The magnetic susceptibility (χ) and remanent magnetization of an ash flow sheet are profoundly influenced by cooling history after emplacement. Spatial variations in these properties, therefore, provide a potentially useful tool for geological studies. Maxima and minima in χ measured along profiles at Yucca Mountain, Nevada, identify persistent magnetic marker horizons within vitric portions of the Tiva Canyon and underlying Topopah Spring Members of the Paintbrush Tuff. The correlatable variations of χ with vertical position in these units exist over the 8 km of strike examined. The observed stratigraphic changes in magnetic properties reflect variations in amounts and mineralogy of Fe‐Ti oxide phenocrysts, and the presence, shape, size, and mineralogy of magnetic Fe‐oxide microcrystals that precipitated at high temperature after emplacement of each sheet. Vitrophyre near the top of the Topopah Spring Member has a distinctive maximum in χ, due to titanomagnetite phenocrysts. Susceptibility in the lower portion of the overlying Tiva Canyon Member reaches maximum values of about 1.3×10−2 (SI) near the base of the member, beneath the columnarly jointed vitrophyre, due to the presence of abundant superparamagnetic cubic Fe‐oxide. These elongate microcrystals are several tens of nm long, and are typically deformed and tapered. Up section in the Tiva Canyon Member, the susceptibility decreases, going to a minimum within the columnar vitrophyre. At this stratigraphic level the precipitated Fe‐oxides are larger and carry intense NRM of up to 30 A/m; χ is only 3×10−3. The whisker‐like microcrystals of Fe‐oxide, which typically nucleated on what appears to be chain silicate, grew as needles up to 25 μm in length. The size variations of the precipitated Fe‐oxides, which were established using transmission electron microscopy (TEM) and petrographic observation, are consistent both with variations in magnetic susceptibility measured at the outcrop and with variations in the intensity of remanent magnetization. Several interpretations of the shape anisotropy of the precipitated Feoxide are possible, including growth by a dislocation mechanism. Additionally, the observed elongation of precipitated microcrystals is consistent with theoretical predictions for growth in a uniaxial stress field. Susceptibility variations as established at the outcrop, as well as in the borehole, offer a potentially useful tool for stratigraphic correlation of ash flow sheets.

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The interpretation of regional magnetic anomalies commonly observed over continental crust in both satellite and near-

Magnetic and optical variations in volcanic glasses of the East African Rift have been investigated by means of experimental and analytical study. The results add to the formal understanding of magnetism in volcanic glasses and give us new perspectives on the nature, origin, and significance of fine-grained magnetic minerals and on the magnetic petrology of glassy rocks. The samples that we have studied in detail, glass shards from the KBS Tuff of northern Kenya, contain a superparamagnetic precipitate of microcrystals,-2 to -10 nm in size. The precipitate is likely to have formed at elevated temperature, during or subsequent to eruption, about 1.9 m.y. ago. We have studied these microcrystals by means of transmission electron microscopy (TEM), ac and dc magnetic methods, electron spin resonance (ESR), MiSssbauer spectroscopy, and optical spectroscopy. Our efforts have concentrated on determining the magnetic mineralogy and understanding the mechanism(s) responsible for the optical and magnetic behavior of these glasses. The particle size determinafion for the larger of these microcrystals has been achieved using TEM imaging and M/Sssbauer spectroscopy. The mineralogy is nonstoichiometric magnetite, based on M6ssbauer, X ray diffraction, and electron diffraction results. The ratio of saturation remanent magnetization to saturation magnetization for the larger microcrystals is very nearly that predicted by sh•gle-domain theory for a uniaxial anisotropy, the origin of which may lie with stress or surface effects. We h•terpret magnetic and ESR results to infer the possible existence of microcrystals or amorphous clusters of Fe atoms, -1 nm or smaller, in KBS glasses that appear homogeneous in high-resolution TEM images. Results from optical absorption experiments show that the color variations in KBS shards arise due to optical absorption by the precipitate, a result consistent with classical electromagnetic theory. -10 nm, we have come to a basic understanding of magnetism Early magnetic experiments on colorless and dark glass and optical variations in these particular rapidly cooled glasses separates of KBS shards, using a vibrating sample magnetomeand related glasses in the section.A current application of these ter, showed that at room temperature the colorless glass is results is to the study of more slowly cooled volcanic glasses of paramagnetic and the dark glass is superparamagnetic. Soon ash flow sheets, which demonstrates the existence of stable sin-thereafter transmission electron microscopy (TEM) was used to gle domains thought to be similar in nature and origin (precipi-image the microstructure of these shards, and we observed microcrystals -10 nm in size in the darker shards. Since that time we have investigated the magnetism and optical properties 1Department of Geology and Geophysics, University of Utah, Salt of the KBS glasses to understand better these phenomena in a Lake City. fundamental way. Our initial research concentrated on under-2Naval Research Labs, Washington, D.C. standing the magnetic p...

Granulites from southwest Lofoten, Norway, often carry intense and stable components of natural remanent magnetization (NRM). Coercivities in excess of 100 mT and blocking temperatures up to 580°–645°C are common. Several phases in these rocks can carry stable components of NRM. Clinopyroxenes (augite with exsolved pigeonite and orthopyroxene) usually contain abundant Fe‐Ti oxides, the majority of which are too small to be studied conclusively with visible light. Additionally, these rocks typically contain coarse‐grained (∼100–1000 μm) exsolution intergrowths of rhombohedral Fe‐Ti oxides and coarse‐grained magnetite. We have studied the magnetism and mineralogy of the titanohematite and the inclusion‐bearing augites in one granulite sample using transmission electron microscopy (TEM) and magnetic experiments to understand better the remanence properties of naturally occurring Fe‐Ti oxides and their relationships to silicate minerals. The coarse‐grained titanohematite has a coercivity of 350 mT and a laboratory saturation remanence that unblocks at 570°–600°C. The hysteresis characteristics of this titanohematite are typical of much smaller ∼10μm‐sized single domains, which apparently reflects the presence of exsolved ilmenite lamellae, as small as <10 nm in thickness. These lamellae presumably inhibit (1) domain wall nucleation and (2) movement of walls that manage to nucleate. The inclusions in the augites serve as carriers of lower coercivity remanence with blocking temperatures less than about 560°C. Magnetite, hematite, and ilmenite are present in the augite, and intergrowth of the rhombohedral phases, revealed by TEM, persists down to the 20‐nm scale. The distribution of the magnetite inclusions in the augite appears to be unrelated to any silicate microstructure, while that of the rhombohedral Fe‐Ti oxides is strongly controlled by silicate microstructure, with intergrowths of these oxides commonly decorating lamellae of pigeonite, orthopyroxene, and clinoamphibole within the augite. The NRM of this sample possibly dates from the amphibolite facies metamorphism that the region experienced about 1000 m.y. ago. Paleomagnetic studies of these rocks could readily recover virtual geomagnetic poles dating from this event. In addition to the included oxides, the augites display a number of unusual features that probably result from the protracted polymetamorphic history of these granulites: pigeonite lamellae are not parallel to the b‐crystallographic axis, pigeonite lamellae cut across those of orthopyroxene and numerous included minerals occur. Rare earth elements are concentrated within inclusions that reside in the augite.