Diffusion-driven magnesium and iron isotope fractionation at a gabbro

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Nov 16, 2017 - also indicate that Mg-Fe inter-diffusion can produce large stable isotope .... Here, we report whole-rock Mg and Fe isotopic data for ... One gabbro sample (13DSC-5) and one granite sample ..... 1.34. 2.63. 2.09. 3.25. 3.74. 3.63. 3.77. 4.28 na. 19.9. 12. 6.28. 6.38 ..... Fe are molar concentrations of Mg and Fe.

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ScienceDirect Geochimica et Cosmochimica Acta 222 (2018) 671–684 www.elsevier.com/locate/gca

Diffusion-driven magnesium and iron isotope fractionation at a gabbro-granite boundary Hongjie Wu a,b, Yongsheng He a,b,⇑, Fang-Zhen Teng b, Shan Ke a,⇑, Zhenhui Hou c, Shuguang Li a a

State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China b Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA c CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Anhui 230026, China Received 2 March 2017; accepted in revised form 8 November 2017; Available online 16 November 2017

Abstract Significant magnesium and iron isotope fractionations were observed in an adjacent gabbro and granite profile from the Dabie Orogen, China. Chilled margin and granitic veins at the gabbro side and gabbro xenoliths in the granite indicate the two intrusions were emplaced simultaneously. The d26Mg decreases from 0.28 ± 0.04‰ to 0.63 ± 0.08‰ and d56Fe increases from 0.07 ± 0.03‰ to +0.25 ± 0.03‰ along a 16 cm traverse from the contact to the granite. Concentrations of major elements such as Al, Na, Ti and most trace elements also systematically change with distance to the contact. All the observations suggest that weathering, magma mixing, fluid exsolution, fractional crystallization and thermal diffusion are not the major processes responsible for the observed elemental and isotopic variations. Rather, the negatively correlated Mg and Fe isotopic compositions as well as co-variations of Mg and Fe isotopes with Mg# reflect Mg-Fe inter-diffusion driven isotope fractionation, with Mg diffusing from the chilled gabbro into the granitic melt and Fe oppositely. The diffusion modeling yields a characteristic diffusive transport distance of 6 cm. Consequently, the diffusion duration, during which the granite may have maintained a molten state, can be constrained to 2 My. The cooling rate of the granite is calculated to be 52–107 °C/My. Our study suggests diffusion profiles can be a powerful geospeedometry. The observed isotope fractionations also indicate that Mg-Fe inter-diffusion can produce large stable isotope fractionations at least on a decimeter scale, with implications for Mg and Fe isotope study of mantle xenoliths, mafic dikes, and inter-bedded lavas. Ó 2017 Elsevier Ltd. All rights reserved. Keywords: Mg-Fe inter-diffusion; Iron isotope; Magnesium isotope; Kinetic fractionation

1. INTRODUCTION Magnesium and iron isotope fractionations at high temperature were considered limited but recent studies found

⇑ Corresponding authors at: State Key Laboratory of Geological

Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China. E-mail addresses: [email protected] (Y. He), [email protected] edu.cn (S. Ke). https://doi.org/10.1016/j.gca.2017.11.010 0016-7037/Ó 2017 Elsevier Ltd. All rights reserved.

large equilibrium (>0.5‰) and kinetic (up to several per mil) isotope fractionations (see recent reviews of Dauphas et al., 2017; Teng, 2017; Watkins et al., 2017). For example, large equilibrium inter-mineral Mg isotope fractionation among garnet, spinel, olivine and pyroxenes has been reported in igneous and metamorphic rocks (e.g., Liu et al., 2011; Wang et al., 2015; Li et al., 2016), making it a potential high-precision geothermometry. Large Mg and Fe isotope fractionation driven by chemical diffusion has been reported with applications toward deciphering the

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cooling history preserved in igneous samples (e.g., Dauphas et al., 2010; Teng et al., 2011; Chopra et al., 2012; Sio et al., 2013; Oeser et al., 2015; Pogge von Strandmann et al., 2015; Sio and Dauphas, 2017). Identifying the process that accounts for isotope fractionation is the prerequisite for using them as tracers, geospeedometry, or geothermometry. However, meaningful interpretation of fractionation signatures is often hampered by the fact that they could be ascribed to many processes such as fractional crystallization, partial melting and thermal diffusion (see recent reviews of Dauphas et al., 2017; Teng, 2017; Watkins et al., 2017). The combination of multiple isotopic systems, where possible, can help distinguish processes responsible for isotopic variations. Fractional crystallization and partial melting can dramatically fractionate Fe isotopes (e.g., Weyer et al., 2005; Schoenberg and von Blanckenburg, 2006; Weyer and Ionov, 2007; Teng et al., 2008, 2013; Sossi et al., 2012; Telus et al., 2012), whereas the same processes lead to negligible fractionation of Mg isotopes (e.g., Teng et al., 2007, 2010a; Liu et al., 2010). Thermal diffusion can theoretically fractionate Mg and Fe isotopes in the same direction, yielding a positive correlation between 26 Mg/24Mg and 56Fe/54Fe (e.g., Richter et al., 2008, 2009; Huang et al., 2010). In chemical diffusion experiments using juxtaposing basaltic and rhyolitic melt couples, both Fe and Mg diffuse from the basaltic to the rhyolitic melt, with the latter simultaneously concentrated light Fe and Mg isotopes and thus a positive correlations between 26 Mg/24Mg and 56Fe/54Fe can be expected (Richter et al., 2008, 2009). Since Mg2+ and Fe2+ occupy the same lattice site in silicate minerals due to their identical charges and similar ionic radii, inter-diffusion of Mg and Fe 2þ 2þ 2þ (Mg2þ have been phase1 þ Fephase2 $ Mgphase2 þ Fephase1 ) observed among minerals and between co-existing minerals and magmas (e.g., olivine, spinel, perovskite; see the review of Zhang and Cherniak, 2010). The Fe and Mg would diffuse in opposite directions during inter-diffusion between solid and melt, which is different from the situation of diffusion between basaltic and rhyolitic melts that both Fe and Mg go from the basaltic to the rhyolitic melt. Recently, inter-diffusion of Mg and Fe was proposed to produce negatively correlated Mg-Fe isotope fractionation in olivine (Dauphas et al., 2010; Teng et al., 2011; Sio et al., 2013; Oeser et al., 2015). Therefore, the combined Mg-Fe isotopic effects are quite different during fractional crystallization, partial melting, thermal and chemical diffusion among or between melts and minerals. Previous studies have found that Mg–Fe inter-diffusion profiles in olivine can be used as a geospeedometry (Dauphas et al., 2010; Teng et al., 2011; Sio et al., 2013; Oeser et al., 2015), nonetheless, whether such fractionation could happen between adjacent rocks is still unknown. Here, we report whole-rock Mg and Fe isotopic data for a gabbro-granite boundary profile from the Dabie Orogen, China. The results show a gradient of Mg and Fe isotopic compositions. Negatively correlated Mg and Fe isotopic compositions as well as co-variations of Mg and Fe isotopic compositions with Mg# likely reflect inter-diffusion of Mg and Fe between the chilled gabbro and its coexisting grani-

tic melt. Results of diffusion modeling agree well with the measured profiles and yield a diffusion duration range of 1.5  2.2 My and a cooling rate range of 52 °C/My  107 °C/My. Our results demonstrate the potential application of the diffusion-controlled Mg-Fe isotope fractionation as a geospeedometry of geological events. 2. SAMPLE DESCRIPTION The profile studied here was collected from the boundary between a granite intrusion and a gabbro intrusion, located in the Dabie Orogen, Central China. The gabbro intrusion was generally called Daoshichong pluton, which is one of the major post-collisional mafic-ultramafic plutons in the Dabie Orogen (Dai et al., 2011; Xu et al., 2012). The granite intrusion belongs to a group of Early Cretaceous monzogranite plutons with ages younger than 130 Ma (Xu et al., 2007). Both intrusions are wider than 20 m in the field. Mafic xenoliths in the granite near the boundary, small granitic veins in the gabbro and a chilled margin on the gabbro side indicate the two intrusions were emplaced almost simultaneously under liquid or semi-solid state (Fig. 1). One gabbro sample (13DSC-5) and one granite sample (13DSC-6) were collected 4 m and 0.5 m away from the boundary, respectively, to represent end-members unaffected by diffusion. The gabbro sample (13DSC-5) is medium-grained and contains pyroxene (25%), plagioclase (40%), hornblende (20%) and oxide minerals (10–15%) (see photomicrographs in Appendix A). The granite sample (13DSC-6) is a medium to coarse-grained monzogranite, which consists of quartz (25%), plagioclase (45%), alkalifeldspar (25%), biotite (less than 2%) and accessory minerals such as oxides (see photomicrographs in Appendix A).

Fig. 1. (a) Simplified geological map showing eastern China and major tectonic units, with roughly marked sampling location. (b) Field photograph of our study profile, with detailed captions of sampling. Both intrusions are wider than 20 m in the field.

H. Wu et al. / Geochimica et Cosmochimica Acta 222 (2018) 671–684

One large hand specimen (13DSC-7) collected from the boundary was sliced into nine pieces (13DSC-7-1–13DSC7-9) for a profile study (Figs. 1 and 2). 13DSC-7-1 is the chilled margin of the gabbro, and the others are granite. The sliced granite samples have the same mineral assemblage with 13DSC-6, but their crystal size changes from fine-grained at the contact to medium- to coarse-grained in 13DSC-7-9 (Fig. 2). 13DSC-7-1 has a finely crystalline texture and contains hornblende, chlorite, oxide minerals with minor quartz and plagioclase (Fig. 2). 3. ANALYTICAL METHODS Surfaces of all samples were removed by 0.5–1 cm with a cutting machine (corundum grinding wheel) to avoid the influence of weathering. After washing with Milli-QÒ water, samples were roughly broken by a copper hammer and then crushed into fine powders (10 lg/g and better than 20% for most elements of 95% crystalline degree can be estimated to be about 750 °C for gabbro 13DSC-5 using MELTS, which is higher than the wet solidus line of granite under an intrusion depth (e.g., 10–15 km) (Chen and Grapes, 2007). Therefore, it is possible that the gabbro side was under solid state during element diffusion, with the granitic side simultaneously being still as a semi-solid or molten state. This could explain why Mg-Fe interdiffusion occurred in the profile observed here. The interdiffusion of Mg and Fe is driven by the Mg-Fe exchange reaction between the solid gabbro and the rhyolitic melt, which is analogous to the reaction between olivine and its

surrounding melts. Diffusion after the solidification of both the gabbro and granite should be minimal, since Mg and Fe are dominantly hosted in few mafic mineral grains in the granite, isolated by abundant but almost Mg and Fe-free quartz and feldspar. It is noted that the boundary granite sample 13DSC-7-2 has higher Fe and Mg contents and Mg# than what are expected by inter-diffusion (Figs. 7 and 8). This could be due to mixing between the granitic and the interstitial basaltic melt near the interface. Mixing is also evidenced by the Co and V concentrations in the sample 13DSC-7-2 (Fig. 8). Contamination of mafic minerals from the chilled margin cannot be completely ruled out, since the contact interface is not an absolute plane and it is difficult to separate the chilled margin completely from the granite slices. The isotope profiles seem unaffected by this contamination since they show continuous variations. Diffusion between the gabbroic chilled margin and the granitic melt is also supported by elemental profiles. The diffusion of a component against its own concentration gradient has often been observed in systems involving solid phase, which is called uphill diffusion (see a review of Zhang et al., 1989; Liang, 2010). The compositions of Ti, Li, Zr, Hf and REE in this study do not show monotonic concentration profiles, which are very similar to those reported in Zhang et al. (1989) and Richter et al. (2003) and can also be attributed to uphill diffusion.

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Fig. 5. Measured and modeled Mg and Fe isotopic diffusion profiles. All the symbols are the same as Fig. 4. Distance x is given relative to the interface between the gabbro and granite. The grey dotted lines represent the effect of physical mixing between the gabbro and granite magma. Blue and black curves are modeled lines based on Fick’s second law. Model parameters: T = 750°C; CG,Mg = 1.5 wt.%; CR,Mg = 0.5 wt.%; CG, Fe = 1.00 wt.%; CR,Fe = 0.50 wt.%; DMg-Fe,R = 100 * DMg-Fe,G; bMg = bFe = 0.05. The modeled curve yielding the best fit to the data is qffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Rt Rt Ddt in the figure represents DMgFe;R dt. The two black curves represent the upper and lower limit represented by the blue line. The 0 0 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rt DMgFe;R dt values yielding reasonable fits to the data. (For interpretation of the references to colour in this figure legend, the reader is 0 referred to the web version of this article.)

The elemental and isotopic data along the profile can be fitted by inter-diffusion of Fe and Mg between a solid gabbro and a molten rhyolite. Since the two sides belong to different phases, there would be both partitioning and diffusion of Mg and Fe. The diffusion in the two phases are modeled using a one-dimensional diffusion model separately, as each side requires different diffusivities. The interface between the gabbro and granite is set as x = 0. The initial effective element concentration (element activity) at x < 0 (gabbro) is CG, and that in the x > 0 half (rhyolitic melt) is CR. The partition coefficients of Mg and Fe between the two phases are KMg and KFe. The gabbrorhyolite exchange coefficient KD(Fe-Mg) is KFe/KMg. The Mg and Fe have the same inter-diffusion coefficient (DMg-Fe). The Mg-Fe inter-diffusivity in gabbro half (x < 0) is DMgFe,G, and in the rhyolitic melt half (x > 0) is DMg-Fe,R. Time-dependent diffusion coefficients are adopted here to estimate the cooling rate of the rhyolitic melt by fitting the isotope profiles. The following equations are used: Diffusion equation: Rt Define: a ¼ 0 Ddt,

@C [email protected]

2

¼ @@xC2 .

the diffusion equation becomes:  Initial condition: C t¼0 ¼

CG CR

@C @a

2

¼ @@xC2 .

x0

Using the solution from Zhang (2008) as reference, we get: C ¼ CG þ

cðC R  KCG Þ jxj ; when x < 0; erfc qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rt 1 þ Kc 2 D dt 0

C ¼ CR þ

MgFe;G

KCG  C R x ; when x > 0: erfc qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R t 1 þ Kc D dt 2 0

MgFe;R

where here, t is the duration of the diffusion; erfc is Gauss error function; c = (qR/qG) (DMg-Fe,R/DMg-Fe,G)1/2, qR and qG are densities of the rhyolitic melt (2.5 g/cm3) and the gabbro (3.1 g/cm3), respectively. The diffusivity in rhyolitic melt (DMg-Fe,R) is assumed to be 100 times higher than that in gabbro (DMg-Fe,G). Since an exchange coefficient KD(Fe-Mg) between gabbro and rhyolitic melt is

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Fig. 6. Mg and Fe isotope fractionation of the study profile. Data are reported in Table 1. All the symbols are the same as Fig. 4. The curves illustrate the modeled d56Fe and d26Mg variations corresponding to D26Mg/D24Mg = (24/26)b and D56Fe/D54Fe = (54/ 56)b. The comparison of the two modeled curves suggest that equally increasing bMg and bFe would enlarge the range of isotope fractionation, but the negative correlation between d56Fe and d26Mg still holds. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

unavailable, it is approximated by the KD(Fe-Mg) between pyroxene and silicic melt. Based on an equation between KD(Fe-Mg) and SiO2 content of silicic melt (Be´dard, 2010), the KD(Fe-Mg) is calculated to be 0.22 using the SiO2 content of 13DSC-6. All the isotopes (24Mg, 26Mg, 54Fe, 56Fe) are treated as individual elements and their diffusion curves are modeled separately. The ratio of diffusivities of isotopes was expressed as D2/D1 = (m1/m2)b (Richter et al., 1999). The d26Mg and d56Fe values of 13DSC-5 and 13DSC-6 are considered to represent the values of unaffected gabbro and granite, respectively. The values of b, CG, CR and qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Rt D dt are estimated to best fit the FeO, MgO, Mg-Fe;R 0 Mg# and isotopic profiles (Fig. 5, Fig. 7). The best fit CG, Mg, CR,Mg, CG,Fe and CR,Fe values are 1.5 wt.%, 0.5 wt.%, 1.0 wt.% and 0.5 wt.%, respectively. Obviously, the fitted effective concentrations of Mg (1.5 wt.%) and Fe (1.0 wt. %) in the gabbro are lower than the actual Mg and Fe concentrations (see Table 2 and Appendix A), which may be due to the low element activity in the solid phase. The b is 0.05 for both Mg and Fe. The value for Mg is consistent with that from the experimental study (0.05 ± 0.01, Richter et al., 2008), while the value for Fe is slightly higher than that from the experimental study (0.03 ± 0.01, Richter et al., 2009). This inconsistence could be due to different compositions between our samples and the experiment materials and the uncertainties of our estimation of the eleffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rt ment activities in the gabbro. The D dt (characMg-Fe;R 0 teristic diffusive transport distance) is 6 cm, with an accessible range of 5.5–6.7 cm (both lower or higher values did not yield a reasonable fit, Fig. 5). Although the interdiffusion coefficient between gabbro and the granitic melt is unavailable, an estimation of DMg-Fe,R,0 (DMg-Fe,R at 750 °C) can be approached by the equation for binary ionic diffusion (Zhang, 2008):

Fig. 7. (a) Measured and modeled Mg# profile. The Mg# of 13DSC-7-8 appears as an outlier, and the reason is not well understood. (b) and (c) are plots of d56Fe and d26Mg with Mg# in profile. Color shaded area in each panel represents the isotopic compositions of unaffected gabbro 13DSC-5. The blue lines are the modeled line of d56Fe and d26Mg varying with Mg#. The red hollow triangle represents sample 13DSC-7-8, which is excluded in the Mg# vs. Distance figure due to its abnormal Mg#. Obviously, Mg# of sample 13DSC-7-2 near the gabbro is higher than the modeling results. This could be due to a mixing between the rhyolitic melt and the interstitial basaltic melt near the interface or contamination of mafic minerals from the chilled margin. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

DMg-Fe;R ¼

DMg  DFe  ðC0Mg þ C0Fe Þ ; C0Mg  DMg þ C0Fe  DFe

where C0Mg and C0Fe are molar concentrations of Mg and Fe in rhyolitic melt, DMg and DFe are self-diffusivities of Mg and Fe in rhyolitic melt. Based on a temperature and composition dependent diffusion coefficient function on Mg dif-

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Fig. 8. Concentration profiles for MgO (a), FeO (b), Co (c) and V (d). All the symbols are the same as Fig. 4. The blue curves in (a) and (b) are the modeled MgO and FeO activity variations based on best fit of the Mg#, d56Fe and d26Mg profiles. The activity parameters are displayed in the figures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

fusivities in high silica melt (Zhang et al., 2010), the DMg is calculated to be 3.7  1013 cm2/s using a composition of 13DSC-6 under a temperature of 750 °C. The diffusion coefficient of Fe is calculated from an experimental databased Arrhenius equation, which yields a DFe of 1.1  1012 cm2/s (Todd and Ratliffe, 1990). C0Mg and C0Fe used here are molar concentrations of Mg and Fe in 13DSC-6. Hence, the inter-diffusivity of DMg-Fe,R,0  6.5  1013 cm2/s. If it is assumed that diffusion occurred at the initial condition without cooling, the diffusion timescale would be: Z t s¼ DMg-Fe;R dt=DMgFe;R;0 : 0

As a result, the duration of diffusion can be calculated to be 1.8 My with a range of 1.5 My and 2.2 My. Due to the assumption that diffusion occurred at the initial condition without cooling, the duration estimated above is the lower limit. In other word, the granitic magma was maintained under a semi-solid or molten state after intrusion for at least 1.5 My. Given the diffusion property (the activation energy), the initial temperature T0 and a obtained from fitting the profiles, a cooling rate may be obtained as follows: Consider a thermal history of monotonic cooling represented by the asymptotic cooling model:   t T ¼ T0 ; 1þ sc

where sc is the cooling timescale for temperature to decrease from T0 to T0/2. The relation between s and sc is (Zhang, 2008):   RT0 : s ¼ sc E The cooling rate q is: q¼

dT T 0 D0 RT20 RT20 ¼ ; ¼ ¼ sc Ea Es dtjt¼0

where E is the activation energy, R is the universal gas constant. The T0 used here is 750 °C. Although the activation energy for Mg-Fe inter-diffusion in the granitic melt is unavailable, an approximate E is approached by assuming it is comparable to activation energy of Fe2+ or Mg2+ diffusion in rhyolitic melts. Based on the experimental data of ferrous iron diffusion in aluminosilicate melt (Dunn and Ratliffe, 1990), the activation energy of Fe2+ can be calculated to be 47.73 kJ/mol with uncertainly limites of 40.41 kJ/mol and 55.06 kJ/mol. The activation energy of Mg2+ diffusion in rhyolitic melts is unavailable. Considering the uncertainly range of s, the cooling rate q can be calculated to be 76 °C/My with an uncertainly range from 52 °C/My to 107 °C/My. Previous studies show the ages for the DSC gabbro intrusion is 129 ± 1 Ma (Dai et al., 2011). 40 Ar-39Ar and fission track techniques revealed that North Dabie (where our samples collected) experienced a rapid lifting and cooled to 540 °C at ca. 125 Ma (Chen, 1995). The maximum duration of diffusion is 4 Ma based on the

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time interval between emplacement of intrusions and upliftcooling of the Dabie Orogen, which is longer than the calculated duration (1.5–2.2 My). Therefore, the calculated duration of diffusion and cooling rate are consistent with the cooling history of the North Dabie Orogen.

magmas of quite different compositions or temperatures, e.g., inter-bedded mafic and felsic magmas, xenoliths/ enclaves versus their hosting lavas, and cumulates in a cooling magma chamber, etc. ACKNOWLEDGMENT

6. SUMMARY AND IMPLICATIONS The negatively correlated d26Mg and d56Fe and their correlations with Mg# in the studied gabbro-granite boundary profile from the Dabie Orogen, China can be explained by isotope fractionations driven by Mg-Fe inter-diffusion, likely between the chilled gabbro margin and the granitic melt. Our studies demonstrate the potential application of the diffusion-controlled Mg-Fe isotope fractionation as geospeedometry of geological events by modeling the diffusion profiles. Since the inter-diffusion after the solidification of the granitic magma is negligible, the calculated diffusion duration 1.5–2.2 My indicates the granitic magma was maintained under a semi-solid or molten state

after intrusion for at least 1.5 My. This may produce magmatic zircons with large age ranges and thus affect zircon U-Pb dating results of granite intrusions. Also, this study suggests the combination of Mg and Fe isotopic data can be used to determine whether chemical gradients in natural settings where melts/rocks of different composition were juxtaposed were due to weathering, fluid exsolution, thermal diffusion, chemical diffusion between two melts or inter-diffusion. Furthermore, our results suggest that identifiable Mg and Fe isotope fractionations may occur on a larger scale, e.g., decimeters or larger, in cases of Mg-Fe interdiffusion. This is of critical importance in interpretation of Mg and Fe isotopic compositions of igneous samples that generally once have been coexisted with country rocks or

We deeply appreciate editorial handling by Dr. Weidong Sun and constructive comments from Dr. Stefan Weyer and two other anonymous reviewers who tremendously improved this manuscript. We are grateful to Qiang Zeng and Xiaolei Liu for their help in sample preparation. This work was financially supported by the DREAM project of Most China (No. 2016YFC0600408); the National Natural Science Foundation of China (41473016); the Fundamental Research Funds for the Central Universities (2652014056) and State Key Lab. Of Geological Processes and Mineral Resources. This is CUGB petrogeochemical contribution No. PGC-201524.

APPENDIX A

Photomicrographs of sample 13DSC-5 and 13DSC-6 under plane polarized light (left) and perpendicular polarized light (right). Q = quartz; Plg = plagioclase; Afs = alkalifeldspar; Bi = biotite; Py = pyroxene; Hbl = hornblende. REFERENCES Be´dard J. H. (2010) Parameterization of the Fe=Mg exchange coefficient (Kd) between clinopyroxene and silicate melts. Chem. Geol. 274, 169–176. Brenot A., Cloquet C., Vigier N., Carignan J. and France-Lanord C. (2008) Magnesium isotope systematics of the lithologically varied Moselle river basin, France. Geochim. Cosmochim. Acta 72, 5070–5089.

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