| Abstrakt: |
We present a systematic examination of the volatile (Cl, F, Br, S) and trace element (Na, Mg, V, Mn, Fe, Sr, Y, Zr, Ba, REEs, Th, U) abundances in apatite from 29 samples from base to top of the Layered Series in the Skaergaard intrusion, East Greenland. Apatite occurs as an interstitial phase in the lower zones, but joins the liquidus assemblage late in the crystallisation history (c. 90% crystallised), defining the base of Upper Zone b. The apatite can be classified as fluorapatite with low Cl/F ratios (< 0.4), confirming previous result. The REE content of interstitial apatite decreases from the base up to the middle part of the intrusion, followed by an increase towards the base of Upper Zone b. When apatite becomes a primocryst (liquidus phase) it shows significant difference in composition, including elements such as REEs + Y, Cl, Sr, and Eu compared to interstitial apatite. Whilst Sr and Eu are enriched in primocryst, the other REEs + Y and Cl are depleted compared to interstitial grains. Whilst the REE composition of primocryst apatite can be explained by fractional crystallisation of the Skaergaard parental magma, the peculiar trend and enrichment of REEs in interstitial apatite can be reproduced using a simple mass balance model (i.e. a simple mass balance equilibrium model where the bulk concentration of an element is evenly distributed between all phases in the final cumulate rock according to the respective partitioning coefficients). In this regard, the difference in composition between interstitial and primocryst apatite is explained by different formation environments (i.e. crystallising (late) from trapped liquid vs. main magma). Compared to primocryst apatite, interstitial apatite in Skaergaard is more REE (and Cl) rich due to crystallisation and continued reaction with a highly fractionated trapped interstitial melt. During crystallisation and continued reaction with the interstitial melt and sub-liquidus equilibration, Sr and Eu partition into plagioclase, depleting interstitial apatite in Eu and Sr compared to primocryst apatite. The mass balance model demonstrates that the REE content of interstitial apatite can be attributed to partitioning competition within the present mineral assemblage during crystallisation of the interstitial melt and diffusive re-equilibration. In this regard, the proportion of trapped interstitial melt relative to the proportion of different primocryst minerals is crucial. It is shown that especially the modal abundance of clinopyroxene exerts strong control on the distribution of REE in interstitial apatite. [ABSTRACT FROM AUTHOR] |
Full text from SpringerLink