HR: 1340h
AN: V13C-0558    [Abstracts]
TI: An Air Noble Gas Component in the Mantle
AU: * Sarda, P
EM: sarda@geol.u-psud.fr
AF: University Paris 11 at Orsay - UMR CNRS 8148, bat . 504, Orsay, 91405 France
AB: Noble gas geochemistry has for long attempted to recover isotopic signatures of mantle components through analyses of basalt glass or xenoliths, but this quest has been plagued by the occurrence of a conspicuous air component, which appears to have both the isotopic and elemental composition of air (except for helium). It is classically considered to be air added to samples close to the surface, in a poorly understood process called "contamination", due to the interaction of rocks and melts with air or water on emplacement. Focusing on Mid-Ocean Ridge Basalts (and Ocean Island Basalts), gases are mostly borne by vesicles and a number of puzzling observations can be made:
- vesicles appear to be heterogeneous at the scale of a centimeter, as shown by stepwise crushing experiments, some vesicles having air, some having mantle gases, some having mixtures of both,
- vesicles appear over-pressured (P > 1 bar) in fresh samples, as shown by highly vesiculous samples such as Popping Rocks,
- the air component appears to be borne by the largest vesicles, as it is recovered in the first steps of stepwise crushing analyses,
- larger samples seem to have more of the air component than smaller ones,
- in Popping Rocks, the air component borne by the largest vesicles is overwhelming,
- the isotopic composition of Pb-Sr-Nd in Popping Rocks was interpreted as indicating a recycled component (related to the HIMU and EM1 mantle end-members). The air noble gas component was suggested recently to be not seawater, but modern air located in fractures of the glass, which should have opened on cooling and resealed immediately [1]. This model faces some difficulties, such as keeping pressure high in the vesicles. I suggest another interpretation, namely that a large part of the air noble gases in oceanic basalts is recycled in origin [2]. It would have been carried down into the mantle at subduction zones, even if most (typically 90%) of the air noble gases in the slab returns to the atmosphere there [3]. The upper mantle would thus be a marble cake and, on melting, the recycled, more fertile component would melt first. The first vesicles would then bear the atmospheric noble gas component, while the next liquids would be produced more from the peridotite matrix, and would eventually generate vesicles with mantle noble gases (⁴⁰Ar/³⁶Ar > 10,000, ¹²⁹Xe/¹³⁰Xe > 7, . . .). A simple model for this double-step melting, using classical equations for trace elements and assuming that vesicles appear as soon as melt is generated, reproduces the ⁴⁰Ar/³⁶Ar pattern seen on step-crushing of popping rocks, with reasonable values for partition coefficients and proportion of recycled matter. Although this model remains to be investigated, part of the atmospheric noble gases in oceanic basalts may be recycled in origin. This hypothesis simply explains several of the observations above. Especially, it can reconcile noble gas and non-gaseous isotopic tracers by allowing recycled noble gases to occur together with mantle noble gases, in vesicles with different sizes. [1] C.J. Ballentine and D.N. Barfod, The origin of air-like noble gases in MORB and OIB, Earth Planet Sci. Lett. 180, 39-48, 2000. [2] Ph. Sarda, Surface noble gas recycling to the terrestrial mantle, Earth Planet Sci. Lett. 228, 49-63, 2004. [3] T. Staudacher and C.J. AllŠgre, Recycling of oceanic crust and sediments: the noble gas subduction barrier, Earth Planet. Sci. Lett. 89, 173-183, 1988.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
DE: 8427 Subaqueous volcanism
DE: 8430 Volcanic gases
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005