Kiglapait Mineralogy II: Fe-Ti Oxide Minerals and the Activities of Oxygen and Silica

Cumulus titanomagnetite and subordinate ilmenite first appearin the Upper Zone of the Kiglapait intrusion. They arrive graduallyand then reach abnormal abundances before falling to a sustainedcotectic mode near seven volume per-cent. The most Ti-rich titanomagnetites(to Usp ₆₆) are preserved in ore bands (layers) which solidifiedby adcumulus growth leading to the complete expulsion of interstitialsilicate liquid. Analyses from three of these ore bands, appliedto the solution model of Lindsley (1977), form a single lineararray in f_o2 versus 1/T ?K, with log f_o2 = (–28,283 ?89)/T + 11.03 ? 0.25. This array implies log f_o2 = –9.65at 1094 ?C, the model temperature of the Main Ore Band, consistentwith primary mineral compositions Ilm₈₉, Usp₇₉ and a weightmode of 18 per cent ilmenite. Silicate rocks yield another linear array, i.e. log f_o2 = (–37,910? 102)/T + 22.57 ? 0.61. This array is ascribed to closure ofsubsolidus reactions from initial compositions near Usp_79–80Ilm₉₀. The center of gravity of the data falling on this arraysuggests a primary mode of about 50 per cent ilmenite for thesilicate rocks, implying somewhat more reducing conditions ofcrystallization than for the ore bands. The modal overproductionrepresented by the ore bands is attributed to super-saturationin oxygen, which is demonstrated by the Al-depleted compositionsof titanomagnetite in ore bands, by direct evidence for elevatedf_o2 at the top of the Main Ore Band, and by abnormally magnesiansilicate mineral compositions in and near the ore bands. The primary titanomagnetite composition for average rocks isestimated at Usp₈₀ for the base, and Usp₇₃ for the top of theUpper Zone, from rock and mineral chemistry and observed textures.The idealized magma path for the Upper Zone runs from (T andlog f_o2) 1154 ?C, –9.0 to 960 ?C, –12.2. The orebands lie above this path and are interpreted as lying on themetastable extension of the Lower Zone path, which originatesat 1250 ?C, –8.1, on the WM buffer at 4 kbar total pressure. Silica activity is estimated from mineral compositions nearthe ore bands as applied to the FMQ equilibrium, and mappedfor the Lower Zone by an adjustment downward from the En-Fo-Silequilibrium, with resultant values near 0.55 relative to quartz= 1.0. The logarithmic oxygen/silica activity ratio (OSAR) coincideswith that of the Skaergaard intrusion in the Lower Zones. TheSkaergaard OSAR is offset downward from the Kiglapait trendduring MZ time, and remains below it at the end of crystallization.The more highly silicated Skaergaard magma was initially moreoxidized than the Kiglapait magma, but this relation was reversedafter the loss of olivine in the Skaergaard intrusion, as couldhave been predicted from theory and the mineralogy of the twointrusions.