A halogen budget of the bulk silicate Earth points to a history of early halogen degassing followed by net regassing

Halogens are important tracers of various planetary formation and evolution processes, and an accurate understanding of their abundances in the Earth’s silicate reservoirs can help us reconstruct the history of interactions among mantle, atmosphere, and oceans. The previous studies of halogen abundances in the bulk silicate Earth (BSE) are based on the assumption of constant ratios of element abundances, which is shown to result in a gross underestimation of the BSE halogen budget. Here we present a more robust approach using a log-log linear model. Using this method, we provide an internally consistent estimate of halogen abundances in the depleted mid-ocean ridge basalts (MORB)-source mantle, the enriched ocean island basalts (OIB)-source mantle, the depleted mantle, and BSE. Unlike previous studies, our results suggest that halogens in BSE are not more depleted compared to elements with similar volatility, thereby indicating sufficient halogen retention during planetary accretion. According to halogen abundances in the depleted mantle and BSE, we estimate that ∼87% of all stable halogens reside in the present-day mantle. Given our understanding of the history of mantle degassing and the evolution of crustal recycling, the revised halogen budget suggests that deep halogen cycle is characterized by efficient degassing in the early Earth and subsequent net regassing in the rest of Earth history. Such an evolution of deep halogen cycle presents a major step toward a more comprehensive understanding of ancient ocean alkalinity, which affects carbon partitioning within the hydrosphere, the stability of crustal and authigenic minerals, and the development of early life.

Geochemical characteristics of back-arc basin lower crust and upper mantle at final spreading stage of Shikoku Basin: an example of Mado Megamullion

This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling.

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

Identifying the origin of noble gases in Earth’s mantle can provide crucial constraints on the source and timing of volatile (C, N, H2O, noble gases, etc.) delivery to Earth. It remains unclear whether the early Earth was able to directly capture and retain volatiles throughout accretion or whether it accreted anhydrously and subsequently acquired volatiles through later additions of chondritic material. Here, we report high-precision noble gas isotopic data from volcanic gases emanating from, in and around, the Yellowstone caldera (Wyoming, United States). We show that the He and Ne isotopic and elemental signatures of the Yellowstone gas requires an input from an undegassed mantle plume. Coupled with the distinct ratio of129Xe to primordial Xe isotopes in Yellowstone compared with mid-ocean ridge basalt (MORB) samples, this confirms that the deep plume and shallow MORB mantles have remained distinct from one another for the majority of Earth’s history. Krypton and xenon isotopes in the Yellowstone mantle plume are found to be chondritic in origin, similar to the MORB source mantle. This is in contrast with the origin of neon in the mantle, which exhibits an isotopic dichotomy between solar plume and chondritic MORB mantle sources. The co-occurrence of solar and chondritic noble gases in the deep mantle is thought to reflect the heterogeneous nature of Earth’s volatile accretion during the lifetime of the protosolar nebula. It notably implies that the Earth was able to retain its chondritic volatiles since its earliest stages of accretion, and not only through late additions.

Low recycling efficiency of boron into the deep mantle

<p>Boron is a distinctly crustal element in that it is strongly enriched in the surface reservoirs, such as continental crust, seawater, sediments, serpentinites and altered oceanic crust, relative to the mantle. These B-enriched reservoirs are also isotopically very distinct from the regular depleted upper mantle (d<sup>11</sup>B = -7.1 ±0.9 ‰ [10.1016/j.gca.2017.03.028]). This has encouraged the idea that boron could be an ideal tracer for subducted surface materials in the deep mantle in the form of isotopically anomalous recycled components in ocean island basalts (OIB) and enriched MORB. Yet, the potential of a geochemical tracer of this type is weakened by its extraction from the slab at the onset of subduction by dewatering and metamorphic dehydration, because this process depletes the recycled components in fluid-mobile elements. As such, this “subduction barrier” diminishes the deep recycling efficiency of incompatible, fluid-mobile tracers like B.</p><p>This study focuses on the B abundances and B isotopic compositions of glasses and melt inclusions that show low Cl/K ratios and are thought to represent the uncontaminated mantle signal from the HIMU (Tuvalu and Mangaia), EM1 (Pitcairn) and EM2 (Samoa) sources. Strikingly, all samples are depleted in boron by a factor of approximately 1.5 to 4 relative to non-fluid-mobile elements of similar incompatibility (e.g. LREE, P, Be). This negative boron anomaly is ubiquitous in OIB and is consistent with the results of previous studies [10.1016/0016-7037(95)00402-5; 10.1016/j.epsl.2018.12.005]. It also mirrors their characteristic negative Pb anomaly. These anomalies show that the mantle sources of OIB are depleted in B (and Pb) relative to non-fluid-mobile elements of similar incompatibility and relative to the MORB-source mantle. This is best explained by the presence in the OIB sources of recycled components that are enriched in all incompatible elements except for the fluid-mobile B (and Pb). The fluid mobile elements must have been preferentially extracted in the subduction barrier and returned to the surface on the short path via arc magmas. Arc magmas consistently show a general enrichment in isotopically heavy boron [10.1007/978-3-319-64666-4_9] with positive B anomalies.</p><p>Despite of the low recycling efficiency of boron into the convecting mantle, OIB still have B isotope signatures that are distinct from those of MORB. Previous studies have reported OIB signatures slightly lighter than MORB and the primitive mantle [10.1016/j.epsl.2018.12.005]. However, our study exclusively finds isotopically heavy B with a range in d<sup>11</sup>B from MORB-like values (-8.6 ±2.0 ‰) up to -2.5 ±1.5‰ for EM1 and HIMU lavas. The total OIB range is small but significant, and is consistent with the deep recycling of material that is strongly depleted in boron, but isotopically distinct (with isotopically heavy B in the case of our EM1 and HIMU samples). The B depletion combined with the B isotopic anomaly in OIB shows that B is efficiently (but not quantitatively) removed from the slab during subduction, and that isotopically distinct mantle domains are thus produced. The subduction barrier for boron increases its strength as a tracer in arcs, but it diminishes its potential as a tracer of deep mantle recycling.</p>