Magnesium Isotope Fractionation Between Brucite

Earth and Planetary Science Letters 394 (2014) 82–93

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Earth and Planetary Science Letters

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Magnesium isotope fractionation between brucite [Mg(OH)2]andMg aqueous species: Implications for silicate weathering and biogeochemical processes ∗ Weiqiang Li a,b, , Brian L. Beard a,b, Chengxiang Li c, Clark M. Johnson a,b a University of Wisconsin-Madison, Department of Geoscience, 1215 West Dayton Street, Madison WI 53706, United States b NASA Astrobiology Institute, United States c State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, PR China article info abstract

Article history: Brucite, with its octahedral structure, has a lattice conﬁguration that is similar to the Mg-bearing Received 16 October 2013 octahedral layers in phyllosilicates. Understanding stable Mg isotope fractionation between brucite and Received in revised form 11 March 2014 aqueous solution therefore bears on interpretation of Mg isotope data in natural weathering systems. Accepted 12 March 2014 In this study, we experimentally determined Mg isotope fractionation between brucite and two Mg Available online xxxx + − aqueous species, the free Mg aquo ion ([Mg(OH ) ]2 ) and EDTA-bonded Mg (Mg-EDTA2 ). Results from Editor: G. Henderson 2 6 recrystallization and brucite synthesis experiments suggest mild preferential partitioning of light Mg 26 + Keywords: isotopes into brucite compared to Mg aquo ions at low temperatures, where measured Mgbrucite-Mg2 ◦ ◦ ◦ Mg isotope fractionation increased from ca. −0.3h at 7 C, to ca. −0.2h at 22 C, to ca. 0h at 40 C. MgO brucite 26 hydrolysis experiments in EDTA-bearing solutions suggest that the Mgbrucite-Mg-EDTA fractionation is ◦ weathering + 2.0h at 22 C, indicating that light Mg isotopes strongly partition into Mg-EDTA complex relative to Mg(OH)2 brucite, as well as relative to Mg aquo ions. Magnesium atoms in brucite, Mg aquo ions, and Mg-EDTA fractionation complexes are all octahedrally coordinated, and the measured Mg isotope fractionations correlate with EDTA average bond lengths for Mg. Aqueous Mg ions have the shortest bond length among the three phases, and enrich heavy Mg isotopes relative to brucite and Mg-EDTA. In contrast, Mg-EDTA has the longest average bond length for Mg, and enriches light Mg isotopes relative to Mg aquo ions and brucite; the relatively long Mg-EDTA bond suggests that organically bound Mg may commonly have low 26Mg/24Mg ratios, which may explain proposed “vital” effects for stable Mg isotopes. Such relations between bond length and Mg isotope fractionation could be extended to other phyllosilicates such as serpentine- and clay-group minerals where Mg is also octahedrally coordinated. © 2014 Elsevier B.V. All rights reserved.

1. Introduction of tetrahedral layers and octahedral layers (Fig. 1; Bailey, 1988). In + Mg-bearing phyllosilicates, octahedrally coordinated Mg2 cations Chemical weathering of silicate rocks plays a key role in global occur in a sheeted structure that is the same as brucite (Fig. 1; cycling of CO2, as well as major cations in the hydrologic-ocean Bailey, 1988; Ryu et al., 2011). Magnesium isotope fractionation system such as Mg and Ca. Stable magnesium isotopes show signif- between brucite and aqueous solution, therefore, potentially sheds icant fractionation during silicate weathering and are a promising insight into the origin of Mg isotope variations during weathering tool for tracing such processes (Bolou-Bi et al., 2012; Huang et and alteration of silicate rocks. al., 2012; Li et al., 2010; Opfergelt et al., 2012, 2014; Pogge von Brucite is also an important secondary mineral that occurs in a Strandmann et al., 2008a, 2008b, 2012; Teng et al., 2010; Tipper wide range of geological settings, including serpentinization zones et al., 2010, 2006, 2008; Wimpenny et al., 2010, 2011). Phyllosil- of ultra-maﬁc rocks (Bach et al., 2006; Hostetler et al., 1966; Neal icates such as clay minerals and serpentine-group minerals are and Stanger, 1984; Wicks and Whittaker, 1977), weathering pro- common secondary minerals that form as weathering and alter- ﬁles of limestone (Alabaster, 1977), and metamorphic contacts be- ation products of silicate rocks, and they consist of basic structures tween dolomite/marble and igneous intrusions (Brown et al., 1985; Carpenter, 1967; Tupper, 1963). Due to its high reactivity with CO2, interest in using natural brucite as a geological reservoir for car- * Corresponding author. bon storage is increasing (Zhao et al., 2009). In addition, brucite E-mail addresses: [email protected], [email protected] (W. Li). has been reported to occur in skeletons of some species of living http://dx.doi.org/10.1016/j.epsl.2014.03.022 0012-821X/© 2014 Elsevier B.V. All rights reserved. W. Li et al. / Earth and Planetary Science Letters 394 (2014) 82–93 83

Fig. 1. Structures of representative Mg-bearing phyllosilicates, including the serpentine group chrysotile (A), clay group talc (B), and structures of brucite (C), Mg aquo ion 2+ ® ([Mg(OH2)6] , D), and Mg-EDTA complex (E). Mineral structures are generated using the Vesta software and standard mineral structures from the American Mineralogist Crystal Structure Database. Note the scales for these structures do not necessarily match. coral (Buster and Holmes, 2006; Nothdurft et al., 2005) and cal- Micromeritics Gemini surface area analyzer and the N2 multipoint careous algae (Weber and Kaufman, 1965), implying that brucite BET method. Accuracy of the surface area measurements was bet- plays a role in bio-mineralization of some oceanic organisms. Ulti- ter than ±8% (2 standard deviation, or 2SD), based on long-term mately, Mg isotope compositions of brucite may be useful tracers analyses of a well-characterized kaolinite standard. X-ray diffrac- for these processes. tion patterns were obtained using a Rigaku Rapid II X-ray diffrac- Experimental calibration of Mg isotope fractionation between tion system, using a Mo target X-ray source (Mo Kα1 = 0.70930 Å). ◦ brucite and aqueous solution at 80 C has been recently reported The pH of solutions was measured using an Accumet model (Wimpenny et al., 2014). It remains unknown, however, whether 20 pH meter with a MI-407 needle pH Microelectrode. The pH the isotopic fractionations are similar for brucite at supergene tem- meter was calibrated with NIST-traceable standard buffer solu- ◦ peratures (e.g., <40 C), which are more pertinent to chemical tions and the accuracy was estimated at ±0.10 (2SD) based on weathering and biogeochemical processes. In this study, we ex- long-term measurements of calibration standards. Magnesium con- perimentally determined Mg isotope fractionation factors between centrations of solutions was measured by MC-ICP-MS analysis + + brucite and two aqueous species of Mg2 ions, hydrated Mg2 with 4–6 variable-concentration standards, following purification 2+ 2+ ([Mg(OH2)6] ) and chelated Mg (Mg-EDTA) (Fig. 1). Hydrated by ion-exchange chromatography; the concentration uncertainty is + + Mg2 is the dominant and most common Mg2 species in natu- estimated at <±5% (2SD, n = 14), based on multiple analyses of ral fluids, whereas the Mg-EDTA complex may provide insight into test solutions. + organic molecule–Mg2 interactions that may occur during weathering and many biogeochemical processes. In our experiments, we 2.2. Recrystallization experiments avoided unidirectional reactions such as synthesis of brucite by + − direct precipitation of Mg2 with OH because such approaches Recrystallization experiments are denoted as Bex1 and Bex2 carry a higher risk of kinetic isotope effects. Instead, we at- (Table 1). Isotope exchange was promoted via a recrystallization tempted to increase the likelihood of attaining isotopic equilibrium process (“Ostwald ripening”), where the growth of larger crystals through two-direction reactions, including recrystallization and occurred through dissolution of smaller crystals (Li et al., 2011; mineral conversion by MgO hydrolysis in weak acid. Our results Stoffregen et al., 1994) in a saturated solution. A “three-isotope” indicate that light Mg isotopes preferentially partition into brucite method was employed during the recrystallization experiments to + and Mg-EDTA compared to hydrated Mg2 at low temperatures. evaluate the degree of isotope exchange by labeling either the mineral or the solution with an enriched 25Mg tracer. Details may be 2. Experimental methods found in Li et al. (2011). Two components were prepared: first, brucite crystals, and 2.1. Reagents and analytical methods second, brucite-saturated Mg solutions. Brucite was synthesized through addition of a saturated MgSO4 solution that was either High-purity magnesium oxide (>99.998% pure MgO, Alfa 25Mg-enriched or isotopically “normal”, to a constantly stirred 1 M ® Aesar ), and reagent grade NaOH, EDTA acid, and Na4-EDTA were NaOH solution, where pH was kept above 13. The very high pH 2+ used. Solutions were made using >18.2M de-ionized H2O, dis- ensured quantitative precipitation of Mg , and prevented poten- 2+ tilled HNO3, and doubly distilled HCl. Molarity of acid was de- tial issues of forming isotopically zoned brucite by partial Mg termined by titration using certified 1 M NaOH to better than precipitation with insufficient NaOH. Gel-like Mg(OH)2 formed im- ±0.05 M for 6 M HCl, ±0.03 M for 1.0 M and 1.5 M HNO3, mediately, which was placed in an ultrasonic bath for >2hours and ±0.01 M for 0.20 M HCl. Prior to experiments, bottles and to make fine-grained brucite. Brucite was washed using de-ionized centrifuge vials were pre-leached in 6 M HCl and rinsed with de- water and 100% ethanol and centrifugation, and dried in oven at ◦ ionized water. 70 C overnight. Brucite-saturated MgCl2 solutions were prepared Morphology of minerals was characterized using a Leo1530 by adding 1 M NaOH to isotopically “normal” and 25Mg-enriched Field Emission Scanning Electron Microscope (FE-SEM). Gold- MgCl2 solutions to a pH of ∼9.4, and this was left to stabilize for coated samples were imaged at 3 kV, and spatial resolution was one week. The solution was then separated from brucite using a better than 5 nm. Surface area of brucite was measured using a 0.45 μm syringe filter; the brucite-saturated solutions were then 84 W. Li et al. / Earth and Planetary Science Letters 394 (2014) 82–93

Table 1 Summary of experiments.

Experiment Temperature Starting material Molar ratio of magnesium ◦ series ID ( C) Mgsol/Mgbrucite Recrystallization Bexl 22 ± 1 0.0907 g isotopically normal brucite +4.75 ml 0.085 M 25Mg-enriched 1:9.3 MgCl2 solution 25 Bex2 22 ± 1 0.0622 g Mg-enriched brucite +5 ml 0.100 isotopically normal MgCl2 1:5.0 solution Synthesis 2012L 7 ± 1 0.0776 g MgO +7ml0.2MHCl 1.8:1 2012M 7 ± 1 0.0382 g MgO +5ml0.2MHCl 1:1.1 2012A 22 ± 1 0.4175 g MgO +50 ml 0.2 M HCl 1:1 2012C 22 ± 1 0.4130 g MgO +1.4844 g EDTA acid crystal +55 ml 0.2 M NaOH 1:1 2012J 22 ± 1 0.1452 g MgO +10 ml 0.1 M Na4EDTA solution 1:2.6 2012N 40 ± 0.1 0.0192 g MgO +2ml0.2MHCl 1:1.4 2012O 40 ± 0.1 0.0460 g MgO +4ml0.2MHCl 1:1.9 used for the recrystallization experiments. Production of a Mg so- brucite suspension in a centrifuge vial with a 0.22 μm filter bot- lution that was demonstrably saturated with respect to brucite was tom. For experiments at higher temperatures, solutions were sep- important to minimize net dissolution or precipitation so that the arated from brucite using a syringe and a 0.45 μm syringe filter. results would closely reflect true isotopic exchange reactions. Brucite was obtained by transferring the brucite-rich bottom so- Two recrystallization experiments were run in 15 ml plastic lution to a 2 ml centrifuge tube using a pipettor, then decanting ◦ centrifuge tubes in an air-conditioned room at 22 ± 1 C. One ex- the supernatant, leaving brucite with < 0.1 mL interstitial solution. periment used fine-grained, isotopically “normal” brucite to recrys- In all cases, the centrifuge, syringes, filters, centrifuge tubes, and 25 tallize in an Mg-enriched, brucite-saturated MgCl2 solution (Exp. pipettor tips were equilibrated at the experiment temperature for Bex1, Table 1), and the other experiment used fine-grained, 25Mg- >3 h prior to separation. Separation operations for experiments at ◦ enriched brucite and an isotopically “normal”, brucite-saturated 40 C were done within 1 min, and the temperature drop during ◦ MgCl2 solution (Exp. Bex2, Table 1).Thecentrifugetubeswere separation was estimated to be <5 C. stirred once every weekday, and were sampled in a time series in an exponential fashion. Loss of water by evaporation was 2.4. Isotope analysis and data reporting <0.23 mg/day, which is insignificant relative to the ca. 5 ml MgCl2 solution that was used for the experiments. During sampling, the Prior to isotopic analysis, aliquots of the harvested brucite and centrifuge tube was stirred thoroughly to form a homogeneous solution were treated using concentrated HNO3 and dried repeat- brucite suspension, then ca. 0.5 ml suspension was quickly trans- edly, to transform cations to nitrite form for ion-exchange chromatography. EDTA-bearing solutions were dried and transferred to ferred to a centrifuge vial with a 0.22 μm filter bottom. MgCl2 ◦ solutions were separated and collected by centrifuging. The brucite quartz crucibles, followed by combustion at 700 Covernightto was further washed using de-ionized water and centrifuged twice decompose organic matters. Magnesium was recovered from the quartz crucibles using 7 M HNO , and then dried and treated us- to remove interstitial MgCl2 solution. 3 ing concentrated HNO3 repeatedly for ion-exchange chromatography. Mg-bearing samples were dissolved in 0.2 mL 0.5 M HNO 2.3. Synthesis experiments 3 and loaded onto an ion-exchange column that contained 0.3 mL of Biorad AG50W × 8 cation exchange resin that had been pre- Brucite synthesis experiments are denoted as 2012X, all of conditioned with 0.5 M HNO . Sodium, the main matrix element which produced brucite through reaction of MgO in HCl and EDTA 3 for Mg in this study, was eluted using 5 mL of 0.5 M HNO3, and solutions (Table 1). MgO powder was added to 0.20 M HCl solu- + Mg2 was quantitatively removed using 1 M HNO and 1.5 M tions in tightly sealed plastic or glass bottles at three different 3 ◦ ◦ HNO3 (Li et al., 2012). Recovery of Mg was 99.2 ± 4.6% (2SD, temperatures (7 ± 1 C in a walk-in refrigerator, 22 ± 1 Cinan ◦ n = 14), as determined by MC-ICP-MS analysis of test solutions air-conditioned room, 40 ± 0.1 C in a water bath; Table 1). Two that were treated as samples. Magnesium loss and total procedu- reactions occurred: ral blanks for the ion-exchange procedure were negligible (Li et al., 2012). + + → 2+ + MgO 2H Mg H2O Magnesium isotope compositions are reported using the standard per mil (h)notationofδ26Mg for the 26Mg/24Mg isotope MgO + H2O → Mg(OH)2 ratios relative to DSM3 Mg isotope standard (Galy et al., 2003), + Weak HCl (0.20 M) was used to release Mg2 from MgO to solu- where tion, which exchanged with the newly formed brucite. 26 = 26 24 26 24 − × In addition, two experiments involved EDTA at room tempera- δ Mg Mg/ Mgsample / Mg/ MgDSM3 1 1000 ture (Table 1). In one experiment (2012C), MgO powder and crys- (1) tals of EDTA acid (H4EDTA) were placed in a 0.2 M NaOH solution; 25 25 24 in the other experiment (2012J), MgO powder was added to a δ Mg values for the Mg/ Mg ratios are reported using a similar formulation. Fractionation in Mg isotopes between two phases A 0.1 M Na4EDTA solution. In both experiments, EDTA base chelated + free Mg2 in solution and form a soluble Mg-EDTA complex: and B is expressed as: 26 26 26 3 26/24 + − − Mg − = δ Mg − δ Mg ≈ 10 ln α − (2) Mg2 + EDTA4 → Mg-EDTA2 A B A B A B The error in Mg isotope fractionation factors is calculated by the Concurrent with this reaction, MgO reacted with H2Otoform error propagation function: brucite. For experiments performed in the walk-in refrigerator and = 2 + 2 1/2 air-conditioned room, samples were harvested by centrifuging the Err MgA−B (Err δMgA) (Err δMgB) (3) W. Li et al. / Earth and Planetary Science Letters 394 (2014) 82–93 85

Fig. 2. Representative SEM images of solids in this study. A. Starting brucite for recrystallization experiment. B. Brucite after 198 days of recrystallization in Exp. Bexl. ◦ C. Aggregates of high purity MgO for brucite synthesis experiments. D. Nanometer sized cubic crystals of MgO. E. Brucite synthesized by MgO hydrolysisin0.2MHClat7 C after 123 days (Exp. 2012L). F. Brucite synthesized by MgO hydrolysis in 0.1 M Na4EDTA solution after 346 days (Exp. 2012J). where Err MgA−B is the error of Mg isotope fractionation factor, cedure noted above. The test solutions were made from the in- and Err δMgA and Err δMgB are the analytical errors for A and B, house Mg standard HPS909104 and Na-compounds (NaCl, Na- respectively. EDTA). The measured isotope compositions of the test solutions Magnesium isotope measurements were made using two MC- match that of HPS909104, with a reproducibility of better than ICP-MS instruments. The first set of analyses were done using a 0.11h for 26Mg/24Mg (2SD, n = 19); specifically, six NaCl–Mg test Micromass IsoProbe MC-ICP-MS, a single-focusing mass spectrom- solutions gave δ26Mg values of −0.66 ± 0.06h, and two EDTA- eter that uses a collision cell to thermalize the ion beam. Re- Mg test solutions gave δ26Mg values of −0.59 ± 0.02h, com- peat analyses for majority of the samples were then done using a parable with the δ26Mg value of −0.66h for HPS909104 (Ap- Nu Instruments Nu Plasma II MC-ICP-MS, a double-focusing mass pendix 1). The accuracy of the total analytical method is further spectrometer. Magnesium isotope ratios were measured using a confirmed by the measured δ26Mg value of −0.83 ± 0.02h for standard-sample-standard bracketing technique and an in-house seawater (Appendix 1), which is consistent with published stud- Mg standard (HPS909104, δ26Mg =−0.66h relative to DSM3, Li ies (e.g., Teng et al., 2010), and this additionally tests the fi- et al., 2011, 2012). Detailed running conditions of the two instru- delity of ion-exchange chromatography for a complex natural ma- ments are provided in Appendix 1. The 2SD long-term external trix. reproducibility of Mg isotope analysis is better than 0.16h for 26Mg/24Mg and 0.10h for 25Mg/24Mg with IsoProbe over three 3. Results years, and better than 0.11h for 26Mg/24Mg and 0.06h for 25Mg/24Mg with Nu Plasma II over two months (Appendix 1), based 3.1. Recrystallization experiments on repeat analysis of international Mg isotope standards against in- ◦ house stock solutions. The accuracy of Mg isotope measurements For recrystallization experiments at 22 C, the solid products is demonstrated by measured δ26Mg values for Cambridge1, a sec- changed substantially in crystal size and morphology over the ond international Mg isotope standard (δ26Mg =−2.56 ± 0.16h, 198-day period. Brucite that was isotopically “normal” and that IsoProbe; −2.55 ± 0.08h, Nu Plasma II), which are consistent with was enriched in 25Mg had very similar initial surface areas of published values (e.g., −2.58 ± 0.14h, Galy et al., 2003; −2.60 ± 49.8 and 51.2 m2/g, respectively. Consistent with surface area 0.14h, Tipper et al., 2006; −2.58±0.04h, Hippler et al., 2009), as measurements, FE-SEM imaging showed that brucite initially had well as three other in-house standards that gave values consistent a homogeneous, granular-platy morphology and 40–100 nm-size with previous studies (Li et al., 2011, 2012; Appendix 1). During crystals (Fig. 2A). During recrystallization, brucite in all experi- analysis, at least two of these international or in-house Mg iso- ments evolved to petaloid-shaped, 0.5–2 μm sized brucite crys- tope standards were analyzed for every 10 samples for assurance tals, indicating extensive recrystallization after 198 days (Fig. 2B). of accuracy. Meanwhile, pH and Mg concentration of the solution remained The accuracy of the total analytical method was monitored constant within analytical uncertainty throughout the experiment by analysis of pure in-house Mg standard and test solutions that (Fig. 3, Table 2), implying that no net mass transfer of Mg occurred were processed along with samples using the ion-exchange pro- during the recrystallization experiments and therefore brucite 86 W. Li et al. / Earth and Planetary Science Letters 394 (2014) 82–93

Fig. 3. Plots of pH and Mg content of aqueous solution, as well as δ25Mg and δ26Mg values of brucite and aqueous solution, as a function of time for recrystallization experiments Bex1 and Bex2. Error bar of the data points denotes 2SD uncertainty. The shaded bars denote initial values with a 2SD error envelope. It should be noted that the apparent high initial Mg concentration for Exp. Bex2 (plot B) is probably caused by erroneous measurement, and it does not reflect a net precipitation of brucite, as δ25Mg data does not support brucite net precipitation. recrystallized via Ostwald ripening. The drop in Mg concentration ( Fig. 2C, 2D). The major peaks for brucite were identified in XRD ◦ at the beginning of Exp. Bex2 is believed to be caused by an er- spectra after 2 days of synthesis experiments at 7 C(Fig. 4B), roneous concentration measurement for the initial Mg solution. where the reaction would be expected to have been the slow- XRD analyses confirmed that solid phases remained as brucite, est. Most of the samples from the synthesis experiments were and, critically, no Mg-carbonate was formed throughout the re- collected after 100 days (Table 3), and XRD analyses confirmed crystallization experiment (Fig. 4A). that all the solid samples collected for isotope analyses were pure Magnesium isotope compositions of the solution and brucite brucite, even after 700 days (Fig. 4B); XRD spectra did not con- 25 changed over time. In Exp. Bex1, after 198 days, the δ Mg value of tain any peaks for MgO or Mg-carbonate when the synthesis ex- ± solution decreased continuously from the starting value of 18.18 periments were stopped (Fig. 4B). For synthesis experiments that h ± h 25 0.13 to 10.26 0.10 , whereas the δ Mg value of brucite in- used 0.2 M HCl, petaloid-shaped, 0.5–1 μm-sized brucite was pro- ± h ± h creased from the starting value of 0.05 0.06 to 0.79 0.06 duced (Fig. 2E). Synthesis experiments that used EDTA produced 25 (Fig. 3, Table 2). The approximately 10 times difference in δ Mg brucite as 100–200 nm grainy-platy nano-crystals (Fig. 2F), very changes for solution and brucite is consistent with the molar ra- different from those in recrystallization experiments and synthesis tio of Mg between the two phases, where Mg /Mg = brucite solution experiments in weak HCl, but more similar to that of the starting 9.3. The δ26Mg value of solution increased from −0.26 ± 0.14h MgO. to −0.06 ± 0.09h over 198 days, whereas the δ26Mg value of ◦ HCl-based synthesis experiments at 7 and 22 C had δ26Mg brucite remained around 0.12h, within analytical error (Table 2), values of brucite that were consistently lower than those of the which is expected based on molar ratio of Mg in solution to solid. 25 co-existing solutions by ca. 0.1–0.4h (Table 3). For experiments at In Exp. Bex2, where brucite was Mg-enriched and the solution ◦ 26 25 40 C, the δ Mg values of brucite and solution were almost identi- was isotopically normal, an opposite trend in δ Mg values was 26 25 cal within error (Table 3), although the difference in δ Mg values observed. Over 198 days, δ Mg values of the solution increased 26 from −0.18 ± 0.08h to 5.39 ± 0.06h, whereas δ25Mg values of between brucite and solution ( Mgbrucite-solution) changed from brucite decreased from 18.28 ± 0.10h to 17.37 ± 0.12h. δ26Mg slightly positive values to slightly negative values from day 123 values of the solution in Exp. Bex2 increased by about 0.3h over to day 186 (Table 3). For synthesis experiments involving EDTA, 26 198 days, from −0.34 ± 0.13h to −0.08 ± 0.11h, whereas the the δ Mg values of brucite were significantly higher than those of δ26Mg value of brucite remained around 0.2h, within analytical co-existing solutions. In experiments that used EDTA acid crystals, 26 + h error (Table 2). the measured Mgbrucite-solution fractionation was around 2.0 ◦ at 22 C(Table 3). In the experiment that used Na4-EDTA, the 26 + h 3.2. Synthesis experiments measured Mgbrucite-solution fractionations were about 1.5 at ◦ 22 C. These apparent fractionations are remarkably different from The MgO used for the synthesis experiments are 3–8 μm ag- those in the synthesis experiments that involved 0.2 M HCl (Ta- gregates of cubic MgO nano-crystals that are 50 nm in size ble 3). W. Li et al. / Earth and Planetary Science Letters 394 (2014) 82–93 87 2SD notation. . Mg 2SD bru 25 / F 9900 0.16 67 0.13 67 0.10 04 0.12 04 0.14 0.14 080511 0.12 0.07 0.11 0406 0.11 0.10 101418 0.10 21 0.13 0.13 0.18 0918 0.12 0.14 ...... 26 0 0 0 0 0 0 0 0 0 0 0 − − − − − − − − − − − 2SD . Mg 2SD sol 01 0.151080 0 0.15 0.1278 1 03 0 0.13 0.0902 0 0 0.09 0 0403 0.10 08 0.09 0.09 05 0.08 06 0.10 0711 0.07 13 0.11 22 0.16 0.10 0916 0.08 0.07 ...... F 0.10 0.01 0.09 0.02 0.22 0.01 0.19 0.01 0.190.21 0.010.24 0.00 0.21 0.00 0.26 0.06 0.25 0.06 0.05 0.17 0.01 0.14 0.05 0.00 0.01 0.00 0.03 0.30 0.00 0.36 0.07 0.36 0.00 0.30 0.04 0.14 0.01 0.170.27 0.04 0.00 0.31 0.04 0.15 0.010.20 0.13 0.01 0.01 0.18 0.03 0.48 0.01 0.42 0.03 0.00 0.01 0.00 0.03 0.25 0.01 0.21 0.04 25 0 0 0 0 0 0 0 0 0 0 0 − − − − − − − − − − − Mg 23 80 . . c 1 25 e δ δ 15 Mg 2SD a 99 0.291047 1 0.28 0.1845 1 09 0 0.14 0.2105 0 0 0.17 0 1208 0.18 18 0.13 0.20 10 0.17 11 0.19 1724 0.15 32 0.21 44 0.23 0.20 1732 0.19 0.21 ...... ) 26 0 0 0 0 0 0 0 0 0 0 0 N Fractionation ( − − − − − − − − − − − n a ) N ( notation, whereas fractionation factors are noted in n δ Mg 2SD 28 0.03 3(1) 46 0.06 6(1) 45 0.07 9(2) 22 0.12 4(1) 383845 0.10 0.11 6(1) 0.14 4(1) 3(1) 362524 0.06 0.11 3(1) 0.03 3(1) 4(1) 67 0.09 7(1) 97 0.20 7(1) 57 0.12 9(4) 12 0.09 12(3) 73 0.18 6(1) 06 0.09 10(2) 25 ...... / 0 0 0 0 0 0 0 17 17 17 17 17 16 17 17 26 − − − − − − − δ Mg 2SD − − − − − − − − 25 798083 0.0425 0.05 7(1) 0.10 7(1) 22 0.12 4(2) 70 7(1) 0.1067 0.07 6(1) 1 70 8(1)75 0.11 2 72 0.11 4(1) 1 77 0.08 6(1) 0.11 6(1) 1 0.04 7(1) 0 7(1) 0 768483 0.0787 0.10 8(1) 0.06 6(1) 0.12 6(1) 6(1) 8487 0.07 0.11 7(1) 7(1) / ...... 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 26 δ itions are noted in − − − − − − − − − − − − − − − − − . Mg 2SD 35 0.08 59 0.08 01 0.06 4150 0.11 48 0.10 0.08 71 0.03 79 0.06 8985 0.03 74 0.17 0.09 68 0.12 37 0.12 62 0.11 28 0.10 05 0.06 0 4.2 ...... 25 δ Mg 2SD 8487 0.02 88 0.05 32 0.08 0.12 2272 0.12 0.09 7278 0.05 82 0.06 81 0.06 84 0.09 0.07 8186 0.06 88 0.08 94 0.06 0.09 9092 0.04 0.06 ...... 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 25 δ − − − − − − − − − − − − − − − − − Mg 2SD 26 δ 0.12 0.09 0 0.23 0.17 18 Brucite Mg 2SD 6266 0.05 70 0.08 56 0.19 0.22 4442 0.23 0.14 3948 0.11 57 0.17 58 0.12 60 0.15 0.08 5769 0.11 72 0.14 81 0.06 0.18 7478 0.08 0.15 ...... 3 3 3 2 2 3 3 3 3 3 3 3 3 3 3 3 3 26 δ Brucite − − − − − − − − − − − − − − − − − b b a a ) ) , for details see text in Section N . N ( ( n n bru F and . sol F Mg 2SD Mg 2SD 25 717572 0.1124 0.05 9(1) 0.04 7(1) 22 0.11 4(2) 37 9(1) 0.0935 0.07 7(1) 74 5(1) 71 0.0476 0.08 4(1) 71 0.07 6(1) 0.08 6(1) 0.09 6(1) 7(1) 667065 0.0766 0.09 6(1) 0.11 7(1) 0.13 6(1) 5(1) 7569 0.09 0.09 7(1) 7(1) / 82 0.08 7(1) 0.06 0.08 0 82 0.12 7(1) 0.11 0.12 0 1203 0.0862 26(2) 13 0.12 0.05 4(1) 0.08 4(1) 7(1) 0.04 0.13 0.03 0.21 0.19 0 0.22 0 0 67 0.04 4(1) 0.25 0.10 18 22 0.11 8(1) 0.1040 0.14 0.10 0 8(1) 0.17 0.08 17 21 0.09 7(1)3471 0.1211 0.15 0.12 0.15 6(1) 0.09 7(1) 0 5(1) 0.19 0.29 0.20 0.11 0.17 17 0.10 17 17 45 0.08 9(1) 0.11 0.14 17 16 0.09 25(2) 83 0.12 8(1) 0.21 0.14 17 ...... 25 ...... / 1 1 1 2 2 2 2 1 1 1 1 1 1 1 1 1 1 26 0 1 3 2 2 3 5 3 δ 15 13 18 15 14 14 10 13 26 − − − − − − − − − − − − − − − − − − − − − − − − − δ − − − − − − − − Mg 2SD 8084 0.10 80 0.07 33 0.04 0.09 3252 0.10 0.08 5180 0.12 77 0.07 83 0.04 78 0.03 0.08 7476 0.04 74 0.08 71 0.15 0.06 8277 0.07 0.05 ...... 1 1 1 2 2 2 2 1 1 1 1 1 1 1 1 1 1 25 Mg 2SD 88 0.15 84 0.05 42 0.12 18 0.13 0967 0.15 21 0.07 0.05 23 0.09 1452 0.08 92 0.15 0.17 26 0.10 27 0.06 39 0.06 18 0.08 70 0.13 δ ...... − − − − − − − − − − − − − − − − − 0 25 δ − Mg 2SD 5158 0.18 52 0.10 55 0.07 0.20 5389 0.17 0.10 8457 0.08 47 0.13 60 0.12 49 0.09 0.17 4045 0.10 39 0.16 37 0.22 0.07 5746 0.18 0.14 ...... denotes number of replicates. denotes number of replicates. 3 3 3 4 4 4 4 3 3 3 3 3 3 3 3 3 3 26 Solution δ − − − − − − − − − − − − − − − − − N N Mg 2SD 21 0.23 15 27 0.14 1 17 0.15 13 19 0.17 3 26 0.141914 18 0.2313 0.08 15 0.10 14 14 2219 0.2119 0.20 2 0.16 2 2 34 0.13 06 0.09 10 08 0.11 5 11 0.09 13 16 0.23 3 ...... 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 26 Solution δ − − − − − − − − − − − − − − − − Time (days) C) 5 186 5 186 5 123 5 123 ◦ 2082 Mg conc. (ppm) 2465 1944 2397 T 40 186 40 186 40 123 40 123 22 358 22 346 ( 2222 358 706 2222 7 126 22 7 22 126 22 115 + pH HCl HCl HCl NaOH HCl HCl 06 9.36 2072 06 9.43 2413 + + + + + + + . . ys) EDTA 4 Time (da Reactant MgO MgO MgO MgO EDTA MgO Na MgO MgO denotes total number of isotope analysis, denotes total number of isotope analysis, The Mg isotope compositionsCalculated of using the isotope starting mass solutions balance were of analyzed starting in materials, multiple and analytical is sessions used over for three calculating years (2011–2013). n n Bex1-2 0 Bex2-l 0 9.47 2738 Bex1-1 0 9.53 2049 Bex2-3 1 9.26 2427 Bex1-3 1Bex1-7 9.45 35 2070 Bex2-7 35 Bex1-4Bex1-5Bex1-6 3 8 15 9.49 9.52 2075 9.50 2062 2114 Bex2-5 8 9.42 2479 Bex2-4Bex2-6 3 15 9.48 2422 9.47 2537 Bex2-2 0 Bex2: Bex1-8 198 Bex2-8 198 EXP No. Bex1: c a a b 2012J 2012N 2012C 2012O 2012M 2012A Exp. No. 2012L Table 3 Magnesium isotope composition of solutions and brucite in synthesis experiments and corresponding isotope fractionation factors. Isotope compos Table 2 Magnesium isotope composition of solution and brucite, and pH and Mg content of solution for recrystallization experiments. 88 W. Li et al. / Earth and Planetary Science Letters 394 (2014) 82–93

− charged Mg-EDTA2 onto the brucite surface, given its negative surface charge at pH > 11. In the experiments that used crystals of EDTA acid, the solution pH was ∼10.6, which is slightly lower than the PZC for brucite, although the uncertainty of PZC determination is up to ±0.4(Pokrovsky and Schott, 2004). The maximum amount − of adsorbed Mg-EDTA2 in this case is calculated assuming a brucite surface charge density of 2 μmol/m2 (Pokrovsky and Schott, 2004), and brucite surface area of 50 m2/g. Using these conser- − vative parameters, the calculated adsorbed Mg-EDTA2 could not − have exceeded 2% of the Mg-EDTA2 in solution (Table 1). Speciation and sorption considerations, therefore, indicate that the measured Mg isotope fractionations for the brucite recrys- 2+ tallization and synthesis experiments will reflect [Mg(OH2)6] - brucite fractionations, and the EDTA experiments will reflect − Mg-EDTA2 -brucite fractionations. The avoidance of adsorption issues in this study excludes the complexities in adsorption- desorption related Mg isotope fractionation, which has been reported from field studies for clay minerals (Bolou-Bi et al., 2012; Huang et al., 2012; Opfergelt et al., 2012, 2014; Pogge von Strand- mann et al., 2012; Tipper et al., 2010, 2012; Wimpenny et al., 2014), but is not well understood in experimental systems.

4.2. Mg isotope fractionation between brucite and magnesium aquo ions

In the recrystallization experiments, changes in δ25Mg values for brucite and co-existing solutions suggest that isotope exchange occurred continuously throughout the course of experiments. The degree of isotope exchange (F ) during the recrystallization experi- Fig. 4. XRD patterns of brucite in crystallization experiment (A) and synthesis exper- ment was calculated using the standard equation iments (B), pure MgO (B), and artificial mixtures of brucite and Mg-carbonates (C). The XRD analyses were performed using a Rigaku Rapid II XRD spectrometer with F = (δt − δi)/(δe − δi) (4) a Mo X-ray source. The XRD peaks of brucite were identified using a Jade 9.0® 25 software, and confirmed to be of brucite (e.g., (A)), the corresponding lattice plane where δt is the isotopic composition (δ Mg values for this study) for the brucite XRD peaks were marked in (A). (C) was made for demonstrating at a given time t, and δi and δe are the initial and equilibrium iso- the purity of brucite in our experiments and the sensitivity of XRD measurement. 25 For (C), two artificial mixtures were made using a synthetic brucite, a natural mag- topic compositions (δ Mg values), respectively (e.g., Johnson et al., nesite (MgCO3), and a synthetic nesquehonite (MgCO3 ∗ 3H2O). The mixtures were 2002; Li et al., 2011). In a two-component system where the equi- weighed and mixed by grinding in acetone before XRD analysis. librium isotope fractionation is small relative to the initial isotopic contrast between the two components, δe can be conveniently set 4. Discussion to the mass balance value of the two components (Johnson et al., 2002; Welch et al., 2003), which can be measured from bulk dis- 4.1. Speciation and adsorption of magnesium species solution or calculated from the mass and isotopic compositions of reactants. Using the calculated system mass balance and Eq. (4), In many natural environments, and the experimental condi- the calculated F from δ25Mg values of either solution or brucite − tions in this study, where Cl is the dominant anion, Mg ions agree within 7% for both recrystallization experiments (Table 2), as in aqueous solutions are dominantly in the form of an octahedral expected in a two-component system without net mass transfer. As 2+ aquo ion ([Mg(OH2)6] ) (e.g., Richens, 1997). This is supported brucite contains the majority of Mg in the experiments (Table 1), by speciation calculations using PHREEQC for the experiments that the sensitivity of δ25Mg to F is lower for brucite due to mass bal- used weak HCl, which indicate that the second most-abundant ance. F values calculated from δ25Mg values of solution are better + Mg species, MgOH , accounts for less than 0.1% of the total Mg indicators for degrees of isotope exchange because solution is the in the solution (Appendix 2). In contrast, in the brucite synthesis minor component of the system, and hence are used hereafter. experiments that used EDTA, speciation calculations suggest that Calculated F values suggest that about 49% and 36% of Mg − Mg-EDTA2 was the dominant Mg species, and the abundances of has undergone isotope exchange in Exp. Bex1 and Exp. Bex2,re- + + hydrated Mg2 and MgOH were two orders of magnitude lower spectively, after 198 days (Table 2). In cases of partial isotope − (Appendix 2). The dominance of Mg-EDTA2 reflects the strong exchange in a two-component system, the isotope fractionation chelating effects of EDTA for divalent cations, and the high pH factor at 100% exchange can be obtained by extrapolation using (>10) of the solution, as buffered by brucite. the “three-isotope method” (Beard et al., 2010; Macris et al., 2013; Brucite has a point of zero surface charge (PZC) at pH of Matsuhisa et al., 1978; Matthews et al., 1983a, 1983b; Shahar et 10.8–11.0, and develops a positive surface charge at pH < 10.8 al., 2008). The extrapolation can be done on a δ–F diagram, where and a negative surface charge at pH > 11.0(Pokrovsky and Schott, F values of the experiment are plotted along x-axis and the ra- 2004; Schott, 1981). In the recrystallization experiments, as well tios of non-spiked isotopes for the components are plotted along as the brucite synthesis experiments that used weak HCl, solution y-axis, and extrapolation is projected to 100% isotope exchange (Li pH was below 10 (e.g., Table 2), and there should have been no et al., 2011; Wu et al., 2011, 2012). The method of extrapolation in 2+ adsorption of [Mg(OH2)6] to the positively-charged brucite sur- a δ–F diagram is applied to Exp. Bex1 and Exp. Bex2 and plotted in face. Solution pH for the experiments that used MgO and Na4EDTA Fig. 5. Combining the three-isotope method (Fig. 5) and Eq. (3),the ◦ 26 + was 12.9, which should have prevented adsorption of negatively- Mgbrucite-Mg2 fractionation at complete exchange at 22 Cis W. Li et al. / Earth and Planetary Science Letters 394 (2014) 82–93 89

Fig. 6. Summary of apparent Mg isotope fractionation factors between brucite and Mg aquo ions obtained from synthesis and recrystallization experiments. Error bar denotes 2SD uncertainty.

light isotopes in the product during ion transport, dehydration, and attachment (DePaolo, 2011). In the brucite synthesis experiments + in this study, however, both aqueous Mg2 andbrucitewereprod- ucts of reactions involving solid MgO, and the amount of Mg in the two components were about equal in magnitude (Table 1). It

26 is possible, therefore, that kinetic isotope effects during the reac- Fig. 5. δ–F plot for S Mg values of brucite and aqueous solutions as a function 2+ of degree of isotope exchange (F) in the recrystallization experiment Bex1 (plot A), tions were to some extent canceled between aqueous Mg and and Bex2 (plot B). For calculation of F, see Section 4.2. Error bar denotes 2SD un- brucite. Furthermore, AFM studies suggest that brucite forms at certainty from multiple analyses. The extrapolation lines were forced through the the surface of MgO during hydrolysis, where the brucite is proba- points of the starting materials, the uncertainties (2SE) for the starting materials bly poorly ordered (Jordan et al., 1999). The poorly-ordered nature have been accounted in regression functions; the regression function was obtained of the newly formed brucite during MgO hydrolysis is supported using Origin®, the error in regression functions denotes 95% confidence in slope. by the wide XRD peaks for brucite at the initial stages of MgO + hydrolysis (Fig. 4B), and the fact that transient high Mg2 con- calculated to be −0.21 ± 0.23h for Exp. Bex1 and −0.38 ± 0.34h tents that reflect brucite supersaturation commonly occur during for Exp. Bex2. MgO hydrolysis (Rocha et al., 2004).Thehighfreeenergyinthe Kinetic effects may play a role in Mg isotope fractionation dur- + newly formed brucite by MgO hydrolysis would be expected to ing precipitation of Mg2 from solution, and hence even results drive newly formed brucite toward ordered structures through re- extrapolated to 100% exchange may not record equilibrium isotopic crystallization, during which isotopic exchange may occur between fractionations. For example, a positive relation between precipita- 26 brucite and aqueous Mg. It should be noted that there is ample tion rates of calcite and apparent Mg 2+ fractionation calcite-Mg chance for the aqueous Mg to exchange with the newly formed has been reported (Immenhauser et al., 2010; Mavromatis et al., brucite during MgO hydrolysis, as the estimated rate of MgO reac- 2013), where increasing precipitation rates produced less nega- 26 tion in weak acids is up to two orders of magnitude lower than tive Mg 2+ fractionations. Mavromatis et al. (2013) sug- calcite-Mg the rate of external mass transfer (Raschman and Fedorocková,ˇ gested that Mg isotope equilibrium can be reached when calcite −8.5 2 2008). Based on these considerations, the brucite synthesis ap- growth rate is below 10 mol/m /s, where the implication is 26 proach in this study may have produced Mg 2+ frac- that for crystallization rates that are sufficiently slow, the kinetics brucite-Mg tionations that are close to equilibrium. This conclusion is further of Mg dehydration will not impart kinetic isotope effects. supported by an O–H isotope study of brucite, which demon- In the brucite recrystallization experiments in this study, Mg strate that O and H isotope equilibrium between brucite and water isotope exchange was promoted by growth of new brucite crys- can be achieved by the MgO hydrolysis technique (Xu and Zheng, tals at the expense of brucite dissolution (i.e., Ostwald ripening). 1999). Based on the δ25Mg values that constrain the extent of exchange, 2 Combining the recrystallization and synthesis experiments at brucite masses, and a surface area of 2.5m/gfortherecrys- ◦ 26 22 C, the Mg 2+ fractionations cluster between −0.1 tallized brucite, the integrated/average brucite precipitation rates brucite-Mg − − 26 were 10 9.7 mol/m2/s for Exp. Bex1 and 10 9.9 mol/m2/s for Exp. and −0.3h (Fig. 6), and the weighted average for Mg frac- 2+ ◦ − ± Bex2. These rates are over one order of magnitude lower than the tionation between brucite and [Mg(OH2)6] at 22 Cis 0.17 limit suggested by Mavromatis et al. (2013) to record equilibrium 0.07h (95% confidence, MSWD = 1.0, n = 7), as calculated us- 26 + fractionations for Mg calcite. Assuming kinetic isotope effects lie ing Isoplot (Ludwig, 1999). The weighted average Mgbrucite-Mg2 primarily in the Mg dehydration step, we infer that the very slow fractionation is −0.27 ± 0.20h (95% confidence, MSWD = 1.7, ◦ precipitation rates for brucite in the exchange experiments may n = 4) for experiments at 7 C, and −0.03 ± 0.09h (95% confi- ◦ reflect equilibrium effects, although it is difficult to prove equilib- dence, MSWD = 1.1, n = 4) for experiments at 40 C, respectively. rium. Our experiments indicate that brucite mildly enriches light Mg + ◦ 26 + 2 The brucite synthesis experiments produced Mgbrucite-Mg2 isotopes relative to aqueous Mg at temperatures below 40 C. fractionation factors that are consistent with the recrystallization These results contrast with the experimental results by Wimpenny experiments, which in turn provide an assessment of the likeli- et al. (2014), who reported that heavy Mg isotopes preferentially + hood of equilibrium attainment. Because mineral synthesis gen- partition into brucite relative to aqueous Mg2 by +0.5h at ◦ erally involves unidirectional processes during precipitation from 80 C. In contrast to the experimental approaches of our study, solution, kinetic effects may be present that tend to enrich the Wimpenny et al. (2014) conducted brucite synthesis experiments 90 W. Li et al. / Earth and Planetary Science Letters 394 (2014) 82–93

+ by partial precipitation of Mg2 using NaOH, followed by aging Our results provide insight into the role of Mg bonding and the ◦ at 80 CinaCO2-free environment to avoid formation of Mg car- expected fluid–mineral Mg isotope fractionations during weather- + bonate, which can produce large Mg2 -carbonate fractionations ing that are independent of effects due to sorption and surface 26 + (e.g., Li et al., 2012). The discrepancy in Mgbrucite-Mg2 be- reactions on clays. Brucite, a serpentine-group mineral, and clay- tween our study and that by Wimpenny et al. (2014) cannot be group minerals, all contain octahedral layers (Fig. 1), where each + explained by Mg-carbonate formation in our experiments, because divalent cation such as Mg2 is surrounded by six oxygen atoms. ∼ 2+ such a scenario would require 35% of Mg to precipitate as Mg- The difference between the three octahedral structures lies in the 26 + carbonate to move an apparent Mgbrucite-Mg2 fractionation of bonding environment for the apical O atoms. The apical O atoms +0.5h as observed by Wimpenny et al. (2014) to the apparent in both sides of the octahedral layer of brucite are bonded with 26 + − h Mgbrucite-Mg2 fractionation of 0.2 that we observe, giving hydroxyl hydrogen atoms. In 2:1 clay group minerals, the apical 26 − h a Mgcarb-aq fractionation of 1.5 during Mg-carbonate for- O atoms in both sides of the octahedral layer are shared with mation (Mavromatis et al., 2012). XRD analyses of the brucite in tetrahedral layers (Fig. 1). For serpentine-group minerals, the api- our experiments do not reveal any detectable level of carbonate cal O atoms on one side of the octahedral layer are shared with in our experiments, confirmed by tests using artificial mixtures of a tetrahedral layer, but O atoms on the other side are not (Fig. 1). brucite and Mg-carbonates (Figs. 4B, 4C). Interactions between the octahedral layer and tetrahedral layers in 26 It is possible that Mg 2+ fractionation reverses from brucite-Mg ◦ ◦ phyllosilicates result in distortion of the Mg–O octahedrons. positive to negative as temperature decreases from 80 Cto40 C. The systematic changes in bonding environment for Mg be- Although relative rare, isotopic fractionation reversals with tem- tween brucite, serpentine-group, and clay-group minerals is re- perature has been reported for O, H, and C isotopes (Chacko et flected in the average Mg–O distance in the octahedrons, which are al., 2001). Another possible cause of the discrepancy between our 2.100–2.093 Å for brucite (Catti et al., 1995; Chakoumakos et al., results and those of Wimpenny et al. (2014) lies in effectively 1997), 2.085 Å for chrysotile, a serpentine-group Mg-rich mineral quenching the experiments at high temperatures. We note that in ◦ (Wittaker, 1956), 2.071 Å for talc (Perdikatsis and Burzlaff, 1981), our work, experiments were attempted at 55 C (Appendix 3), but 26 and 2.077–2.080 Å for hectorite, a Mg-rich clay mineral (Breu et the measured Mg 2+ fractionations varied wildly from brucite-Mg al., 2003; Kalo et al., 2012). For comparison, the average Mg–O −0.1h to ca. +1.0h (Appendix 3), and recoveries were incon- 2+ distance in the octahedron of [Mg(OH2)6] is 2.08 Å (Pavlov et sistent. We speculate that kinetic Mg isotope fractionation may al., 1998). The average Mg–O distance in octahedrons in brucite have occurred to various extents through brucite dissolution and/or 2+ and chrysotile are longer than the Mg–O distance in [Mg(OH2)6] , precipitation during the large temperature drops that invariably 2+ which correlates to a negative mineral-[Mg(OH2)6] fractionation occurred during brucite separation of the high-temperature exper- ◦ factor for 26Mg/24Mg at temperatures below 40 C (brucite, this iments. Given the uncertainties for our experiments at elevated ◦ study; chrysotile, Wimpenny et al., 2010). In contrast, the average temperature (i.e., 55 C), the discussions below concentrate on 26 Mg–O distance in octahedrons in clay minerals is slightly shorter Mg 2+ fractionations at supergene temperatures (i.e., ◦ brucite-Mg 2+ 40 C), which are more pertinent to weathering and biogeochem- than the Mg–O distance in [Mg(OH2)6] , and clay minerals are 2+ ical processes, and for which sample handling issues were mini- generally thought to have a positive mineral-[Mg(OH2)6] frac- 26 24 mal. tionation factor for Mg/ Mg at surface temperatures (Brenot et al., 2008; Opfergelt et al., 2012; Shen et al., 2009; Teng et al., 2010; 4.3. Implications for Mg isotope fractionation between secondary Tipper et al., 2010, 2006). Such correlation is also found in salts, minerals formed during weathering where the average Mg–O distance in octahedrons in epsomite is 2+ 2.065 Å (Baur, 1964), shorter than that of aqueous [Mg(OH2)6] , Significant Mg isotope fractionation during weathering of sili- and Mg in epsomite is isotopically heavier than MgSO4 solution by h 26 24 cate rocks has been reported in a number of field studies. In gen- 0.6 in Mg/ Mg ratios at room temperature (Li et al., 2011). In eral, the solid residues of silicate weathering have higher δ26Mg summary, these correlations are consistent with the general rule valuesthanthesourcerocks(Brenot et al., 2008; Opfergelt et al., of isotopic fractionation where heavy isotopes favor shorter, stiffer 2012; Shen et al., 2009; Teng et al., 2010; Tipper et al., 2010, 2006), bonds given the same coordination (e.g., Schauble, 2004). except that some weathering products of basalts that contain allo- The importance of Mg coordination on isotope fractionation phane and kaolinite have low δ26Mg values (Huang et al., 2012; is well demonstrated by comparison of mineral–fluid fractiona- Pogge von Strandmann et al., 2008a). Preferential partitioning of tions that are found in minerals where Mg is octahedrally coor- heavy Mg isotopes into Mg-bearing secondary minerals (e.g., smec- dinated by O, such as carbonates where cations are coordinated − tite) during weathering is supported by the fact that rivers that 2 by CO3 groups. As discussed above, for minerals with Mg–O drain silicate rock terrains have δ26Mg values that are lower than octahedral structures, mineral–fluid fractionation factors are rel- 26 those of the bedrock (Brenot et al., 2008; Tipper et al., 2006). The atively small (e.g., Mg 2+ =+0.6h, Li et al., 2011; 26 24 epsomite-Mg apparent Mg/ Mg isotope fractionation between bulk saprolite 26 + =− h Mgbrucite-Mg2 0.2 , this study) as compared to carbonate and fluid during weathering of a diabase dike has been calcu- 26 minerals (e.g., Mg 2+ =−2.5to−3.5h, Li et al., 2012; lated to be 0.05 to 0.4h (Teng et al., 2010). The crystallographic calcite-Mg control of Mg isotope fractionation between specific Mg-bearing Mavromatis et al., 2013). These observations suggest that although secondary minerals and aqueous solution during weathering of bond length is important, as discussed above, large contrasts in silicate rocks, however, remains poorly understood. In addition coordination and crystal structure may impart an even larger to bulk mineral–fluid isotopic fractionations, the focus of our effect on Mg isotope fractionations. Such a conclusion is sup- study, it has been increasingly realized that adsorption–desorption ported by significant high-temperature Mg isotope fractionations processes and mineral–surface exchange (Bolou-Bi et al., 2012; between minerals that have different coordination numbers for Huang et al., 2012; Opfergelt et al., 2012, 2014; Pogge von Strand- Mg. For example, theoretical, experimental, and field studies con- 26 h mann et al., 2012; Tipper et al., 2010, 2012; Wimpenny et al., firm a Mgspinel-olivine fractionation of 0.25–1.29 between 2014), as well as preferential dissolution of isotopically distinct spinel, where Mg is tetrahedrally coordinated, and olivine, where ◦ phase/mineral (Ryu et al., 2011), may play an important role in Mg is octahedrally coordinate, at temperatures at 600–1150 C fractionating Mg isotopes during silicate weathering. (e.g., Liu et al., 2011; Macris et al., 2013; Young et al., 2009). W. Li et al. / Earth and Planetary Science Letters 394 (2014) 82–93 91

4.4. Mg isotope fractionation between brucite and Mg-EDTA and implications for biogeochemical processes

Our results on Mg isotope fractionation in EDTA-bearing systems provide insight into the possible roles of organic compounds in producing Mg isotope fractionations that are distinct from equilibrium conditions in inorganic systems, or so-called “vital” ef- 26 fects. In this study, the measured Mgbrucite-Mg-EDTA fractiona- ◦ tion at 22 Cwasca.+1.5h in the experiment of MgO hydrolysis in Na4EDTA solution, and ca. +2.0h in the experiment of MgOhydrolysisinNaOHsolutionwithEDTAacidcrystals(Ta- 26 ble 3). The difference in Mgbrucite-Mg-EDTA fractionation likely reflects the difference in availability of free EDTA chelator during MgO hydrolysis between the two experiments. In the exper- 4− iment that used Na4EDTA solution, free EDTA concentrations − were high initially, then decreased as chelation between EDTA4 + and Mg2 occurred during MgO hydrolysis. In the experiment that Fig. 7. Cartoon that shows the conceptual model of organically bonded Mg affecting − used EDTA acid crystals, the free EDTA4 concentration was low the Mg isotope fractionation during plant uptake. initially, due to the low solubility of EDTA acid in pure water (<1.5μM; Battaglia et al., 2008), and dissolution of EDTA acid 5. Conclusions and outlook crystals was driven by reaction of dissolved EDTA acid with NaOH + and Mg2 that were gradually released during MgO hydrolysis. In this study, Mg isotope fractionations between brucite, aque- + The differences in reaction mechanisms and measured apparent ous Mg2 , and EDTA-chelated Mg were experimentally deter- 26 Mgbrucite-Mg-EDTA fractionations between the two experiments mined. All Mg species in this study contain octahedrally-coordi- suggest the influence of kinetic effects in at least one of the ex- nated Mg, and the fractionation factors between them appear to 26 periments, and the Mgbrucite-Mg-EDTA fractionation in the slower correlate with average bond length for Mg, where heavy Mg iso- experiment that used EDTA acid crystals is probably closer to the topes preferentially partition into species that have shorter average equilibrium fractionation. We therefore infer that the equilibrium bond lengths for Mg. Recrystallization and MgO hydrolysis experi- 26 ◦ Mg fractionation is likely +2 0h at 22 C. Such 26 + brucite-Mg-EDTA . ments gave mildly negative Mgbrucite-Mg2 fractionation factors ◦ a fractionation factor also correlates with bond-length relations, at low temperatures (i.e., ca. −0.3h at 7 C, and ca. −0.2h at ◦ because Mg in Mg-EDTA complex is also octahedrally coordinated, 22 C), correlating with a slightly longer Mg–O bond in brucite and the average Mg–O/N distance is 2.17 Å (Pozhidaev et al., 1974), 2+ (2.100–2.093 Å) than that in Mg(H2O)6 (2.08 Å) at low tem- longer than the Mg–O distance for octahedron in brucite. peratures. MgO hydrolysis experiments in EDTA solutions suggest 26 + 26 Based on the Mgbrucite-Mg2 and Mgbrucite-Mg-EDTA frac- that heavy Mg isotopes strongly partition into brucite relative to 2+ 26 h ◦ tionations, the [Mg(OH2)6] -Mg-EDTA Mg isotope fractionation is EDTA, with Mgbrucite-Mg-EDTA > 2 at 22 C, which correlates constrained to be +2.2h. The direction of this isotopic fraction- to a significantly longer average Mg–O/N bond length (2.17 Å) for ation is consistent with the study of Black et al. (2006), who found the Mg-EDTA complex than the Mg–O bond in brucite. This corre- that Mg in chlorophyll-a (Chl) is isotopically lighter than the cul- lation is consistent with preferential partitioning of light isotopes 26 24 ture medium by 0.7–0.5h in Mg/ Mg ratios, as well as the into chrysotile (Wimpenny et al., 2010), which has a longer average study of Bolou-Bi et al. (2010), who reported that Mg in plant Mg–O bond length, and preferential partitioning of heavy isotopes root cells is isotopically lighter than the Mg adsorbed onto the into epsomite (Li et al., 2011), which has a shorter Mg–O bond + root surface. In both studies, such Mg isotope fractionation has length, relative to aqueous Mg2 . been tentatively attributed to chelation of Mg by organic matter. The correlation between Mg–O bond length and Mg isotope It should be noted that Black et al. (2007) suggested that the fractionation between Mg-centered octahedron bearing species is 2+ chlorophyll-[Mg(OH2)6] fractionation should be positive based consistent with the general rule of isotopic fractionation, and pro- on quantum mechanical calculations, although the accuracy of the vides insights into Mg isotope fractionation during silicate weath- prediction was later questioned by the same group (Rustad et ering and phyllosilicate formation. In addition, results of EDTA- al., 2010). Additional experimental calibration and modeling are experiments suggest that organically bound Mg tends to be iso- needed to resolve the discrepancies. topically light. This in turn provides a possible explanation for Experimental studies to date suggest that there is a significant, some of the relatively large Mg isotope fractionations that are negative Mg isotope fractionation between organically-bound Mg 2+ observed in plants and calcifying marine organisms. Organic com- and [Mg(OH2)6] . For plant growth, uptake of Mg occurs via root 2+ plexation may therefore be responsible for some of the isotopic cell surface cation exchange for [Mg(OH2)6] (e.g., Haynes, 1980). + “vital” effects that have been observed for Mg. In an organic-rich soil environment, if a proportion of Mg2 in pore water is chelated by organic compounds (e.g., humic acid), and hence is isotopically light, isotope mass balance would shift Acknowledgements 2+ 26 24 [Mg(OH2)6] to higher Mg/ Mgratios.Thisinturnwould 2+ cause preferential uptake of the isotopically heavier, [Mg(OH2)6] Fangfu Zhang assisted in BET analysis, Zhizhang Shen assisted by plants (Fig. 7), given the fact that organically bound Mg is in XRD analysis and mineral lattice data interpretation. Jim Kern neutral or negatively charged and hence unavailable for cation ex- and Reinhard Kozdon assisted in gold coating of brucite samples. change and plant uptake. Such relations are consistent with the Prof. Huifang Xu provided a natural magnesite sample and access field observation that vegetation has higher δ26Mg values than to water bath. This paper benefited from constructive comments those of pore fluids in the root zones (Opfergeltetal.,2014; from J. Wimpenny, P. Pogge von Strandmann, and two anonymous Tipper et al., 2010), as well as laboratory experiments that show reviewers, as well as editorial comments by G. Henderson. This an enrichment in 26Mg/24Mg ratio by 0.3–1.1h in plants relative study was supported by grants from the NASA Astrobiology Insti- tothenutrientsource(Black et al., 2008; Bolou-Bi et al., 2010). tute and the Department of Energy. 92 W. Li et al. / Earth and Planetary Science Letters 394 (2014) 82–93

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