Tracing helium isotope compositions from mantle source to fumaroles at Oldoinyo Lengai volcano, Tanzania

Type Article
Date 2018-03
Language English
Author(s) Mollex Gaelle1, Furi EvelynORCID1, Burnard Pete1, Zimmermann Laurent1, Chazot Gilles2, Kazimoto Emmanuel O.3, Marty Bernard1, France Lyderic1
Affiliation(s) 1 : Univ Lorraine, CNRS, UMR 7358, CRPG, 15 Rue Notre Dame Pauvres, F-54501 Vandoeuvre Les Nancy, France.
2 : Univ Brest, CNRS, UMR Domaines Ocean 6538, Inst Univ Europeen Mer, Pl Copernic, F-29280 Plouzane, France.
3 : Univ Dar Es Salaam, Dept Geol, POB 35052, Dar Es Salaam, Tanzania.
Source Chemical Geology (0009-2541) (Elsevier Science Bv), 2018-03 , Vol. 480 , P. 66-74
DOI 10.1016/j.chemgeo.2017.08.015
WOS© Times Cited 14
Keyword(s) Carbonatite, Neon, 2007-2008 eruption, SCLM, Cognate xenoliths, Helium, Neon, Argon

Oldoinyo Lengai is the only volcano on Earth currently erupting natrocarbonatites, of which the source and genesis remain controversial. Cognate xenoliths and fumaroles were sampled at the summit of Oldoinyo Lengai, and deep crustal xenoliths from Oltatwa maar, in 2010 and 2014, after the 2007-2008 sub-Plinian eruption. The summit cognate xenoliths provide direct information on the isotopic composition of the mid-crustal magma chamber that was active during the 2007-2008 explosive eruption. Cognate xenolith-hosted pyroxenes from Oldoinyo Lengai have an average He-3/He-4 = 6.58 +/- 0.46 R-A, similar to values from nearby silicate volcanoes (4.95-7.30 R-A), and reflecting a sub-continental lithospheric mantle (SCLM) signature. This similarity implies that Oldoinyo Lengai carbonatites form from a similar mantle reservoir as the nearby silicate volcanoes. We identify SCLM, metasomatized by fluids/melts derived from the depleted convective mantle, as the common source of magmas in the Arusha volcanic province. Fumarole measurements highlight that fumarolic He-3/He-4 values have been relatively constant since at least 1988, indicating that dramatic changes to the crater region morphology during the 2007-2008 eruption did not affect the architecture of the hydrothermal system, which is probably connected to the crustal magma chamber(s). Moreover, the similarity between He-3/He-4 values from the mid-crustal magma chamber (6.58 +/- 0.46 R-A) and fumaroles (7.31 +/- 0.24 R-A) of Oldoinyo Lengai attests that helium is not subjected to atmospheric contamination or crustal assimilation during transport to the surface.

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