Zinc isotopic composition of the lower continental crust estimated from lower crustal xenoliths and granulite terrains

doi:10.1016/j.gca.2020.02.030

Geochimica et Cosmochimica Acta

Volume 276, 1 May 2020, Pages 92-108

https://doi.org/10.1016/j.gca.2020.02.030 Get rights and content

Abstract

This study presents high precision Zn stable isotope analyses for lower crustal rocks (9 granulites from Archean terrains and 30 lower crustal xenoliths) from the north margin of the North China Craton (NCC) to understand the behavior of Zn isotopes during deep crustal processes and the Zn isotopic composition of the lower continental crust (LCC). The lower crustal xenoliths range in composition from mafic to felsic with MgO contents of between 0.4 to 20.2 wt.%. The δ⁶⁶Zn (the permil deviation of the ⁶⁶Zn/⁶⁴Zn ratio from the JMC-Lyon standard) values of lower crustal xenoliths range from −0.17‰ to 0.38‰. The lack of clear correlation of δ⁶⁶Zn with geochemical indicators (e.g., Al₂O₃, Cr, Ba/La, ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd) indicates that assimilation of Precambrian lower crust, fluid metasomatism, and accumulation of pyroxene and plagioclase had a limited effect on Zn isotopic compositions of these lower crustal xenoliths. The δ⁶⁶Zn values (0.18‰ to 0.34‰) of garnet-bearing mafic granulites decrease with increasing FeO_(T) and V contents, which are likely results of the accumulation of Fe-Ti oxides.

The average δ⁶⁶Zn of the lower continental crust is estimated to be 0.29 ± 0.02‰ (95% SE) using lower crustal xenoliths from the Neogene Hannuoba basalts. This δ⁶⁶Zn value is similar to the estimated value (0.28 ± 0.04‰, 95% SE) obtained from the granulites from Archean terrains, suggesting that there is no significant difference in the Zn isotopic composition between the Archean and present-day lower continental crust. Combining the δ⁶⁶Zn data of the lower crustal xenoliths and granulite terrains and different weighting methods, the Zn isotopic composition of the lower continental crust is estimated to be 0.28 ± 0.04‰ (95% SE).

Introduction

Zinc is a first group transitional element and has five natural stable isotopes (⁶⁴Zn, ⁶⁶Zn, ⁶⁷Zn, ⁶⁸Zn and ⁷⁰Zn). Over the past twenty years, zinc isotopes have been widely applied to trace processes in a variety of environments (see Moynier et al. (2017) for a recent review). One aspect of Zn isotopes that remains to be better understood is the compositions of different reservoirs on the Earth. Chen et al. (2013) through the study of volcanic rocks from Kilauea Iki and Hekla, suggested that Bulk Silicate Earth (BSE) has δ⁶⁶Zn (permil deviation of the ⁶⁶Zn/⁶⁴Zn ratio from the JMC-Lyon standard) of 0.28 ± 0.05‰ (2SD). However, some recent studies on non-metasomatized peridotites, komatiites, and picrites have estimated values for the BSE between 0.16‰ to 0.20‰ (Wang et al., 2017, McCoy-West et al., 2018, Sossi et al., 2018). Compared to the relatively homogenous composition of the mantle, the continental crust exhibits larger Zn isotopic variation (−0.03‰ to 0.88‰; Telus et al., 2012, Xia et al., 2017, Doucet et al., 2018, Xu et al., 2019). A recent study on Archean crustal granitoids from the northern Kaapvaal Craton suggests that partial melting of the early felsic crust does fractionate Zn isotopes by up to 0.15‰ (Doucet et al., 2018). Furthermore, fluid exsolution during the later stages of magmatic differentiation in the continental crust could form pegmatites with heavy Zn isotopic compositions (δ⁶⁶Zn from 0.53‰ to 0.88‰; Telus et al., 2012). Zinc isotopic compositions of the leucosome and melanosome phases in migmatites from the Dabei orogenic belt also indicate that the Zn isotopic compositions of leucosomes can be modified by fluid exsolution (Xu et al., 2019). Additionally, Little et al. (2016) suggest that the average isotopic composition of lithogenic Zn (δ⁶⁶Zn) is 0.27 ± 0.07‰ (1SD).

Although many studies have focused on mantle peridotites, mantle-derived melts and shallow crustal rocks, the Zn isotopic composition of the lower continental crust remains poorly constrained. To better understand the behavior of Zn isotopes during lower crustal processes and to constrain the Zn isotopic composition of the lower continental crust, it is necessary to conduct a detailed investigation of lower crustal rocks. Granulite-facies xenoliths brought to the Earth’s surface by fast-rising magmas and exposed granulite terrains can provide valuable insight into the lower continental crust (Rudnick and Taylor, 1987, Rudnick and Fountain, 1995, Villaseca et al., 1999, Hacker et al., 2011, Rudnick and Gao, 2014 and references therein). However, major differences exist between these two types of lower crustal rocks. Granulite xenoliths are mostly found in Phanerozoic basalts or kimberlites and are dominated by mafic compositions, while exposed granulite terrains are mainly formed in the Archean and are richer in silica (e.g., Rudnick and Taylor, 1987, Rudnick and Fountain, 1995, Gao et al., 2000). In order to estimate the Zn isotopic composition of the lower continental crust and its possible evolution from the Archean to the Phanerozoic, it is necessary to study both types of lower crustal rocks.

Lower crustal xenoliths from the Neogene Hannuoba basalts and the nearby Archean granulite terrains from the north margin of the North China Craton provide an excellent opportunity to study the Zn isotopic composition of the lower continental crust. First, lower crustal xenoliths and Archean granulite terrains coexist in the same area (Gao et al., 2000, Zhai et al., 2001), which can be used to compare the Zn isotopic composition of the present-day and Archean lower continental crust. Second, the different types of lower crustal xenoliths derive from different depths of the lower crust (Liu et al., 2001). Thus, the Zn isotopic composition of the lower continental crust can be estimated according to the proportions of the different xenoliths from different crustal depths. Finally, the lower crustal xenoliths range in composition from mafic to felsic and have been previously well characterized (major and trace elements, Sr-Nd-Pb-Li-Mg isotopes, and U-Pb zircon ages; Gao et al., 2000, Chen et al., 2001, Liu et al., 2001, Liu et al., 2004, Liu et al., 2010, Zhou et al., 2002, Wilde et al., 2003, Teng et al., 2008, Zheng et al., 2009, Jiang and Guo, 2010, Wei et al., 2015, Yang et al., 2016). Here, 30 lower crustal xenoliths from the Neogene Hannuoba basalts and 9 granulites from Archean terrains were analyzed to determine the behavior of Zn isotopes during lower crustal processes and estimate the average Zn isotopic composition of the lower continental crust.

Section snippets

Geological setting

The North China Craton (NCC), with the remnants of 3.8 Ga crust (Liu et al., 1992), is one of the oldest continental blocks in the world. The NCC is bounded to the north by the Central Asian Orogenic Belt, the southwest by the Qinling-Dabie Belt, and the southeast by the Sulu Belt (Wang and Mo, 1995) (Fig. 1). Based on ages, lithological assemblages, tectonic evolution, and P-T-t paths, the NCC can be divided into the Eastern Block, the Trans-North China Orogen, and the Western Block (Zhao et

In-situ major and trace elements

The in-situ major and trace element (V, Cr, Ni, and Zn) compositions of oxide minerals in the garnet-bearing mafic granulites were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS; GeoLas 2005 + Agilent 7500a) with a spot size of 44 µm at the China University of Geosciences (Wuhan). The element contents were calibrated against BCR-2G, BHVO-2G and BIR-1G (USGS reference glasses) without applying internal standardization (Liu et al., 2008a). The time-drift

Major and trace elements

Whole-rock trace element concentrations are provided in Table S3. The Zn contents (51.7 to 91.2 ppm) of garnet-bearing mafic granulites show strong positive correlations with FeO_(T) (8.9 to 15.4 wt.%) and V contents (135 to 563 ppm) (Fig. 4a, b).

The results for oxide minerals from garnet-bearing mafic granulites indicate that they are mainly composed of iron and titanium, and the TiO₂ and FeO_(T) contents show extensive ranges of variation with 21.6 wt.% to 52.4 wt.% and 38.4 wt.% to 68.6 wt.% (

Discussion

The formation and evolution of the lower continental crust involved many geological processes, including, for example, ancient crustal assimilation, fluid metasomatism, addition of supracrustal sediments, magmatic differentiation and mineral accumulation. These geological processes may affect the Zn isotopic composition of the lower continental crust. Here, the Zn isotopic composition of the lower continental crust is estimated using lower crustal xenoliths and Archean granulite terrains.

Conclusions

The main conclusions of this study on the Zn isotopic compositions of different types of lower crustal rocks are:

1.
The δ⁶⁶Zn values of lower crustal rocks from the north margin of the NCC range from 0.13‰ to 0.38‰ (37 out of 39 samples), indicating that the lower continental crust has a limited Zn isotopic variation relative to the shallow crust. Combining the δ⁶⁶Zn values of the lower crustal xenoliths and granulite terrains and different weighting methods, a weighted average δ⁶⁶Zn of

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.

Acknowledgments

We thank Dr. ShengAo Liu for kind support during the development of the analytical method for Zn isotopes, and Dr. Fang Huang for providing NIST SRM 683 solution. Dr. Mark Rehkämper, Dr. Alex McCoy-West, Dr. Luc S. Doucet and four anonymous reviewers are thanked for thorough reviews that have greatly improved the quality of the manuscript. We thank Dr. Edward Inglis and Dr. Michael Antonelli for their comments and language editing. This research is co-supported by the National Natural Science

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Notably, the loess average overlaps with the δ66Zn data obtained for the igneous crustal rocks AGV-2 (0.28 ± 0.03 ‰) and BCR-2 (0.28 ± 0.06) and is identical, within error, to the published ‘lithogenic’ Zn isotope value of δ66Zn = 0.27 ± 0.14 ‰, which Little et al. (2016) calculated from data for 50 crustal igneous rocks, sediments and aerosols. The loess data are, furthermore, indistinguishable from a recent lower continental crust δ66Zn estimate of 0.28 ± 0.04 ‰ (95 % SE), as defined by lower crustal xenoliths and granulite terrains from the North China Craton (Zhang et al., 2020). Together, these observations suggest that the Zn isotope composition of the continental crust is relatively homogeneous, on both a global basis and with depth, and similar to that of mantle-derived basalts (Fig. 1).
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Zinc isotopic composition of the lower continental crust estimated from lower crustal xenoliths and granulite terrains

Abstract

Introduction

Section snippets

Geological setting

In-situ major and trace elements

Major and trace elements

Discussion

Conclusions

Declaration of Competing Interest

Acknowledgments

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