GP21B-0155 INVITED 0800h
Plagioclase-Hosted Magnetite Inclusions From the Bushveld Complex
Gabbros from the Main Zone of the 2.064 Ga Bushveld Complex have long been known to possess unusually stable magnetizations due to the presence of high coercivity, exsolved magnetite inclusions in plagioclase and clinopyroxene. The paleomagnetic pole for these rocks has been used to anchor apparent polar wander paths for the Kaapval craton during the Early-Mid Proterozoic. To better understand the rock magnetic properties of silicate-hosted magnetite inclusions, oriented paleomagnetic samples of gabbro were collected from quarries near Belfast and Rustenberg, South Africa, sampling the eastern and western limbs of the Complex, respectively. Plagioclase composition at both sites ranges from An$_{55}$ (rims) to An$_{65}$ (cores) based on optical and electron microprobe data. Four kinds of inclusions are present within the plagioclase: elongate magnetite needles, nanometer-scale magnetite particles (responsible for the "cloudy" appearance of some crystals), translucent brown hematite/ilmenite platelets, and colorless euhedral inclusions of pyroxene and/or feldspar. Magnetite inclusions are most abundant at the cores of the plagioclase crystals. Orientations of the needles and the platelets are crystallographically controlled by the silicate host. Although the elongation direction of the magnetite inclusions can occur in any of five possible orientations, only two or three of these directions dominates each plagioclase crystal. Alternating field demagnetization of bulk samples (NRM = 1.5 x 10$^{1}$ A m$^{-1}$) shows univectorial remanence with average median destructive fields (MDF) of 115 mT (Belfast) and 90 mT (Rustenberg). AF demagnetization of single plagioclase crystals (NRM = 100 A m$^{-1}$) also shows single component remanence with average MDFs $>$150 mT. The NRM coercivity spectra of single plagioclase crystals are indistinguishable from that of the bulk samples. When normalized to their abundance in bulk samples the magnetite-bearing plagioclase fully accounts for the NRM of Bushveld gabbros at both sites. Close examination of the inclusions' interiors using magnetic force microscopy shows no ulvöspinel exsolution as observed in other silicate exsolved titanomagnetites with comparably high coercivities. Consequently, we interpret the high coercivities of the inclusions to be a product of their small size and extreme shape anisotropy. Single crystals of plagioclase demonstrate a strong anisotropy of IRM acquisition (see Scott, et al. this conference). Additionally, electron backscatter diffraction (EBSD) orientation indexing shows a strongly preferred orientation for plagioclase and pyroxene (with (010)$_{plag}$ and (100)$_{pyr}$ parallel to subhorizontal layering) consistent with gravitational settling within a magma chamber. Thus, there are two anisotropies (silicate preferred orientation and magnetite inclusion remanence) to consider when describing the ancient magnetic field present during the emplacement of the Bushveld.
GP21B-0156 INVITED 0800h
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.
GP21B-0157 INVITED 0800h
Magnetic properties of natural and synthetic olivines: high-field measurements
Olivine [(Fe$_{x}$, Mg$_{1-x}$)$_{2}$ SiO$_{4}$] is an orthosilicate solid solution between fayalite [Fe$_{2}$ SiO$_{4}$] and forsterite [Mg$_{2}$ SiO$_{4}$]. Olivine is a major constituent of the Earth mantle that is abundant in oceanic and continental peridotites and mantle xenoliths. The magnetic properties of olivines have been previously investigated using gem quality natural crystals known as peridots (Zabargad) or using laboratory grown synthetic crystals. Magnetic investigations are generally performed using low magnetic field or neutron diffraction techniques. Optical microscopy and TEM imagery reveal that most olivine crystals host iron oxides formed by exsolution during cooling. Theoretically, the magnetic susceptibility of olivine should decrease linearly from fayalite to fayalite as a function of the Fe content. The magnetic behavior should range from antiferromagnetic at high Fe content, paramagnetic at intermediate Fe contents and diamagnetic at very low Fe contents. New magnetic measurements, performed on various high field instruments (vibrating sample magnetometer, torque magnetometer, cantilever magnetometer), both on natural and synthetic samples, display ferromagnetic behavior, interpreted as due to the systematic presence of titanomagnetite inclusions in olivine crystals. These results emphasize the need to conduct measurements in high field in order to isolate the intrinsic paramagnetic properties of olivines. These measurements demonstrate the orthorhombic nature of the intrinsic paramagnetic properties, but also yield new data concerning the relationship between crystallographic axes, magnetic anisotropy and other physical anisotropies: [100] = K$_{1}$, [010] = K$_{2}$ and [001] = K$_{3}$. Preliminary results also indicate substantial variations in degree of paramagnetic anisotropy (P) and paramagnetic shape factor (T). For Fo$_{92}$, P = 1.359 and T = -0.845. These intrinsic paramagnetic properties are used to model the magnetic behavior of olivine across a range of temperatures relevant to planetary exploration. They are also used to evaluate the possible effects of olivine deformation on its intrinsic properties. For example, deformation-induced striped iron zoning is anticipated to strongly modify crystal magnetic anisotropy.
GP21B-0158 INVITED 0800h
Significance of Elevated Contents of $\Box$Si$_{4}$O$_{8}$ and AlAl$_{3}$SiO$_{8}$ End Members in Black Clouded Plagioclase From Anorthosite: Evidence for Oxidation-Induced Postmagmatic Exsolution of Fe-Ti Oxide Inclusions From Plagioclase
Plagioclase phenocrysts from large Proterozoic massif anorthosites that have escaped metamorphic recrystallization commonly have a distinctive black color that is attributed to myriads of minute Fe-Ti oxide inclusions. These inclusions are responsible for the black color of the feldspar. The orientation of these inclusions along specific crystallographic directions in the plagioclase is strong evidence that these formed by exsolution from the plagioclase. Plagioclase from the Proterozoic Lac Saint-Jean anorthosite in Qu$\'{e}$bec is a good example of such clouded feldspar. In this plagioclase we recognize four types of Fe-Ti oxide inclusions: 1) Myriads of fine, submicroscopic and unidentifiable dust-like inclusions, presumably Fe-Ti oxides, and responsible in great part for the black color. 2) Small $\sim$1 $\mu$m sized crystals of hemo-ilmenite that occur in regions of the plagioclase that are free of type 1 dust, suggesting that these originated by Ostwald ripening from the dusty material. 3) Larger aligned and acicular crystals of hematite, $\sim$10 $\mu$m in diameter and with good rhombohedral sections. These occur, like type 2 inclusions, in zones free of type 1 dust suggesting again that they result from Ostwald ripening. 4) Millimetric inclusions of ilmenite or magnetite of magmatic origin, often rimmed by amphibole or biotite, signs of late hydration. As part of our study, EPMA analyses of plagioclase from Lac Saint-Jean were done. In addition, a large single crystal, $\sim$30 cm in diameter was analyzed by bulk XRF. Our results show that plagioclase contains significant amounts of the $\Box$Si$_{4}$O$_{8}$ (up to 6.6 mole $%$) and AlAl$_{3}$SiO$_{8}$ (up to 2.2 mole $%$) endmembers. A positive relationship is observed between the content of these two endmembers. Furthermore, the ratio of $\Box$Si$_{4}$O$_{8}$ to AlAl$_{3}$SiO$_{8}$ is approximately 3:4. We believe that the high content of $\Box$Si$_{4}$O$_{8}$ and AlAl$_{3}$SiO$_{8}$ in plagioclase, and the presence of minute Fe-Ti oxide inclusions are both products of a late postmagmatic oxidation via the following chemical reactions: 1) 4Fe$^{2+}$Al$_{2}$Si$_{2}$O$_{8}$ + 4CaFe$^{2+}$Si$_{3}$O$_{8}$ + 2O$_{2}$ $\rightarrow$ 4Fe$_{2}$O$_{3}$ + 4CaAl$_{2}$Si$_{2}$O$_{8}$ + 3$\Box$Si$_{4}$O$_{8}$ 2) 8Fe$^{2+}$Al$_{2}$Si$_{2}$O$_{8}$ + 2O$_{2}$ $\rightarrow$ 4Fe$_{2}$O$_{3}$ + 3$\Box$Si$_{4}$O$_{8}$ + 4 AlAl$_{3}$SiO$_{8}$ 3) 8CaAl$_{2}$SiTiO$_{8}$ + 8Fe$^{2+}$Al$_{2}$Si$_{2}$O$_{8}$ $\rightarrow$ 8CaAl$_{2}$Si$_{2}$O$_{8}$ + 8FeTiO$_{3}$ + $\Box$Si$_{4}$O$_{8}$ + 4 AlAl$_{3}$SiO$_{8}$ These reactions indicate that significant amounts of Fe$^{2+}$ were originally present in the tetrahedral and cavity sites of the plagioclase and that Ti$^{4+}$ was also present in the tetrahedral sites. We conclude that plagioclase in the Lac Saint-Jean anorthosite crystallized under fairly reducing conditions and later underwent postmagmatic oxidation when the plutonic mass was uplifted and came into contact with crustally-derived oxygenated fluids. These findings have important implications for studies of magnetic remanence and anisotropy fabrics in anorthositic rocks.
GP21B-0159 0800h
Anisotropy of Silicate-Hosted Magnetite Inclusions
Anisotropy of magnetic properties is a hallmark of silicate crystals with oriented iron-oxide inclusions. Strongly magnetic magnetite-bearing silicates (10$^{-1}$ A m$^{-1}$) are common components of gabbros and layered intrusions, contributing to local and regional magnetic anomalies. Additionally, these iron-oxide silicates hold the promise of being exceptional paleomagnetic recorders owing to their features of: physical/chemical isolation from altering fluids, chemical equilibrium with their silicate host, and long relaxation times (enhanced coercivity). However, anisotropy of remanence must be understood before these advantageous features can be utilized. Measurements of single crystals of clinopyroxene and plagioclase (10$^{-4}$ g) show anisotropy in direction and intensity that directly reflect the crystallography of the silicate host. The host controls both the crystallographic orientation of the magnetite (magnetocrystalline anisotropy) and the elongation direction of the magnetite inclusion (shape anisotropy). We have found another source of anisotropy that involves an internal exsolution of ulv\"{o}spinel within titanomagnetite inclusions. This also reflects a host control as this second exsolution occurs along the magnetite \{100\}. This fixed wall shape anisotropy creates an array of interacting single domain magnetite parallelepipeds, parallel to \{100\}. Each of these anisotropies contributes to enhanced coercivity of remanence, which significantly exceeds the IRM saturation magnetization for magnetite (300 mT). The anisotropy of IRM (aIRM@ 1.1 T) of magnetite-bearing clinopyroxene and plagioclase shows clustering of directions, reflecting the mixture of variables that include: inclusion elongation direction and abundance, orientation of magnetite easy axes relative to the applied field, inclusion aspect ratio and diameter, and pre-existing magnetic domain structure. For pyroxene (monoclinic) with two arrays of needle-shaped magnetite inclusions, the aIRM is strongly planar, showing two sets of antipodal direction clusters. The total intensity is uniform over $\sim$60% of the remanence plane. The component of magnetization perpendicular to the applied IRM (transverse remanence) exceeds the parallel remanence in some crystallographic directions. For plagioclase (triclinic) with three to five arrays of needle-shaped magnetite inclusions (see Feinberg, et al. this conference), the aIRM is subspherical with numerous minor clusters of directions. Transverse remanence is less important than parallel remanence in plagioclase. Thus, the greater number of inclusion arrays in plagioclase decreases the severity of the IRM anisotropy relative to the two arrays in pyroxene.
GP21B-0160 0800h
Diagenetic Origin of Iron Oxides in Plagioclase
Oriented iron oxide inclusions in plagioclase are typically related to high-temperature phenomena, resulting from exsolution and/or oxidation during crystallization and cooling of igneous rocks. Empirical observations suggest that low-temperature alteration, associated with diagenesis, can account for some oriented iron oxides in plagioclase. These late oxides occur adjacent to margins and fractures in plagioclase crystals and have implications to the interpretation of remanence. Examples include plagioclase from ashflow tuffs (Bishop Tuff, California) and basalt (Parana basalt, Vargeao impact structure, Brasil).