Noble gases and halogens in altered MORB and implications for seawater recycling

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Author(s): Chavrit, D.; Burgess, R.; Weston, B.; Abbott, L.; Ballentine, C. J.; Teagle, D. A.; Droop, G.; Pawley, A.
Author Affiliation(s): Primary:
University of Manchester, School of Earth, Atmospheric and Environmental Science, Manchester, United Kingdom
University of Southampton, United Kingdom
Volume Title: AGU 2011 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2011; American Geophysical Union 2011 fall meeting, San Francisco, CA, Dec. 5-9, 2011. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English. Accessed on Sept. 12, 2014. 2 refs.
Summary: Recent studies suggested that the heavy noble gases and halogens are significantly recycled into the mantle through the subduction zone, since a seawater derived signature has been recorded in the mantle [1,2]. However, the processes of how these elements preserve the unique elemental composition of seawater from the ocean floor to the subduction process are unknown. Thus, we propose here a study of altered oceanic crust to identify the respective major host phases of both noble gases and halogens in pre-subducted material. Ten altered MORB coming from the ODP sites 504, 896 and 1256 in the Eastern Pacific Ocean (5.9 and 15 Ma) are being studied. Noble gas isotopes and abundances have been determined using an upgraded VG5400 mass spectrometer. Halogens (Cl, Br, I) are obtained by measuring noble gases on irradiated samples using a MS1 mass spectrometer. The analyses were all obtained from in vacuo crushing release of whole rock samples. Preliminary results show Ne to Xe isotopically identical to air. 3He/4He ratios vary from 4.5±0.5 (R/RA) to MORB-like values. Heavy noble gas elemental ratios for 9 samples fall within a narrow range, with 130Xe/36Ar and 84Kr/36Ar ratios varying respectively by 10% and 30%. They range from values close to seawater (± air) to values enriched in Xe and Kr going towards mantle values [1]. One altered MORB shows 130Xe/36Kr and 84Kr/36Ar ratios respectively 2 and 5 times greater than the average of the other samples, which suggests the contribution of a sediment component in this sample. Halogen data obtained on 4 samples are in accordance with noble gases results. The Br/Cl molar ratio is constant (1.97±0.15.10-3) while the I/Cl molar ratio varies significantly by up to one order of magnitude, ranging from 1.2±0.3.10-6 to 9.9±0.8.10-6. These results are in favor of a mixing between a seawater endmember and a sediment pore fluid endmember, which remains to be identified. The measurements will be extended to gabbros and sediments forming the oceanic crust. Also, the addition of data obtained by heating to those obtained by crushing will allow us to better constrain the partitioning of both the noble gases and the halogens in pre-subducted material, the identification of the host phases, as well as the controls of the seawater noble gases interaction with the oceanic crust. [1] Holland & Ballentine (2006), Nature 441, 186-191. [2] Sumino et al. (2010), EPSL 294, 163-172.
Year of Publication: 2011
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 05 Petrology, Igneous and Metamorphic; Argon; Basalts; Br/Cl; Bromine; Chemical composition; Chlorine; Crust; DSDP Site 504; East Pacific; Equatorial Pacific; Halogens; He-4/He-3; Helium; I/Cl; Igneous rocks; Iodine; Isotope ratios; Isotopes; Kr-84/Ar-36; Krypton; Leg 148; Mass spectra; Mid-ocean ridge basalts; Noble gases; North Pacific; Northeast Pacific; ODP Site 1256; ODP Site 896; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Plate tectonics; Sea water; Spectra; Stable isotopes; Subduction; Volcanic rocks; Xe-130/Kr-36; Xenon
Coordinates: N011335 N011338 W0834348 W0834357
N011301 N011301 W0834323 W0834323
N064400 N064400 W0915600 W0915600
Record ID: 2014092548
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