Propagation of Magma-Filled Cracks

The conventional rules, derived from empirical and theoretical considerations, for the interpretation of anisotropy of magnetic susceptibility (AMS) in terms of microstructure and deformation are subject to numerous exceptions as a result of particular rock magnetic effects. Unusual relationships between structural and magnetic axes (so-called inverse or intermediate magnetic fabrics) can occur because of the presence of certain magnetic minerals, either single domain magnetite or various paramagnetic minerals. When more than one mineral is responsible for magnetic susceptibility, various problems appear, in particular the impossibility of using anisotropy to make quantitative inferences on the intensity of the preferred orientation and consequently on strain. In ferromagnetic grains, AMS may also be influenced by the magnetic memory of the grains (including natural remanence). The effect of alternating field or thermal demagnetization on AMS is briefly discussed. As discussed in this article, various rock magnetic techniques, specific to AMS interpretation, have to be developed for a better assessment of the geological significance of AMS data. These techniques mainly rely on measurements of susceptibility versus magnetic field and temperature, together with anisotropy of remanence. 93 refs., 11 figs., 1 tab.

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Rock magnetism and the interpretation of anisotropy of magnetic susceptibility

Pyoclastic density currents are awesome volcanic phenomena that can wreak destruction on a regional scale and can impact global climate. They deposit ignimbrites, which include vast impact lansdscape-modifying sheets with volumes exceeding 1000 km3.This book takes stock of our understanding of pyroclastic density currents and presents a new conceptual framework for investigating how ignimbrites are deposited. It integrates the results of field-based studies, laboratory experiments and numerical modelling, including work on clastic sedimentologym and industrial particle transport. Topics covered include the behaviour or particulate currents, mechanisms of clast support and segregation, interpreting ignimbrite lithofacies and architectures, and future research directions. The new approach focuses on processes and conditions within the lower flow-boundary zone of currents. Superb diagrams explain many new concepts, while the 95 photographs make an explanatiry atlas of deposit types. This is essential reading for workers investigating volcanic hazards, and for anyone wishing to interpret modern or ancient ignimbrites, as well as other catastrophically emplaced sediments.

“Given the depth of scholarship that they have brought to the subject, the power of their arguments, and the degree of synthesis with other fields, this would seemto qualify as a seminal work… I think that this will be the paper on the topic that others will have to contend with for many years to come.” Marcus Bursik, State University of New York

Pyroclastic density currents and the sedimentation of ignimbrites

Allanite, an epidote group mineral, occurs as a common accessory mineral in igneous, metamorphic rocks and hydrothermal deposits. It contains radioactive elements Th and U, making it a valuable U–Th–Pb geochronometer for various geological processes. Due to the structural and compositional complexity of allanite, in situ dating method is a better choice when the allanite is heterogeneous or has crystallized in multi-stage events. The wide variation of chemical compositions, however, raises the potential for matrix effects and complicates the use of in situ methods for allanite U–Th–Pb chronology. To address this issue, we made a comprehensive investigation on allanite U–Th–Pb chronology using Secondary Ion Mass Spectrometry (SIMS). Five allanite samples (CAP, Daibosatsu, SQ-51, Toba OTT, TARA allanite) with variable elemental compositions and common lead concentrations have been used. Allanites with variations in FeO (from 12.8 to 16.1 wt%) and similar ThO2 contents (around 1–2%) do not show significant matrix effects, as suggested by a previous study. While, obvious changes in calibration parameters (UO2+/U+, ThO2+/Th+) were noticed for allanites with a large range of Th contents (from 300 ppm to 2.0 wt%). By employing power law relationships between Pb+/U+versus UO2+/U+, Pb+/Th+versus ThO2+/Th+ with suitable exponentials, we do not observe obvious matrix effects with ThO2 concentrations variations. This study demonstrates that allanite can be a good geochronometer for multi-stage mineralization of hydrothermal deposits, providing valuable complementary information to zircon and monazite.

Allanite U–Th–Pb geochronology by ion microprobe

Environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport, deposition, or transformation of magnetic grains is influenced by environmental processes in the atmosphere, hydrosphere, and lithosphere. The first explicit description of environmental magnetism as a distinct field was in 1980. Since that time environmental magnetism has become a broad field that is finding application in an ever-increasing array of scientific disciplines. In this review of the present state of environmental magnetic studies, we divide the field into three broad, but arbitrary, categories. The first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments. This category includes the correlation of sediment cores using magnetic susceptibility measurements, studies of geomagnetic field behavior, the analysis of depositional and postdepositional mechanical processes that affect sediments, and the examination of magnetic parameters that might represent proxies for paleoclimatic variation. The second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment. These provenance investigations include studies of changes in catchment-derived sediment in lakes, fluctuations in contributions from terrigenous, aeolian and glaciogenic components in deep-sea sediments, and the origin of atmospheric particulates. The final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments, including pedogenesis, authigenetic/diagenetic formation of ferrimagnetic phases, dissolution of magnetic minerals due to reductive diagenesis, and contributions of biomagnetism to sedimentary magnetism. Because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques, it is likely that the scope of environmental magnetism will continue to expand rapidly. Environmental magnetism is capable of providing important data for studies of global environmental change, climatic processes, and the impact of humans on the environment, all of which are major research initiatives in the international scientific community. These factors suggest that environmental magnetism has a bright and diverse future.

Environmental magnetism: Past, present, and future

We compared the magnetic fabric with the macroscopic surface lineation direction (previously assumed to be parallel with magma flow direction) for 35 dikes of the Koolau complex, Oahu. Anisotropy of magnetic susceptibility (AMS) was determined for 530 samples from 59 dikes (including a sill and a pluglike intrusive body) and two filled lava tubes, and statistically significant AMS clusters were found in all but five dikes. For the 35 lineated dikes, the mean maximum AMS direction (χ1) coincides with the macroscopic lineations to within 25° in 18 and to within 15° in eight. The AMS ellipsoid shape in about half of the samples is prolate (cigar shaped) and in about half is oblate (pancake shaped). We interpret the AMS to be determined by shape anisotropy of ferromagnetic grains. The mean total anisotropy is 2.2%, and total anisotropy (H) ranges from 0.24% to as high as 14.12%. The principal AMS axes for samples from a dike are tightly grouped on equal-area plots, and for most of the dikes the minimum AMS axes plot near to the pole of the dike. Our AMS measurements enable us to infer absolute magma flow directions for half the dikes studied; the maximum AMS axes of samples from one side of the dike form a tight cluster on an equal-area plot, and those from the other side of the dike form another tight cluster. The two clusters are close together (separated by 10–30°) for narrow dikes (<100 cm wide), and are farther apart (30–60°) for wide dikes and are symmetrically disposed on either side of the plane of the dike. We infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient, and hence indicate the flow azimuth, i.e., the absolute magma flow direction. Nearly half of the dikes measured (25) gave paired AMS plots; in 12 the magma flowed up at low oblique angles 5–30° from the horizontal, in five it flowed down at 5–20° in four it flowed horizontally (<5°), in three it flowed up at a steep angle (50–75°), and in one it flowed down at a steep angle. The magma flow in general was rarely vertical or horizontal.

Magma flow directions in dikes of the Koolau Complex, Oahu, determined from magnetic fabric studies

The Toba depression in north central Sumatra is a complex of several overlapping calderas resulting from three major ignimbrite-forming eruptions. Within the depression, the upland masses of Samosir and northern Uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment, hitherto interpreted to be two parts of a single resurgent dome. This study has demonstrated that the welded tuffs of Samosir and Uluan have different magnetic polarities and therefore at least two different ignimbrites are present; the Samosir/Uluan massif may consist of parts of two resurgent domes. The first ignimbrite eruption occurred at 0.84 Ma and produced a very thick (>400 m), densely welded unit having a reversed polarity. Anisotropy of magnetic susceptibility (AMS) flow direction and lithic size data indicate that the source lies in the southern part of the Toba depression, and the thick deposit of Uluan is thought to have ponded in a 40-km-wide caldera. The second ignimbrite is normally magnetized. AMS flow direction data indicate two separate source vents, one to the north in the Haranggaol caldera, and another to the south. The thick deposit at Samosir is thought to have ponded in the southern caldera. Coarse sediments then accumulated over Samosir and northern Uluan and were capped by lacustrine deposits. A renewed episode of resurgence then uplifted Samosir Island and possibly the northern part of Uluan. At approximately 0.075 Ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the Toba depression. This ignimbrite is mostly nonwelded and normally magnetized. Part of the Uluan dome was destroyed by collapse of the Sibandung caldera and Latung graben and concomitant with renewed subsidence of the Haranggaol and Porsea calderas.

Stratigraphy, paleomagnetism, and magnetic fabric of the Toba Tuffs: Constraints on the sources and eruptive styles

Paleomagnetic and 40Ar/39Ar data indicate that two tuffs are present at Siguragura, North Sumatra, Indonesia: a reversely magnetized tuff (the earliest eruptive unit of the Toba caldera) of Matuyama age (0.84 Ma) and a normally magnetized tuff (the last eruptive unit of the Toba caldera) of late Brunhes age. No reversely magnetized tuffs of late Brunhes age are present at this location as suggested by previous investigators.

Michael D. Knight

Papers

Magma flow directions in dikes of the Koolau Complex, Oahu, determined from magnetic fabric studies

Stratigraphy, paleomagnetism, and magnetic fabric of the Toba Tuffs: Constraints on the sources and eruptive styles

No short reversals of Brunhes Age recorded in the Toba Tuffs, north Sumatra, Indonesia