14:15 〜 14:45
[SGC32-06] Volatile characteristics of Central American geothermal fluids8.6.0
★Invited Papers
キーワード：Carbon, Helium, Nitrogen, Central America, Costa Rica, Panama
Earth’s various mantle reservoirs (e.g., depleted mid ocean ridge basalt (MORB) mantle, plume mantle source) retain distinct volatile geochemical features that inform our understanding of planetary accretion, mantle convection and mixing, as well as subduction-driven recycling processes. At subduction zones, volatile elements (e.g., helium, carbon, nitrogen) are actively cycled between terrestrial reservoirs by plate tectonics. The efficiency of volatile transfers between Earth’s interior (crust and mantle) and exterior (atmosphere and oceans) controls Earth’s redox conditions, mantle heterogeneities and atmospheric evolution. The distribution of terrestrial volatiles, which is controlled by the long-term balance between volcanism and recycling, has enabled conditions favorable for life on Earth. Despite the importance of volatile elements on a global scale, their fluxes and sources (slab vs. mantle), and sinks however remain under-constrained, particularly in subduction-related fluids.
The Central America Volcanic Arc (CAVA), is one of the best studied1,2,3 arc segments globally. However, relatively few studies have focused on constraining volatile fluxes throughout the southern segment of the CAVA due to the lack of active volcanism in the region. This area is tectonically unique, as the border between Costa Rica and Panama represents the transition between the orthogonal subduction of the Cocos Plate and the oblique subduction of the Nazca Plate relative to the Caribbean Plate. Here, we present unpublished He, CO2 and N2 isotope and relative abundance data from geothermal fluids in southern Costa Rica and western Panama. In total, isotope data are reported for 65 deeply sourced wells, seeps and springs, as well as gas samples. These data reveal a clear southeastward increase in 3He/4He, from typical volcanic arc values (about 7 times the atmospheric ratio, RA) in Costa Rica, up to MORB-like values (8 plus/minus1 RA) in the volcanically-dormant region of western Panama. Remarkably, "cold" seeps in Panama display 3He/4He up to 8.9 RA, equivalent to the highest values ever reported for active volcanic arc settings worldwide. These data suggest contribution from a 3He-rich mantle source under western Panama, potentially associated with the Galapagos plume. d13C vary from –29.7‰ to +6.7‰ vs. PDVB, suggesting CO2 loss due to calcite precipitation, as it has been recently shown in Costa Rica’s Nicoya Peninsula4. N isotopes and clumped N isotopologues5 in CAVA samples cluster around +5‰ vs. air, suggesting N is mainly derived from sediments. Flux considerations suggest that subducting N may be quantitatively recycled into the arc, although uncertainties on fluxes allow up to 63% of slab-N to be transported into the mantle past the arc melting region, which is consistent with previous studies6,7. In summary, we show evidence for 1) the existence of a pervasive (plume) mantle component in southern CAVA samples, and 2) extensive CO2 fractionation during low temperature C sequestration as calcite in the forearc region and 3) strong N sediment signatures in arc fluids.
By combining these findings with other geochemical tracers (clumped N-isotopes, Pb isotopes, Ce/Pb8) and high-resolution simulations of global mantle flow around the Galapagos plume9, we conclude that there may be an asthenospheric “pipeline” connecting the Galapagos plume to a slab window that formed via subduction of a spreading ridge under western Panama10,11. This finding is globally significant, as the lateral entrainment and dispersal of plume material by large-scale asthenospheric mantle winds provides a straightforward explanation for the worldwide observation of OIB-like geochemical signatures in mantle domains that are remote from plume sources.
[1] Shaw et al., 2003.
[2] De Leeuw et al., 2007.
[3] de Moor et al., 2017.
[4] Barry et al., 2019.
[5] Labidi et al., 2021 (In Review - EPSL).
[6] Busigny et al. (2019)
[7] Bekaert et al., 2021.
[8] Gazel et al. 2011.
[9] Conrad and Behn 2010.
[10] Herrstrom et al. 1995.
[11] Abratis and Wörner 2001. 8.6.0
The Central America Volcanic Arc (CAVA), is one of the best studied1,2,3 arc segments globally. However, relatively few studies have focused on constraining volatile fluxes throughout the southern segment of the CAVA due to the lack of active volcanism in the region. This area is tectonically unique, as the border between Costa Rica and Panama represents the transition between the orthogonal subduction of the Cocos Plate and the oblique subduction of the Nazca Plate relative to the Caribbean Plate. Here, we present unpublished He, CO2 and N2 isotope and relative abundance data from geothermal fluids in southern Costa Rica and western Panama. In total, isotope data are reported for 65 deeply sourced wells, seeps and springs, as well as gas samples. These data reveal a clear southeastward increase in 3He/4He, from typical volcanic arc values (about 7 times the atmospheric ratio, RA) in Costa Rica, up to MORB-like values (8 plus/minus1 RA) in the volcanically-dormant region of western Panama. Remarkably, "cold" seeps in Panama display 3He/4He up to 8.9 RA, equivalent to the highest values ever reported for active volcanic arc settings worldwide. These data suggest contribution from a 3He-rich mantle source under western Panama, potentially associated with the Galapagos plume. d13C vary from –29.7‰ to +6.7‰ vs. PDVB, suggesting CO2 loss due to calcite precipitation, as it has been recently shown in Costa Rica’s Nicoya Peninsula4. N isotopes and clumped N isotopologues5 in CAVA samples cluster around +5‰ vs. air, suggesting N is mainly derived from sediments. Flux considerations suggest that subducting N may be quantitatively recycled into the arc, although uncertainties on fluxes allow up to 63% of slab-N to be transported into the mantle past the arc melting region, which is consistent with previous studies6,7. In summary, we show evidence for 1) the existence of a pervasive (plume) mantle component in southern CAVA samples, and 2) extensive CO2 fractionation during low temperature C sequestration as calcite in the forearc region and 3) strong N sediment signatures in arc fluids.
By combining these findings with other geochemical tracers (clumped N-isotopes, Pb isotopes, Ce/Pb8) and high-resolution simulations of global mantle flow around the Galapagos plume9, we conclude that there may be an asthenospheric “pipeline” connecting the Galapagos plume to a slab window that formed via subduction of a spreading ridge under western Panama10,11. This finding is globally significant, as the lateral entrainment and dispersal of plume material by large-scale asthenospheric mantle winds provides a straightforward explanation for the worldwide observation of OIB-like geochemical signatures in mantle domains that are remote from plume sources.
[1] Shaw et al., 2003.
[2] De Leeuw et al., 2007.
[3] de Moor et al., 2017.
[4] Barry et al., 2019.
[5] Labidi et al., 2021 (In Review - EPSL).
[6] Busigny et al. (2019)
[7] Bekaert et al., 2021.
[8] Gazel et al. 2011.
[9] Conrad and Behn 2010.
[10] Herrstrom et al. 1995.
[11] Abratis and Wörner 2001. 8.6.0