Oriented Exsolution of Fe-Ti Oxides in Augite from the Kiglapait Intrusion

Augite is the black mineral that defines the Upper Zone of the Kiglapait Intrusion (KI), Labrador, at the 84 percent solidified (PCS) level in the field. It is black because it is heavily charged with opaque lamellae of titanomagnetite in the nominal a and c directions in (010), and with thin ilmenite lamellae in the b direction in (100). Chemically, however, the augite crystals are no richer in Ti or Fe3+ than similar unexsolved augites in the smaller Skaergaard Intrusion; the exsolution is a result of very slow cooling and annealing at very high temperatures, and it occurred concomitantly with exsolution of Opx in (100) and Pig in (001). All references to the indices and crystallographic directions here imply the relevant optimal phase boundary (e.g., Robinson et al. 1971 Am Min 56: 909; Feinberg et al. 2004 Am Min 89: 462), not necessarily the precise axial direction. We designate as "A plates" and "C plates" the cloth-textured, exsolved Mt-Usp (spinel) objects nominally parallel to a and c respectively, and as "B plates" the thin, translucent brown ilmenite lamellae in (100). Styles of exsolution vary with stratigraphic height (hence composition) in the intrusion. Densely-crowded C plates from 84-94 PCS give the crystals a hairy appearance. The equal abundance of A and C plates above 94 PCS characteristically shows the nominal beta crystallographic angle. The orientations are such that the anion nets in spinels and augites are in register. The Usp content of spinels decreases from 60 mole percent at lower levels of the stratigraphy to 0-30 mole percent at uppermost levels. Pyroxene exsolution lamellae are commonly Opx in (100) to 93.6 PCS, where the augite Mg ratio is En66. Thin pigeonite lamellae first appear at 91.2 PCS and persist to the end of crystallization. The permanent switch from Opx to Pig lamellae at 93.6 PCS, En66, is similar to the switch in external grains of the Skaergaard Intrusion at En69. The switch is evidently a function of T and bulk composition. An exsolution origin of the spinel and ilmenite plates is shown by both chemical and textural evidence. Unexsolved Skaergaard augites even have somewhat higher Ti and Fe3+ and are also olivine-normative. Texturally, minute oriented oxide plates are visible wholly within (00l) Pig lamellae in the KI and could not have been produced by extrinsic oxidation unless that selectively affected the lamellae. Additional compositional evidence resides in the bulk metal:oxygen ratio of the pyroxenes, which are olivine-normative and deficient in oxygen by 1 atom in about 416; the 3:4 oxide component is intrinsic, as in all Fo-saturated experimental diopsides. The KI augite series is accompanied by olivine, without any external Ca-poor pyroxene, throughout the composition range En73 to En0, and the Aug-Ol pair allows retrieval of the silica activity, via QUILF equilibria, varying from a(SiO2) 0.59 to 0.98 over the range 94-99.985 PCS. The A and C plates of Ti-Mt carry a strong and stable remanent magnetization. Hysteresis measurements indicate the bulk response of these exsolutions is of single domain nature with ratios of Mrs/Ms of 0.54, and Hcr/Hc of 1.5. Bulk coercivity is very high at 65 mT. Low temperature remanence measurements show a pseudo-single-domain component with a significant remanence loss at the Verwey transition (Tv). Upon warming through Tv, remanence increases, indicating there may be exchange coupling between Usp and Mt. Low temperature frequency of susceptibility measurements show little dispersion indicating that the exsolved titanomagnetites are above the superparamagnetic threshold.