Apatite in mafic rocks

Apatite is a widespread accessory mineral in xenoliths of Phanerozoic lithospheric mantle brought to the surface by volcanic processes. It is important in understanding the mantle residence of volatiles such as Cl and F further, it is a major host for REE and some LILE (such as U, Th, Sr) in the mantle (O Reilly and Griffin 2000). 

The apatite in mantle rocks studied by O Reilly and Griffin (2000) include an unusual carbonate-bearing (0.7-1.7 wt %) hydroxyl-chlorapatite (1.5-2.5 wt % Cl, 6-40 ppm Br, 1 wt % F) associated with C02-rich fluid inclusion-bearing veins in mantle Iherzolites (from both Australia and Alaska), and also a hydroxyl-fluorapatite variant that is igneous in origin. The former occurs in metasomatized mantle wall-rock peridotites. 

A number of workers have stressed the important role of Cl, F and H2O in magmatic/hydrothermal processes, and the role of apatite in aiding in the determination of both the fugacities and concentrations of the halogens and their related compounds. We would like to emphasize some salient points about apatite chemistry in magmatic and hydrothermal systems  

1) For metaluminous systems, apatite crystallizes relatively early in the crystallization sequence of felsic magmas, and crystallizes comparatively later in more mafic melts. For example, it was estimated that apatite is a near liquidus phase in the Johnson Granite Porphyry, but began to crystallize after about 1/3 of the Kuna Crest (quartz diorite) magma crystallized (Piccoli and Candela 1994). Ultimately, apatite often first appears as an intercumulus phase in gabbros and related rocks (e g., Boudreau et al. 1986). 

2) If Cl, F and H2O were to behave as perfectly incompatible substances and therefore increase in concentration in the melt, then the ratio of Cl, F and OH in successive aliquots of apatite will be constant as crystallization progresses (Candela 1986), if the effect of temperature is neglected. 

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