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Dating Clinopyroxene Phenocrysts in Submarine Basalts Using ^(40)Ar/^(39)Ar Geochronology

Konrad, Kevin and Koppers, Anthony A. P. and Balbas, Andrea M. and Miggins, Daniel P. and Heaton, Daniel E. (2019) Dating Clinopyroxene Phenocrysts in Submarine Basalts Using ^(40)Ar/^(39)Ar Geochronology. Geochemistry, Geophysics, Geosystems, 20 (2). pp. 1041-1053. ISSN 1525-2027. https://resolver.caltech.edu/CaltechAUTHORS:20190404-083603450

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Abstract

Dating submarine basalts using ^(40)Ar/^(39)Ar geochronology is often hindered by a lack of potassium‐bearing phenocrystic phases and severe alteration in the groundmass. Clinopyroxene is a common phenocrystic phase in seafloor basalts and is highly resistive to low‐temperature alteration. Here we show that clinopyroxene phenocrysts separated from marine basalts are a viable phase for ^(40)Ar/^(39)Ar incremental heating age determinations. We provide results from a pilot study comprising 16 age experiments from nine clinopyroxene separates, five of which from samples with dated coeval phases. The clinopyroxene ages range from 11.5 to 112 Ma with relatively high uncertainties (ranging from 0.8% to 7.1%; median of 1.9%) compared to more traditional phases. The clinopyroxene age plateaus form at low to moderate temperature steps and are characterized by relatively elevated K/Ca of 0.002–0.4, suggesting that other K‐bearing phases hosted within the clinopyroxene are likely degassing to yield the ^(40)Ar/^(39)Ar age information. There are three possible origins for the K and corresponding ^(40)Ar* including films of trapped melt/nanomineral inclusions along grain defects, secondary melt inclusion bands, or variations in degassing behaviors between lower and higher crystalline Ca pyroxene phases. Regardless of the source of the K, the age determinations are successful with 75% of the experiments producing long plateaus (>60% ^(39)Ar released) with mean square of the weighted deviations ranging from 0.6 to 1.5 and probability of fit values >0.05. We conclude that clinopyroxene dating by the ^(40)Ar/^(39)Ar method has the potential to provide a wealth of information for previously undated, altered seafloor lithologies and continental equivalents.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1029/2018gc007697DOIArticle
ORCID:
AuthorORCID
Konrad, Kevin0000-0002-4349-1618
Koppers, Anthony A. P.0000-0002-8136-5372
Balbas, Andrea M.0000-0002-4855-3507
Miggins, Daniel P.0000-0001-8256-8377
Heaton, Daniel E.0000-0003-1255-7642
Additional Information:© 2019 American Geophysical Union. Received 18 MAY 2018; Accepted 13 JAN 2019; Accepted article online 28 JAN 2019; Published online 18 FEB 2019. We thank Jasper Konter for providing the Western Pacific Seamount Province clinopyroxene separates. Extended ^(40)Ar/^(39)Ar age determination data for these samples are available in the online EarthRef.org Digital Archive (https://earthref.org/ERDA). Fred Jourdan and an anonymous referee are thanked for reviews and discussion that improved the quality of this work.
Subject Keywords:40Ar/39Ar geochronology; clinopyroxene; basalts; melt inclusions; submarine alteration
Issue or Number:2
Record Number:CaltechAUTHORS:20190404-083603450
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20190404-083603450
Official Citation:Konrad, K., Koppers, A. A. P., Balbas, A. M., Miggins, D. P., & Heaton, D. E. (2019). Dating clinopyroxene phenocrysts in submarine basalts using 40Ar/39Ar geochronology. Geochemistry, Geophysics, Geosystems, 20, 1041–1053. https://doi.org/10.1029/2018GC007697
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:94444
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:04 Apr 2019 16:08
Last Modified:03 Oct 2019 21:04

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