HR: 1340h
AN: DI33A-1130    [Abstracts]
TI: Magma Dynamics at Mid-Ocean Ridges by Noble Gas Kinetic Fractionation: Assessment of Magmatic Ascent Rates and Mantle Composition
AU: * Paonita, A
EM: a.paonita@pa.ingv.it
AF: Istituto NAzionale di Geofisica e Vulcanologia, Sezione di Palermo, Via Ugo La Malfa 153, 90146 Palermo Italy, Palermo, ita 90146, Italy
AU: Martelli, M
EM: m.martelli@pa.ingv.it
AF: Istituto NAzionale di Geofisica e Vulcanologia, Sezione di Palermo, Via Ugo La Malfa 153, 90146 Palermo Italy, Palermo, ita 90146, Italy
AB: Topical scientific literature on magma degassing at mid-ocean ridges more and more focuses on exsolution processes occurring under conditions that are far from thermodynamic equilibrium between bubbles and silicate melt. Indeed, the dynamics of magma ascent and decompression can be faster than that of CO2 diffusion into bubbles, in which case the diffusivity ratios among volatiles are the main control of the composition of the exsolving gas phase. We have developed a model of bubble growth in silicate melts that calculates the extent of both CO2 supersaturation and kinetic fractionation among noble gases in vesicles in relation to the decompressive rate of basaltic melts. The model predicts that, due to comparable Ar and CO2 diffusivity, magma degassing at low pressure fractionates both He/Ar and He/CO2 ratios by a similar extent, while the slower CO2 diffusion at high pressure causes early kinetic effects on Ar/CO2 ratio and dramatically changes the degassing paths. By using this tool, we have reviewed the global He-Ar-CO2 dataset of fluid inclusions in mid-ocean-ridge glasses. We display that non-equilibrium fractionations among He, Ar and CO2, driven by their different diffusivities in silicate melts, are common in most of the natural conditions of magma decompression and their signature strongly depends on pressure of degassing. The different geochemical signatures among suites of data coming from different ridge segments mainly depend on the depth of the magma chamber where the melt was stored. Moreover, variations inside a single suite emerge due to the interplay between variable ascent speed of magma and cooling rate of the emplaced lava. As a result, two data groups coming from the Pito Seamount suite (Easter Microplate East ridge), showing different degree of CO2 supersaturation and He/Ar fractionation, provide ascent rates which differ by ten folds or even more. The large variations in both the He/CO2 and Ar/CO2 ratios at almost constant He/Ar, displayed in products coming from the Mid-Atlantic Ridge 24°N segment and the Rodriguez Triple Junction, require magma storage and degassing processes occurring at high-pressure conditions. In contrast, the simultaneous increase in both He/CO2 and He/Ar of the East Pacific Rise and South-East Indian Ridge data sets suggests the dominance of low-pressure fractionation, implying that the shallow magma chambers are at a lower depth than those of the Mid-Atlantic Ridge 24°N and Rodriguez Triple Junction. Our conclusions support the presence of a relationship between spreading rate and depth of high-temperature zones below ridges, and are consistent with the depth of magma chambers as suggested from seismic studies. Finally, the non-equilibrium degassing model provides striking constraints on the compositions of noble gases and carbon in mantle-derived magmas. Our results dispense in fact with the supposed need for He-Ar-CO2 heterogeneities in the upper mantle, because the degassing of a single, popping-rock-like primary magma is able to explain all the available data.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 8439 Physics and chemistry of magma bodies
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting