Geochemistry, Geophysics, Geosystems RESEARCH ARTICLE Carbon Fluxes and Primary Magma CO2 Contents Along 10.1029/2018GC007630 the Global Mid-Ocean Ridge System Special Section: Marion Le Voyer1,2, Erik H. Hauri1,3 , Elizabeth Cottrell2 , Katherine A. Kelley4 , Carbon degassing through Vincent J. M. Salters5 , Charles H. Langmuir6 , David R. Hilton7,8, Peter H. Barry9 , volcanoes and active tectonic 10 regions and Evelyn Füri 1Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC, USA, 2Smithsonian Institution, Key Points: National Museum of Natural History, Washington, DC, USA, 3Deceased 5 September 2018, 4Graduate School of Oceanography, • New analyses (753) and compiled University of Rhode Island, Narragansett, RI, USA, 5National High Magnetic Field Laboratory and Department of Earth, Ocean data (2,446) for volatiles CO ,HO, F, 2 2 and Atmospheric Sciences, Florida State University, Tallahassee, FL, USA, 6Department of Earth and Planetary Sciences, S, and Cl are reported in a global suite 7 of mid-ocean ridge basalts Harvard University, Cambridge, MA, USA, Scripps Institution of Oceanography, University of California San Diego, San Diego, 8 9 10 • Estimated primary MORB CO2 CA, USA, Deceased 7 January 2018, Department of Earth Sciences, University of Oxford, Oxford, UK, Centre de Recherches contents vary from 104 ppm to Pétrographiques et Géochimiques, UMR7358, CNRS - Université de Lorraine, Vandoeuvre-lès-Nancy, France 1.9 wt% and are the result of variations in mantle composition • fl High CO2 uxes at ridges correlate Abstract The concentration of carbon in primary mid-ocean ridge basalts (MORBs), and the associated with high primary CO2 contents and are due to the presence of fluxes of CO2 outgassed at ocean ridges, is examined through new data obtained by secondary ion mass subduction components in MORB spectrometry (SIMS) on 753 globally distributed MORB glasses. MORB glasses are typically 80–90% degassed sources of CO2. We thus use the limited range in CO2/Ba (81.3 ± 23) and CO2/Rb (991 ± 129), derived from undegassed MORB and MORB melt inclusions, to estimate primary CO2 concentrations for ridges that have Supporting Information: • Supporting Information S1 Ba and/or Rb data. When combined with quality-controlled volatile-element data from the literature • Data Set S1 (n = 2,446), these data constrain a range of primary CO2 abundances that vary from 104 ppm to 1.90 wt%. Segment-scale data reveal a range in MORB magma flux varying by a factor of 440 (from 6.8 × 105 to 3.0 × 108 3 3 m /year) and an integrated global MORB magma flux of 16.5 ± 1.6 km /year. When combined with CO2/Ba Correspondence to: and CO /Rb-derived primary magma CO abundances, the calculated segment-scale CO fluxes vary by K. A. Kelley, 2 2 2 7 10 [email protected] more than 3 orders of magnitude (3.3 × 10 to 4.0 × 10 mol/year) and sum to an integrated global MORB CO2 fl : þ0:77 12 fl ux of 1 320:85 ×10 mol/year. Variations in ridge CO2 uxes have a muted effect on global climate; however, because the vast majority of CO degassed at ridges is dissolved into seawater and enters the marine Citation: 2 Le Voyer, M., Hauri, E. H., Cottrell, E., bicarbonate cycle. MORB degassing would thus only contribute to long-term variations in climate via degassing Kelley, K. A., Salters, V. J. M., Langmuir, directly into the atmosphere in shallow-water areas or where the ridge system is exposed above sea level. C. H., et al. (2019). Carbon fluxes and primary magma CO2 contents along the Plain Language Summary Estimated CO2 contents of primary mid-ocean ridge basalts (MORB), global mid-ocean ridge system. calculated on a segment-by-segment basis, vary from 104 ppm to 1.9 wt%. CO -enriched MORB are Geochemistry, Geophysics, Geosystems, 2 20, 1387–1424. https://doi.org/10.1029/ present in all ocean basins, in particular, in the Atlantic Ocean basin, which is younger and more likely to 2018GC007630 contain admixed material from recent subduction compared to the much older Pacific Ocean basin. CO2 fluxes at individual ridge segments vary by 3 orders of magnitude due primarily to large variability in primary Received 18 APR 2018 CO content. This study provides the most detailed and accurate estimate to date of the integrated total flux Accepted 26 SEP 2018 2 : þ0:77 12 Accepted article online 23 NOV 2018 of CO2 from mid-ocean ridges of 1 320:85 ×10 mol/year. Published online 14 MAR 2019 1. Introduction The 56,000-km length of the global mid-ocean ridge system represents the dominant locus of magma production and heat loss on Earth. Decades of deep-water oceanographic research have revealed that new seafloor is produced over a limited range of crustal thicknesses (6 ± 1 km, Behn & Grove, 2015; Klein & Langmuir, 1987; Van Avendonk et al., 2017; White et al., 1992) and at variable spreading rates that are well-described globally (e.g., Gale et al., 2013). As a result, magma production rates at mid-ocean ridges are well understood and the fluxes of nonvolatile elements emerging from the mantle at ridges are well constrained. These same research cruises have retrieved tens of thousands of individual submarine mid-ocean ridge basalt (MORB) samples, and the study of these samples has revealed that chemical variations in MORB magmas are produced by polybaric melting of mantle sources at variable temperatures, ©2018. American Geophysical Union. whereas local deviations from global trends are primarily due to variations in mantle composition (Gale et al., All Rights Reserved. 2014; Kelley et al., 2013; Kinzler & Grove, 1992; Klein & Langmuir, 1987; Langmuir et al., 1992). LE VOYER ET AL. 1387 Geochemistry, Geophysics, Geosystems 10.1029/2018GC007630 Carbon is present in MORBs in trace amounts as CO2-rich vesicles and as carbonate ions dissolved in quenched seafloor glass (Cartigny et al., 2008; Dixon & Stolper, 1995; Javoy & Pineau, 1991; Marty & Tolstikhin, 1998; Michael, 1995). The solubility of CO2 in basaltic magma is low at hydrostatic pressures corre- sponding to seafloor water depths (e.g., Dixon et al., 1995; Jendrzejewski et al., 1997; Shishkina et al., 2010), and thus nearly all magmas erupted at mid-ocean ridges have degassed the majority of their carbon. This flux of CO2 into the oceans contributes to the global surface carbon cycle, and changes in basaltic magma pro- duction and associated degassing have been proposed as one of several important forcing mechanisms that have influenced past global climate variations (Burley & Katz, 2015; Robock, 2000; Shindell et al., 2003; Tolstoy, 2015; Zhong et al., 2010). At the same time, it is now widely recognized that most of the Earth’s carbon is pre- sent in the Earth’s deep interior, rather than at Earth’s surface (Marty et al., 2013; Marty & Tolstikhin, 1998). Importantly, basaltic magmatism and subduction of oceanic crust represent the two largest outgassing and ingassing fluxes at the interface between the surface and deep-Earth carbon cycles. These observations provide the motivation of the present study, in which we apply the rapid, precise, and accurate measurement of volatiles by secondary ion mass spectrometry (SIMS; Hauri et al., 2002) to characterize the extensive worldwide sample collection of MORBs to examine in detail the flux of CO2 from the global ridge system. In this study we report new SIMS data on 753 geographically distributed MORB samples, and combine these data with a quality-controlled database of published data from an additional 2,446 samples (Figure 1). This combined data set permits a description of CO2 fluxes, and estimated primary magma CO2 contents, for 381 of the 458 global ridge segments with major element data, in addition to examination of the variability of primary MORB CO2 contents, the role of carbon in the melting processes that produce MORB, and the role of the oceanic crust in the context of the Earth’s deep carbon cycle. 2. Methods Here we briefly describe the methods used in this study for sample preparation, SIMS analysis of volatile elements, major elements, and trace elements. 2.1. Sample Preparation The majority of the basalt glasses analyzed in this study were obtained from the Rock and Ore collection of the Smithsonian Institution’s National Museum of Natural History. The equatorial mid-Atlantic Ridge glasses were obtained from the Schilling Volcanic Glass collection at the University of Rhode Island (University of Rhode Island Graduate School of Oceanography Marine Geological Samples Laboratory, 2018). Additional glass samples were provided from the Pacific-Antarctic Ridge (PACANTARCTIC 1 and 2 cruises, Clog et al., 2013; Hamelin et al., 2010; Labidi et al., 2014; Moreira et al., 2008; Vlastelic et al., 1999, 2000), the Central Indian Ridge (GIMNAUT, CD127, and KNOX11RR cruises, Füri et al., 2011; Murton et al., 2005; Barry, 2012), the Southwest Indian Ridge (PROTEA-5, MD34, AG22, and AG53 cruises, Arevalo & McDonough, 2010; B. Hamelin & Allègre, 1985; Janney et al., 2005; Le Roex et al., 1989; Mahoney et al., 1992), and the East Pacific Rise (CHEPR and PANORAMA-1 cruises, Langmuir, 2014; Salters et al., 2011). The major element, trace ele- ment, and isotopic ratio data for the glasses measured in this study were compiled from the literature (Clog et al., 2013; Cottrell & Kelley, 2011; C. Hamelin et al., 2010; Jenner & O’Neill, 2012; Labidi et al., 2014; Le Voyer et al., 2015; Moreira et al., 2008; Vlastelic et al., 2000, 1999; Nauret et al., 2006). We selected 1- to 3-mm glass chips that did not show any sign of alteration, crystallization, or devitrification. The chips were pressed into indium mounts together with secondary standards (ALV519-4-1 and VG2), and polished using silicon carbide papers, diamond paste, and finally 0.3-μm alumina paste.