A re-examination of hydrothermal alteration and Fe-Ti oxide relationships in the Skaergaard Layered Mafic Intrusion, East Greenland

Abstract

Fractionation between Fe isotopes in igneous systems and hydrothermally-altered rocks is small but significant, and detailed petrographic understanding of rocks is needed to discern the origin of such fractionations. One hypothesis proposed to explain the causes of Fe isotope fractionation in igneous rocks involves hydrothermal alteration. Movement of hydrothermal fluids through the system could result in removal of light Fe isotopes, leaving a heavier Fe isotope composition in the rocks. Previous oxygen isotope ([delta]18O) analyses of bulk-rocks, pyroxene, and plagioclase indicate that the Skaergaard intrusion hosted its own hydrothermal system. To provide petrographic context to interpret bulk-rock, bulk-mineral, and in-situ Fe isotope compositions (of Fe-Ti oxides), this study characterizes hydrothermal alteration via oxygen isotope geochemistry of rocks and minerals and the nature of fine-scale exsolution relationships among Fe-Ti oxides in the Skaergaard intrusion. Petrographic, SEM-EDS, TEM, and XRD analyses show exsolutions of ilmenite, hematite, and spinel of different sizes and shapes within Ti-rich magnetite and ilmenite. Characterization of the textural relationships show the following associations: 1) discrete magnetite and ilmenite crystals (0.2-1 mm) in mutual contact, in which ilmenite exsolved from original titanomagnetite or crystalized separately; 2) very thin (1 [micro]m) exsolutions of hematite within ilmenite; 3) coarse (~50 [micro]m) triangular exsolutions of ilmenite within magnetite; 4) very-fine scale (0.1[micro]m), box-like exsolutions of Fe-Ti oxide within titanomagnetite visible only via SEM at 24,000x magnification; 5) very fine-grained (0.5-5 [micro]m) exsolutions of a transparent mineral phase. TEM analysis indicates that the very fine, box-type exsolutions within magnetite are secondary ilmenite containing small amounts of Mg, Al, and Mn, whereas thicker, triangular lamellae are early ilmenite close to pure Ti-Fe oxide in composition. Very fine-grained transparent phases correspond to Fe-Mg-Zn spinel ([Fe, Mg, Zn]Al2O4). Complex, fine-scale exsolutions complicate in-situ analysis of Fe-Ti oxides and mask variations in isotope and major- and trace-element contents. Mass-balance calculations and simple binary-mixing modeling of iron isotope compositions ([delta]56Fe) of magnetite and ilmenite and various mixtures of these oxides indicate that previously reported Fe isotope compositions measured in-situ in magnetite, as well as in bulk-magnetite separates, are unlikely to represent pure magnetite, and instead reflect a mixture of magnetite and ilmenite in varying proportions. New oxygen isotope compositions obtained by traditional laser-fluorination mass spectrometry are presented for bulk-rock gabbro and ferrodiorite (n=35) and bulk-mineral separates for the same rocks for olivine (n=10), pyroxene (n=23), Fe-Ti oxides (n=29), and amphibole (n=5) from the least to most hydrothermally altered layers of the Skaergaard intrusion. Bulk-rock [delta]18OVSMOW values range from -0.03±0.08[per mile] to 5.5±0.14[per mile] (1SD). Mineral separates have a range of [delta]18O from -0.61±0.26[per mile] to +5.29±0.16[per mile] for pyroxene, from +4.47±0.26[per mile] to +4.77±0.13[per mile] for olivine, from -3.13±0.16[per mile] to +3.66±0.16[per mile] for Fe-Ti oxides (magnetite-ilmenite-spinel), and from -0.12±0.13[per mile] to +1.91±0.13[per mile] (2SD) for amphibole. Inter-mineral oxygen isotope fractionation factors between olivine and pyroxene range from -0.09 to 0.28[per mile] and yield temperatures of 900-1200 [degrees] C. Fractionation factors for olivine-oxide and pyroxene-oxide pairs range from 1.19 to 1.82[per mile] and 0.45 to 2.13[per mile], respectively, and reflect hydrothermal alteration at temperatures of 700-800°C, and 800-1000°C respectively.