Browsing by Subject "Alps"
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Item Book Review of The Alps by Shiro Shirahata(Library Journal, 1981-01-15) Sandy, John H.Item Evidence from high-temporal-resolution strain rates for strain softening due to episodic fluid influx at Passo del Sole, Central Swiss Alps(2012-12) Stacy, Sarah Jean; Carlson, William, 1952-; Cloos, Mark; Ketcham, Richard ABerg (2007) determined hand-sample-scale high-temporal-resolution strain rates from rotated garnet for two samples of quartzose pelitic gneiss at Passo del Sole, Central Swiss Alps, documenting a correlation between dramatic increases in strain rate and compositionally anomalous garnet growth zones. Considering additional evidence that these anomalous zones resulted from externally derived ephemeral fluid flow, he concluded that increased strain rates at Passo del Sole are the result of strain softening caused by fluid influx. This study tests Berg's interpretation by calculating new hand-sample-scale high-temporal-resolution strain rates for two additional samples of the same gneiss: a control sample (Sample PDS 03-30) that shows no unusual zoning patterns, implying that it was unaffected by fluids; and another (Sample PDS 03-2) that features a prominent Ca spike, suggesting that it has been affected by fluid influx. Unique features of garnet from this locality--contemporaneity of chemical zones, near-simultaneous nucleation, size-proportional growth, and rock-wide chemical equilibrium--were exploited to calculate strain rates of unprecedentedly high temporal resolution. Thermodynamically modeled temperatures for several growth-zone boundaries in each garnet crystal were combined with a regional heating rate of 11.5 ± 3.5 °C/Myr (presumed constant) and measured deflections of inclusion trails in each zone to calculate strain rates for several discrete time increments during garnet growth. Sample PDS 03-2 displays a 2- to 16-fold increase in strain rate that correlates with growth of the high-Ca zone; strain rates are 0.4 x 10⁻¹⁴ s⁻¹ to 4.1 x 10⁻¹⁴ s⁻¹ for zones with normal Ca concentration and 9.1 x 10⁻¹⁴ s⁻¹ to 17.9 x 10⁻¹⁴ s⁻¹ for the high-Ca zone. Distinct amongst all analyzed samples from Passo del Sole, Sample PDS 03-30--which has not been affected by fluid influx--shows no fluctuations in strain rates, which remain low and similar (0.2 x 10⁻¹⁴ s⁻¹ to 2.6 x 10⁻¹⁴ s⁻¹) across all zones. Results from this study therefore further substantiate the correlation of high strain rates with compositionally anomalous zones, strengthening the interpretation that elevated strain rates at Passo del Sole result from strain softening caused by episodic, externally controlled flow of fluids through the system during synkinematic garnet growth.Item Novel (U-Th)/He thermochronometric constraints on serpentinized ultramafic rocks(2017-08) Cooperdock, Emily Jane; Stockli, Daniel F.; Ketcham, Richard; Barnes, Jaime; Lavier, Luc; Müntener, Othmar; Klein, FriederSerpentinization, the hydration of peridotite, is a widespread process that impacts rheological properties along plate boundaries, fluid-mobile element cycling and biogeochemical processes. While numerous studies exist on the petrology, structure and geochemistry of serpentinites, geochronological analysis has been elusive due to the lack minerals with well-established dating techniques. With technological advances in radiogenic isotope analysis in the past decade, it is now possible to address the timing of serpentinization by dating the growth of magnetite that forms as a direct result of the breakdown of primary peridotite phases in the presence of water. This study develops and applies a (U-Th)/He-based method that is tailored to analyze magnetite that grow during the alteration of ultramafic rocks. The technique includes procedures to screen for grain quality in opaque phases with X-Ray Computed Tomography, physical air abrasion to remove the alpha implantation/ejection zone in the outer grain boundary, and analytical procedures to measure low parent nuclide concentrations typical of these lithologies. The method is then modified to test the applicability to date magmatic spinel, a primary mineral phase in peridotite, to date peridotite exhumation by tectonic and volcanic processes. The magnetite (U-Th)/He technique is applied to serpentinites within an exhumed subduction complex in Syros, Greece, uplifted serpentinites within the Malenco Ultramafic Unit, Italian Alps, and to carbonated serpentinite in Wadi Fins, Oman. The results of these studies provide evidence for episodic or continuous magnetite growth over millions of years, which implies that magnetite formation can continue to occur in serpentinites under changing metamorphic conditions. These studies also show that multiple generations of magnetite growth can be distinguished based on magnetite grain size and trace element chemistry, and that hydrothermal magnetite can record changing chemical conditions and act as an important host phase of U and Th in serpentinites. Overall, the magnetite (U-Th)/He chronometer can be used to distinguish between multiple episodes of fluid-rock alteration, which has implications for the cooling history and geochemical exchanges in serpentinized peridotite. This technique, coupled with spinel (U-Th)/He, has broad applicability to investigate the timescales of mantle exhumation and alteration of ultramafic rocks in a variety of tectonic settings.