Boron isotope evidence for devolatilized and rehydrated recycled materials in the Icelandic mantle source

Marshall, Edward W.; Ranta, Eemu; Halldórsson, Sæmundur A.; Caracciolo, Alberto; Bali, Enikő; Jeon, Heejin; Whitehouse, Martin J.; Barnes, Jaime D.; Stefánsson, Andri

doi:10.1016/j.epsl.2021.117229

Cited by 12 publications

(4 citation statements)

References 62 publications

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“…Overall, the model shows that the distribution of magma compositions from Iceland's rift zones (Marshall et al, 2022) is consistent with the existence of DM and EM components that undergo partial melting. In particular, the δ 18 O values of mixtures lying along the mantle array (gray field in Fig.…”

Section: Discussion the Role Of The Crust In Masking Mantle Heterogen...mentioning

confidence: 54%

Oxygen isotope evidence for progressively assimilating trans-crustal magma plumbing systems in Iceland

Caracciolo

Halldórsson

Bali

et al. 2022

Geology

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The oxygen isotope composition of mantle-derived melts can place important constraints on how magmas are processed as they traverse the crust. Assimilation of crustal material is a crucial aspect of basalt petrogenesis, as it affects the chemical and rheological characteristics of eruptive magmas at active volcanoes. We report oxygen isotope (δ18O) and trace element (TE) data from a suite of well-characterized basaltic melt inclusions and groundmass glasses from the Bárðarbunga volcanic system in Iceland to assess how and where in the plumbing system crustal rocks interact with ascending magmas. While both melt inclusions and groundmass glasses record a large range in δ18O values (+3.2‰ to +6.4‰ and +2.6‰ to +5.5‰, respectively) groundmass glasses record lower values on average. Relationships between incompatible trace element (e.g., Zr/Nb) and oxygen isotope ratios are best explained with three-component mixing, where primary melts derived from depleted and enriched mantle components with distinct δ18O values mix and acquire a low-δ18O character upon progressive contamination with altered Icelandic crust. The majority (60%) of melt inclusions require 10–30% exchange of oxygen with the Icelandic crust. In addition, for the first time, we link the extent of oxygen isotope exchange with melt equilibration depths, showing that most of the contamination occurs at 1–2 kbar (3–7 km depth). We propose that a progressively assimilating, multi-tiered plumbing system is a characteristic feature of the Bárðarbunga volcanic system, whereby chemical modifications resulting from interaction with the crust systematically increase as melts migrate through higher crustal levels. We show that similar processes may also occur across the active rift zone in Iceland.

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Section: Discussion the Role Of The Crust In Masking Mantle Heterogen...mentioning

confidence: 54%

Oxygen isotope evidence for progressively assimilating trans-crustal magma plumbing systems in Iceland

Caracciolo

Halldórsson

Bali

et al. 2022

Geology

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“…While our DFT-MD simulations are performed with the goal of first-order constraints on the lithophile and siderophile characteristics of boron on three P-T points (Figure 3 and S10 in Supporting Information S1), extrapolating 𝐴𝐴 log 10 𝐷𝐷 m∕s B for elemental B at 10 GPa to higher P tentatively puts the transition between lithophile and siderophile behavior at ∼20 GPa. With metal saturation in the transition zone and below, our results suggest that the amount of boron in metallic reservoirs in the mantle can be significantly larger than that recycled into the lower mantle via subduction, particularly because dehydration at subarc depths (Chemia et al, 2015;Syracuse et al, 2010) leads to a removal of >99% boron from the slab crust during subduction (Marshall et al, 2022;McCaig et al, 2018).…”

Section: Boron Metal-silicate Partitioningmentioning

confidence: 88%

Possible Control of Earth's Boron Budget by Metallic Iron

Yuan

Steinle‐Neumann

2022

Geophysical Research Letters

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Boron is considered a principal tracer for crustal recycling into the depleted mantle, and high boron anomalies and isotopically light boron ( 10 B) in mantle-derived samples have been interpreted as the result of subduction. Using density functional theory, we predict that boron behavior changes from lithophile to siderophile, with the calculated boron partition coefficients (D m/s ) between liquid metal and silicate ranging from log 10 D m/s ≤ −0.8 at low pressure-temperature conditions (10 GPa, 3000 K) to log 10 D m/s ≥ 3.7 at high pressure and temperature (130 GPa, 5000 K). We further predict that the silicate becomes enriched in the heavier boron isotope ( 11 B) by >1‰ relative to metal at high pressure-temperature. Our results suggest that Earth's core may hold >50% of Earth's boron budget and that a high boron content in deep diamonds and isotopically light boron in ocean islands basalts may reflect contributions from metallic reservoirs, rather than being attributed to crustal subduction and recycling.Plain Language Summary Plate tectonics promotes the transport of surface rocks into the mantle, producing much of its chemical heterogeneity. Boron, a quintessential crustal element, is often used as a proxy for crustal contributions when found in mantle rocks and is, therefore, one of the central tools in geochemistry to trace recycling/mixing in the mantle. Using quantum mechanical calculations, we find that the chemical behavior of boron changes from lithophile (rock-loving) to siderophile (iron-loving) under pressure-temperature conditions relevant to core formation. Thus, much boron may have been transported to the core, and the core may be Earth's largest boron reservoir, rather than the crust. Chemical heterogeneity in the mantle inferred from boron signatures could therefore also result from chemical exchange between the core and mantle or equilibration with metallic iron dispersed throughout the mantle.

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“…Thus, Icelandic primitive melts appear to have a have low S 6+ /S T . A number of studies have shown that the mantle beneath Iceland is heterogeneous with respect to the volatile origin (and probably, volatile concentrations) on a regional scale (e.g., Füri et al., 2010; Halldórsson, Barnes, et al., 2016, Halldórsson, Hilton, et al., 2016; Harðardóttir et al., 2018; Hilton et al., 2000; Kurz et al., 1985; Marshall et al., 2022; Matthews et al., 2021; Miller et al., 2019; Nichols et al., 2002; Poreda et al., 1986; Ranta et al., 2022). Given this, and that the degree of mantle melting is largely controlled by the tectonic setting and lithospheric thickness—melting degree being higher near the rift zones, and lower in the off‐rift zones (Harðardóttir et al., 2022)—regional variations are expected to be expressed in MI volatile concentrations.…”

Section: Discussionmentioning

confidence: 99%

Magmatic Controls on Volcanic Sulfur Emissions at the Iceland Hotspot

Ranta,

Halldórsson,

Óladóttir

et al. 2024

Geochem Geophys Geosyst

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Outgassing of sulfur (as SO2) is one of the principal hazards posed by volcanic eruptions. However, S emission potentials of most volcanoes globally are poorly constrained due to a short observational record and an incomplete understanding of the magmatic processes that influence pre‐eruptive S concentrations. Here, we use a compilation of published and new data from melt inclusions (MIs)—which can preserve magmatic S concentrations prior to eruptive degassing—from the Iceland hotspot to evaluate the effects of mantle melting and crustal magmatic processes on the S budgets of Icelandic melts. We use MI data to estimate S emission potentials (ΔSmax, in ppm S) for 73 eruptions from 22 of Iceland's presently active ∼33 volcanic systems. We show that the S systematics of Icelandic melts are strongly regulated by the sulfide solubility limit. Sulfide‐saturated conditions during lower‐degree mantle melting, prevalent at off‐rift zones, likely explains observed decoupling between S and Cl. During magmatic differentiation, a local maximum in modeled sulfide solubility occurs in evolved basalts (4–6 wt.% MgO), coinciding with highest MI S concentrations. Highest ΔSmax values (2,100–2,600 ppm) are found in the Hekla 1913 CE, Eldgjá 939 CE, and Surtsey 1963–1967 CE eruptions in the South Iceland Volcanic Zone. Our results extend the record of volcanic sulfur emissions back in time and can be used to assess volcanic gas hazards at Icelandic volcanoes where no direct measurements are available. Broadly, the results underline the governing role of sulfide saturation during melting and magma differentiation in controlling the eruptible S contents of Icelandic magmas.

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Boron isotope evidence for devolatilized and rehydrated recycled materials in the Icelandic mantle source

Cited by 12 publications

References 62 publications

Oxygen isotope evidence for progressively assimilating trans-crustal magma plumbing systems in Iceland

Oxygen isotope evidence for progressively assimilating trans-crustal magma plumbing systems in Iceland

Possible Control of Earth's Boron Budget by Metallic Iron

Magmatic Controls on Volcanic Sulfur Emissions at the Iceland Hotspot

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