Gareth Seward

Strain within the ultrahigh-pressure Western Gneiss region of Norway recorded by quartz CPOs

Abstract Electron back-scatter diffraction (EBSD) was used to measure the crystal preferred orientations (CPOs) from 101 samples across the ultrahigh-pressure Western Gneiss region of Norway to assess slip systems, sense of shear, CPO strength, and strain geometry. The CPOs suggest a dominance of prism ⟨a⟩ slip, with lesser amounts of prism [c] slip and basal ⟨a⟩ slip; there are few Type I and Type II girdles. The major structural feature in the study area – the high-strain, top-W, normal-sense Nordfjord–Sogn Detachment Zone – is characterized by asymmetric and strong CPOs; an eastern domain with strong asymmetric CPOs shows top-E shear. Strain throughout the study area was characterized by a mix of plane strain and constriction with no evidence of flattening. Adjacent gneiss and quartzite/vein samples have similar CPOs.

Localized ductile shear below the seismogenic zone: Structural analysis of an exhumed strike‐slip fault, Austrian Alps

[1] The Miocene Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) strike-slip fault in Austria allows study of the internal structure of a fault zone from the near surface to � 30 km depth. As it enters the Tauern Window along the Rinderkarsee shear zone, the SEMP fault passes from a dominantly brittle to a dominantly ductile structure. The shear zone consists of three 1- to 100-m-wide zones of brittle-ductile and ductile deformation separated by 500-m-wide zones of less deformed rocks. The southern shear zone is mylonitic, with ductile amphibole and plagioclase; weak crystal preferred orientations imply that the main deformation mechanism was dislocation-accommodated grain boundary sliding. The northern and central shear zones are characterized by discrete millimeter-wide shear zones with ductile quartz, muscovite, and biotite and brittle feldspar. Shear zone nucleation at the grain scale involved dislocation creep and the transformation of plagioclase to muscovite; strain then localized in muscovite-rich grain boundary shear zones that linked to form throughgoing shear zones.

The Potential of Combined In-Situ Heating Experiments and Detailed EBSD Analysis in the Investigation of Grain Scale Processes such as Recrystallization and Phase Transformation

Experiments in which the microstructural development can be observed at the same time as the crystallography is described fully opens up new, powerful ways to advance our understanding of microstructural processes such as grain growth, primary and secondary recrystallization and phase transformations. In addition, comparison of results of experiments in different materials can be used to develop general laws for the investigated processes. In this study, we briefly review and compare the results from various ongoing studies undertaken in a variety of materials with emphasis on highlighting (a) the scientific potential of such experiments and (b)similarities and differences in their microstructural evolution. Materials studied include metals e.g. Ti, Ni, Al, Mg, Ti-SULC steel and geological materials such as rocksalt (NaCl), hematite and magnetite. Here, we present experimental results and their interpretation in terms of subgrain to grain-scale processes.

Deformational history and thermochronology of Wrangel Island, East Siberian Shelf and coastal Chukotka, Arctic Russia

Abstract In Arctic Russia, south of Wrangel Island, Jura–Cretaceous fold belt structures are cut by c. 108–100 Ma plutonic rocks and a c. 103 Ma migmatitic complex (U–Pb, zircon) that cooled by c. 96 Ma (40Ar/39Ar biotite); the structures are unconformably overlain by c. 88 Ma and younger (U–Pb, zircon) volcanic rocks. Wrangel Island, with a similar stratigraphy and added exposure of Neoproterozoic basement rocks, was thought to represent the westwards continuation of the Jura–Cretaceous Brookian thrust belt of Alaska. A penetrative, high-strain, S-dipping foliation formed during north–south stretching in Triassic and older rocks, with stretched pebble aspect ratios of c. 2:1:0.5 to 10:1:0.1. Deformation was at greenschist facies (chlorite+white mica; biotite at depth; temperature c. 300–450°C). Microstructures suggest deformation mostly by pure shear and north–south stretching; the quartz textures and lattice preferred orientations suggest temperatures of c. 300–450°C. 40Ar/39Ar K-feldspar spectra (n=1) and muscovite (n=3) (total gas ages c. 611–514 Ma) in Neoproterozoic basement rocks are consistent with a short thermal pulse during deformation at 105–100 Ma. Apatite fission track ages (n=7) indicate cooling to near-surface conditions at c. 95 Ma. The shared thermal histories of Wrangel Island and Chukotka suggest that Wrangel deformation is related to post-shortening, north–south extension, not to fold–thrust belt deformation. Seismic data (line AR-5) indicate a sharp Moho and strong sub-horizontal reflectivity in the lower and middle crust beneath the region. Wrangel Island probably represents a crustal-scale extensional boudin between the North Chukchi and Longa basins.

Controls on Trace Element Uptake in Metamorphic Titanite: Implications for Petrochronology

Petrochronology—the interpretation of isotopic dates with complementary elemental data— requires understanding the relationship between trace elements in chronometers and the petrological evolution of their host rocks. Titanite is a useful petrochronometer for crustal processes, but how titanite records host rock evolution is uncertain. We present an extensive titanite U–Pb and chemical dataset from felsic gneisses and leucosomes in the Western Gneiss Region (WGR) of Norway. Mineral textures, U–Pb dates, and major, minor, and trace element chemistry reveal three titanite populations: (1) Precambrian igneous titanite [high light rare earth elements (LREE), Th, Pb, Zr; low Al, F]; (2) Caledonian recrystallized titanite (low LREE, Th, Pb) that formed from dissolution–reprecipitation of the Precambrian titanite and co-crystallized with allanite; (3) Caledonian neocrystallized titanite (high Al, F and variable REE). Although titanite records multiple igneous and metamorphic events in the WGR, we use a principal components analysis to identify distinct petrological and thermal effects on trace element uptake that hold across all titanite populations. Coupled with textural observations, these data show that different trace element patterns between populations predominantly represent the activity of different rock reactions during continental subduction and exhumation; using correlations between principal component scores and trace element abundances or ratios, we discriminate which phases co-crystallized with titanite. Our results further demonstrate that thermal and fluid partitioning effects can complicate interpretations of rock petrology from titanite trace elements, but these factors can be assessed by measuring specific trace elements (e.g. Al, Zr).

Non-conventional synthesis and magnetic properties of MAX phases (Cr/Mn)2AlC and (Cr/Fe)2AlC

A few years after the theoretical prediction of magnetic MAX phases, a number of such materials have been experimentally reported, especially in the form of thin films. Yet, due to a relatively small number of studies, we have only just begun to discover the intriguing magnetic properties that are associated with this class of materials. The preparation of bulk MAX phases with later transition metals has been proven to be particularly challenging. Consequentially, there is a great need to develop synthetic strategies to obtain the respective materials in suitable quantities for magnetic investigations. Here, bulk Mn- and Fe-substituted Cr2AlC are prepared using non-conventional synthesis methods such as microwave heating and spark plasma sintering. Synchrotron X-ray diffraction coupled with detailed elemental analyses is used to confirm the successful doping of the MAX phase with the later transition metals as well as to elucidate the microstructure of the obtained dense materials. 57Fe Mossbauer spectroscopy data are presented showing signals of the doped MAX phase and Fe-containing secondary phases. Based on PPMS and SQUID measurements the non-trivial magnetic behavior of the obtained samples is discussed in the context of the existing studies.

Metamorphic records of multiple seismic cycles during subduction

Garnets record subduction earthquakes. Large earthquakes occur in rocks undergoing high-pressure/low-temperature metamorphism during subduction. Rhythmic major-element zoning in garnet is a common product of such metamorphism, and one that must record a fundamental subduction process. We argue that rhythmic major-element zoning in subduction zone garnets from the Franciscan Complex, California, developed in response to growth-dissolution cycles driven by pressure pulses. Using electron probe microanalysis and novel techniques in Raman and synchrotron Fourier transform infrared microspectroscopy, we demonstrate that at least four such pressure pulses, of magnitude 100–350 MPa, occurred over less than 300,000 years. These pressure magnitude and time scale constraints are most consistent with the garnet zoning having resulted from periodic overpressure development-dissipation cycles, related to pore-fluid pressure fluctuations linked to earthquake cycles. This study demonstrates that some metamorphic reactions can track individual earthquake cycles and thereby opens new avenues to the study of seismicity.

Rapid Microwave Preparation and Composition Tuning of the High-Performance Magnetocalorics (Mn,Fe) 2 (P,Si)

Rapid preparation utilizing assisted microwave heating permits significantly shorter preparation times for magnetocaloric compounds in the (Mn,Fe)2(P,Si) family, specifically samples of (Mn,Fe)2-δP0.5Si0.5 with starting compositions of δ = 0, 0.06, and 0.12. To fully understand the effects of processing and composition changes on structure and properties, these materials are characterized using synchrotron powder diffraction, neutron powder diffraction, electron microprobe analysis (EMPA), X-ray fluorescence (XRF), and magnetic measurements. The diffraction analysis reveals that increasing δ results in decreasing amounts of the common Heusler (Mn,Fe)3Si secondary phase. EMPA shows (Mn,Fe)2(P,Si) in all three samples to be Mn and P rich, whereas XRF demonstrates that the bulk material is Mn rich yet P deficient. Increasing δ brings the Mn/Fe and P/Si ratios closer to their starting values. Measurements of magnetic properties show an increase in saturation magnetization and ordering temperature with increasing δ, consistent with the increase in Fe and Si contents. Increasing δ also results in a decrease in thermal hysteresis and an increase in magnetic entropy change, the latter reaching values close to what have been previously reported on samples that take much longer to prepare.