Geochemistry of the Halogens (I, Br, Cl) and Evolution of the Seawater During the Archean

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Guillaume Avice1, Bernard Marty1, Ray Burgess2 and Sarah L Goldsmith2, (1)CRPG Centre de Recherches Pétrographiques et Géochimiques, Vandoeuvre-Les-Nancy, France, (2)University of Manchester, SEAES, Manchester, United Kingdom
Halogens (I, Cl, Br) on Earth are depleted relative to the results of the condensation sequence of elements from the solar nebula [1]. The cause of this depletion remains unknown but might be related to loss from impacts or to the presence of an hidden reservoir (e.g. core). The presence of fluid inclusions in hydrothermal quartz of various ages is an opportunity to follow the evolution of the composition of the atmosphere (N, noble gases) through time [2] but also of halogens in seawater (their main reservoir). Such studies may help to constrain what is the origin of these volatile elements on Earth. For this purpose, we analysed quartz from hydrothermal veins and pods of various localities and ages (e.g. Barberton Greenstone Belt (South Africa; 3.2 Ga), Fortescue Group (Australia; 2.7 Ga)) by crushing and step-heating following the extended Ar-Ar method [3]. In Barberton and Fortescue Gp. quartz, a hydrothermal component characterized by excess 40Ar (40ArE) and closely associated with chlorine is clearly identified from crushing and step-heating results and permits to define a 40ArE/Cl used to correct Ar-Ar data for this contribution. In the case of Barberton, the correction for the hydrothermal component leads to an approximate formation age of the quartz of 3.2 (+/- 0.1) Ga. In Barberton quartz, halogens elemental ratios obtained during crushing and step-heating experiments are highly enriched in iodine and bromine (I/Cl=2-7x10-4; Br/Cl=4-10x10-3) relative to modern seawater. These results are similar to those found in a previous study of Barberton fluid inclusions [3] and can be interpreted as indicators that organic carbon was not sufficiently abundant at this time and/or this location to efficiently pump Br and I during sediment burial. Iodine and bromine, in Fortescue Gp. quartz, are less enriched suggesting that, in that case, organic activity already acted as pump for these elements. [1] Sharp & Draper (2013) EPSL, 369-370, p. 71-77 [2] Pujol et al. (2011) EPSL, 308, p. 298-306 [3] Channer et al. (1997) EPSL, 150, p. 325-335