TY - JOUR
T1 - Deep pooling of low degree melts and volatile fluxes at the 85°E segment of the Gakkel Ridge
T2 - Evidence from olivine-hosted melt inclusions and glasses
AU - Shaw, Alison M.
AU - Behn, Mark D.
AU - Humphris, Susan E.
AU - Sohn, Robert A.
AU - Gregg, Patricia M.
N1 - Funding Information:
We thank Andrey Gurenko for assistance with ion probe measurements on WHOI's 1280 and Nilanjan Chatterjee for assistance with major element analyses at the MIT Electron Microprobe Facility. We also thank Erik Hauri for his hospitality and providing access to the Carnegie Institution of Washington's 6f ion microprobe for trace element analyses and Gary Huss for his assistance with the 1280 ion microprobe at the University of Hawaii. We thank Alberto Saal, Laurence Coogan, and Rajdeep Dasgupta for their thoughtful and thorough reviews of this manuscript. This study benefited from conversations with M. Laubier, J. Standish, A. Soule, G. Hirth, C. Pontbriand, T.L. Grove, and H.J.B. Dick. Funding was provided by NSF Grants OCE-0646694 & OCE-0926422 (AS), OCE-0649103 (MB), OCE-042538 (RS & SH), NASA , and a WHOI Mellon Joint Initiatives Award for Interdisciplinary Research (AS & MB).
PY - 2010/1/31
Y1 - 2010/1/31
N2 - We present new analyses of volatile, major, and trace elements for a suite of glasses and melt inclusions from the 85°E segment of the ultra-slow spreading Gakkel Ridge. Samples from this segment include limu o pele and glass shards, proposed to result from CO2-driven explosive activity. The major element and volatile compositions of the melt inclusions are more variable and consistently more primitive than the glass data. CO2 contents in the melt inclusions extend to higher values (167-1596 ppm) than in the co-existing glasses (187-227 ppm), indicating that the melt inclusions were trapped at greater depths. These melt inclusions record the highest CO2 melt concentrations observed for a ridge environment. Based on a vapor saturation model, we estimate that the melt inclusions were trapped between seafloor depths (∼ 4 km) and ∼ 9 km below the seafloor. However, the glasses are all in equilibrium with their eruption depths, which is inconsistent with the rapid magma ascent rates expected for explosive activity. Melting conditions inferred from thermobarometry suggest relatively deep (25-40 km) and cold (1240°-1325 °C) melting conditions, consistent with a thermal structure calculated for the Gakkel Ridge. The water contents and trace element compositions of the melt inclusions and glasses are remarkably homogeneous; this is an unexpected result for ultra-slow spreading ridges, where magma mixing is generally thought to be less efficient based on the assumption that steady-state crustal magma chambers are absent in these environments. All melts can be described by a single liquid line of descent originating from a pooled melt composition that is consistent with the aggregate melt calculated from a geodynamic model for the Gakkel Ridge. These data suggest a model in which deep, low degree melts are efficiently pooled in the upper mantle (9-20 km depth), after which crystallization commences and continues during ascent and eruption. Based on our melting model and the assumption that CO2 is perfectly incompatible, we show that the highest CO2 concentrations of the melt inclusions (∼ 1600 ppm) are consistent with the calculated CO2 concentrations of primary undegassed melts. The highest measured CO2/Nb ratio (443) of Gakkel Ridge melt inclusions predicts a mantle CO2 content of 134 ppm and would result in a global ridge flux of 2.0 × 1012 mol CO2/yr.
AB - We present new analyses of volatile, major, and trace elements for a suite of glasses and melt inclusions from the 85°E segment of the ultra-slow spreading Gakkel Ridge. Samples from this segment include limu o pele and glass shards, proposed to result from CO2-driven explosive activity. The major element and volatile compositions of the melt inclusions are more variable and consistently more primitive than the glass data. CO2 contents in the melt inclusions extend to higher values (167-1596 ppm) than in the co-existing glasses (187-227 ppm), indicating that the melt inclusions were trapped at greater depths. These melt inclusions record the highest CO2 melt concentrations observed for a ridge environment. Based on a vapor saturation model, we estimate that the melt inclusions were trapped between seafloor depths (∼ 4 km) and ∼ 9 km below the seafloor. However, the glasses are all in equilibrium with their eruption depths, which is inconsistent with the rapid magma ascent rates expected for explosive activity. Melting conditions inferred from thermobarometry suggest relatively deep (25-40 km) and cold (1240°-1325 °C) melting conditions, consistent with a thermal structure calculated for the Gakkel Ridge. The water contents and trace element compositions of the melt inclusions and glasses are remarkably homogeneous; this is an unexpected result for ultra-slow spreading ridges, where magma mixing is generally thought to be less efficient based on the assumption that steady-state crustal magma chambers are absent in these environments. All melts can be described by a single liquid line of descent originating from a pooled melt composition that is consistent with the aggregate melt calculated from a geodynamic model for the Gakkel Ridge. These data suggest a model in which deep, low degree melts are efficiently pooled in the upper mantle (9-20 km depth), after which crystallization commences and continues during ascent and eruption. Based on our melting model and the assumption that CO2 is perfectly incompatible, we show that the highest CO2 concentrations of the melt inclusions (∼ 1600 ppm) are consistent with the calculated CO2 concentrations of primary undegassed melts. The highest measured CO2/Nb ratio (443) of Gakkel Ridge melt inclusions predicts a mantle CO2 content of 134 ppm and would result in a global ridge flux of 2.0 × 1012 mol CO2/yr.
KW - CO fluxes
KW - mantle melting
KW - ultra-slow spreading ridges
KW - volatiles
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U2 - 10.1016/j.epsl.2009.11.018
DO - 10.1016/j.epsl.2009.11.018
M3 - Article
AN - SCOPUS:73449129294
VL - 289
SP - 311
EP - 322
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
SN - 0012-821X
IS - 3-4
ER -