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U-Th baddeleyite geochronology and its significance to date the emplacement of silica undersaturated magmas

Wan N. Wu, Axel K. Schmitt and Lucia Pappalardo
U-Th baddeleyite geochronology and its significance to date the emplacement of silica undersaturated magmas
American Mineralogist (October 2015) 100 (10): 2082-2090


Baddeleyite is a frequently found accessory mineral in silica undersaturated lavas. Because it is typically enriched in uranium, while having low initial lead, baddeleyite has long been a prime target for U-Pb geochronology of mafic rocks. The difficulties in retrieving small baddeleyite grains from volcanic samples and the lack of a detailed understanding of baddeleyite occurrence, however, have limited baddeleyite chronology largely to coarse-grained mafic intrusive rocks. Here, the development of U-Th in situ baddeleyite analysis using secondary ionization mass spectrometry (SIMS) is presented together with an assessment of baddeleyite occurrence in Quaternary silica-undersaturated lavas from Campi Flegrei (Naples, Italy). Samples studied comprise the pre- and post Campanian Ignimbrite (ca. 40 ka) lava domes of Cuma and Punta Marmolite, and Astroni and Accademia, respectively. The in situ sample preparation required initial identification of baddeleyite crystals from sawed and polished rock billets using scanning electron microscope (SEM) backscatter imaging and energy-dispersive X-ray analysis. U-Th baddeleyite isochron ages for intra-caldera Astroni and Accademia lava domes are 5.01 (super +2.61) (sub -2.55) ka (MSWD=2.0; n=17) and 4.36 (super +1.13) (sub -1.12) ka (MSWD=2.9; n=24), respectively. The ages for Punta Marmolite (62.4 (super +3.9) (sub -3.8) ka; MSWD=1.2; n=11) and Cuma (45.9 (super +3.6) (sub -3.5) ka; MSWD=2.2; n=11) predate the Campanian Ignimbrite. Rapid baddeleyite crystallization at the time of emplacement is supported by petrologic observations that >50% of the baddeleyite crystals documented in this study occur either in vesicles or in vesicle-rich regions of the host lavas whose textures developed over timescales of a few years to a few decades based on microlite crystal size distribution (CSD) analysis. Radiometric U-Th baddeleyite ages are mostly in agreement with previously determined K-Ar eruption ages, except for the Punta Marmolite lava dome whose U-Th baddeleyite age predates the K-Ar age by ca. 15 ka. Baddeleyite thus records eruptive emplacement with little evidence for significant pre-eruptive crystal residence, and has potential as an eruption chronometer for Quaternary silica-undersaturated volcanic rocks.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 100
Serial Issue: 10
Title: U-Th baddeleyite geochronology and its significance to date the emplacement of silica undersaturated magmas
Affiliation: University of California at Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, CA, United States
Pages: 2082-2090
Published: 201510
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 72
Accession Number: 2016-007292
Categories: GeochronologyIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
N40°49'60" - N40°49'60", E14°07'60" - E14°07'60"
Secondary Affiliation: Istituto Nazionale di Geofisica e Vulcanologia, ITA, Italy
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201604
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