Halogens in chondritic meteorites and terrestrial accretion

Abstract

Volatile element delivery and retention played a fundamental part in Earth's formation and subsequent chemical differentiation. The heavy halogens-chlorine (Cl), bromine (Br) and iodine (I)-are key tracers of accretionary processes owing to their high volatility and incompatibility, but have low abundances in most geological and planetary materials. However, noble gas proxy isotopes produced during neutron irradiation provide a high-sensitivity tool for the determination of heavy halogen abundances. Using such isotopes, here we show that Cl, Br and I abundances in carbonaceous, enstatite, Rumuruti and primitive ordinary chondrites are about 6 times, 9 times and 15-37 times lower, respectively, than previously reported and usually accepted estimates. This is independent of the oxidation state or petrological type of the chondrites. The ratios Br/Cl and I/Cl in all studied chondrites show a limited range, indistinguishable from bulk silicate Earth estimates. Our results demonstrate that the halogen depletion of bulk silicate Earth relative to primitive meteorites is consistent with the depletion of lithophile elements of similar volatility. These results for carbonaceous chondrites reveal that late accretion, constrained to a maximum of 0.5 ± 0.2 per cent of Earth's silicate mass, cannot solely account for present-day terrestrial halogen inventories. It is estimated that 80-90 per cent of heavy halogens are concentrated in Earth's surface reservoirs and have not undergone the extreme early loss observed in atmosphere-forming elements. Therefore, in addition to late-stage terrestrial accretion of halogens and mantle degassing, which has removed less than half of Earth's dissolved mantle gases, the efficient extraction of halogen-rich fluids from the solid Earth during the earliest stages of terrestrial differentiation is also required to explain the presence of these heavy halogens at the surface. The hydropilic nature of halogens, whereby they track with water, supports this requirement, and is consistent with volatile-rich or water-rich late-stage terrestrial accretion.

The composition of Mars

“…Errors are reported as 1 standard deviations, unless otherwise noted. For a CI chondritic composition, we adopted the value proposed by for most elements, with some modifications for halogens (Clay et al, 2017), Mo, Tl, Bi (Wang et al, 2015), highly siderophile elements (Day et al, 2016), and U (Wipperfurth et al, 2018) (Table 2).…”

The late Earth's accretion: Processes and materials

“…Thus, new estimate of the average iodine concentration in carbonaceous chondrites, 127 I CH = 4.5 × 10 −10 mol g −1 [Clay et al, 2017], gives 129 Xe(I)/ 136 Xe(Pu) CH = 1.5 × 10 3 , which is quite a large value still.…”

“…Regarding the abundance and source of terrestrial iodine, it should be noted that to provide the planet with volatile components, it is necessary to add ≈ 0.015× M EARTH of a chondrites-like material [Clay et al, 2017;Kramers, 2003;Marty, 2012]. Such an amount corresponds to 127 I BSE ≈ 10 × 10 −9 g g −1 , which is close to the maximum value used by Caracausi et al [2016].…”

The chlorine isotope composition of iron meteorites: Evidence for the Cl isotope composition of the solar nebula and implications for extensive devolatilization during planet formation

“…The necessary chondritic component is increased further if lower chlorine contents are used (i.e., Clay et al. ). Additionally, an independent estimate that satisfies other isotopic constraints (i.e., Dauphas et al.…”

“…To account for 85% of Earth's chlorine by ECs requires 3.6% EC addition when using the highest range of chlorine concentrations . The necessary chondritic component is increased further if lower chlorine contents are used (i.e., Clay et al 2017). Additionally, an independent estimate that satisfies other isotopic constraints (i.e., Dauphas et al 2014) using some amount of ordinary chondrites would require an even larger chondritic component.…”