The Fe and Zn isotope composition of deep mantle source regions: Insights from Baffin Island picrites

Young (61 Ma) unaltered picrites from Baffin Island, northwest Canada, possess some of the highest 3He/4He (up to 50 Ra) seen on Earth, and provide a unique opportunity to study primordial mantle that has escaped subsequent chemical modification. These high-degree partial melts also record anomalously high 182W/184W ratios, but their Sr-Nd-Hf-Pb isotopic compositons (including 142Nd) are indistinguishable from those of North Atlantic mid-ocean ridge basalts. New high precision Fe and Zn stable isotope analyses of Baffin Island picrites show limited variability with δ56Fe ranging from −0.03‰ to 0.13‰ and δ66Zn varying from 0.18‰ to 0.28‰. However, a clear inflection is seen in both sets of isotope data around the composition of the parental melt (MgO = 21 wt %; δ56Fe = 0.08 ± 0.04‰; and δ66Zn = 0.24 ± 0.03‰), with two diverging trends interpreted to reflect the crystallisation of olivine and spinel in low-MgO samples and the accumulation of olivine at higher MgO. Olivine mineral separates are significantly isotopically lighter than their corresponding whole rocks (δ56Fe ≥ −0.62‰ and δ66Zn ≥ −0.22‰), with analyses of individual olivine phenocrysts having extremely variable Fe isotope compositions (δ56Fe = −0.01‰ to −0.80‰). By carrying out modelling in three-isotope space, we show that the very negative Fe isotope compositions of olivine phenocryst are the result of kinetic isotope fractionation from disequilibrium diffusional processes. An excellent correlation is observed between δ56Fe and δ66Zn, demonstrating that Zn isotopes are fractionated by the same processes as Fe in simple systems dominated by magmatic olivine. The incompatible behaviour of Cu during magmatic evolution is consistent with the sulfide-undersaturated nature of these melts. Consequently Zn behaves as a purely lithophile element, and estimates of the bulk Earth Zn isotope composition based on Baffin Island should therefore be robust. The ancient undegassed lower mantle sampled at Baffin Island possesses a δ56Fe value that is within error of previous estimates of bulk mantle δ56Fe, however, our estimate of the Baffin mantle δ66Zn (0.20 ± 0.03‰) is significantly lower than some previous estimates. Comparison of our new data with those for Archean and Proterozoic komatiites is consistent with the Fe and Zn isotope composition of the mantle remaining constant from at least 3 Ga to the present day. By focusing on large-degree partial melts (e.g. komatiites and picrites) we are potenitally biasing our record to samples that will inevitably have interacted with, entrained and melted the ambient shallow mantle during ascent. For a major element such as Fe, that will continuosly participate in melting as it rises through the mantle, the final isotopic compositon of the magama will be a weighted average of the complete melting column. Thus it is unsuprising that minimal Fe isotope variation are seen between localities. In contrast, the unique geochemical signatures (e.g. He and W) displayed by the Baffin Island picrites are inferred to solely originate from the lowermost mantle and will be continuously diluted upon magma ascent

Heterogeneous nickel isotopic compositions in the terrestrial mantle – Part 1: Ultramafic lithologies

High precision nickel stable isotopic compositions (δ⁶⁰/⁵⁸Ni) are reported for 22 peridotite xenoliths from the USA (Kilbourne Hole, New Mexico), Tanzania, and Cameroon. For a subset of these, δ⁶⁰/⁵⁸Ni is also reported for their constituent mineral separates (olivine, orthopyroxene, clinopyroxene, and spinel). Bulk peridotites show significant heterogeneity in Ni isotopic composition, ranging from +0.02‰ to +0.26‰. Unmetasomatised fertile peridotites from three localities, define an average δ⁶⁰/⁵⁸Ni of +0.19±0.09‰ (n = 18). This value is comparable to previous estimates for the δ⁶⁰/⁵⁸Ni of the bulk silicate earth (BSE), but is unlikely to be representative, given observed heterogeneity, presented here and elsewhere. Samples with reaction rims and interstitial glass (interpreted as petrographic indications of minor metasomatism) were excluded from this average; their Ni isotopic compositions extend to lighter values, spanning nearly the entire range observed in peridotite worldwide. Dunites (n = 2) are lighter in δ⁶⁰/⁵⁸Ni than lherzolites and harzburgites from the same location, and pyroxenites (n = 5) range from +0.16‰ to as light as −0.38‰. The δ⁶⁰/⁵⁸Ni in the Kilbourne Hole xenoliths correlate negatively with bulk-rock Fe concentration and positively with ¹⁴³Nd/¹⁴⁴Nd, providing evidence that light δ⁶⁰/⁵⁸Ni is associated with mantle fertility and enrichment. The trend between δ⁶⁰/⁵⁸Ni and Fe concentration in bulk rocks appears to be global, replicated across the peridotites in this work from other localities, and in literature data. The inter-mineral fractionations are small; the maximum difference between heaviest and lightest phase is 0.12‰. This provides evidence that bulk rock δ⁶⁰/⁵⁸Nii variation does not result from differences in modal mineralogy, fractional crystallization or degrees of partial melting. The δ⁶⁰/⁵⁸Ni fractionation appears to be an equilibrium effect and usually is in the decreasing order spinel > olivine = orthopyroxene > clinopyroxene. However, the fractionation between clinopyroxene and orthopyroxene varies in magnitude and sign, and is correlated with pyroxene Si/Fe positively, and Fe/Mg negatively. The magnitude of inter-pyroxene fractionation also correlates with other pyroxene compositional ratios (e.g. La/Sm_clinopyroxene); as well as bulk rock δ⁶⁰/⁵⁸Ni, and [U]. These data provide evidence that Ni isotopes fractionate at the bulk rock and mineral scale in response to mantle enrichment processes, possibly related to recycling of isotopically light subducted components

Heterogeneous nickel isotope compositions of the terrestrial mantle – Part 2: Mafic lithologies

We report stable Ni isotope compositions (δ⁶⁰/⁵⁸Ni, relative to SRM986) for mafic lavas with a range of -0.16 ‰ to +0.20 ‰ (n=44), similar to that of peridotite samples. Ocean island basalts (OIB) have been analysed from Iceland (n=6), the Azores (n=3), the Galápagos Islands (n=2), and Lōʻihi, Hawaii (n=1). Samples from Iceland (average δ⁶⁰/⁵⁸Ni = +0.13±0.16‰, 2s, n=7) display the greatest range in Ni isotope compositions from a single OIB location in this work, of +0.01 ‰ to +0.23 ‰. Samples from the Azores (average δ⁶⁰/⁵⁸Ni = -0.10±0.10 ‰, 2s) and Galápagos (average δ⁶⁰/⁵⁸Ni = -0.01±0.04 ‰, 2s) are generally isotopically lighter. The single Lōʻihi sample has a δ⁶⁰/⁵⁸Ni of +0.17 ‰. The lightest analysed bulk rock δ⁶⁰/⁵⁸Ni in this work, -0.16 ‰, is from the Azores island, Pico. Enriched mid ocean ridge basalts (E-MORB), which have (La/Sm)_N>1, are isotopically lighter than normal type MORB (N-MORB), as shown by data from the Mid Atlantic Ridge (n=9) and East Pacific Rise (n=3). All E-MORB average δ60/58Ni = +0.00±0.06 ‰ (2s, n=7), whereas N-MORB average δ60/58Ni = +0.14±0.10 ‰ (2s, n=5). A suite of 15 mafic samples from the Cameroon Line, comprising lithologies ranging from nephelinites to hypersthene-normative basalts, have Ni isotope compositions that are identical within analytical uncertainty (average δ⁶⁰/⁵⁸Ni = +0.08±0.06 ‰, 2s). Similarly, MORB samples display no relationship between δ⁶⁰/⁵⁸Ni and geochemical indicators of degree of partial melting or fractional crystallisation. Host lavas for two previously analysed ultramafic xenolith suites have δ⁶⁰/⁵⁸Ni identical to the average δ⁶⁰/⁵⁸Ni of their respective xenolith suites. This is consistent with previously published evidence from peridotites and komatiites that Ni isotopes are not greatly fractionated by melting. Therefore, mafic rocks may preserve the δ⁶⁰/⁵⁸Ni of their mantle source. Sampling a greater volume of mantle, their average Ni isotope composition +0.07±0.17 ‰ (2s, n=44) may also be a better representation of the Bulk Silicate Earth (BSE), than estimates based purely on peridotites. The δ⁶⁰/⁵⁸Ni of MORB co-varies with La/Sm, Rb/Sr, europium anomaly (Eu/Eu*), and K₂O/(K₂O+Na₂O). The relationships between these parameters and δ⁶⁰/⁵⁸Ni are consistent with mixing between two model endmembers. One could be depleted MORB or depleted MORB mantle (DMM) with a relatively heavy Ni isotope composition; the other a more enriched endmember that has isotopically lighter δ⁶⁰/⁵⁸Ni. The link between lighter δ⁶⁰/⁵⁸Ni and enriched lithologies in the mantle is further supported by published evidence of light Ni isotope compositions associated with some pyroxenite xenoliths. However, the curvature of the apparent mixing arrays defined by basalts is hard to reconcile with admixing of geochemically enriched but isotopically fractionated oceanic crustal lithologies. High [Ni] enriched magmas such as kimberlites may be a closer match to the enriched endmember. However, this needs further study

Extremely high He isotope ratios in MORB-source mantle from the proto-Iceland plume

The high <sup>3</sup>He/<sup>4</sup>He ratio of volcanic rocks thought to be derived from mantle plumes is taken as evidence for the existence of a mantle reservoir that has remained largely undegassed since the Earth's accretion. The helium isotope composition of this reservoir places constraints on the origin of volatiles within the Earth and on the evolution and structure of the Earth's mantle. Here we show that olivine phenocrysts in picritic basalts presumably derived from the proto-Iceland plume at Baffin Island, Canada, have the highest magmatic <sup>3</sup>He/<sup>4</sup>He ratios yet recorded. A strong correlation between <sup>3</sup>He/<sup>4</sup>He and <sup>87</sup>Sr/<sup>86</sup>Sr, <sup>143</sup>Nd/<sup>144</sup>Nd and trace element ratios demonstrate that the <sup>3</sup>He-rich end-member is present in basalts that are derived from large-volume melts of depleted upper-mantle rocks. This reservoir is consistent with the recharging of depleted upper-mantle rocks by small volumes of primordial volatile-rich lower-mantle material at a thermal boundary layer between convectively isolated reservoirs. The highest <sup>3</sup>He/<sup>4</sup>He basalts from Hawaii and Iceland plot on the observed mixing trend. This indicates that a <sup>3</sup>He-recharged depleted mantle (HRDM) reservoir may be the principal source of high <sup>3</sup>He/<sup>4</sup>He in mantle plumes, and may explain why the helium concentration of the 'plume' component in ocean island basalts is lower than that predicted for a two-layer, steady-state model of mantle structure

Extensive crustal extraction in Earth’s early history inferred from molybdenum isotopes

Estimates of the volume of the earliest crust based on zircon ages and radiogenic isotopes remain equivocal. Stable isotope systems, such as molybdenum, have the potential to provide further constraints but remain underused due to the lack of complementarity between mantle and crustal reservoirs. Here we present molybdenum isotope data for Archaean komatiites and Phanerozoic komatiites and picrites and demonstrate that their mantle sources all possess subchondritic signatures complementary to the superchondritic continental crust. These results confirm that the present-day degree of mantle depletion was achieved by 3.5 billion years ago and that Earth has been in a steady state with respect to molybdenum recycling. Mass balance modelling shows that this early mantle depletion requires the extraction of a far greater volume of mafic-dominated protocrust than previously thought, more than twice the volume of the continental crust today, implying rapid crustal growth and destruction in the first billion years of Earth’s history

Deep formation of Earth's earliest continental crust consistent with subduction

About four billion years ago, Earth’s outer layer is thought to have been composed mostly of a 25- to 50-km-thick basaltic crust that differentiated to form the oldest stable continental crust. However, the tectonic processes responsible for the formation of this continental material remain controversial. Suggested explanations include convergent plate boundary processes akin to subduction operating today and a variety of relatively shallow (50 km) subduction-like environments. Our results support previous Eoarchaean field evidence and analyses of igneous rocks that date to 4.0–3.6 billion years ago, which are consistent with subduction-like processes and suggest a primitive type of plate tectonics operated as long as 4 billion years ago on early Earth