Copper and zinc isotope systematics of altered oceanic crust at IODP Site 1256 in the eastern equatorial Pacific

Abstract

This paper presents the first combined Cu and Zn isotopic study of altered oceanic crust at Integrated Ocean Drilling Program (IODP) Hole 1256D that penetrates a volcanic section, a lava‐dyke transition zone, a sheeted dyke complex, and a plutonic complex. In the volcanic section, all but one rocks have Cu and Zn isotopic compositions similar to fresh mid‐ocean ridge basalt (MORB), reflecting restricted seawater circulation and low oxygen fugacity. Rocks in the transition zone have MORB‐like δ65Cu and δ66Zn, indicating the dominant influence of basalt‐derived Cu and Zn during alteration. Rocks in the dyke complex have more variable δ65Cu (−0.50–0.90‰) and δ66Zn (0.19–0.55‰) and those in the plutonic complex have δ65Cu of −0.43 to 0.20‰ and δ66Zn of 0.21 to 0.41‰. The rocks with heavier δ66Zn and heavier or lighter δ65Cu relative to MORB are characterized by Cu‐Zn depletions, low Li/Yb (<1.0) and low δ18O (<5‰), suggesting that hydrothermal extraction during high temperature alteration of oceanic crust can result in significant Cu and Zn isotope fractionation. Such large Cu and Zn isotopic variations are the results of redox transformation of Cu as well as Cu and Zn isotope fractionation between altered basaltic rocks and dissolved Cu and Zn species in hydrothermal fluids (e.g., [CuCl3]1−, Zn(HS)42−). This work is the first to define the distribution of Cu and Zn isotopes in an intact oceanic crust with concentration‐weighted averages of δ65Cu (0.05 ± 0.03‰) and δ66Zn (0.27 ± 0.01‰). The potential implications of these new observations are discussed.

Mantle Zn Isotopic Heterogeneity Caused by Melt‐Rock Reaction: Evidence From Fe‐Rich Peridotites and Pyroxenites From the Bohemian Massif, Central Europe

“…Analyses of in‐house and international reference standards processed with the studied peridotites and pyroxenites yield δ 66 Zn = 0.28 ± 0.03‰ (2SD, n = 60) for IRMM3702, δ 66 Zn = 0.31 ± 0.05‰ (2SD, n = 6) for BHVO‐2, 0.22 ± 0.05‰ (2SD, n = 8) for JP‐1, and 0.26 ± 0.04‰ for DTS‐2 (2SD, n = 4) (Table ). These values agree well within error with previously published values (Chen et al, , ; Doucet et al, ; Huang et al, ; Huang, Chen, et al, ; Huang, Zhang, et al, ; Moynier et al, ; Sossi et al, ; Wang et al, ). This, combined with consistent results for repeated analyses (Table ), assures the accuracy and precision of our Zn isotopic data.…”

“…Nb/Nb* = Nb PM /(Th PM × La PM ) 0.5 , PM denotes primitive mantle normalization. Nb/Nb* and Sr isotopic data are taken from Hart et al (), and Zn isotopic data are taken from Huang et al () for altered lower (ALOC) and upper (AUOC) oceanic crust. Zinc isotopic data for deep‐sea sediments are taken from Maréchal et al (), Bentahila et al (), and Little et al ().…”

“…Zinc isotopes can be significantly fractionated during continental weathering, carbonate and sulfide precipitation, and fluid‐rock reactions (e.g., Albarède, ; Moynier et al, ). These processes cause ~2.0‰ fractionation in δ 66 Zn (δ 66 Zn = [( 66 Zn/ 64 Zn) sample /( 66 Zn/ 64 Zn) JMC Lyon − 1] × 1000‰) of potential recycling components in subduction zones (e.g., sediments, altered oceanic crust, and serpentinite; Bentahila et al, ; Huang et al, ; Little et al, , ; Maréchal et al, ; Pichat et al, ; Pons et al, , ). The variable δ 66 Zn of recycling components compared to the relatively uniform δ 66 Zn of the normal mantle (0.16 ± 0.06‰, Sossi et al, ) highlights the potential use of Zn isotopes to trace crustal recycling and crust‐mantle interaction (Liu et al, ; Wang et al, ).…”

Mantle heterogeneity through Zn systematics in oceanic basalts: Evidence for a deep carbon cycling

“…( 87/86 Sr AOC = 0.70458, Staudigel et al, 1995). Nonetheless, this process does not alter its Nd and Zn isotope composition ( 143/ 144 Nd AOC = 0.51308; δ 66 Zn AOC = +0.27 ± 0.01‰, Staudigel et al, 1995;Huang et al, 2016). Consequently, we argue that MORB-eclogite cannot account for the extreme Zn enrichment and global trends between Zn and Zn-Sr-Nd isotopes in oceanic basalts.…”

The behavior of iron and zinc stable isotopes accompanying the subduction of mafic oceanic crust: A case study from Western Alpine ophiolites

“…Recently Zn isotopes have been shown to be sensitive to mantle partial melting [ Doucet et al ., ; Wang et al ., ] and igneous differentiation [ Chen et al ., ], but owing to the complex metasomatic and metamorphic history of the studied samples, coupled with the lack of a comprehensive study of Zn isotopes in global MORB and oceanic gabbros, it is difficult to conclude if the variations in Zn isotope composition observed here reflect primary magmatic process or modification by late stage alteration and metasomatic processes. While it has previously been stated that the process of low‐temperature seafloor alteration of the upper, basaltic oceanic lithosphere has little effect on Zn isotopes, the same study demonstrated that high‐temperature (>350°C) hydrothermal circulation and complexing of light Zn isotopes in hydrothermal fluids could drive the Zn isotope composition toward heavier δ 66 Zn values in the gabbroic portion of the oceanic lithosphere [ Huang et al ., ]. This observation could be invoked to explain the range of δ 66 Zn values observed in the Zermatt and Queyras metagabbros, but as the samples now preserve a subduction‐related, Alpine overprint to their mineralogy it is not possible to unambiguously conclude on the effect of seafloor hydrothermal activity on the Zn isotope compositions of these rocks.…”