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Nucleation rates of spherulites in natural rhyolitic lava

James E. Gardner, Kenneth S. Befus, James M. Watkins and Travis Clow
Nucleation rates of spherulites in natural rhyolitic lava
American Mineralogist (November 2016) 101 (11): 2367-2376

Abstract

The rates of nucleation and crystal growth from silicate melt are difficult to measure because the temperature-time path of magma is often unknown. We use geochemical gradients around spherulites in obsidian glass to estimate the temperature-time interval of spherulite crystallization. This information is used in conjunction with new high-resolution X-ray computed tomography (HRXCT) data on the size distributions of spherulites in six samples of rhyolite obsidian lava to infer spherulite nucleation rates. A large data set of geochemical profiles indicate that the lavas cooled at rates of 10 (super -2.2) to 10 (super -1.2) degrees C/h, and that the spherulites grew at rates that decreased exponentially with time, with values of 10 (super -0.70) to 10 (super 0.30) mu m/h at 600 degrees C. Spherulites are estimated to have begun nucleating when undercooling [Delta T, = liquidus T ( approximately 800 degrees C) minus nucleation T] reached 100-277 degrees C, and stopped when Delta T = 203-365 degrees C, with exact values dependent on assumed cooling and growth rates. Regardless of rates, we find that spherulites nucleated within a approximately 88-113 degrees C temperature interval and, hence, began when Delta T nearly equal 0.65-0.88 X T (sub L) , peaking when Delta T nearly equal 0.59-0.80 X T (sub L) A peak rate of nucleation of 0.072 + or - 0.049 cm (super -3) h (super -1) occurred at 533 + or - 14 degrees C, using cooling and growth rates that best fit the data set of geochemical profiles. While our inferred values for Delta T overlap those from experimental studies, our nucleation rates are much lower. That difference likely results from experimental studies using hydrous melts; the natural spherulites grew in nearly anhydrous glass.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 101
Serial Issue: 11
Title: Nucleation rates of spherulites in natural rhyolitic lava
Affiliation: University of Texas at Austin, Jackson School of Geosciences, Austin, TX, United States
Pages: 2367-2376
Published: 201611
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 40
Accession Number: 2016-110932
Categories: Igneous and metamorphic petrologyGeneral mineralogy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
N44°10'00" - N45°00'00", W111°04'60" - W109°49'60"
Secondary Affiliation: Baylor University, USA, United StatesUniversity of Oregon, USA, United States
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: 201652
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