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Depth and timing of calcite spar and "spar cave" genesis; implications for landscape evolution studies

David D. Decker, Victor J. Polyak and Yemane Asmerom
Depth and timing of calcite spar and "spar cave" genesis; implications for landscape evolution studies (in Caves and karst across time, Joshua Feinberg (editor), Yongli Gao (editor) and E. Calvin Alexander (editor))
Special Paper - Geological Society of America (June 2015) 516: 103-111


Calcite spar (crystals >1 cm in diameter) are common in limestone and dolostone terrains. In the Guadalupe Mountains, New Mexico and west Texas, calcite spar is abundant and lines small geode-like caves. Determining the depth and timing of formation of these large scalenohedral calcite crystals is critical in linking the growth of spar with landscape evolution. In this study, we show that large euhedral calcite crystals precipitate deep in the phreatic zone (400-800 m) in these small geode-like caves (spar caves), and we propose both are the result of properties of supercritical CO (sub 2) at that depth. U-Pb dating of spar crystals shows that they formed primarily between 36 and 28 Ma. The (super 87) Sr/ (super 86) Sr values of the euhedral calcite spar show that the spar has a significantly higher (super 87) Sr/ (super 86) Sr (0.710-0.716) than the host Permian limestone (0.706-0.709). This indicates the spar formed from waters that are mixed with, or formed entirely from, a source other than the surrounding bedrock aquifer, and this is consistent with hypogene speleogenesis at significant depth. In addition, we conducted highly precise measurements of the variation in nonradiogenic isotopes of strontium, (super 88) Sr/ (super 86) Sr, expressed as delta (super 88) Sr, the variation of which has previously been shown to depend on temperature of precipitation. Our preliminary delta (super 88) Sr results from the spar calcite are consistent with formation at 50-70 degrees C. Our first U-Pb results show that the spar was precipitated during the beginning of Basin and Range tectonism in a late Eocene to early Oligocene episode, which was coeval with two major magmatic periods at 36-33 Ma and 32-28 Ma. A novel speleogenetic process that includes both the dissolution of the spar caves and precipitation of the spar by the same speleogenetic event is proposed and supports the formation of the spar at 400-800 m depth, where the transition from supercritical to subcritical CO (sub 2) drives both dissolution of limestone during the main speleogenetic event and precipitation of calcite at the terminal phase of speleogenesis. We suggest that CO (sub 2) is derived from contemporaneous igneous activity. This proposed model suggests that calcite spar can be used for reconstruction of landscape evolution.

ISSN: 0072-1077
EISSN: 2331-219X
Serial Title: Special Paper - Geological Society of America
Serial Volume: 516
Title: Depth and timing of calcite spar and "spar cave" genesis; implications for landscape evolution studies
Title: Caves and karst across time
Author(s): Decker, David D.Polyak, Victor J.Asmerom, Yemane
Author(s): Feinberg, Joshuaeditor
Author(s): Gao, Yonglieditor
Author(s): Alexander, E. Calvin, Jr.editor
Affiliation: University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM, United States
Pages: 103-111
Published: 20150624
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
Number of pages: 11
References: 35
Accession Number: 2015-071692
Categories: Geomorphology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, geol. sketch map
N32°01'60" - N32°10'00", W104°42'00" - W104°22'60"
Source Note: Online First
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201531
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