Elsevier

Chemical Geology

Volume 565, 30 March 2021, 120075
Chemical Geology

Large Mgsingle bondFe isotope fractionation in volcanic rocks from northeast China: The role of chemical weathering and magma compositional effect

https://doi.org/10.1016/j.chemgeo.2021.120075Get rights and content

Abstract

This study presents Mg and Fe isotopic data for a suite of volcanic rocks including basalt, trachyte and comendite from Changbaishan, northeast China. Our results show that the Millennium Eruption (ME) trachytes and comendites have δ26Mg ranging from −0.37 to 0.14‰. Combined with literature data, the positive correlation between δ26Mg and 87Sr/86Sr ratios and the almost constant value for Nd isotopes suggest that the heavy Mg isotopic composition most likely derived from involvement of weathered products of basalt instead of the old supracrustal/basement rocks. Iron isotopic composition is significantly varied throughout our suite of samples, with δ56Fe values of 0.17 ± 0.04‰ and 0.16 ± 0.03‰ for basalts, 0.09 to 0.23‰ for cone-construction trachytes, and 0.23 to 0.37‰ for the ME samples. The slightly higher δ56Fe than MORB observed in basalts reflect the role of subducted materials that could elevate source Fe3+/ΣFe ratios via redox reaction. The trachytes from the cone construction, thought to be significantly affected by chemical weathered product, have a restricted Fe isotopic variation, while those ME samples with less weathered product input have overall higher δ56Fe values. This contrasting Fe isotopic signature suggests that the paleo-weathering event likely occurred under oxidized environment, leaving the residue with little Fe isotope fractionation. By contrast, the good correlations between Fe isotopes and indicators of magmatic differentiation (e.g., tFe2O3, SiO2, Mg#) in all rocks reflect that fractional crystallization under an open system most likely accounts for the Fe isotopic variation, which was enhanced by the compositional effect as suggested by the broadly positive relationship between δ56Fe and (Na + K)/(Ca + Mg). This study therefore highlights the potential applications of Mgsingle bondFe isotopes as great tracers of redox condition and ancient climate.

Introduction

Magnesium and iron isotopes are proven to be largely fractionated at low temperature environments (e.g., Teng et al., 2010a; Liu et al., 2014). Studies have found over 1.5‰ Mg isotope variation in sedimentary rocks (e.g., shales, pelites, and loess) and residues of silicate rocks formed by chemical weathering (e.g., Li et al., 2010), making it as an effective probe for the recycled supracrustal materials (Li et al., 2017a; Teng, 2017). Iron is a redox-sensitive element and its isotopes are readily fractionated relevant to reducing weathering but limited fractionation under oxidizing conditions (Liu et al., 2014). At high temperature, both partial melting and subsequent fractional crystallization do not significantly fractionate Mg isotopes but can cause large Fe isotopic variation (Teng et al., 2008, Teng et al., 2010b, Teng et al., 2013). More importantly, the combination of Mg and Fe isotopes have been widely applied to illustrate/distinguish complicated geological processes such as tracing the deep oxygen cycling (He et al., 2019), tracing the crust evolution (Shen et al., 2009; Li et al., 2020), distinguishing the chemical heterogeneity derived from crystal growth or kinetic processes (e.g., Teng et al., 2011; Sio et al., 2013) or using them as geospeedometry (Teng et al., 2011; Wu et al., 2018). For example, Tian et al. (2020) reports a large-scale of diffusion-driven Mgsingle bondFe isotope fractionation in ilmenites from Panzhihua layered mafic intrusion, suggesting that the interstitial liquid immiscibility plays a crucial role in the origins of Fe-Ti-rich ore deposits. Therefore, the Mgsingle bondFe isotopes can help advance our understanding of what magmatic processes have contributed to the formation and evolution of the magmas.

Recently, the remarkably heavy Mg isotopic compositions (up to 0.94‰) have been reported in Changbaishan trachytes and attributed to the contribution of previously weathered residue of basaltic rocks based on their decoupled Mgsingle bondSr and Nd isotopic ratios (Tian et al., 2019). If this explanation is correct, the Fe isotopic compositions of Changbaishan trachytes with heavy Mg isotopes should also record the geochemical fingerprints for such weathered components and can be used to further decipher the redox state of the chemical weathering during that period. This is important for the interpretation of the past climate in northeastern China during the period when the weathered products formed. To date, no available Fe isotopic data have been reported for Changbaishan volcanic rocks.

In this paper, Fe isotopic compositions for selected basalts, cone-construction trachytes and ME trachyte-comendite samples were investigated (Fig. 1). These samples cover most of the geochemical characteristics and lithologies of the volcanic rocks distributed in Changbaishan volcanic region. We have also made Sr-Nd-Mg isotopic analyses for highly evolved ME products in order to examine whether a signal of the weathered residue occurs in these materials. Significant Fe (~0.3‰ for δ56Fe) and Mg (~0.5‰ for δ26Mg) isotopic variations are observed in these volcanic rocks. These isotopic data are then used to constrain whether the previously weathered basalt continuously participated in the Changbaishan volcanic rocks, the mechanism of Fe isotope fractionation, and to investigate any connections between Mg and Fe isotopes.

Section snippets

Samples

Changbaishan Tianchi volcano located on the China-North Korea boundary belongs to a shield volcano built on the Neogene basalt lavas (also named as the Gaima Plateau) (Fig. 1). This stratovolcano predominantly consists of pre-shield basalts, shield-forming basalts, cone-construction trachytes and Holocene comendite-trachyte series from the bottom to the top (see review of Zhang et al., 2018). Those basalts are generally younger than 10 Mys and widely spread around the volcano with an estimated

Results

The trace elemental concentrations for ME samples along with major oxides data from literature are summarized in Table S1 and the isotopic compositions analyzed here are listed in Table 1. The ME trachyte-comendite series exhibit a negative relationship between total alkali and silica contents (Fig. S1). Together with literature data, most studied trachytes and comendites lay along the fractional crystallization trends (Fig. S1). They display weakly to extremely negative Sr, P, Eu and Ti

Discussion

The newly analyzed ME trachyte-comendite series show a substantial variation in δ26Mg and are generally enriched in heavy Mg isotopes (Fig. 2a). This isotopic variation is unlikely induced by surface chemical weathering since the degree of isotopic fractionation is independent on the LOI (loss on ignition) (Fig. 3a). In a plot of δ56Fe versus LOI (Fig. 3b), the ME trachyte-comendite series have higher LOI than the basalt and cone-construction trachyte. The former seems to have heavy Fe isotopic

Summary and implications

In present work, the ME trachyte-comendite series display a large Mg isotopic fractionation with δ26Mg values ranging from −0.37 to +0.14‰, similar to that of the cone-construction trachytes. Such an isotopic variation observed here and the good correlations between Mg isotopes and 87Sr/86Sr ratios indicate the continuous participation of formerly weathered residue of basalts in the magmatic evolution. This interpretation is somewhat similar to the oxygen isotopic study that reported low δ18O

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We would like to thank the associate editor Christian France-lanord, Dr. Jian Huang and an anonymous reviewer for their insightful comments, which greatly improved the manuscript. We also thank Prof. Fang-Zhen Teng for the discussion and Dr. Xin-Yang Chen for help during Mg and Fe isotopic analyses. This work is financially funded by the National Natural Science Foundation of China (41730214, 41861144025), National Postdoctoral Program (2018M631566), Fundamental Scientific Research Projects of

References (66)

  • J.G. Konter et al.

    Unusual δ56Fe values in Samoan rejuvenated lavas generated in the mantle

    Earth Planet. Sci. Lett.

    (2016)
  • T. Kuritani et al.

    Magma plumbing system for the Millennium Eruption at Changbaishan volcano, China: constraints from whole-rock U–Th disequilibrium

    Lithos

    (2020)
  • W.Y. Li et al.

    Heterogeneous magnesium isotopic composition of the upper continental crust

    Geochim. Cosmochim. Acta

    (2010)
  • S.G. Li et al.

    Deep carbon cycles constrained by a large-scale mantle Mg isotope anomaly in eastern China

    Natl. Sci. Rev.

    (2017)
  • M. Li et al.

    Why was iron lost without significant isotope fractionation during the lateritic process in tropical environments?

    Geoderma

    (2017)
  • R.Y. Li et al.

    Origins of two types of Archean potassic granite constrained by Mg isotopes and statistical geochemistry: Implications for continental crustal evolution

    Lithos

    (2020)
  • S.A. Liu et al.

    Copper and iron isotope fractionation during weathering and pedogenesis: Insights from saprolite profiles

    Geochim. Cosmochim. Acta

    (2014)
  • J.L. Ma et al.

    Mobilization and re-distribution of major and trace elements during extreme weathering of basalt in Hainan Island, South China

    Geochim. Cosmochim. Acta

    (2007)
  • D. McLean et al.

    Identification of the Changbaishan 'Millennium' (B-Tm) eruption deposit in the Lake Suigetsu (SG06) sedimentary archive, Japan: Synchronisation of hemispheric-wide palaeoclimate archives

    Quat. Sci. Rev.

    (2016)
  • C. Oppenheimer et al.

    Multi-proxy dating the 'Millennium Eruption' of Changbaishan to late 946 CE

    Quat. Sci. Rev.

    (2017)
  • B. Pan et al.

    The VEI-7 Millennium eruption, Changbaishan-Tianchi volcano, China/DPRK: New field, petrological, and chemical constraints on stratigraphy, volcanology, and magma dynamics

    J. Volcanol. Geotherm. Res.

    (2017)
  • F. Poitrasson et al.

    Heavy iron isotope composition of granites determined by high resolution MC-ICP-MS

    Chem. Geol.

    (2005)
  • J.A. Schuessler et al.

    Iron and lithium isotope systematics of the Hekla volcano, Iceland: evidence for Fe isotope fractionation during magma differentiation

    Chem. Geol.

    (2009)
  • C.K.I. Sio et al.

    Discerning crystal growth from diffusion profiles in zoned olivine by in situ Mg-Fe isotopic analyses

    Geochim. Cosmochim. Acta

    (2013)
  • A.C.S. Cheong et al.

    Latest Pleistocene crustal cannibalization at Baekdusan (Changbaishan) as traced by oxygen isotopes of zircon from the Millennium Eruption

    Lithos

    (2017)
  • M. Telus et al.

    Iron, zinc, magnesium and uranium isotopic fractionation during continental crust differentiation: the tale from migmatites, granitoids, and pegmatites

    Geochim. Cosmochim. Acta

    (2012)
  • F.Z. Teng et al.

    Contrasting lithium and magnesium isotope fractionation during continental weathering

    Earth Planet. Sci. Lett.

    (2010)
  • F.Z. Teng et al.

    Magnesium isotopic composition of the Earth and chondrites

    Geochim. Cosmochim. Acta

    (2010)
  • F.Z. Teng et al.

    Diffusion-driven magnesium and iron isotope fractionation in Hawaiian olivine

    Earth Planet. Sci. Lett.

    (2011)
  • F.Z. Teng et al.

    Iron isotopic systematics of oceanic basalts

    Geochim. Cosmochim. Acta

    (2013)
  • H.C. Tian et al.

    Approach to trace hidden paleo-weathering of basaltic crust through decoupled Mg-Sr and Nd isotopes recorded in volcanic rocks

    Chem. Geol.

    (2019)
  • H.C. Tian et al.

    Diffusion-driven extreme Mg and Fe isotope fractionation in Panzhihua ilmenite: Implications for the origin of mafic intrusion

    Geochim. Cosmochim. Acta

    (2020)
  • H.Q. Wei et al.

    Timescale and evolution of the intracontinental Tianchi volcanic shield and ignimbrite-forming eruption, Changbaishan, Northeast China

    Lithos

    (2007)
  • Cited by (2)

    1

    Haiquan Wei and Heng-Ci Tian contributed equally to this work.

    View full text