Joshua Moser Feinberg

Exsolved magnetite inclusions in silicates: Features determining their remanence behavior

Submicroscopic, needle-shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts, making them excellent candidates for paleomagnetic studies. A suite of samples containing clinopyroxene- and plagioclase-hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions magnetic domain state. Alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others. The two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions. Magnetite-ulvospinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103–105 magnetite prisms separated by thin ulvospinel lamellae. The conversion of multidomain-sized needles into assemblages of interacting single domains increases the coercivity (and hence relaxation time) of the inclusions, and results in a thermochemical magnetic remanence. In samples without this exsolution microstructure, the inclusions diameters determine coercivity and their magnetization is thermoremanent. Both styles of high-coercivity inclusions successfully record paleomagnetic directions in Mesozoic rocks, and their ubiquity within silicate minerals (clinopyroxene and plagioclase) of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field, marine magnetic anomalies, and crustal kinematics.

Oriented inclusions of magnetite in clinopyroxene: Source of stable remanent magnetization in gabbros of the Messum Complex, Namibia

[1]xa0Crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the Early Cretaceous Messum Complex, Namibia. Rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single-domain magnetite, consistent with the observed sizes and shape anisotropy of the magnetite inclusions. As in previous studies of similar inclusions, these are inferred to have formed by exsolution. Two arrays of inclusions are regularly present in the Messum clinopyroxenes, inclined at about 74°, consistent with formation at about 800°C deduced from optimization of phase boundary orientations. Virtual geomagnetic poles from these rocks are consistent with reference data, confirming that the magnetization is of thermoremanent origin. Bipolar magnetizations are recorded at one site as well in individual clinopyroxene crystals, suggesting that remanence acquisition upon initial cooling of the gabbro spanned a geomagnetic polarity reversal.

Epitaxial relationships of clinopyroxene-hosted magnetite determined using electron backscatter diffraction (EBSD) technique

Abstract Crystallographic relationships between exsolved phases and their hosts are typically characterized using transmission electron microscopy (TEM) or single crystal X-ray diffraction (XRD). In this investigation, electron backscatter diffraction (EBSD) was used to determine the epitaxial relationships of exsolved laths of magnetite in clinopyroxenes from three sampling sites in the Cretaceous Messum Complex of Namibia. Two orientations of magnetite inclusions are found with their long axes subparallel to [100] and [001] of the host clinopyroxene. Inclusions subparallel to [100]c have [1̅10]m // [010]c, (1̅1̅1)m // (1̅01)c, and [112]m // [101]c. Inclusions subparallel to [001]c have [1̅10]m // [010]c, (111)m // (100)c, and [1̅1̅2]m // [001]c. The EBSD-derived orientation relationships agree well with previous TEM and XRD studies on similar materials. The crystallographic relationships obtained with EBSD are used in conjunction with optimal phase boundary theory to determine the exsolution temperature of the magnetite inclusions, which is of importance to paleomagnetic studies. For one sample, this temperature (840 ± 50 °C) can be compared with that (865 ± 25 °C) derived from a more widely used cation exchange geothermometer. Thus it appears clear that exsolution occurred well above the Curie temperature of pure magnetite (580 °C).

Early Triassic Ophiuroids: Their Paleoecology, Taphonomy, and Distribution

Abstract The Permian–Triassic transition was an important time for many marine groups, including echinoderms. However, the fossil record of ophiuroids through this interval is poorly understood. Recent discoveries in Lower Triassic rocks from northern Italy and western US suggest that ophiuroids were more common during this time than previously has been appreciated. Evidence from resting traces (Asteriacites lumbricalis) indicates that Early Triassic ophiuroids lived on fine-grained siliciclastic sediments in oxygenated, shallow-marine environments within storm wave base. No resting traces have been recorded from deeper and/or oxygen-restricted settings. However, following death and decay, ophiuroid skeletal elements sometimes were transported into offshore, low-oxygen environments. All known occurrences of Early Triassic ophiuroids fall within the paleotropics, but this is attributed to sampling bias. Articulated body fossils and their trace fossils are most common in latest Lower Triassic (Olenekian) rocks, but disarticulated ossicles occur throughout the Induan and Olenekian. At times and places, ophiuroids were the dominant marine benthos, carpeting the substrate in prodigious numbers. However, at other times, they also comprised minor components of diverse benthic assemblages, living alongside crinoids (Holocrinus), bryozoans, bivalves, gastropods, and brachiopods. Morphological comparison with extant ophiuroids suggests that all known Early Triassic taxa were small surface-dwellers.