Kiglapait geochemistry V: Strontium

Abstract

The Kiglapait intrusion contains 330 ppm Sr and has Sr Ca = 5 × 10 −3 and Rb Sr = 3 × 10 −3 , as determined by summation over the Layered Group of the intrusion. Wholerocks in the Lower Zone contain 403 F L 0.141 ppm Sr, where F L is the fraction of liquid remaining; Sr drops to 180 ppm at the peak of augite production ( F L = 0.11) and rises to a maximum of 430 ppm in the Upper Zone before decreasing to 172 ppm at the end of crystallization. Feldspars in the Lower Zone contain 532 F L 0.090 ppm Sr, increasing to 680 ppm in the Upper Zone before decreasing to 310 ppm at the end. Clinopyroxenes contain 15 to 30 ppm Sr and have a mineral-melt distribution coefficient D = 0.06 except near the top of the intrusion where D = 0.10. The calculated feldspar-liquid distribution coefficient has an average value near 1.75 but shows four distinct trends when plotted against X An of feldspar. The first two of these are strongly correlated with the modal augite content of the liquid, on average by the relation D = 1.4 + 0.02 Aug L . The third (decreasing) trend is due to co-crystallization of apatite, and the fourth (increasing) trend can best be attributed to a triclinic-monoclinic symmetry change in the feldspar at An 26, 1030°C. The compound feldspar-liquid distribution coefficient K D for Sr Ca bears out these deductions in detail and yields ΔG r for the Sr-Ca exchange ranging from nearly zero at the base of the Lower Zone to −26 k J/gramatom at the end of crystallization. The compound feldspar-liquid distribution coefficient K D for Rb Sr varies from 0.3 in the Lower Zone to 1.1 at the end of crystallization. The ratio Ca F Ca L is about 1.45 for troctolitic liquids containing 5% augite, for which K D (Sr-Ca) = 1.0 and D Ca = D Sr . For common basaltic liquids containing 20% augite, the Kiglapait data predict solSr F Sr L = 1.8 , as commonly found elsewhere. The strong dependence of D sr on augite content of the liquid illuminates the role of liquid composition and structure in determining the feldspar-liquid distribution coefficient. Conversely, a discontinuous change in the trend of D Sr when apatite arrives shows that the effect is due to apatite crystallization itself, not to the continuous variation of the liquid as it becomes enriched in apatite component.