KIGLAPAIT MINERALOGY .1. APATITE, BIOTITE, AND VOLATILES

Electron microprobe analyses show that Upper Zone apatites in the Kiglapait intrusion are fluorine rich and contain minor chlorine and hydroxyl (calculated). Apatite from the Outer Border Zone has a higher Cl content.The refractive indices of UZ apatites have the following ranges: ω = 1.6345-1.6379, ω = 1.6326-1.6352, and B = 0.0020-0.0028. The birefringence is low for apatites with these refractive indices. Some Outer and Upper Border Zone apatites have higher indices of refraction and normal birefringence.Fractional crystallization of the basaltic Kiglapait magma produced cumulus apatite beginning at the 94 per cent solidified level when P2O5 reached saturation in the liquid. The amounts of P2O5 and modal apatite decreased gradually from the 94 per cent to the 99.99 per cent solidified level as the liquid was depleted in P2O5. F and Cl appear to be equally partitioned between the liquid and apatite because no fractionation trends are noted between the two halogens. There is a slight decrease in the calculated ratio OH/F in apatite which suggests possible depletion of OH in the liquid with fractionation.Kiglapait apatites appear to be stoichiometric, based on microprobe chemistry, refractive index, and unit cell dimensions. However, infrared absorption analyses show no detectable water, which leaves approximately 11 per cent of the monovalent anion site unaccounted for. Anion deficiencies in apatites from low-H2O environments may be explained either by substitution of O for F, or domains of tetracalcium phosphate.Non-cumulus biotite occurs in minor quantities in the intrusion. Electron microprobe analyses of Upper Zone biotites show that they contain an average (by weight) pf 0.4 per cent F, 0.07 per cent Cl, and 4.0 per cent H2O (calculated).The volatile chemistry of the Kiglapait intrusion is calculated from apatite and biotite chemistry. The intrusion contains an estimated 900 ppm P2O5, 166 ppm F, and 12 ppm Cl. There is a maximum of 68 ppm H2O using calculated H2O from microprobe data, or a minimum of 8 ppm using H2O from infrared analysis. It is proposed that the anhydrous basaltic Kiglapait magma was a second partial melt of amphibole-bearing mantle rock. © 1979 Oxford University Press.