Copper isotopes track the Neoproterozoic oxidation of cratonic mantle roots

Abstract

The oxygen fugacity (fO₂) of the lower cratonic lithosphere influences diamond formation, melting mechanisms, and lithospheric evolution, but its redox evolution over time is unclear. We apply Cu isotopes (δ⁶⁵Cu) of ~ 1.4 Ga lamproites and < 0.59 Ga silica-undersaturated alkaline rocks from the lithosphere-asthenosphere boundary (LAB) of the North Atlantic Craton to characterize fO₂ and volatile speciation in their sources. The lamproites' low δ⁶⁵Cu (-0.19 to -0.12‰) show that the LAB was metal-saturated with CH₄ + H₂O as the dominant volatiles during the Mesoproterozoic. The mantle-like δ⁶⁵Cu of the < 0.59 Ga alkaline rocks (0.03 to 0.15‰) indicate that the LAB was more oxidized, stabilizing CO₂ + H₂O and destabilizing metals. The Neoproterozoic oxidation resulted in an increase of at least 2.5 log units in fO₂ at the LAB. Combined with previously reported high fO₂ in peridotites from the Slave, Kaapvaal, and Siberia cratonic roots, this oxidation might occur in cratonic roots globally.

Fig. 1. Distribution of alkaline rocks and their Cu isotopes.

a Age distribution of the three stages of rift-related alkaline rocks in Labrador. b Cu isotope compositions of rift-related rocks from Labrador in this study. Modern mid-ocean ridge basalts and the bulk silicate Earth are shown for comparison. The error bars for δ⁶⁵Cu values of the alkaline rocks at the three stages and MORBs are 2sd. c and d Frequency distribution of ages of global nonorogenic lamproites (Supplementary Data 1), carbonatites, and kimberlites. The cumulative U–Pb age data for detrital zircons interpreted to originate from carbonatite-alkaline rocks in Neoproterozoic–Triassic sandstones from Antarctica (red line, n = 493) and the C–O–H species in their mantle sources^, are also shown.

Fig. 2. Copper isotope fractionation among phases.

a Cu isotope fractionation between metal (and sulfide) and silicate melt from high-pressure experiments^,. The gray solid line represents Cu isotope fractionation between sulfate and sulfide, extrapolated from calculations in a low-temperature system. b Copper isotope compositions of the lamproites from Labrador, average mid-ocean ridge basalts and komatite, and modeled Cu isotope fractionation during partial melting of a peridotite and metal-saturated pyroxenite. Cu isotope fractionation factor α_{Iron nickel sulfide-Melt} of 1 and α_{Iron sulfide-Melt} of 0.99976 are from Xia et al., and α_{Iron sulfide-Melt} of 0.99990 is from Savage et al.. For detailed modeling calculation (see Supplementary Data 2 and 4). The average melting degrees of the mid-ocean ridge basalts and komatiites are shown for comparison^,, and the average melting degree of the Labrador lamproites is assumed to be 10%. c Modeled Cu isotope fractionation during magmatic differentiation of a modern mid-ocean ridge basalt-like melt. δ⁶⁵Cu values of the mid-ocean ridge basalts^,,, (MgO > 5 wt%) are shown. The mid-ocean ridge basalts and Labrador lamproites show no relationship between δ⁶⁵Cu value and MgO content. For detailed modeling calculations, see Supplementary Data 3. The error bars for all δ⁶⁵Cu values in this figure are 2sd.

Fig. 3. Copper systematics of magmatic rocks.

Cu versus MgO contents in rift-related alkaline rocks from Labrador compared to global mid-ocean ridge basalts, komatiites and picrites^,,.

Fig. 4. Evolution of fO₂ of cratonic roots and subduction.

a Redox evolution of the LAB at ≈200 km depth. The gray dotted lines are the oxygen fugacities for melts with different carbonate contents (CO₃^2-, molar percentage) in equilibrium with diamond-bearing peridotites. The fO₂ of the LAB of the North Atlantic Craton at a depth of about 200 km did not significantly change since the Late Proterozoic. b The proportions of carbonate rocks and evaporites in marine sediments and seawater SO₄^2- concentration^, over time. Subduction geotherms are constrained by continental crust thickness.

Fig. 5. Illustration of mantle oxidation evolution.

The oxidation state of the cratonic mantle over time and the redox melting caused by the change of the speciation of volatiles in the mantle during magmatic episodes in the Mesoproterozoic (a) and Late Neoproterozoic (b). aThe fO₂ of the lithospheric mantle decreases with depth at a rate of about 0.4–0.6 log units per GPa^, until the metal saturation with the nickel precipitation curve. bThe lithosphere–asthenosphere boundary was significantly oxidized during the Neoproterozoic.

Fig. 6. Redox evolution in the lower cratonic lithosphere.

∆logfO₂ (∆FMQ) versus pressure in GPa for the lamproites and aillikites in Labrador, and published peridotite xenolith data from the Slave, Kaapvaal, and Siberian Cratons^,,. The gray and light red regions are defined by Yaxley et al. and encompass depleted and enriched Slave Craton samples, respectively. The enriched Kaapvaal samples also fall inside the light red field. The gray-filled symbols for the Kaapvaal and Siberia Craton samples are not defined as “depleted” or “enriched” in previous publications. The FeNi metal precipitation curve, graphite/diamond transition, and the reaction for carbonate/diamond stability in harzburgitic assemblages (enstatite + magnesite = forsterite + graphite/diamond) are shown for reference. All fO₂ for peridotites were calculated by Yaxley et al. and Tappe et al. using the experimental calibration for garnet peridotite assemblages of Stagno et al. and garnet Fe³⁺/ΣFe. The source fO₂ of the Labrador lamproites is constrained by metal-saturated conditions at 6 GPa. The source fO₂ of the aillikites was calculated using the function of Stagno et al..