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 mineralmelt distribution coefficient D = 0.06 except near the top of the intrusion where D = 0.10. The calculated feldsparliquid 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 cocrystallization of apatite, and the fourth (increasing) trend can best be attributed to a triclinicmonoclinic symmetry change in the feldspar at An _{26}, 1030°C. The compound feldsparliquid distribution coefficient K_{D} for Sr/Ca bears out these deductions in detail and yields ∆G_{r} for the SrCa exchange ranging from nearly zero at the base of the Lower Zone to 26 k J/gramatom at the end of crystallization. The compound feldsparliquid 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} (SrCa) = 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 feldsparliquid 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.
 Publication:

Geochimica et Cosmochimica Acta
 Pub Date:
 February 1982
 DOI:
 10.1016/00167037(82)902496
 Bibcode:
 1982GeCoA..46..223M