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The behavior of magnesium isotopes in low-grade metamorphosed mudrocks

Shui-Jiong Wang, Fang-Zhen Teng, Roberta L. Rudnick, Shu-Guang Li

PII: S0016-7037(15)00402-0

DOI: http://dx.doi.org/10.1016/j.gca.2015.06.019

Reference: GCA 9334

To appear in: Geochimica et Cosmochimica Acta

Received Date: 13 February 2015

Accepted Date: 17 June 2015

Please cite this article as: Wang, S-J., Teng, F-Z., Rudnick, R.L., Li, S-G., The behavior of magnesium isotopes in

low-grade metamorphosed mudrocks, Geochimica et Cosmochimica Acta (2015), doi: http://dx.doi.org/10.1016/

j.gca.2015.06.019

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The behavior of magnesium isotopes in low-grade

metamorphosed mudrocks

Shui-Jiong Wang1*, 2, Fang-Zhen Teng2*, Roberta L. Rudnick3, Shu-Guang Li1, 4

1State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences,

Beijing 100083, China

2Isotope Laboratory, Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195-

1310, USA

3Geochemical Laboratory, Department of Geology, University of Maryland, College Park, MD 20742, USA

4CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences,

University of Science and Technology of China, Hefei 230026, Anhui, China

Abstract: 324 words

Text: 4887 words

Table: 1

Figures: 9

Revised version submitted to GCA (June 16, 2015)

*Corresponding authors: Email: [email protected] (S.-J. Wang); [email protected] (F.-

Z. Teng)

mailto:[email protected]

mailto:[email protected]

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Abstract

Magnesium isotopic compositions of mudrocks metamorphosed at sub-greenschist facies from

three lower Paleozoic basins (northern Lake District, southern Lake District, and Southern

Uplands) in the British Caledonides were measured in order to understand the behavior of Mg

isotopes during diagenesis and low-grade metamorphism. Carbonate-free mudrocks from the

northern Lake District have heavy δ26Mg values varying from -0.17 to +0.25. By contrast, Mg

isotopic compositions of carbonate-bearing mudrocks from the southern Lake District and

Southern Uplands vary more widely, with δ26Mg ranging from -0.74 to -0.08. Acid leaching

experiments on the latter show that the leachates have higher Ca/Al and Ca/K ratios than the

residues due to the dissolution of leachable carbonates. The δ26Mg values of leachates (-1.54 to -

0.21) are always lighter than the corresponding residues (δ26Mg = -0.39 to +0.09), consistent

with isotopically light Mg in carbonates. A rough, negative correlation between δ26Mg and

Mg/Al for the residual silicate fraction of mudrocks suggests that their Mg isotopic compositions

are controlled by the relative proportion of illite/muscovite and chlorite. Global clastic sediments

display highly variable Mg isotopic compositions that are negatively correlated with CaO/Al2O3

and CaO/TiO2, implying that carbonates introduce light Mg isotopes to sediments, although the

silicate end member itself has a wide range of δ26Mg, depending on its mineralogy. Magnesium

isotopic compositions of mudrocks, as well as their silicate and carbonate fractions, do not vary

systemically as metamorphism proceeds from diagenesis to low-grade metamorphism,

suggesting limited Mg isotope fractionation during low-temperature metamorphic dehydration

(<300ºC). The general decrease of Mg fraction (by mass) contributed by carbonate with

increasing metamorphic grade suggests that dissolution or decomposition of carbonates during

metamorphism expelled light Mg isotopes. Thus, the Mg isotopic compositions of the silicate

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fractions in clastic sediments more faithfully reflect their provenance signatures. Our study

shows that Mg isotopes can be used to study sedimentary diagenesis, and Mg isotopes may prove

a useful tracer of sediments recycled into the mantle given their heterogeneous δ26Mg values.

Keywords: Magnesium isotopes, metamorphic dehydration, mudrock, carbonate, leaching

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1. Introduction

Magnesium (Mg), with its three isotopes of 24Mg, 25Mg and 26Mg, is a soluble major

element in Earth’s mantle and crust. The significant mass difference among the three isotopes

(e.g., >8% between 24Mg and 26Mg) leads to potentially large isotope fractionations associated

with geological processes. The terrestrial mantle, as represented by peridotite xenoliths and

oceanic basalts, displays a restricted range of Mg isotopic composition (Teng et al., 2007b, 2010a;

Handler et al., 2009; Yang et al., 2009; Bourdon et al., 2010; Dauphas et al., 2010; Bizzarro et al.,

2011; Huang et al., 2011; Pogge von Strandmann et al., 2011; Xiao et al., 2013; Lai et al., 2015),

with the average δ26Mg of -0.25 ± 0.07 (2SD; Teng et al., 2010a). Although the average δ26Mg

value of bulk upper continental crust (-0.22) is estimated to be similar to the normal mantle value

(Li et al., 2010), significant heterogeneity (varying by up to 7‰) has been documented in

sedimentary rocks and soils (e.g., Galy et al., 2002; Tipper et al., 2006b; Pogge von Strandmann

et al., 2008a; Immenhauser et al., 2010; Huang et al., 2013; Liu et al., 2014). For example,

carbonate minerals have the lowest δ26Mg of terrestrial rocks (e.g., Higgins and Schrag. 2010),

whereas weathered regoliths have heavy δ26Mg values up to +1.81 (e.g., Liu et al., 2014). With

respect to the hydrosphere, seawater has a homogenous δ26Mg around -0.83 (Foster et al., 2010;

Ling et al., 2011), while global rivers have variable δ26Mg values, with the average (-1.09)

generally lighter than that of seawater (Tipper et al., 2006b, 2008; Brenot et al., 2008; Pogge von

Strandmann et al., 2008b; Wimpenny et al., 2011). The large Mg isotopic variations observed in

sedimentary rocks and the systematic Mg isotopic difference between lithosphere and

hydrosphere are thought to result from low-temperature chemical weathering and sedimentation

(Tipper et al., 2006a, b, 2008, 2010; Brenot et al., 2008; Teng et al., 2010b; Wimpenny et al.,

2010, 2011, 2014a; Huang et al., 2012; Opfergelt et al., 2012, 2014; Pogge von Strandmann et al.,

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2008b, 2012; Liu et al., 2014; Ma et al., 2015).

Chemical weathering transports isotopically light Mg from silicate rocks to the hydrosphere,

leaving heavier Mg isotopes in the weathered residues (e.g., Teng et al., 2010b; Tipper et al.,

2010; Huang et al., 2012; Liu et al., 2014). However, although chemical weathering produces a

wide range of δ26Mg in the secondary minerals, this process cannot solely account for the Mg

isotopic heterogeneity (-1.64 to +0.92) seen in clastic sediments (Li et al., 2010, 2014; Huang et

al., 2013; Wimpenny et al., 2014b). Huang et al. (2013) and Wimpenny et al. (2014b) found that

the presence of carbonate minerals in loess deposits exerts a large impact on their bulk Mg

isotopic compositions. Because carbonate minerals contain Mg that is characteristically light, the

addition of carbonates potentially introduces light Mg isotopes to the bulk clastic sediments.

However, the influence of carbonates on the Mg isotopic compositions of water-lain clastic

sediments has yet to be characterized. Moreover, clastic sediments commonly experience low-

temperature metamorphism (<300ºC; Wintsch and Kvale, 1994; Sutton and Land, 1996; Milliken,

2003; Merriman et al., 2009). As Mg is water soluble, Mg isotope frationation due to

metamorphic dehydration might occur at low temperatures. Although previous studies found

high-temperature (>300ºC) metamorphic dehydration causes limited Mg isotope fractionation (Li

et al., 2010, 2014; Teng et al., 2013; Wang et al., 2014a, 2015), the behavior of Mg isotopes

during low-temperature metamorphism (e.g., diagenesis and low-grade metamorphism) remains

unknown.

To address these questions, we carried out leaching experiments and Mg isotopic analyses

on three suites of low-grade metamorphosed mudrocks from lower Paleozoic basins within the

British Caledonides that had previously been analyzed for major, trace element and Nd, Sr, and

Li isotopic compositions (Qiu et al., 2009), as well as their metamorphic mineralogy (Merriman,

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2006). Mudrocks make up ~50% of sedimentary rocks, and represent the most typical clastic

sediments on the Earth’s surface (e.g., Taylor and McLennan, 1985), thus, they are suitable for

studying the behavior of Mg isotopes in clastic sediments. We found large (~1‰) Mg isotopic

variations in the mudrocks regardless of the metamorphic grade, and up to 1.59‰ Mg isotopic

differences between residues and leachates. These findings suggest that diagenesis and low-grade

metamorphic dehydration do not cause significant Mg isotope fractionation in bulk mudrocks,

but the addition of carbonate minerals may impart light Mg isotopic signatures to the clastic

sediments. Thus, only Mg isotopic compositions of the silicate fraction reflect the signature of

their provenance.

2. Geological Background and Samples

Mudrocks were collected from three Ordovician to Silurian sedimentary basins in the

British Caledonides (Fig. 1): the northern Lake District, southern Lake District, and Southern

Uplands (Merriman et al., 2009). The northern Lake District basin was formed in an extensional

setting on the southern margin of the Iapetus Ocean during the early to mid-Ordovician (Stone

and Merriman, 2004). The southern Lake District was formed following the flexural subsidence

of the crust when the Iapetus Ocean was closed during the late Ordovician and early Silurian,

while the Southern Uplands basin was developed as an accretionary thrust complex at the

Laurentian continental margin (Leggett et al., 1979; Stone et al., 1987; Kneller, 1991). Deposits

in these basins are commonly turbidite-dominated, mudrock sequences that were overprinted by

diagenesis and low-grade metamorphism (Merriman et al., 2009). They consist mainly of clay

assemblages dominated by illite, muscovite and chlorite, with non-clay fractions composed of

quartz (<40%), albite (<15%), and minor amounts (<5%) of dolomite, calcite, K-feldspar,

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hematite, pyrite, and trace amounts (<1%) of rutile or anatase (Merriman et al., 2009). Samples

were selected from previous collections that were used to study the metamorphic patterns of the

three basins (Fortey, 1989; Merriman and Roberts, 2000; Johnson et al., 2001), and later used to

investigate their major/trace elements and Sr, Nd, and Li isotopic geochemistry (Merriman et al.,

2009; Qiu et al., 2009).

The northern Lake District mudrocks are from the Skiddaw Group (Fig. 1) that was

metamorphosed under relatively high heat flow conditions (30-50ºC km-1) in an extensional

setting (Stone and Merriman, 2004). The lower-grade mudrocks from the deep diagenetic zone

and low anchizone are carbonate-free and contain authigenic illite as the major mineral phase,

accompanied by variable amounts of intermediate Na/K-mica, illite-smectite (I-S) and chlorite

(Merriman, 2006). The higher-grade mudrocks from the high anchizone and epizone consist

mainly of authigenic muscovite and chlorite, with paragonite and intermediate Na/K-mica, and

minor pyrophyllite, albite, rutile, and quartz (Merriman, 2006). The provenance of the northern

Lake District mudrocks is a highly weathered ancient upper continental crust (Qiu et al., 2009).

The southern Lake District mudrocks are from the Windermere Supergroup (Fig. 1), which

was metamorphosed under low heat flow conditions (<20ºC km-1) (Soper and Woodcock, 2003).

Clay minerals consist of K-mica and chlorite, with minor corrensite. Paragonite and pyrophyllite

are not recorded in these mudrocks, but carbonate minerals are usually present (Merriman, 2006).

Both weathered upper continental crust and juvenile arc volcanic materials are present in the

provenance of the southern Lake District basin (Qiu et al., 2009).

The Southern Uplands mudrocks were deposited in the Ordovician to Silurian (Fig. 1), and

were metamorphosed under a low geothermal gradient of <25ºC km-1 (Merriman and Roberts,

2000). Chlorite and K-mica are the dominant clay minerals. Minor amounts of albite, dolomite,

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corrensite, kaolinite, and intermediate Na/K-mica are also present (Merriman, 2006). The

provenance of the Southern Uplands basin is, like that of the southern Lake District basin,

composed of a mixture of arc lavas and weathered upper continental crust (Qiu et al., 2009).

3. Methods

All experiments were carried out at the Isotope Laboratory of the University of Washington,

Seattle. For bulk rock powders, approximately two to six mg of samples was weighed, and

treated sequentially with Optima-grade HF-HNO3, HNO3-HCl, and HNO3. After complete

dissolution, the samples were evaporated to dryness at ~160ºC, and finally taken up in 1N HNO3

for chromatographic separation.

Leaching experiments were carried out on mudrocks from the southern Lake District and

Southern Uplands to remove carbonate minerals. Dilute acetic and hydrochloric acids are

commonly used in leaching experiments (e.g., Ostrom, 1961). Since dilute acetic acid may not

completely dissolve dolomite minerals in sediments (Wimpenny et al., 2014b), we also used

dilute hydrochloric acid. Previous studies suggest that 0.3N HCl may have a negligible effect on

either well or poorly crystallized illite and chlorite that are the two major Mg-bearing clays in the

mudrocks (Ostrom, 1961). Therefore, we used 0.3N HCl in our leaching experiments. For each

sample, approximately 12 to 24 mg of rock powder was immersed in 10 ml of 0.3N HCl at room

temperature. The slurries were ultra-sonicated for ~45 minutes and then centrifuged to separate

the supernatant and the residue. After separation, the supernatants were evaporated to dryness at

~160ºC, and re-dissolved in 12N Optima-grade HCl. The residues were cleaned using Milli-Q

water (18.2 MΩ cm) three times before being dissolved in a mixture of Optima-grade HF-HNO3-

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HCl acids.

The separation of Mg was achieved by cation exchange chromatography using Bio-Rad

200-400 mesh AG50W-X8 resin in 1N HNO3 (Teng et al., 2007b; 2010a; Yang et al., 2009; Li et

al., 2010). An additional chromatographic step was processed for the leachates to separate Mg

from Ca using Bio-Rad 200-400 mesh AG50W-X12 resin in 12N HCl (Ling et al., 2013; Wang et

al., 2014b). Three standards, Kilbourne Hole (KH) olivine, San Carlos (SC) olivine, and seawater,

were processed together with samples for each batch of column chemistry. The same column

procedure was performed twice to obtain pure Mg solutions for mass spectrometry. The total

procedural blank is <10 ng, which represents <0.1% of the Mg loaded on the column (Teng et al.,

2010a).

Magnesium isotopic ratios were determined using the standard-sample bracketing protocol

on a Nu Plasma MC-ICPMS with a “wet” plasma introduction system (Teng and Yang, 2014).

The 26Mg, 25Mg and 24Mg were measured simultaneously in separate Faraday cups (H5, Ax, and

L4). The background Mg signal for 24Mg was <10-4 V, which is negligible relative to the sample

signals of 3-4 V. Magnesium isotopic results are reported in δ notation in per mil relative to

DSM-3: δxMg = [(xMg/24Mg)sample/(xMg/24Mg)DSM-3 - 1] × 1000, where x refers to mass 25 or 26.

Three in-house standards were analyzed during the course of this study: KH olivine, SC olivine

and seawater, and yielded average δ26Mg values of -0.25 ± 0.03 (2SD; n = 3), -0.25 ± 0.04 (2SD;

n = 3), and -0.84 ± 0.03 (2SD; n = 6), respectively (Supplementary Table 1), which are in

agreement with previously reported values (e.g., Yang et al., 2009; Foster et al., 2010; Li et al.,

2010; Teng et al., 2010a, 2015; Ling et al., 2011).

The Ca/Al, Ca/K, and Mg/Al ratios of the leachates and residues were determined on the Nu

Plasma MC-ICPMS. Fractions of the dissolved aliquots of the leachate and residue solutions

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were diluted in 3% HNO3 prior to analysis. Four gravimetrically prepared SPEX ClaritasTM

ICPMS elemental standard solutions with a wide range of elemental ratios (covering the ratios of

unknowing samples being analyzed) were analyzed to generate a calibration curve. This set of

standard solutions was analyzed several times to verify that the calibration curve had remained

unchanged during the course of the analyses of sample solutions. Five rock standards with

known elemental ratios, including a basalt from Nancy, France (BR), three Chinese reference

materials (GBW07112 gabbro, GBW07111 granodiorite, and GBW07103 granite), and a USGS

shale standard from Wyoming, USA (SCo-1), were also analyzed together with the sample

solutions, to monitor accuracy and precision. The uncertainty of Ca/Al, Ca/K and Mg/Al ratios

are better than 10% (Supplementary Table 2).

4. Results

Major elemental ratios and Mg isotopic data of bulk rocks, leachates and residues are

reported in Table 1. Magnesium isotopic compositions of all samples fall on a single mass-

dependent fractionation line on the three-isotope diagram with a slope of 0.510 (not shown).

The δ26Mg of the northern Lake District mudrocks vary from -0.17 to +0.25 (Fig. 2).

Mudrocks from the southern Lake District and Southern Uplands have more variable δ26Mg,

ranging from -0.74 to -0.09 and from -0.74 to -0.08 (Fig. 2), respectively.

The acid leachates contain considerable amounts of Al and K in addition to Mg and Ca

(Table 1). Leachates of the southern Lake District mudrocks have Ca/Al of 0.36 ~ 8.40 and Ca/K

of 0.79 ~ 59.3 (Fig. 3). The corresponding residues have significantly lower Ca/Al and Ca/K

ratios of 0.01 ~ 0.14 and 0.03 ~ 0.47 (Fig. 3), respectively. Similarly, leachates of the Southern

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Uplands mudrocks have high Ca/Al of 0.10 ~ 4.77 and Ca/K of 0.13 ~ 11.6 (Fig. 3); whereas the

residues have low Ca/Al of 0.004 ~ 0.10 and Ca/K of 0.01 ~ 0.27 (Fig. 3).

The Mg isotopic compositions of the leachates are always lighter than the residues (Fig. 4).

Leachate δ26Mg values range from -1.54 to -0.84 for the southern Lake District mudrocks and

from -1.53 to -0.21 for the Southern Uplands mudrocks; residue δ26Mg values vary from -0.39 to

+0.05 for the southern Lake District mudrocks and from -0.32 to +0.09 for the Southern Uplands

mudrocks (Fig. 4). Correspondingly, Mg isotopic differences between the residue and leachate

(expressed as ∆26Mgresidue-leachate = δ26Mgresidue - δ26Mgleachate) are in the range of 0.05 ~ 1.59‰.

5. Discussion

In this section, we first examine the mineralogical controls on Mg isotopic compositions of

the clastic sediments; then evaluate the metamorphism and provenance effects on Mg isotopic

compositions of these mudrocks. Finally, we discuss the use of Mg isotopes in tracing the

recycling of sediments.

5.1 Mineralogical controls on magnesium isotopic compositions of mudrocks

The large Mg isotopic variations observed in mudrocks may reflect the variation in the

proportions of Mg-bearing phases that have distinct Mg isotopic compositions, such as clays and

carbonates. The northern Lake District mudrocks are carbonate-free (Merriman et al., 2006), and

have consistently heavy Mg isotopic compositions (Fig. 2). By contrast, the southern Lake

District and Southern Uplands mudrocks contain variable amounts of carbonates (Merriman et

al., 2006), and display significantly more variable δ26Mg, extending to very low values (Fig. 2).

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As carbonate minerals (e.g., Mg-bearing calcite and dolomite) contain variable Mg that is

isotopically light (e.g., Galy et al., 2002), the presence of carbonates in mudrocks will impact

bulk δ26Mg values.

Our acid leaching experiments show that Mg isotopic compositions of the leachates are

always lighter than the residues (Fig. 4), consistent with isotopically light Mg leached from

carbonates (Wimpenny et al., 2014b). Nonetheless, these leachates also contain considerable

amounts of Al and K, which must derive from the clays (Table. 1), implying that acid leaching

also removed components from clay minerals, in addition to carbonates. The clay minerals in

mudrocks from the southern Lake District and Southern Uplands are Mg-rich illite-muscovite

and chlorite (Merriman et al., 1995, 2009), which are expected to be enriched in 26Mg (Teng et

al., 2010b; Tipper et al., 2010; Huang et al., 2012; Opfergelt et al., 2012, 2014; Pogge von

Strandmann et al., 2012; Wimpenny et al., 2014a). This is supported by the high δ26Mg values of

northern Lake District mudrocks (Fig. 2), which contain no carbonates but have similar clay

mineral assemblage of illite-muscovite + chlorite (Merriman et al., 1995, 2009). Leachates of

certain low-CaO sediments (e.g., BRS781 and BRS829 from the Southern Uplands) have

comparable δ26Mg values to corresponding residues (Fig. 4), suggesting that no Mg isotope

fractionation occurred during the acid leaching of clay minerals. The light Mg isotopic

compositions of leachates therefore reflect the maximum δ26Mg values of the carbonate fraction

in mudrocks. These δ26Mg values also fall within the range of carbonate leachates from loess

(Wimpenny et al., 2014b). Further support for the contribution of carbonate to the leachate

comes from the elemental ratios. The leachates have significantly higher Ca/Al ratios than

corresponding residues and bulk rocks (Fig. 3a), reflecting the preponderance of carbonates.

Alkali and alkaline earth elements behave similarly during acid leaching of clay minerals (e.g.,

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Grim, 1953); thus, the Ca/K ratio of leachates is used as an indicator of carbonate contribution.

Likewise, leachate Ca/K ratios are significantly higher than the residues (Fig. 3b). Therefore,

both Ca/Al and Ca/K ratios indicate the dominance of carbonate in controlling the composition

of the leachates.

The influence of carbonates on bulk Mg isotopic compositions of these mudrocks is

evaluated using the Mg isotopic difference between bulk rocks and residues (expressed as

Δ26Mgresidue-bulk = δ26Mgresidue - δ26Mgbulk). The carbonate-free sediments or those containing low

carbonate contents should yield similar δ26Mg values between bulk rocks and residues (Fig. 2). A

larger difference thus corresponds to higher Mg fraction contributed by carbonate minerals.

Calcite is low in Mg, so its dissolution can increase the Ca/Alleachate but may not significantly

influence the bulk Mg isotopic composition or Mg/Alleachate unless it is present in large quantities

(Fig. 5a, b). By contrast, dolomite, because of its high Mg and Ca concentrations, can have larger

impacts on the bulk Mg isotopic composition, as well as Mg/Alleachate and Ca/Alleachate (Fig. 5a, b).

Residues from acid leaching represent the silicate fraction. As clay assemblages dominate

the Mg budget of silicate fraction of these mudrocks, the Mg isotopic variations in residues

reflect the relative proportions of illite/muscovite and chlorite that are produced by two parallel

clay metamorphic mineral reaction series (Merriman, 2006): (1) smectite à mixed-layer

illite/smectite à illite à muscovite; and (2) smectite à mixed-layer chlorite/smectite à

chlorite. Chlorite has high Mg/Al of 0.8 ~ 1.1, whereas illite and muscovite have low Mg/Al of

0.06 ~ 0.22 (Merriman et al., 1995). The Mg/Alresidue is thus an indicator of the varying

proportions of illite/muscovite vs. chlorite (Fig. 6). The carbonate-free mudrocks from the

northern Lake District have the lowest Mg/Al ratios (Fig. 6), which is consistent with their high

modal abundance of illite and muscovite. The heavy Mg isotopic compositions suggest

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enrichment of 26Mg in illite and muscovite. By contrast, residues of mudrocks from southern

Lake District and Southern Uplands have generally higher and more variable Mg/Al ratios,

which show a negative correlation with δ26Mg (Fig. 6). The correlation implies that illite and

muscovite are isotopically heavier than chlorite in Mg isotopes.

Overall, the relative abundances of illite/muscovite and chlorite control the Mg isotopic

compositions of the silicate fraction of these mudrocks; and the presence of carbonate minerals

introduces isotopically light Mg to the bulk sediments. This is further highlighted when all

published δ26Mg values for clastic sedimentary rocks are compiled (Fig. 7). The CaO/Al2O3 and

CaO/TiO2 ratios are indicators of carbonate proportions in sediments, as they are extremely high

in carbonate minerals but low in clays. Globally, δ26Mg values of clastic sedimentary rocks show

rough, negative correlations with CaO/Al2O3 and CaO/TiO2 (Fig. 7). In general, sediments with

low CaO/Al2O3 and CaO/TiO2 ratios, for instance, lower than the average Post-Archean

Australian Shales (PAAS), have variable Mg isotopic compositions, but are mostly heavier than

the average value for bulk upper continental crust (-0.22; Fig. 7). The heterogeneity of δ26Mg

may be caused by the mineralogical variation of silicate phases. On the other hand, sediments

with high CaO/Al2O3 and CaO/TiO2 ratios have lighter Mg isotopic compositions, reflecting the

incorporation of carbonate phases in these rocks (Fig. 7). While previous studies have shown

clear evidence that Mg isotopic compositions of loess sediments are influenced by the presence

of carbonate minerals (Huang et al., 2013; Wimpenny et al., 2014b), our results indicate that Mg

isotopic compositions of water-lain sediments are also controlled by silicate-carbonate mixing.

5.2 Effects of metamorphism on magnesium isotopes of mudrocks

Metamorphism of crustal rocks leads to the breakdown of hydrous minerals and release of

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hydrous fluids. Magnesium is soluble with light Mg isotopes preferentially partitioning into the

fluid during low-temperature water-rock interactions (e.g., Teng et al., 2010b; Tipper et al., 2010;

Liu et al., 2014). Thus, δ26Mg values of metamorphic rocks are expected to become heavier with

increasing metamorphic grade.

The metamorphic grade for the mudrocks has been determined using the Kübler index (KI),

which is a measure of small changes in the width at half-height of the illite-muscovite ~10 Å X-

Ray Diffraction (XRD) peak (Peacor, 1992). This width varies in response to diagenesis and low-

grade metamorphism. With increasing metamorphic grade, KI values decrease from >0.42 for the

deep diagenetic zone, to 0.42-0.25 for the anchizone, and finally to <0.25 for the epizone/lower

greenschist-facies (Merriman and Peacor, 1999). The KI values of the mudrocks from the British

Caledonides range from 0.20 to 0.63 for the northern Lake District, from 0.26 to 0.66 for the

southern Lake District, and from 0.20 to 0.50 for the Southern Uplands, consistent with

metamorphism from the deep diagenetic zone to the high anchizone. Our results show that bulk

Mg isotopic compositions of these mudrocks do not vary systematically with increasing

metamorphic grade, as represented by the decrease of KI (Fig. 8a). Neither silicate fraction nor

carbonate fraction shows a correlation between their δ26Mg and KI (not shown). These

observations suggest that the Mg isotopic compositions of the bulk mudrocks, as well as their

components, may not be directly influenced by diagenesis or low-grade metamorphic

dehydration, and that the original Mg is accommodated in newly formed mineral phases rather

than being partitioned into the metamorphic fluids. A similar conclusion was also reached for Li

(which is more soluble than Mg) and its isotopes (Teng et al., 2007a; Qiu et al., 2009, 2011a,b).

This observation, together with the absence of Mg isotope fractionation during high-temperature

metamorphism (>300ºC; Li et al., 2011, 2014; Teng et al., 2013; Wang et al., 2014a, 2015),

16

indicates that metamorphic dehydration has an insignificant influence on Mg isotopic

compositions of metamorphic rocks.

Mudrocks with relatively high δ26Mg values (e.g., comparable to those of carbonate-free

mudrocks from northern Lake District) are considered to contain no or very little carbonate. If

such high-δ26Mg samples from the southern Lake District and Southern Uplands are excluded,

Mg isotopic compositions of the remainder tend to become more positive with increasing

metamorphic grade (Fig. 8a). One possible explanation for this relationship is that the dissolution

or decomposition of carbonate minerals during prograde metamorphism expelled light Mg

isotopes from the mudrocks. As such, the Mg fraction contributed by the carbonate (f) is

estimated using two end-memeber mixing: f = (δ26Mgresidue - δ26Mgbulk)/(δ26Mgresidue -

δ26Mgleachate), which ranges from 2 ± 7% to 47 ± 9% (Fig. 8b). Two samples from the Southern

Uplands (BRS781 and BRS829) yield extremely large error bars for f because of the identical

δ26Mg value between residue and leachate. If these samples are excluded, the majority shows a

rough, positive correlation between f and KI (Fig. 8b). Indeed, most mudrocks yield Δ26Mgresidue-

bulk < 0.14‰ (Figs. 2 and 5), which is insignificant given the external uncertainty of 0.10‰ for

the 26Mg/24Mg, and thus the corresponding f cluster around 10 ± 10% (Fig. 8b). Consequently,

sediments with relatively larger Δ26Mgresidue-bulk (e.g., >0.20‰ in Fig. 2) give more reliable f, and

again they show a positive correlation with KI for the mudrocks from each basin (Fig. 8b). This

suggests that the influence of carbonates on the bulk Mg isotopic compositions is weakened with

increasing metamorphic grade. However, this explanation is not unique and future Mg isotopic

studies of carbonate-bearing clastic sediments are desirable.

5.3 Effects of provenance on magnesium and lithium isotopes of silicate fraction

The above discussion suggests that bulk Mg isotopic compositions of clastic sediments do

17

not always reflect their source characteristics due to the influence of carbonates. By contrast,

bulk Li isotopic compositions are controlled by the silicate δ7Li values, as carbonates contain

insignificant amount of Li compared to the clays. Because Mg and Li isotopes of silicate

fractions are unaffected by metamorphic dehydration (Teng et al., 2007a, 2013; Qiu et al., 2009;

2011a,b; Li et al., 2011, 2014; Wang et al., 2014a, 2015), their variations may reflect the

differences in the provenance.

A previous Sr-Nd-Li isotopic and trace elemental study found that northern Lake District

mudrocks are derived from a highly weathered, old upper continental crust source; whereas

mudrocks from the southern Lake District and Southern Uplands reflect a mixed provenance of

arc lava and PAAS-like upper continental crust (Qiu et al., 2009). Because weathering processes

preferentially release heavy Li and light Mg to the hydrosphere (e.g., Teng et al., 2004; 2010b;

Tipper et al., 2010; Liu et al., 2014), the residues of intense crustal weathering are characterized

by light δ7Li (down to -20; Rudnick et al., 2004) and heavy δ26Mg (up to +1.81; Liu et al., 2014).

Mantle-derived basalts, however, have relatively homogenous δ7Li of +4.7 ± 1.8 (for basaltic arc

lavas; Qiu et al., 2009 and reference therein) and δ26Mg of -0.25 ± 0.07 (Teng et al., 2010a). The

lightest δ7Li and heaviest δ26Mg values of the northern Lake District mudrocks (Fig. 9), are

consistent with their derivation from a highly weathered continental provenance (Qiu et al.,

2009). Mudrocks from the Southern Uplands contain the greatest proportion of arc component

and thus have the heaviest δ7Li (Qiu et al., 2009). Mixing calculations show that the majority of

southern Lake District mudrocks fall on an array between arc basalt and northern Lake District

mudrocks; while the Southern Uplands mudrocks represent a mixture between arc basalt and

PAAS-like upper continental crust (Fig. 9). Consequently, mixing among average basaltic arc

lavas, PAAS-like upper continental crust, and a highly weathered component, as represented by

18

the northern Lake District mudrocks, can reproduce most of the δ7Li and δ26Mg variations

observed in these mudrocks (Fig. 9). However, as pointed by Qiu et al., (2009), this is an

oversimplified scenario, and different mixing end members most likely apply to different

samples (Fig. 9).

5.4 Implication for sediment recycling

Subducted sediments can influence on the compositions of oceanic island basalts as well as

arc lavas (e.g., Plank and Langmuir, 1998; Plank, 2014); for example, sediment addition enriches

highly fluid moible elements (such as Li, Be, Ba and Sr) in arc laves and may give rise to the

EM I end member in the mantle (e.g., Ryan and Chauvel, 2014). Deeply subducted sedimentary

rocks may largely retain their δ26Mg signatures, as Mg isotopes are unaffected by either

metamorphism or partial melting durnig crustal subduction (e.g., Li et al., 2011, 2014; Teng et al.,

2010b, 2013; Wang et al., 2014a, 2015). The recycling of dolomite into the mantle might impact

light Mg isotopic compositions to the mantle-derived rocks (Yang et al., 2012; Huang et al.,

2015). However, addition of a low-MgO clastic sedimentary component to the mantle source

may not significantly modify the mantle Mg isotopic compositions, owing to the large Mg

budget of the mantle (~38 wt.%; McDonough and Sun, 1995). On the other hand, Mg isotopes

may be potentially good tracers of sediment assimilation during magma ascent. This is due to the

fact that basaltic magmas have much lower MgO content (~8 wt.%) than peridotite, and therefore

their Mg isotopic compositions are relatively more sensitive to sediment or sediment-derived

melt addition. Bulk mixing models suggest that 10~20% sediment addition to a basaltic magma

would potentially produce δ26Mg exceeding the range of oceanic basalts.

Burial of sedimentary rocks will produce an isotopically heterogenous middle-lower

19

continental crust with repect to Mg isotopes (Teng et al., 2013; Wang et al., 2015; Yang et al.,

2015). A wide range of δ26Mg in a variety of granitoids (e.g., I, S and A types) as well as in

granulite xenoliths bear witness to the sedimentary recycling process (Shen et al., 2009; Li et al.,

2010; Telus et al., 2012; Teng et al., 2013; Wang et al., 2015; Yang et al., 2015). Although light-

δ26Mg signatures are generally seen in carbonate-bearing clastic sediments, it is also inferred that

thermal evolution of a granulite-facies lower continental crust would induce carbonate-silicate

Mg isotopic exchange or decarbonation of carbonate-bearing sediments leaving a silicate residue

enriched in light Mg isotopes (Shen et al., 2013; Wang et al., 2014b; Yang et al., 2015).

6. Conclusions

Leaching experiments and Mg isotopic analyses carried out on three suites of low-grade

metamorphosed mudrocks from lower Paleozoic basins within the British Caledonides (northern

Lake District, southern Lake District, and Southern Uplands) demonstrate that:

(1) The δ26Mg varies widely in these mudrocks, i.e., -0.17 to +0.25 in carbonate-free

northern Lake District mudrocks, and -0.74 to -0.08 in carbonate-bearing mudrocks

from southern Lake District and Southern Uplands.

(2) Large Mg isotope differences, up to 1.59‰, occur between leachates and residues. The

δ26Mg of the leachates are always lighter than that of the residues, due to the dissolution

of leachable carbonate minerals. Mg isotopic compositions of global clastic sediments

are controlled by silicate-carbonate mixing, while the silicate mineralogy determines the

δ26Mg of silicate fraction in sediments.

(3) The Mg isotopes are not directly affected by low-temperature (<300ºC) metamorphic

20

dehydration, based on the absence of correlations between bulk δ26Mg and KI values for

the mudrocks. However, δ26Mg of carbonate-bearing mudrocks may become heavier as

metamorphism progresses, due to carbonate dissolution or decomposition with

increasing metamorphic grade.

(4) Magnesium isotopic compositions of the silicate fraction in clastic sediments more

faithfully reflect provenance signatures, as bulk Mg isotopic compositions may be

influenced by the presence of carbonate minerals.

(5) Burial of clastic sediments may produce Mg isotopic heterogeneity in the middle-lower

continental crust, as well as their derivatives (e.g., S, I, and A type granites). Sediment

assimilation in basaltic magmas may leave distinguishable Mg isotopic signatures in

some extreme case.

Acknowledgements

We thank Richard Merriman for providing samples, Melissa Hornick for help in the clean

lab, Jody Bourgeois, Charlotte Schreiber, and Aaron Brewer for their thoughtful discussions, Ed

Tipper and two anonymous reviewers for insightful comments, and Shichun Huang for careful

and efficient handling. This work was supported by the National Science Foundation (EAR-

0838227, EAR-1056713 and EAR-1340160) and the National Nature Science Foundation of

China (41230209 and 41090372).

21

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Figure Captions

Fig. 1: Map showing the location of the three sedimentary basins from which the mudrocks

derive: Southern Uplands, Northern Lake District, and Southern Lake District (after Merriman et

al., 2009).

Fig. 2: δ26Mg of bulk-rocks, leachates and residues of mudrocks from the northern Lake District,

southern Lake District and Southern Uplands. The gray line represents the δ26Mg of average

upper continental crust (UCC = -0.22; Li et al., 2010). Data are reported in Table 1.

Fig. 3: Ca/Al (a) and Ca/K (b) ratios in leachates and residues versus that in the bulk rocks. Data

are reported in Table 1

Fig. 4: Comparison of δ26Mg values between leachates (δ26Mgleachate) and residues (δ26Mgresidue).

Data are reported in Table 1.

Fig. 5: Variation of Mg isotopic difference between bulk rock and residue (Δ26Mgresidue-bulk =

δ26Mgresidue - δ26Mgbulk) as a function of (a) leachate’s Ca/Al ratios (Ca/Alleachate), and (b)

leachate’s Mg/Al ratios (Mg/Alleachate). The arrows indicate the trends created by calcite or

dolomite dissolution. Data are reported in Table 1.

Fig. 6: Variation of δ26Mgresidue as a function of residue’s Mg/Al ratios (Mg/Alresidue). Average

upper continental crust δ26Mg shown by the gray line (Li et al., 2010). Data are reported in Table

34

1.

Fig. 7: Plot of δ26Mg of clastic sedimentary rocks versus a) CaO/Al2O3 and b) CaO/TiO2. The

Mg isotopic data for clastic sedimentary rocks are from Wombacher et al. (2009), Li et al. (2010,

2014), Huang et al., (2013), Wimpenny et al., (2014b), and this study; The corresponding major

elemental data are from Gallet et al. (1998), Gao et al. (1998), Jahn et al. (2001), Nance and

Taylor (1976), Taylor et al. (1983), and Qiu et al. (2009). Also shown are the average Post-

Archean Australian Shales (PAAS) from Li et al., (2010). See text for discussion.

Fig. 8: (a) Variation of bulk δ26Mg as a function of Kübler Index (KI); (b) Variation of the Mg

fraction contributed by carbonate (f) as a function of KI. The KI value decreases as

metamorphism progresses (arrows). KI values are from Merriman et al. (2009). The gray line in

(a) represents the average upper continental crust δ26Mg (Li et al., 2010). The gray area in (b)

represents f = 10 ± 10%. δ26Mg values are reported in Table 1. See text for discussion.

Fig. 9: Coupled δ26Mg-δ7Li variations for the silicate fraction of mudrocks from the three basins.

The gray lines are mixing trends between basaltic arc lava and PAAS, and between basaltic arc

lava and northern Lake District mudrocks, respectively. The [Li] and δ7Li of arc basalts, PAAS

and LC507 are from Qiu et al. (2009 and references therein). The MgO used for arc basalts and

PAAS are ~6 wt.% and 2.3 wt.%, respectively; the δ26Mg used for arc basalts and PAAS are -

0.25 and +0.07, respectively. Sample LC507 from the northern Lake District mudrocks is taken

to represent the end member of highly weathered upper continental crustal provenance. δ26Mg

35

values are reported in Table 1. See text for further details.

Table 1: Magnesium isotopic compositions and elemental ratios for the bulk rock, leachate andresidue of mudrocks from the northern Lake District, southern Lake District and SouthernUplands.

Sample δ26Mg 2SD δ25Mg 2SD KI CIA Mg/Al Ca/Al Ca/K

Northern Lake DistrictLC348 Bulk-rock -0.09 0.06 -0.03 0.03 0.2 79 0.08 0.02 0.06LC199o Bulk-rock -0.13 0.06 -0.05 0.04 0.22 80 0.09 0.02 0.06LC142 Bulk-rock -0.04 0.06 -0.03 0.03 0.32 80 0.14 0.02 0.08LC521r Bulk-rock +0.02 0.07 +0.02 0.05 0.39 81 0.09 0.02 0.08LC482r Bulk-rock -0.13 0.07 -0.04 0.05 0.4 85 0.08 0.01 0.03LC507 Bulk-rock +0.25 0.07 +0.16 0.05 0.46 83 0.11 0.02 0.09LC434 Bulk-rock -0.17 0.06 -0.08 0.03 0.51 82 0.1 0.01 0.06LC495r Bulk-rock -0.10 0.07 -0.06 0.05 0.63 81 0.07 0.01 0.06

Southern Lake District

LC940

Bulk-rock -0.36 0.04 -0.19 0.03 0.26 61 0.3 0.29 0.88Residue -0.31 0.07 -0.19 0.07 0.27 0.05 0.16Leachate -0.85 0.10 -0.43 0.08 0.34 3.4 12.24

Leachate R -0.83 0.07 -0.41 0.06average -0.84 0.06 -0.42 0.05

SH24oBulk-rock -0.28 0.06 -0.15 0.03 0.27 73 0.31 0.02 0.06Residue -0.26 0.07 -0.15 0.06 0.28 0.01 0.03Leachate -1.54 0.10 -0.76 0.07 0.33 0.63 0.79

SH19Bulk-rock -0.33 0.04 -0.15 0.03 0.29 71 0.3 0.06 0.17Residue -0.25 0.07 -0.11 0.06 0.21 0.03 0.1Leachate -0.76 0.10 -0.39 0.07 0.32 0.79 1.71



LC1606r,oBulk-rock -0.31 0.07 -0.17 0.05 0.34 72 0.3 0.02 0.05Residue -0.29 0.07 -0.14 0.06 0.28 0.01 0.04Leachate -0.84 0.10 -0.42 0.07 0.32 0.36 0.54

LC1570r,oBulk-rock -0.48 0.06 -0.24 0.03 0.4 74 0.15 2.54 10.18Residue +0.05 0.10 +0.01 0.08 0.17 0.14 0.47Leachate -1.54 0.07 -0.77 0.06 0.16 8.4 59.33

LC1617Bulk-rock -0.75 0.05 -0.39 0.02 0.45 68 0.37 0.53 1.48

Bulk-rock R -0.71 0.07 -0.36 0.04

36

average -0.74 0.04 -0.38 0.02Residue -0.25 0.07 -0.11 0.07 0.25 0.06 0.16Leachate -1.31 0.10 -0.66 0.08 0.72 2.38 12.39


LC1618r,oBulk-rock -0.09 0.06 -0.03 0.04 0.66 74 0.13 0.32 1.24Residue +0.03 0.07 +0.01 0.06 0.12 0.06 0.22Leachate -1.00 0.10 -0.52 0.07 0.12 2.39 8.04

Southern Uplands

BRS781rBulk-rock -0.24 0.07 -0.12 0.05 0.2 72 0.34 0.02 0.05Residue -0.21 0.07 -0.08 0.06 0.26 0.01 0.03Leachate -0.25 0.10 -0.10 0.07 0.18 0.18 0.26

BRS790r



BRS807r



BRS824r

Bulk-rock -0.20 0.06 -0.09 0.04 0.23 73 0.26 0.03 0.08Residue -0.09 0.07 -0.03 0.06 0.25 0.01 0.04

Residue R -0.13 0.08 -0.05 0.05 0.22 0.4 0.68average -0.11 0.05 -0.04 0.04

Leachate -0.52 0.07 -0.21 0.06

BRS882Bulk-rock -0.36 0.07 -0.18 0.05 0.28 69 0.3 0.03 0.07Residue +0.03 0.07 +0.05 0.06 0.28 0.02 0.04Leachate -1.17 0.10 -0.58 0.08 0.31 0.43 0.66

BRS879rBulk-rock -0.16 0.06 -0.06 0.04 0.29 71 0.28 0.05 0.11Residue -0.32 0.07 -0.15 0.06 0.28 0.05 0.12Leachate -0.66 0.10 -0.34 0.07 0.16 0.22 0.71

BRS753r


Residue R -0.30 0.08 -0.18 0.05average -0.27 0.05 -0.15 0.04

Leachate -0.44 0.10 -0.23 0.07 0.12 0.24 0.68Leachate R -0.45 0.08 -0.22 0.05

average -0.45 0.06 -0.23 0.04

BRS1028Bulk-rock -0.54 0.07 -0.29 0.05 0.44 74 0.22 0.24 0.61Residue -0.20 0.07 -0.06 0.06 0.14 0.1 0.27

Residue R -0.14 0.08 -0.08 0.05

37

average -0.18 0.05 -0.07 0.04Leachate -1.02 0.10 -0.55 0.08 0.7 1.73 7.29

BRS742r,o

Bulk-rock -0.11 0.06 -0.07 0.04 0.45 73 0.16 0.01 0.04Residue +0.08 0.10 +0.01 0.07 0.16 0.01 0.02

Residue R +0.09 0.07 +0.02 0.06average +0.09 0.06 +0.02 0.05

Leachate -0.46 0.10 -0.30 0.07 0.2 0.12 0.25

BRS829r,o

Bulk-rock -0.08 0.06 -0.02 0.04 0.48 73 0.18 0.01 0.02Residue -0.07 0.10 -0.05 0.07 0.17 0 0.01Leachate -0.21 0.07 -0.07 0.06 0.19 0.1 0.13


BRS710r, o


Residue R -0.07 0.07 -0.06 0.06average -0.09 0.06 -0.07 0.05

Leachate -1.49 0.10 -0.76 0.08 1.85 3.92 11.59Leachate R -1.54 0.07 -0.76 0.06

average -1.53 0.06 -0.76 0.05

KI values are from Merriman et al. (2009); Bulk-rock major elemental ratios are from Qiu et al. (2009), andelemental ratios of leachates and residues are from this study.r: Re-sample from the original sample of Merriman et al. (2009)o: Sample with organic carbonR: Repeat column chemistry and instrumental analysis.average = weighted average value;2SD = 2 times the standard deviation of the population of n (n>20) repeated measurements of the standardsduring an analytical session.

the behavior of magnesium isotopes in low-grade ... filethe behavior of magnesium isotopes in...

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