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Melt inclusion CO (sub 2) contents, pressures of olivine crystallization, and the problem of shrinkage bubbles

Paul J. Wallace, Vadim S. Kamenetsky and Pablo Cervantes
Melt inclusion CO (sub 2) contents, pressures of olivine crystallization, and the problem of shrinkage bubbles
American Mineralogist (April 2015) 100 (4): 787-794

Abstract

The H (sub 2) O and CO (sub 2) contents of melt inclusions can potentially be used to infer pressures of crystallization and inclusion entrapment because the solubility of mixed H (sub 2) O-CO (sub 2) vapor has been determined experimentally for a wide range of melt compositions. However, melt inclusions commonly develop a shrinkage bubble during post-entrapment cooling and crystallization because these processes cause a pressure drop in the inclusion. This pressure drop causes a vapor bubble to nucleate, leading to exsolution of low-solubility CO (sub 2) from the trapped melt. To investigate the loss of CO (sub 2) into such bubbles, we experimentally heated large, naturally glassy melt inclusions in olivine (Fo contents of 88.1 + or - 0.2) from a Mauna Loa picrite to rehomogenize the inclusions. Rapid heating to 1420 degrees C using a high-temperature heating stage dissolved the shrinkage bubbles into the melt. CO (sub 2) contents measured by FTIR spectroscopy and recalculated for melt in equilibrium with the olivine host are 224-505 ppm (n = 11) for heated inclusions, much higher than the CO (sub 2) contents of naturally quenched inclusions from the same sample (38-158 ppm; n = 8). Pressures of inclusion entrapment calculated from the H (sub 2) O and CO (sub 2) data for the heated inclusions range from 0.5 to 1.1 kbar, indicating that Mg-rich olivine crystallized at very shallow depths beneath the surface of Mauna Loa. Our results indicate that 40-90% (average 75%) of the original CO (sub 2) dissolved in the melt at the time of inclusion entrapment can be lost to the shrinkage bubble during post-entrapment cooling. We show that the computational method of Riker (2005), which predicts the pre-eruption shrinkage bubble size as a function of the difference between trapping temperature and pre-eruption temperature, successfully reproduces our experimental results. Our results demonstrate that the mass of CO (sub 2) contained in shrinkage bubbles must be considered to accurately infer original pressures of crystallization for melt inclusions. However, the effect is expected to be smaller for more H (sub 2) O-rich melt inclusions than those studied here because the vapor bubble in such inclusions will have lower mole fractions of CO (sub 2) than the low-H (sub 2) O inclusions in our study.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 100
Serial Issue: 4
Title: Melt inclusion CO (sub 2) contents, pressures of olivine crystallization, and the problem of shrinkage bubbles
Affiliation: University of Oregon, Department of Geological Sciences, Eugene, OR, United States
Pages: 787-794
Published: 201504
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 40
Accession Number: 2015-052269
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Secondary Affiliation: University of Tasmania, AUS, AustraliaTexas A&M University, 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: 201524
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