1989Holness et al, 2007b). Part of the reason for the continuing debate is the inherent nature of the rocks, which are mixtures of early-formed minerals and trapped melt.…”