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Title: Effect of paleoseawater composition on hydrothermal exchange in midocean ridges

Abstract

Variations in the Mg, Ca, Sr, and SO 4 concentrations of paleoseawater can affect the chemical exchange between seawater and oceanic basalt in hydrothermal systems at midocean ridges (MOR). Here, we present a model for evaluating the nature and magnitude of these previously unappreciated effects, using available estimates of paleoseawater composition over Phanerozoic time as inputs and 87Sr/ 86Sr of ophiolite epidosites and epidote-quartz veins as constraints. The results suggest that modern hydrothermal fluids are not typical due to low Ca and Sr relative to Mg and SO 4 in modern seawater. At other times during the last 500 million years, particularly during the Cretaceous and Ordovician, hydrothermal fluids had more seawater-derived Sr and Ca, a prediction that is supported by Sr isotope data. The predicted 87Sr/ 86Sr of vent fluids varies cyclically in concert with ocean chemistry, with some values much higher than the modern value of ~0.7037. The seawater chemistry effects can be expressed in terms of the transfer efficiency of basaltic Ca and Sr to seawater in hydrothermal systems, which varies by a factor of ~1.6 over the Phanerozoic, with minima when seawater Mg and SO 4 are low. This effect provides a modest negative feedback onmore » seawater composition and 87Sr/ 86Sr changes. For the mid-Cretaceous, the low 87Sr/ 86Sr of seawater requires either exceptionally large amounts of low-temperature exchange with oceanic crust or that the weathering flux of continentally derived Sr was especially small. Lastly, the model also has implications for MOR hydrothermal systems in the Precambrian, when low-seawater SO 4 could help explain low seawater 87Sr/ 86Sr.« less

Authors:
ORCiD logo [1];  [2];  [3];  [3]
  1. Univ. of California, Berkeley, CA (United States). Department of Earth and Planetary Science
  2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Earth and Environmental Sciences
  3. Univ. of California, Berkeley, CA (United States). Department of Earth and Planetary Science; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Earth and Environmental Sciences
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1437974
Grant/Contract Number:  
AC02-05CH11231
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 114; Journal Issue: 47; Related Information: © 2017, National Academy of Sciences. All rights reserved.; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; paleoseawater; Sr isotopes; hydrothermal systems; midocean ridges

Citation Formats

Antonelli, Michael A., Pester, Nicholas J., Brown, Shaun T., and DePaolo, Donald J. Effect of paleoseawater composition on hydrothermal exchange in midocean ridges. United States: N. p., 2017. Web. doi:10.1073/pnas.1709145114.
Antonelli, Michael A., Pester, Nicholas J., Brown, Shaun T., & DePaolo, Donald J. Effect of paleoseawater composition on hydrothermal exchange in midocean ridges. United States. doi:10.1073/pnas.1709145114.
Antonelli, Michael A., Pester, Nicholas J., Brown, Shaun T., and DePaolo, Donald J. Mon . "Effect of paleoseawater composition on hydrothermal exchange in midocean ridges". United States. doi:10.1073/pnas.1709145114.
@article{osti_1437974,
title = {Effect of paleoseawater composition on hydrothermal exchange in midocean ridges},
author = {Antonelli, Michael A. and Pester, Nicholas J. and Brown, Shaun T. and DePaolo, Donald J.},
abstractNote = {Variations in the Mg, Ca, Sr, and SO4 concentrations of paleoseawater can affect the chemical exchange between seawater and oceanic basalt in hydrothermal systems at midocean ridges (MOR). Here, we present a model for evaluating the nature and magnitude of these previously unappreciated effects, using available estimates of paleoseawater composition over Phanerozoic time as inputs and 87Sr/86Sr of ophiolite epidosites and epidote-quartz veins as constraints. The results suggest that modern hydrothermal fluids are not typical due to low Ca and Sr relative to Mg and SO4 in modern seawater. At other times during the last 500 million years, particularly during the Cretaceous and Ordovician, hydrothermal fluids had more seawater-derived Sr and Ca, a prediction that is supported by Sr isotope data. The predicted 87Sr/86Sr of vent fluids varies cyclically in concert with ocean chemistry, with some values much higher than the modern value of ~0.7037. The seawater chemistry effects can be expressed in terms of the transfer efficiency of basaltic Ca and Sr to seawater in hydrothermal systems, which varies by a factor of ~1.6 over the Phanerozoic, with minima when seawater Mg and SO4 are low. This effect provides a modest negative feedback on seawater composition and 87Sr/86Sr changes. For the mid-Cretaceous, the low 87Sr/86Sr of seawater requires either exceptionally large amounts of low-temperature exchange with oceanic crust or that the weathering flux of continentally derived Sr was especially small. Lastly, the model also has implications for MOR hydrothermal systems in the Precambrian, when low-seawater SO4 could help explain low seawater 87Sr/86Sr.},
doi = {10.1073/pnas.1709145114},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 47,
volume = 114,
place = {United States},
year = {Mon Nov 06 00:00:00 EST 2017},
month = {Mon Nov 06 00:00:00 EST 2017}
}

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