Exceptional REE-enrichment in apatite during the low pressure fractional crystallisation of alkali olivine basalt; an example from the British Tertiary Igneous Province

The Cnoc Rhaonastil dolerite boss on Islay, NW Scotland represents a body o f alkali-olivine basalt magma which differentiated at low pressure and in s itu, from dolerite through teschenite to minor nepheline-syenite. The syeni tes occur as isolated pods and pegmatitic schlieren within the leucodolerit e, and have an exotic mineralogy including Zr-aegirine, Zr-arfvedsonite, Ca -catapleiite, zirconolite and aenigmatite. Fluor-apatite occurs as an acces sory phase in the dolerite, but becomes more abundant within the teschenite and syenites. Total REE contents within apatites in the dolerites are typi cally low (Sigma REE = 0.57-3.21 wt.% oxide), the highest REE contents occu rring in irregular, deuterically altered rims and internal patches. The REE -enriched rims also have slightly elevated SiO2 contents at 0.81-0.95 wt.%, suggesting that the substitution scheme Ca2+ + P5+ double left right arrow REE3+ + Si4+ was operating. These apatites have up to 0.08 wt.% Cl and 3.7 wt.% F, with most being almost pure end-member fluor-apatite. The majority of the teschenite apatites show the least REE-enrichment (Sigma REE = 0.27 -0.45 wt.%), coupled with low Na (<0.12 wt.%) and low SiO2 (<0.39 wt.%) con tents. However, within the syenites two distinct populations of apatite exi st. The first, most common, variety consists of unzoned, low-REE apatites ( max. 3.1 wt.% Sigma REE, again in irregular rims and patches), whereas the second variety is often complexly zoned, and has variably enriched zones up to a maximum Sigma REE content of 42 wt.%; this is by far the most REE-enr iched natural fluor-apatite so far reported from the British Isles. The REE -enriched zones are often less than 3 mum wide, and have Na content up to 5 .4 wt.% Na2O, implying that the substitution scheme Na++REE(3+)double left right arrow 2Ca(2+) dominated over the more typical scheme involving Si4+ w hich operated in the dolerites and teschenite. Other zones are either varia bly enriched in Y (up 2.1 wt.% Y2O3) or Th (up to 0.85 wt.% ThO2). However, there is no correlation between Y and REE contents, suggesting that crysta llographic factors were involved in apatite Y and REE partitioning. The REE -rich apatites have very low Cl content (<0.04 wt.%), but high F concentrat ions (<2.8 wt.%). It is believed that these strongly enriched apatites crys tallised under disequilibrium conditions from isolated, variably REE-enrich ed domains, within the fluid-rich residual syenitic magma. These domains ma y have been generated by the prior crystallisation of monazite, Ca-cataplei ite or zirconolite, which can be found as small inclusions within albite an d interstitial analcime. The dynamic process of slumping of the denser tesc henite back into the leucodolerite crystal mush is believed to have played an important role in the release of deuteric fluids and the concentration o f residual magmas.