Dating exhumed peridotite with spinel (U-Th)/He chronometry

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doi: 10.1016/j.epsl.2018.02.041
Author(s): Cooperdock, Emily H. G.; Stockli, Daniel F.
Author Affiliation(s): Primary:
University of Texas at Austin, Department of Geological Sciences, Austin, TX, United States
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, Vol.489, p.219-227. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. Includes appendix. 50 refs.; illus., incl. 2 tables, geol. sketch maps
Summary: The timing of cooling and exhumation of mantle peridotites in oceanic and continental settings has been challenging to determine using traditional geo- and thermochronometric techniques. Hence, the timing of the exhumation of mantle rocks to the Earth's surface at mid-ocean ridges, rifted and passive continental margins, and within continental volcanic and orogenic systems has remained largely elusive or only loosely constrained by relative age bracketing. Magmatic spinel [(Mg, Fe)(Al,Cr)2O4] is a ubiquitous primary mineral phase in mantle peridotites and is often the only primary mineral phase to survive surface weathering and serpentinization. This work explores spinel (U-Th)/He thermochronology as a novel tool to directly date the exhumation and cooling history of spinel-bearing mantle peridotite. Samples were chosen from a range of tectonic and petrologic settings, including a mid-ocean ridge abyssal peridotite (ODP Leg 209), an orogenic tectonic sliver of sub-continental mantle (Lherz massif, France), and a volcanic-rock hosted mantle xenolith (Green Knobs, NM). Spinel grains were selected based on grain size and morphology, screened for internal homogeneity using X-ray computed tomography, and air abraded to eliminate effects of alpha ejection/implantation. These case studies yield spinel He age results that are reproducible and generally in good agreement with independent age constraints. For ODP Leg 209, a spinel He age of 1.1 ± 0.3 Ma (2 SE) (n = 8) is consistent with independent U-Pb and magnetic anomaly ages for the exhumation of oceanic crust by detachment faulting along this segment of the slow-spreading ridge. Spinel from the Lherz massif yield He ages from 60-70 Ma (n = 3), which correspond well with independent thermochronometric constraints for cooling associated with Pyrenean collisional exhumation. Spinel from a mantle xenolith within a previously undated kimberlite diatreme at Green Knobs, New Mexico, generate a reproducible mean He age of 11.7 ± 1.8 Ma (2 SE) (n = 3) that appears to record young volcanism in the area or age resetting by post-emplacement re-heating or alteration. The combined results of these case studies demonstrate the viability for spinel He thermochronometry to resolve cooling histories of peridotite exhumed through tectonic and volcanic processes.
Year of Publication: 2018
Research Program: ODP Ocean Drilling Program
Key Words: (U-Th)/He; 03 Geochronology; 05 Petrology, Igneous and Metamorphic; Absolute age; Atlantic Ocean; Cenozoic; Cretaceous; Dates; Detachment faults; Europe; Exhumation; Faults; France; Green Knobs; Igneous rocks; Inclusions; Leg 209; Lherz; Lower Paleocene; Mantle; Mesozoic; Mid-Atlantic Ridge; Miocene; Neogene; New Mexico; North Atlantic; ODP Site 1272; Ocean Drilling Program; Oxides; Paleocene; Paleogene; Peridotites; Pleistocene; Plutonic rocks; Quaternary; Spinel; Tectonics; Tertiary; Thermal history; Thermochronology; Ultramafics; United States; Upper Cretaceous; Volcanism; Western Europe; Xenoliths
Coordinates: N424700 N424700 E0012230 E0011223
N150600 N150600 W0445800 W0445800
N355700 N355700 W1090300 W1090300
Record ID: 2018047309
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands