The new CLOCIT irradiation facility for 40Ar/39Ar geochronology: Characterization, comparison with CLICIT, and implications for high-precision geochronology

Rutte, Daniel; Becker, Tim; Deino, Al; Reese, Steven; Renne, Paul R.; Schickler, Robert

doi:10.31223/osf.io/2us4z

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“…We scanned for double‐charged 82 Kr on m/z 41 to monitor possible interference from double‐charged 80 Kr on m/z 40, produced from Br during irradiation (Rutte et al, 2017); no significant contents were measured. Correction factors for collateral Ar isotope production and 37 Ar Ca / 39 Ar K production ratio for CLICIT irradiation are taken from Renne et al (2015); correction factors for CLOCIT are from Rutte et al (2018), and we determined and used a 37 Ar Ca / 39 Ar K production ratio of 1.99 ± 0.08 for CLOCIT using Hb3Gr in irradiation 476.…”

Section: Methodsmentioning

confidence: 99%

An Exhumation Pulse From the Nascent Franciscan Subduction Zone (California, USA)

et al. 2020

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We investigated a suite of metabasite blocks from serpentinite matrix and shale matrix mélanges of the California Coast Ranges. Our new data set consists of 40 Ar/ 39 Ar dates of amphibole and phengite and U-Pb dates of metamorphic zircon. Combined with published geochronology, including prograde Lu-Hf garnet ages from the same blocks, we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks. In particular we find that exhumation from amphibole-eclogite facies conditions occurred as a single episode at 165-157 Ma, with an apparent southward younging trend. The rate and timing of exhumation were initially uniform (when comparing individual blocks) and fast (with cooling rates up to~140°C/Ma). In the cooler and shallower blueschist facies, exhumation slowed and became less uniform among blocks. Considering the subduction zone system, the high-grade exhumation temporally correlates with a magmatic arc pulse (Sierra Nevada) and the termination of forearc spreading (Coast Range Ophiolite). Our findings suggest that a geodynamic one-time event led to exhumation of amphibole-eclogite facies rocks. We propose that interaction of the Franciscan subduction zone with a spreading ridge led to extraction of the forearc mantle wedge from its position between forearc crust and subducting crust. The extraction led to fast and uniform exhumation of subducted rocks into the blueschist facies. We also show that the Franciscan subduction zone did not undergo significant cooling over time and that its initiation was not coeval with blueschist-facies metamorphism of the Red Ant schist of the Sierra Nevada foothills. Plain Language Summary A subduction zone is where oceanic crust of one tectonic plate dives under the crust of another plate. These zones are important drivers of the movement of the continental plates. Special rocks-blueschists and eclogites-form in subduction zones at high pressure and relatively low temperature. The Franciscan Complex forms most of the geology of western California. It consists of rocks that dived into a subduction zone and returned back to the surface. We investigated the history of some of these rocks by determining the age and conditions of their movement. We find that the rocks that subducted to depths of up to 80 km all came up at the same time around 160 million years before present. This suggests that a specific tectonic event occurred at that time; other regional data show that the effects of this event were felt elsewhere along the paleo-Pacific margin. We also investigate rocks in the Sierra Nevada foothills that other researchers suggested to have formed at about the same time and found that they are not correlated.

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Section: Methodsmentioning

confidence: 99%

An Exhumation Pulse From the Nascent Franciscan Subduction Zone (California, USA)

et al. 2020

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The new CLOCIT irradiation facility for 40Ar/39Ar geochronology: Characterization, comparison with CLICIT, and implications for high-precision geochronology

Abstract: 9The Cadmium-Lined Outer-Core Irradiation Tube (CLOCIT) is a new irradiation facility for

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An Exhumation Pulse From the Nascent Franciscan Subduction Zone (California, USA)

An Exhumation Pulse From the Nascent Franciscan Subduction Zone (California, USA)

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