Plate tectonic cycling modulates Earth's 3He/22Ne ratio

Abstract

The ratio of 3 He and 22 Ne varies throughout the mantle. This observation is surprising because 3 He and 22 Ne are not produced in the mantle, are highly incompatible during mantle melting, and are not recycled back into the mantle by subduction of oceanic sediment or basaltic crust. Our new compilation yields average 3 He/ 22 Ne ratios of 7.5±1.2 and 3.5±2.4 for mid-ocean ridge basalt (MORB) mantle and ocean island basalt (OIB) mantle sources respectively. The low 3 He/ 22 Ne of OIB mantle approaches planetary precursor 3 He/ 22 Ne values; ~1 for chondrites and ~1.5 for the solar nebula. The high 3 He/ 22 Ne of the MORB mantle is not similar to any planetary precursor, requiring a mechanism for fractionating He from Ne in the mantle and suggesting isolation of distinct mantle reservoirs throughout geologic time. New experimental results reported here demonstrate that He and Ne diffuse at rates differing by one or more orders of magnitude at relevant temperatures in mantle materials. We model the formation of a MORB mantle with an elevated 3 He/ 22 Ne ratio through kinetically modulated chemical exchange between dunite channel-hosted basaltic liquids and harzburgite wallrock beneath mid-ocean ridges. Over timescales relevant to mantle upwelling beneath spreading centers, He may diffuse tens to hundreds of meters into wallrock while Ne is effectively immobile, producing a mantle lithosphere regassed with respect to He and depleted with respect to Ne, with a net elevated 3 He/ 22 Ne. Subduction of high 3 He/ 22 Ne mantle lithosphere throughout geologic time would generate a MORB source with high 3 He/ 22 Ne. Mixing models suggest that to preserve a high 3 He/ 22 Ne reservoir, MORB mantle mixing timescales must be on the order of hundreds of millions of years or longer, that mantle convection has not been layered about the transition zone for most of geologic time, and that Earth's convecting mantle has lost at least 96% of its 3 primordial volatile elements. The most depleted, highest 3 He/ 22 Ne mantle may be best preserved in the lower mantle where relatively high viscosities impede mechanical mixing.

Identification of chondritic krypton and xenon in Yellowstone gases and the timing of terrestrial volatile accretion

“…The 40 Ar/ 36 Ar of the Brimstone Basin samples reach values greater than 1,400, which are the highest values yet measured in Yellowstone magmatic gas samples (37) and significantly higher than that measured in Mud Volcano and Turbid Lake (SI Appendix, Table S2). Importantly, crustal Ar contributions in the Brimstone Basin samples can be accounted for to derive the underlying mantle argon compositions (SI Appendix).…”

“…Importantly, crustal Ar contributions in the Brimstone Basin samples can be accounted for to derive the underlying mantle argon compositions (SI Appendix). Following correction for crustal 40 Ar* contribution, the 40 Ar/ 36 Ar of Brimstone Basin sample 4B is 1,001 ± 42, which is still in excess of atmosphere. The corrected 40 Ar/ 36 Ar of Brimstone Basin remains higher than the other samples, suggesting that Brimstone Basin has the lowest degree of air contamination and greatest contribution of mantle-derived Ar.…”

“…Following correction for crustal 40 Ar* contribution, the 40 Ar/ 36 Ar of Brimstone Basin sample 4B is 1,001 ± 42, which is still in excess of atmosphere. The corrected 40 Ar/ 36 Ar of Brimstone Basin remains higher than the other samples, suggesting that Brimstone Basin has the lowest degree of air contamination and greatest contribution of mantle-derived Ar. Unlike Mud Volcano and Turbid Lake, Brimstone Basin is not a thermal area but is still a significant degassing site (38).…”

“…To account for the Ne isotopic dichotomy between the plume and MORB mantle reservoirs, it has been suggested that the MORB mantle originated from the plume mantle but was subsequently modified by additions of chondritic material following the dispersion of the solar nebula (4). This scenario is based on the fact that, in 20 Ne/ 22 Ne versus 36 Ar/ 22 Ne or 130 Xe/ 22 Ne space, plume-influenced basalts and MORBs lie along a mixing line between the solar nebula composition and a chondritic component similar to CI chondrites and/or ocean water values. Both mantle reservoirs would hence have originated with a solar noble gas composition, before being modified to variable extents by the latter addition of chondritic material (4).…”

Magmatism in the North Atlantic Igneous Province; mantle temperatures, rifting and geodynamics

“…The first case requires a helium contribution from primitive, unfractionated, undegassed sources deep in the mantle or in the core (Bouhifd et al, 2013;Dygert et al, 2018) that retain near-primordial high 3 He/ 4 He. This primitive mantle must be separated from degassed uppermantle by a boundary layer at 670 km or at the core-mantle boundary, such that hypothetical hot mantle plumes entrain 3 He from the undegassed source.…”