Large MgFe isotope fractionation in volcanic rocks from northeast China: The role of chemical weathering and magma compositional effect
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 MgFe 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 MgFe 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 MgSr 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
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Haiquan Wei and Heng-Ci Tian contributed equally to this work.