# Using stable Mg isotope signatures to assess the fate of magnesium during the in situ mineralisation of CO2 and H2S at the CarbFix site in SW-Iceland

Eric Oelkers, Rhiannon Butcher, Philip A.E. Pogge von Strandmann, Jan Schuessler, Friedhelm von Blanckenburg, Sandra Ó. Snæbjörnsdóttir, Kiflom Mesfin, Edda Sif Aradóttir, Ingvi Gunnarsson, Bergur Sigfússon, et al.

<figure><img src="image_1.png" /></figure>

## To cite this version:

Eric Oelkers, Rhiannon Butcher, Philip A.E. Pogge von Strandmann, Jan Schuessler, Friedhelm von Blanckenburg, et al. Using stable Mg isotope signatures to assess the fate of magnesium during the in situ mineralisation of CO2 and H2S at the CarbFix site in SW-Iceland. Geochimica et Cosmochimica Acta, 2019, 245, pp.542-555. 10.1016/j.gca.2018.11.011. hal-04892008

HAL Id: hal-04892008

https://ut3-toulouseinp.hal.science/hal-04892008v1

Submitted on 17 Jan 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Using stable Mg isotope signatures to assess the fate of magnesium during the *in situ* mineralisation of CO₂ and H₂S at the CarbFix site in SW-Iceland

Eric H. Oelkers¹,²,³, Rhiannon Butcher³, Philip A.E. Pogge von Strandmann³, Jan A. Schuessler⁴, Friedhelm von Blankenburg⁴, Sandra Ó. Snæbjörnsdóttir¹,⁵, Kiflom Mesfin¹, Edda Sif Aradóttir⁵, Ingvi Gunnarsson⁵, Bergur Sigfússon⁵, Einar Gunnlaugsson⁵, Juerg M. Matter⁶,⁷, Martin Stute⁷, Sigurdur R. Gislason¹

¹Institute of Earth Science, University of Iceland, Iceland

²CNRS/UMR 5563, Université Paul Sabatier, France

³London Geochemistry and Isotope Centre (LOGIC), University College London and Birkbeck, University of London, Gower Place, London, UK.

⁴GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany

⁵Reykjavik Energy, Iceland

⁶Ocean and Earth Science, University of Southampton, UK

⁷Lamont-Doherty Earth Observatory, Columbia University, USA

**Abstract** - The *in-situ* carbonation of basaltic rocks could provide a long-term carbon storage solution. To investigate the viability of this carbon storage solution, 175 tonnes of pure CO₂ and 73 tonnes of a 75% CO₂-24% H₂S-1% H₂-gas mixture were sequentially injected into basaltic rocks as a dissolved aqueous fluid at the CarbFix site at Hellisheidi, SW-Iceland. This paper reports the Mg stable isotope compositions of sub-surface fluids sampled prior to, during, and after the CO₂ injections. These Mg isotopic compositions are used to trace the fate of this element during the subsurface carbonation of basalts. The measured Mg isotopic compositions of the monitoring well fluids are isotopically lighter than the dissolving basalts and continue to become increasingly lighter for at least two years after the gas-charged water injection was stopped. The results indicate that the formation of isotopically heavy Mg-clays rather than Mg-carbonates are the predominant Mg secondary phases precipitating from the sampled fluids. Isotope mass balance calculations suggest that more than 70% of the Mg
38 liberated from the basalt by the injected gas charged water was
39 precipitated as Mg-clays, with this percentage increasing with time
40 after the injection, consistent with the continued precipitation of Mg
41 clays over the whole of the study period. The formation of Mg clays
42 in response to the injection of CO2 into basalts, as indicated in this
43 study, could be detrimental to carbon storage efforts because the
44 formation of these minerals consume divalent Mg that could
45 otherwise be used for the formation of carbonate minerals and
46 because such clays could decrease host rock permeability.
47

# 1. Introduction

49 The CarbFix project was created to develop the technology to store carbon dioxide as
50 stable carbonate minerals directly in the subsurface by reacting gas charged injection
51 waters with basaltic rocks (Gislason et al., 2010, 2018; Arádóttir et al., 2011, 2018).
52 Carbon storage in basaltic rocks offers numerous advantages including their ability to
53 promote mineral carbonation and their large potential storage volume (McGrail et al.,
54 2006; Goldberg and Slagle, 2009; Goldberg et al., 2010; Gislason and Oelkers, 2014;
55 Snæbjörnsdóttir et al., 2014). As such, numerous studies have focused on developing the
56 technology to safely store CO2 in basaltic rocks including laboratory (Assayag et al.,
57 2009; Flaathen et al., 2009; Gudbrandsson et al., 2011; Stockmann et al., 2011; Gysi and
58 Stefánsson, 2012; Rosenbauer et al., 2012; Galeczka et al., 2014; Mareni et al., 2018),
59 modelling (McGrail et al., 2006; Van Pham et al., 2012; Goldberg et al., 2009, 2013;
60 Rosenbauer et al., 2012; Bacon et al., 2014) and field efforts (Rogers et al., 2006; Matter
61 et al., 2007; McGrail et al., 2011, 2012; Siggfusson et al., 2015). Basaltic rocks are rich in
62 divalent cations such as Ca²⁺, Mg²⁺, and Fe²⁺. The injection of acidic CO2-charged water
63 promotes the release of these metals, potentially leading to the formation of carbonate
64 minerals such as calcite, magnesite, and siderite as the continued dissolution of basalt
65 increases the pH of the aqueous fluid (Oelkers et al., 2008; Gislason et al., 2010, 2014;
66 Stefánsson et al., 2011; Gislason and Oelkers, 2014; Olsson et al., 2014). About 5% of
67 the continents and most of the oceanic floor are comprised of basaltic rocks, including the
68 mid-oceanic ridges. As such the largest basaltic storage potential lies offshore;
theoretically all CO₂ from the burning of fossil fuel carbon (estimated to be ~5000 Gt; Archer, 2005) could be stored by mineral carbonation along the mid-ocean ridges (Snæbjörnsdóttir et al., 2014). The flanks of these ridges contain highly fractured and permeable basaltic layers (Fisher, 1998) with a pervasive circulation of about 1,000 Gt seawater/yr (Harris and Chapman, 2004). The potential for using these marine systems for carbon storage is confirmed by the results of Wolff-Boenisch et al. (2011), who demonstrated the rapid dissolution of basaltic rocks in CO₂ charged seawater.

The efficiency of mineral carbonation, however, can be limited if Mg clay minerals rather than Mg carbonate minerals form in response to the injection of CO₂ into basalts. Magnesium clay formation is detrimental to carbon storage efforts because these minerals consume divalent Mg that could otherwise be used for the carbonate mineral formation, and because Mg-bearing clays could decrease host rock permeability. The fate of Mg during mineral carbonation efforts in basalts will be assessed in this study through the application of Mg isotopes. The stable isotopes of magnesium (²⁴Mg, ²⁵Mg and ²⁶Mg) are increasingly being used as tracers of geochemical processes, including their use to assess mineral reactions during weathering (Brenot et al., 2008; Huang et al., 2012; Liu et al., 2014; Pogge von Strandmann et al., 2012; Tipper et al., 2006, 2008, 2012a, 2012b; Wimpenny et al., 2011). Like all major elements, magnesium and its isotopes are affected by a number of processes. For example, the magnitude and sign of Mg isotope fractionation during precipitation appears to be dependent on the type of mineral forming (Geske et al., 2012; 2015; Immenhauser et al., 2010; Li et al., 2012, 2015; Saulnier et al., 2012; Wombacher et al., 2011), organic vs. inorganic precipitation (Chang et al., 2004; Pogge von Strandmann, 2008; Saenger and Wang, 2014), precipitation rate (Mavromatis et al., 2013), fractionation mechanism (Buhl et al., 2007), and aqueous speciation (Li et al., 2014; Schott et al., 2016). In addition, isotopic fractionation can also occur due to both the preferential incorporation and preferential adsorption of specific Mg isotopes (Huang et al., 2012; Liu et al., 2014; Opfergelt et al., 2011; Pogge von Strandmann et al., 2008, 2012; Tipper et al., 2012a; Wimpenny et al., 2014).
100 2010, 2014). Finally, the uptake of Mg by plants and microbes causes variable isotope
101 fractionation (Black et al., 2006; Bolou-Bi et al., 2010, 2012; Oelkers et al., 2015;
102 Pokharel et al., 2017; Uhlig et al., 2017).
103

104 In this study the Mg isotope systematics of subsurface fluids sampled prior to, during,
105 and after the injection of CO2-charged waters into subsurface basalts was measured to
106 provide insight into the mineralisation reactions that occurred in response to this
107 injection. This approach takes advantage of the distinct Mg isotope fractionation
108 signatures of primary and secondary phases. The Mg isotope ratio of primary igneous
109 silicate rocks is virtually uniform (Teng et al., 2010a; Pogge von Strandmann et al., 2011)
110 and it differs markedly from that in carbonates (Chang et al., 2004; Pogge von
111 Strandmann, 2008; Wombacher et al., 2011; Li et al., 2012, 2014; Saenger and Wang,
112 2014, Teng 2017). As such, the Mg isotope composition of surface waters is often
113 controlled by the balance of dissolution of silicate to carbonate rocks in the host
114 catchment and/or the relative precipitation rates of Mg-clays compared to Mg-bearing
115 carbonates (Tipper et al., 2006a, 2006b, 2008; Pogge von Strandmann et al., 2008, 2014).
116 It is thus anticipated that a similar approach will provide insight into the identity of Mg
117 minerals formed during subsurface processes occurring at the CarbFix site.
118
119

## 2. Background of the CarbFix Project

120 This study focuses on the fate of Mg liberated from dissolving basalts in response to
121 the injection of CO2-charged water at the CarbFix injection site. A number of past
122 publications have reported the details of this injection, including the temporal evolution
123 of dissolved element compositions and chemical tracers following these injections. A
124 description of the injection method was presented by Sigfússon et al. (2015). The
125 temporal evolution of chemical tracers, dissolved carbon and pH in the first monitoring
126 well downstream from the injection well was reported by Matter et al. (2016), as well as
127 an estimate of the fraction of injected carbon fixed by carbonation reactions. This study
128 concluded that more than 95% of the dissolved carbon injected into the subsurface was
129 fixed as stable carbonate minerals within 2 years. The concentrations of dissolved major
130
elements during and after injection in the first monitoring well, as well as the saturation indices of potential secondary minerals were described by Snæbjörnsdóttir et al. (2017). Reaction path modelling of the carbon dioxide charged injection fluids, as they reacted with the subsurface basalts were reported by Snæbjörnsdóttir et al. (2018).

135

The CarbFix injection site is equipped with a 2000 m deep injection well and 8 monitoring wells ranging in depth from 50 to 1300 m in depth. The subsurface rocks at the injection site are primarily olivine tholeiite basalts consisting of lava flows and hyaloclastite formations (see Fig. 1). The hyaloclastites are relatively low permeability glassy rocks formed under ice and melt water during glaciations; the boundaries between hyaloclastites and lava flows, and those between individual lava flows boundaries are preferential fluid flow pathways (Alfredsson et al., 2013). Some alteration is observed in the hyaloclastite rocks starting at 120 to 300 m depth. The common alteration minerals at this depth are smectite, calcite, Ca-rich zeolites, and poorly crystalline iron-hydroxides (Alfredsson et al., 2013). Fluid injection was targeted at a lava flow sequence located 400–800 m below the surface with the main aquifer located at ~530 m depth. Loss on ignition measurements on rock samples suggest that over 80% of the primary rocks in the target zone are currently unaltered. Tracer tests were conducted both under natural and forced flow conditions from 2008 to 2011 to define the system hydrology (Aradóttir et al., 2012; Gislason et al., 2010; Khalilabad et al., 2008). These tests indicated that the flow from the HN-02 injection well to the first monitoring well (HN-04) consists of relatively homogenous porous media intersected by a low volume and fast flow path that channels about 3% of the tracer flow (Khalilabad et al., 2008).

154

The water in the target zone prior to the injection ranged in temperature from 15 to 35 °C and had *in situ* pH ranging from 8.4 to 9.8. The concentrations of dissolved CO₂ and O₂ collected from the injection well from the target reservoir were 1.27 x10⁻³ and 1.1x10⁻⁷ mol/kg, respectively, prior to the injection (Alfredsson et al., 2013). These values are substantially undersaturated with respect to the composition of these gases in the atmosphere, suggesting that the target injection reservoir was isolated from the surface.
The concentration of Ca and Mg in these waters prior to the injection was limited by secondary mineral precipitation (Alfredsson et al., 2013). All the water samples collected from the target reservoir prior to the injection of CO₂ charged water were supersaturated with respect to Ca-zeolite, analcime, Ca–Mg–Fe smectite, calcite, and aragonite, and some are supersaturated with respect to dolomite and Fe–Mg carbonates (Snæbjörnsdóttir et al., 2017).

Approximately 175 tons of pure commercial CO₂ and 73 tons of a 75%-24%-0.8% mixture of CO₂–H₂S–H₂ gases were dissolved into water during their injection from January until August 2012. This latter gas mixture was captured directly from the adjoining Hellisheidi power plant by its dissolution into water at elevated pressure (Gislason et al., 2018; Sigfusson et al., 2018). The injected water had a temperature of 25 °C and was equilibrated with 26 to 28 bars pressure of the CO₂ gas or 14 bars pressure of the CO₂–H₂S–H₂ mixture. The injected fluid equilibrated with pure carbon dioxide had dissolved CO₂ concentrations of 0.8 mol/kg H₂O and a pH of 3.85, whereas the fluid equilibrated with the gas mixture had a dissolved CO₂ concentration of 0.43 mol/kg H₂O and a pH of 4.0 (Snæbjörnsdóttir et al., 2017). All host rock minerals and glass were strongly undersaturated with respect to the gas charged injection waters (Snæbjörnsdóttir et al., 2018). The interaction of these acidic fluids with the host basalts creates porosity near the injection well by dissolving primary and secondary minerals. As gas-charged water continues to dissolve the basaltic host rock, this fluid becomes increasingly basic and secondary minerals will precipitate, potentially clogging the system. Reaction path calculations reported by Snæbjörnsdóttir et al. (2018) suggested that Mg–Fe–carbonates and siderite became supersaturated in the subsurface fluids at pH <5, whereas Ca-Mg-Fe-carbonates and calcite were saturated or supersaturated at higher pH. Mg bearing clays and Ca, Na-rich zeolites also became saturated in the subsurface fluids at pH >7.

## 3. Sampling and Analytical Methods
A detailed overview of fluid injection and sampling before, during and after the CarbFix injections have been reported by Snæbjörnsdóttir et al. (2017). Sampling of the fluids from the HN-04 monitoring well and other wells surrounding the injection well began in 2008. Water samples for chemical analysis were collected several times prior to the injections, which began during January 2012 (Alfredsson et al., 2013). During the injections and until mid-September 2012 the HN-04 injection well was sampled twice weekly. Weekly sampling continued until mid-July 2013 with few exceptions. Water was pumped from this monitoring well at the rate of 3.5 m³/h throughout this study maintaining a constant head from the injection to the monitoring well.

Fluid samples were collected via a 10 m long, 10 mm diameter stainless steel pipe connected to the 53 mm diameter monitoring well lining pipe extending down to the pump (Alfredsson et al., 2016). The sample pipe was connected directly to a sampling valve inside an on-site field laboratory. After flushing the sampling pipe, the sampled waters were immediately filtered through 0.2 µm Millipore cellulose acetate membranes using silicon tubing and a 140 mm Sartorius® filter holder. All air in the filtration system was expelled through a valve prior to sampling and at least 3 L of water was pumped through the system before the samples were collected in acid washed high density polyethylene bottles for cation and isotopic analysis. The sampling bottles were also washed with the monitoring well fluids three times before the final sampling. These samples were acidified using Suprapur® HNO₃, 1% (v/v) then stored prior to analysis. The fluid sample Si, Ca, Mg concentrations, alkalinity and pH data used in this study were previously reported by Snæbjörnsdóttir et al. (2017) and Alfredsson et al. (2013). Four pre-CO₂-injection fluid samples from different shallow wells and 19 post-injection monitoring well (HN-04) samples were selected for Mg isotope measurements (Table 1). Samples collected prior to the acid gas injections from the deep wells (> 400 m), including the HN-04 monitoring well, could not be analysed for their Mg isotopic ratios due to their low Mg concentrations (see Fig. 2). These concentrations did not exceed 6 µmol kg⁻¹ (Alfredsson et al., 2013; Snæbjörnsdóttir, et al., 2017).

220
**Mg isotope analyses** - Prior to stable Mg isotope analyses, the fluid samples were chemically purified by cation exchange chromatography in a clean laboratory equipped with filtered air laminar flow workstations at GET Toulouse following a protocol similar to that reported by Mavromatis et al. (2012, 2013), Pearce et al. (2012), and Shirokova et al. (2013). Briefly, fluid samples were evaporated to dryness and re-dissolved in 1M HNO₃ prior to loading onto 10 ml Bio-Rad poly-prop columns containing AG-50W-X12 resin (200–400 mesh) for separation of Mg from other elements. A total of 23 samples were processed in two batches of Mg column chemistry. With each batch of samples, the IAPSO seawater reference material and a procedure blank was processed for quality control. Mg recovery after chromatographic separation was >99.5%. After column chemistry, samples were evaporated, treated with H₂O₂/HNO₃ at 150°C to remove any remaining organics, and finally re-dissolved in 0.3M HNO₃.

Mg isotope measurements were performed at the HELGES lab, GFZ Potsdam, following protocols described by Uhlig et al. (2017) and Pokharel et al. (2017). Before Mg isotope ratio analysis, the purity of the Mg sample solutions and the Mg content in procedure blanks were checked by ICP-OES (Varian 720ES) and quadrupole ICP-MS (Thermo iCAP-Qc), respectively. Most of the samples showed a purity of higher than 99% Mg; for some samples that did not, we tested that the remaining impurities of Na and K would cause no analytical bias. Doping DSM-3 with Na and K and measuring the Mg isotopic composition showed that the impurities did not bias the Mg isotope measurements, i.e., the Na-K-doped DSM-3 (δ²⁶Mg = -0.05±0.10 ‰, 2SD) was identical to pure DSM-3 within analytical uncertainty, consistent with previous findings (Pokharel et al., 2017). Procedural blanks contributed less than 0.9% to the Mg processed through column chemistry (15 µg) and are therefore considered insignificant (potential bias in δ²⁶Mg is less than 0.04 ‰). All Mg isotope ratio measurements were performed in medium resolution mode on a Thermo Neptune multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). Samples and the DSM3 bracketing standard were diluted in 0.3 M HNO₃ to 500 ppb. The solutions were introduced to the MC-ICP-MS via a quartz-glass spray chamber (double pass cyclon-scott type, Thermo SIS)
equipped with a self-aspirating ca. 100 µL/min. The Mg isotope signals ($^{24}$Mg$^{+}$, $^{25}$Mg$^{+}$, $^{26}$Mg$^{+}$) were measured simultaneously on Faraday detectors. $^{26}$Mg was measured on the interference-free low mass side of the flat-top peak to avoid interference from $^{12}$C$^{14}$N$^{+}$. Sample signal intensities of ca. 12 V for $^{24}$Mg$^{+}$ were obtained. Background intensities (< 8 mV $^{24}$Mg$^{+}$) were measured on-peak in 0.3 M HNO$_3$ before and after each sample measurement block and were subtracted from sample signal intensities. Instrumental mass bias on measured Mg isotope ratios was corrected by the sample-standard bracketing method using concentration-matched DSM3 as a standard.

We report isotope ratios of samples as per mil deviation of the $^{26}$Mg/$^{24}$Mg and $^{25}$Mg/$^{24}$Mg ratios from the DSM3 international reference material using the delta notation, as $\delta^{26}$Mg and $\delta^{25}$Mg, respectively. All results are consistent with mass dependent isotope fractionation. Average $\delta$-values obtained from 2 to 6 replicate measurements of the same fluid are reported in Table 1 together with twice the standard deviation (2SD), which indicates the instrument repeatability. To assess total analysis uncertainty, IAPSO Atlantic seawater was also analysed with our samples and gave a mean $^{26}$Mg of -0.90±0.08 (n = 12), which is in agreement with literature reference values (-0.83 ±0.09 ‰, 2SD, Foster et al. 2010, Ling et al. 2011 and references therein). The pure Mg solution Cambridge-1 was measured 21 times during this study and gave a $^{26}$Mg of -2.62±0.10‰ (2SD), which is also in close agreement with literature values (-2.59 to 2.78‰ – see compilations by Pogge von Strandmann et al., 2011; An and Huang, 2014). These observations are consistent with previously established long-term uncertainty estimates of the MC-ICP-MS method at GFZ HELGES of ±0.06‰ (2SD) for $\delta^{25}$Mg and ±0.10‰ (2SD) for $\delta^{26}$Mg (Uhlig et al., 2017; Pokharel et al., 2017).

## 4. Results

The Mg isotope compositions of four pre-injection shallow well samples, collected during 2008 and 2010 from wells HK-12, HK-13, and HK-25, which are drilled to depths of 130, 210 and 310 m, respectively,
are shown in Fig. 3, and listed in Table 1; the composition of selected elements in these fluids are provided in Table 2. The measured $\delta^{26}$Mg of these samples ranged from -0.57 to -0.69 ‰, which is ~0.3‰ lower than basaltic rocks from this area and previously measured groundwaters in Iceland (Pogge von Strandmann et al., 2008), but within the range of precipitation-uncorrected Icelandic soil pore waters (Pogge von Strandmann et al., 2012). Note that the formation of Mg-bearing clays in these systems occurs only at depths greater that 120 to 300 m (Alfredsson et al., 2013), such that these shallow water samples are likely relatively unaffected by Mg clay precipitation compared to deeper wells, which is also consistent with the relatively high Mg concentrations of these fluids.

The fluid compositions of the HN-04 monitoring well fluids are provided in Tables 1 and 2. The first of these samples analysed for $\delta^{26}$Mg was collected on 9 February 2012, 17 days after the start of the initial CO$_2$-charged water injection. This sample had a Mg concentration of 36 µmol kg$^{-1}$ and a $\delta^{26}$Mg of -0.84 ‰, which is approximately 0.25 ‰ lower than that of the pre-injection shallow well samples. As the aqueous Mg concentrations of the monitoring well fluids increased with time up to 101 µmol kg$^{-1}$ on 26 March 2012 (Fig. 2b) the $\delta^{26}$Mg values of these fluids decreased to -0.94 ‰. These $\delta^{26}$Mg values continued to decrease with time to about -1.3 ‰ through June 2013 as the dissolved Mg concentration decreased, increased, and decreased again in response to the arrival to the monitoring well of fluids influenced by the injection of the acid gases (see Fig. 2, 3). The isotopically lightest $\delta^{26}$Mg value of -1.34 ‰ was measured in the final sample collected on 8 June 2014.

## 5. Discussion
The basaltic glass in the region of CarbFix injection site has a δ²⁶Mg value of -0.28‰ (Pogge von Strandmann et al., 2008; Wimpenny et al., 2010), which is identical to State University of New York (SUNY) Mid-Ocean Ridge Basalt (MORB; Teng et al., 2007), as well as the bulk silicate Earth (Hin et al., 2017). This glass is susceptible to dissolution in response to the injection of acidic fluids (Gislason et al., 1996; Oelkers, 2001; Oelkers and Gislason, 2001; Gislason and Oelkers, 2003) and has been identified as the major phase dissolving in response to the CarbFix gas charged water injections (Snæbjörnsdóttir et al., 2018). The rapid fluid-flow pathway, channeling ~3% of the injected gas charged fluid was attributed to a fracture network located in crystalline basalts containing mainly plagioclase, olivine, and pyroxene. Nevertheless olivines and crystalline basalts near the CarbFix site have an identical δ²⁶Mg composition to each other and the basaltic glass (Pogge von Strandmann et al., 2008, 2012). This is consistent with other studies suggesting basaltic olivines have a narrow isotopic range, with a similar Mg isotope composition to that of its coexisting basalt glass (Norman et al., 2006; Teng et al., 2007; Wimpenny et al., 2010; Liu et al., 2017). Although such results indicate that the preferential dissolution of distinct primary phases will not cause significant Mg isotope fractionation, it should be noted that both Wimpenny et al. (2010) and Maher et al. (2016) reported an initial preferential loss of light Mg during the dissolution of basaltic glass and olivine. Longer-term experiments reported by Oelkers et al. (2015) however, suggest that for the case of olivine, this preferential release of light Mg is limited to the initial stages of dissolution.

The low Mg concentrations in the target reservoir fluids prior to the gas injection indicate that essentially all the Mg dissolved in the HN-04 monitoring well samples collected in this study after January 2012 originates from the injected fluids or the dissolution of the host minerals in response to fluid-rock interaction. The low Mg concentrations of the target reservoir prior to injection have been attributed to the formation of Mg-bearing secondary minerals in the subsurface reservoirs, and in particular to Mg-bearing clay precipitation. Indeed, the smectite, Mg-saponite, is calculated to be
strongly supersaturated in these fluids prior to injection, whereas dolomite and magnesite are undersaturated (Snæbjörnsdóttir, et al., 2017). These calculated results are consistent with the observed secondary minerals in the target basalt formations (Alfredsson et al., 2013). They also explain the low δ²⁶Mg of the pre-injection shallow reservoir fluids compared to that of the host basalt, as Mg-bearing silicate minerals preferentially incorporate heavy Mg isotopes upon their formation (c.f. Young and Galy, 2004; Tipper et al., 2006; Teng et al., 2007, 2010; Pogge von Strandmann et al., 2008).

It seems likely, therefore, that the increasingly isotopically light composition of the post-injection well samples must either be caused by continued dissolution of an isotopically light and previously formed secondary phase, or the precipitation of an isotopically heavy secondary phase. Evidence suggests the calcite present in the host basalts dissolve as the acidic gas charged injection waters first interact with the subsurface basalts (Snæbjörnsdóttir et al., 2017). This calcite likely has a δ²⁶Mg of no more than -1.4‰ (Saenger and Wang 2014; Wombacher et al., 2011) such that the initial dissolution of calcite would tend to make the fluids lighter. Nevertheless, after a short initial time, saturation state and reactive path calculations indicate that calcite, as well as dolomite and mixed Mg-Ca-Fe carbonates, become supersaturated in the monitoring fluids collected from the HN-4 monitoring well (Snæbjörnsdóttir et al., 2017, 2018). The initially dissolved calcite tends to precipitate from this fluid as its pH increases in response to basalt dissolution. Due to the injection of carbon dioxide, more calcite precipitates in this system than it initially dissolved (Matter et al., 2016; Snæbjörnsdóttir et al., 2018). Such carbonate minerals likely tend to preferentially incorporate light Mg into their structure. For example, Mavromatis et al. (2014) found that apparent dolomite-pore fluid δ²⁶Mg fractionation factors were -2.6 ‰, a value similar to that reported by Higgins and Schrag (2010). Numerous other studies have observed the preferential incorporation of light Mg in Mg-bearing carbonates (Galy et al., 2002; Buhl et al., 2007; Hippler et al., 2009; Immenhauser et al., 2010; Wombacher et al., 2011; Li et al., 2012, 2015; Pearce et al., 2012; Mavromatis et al., 2012, 2013; Shirokova et al., 2013; Beinlich et al., 2014; Prikryl et al., 2018). Such observations suggests that carbonate precipitation as observed at the CarbFix site would drive the sampled monitoring well to heavier rather
than lighter compositions. It seems therefore that carbonate precipitation is not the dominant process controlling the Mg isotope composition of these monitoring well fluids.

The measured formation water composition requires, therefore, the incorporation of heavy Mg into secondary phases. The calculations reported by Snæbjörnsdóttir et al. (2017, 2018) suggest that Mg-smectites are supersaturated in most of the monitoring fluids collected from the HN-4 monitoring well; the saturation state of the monitoring fluids analyzed in this study with respect to Mg-clay are provided in Table 2. The Mg isotopic compositions of clay minerals tend to be heavier than their co-existent fluid phases (Young and Galy, 2004; Tipper et al., 2006; Teng et al., 2007, 2010; Pogge von Strandmann et al., 2008). Similarly, Wimpenny et al. (2010) concluded that the formation of secondary chrysotile, an Mg clay, lead to light Mg release from dissolving basaltic glass and olivine. Ryu et al. (2016) estimated a clay-fluid Mg isotope fractionation factor of 0.54 ‰, for T-O-T clays formed at temperatures from 90 to 250 °C. Moreover, Wimpenny et al. (2014) measured the brucite-fluid δ²⁶Mg fractionation factor to be 0.50‰ at 80 °C and near to neutral pH. This study argued that this system provided a good analogue to the incorporation of Mg into the octahedral sheets of Mg-rich clay minerals. Note also that ²⁶Mg preferentially adsorbs on the surfaces of kaolinite type minerals, including allophane (Huang et al., 2012; Pogge von Strandmann, 2012; Opfergelt et al., 2014). Thus, both precipitation and adsorption of exchangeable Mg onto clay minerals could lead to the observed pore fluid δ²⁶Mg decrease with time.

If, to a first approximation, it can be assumed that the Mg isotopic composition of the HN-04 monitoring fluids stem exclusively from the conservative dissolution of basalt<fnref n="1" /> having an isotopic composition of δ²⁶Mgbasalt = -0.28‰ (Pogge von Strandmann et al., 2008), coupled to the precipitation of a Mg-clay having a Δ²⁶Mgclay-fluid fractionation factor of 0.50‰ (Wimpenny et al., 2014), the fraction of Mg released to the fluid by the basalt and originally injected into the well with the gas charged fluid that was incorporated into clays can be estimated from mass balance constraints. The Mg

<footnote n="1">¹ The term conservative dissolution in this regard refers to a process where the dissolution releases to the fluid Mg having the same isotopic composition as the dissolving rock.</footnote>
concentration of the sampled monitoring well fluids is equal to the sum of contributions from the Mg concentration of the injected gas charged water (c<sub>Mg,inject</sub>), the Mg concentration of the original formation water (c<sub>Mg,fw</sub>), and the change in Mg concentrations due to basalt dissolution (Δc<sub>Mg,basalt</sub>) and Mg-clay precipitation (Δc<sub>Mg,clay</sub>) such that

$$410 \quad c_{\text{Mg,measured}} = X_{\text{fw}} c_{\text{Mg,fw}} + X_{\text{inject}} c_{\text{Mg,inject}} + \Delta c_{\text{Mg,basalt}} - \Delta c_{\text{Mg,clay}} \quad (1)$$

where X<sub>fw</sub> and X<sub>inject</sub> are the fraction of formation water and injected water in the collected monitoring well sample, determined by the concentrations of SF<sub>6</sub> and SF<sub>5</sub>CF<sub>3</sub> tracers in these fluids (Matter et al., 2016) such that X<sub>fw</sub> + X<sub>inject</sub> = 1. If it can be assumed that the fluids mix and basalt dissolved prior to the precipitation of the Mg-bearing clay, the isotopic composition of the fluid before clay precipitation (δ<sup>26</sup>Mg<sub>1</sub>) is given by

$$418 \quad \delta^{26}\text{Mg}_1 = \frac{(\delta^{26}\text{Mg}_{\text{fw}} X_{\text{fw}} c_{\text{Mg,fw}} + \delta^{26}\text{Mg}_{\text{inject}} X_{\text{inject}} c_{\text{Mg,inject}} + \delta^{26}\text{Mg}_{\text{basalt}} \Delta c_{\text{Mg,basalt}})}{(X_{\text{fw}} c_{\text{Mg,fw}} + X_{\text{inject}} c_{\text{Mg,inject}} + \Delta c_{\text{Mg,basalt}})} \quad (2)$$

where δ<sup>26</sup>Mg<sub>i</sub> designates the isotopic composition of the indicated source. The Mg isotope compositions of the monitoring well samples can then be determined from an expression of the Rayleigh equation of the form:

$$423 \quad \delta^{26}\text{Mg}_{\text{sampled fluid}} = \delta^{26}\text{Mg}_1 - f^{\Delta^{26}\text{Mg}_{\text{clay-fluid}}} \quad (3)$$

where δ<sup>26</sup>Mg<sub>sampled fluid</sub> designates the isotopic composition in the sampled monitoring well fluid, Δ<sup>26</sup>Mg<sub>clay-fluid</sub> stands for the indicated isotope fractionation factor, and f, the fraction of the Mg remaining in the fluid following clay mineral precipitation in each sample. Equations (1) to (3) were solved simultaneously to generate the v alues of f shown in Fig. 4. The values of X<sub>fw</sub> and X<sub>inject</sub> in Eqn. (2) required for this calculations were taken from Matter et al. (2016), and values of δ<sup>26</sup>Mg<sub>inject</sub> and c<sub>Mg,inject</sub> are provided in Tables 1 and 2. Note the δ<sup>26</sup>Mg<sub>fw</sub> X<sub>fw</sub> c<sub>Mg,fw</sub> term is negligible compare to the other terms in Eqn. (2) due to the low Mg concentration of the pre-injection formation waters (compare the composition of sample 12KMG01 with the others in Table 2). The
calculated fraction of the Mg remaining in solution into clay minerals, as shown in Fig. 4,
tends to decrease with time. Calculations suggest that this fraction decreases
continuously from 33 to 12 percent with time from February 8, 2012 to June 8, 2014.
This behavior contrasts somewhat with the saturation index of Mg clays, which becomes
undersaturated in the sampled monitoring fluids when the injected acid fluid first arrive at
the monitoring well – see Table 2. It can be seen in Fig. 4 that the distribution of
calculated Mg precipitation fractions exhibit a concave distribution consistent with the
slowing of clay formation rates with time. Consistent with the very dilute Mg
concentrations of the deep well fluid prior to the injection, it seems likely that this
fraction would approach zero over the long-term.

This estimate, however, is likely a minimum estimate as evidence indicates that
calcite dissolved into the gas charged injection waters shortly after they arrive in the
subsurface (Matter et al., 2016; Snæbjörnsdóttir et al., 2017, 2018). The dissolution of
these calcites would likely release some light Mg to the fluid phase. In addition, the
dissolution of basaltic glass and olivine has been reported to initially release light Mg to
the fluid (Wimpenny et al., 2010). Indeed, mass balance and reactive path calculations
reported by Snæbjörnsdóttir et al. (2017, 2018) suggest that more than 95% of the Mg
release by dissolving basaltic glass needed to be incorporated into secondary phases to be
consistent with the measured dissolved Mg concentration of the HN-04 monitoring well
fluids in March 2013. The stable Mg isotope measurements reported here suggest that the
bulk of these secondary Mg phases formed following the CarbFix injections were Mg
bearing clays (e.g. smectite).

It should be emphasized, however, that the calculated results shown in Fig. 4 are
highly uncertain due to poor understanding of the Mg isotope fractionation factor
between Mg clay minerals and the aqueous fluid. For example, Teng et al. (2010)
suggested that the $\Delta^{26}Mg_{clay-fluid}$ deduced from the compositions of South Carolina
saprolites range from 0.05‰ to -0.4‰. In contrast, Huang et al. (2012) reported that
$\Delta^{26}Mg_{clay-fluid}$ based on the compositions of southern Chinese saprolites ranged from -
0.94‰ to -1.94‰. Wimpenny et al. (2014), however, concluded that the Huang et al.
(2012) observations were more consistent with a $\Delta^{26}Mg_{clay-fluid}$ of -0.57‰. The computed value of f, the fraction of the Mg remaining in the fluid following clay mineral precipitation in each sample determined in this study depends strongly on the value chosen for $\Delta^{26}Mg_{clay-fluid}$. A value of $\Delta^{26}Mg_{clay-fluid}$ = -0.40‰ would yield a final f value for the June 8, 2014 sample of 0.07, whereas a $\Delta^{26}Mg_{clay-fluid}$ = -1.00‰ would yield a final f value for the June 8, 2014 sample of 0.34. Moreover, it should also be noted that at the pH and fluid composition changed (Schott et al., 2016), so too would $\Delta^{26}Mg_{clay-fluid}$. Such changes also would alter the calculated values of f. It follows that precise and accurate values of $\Delta^{26}Mg_{clay-fluid}$ are an essential prerequisite to using the compositions the Mg isotopic compositions of minerals and fluids to quantify natural geochemical processes.

The formation of Mg clays in response to the injection of CO₂ into basalts, as indicated in this study, is detrimental to carbon storage efforts for two main reasons. First, the formation of these Mg-bearing silicates consume divalent Mg that could otherwise be used for the formation of carbonate minerals. Second, Mg-bearing clays are voluminous, such that they consume valuable porosity and could decrease host rock permeability. The formation of these clays, however, will occur at a distance from the injection well, as their precipitation requires sufficient basalt dissolution to increase the fluid pH to at least 7 (Snæbjörnsdóttir et al., 2017). As such their formation will not likely clog flow pathways near the injection well. Nevertheless, their formation at a distance from these wells will limit somewhat the efficiency of carbon storage efforts in basalts. This potential challenge might be overcome by injecting at higher temperatures, where the formation of magnesite rather than Mg-bearing clays might be favored, however, Mg-silicate alteration phases including mixed-layered clays, chlorite, and epidote are common alteration phases in basalts to at least 280 °C (Snæbjörnsdóttir et al., 2018). Nevertheless, the continuous injection of acidic CO₂-charged fluids may lead to a propagating reaction front that would progressively move these clay minerals further away from the injection well.
495 **6. Conclusions**

496 The Mg isotope compositions of monitoring well samples
497 collected in this study are lighter than the host basalts that
498 dissolved in response to the injection of gas charged waters into the
499 CarbFix site. This observation is consistent with the consumption of
500 the divalent Mg cations released by basalt dissolution by
501 isotopically heavier Mg clay minerals, rather than carbonate
502 minerals, which tend to favor the incorporation of isotopically
503 light Mg. This conclusion is supported by the fact that Mg-smectites
504 rather than Mg carbonate phases are commonly observed as
505 secondary phases in basalts altered at low temperatures. Such
506 results demonstrate that the Mg isotopic compositions of monitoring
507 well fluids can be used to provide insight into the fate of Mg during
508 subsurface carbon storage efforts.
509

509

510 The rates at which Mg precipitates as clay minerals appear to
511 be relatively slow; mass balance calculations suggest that
512 smectite is still continuing to precipitate 2 years after the
513 termination of the acid gas injection – calculations suggest that ~12
514 percent of the Mg released to the fluid by basalt dissolution
515 remains to be precipitated after two years. This contrasts with
516 calcite, which has demonstrated to precipitate more than 95% of the
517 injected CO2 in less than 2 years (Matter et al., 2016). Although Mg
518 clay mineral precipitation is relatively slow, its precipitation may
519 limit significantly the efficiency of carbon storage efforts based on
520 the enhanced weathering of mafic or ultramafic rocks over the long
521 term (Rigopoulos et al., 2018).

522

523 *Acknowledgements*- We acknowledge funding from the Environmental Fund of
524 Reykjavik Energy, the European Commission through the projects CarbFix (EC
525 coordinated action 283148), CarbFix2 (Grant Agreement No, 764760), Min-GRO (MC-
526 RTN-35488), Delta-Min (PITN-GA-2008-215360), and CO2-REACT (EC Project
527 317235), the U.S. Department of Energy under award number DE-FE0004847, the
528 Nordic fund 11029-NORDICCS, and the Icelandic GEORG Geothermal Research fund
529 (09-02-001). We are indebted to Hólmfríður Sigurðardóttir at Reykjavík Energy, Magnús
530 Þór Arnarson at Mannvit Engineering, Domenik Wolff-Boenisch at Curtin University in
531 Australia, Helgi A. Alfreðsson at the University of Iceland and Wallace S. Broecker at
532 Columbia University for their contributions to the CarbFix project. We thank Einar Örn
533 Þrastarson, Trausti Kristinsson, Vordís Eiríksdóttir, Halldór Bergmann, and Þorsteinn A.
534 Þorgeirsson at Reykjavík Energy; Vigdís Harðardóttir, Finnbogi Óskarsson, Kristján
535 Hrafn Sigurðsson and Steinþór Níelsson at ISOR; Jennifer Hall at Columbia University,
536 Franziska Stamm at GET Toulouse, and Þorsteinn Jónsson, Sveinbjörn Steinþórsson,
537 Iwona Galezcka, Eydís S. Eiríksdóttir, Deirdre Clark, Chris Grimm and Flora Brocza at
538 the University of Iceland for helping the injection and sampling campaign.
539

540 **7. References**

541 Alfredsson, H. A., Oelkers, E. H., Hardarsson, B. S., Franzson, H.,
542 Gunnlaugsson, E., and Gislason, S.R. (2013) The geology and water
543 chemistry of the Hellisheidi, SW-Iceland carbon storage site. *Int. J.
544 Greenhouse Gas Cont.* **12**, 399-418.

545 Alfredsson, H. A., Mesfin, K.G., and Wolff-Boenisch, D. (2016) The syringe
546 sampler: An inexpensive alternative borehole sampling technique for
547 CO2-rich fluids during mineral carbon storage. *Greenhouse Gases: Sci. Tech.* **6**,
548 167-177.

549 An, Y.J., and Huang, F. (2014) A review of Mg isotope analytical methods by
550 MC-ICP-MS. *J. Earth Sci.-China* **25**, 822-840.

551 Aradóttir, E., Sigurdardóttir, H., Sigfússon, B., and Gunnlaugsson, E. (2011).
552 CarbFix: A CCS pilot project imitating and accelerating natural CO2
553 sequestration. *Greenhouse Gas Sci Technol*, **1**, 105-118.

554 Aradóttir, E., and Hjalmarsson, E. (2018) CarbFix – Public engagement and
555 transparency. *Energy Procedia*, **146**, 115-120

556 Archer, D. (2005) Fate of fossil fuel CO2 in geologic time. *J. Geophys. Res.* **110**, C09S05.

557 Assayag, N., Matter, J., Ader, M., Goldberg, D., and Agrinier, P., 2009. Water-rock interactions
during a CO2 injection field-test: Implications on host rock dissolution and alteration effects.
*Chem. Geol.* **265**, 227-235.

560 Bacon, D. H., Ramanathan, R., Schaef, H. T., and McGrail, B. P. (2014) Simulating geologic co-
561 sequestration of carbon dioxide and hydrogen sulfide in a basalt formation. *Int. J. Greenhouse
562 Gas Control* **21**, 165-176.

563 Beinlich, A., Mavromatis, V., Austrheim, H., and Oelkers E.H. (2012) Inter-
564 mineral Mg isotope fractionation during hydrothermal ultramafic rock
565 alteration – Implications for the global Mg-cycle. *Earth Planet. Sci. Let.* **392**,
566 166-76.

567 Black, J.R., Yin, Q.Z., Casey, W.H. (2006) An experimental study of magnesium-isotope
568 fractionation in chlorophyll-a photosynthesis. *Geochim. Cosmochim. Acta* **70**, 4072-4079.

569 Bolou-Bi, E.B., Vigier, N., Brenot, A. and Poszwa, A. (2009) Magnesium isotope
570 compositions of natural reference materials. *Geostand. Geoanal. Res.* **33**, 95-
571 109.
572 Bolou-Bi, E.B., Poszwa, A., Leyval, C., Vigier, N. (2010) Experimental determination of
573 magnesium isotope fractionation during higher plant growth. *Geochim. Cosmochim. Acta* **74**,
574 2523-2537.

575 Bolou-Bi, E.B., Vigier, N., Poszwa, A., Boudot, J.-P., Dambrine, E. (2012) Effects of
576 biogeochemical processes on magnesium isotope variations in a forested catchment in the
577 Vosges Mountains (France). *Geochim. Cosmochim. Acta* **87**, 341–355.

578 Brenot, A., Cloquet, C., Vigier, N., Carignan, J., and France-Lanord, C. (2008)
579 Magnesium isotope systematics of the lithologically varied Moselle
580 river basin, France. *Geochim. Cosmochim. Acta* **72**, 5070-5089.

581 Buhl, D., Immenhauser, A., Smeulders, G., Kabiri, L. and Richter, D.K. (2007) Time series
582 δ²⁶Mg analysis in speleothem calcite: Kinetic versus equilibrium fractionation, comparison
583 with other proxies and implications for palaeoclimate research. *Chem. Geol.* **244**, 715-729.

584 Chang, V.T.C., Williams, R.J.P., Makishima, A., Belshawl, N.S. and O'Nions, R.K. (2004) Mg
585 and Ca isotope fractionation during CaCO₃ biomineralisation. *Biochem. Biophys. Res. Comm.*
586 **323**, 79-85.

587 Croviser, J.L., Honnorez, J., Britz, B., and Petit, J.-C. (1992) Dissolution of subglacial volcanic
588 glasses from Iceland: Laboratory study and modelling. *Appl. Geochem.*, 7, Supplement 1, 55-
589 81.

590 Fisher, A. T. (1998) Permeability within basaltic oceanic crust: *Revs. Geophys.* **36**, 143-182.

591 Flaathen, T. K., Gislason, S. R., Oelkers, E. H., and Sveinbjörnsdóttir, Á. E. (2009) Chemical
592 evolution of the Mt. Hekla, Iceland, groundwaters: A natural analogue for CO₂ sequestration
593 in basaltic rocks. *App. Geochem.* **24**, 463-474.

594 Foster, G.L., Pogge von Strandmann, P.A.E., and Rae, J.W.B. (2010) The boron and magnesium
595 isotopic composition of seawater. *Geochemistry Geophysics Geosystems* **11**, Q08015,
596 doi:10.1029/2010GC003201.

597 Galeczka, I., Wolff-Boenisch, D., Oelkers, E. H., and Gislason, S. R. (2014) An experimental
598 study of basaltic glass–H₂O–CO₂ interaction at 22 and 50°C: Implications for subsurface
599 storage of CO₂. *Geochim. Cosmochim. Acta* **126**, 123-145.

600 Galy, A., Bar-Matthews, M., Halicz, L and O'Nions, R. K. (2002) Mg isotopis
601 compostion of carbonate: In sight from speleothem formation. *Earth*
602 *Planet. Sci. Let.* **202**, 105-115.

603 Galy, A., Yoffe, O., Janney, P.E., Williams, R.E., Cloquet, C., Alard, O., Halicz, L., Wadwha, A.,
604 Hutchen, I.D., Ramon, E. and Carignan, J. (2003) Magnesium isotopes heterogeneity of the
605 isotopic standard SRM980 and new reference materials for magnesium-isotope-ratio
606 measurements. *J. Anal. At. Spectrom.* **18**, 1352-1356.

607 Geske, A., Goldstein, R.H., Mavromatis, V, Richter, D.K., Buhl, D., Kluge, T., John, C.M., and
608 Immenhauser, A. (2015) The magnesium isotope (δ²⁶Mg) signature of dolomites. *Geochim.
609 Cosmochim. Acta* **149**, 131–151.

610 Geske, A., Zorlu, J., Richter, D.K., Buhl, D., Niedermayr, A., and Immenhauser, A. (2012)
611 Impact of diagenesis and low grade metamorphosis on isotope (δ²⁶Mg, δ¹³C, δ¹⁸O and
612 Sr/δ¹³C) and elemental (Ca, Mg, Mn, Fe and Sr) signatures of Triassic sabkha dolomites.
613 *Chem. Geol.* **332–333**, 45–64.
614 Gíslason, S. R., Arnorsson, S. and Armannsson, H. (1996) Chemical Weathering of Basalt in Southwest Iceland; Effects of Runoff, Age of Rocks and Vegetative/glacial Cover. Am. J. Sci. **296**, 837-907.

617 Gíslason, S. R., Broecker, W. S., Gunnlaugsson, E., Snæbjörnsdóttir, S. Ó., Mesfin, K. G., Alfredsson, H. A., Aradottir, E. S., Sigfusson, B., Gunnarsson, I., Stute, M., Matter, J. M., Arnarson, M. T., Galeczka, I. M., Guðbrandsson, S., Stockman, G., Wolff-Boenisch, D., Stefansson, A., Ragnheidardottir, E., Faathen, T., Gysi, A. P., Olssen, J., Didriksen, K., Stipp, S., Menez, B., and Oelkers, E. H. (2014) Rapid solubility and mineral storage of CO₂ in basalt. Energy Procedia **63**, 4561-4574.

623 Gíslason, S.R. and Oelkers, E.H. (2003) Mechanism, rates, and consequences of basaltic glass dissolution: II. An experimental study of the dissolution rates of basaltic glass as a function of pH and temperature. Geochim. Cosmochim. Acta **67**, 3817-3832.

626 Gíslason, S. R., and Oelkers, E. H. (2014) Carbon Storage in Basalt. Science, **344**, 373-374.

628 Gíslason, S. R., Wolff-Boenisch, D., Stefansson, A., Oelkers, E. H., Gunnlaugsson, E., Sigurdardottir, H., Sigfusson, B., Broecker, W.S., Matter, J. W., and Stute, M. (2010) Mineral Sequestration of Carbon Dioxide in Basalt: A Pre-injection Overview of the CarbFix project. Int. J. Greenhouse Gas Cont. **4**, 537-45.

633 Gíslason, S. R., Sigurdardóttir, H., Aradóttir, E. S., and Oelkers, E.H. (2018) A brief history of CarbFix: challenges and victories of the project's phase. Energy Procedia, **146**, 103-114.

635 Goldberg, D. and Slagle, A.L. (2009) A global assessment of deep-sea basalt sites for carbon sequestration. Energy Procedia **1**, 3675-3682.

637 Goldberg, D., Lackner, K., Han, P., and Wang, T. (2013) Co-Location of Air Capture, Subseafloor CO₂ Sequestration, and Energy Production on the Kerguelen Plateau: Envir. Sci. Tech. **47**, 7521-7529.

640 Goldberg, D. S., Takahashi, T., and Slagle, A. L. (2008) Carbon dioxdie sequestration in deep-sea basalt. PNAS **105**, 9920-9925.

642 Goldberg, D. S., Kent, D. V., and Olsen, P. E. (2010) Potential on-shore and off-shore reservoirs for CO₂ sequestration in Central Atlantic magmatic province basalts. PNAS, **107**, 1327-1332.

644 Gudbrandsson, S., Wolff-Boenisch, D., Gislason, S. R., and Oelkers, E. H. (2011) An experimental study of crystalline basalt dissolution from 2 < pH < 11 and temperatures from 5 to 75 °C. Geochim. Cosmochim. Acta **75**, 5496-5509.

647 Gysi, A. P., and Stefánsson, A. (2012) CO₂-water-basalt interaction. Low temperature experiments and implications for CO₂ sequestration into basalts: Geochim. Cosmochim. Acta **81**, 129-152.

650 Harris, R. N., and Chapman, D. S. (2004) Deep-seated oceanic heat flux, heat deficits and hydrothermal circulation, Cambridge, Cambridge University Press, Hydrogeology of the Oceanic Lithosphere.

653 Higgins, J.A. and Schrag, D.P. (2010) Constraining magnesium cycling in marine sediments using magnesium isotopes. Geochim. Cosmochim. Acta **74**, 5039-5053.

655 Hin, R.C., Coath, C.D., Carter, P.J., Nimmo, F., Lai, Y.-L., Pogge von Strandmann, P.A.E., Willbold, M., Leinhardt, Z.M., Walter, M.J., Elliott, T., (2017) Magnesium isotope evidence that accretional vapour loss shapes planetary compositions. Nature **549**, 511-515.
658 Hippler, D., Buhl, D., Witbaars, R.m, Richter, D.K., and Immenhauser, A. (2009) Towards a better understanding of magnesium-isotope rations from marine skeletons. *Geochim. Cosmochim. Acta* **73**, 6134-6146.

661 Huang K.-J., Teng F.-Z., Wei G.-J., Ma J.-L., and Bao Z.-Y. (2012) Adsorption- and desorption-controlled magnesium isotope fractionation during extreme weathering of basalt in Hainan Island, China. *Earth Planet. Sci. Let.* **359–360**, 73–83.

665 Immenhauser, A., Buhl, D., Richter, D., Niedermayr, A., Riechelmann, D., Dietzel, M., and Schulte, U. (2010) Magnesium-isotope fractionation during low-Mg calcite precipitation in a limestone cave - Field study and experiments. *Geochim. Cosmochim. Acta* **74**, 4346-4364.

668 Khalilabad, M.R., Axelsson, G. and Gislason, S.R. (2008) Aquifer characterization with tracer test technique; permanent CO₂ sequestration into basalt, SW Iceland. *Min. Mag.* **72**, 121-125.

670 Li, W., Chakraborty, S., Beard, B.L., Romanek, C.S., and Johnson, C.M. (2012) Magnesium isotope fractionation during precipitation of inorganic calcite under laboratory conditions. *Earth and Planet. Sci. Let.* **333-334**, 304-316.

673 Li, W., Beard, B.L., Li, C., Xu, H., and Johnson, C.M. (2015) Experimental calibration of Mg isotope fractionation between dolomite and aqueous solution and its geological implications. *Geochim. Cosmochim. Acta* **157**, 164-181.

676 Li, W., Beard, B.L., Li, C., and Johnson, C.M. (2014) Magnesium isotope fractionation between brucite [Mg(OH)₂] and Mg aqueous species: Implications for silicate weathering and biogeochemical processes. *Earth and Planet. Sci. Let.* **394**, 82-93.

679 Ling, M.X., Sedaghatpour, F., Teng, F.-Z., Hays, P.D., Strauss, J., Sun, W.D. (2011) Homogeneous magnesium isotopic composition of seawater: an excellent geostandard for Mg isotope analysis. *Rapid. Commun. Mass Sp.* **25**, 2828-2836

682 Liu, P.-P., Teng, F.-Z., Dick, H. J. B., Zhou, M.-F., and Chung S.-L. (2017) Magnesium isotopic composition of the oceanic mantle and oceanic Mg cycling. *Geochim. Cosmochim. Acta* **206**, 151-165.

685 Liu, X-M., F-Z. Teng, F.-Z., Rudnick, R. L., Mcdonough, W. F. and Cummings, M. L. (2014) Massive magnesium depletion and isotope fractionation in weathered basalts. *Geochim. Cosmochim. Acta* **135**, 336-349.

688 Maher, K., Johnson, N.C., Jackson, A., Lammers, L.N., Torchinsky, A.B., Weaver, K.L., Bird, D.K., Brown, G.E. (2016) A spatially resolved surface kinetic model for forsterite dissolution. *Geochim. Cosmochim. Acta* **174**, 313-334.

691 Marieni, C., Prikryl, J., Aradottir, E.S., Gunnarsson, I., and Stefansson, A. (2018) Towards ‘green’ geothermal energy: Co-mineralization of carbon and sulphur in geothermal reservoirs. *Int. J. Greenhouse Gas Cont.*, **77**, 96-105.

694 Matter, J. M., Broecker, W., Stute, M., Gislason, S., Oelkers, E., Stefánsson, A., Björnsson, G. (2009) Permanent carbon dioxide storage into basalt: The CarbFix pilot project, Iceland. *Energy Procedia*, **1**, 3641-3646.

697 Matter, J., Takahashi, T., and Goldberg, D. (2007) Experimental evaluation of *in situ* CO₂-water-rock reactions during CO₂ injection in basaltic rocks: Implications for geological CO₂ sequestration: *Geochem. Geophy. Geosy.* **6**, 19.

700 Matter, J.M., Stute, M., Snæbjörnsdóttir, S.Ó., Oelkers, E.H., Gislason, S.R., Aradottir, E.S., Sigfusson, B., Gunnarsson, I., Sigurdardóttir, H., Gunnlaugsson, E., Axelsson, G., Alfredsson, H.A., Wolff-Boenisch, D., Mesfin, K., Fernandez de la Reguera Taya, D., Hall, J., Dideriksen,
703 K. and Broecker, W.S. (2016) Rapid carbon mineralization for permanent and safe disposal of
704 anthropogenic carbon dioxide emissions. *Science* **352**, 1312-1314.

705 Mavromatis, V., Gautier, Q., Bosc, O. and Schott, J. (2013) Kinetics of Mg
706 partition and Mg stable isotope fractionation during its incorporation
707 in calcite. *Geochim. Cosmochim. Acta* **114**, 188-203.

708 Mavromatis, V., Meister, P. and Oelkers, E.H. (2014) Using stable Mg isotopes to distinguish
709 dolomite formation mechanisms: A case study from the Peru Margin. *Chem. Geol.* **385**, 84-91.

710 Mavromatis, V., Pearce, C.R., Shirokova, L.S., Bundeleva, I.A., Pokrovsky, O.S.,
711 Benezeth, P. and Oelkers, E.H. (2012) Magnesium isotope fractionation
during hydrous magnesium carbonate precipitation with and without
712 cyanobacteria. *Geochim. Cosmochim. Acta* **76**, 161-174.
713

714 McGrail, B. P., Freeman, C. J., Brown, C. F., Sullivan, E. C., White, S. K., Reddy, S., Garber, R.
715 D., Tobin, D., Gilmartin, J. J., and Steffensen, E. J. (2012) Overcoming business model
716 uncertainty in a carbon dioxide capture and sequestration project: Case study at the Boise
717 White Paper Mill. *Int. J. Greenhouse Gas Cont.* **9**, 91-102.

718 McGrail, B. P., Schaef, H. T., Ho, A. M., Chien, Y.-J., Dooley, J. J., and Davidson, C. L. (2006)
719 Potential for carbon dioxide sequestration in flood basalts. *J. Geophys. Res. Solid Earth* **111**,
720 B12201.

721 McGrail, B. P., Spane, F. A., Sullivan, E. C., Bacon, D. H., and Hund, G. (2011) The Wallula
722 basalt sequestration pilot project. *Energy Procedia* **4**, 5653-5660.

723 Norman, M.D., Yaxley, G.M., Bennett, V.C., and Brandon, A.D. (2006) Magnesium isotopic
724 composition of oliving from ther Earth, Mars, Moon and pallasite parent body. *Geophys. Res.
725 Let.*, **33**, L15202.

726 Oelkers, E. H. (2001) General kinetic description of multioxide silicate mineral and glass
727 dissolution. *Geochim. Cosmochim. Acta* **65**, 3703-3719.

728 Oelkers, E.H. and Gislason, S.R. (2001) The mechanism, rates and consequences of basaltic glass
729 dissolution: I. An experimental study of the dissolution rates of basaltic glass as a function of
730 aqueous Al, Si and oxalic acid concentration at 25°C and pH = 3 and 11. *Geochim. Cosmochim. Acta* **65**, 3671-3681.
731

732 Oelkers, E. H., Gislason, S. R., and Matter, J. M. (2008) Mineral carbonation of
733 CO₂. *Elements*, **4**, 333-337.

734 Oelkers, E.H., Benning, L.G., Lutz, S., Mavromatis, V., Pearce, C.R. and Plümper, O. (2015) The
735 efficient long-term inhibition of forsterite dissolution by common soil bacteria and fungi at
736 Earth surface conditions. *Geochim. Cosmochim. Acta* **168**, 222-235.

737 Opfergelt, S., Geors, R.B., Burton. K.W., Guicharnaud, R., Siebert, C., Gislason, S. R., and
738 Halliday, A.N. (2011) Seasonal magnesium isotope variations in soil solutions reflecting
739 physico-chemical processes controlling soil weathering fluxes. *Min. Mag.* **75**, 1571.

740 Opfergelt, S., Burton, K., Georg, R., West, A., Guicharnaud, R., Sigfusson, B.,
741 Seibert, C., Gislason, S.R. and Halliday, A. (2014) Magnesium retention on
742 the soil exchange complex controlling Mg isotope variations in soils,
743 soil solutions and vegetation in volcanic soils, Iceland. *Geochim. Cosmochim. Acta* **125**, 110-130.
744
745 Olsson, J., Stipp, S. L. S., Makovicky, E., and Gislason, S. R. (2014) Metal scavenging by
746 calcium carbonate at the Eyjafjallajökull volcano: A carbon capture and storage analogue.
747 *Chem. Geol.* **384**, 135-148.

748 Pearce, C. R., Saldi, G. D., Schott, J., and Oelkers, E. H. (2012) Isotopic
749 fractionation during congruent dissolution, precipitation and at
750 equilibration: Evidence from Mg isotopes. *Geochim. Cosmochim. Acta* **92**, 170-183.

751 Pogge von Strandmann, P.A.E., Burton, K.W., James, R.H., van Calsteren, P., Gislason, S.R. and
752 Sigfusson, B. (2008) The influence of weathering processes on riverine magnesium isotopes in
753 a basaltic terrain. *Earth Planet Sci. Let.* **276**, 187-197.

754 Pogge von Strandmann, P.A.E., Opfergelt, S., Lai, Y.-J., Sugfusson, B., Gislason, S.R. and
755 Burton, K.W. (2011) Variations of Li and Mg isotope ratios in bulk chondrites and mantle
756 xenoliths. *Geochim. Cosmochim. Acta* **75**, 5247-5268.

757 Pogge von Strandmann, P.A.E., Elliott, T., Marschall, H. R., Coath, C., Lai, Y.-J., Jeffcoate, A.
758 B., and Ioanoc, D. A. (2012) Lithium, magnesium and silicon isotope behaviour
759 accompanying weathering in a basaltic soil and pore water profile in Iceland. *Earth Planet.
760 Sci. Let.* **339-340**, 11-23.

761 Pogge von Strandmann, P. A. E., Forshaw, J., and Schmidt, D. (2014) Modern
762 and Cenozoic records of seawater magnesium from foraminiferal Mg
763 isotopes. *Biogeosciences*, 5155-5168.

764 Pokharel, R., Gerrits, R., Schuessler, J.A., Floor, G., Gorbushina, A.A., and von Blanckenburg, F.
765 (2017) Mg isotope fractionation during uptake by a rock-inhabiting, model microcolonial
766 fungus Knufia petricola at acidic and neutral pH. *Environ. Sci. Tech.* **51**, 9691-9699.

767 Prikryl, J., Stefansson, A., and Pearce, C.R. (2018) Tracing olivine carbonation and
768 serpentinization in CO₂-rich fluids via magnesium exchange and isotopic fractionation.
769 *Geochim. Cosmochim. Acta* **243**, 133-148.

770 Rigopoulos, I., Harrison, A.L., Delimitis, A., Ioannou, I., Efastathiou, A.M., Kyratsi, T., and
771 Oelkers, E.H. (2018) Carbon sequestration via enhanced weathering of peridotes and basalts in
772 seawater. *App. Geochem.* **91**, 197-218.

773 Rogers, K. L., Neuhoff, P. S., Pedersen, A. K., and Bird, D. K. (2006) CO₂ metasomatism in a
774 basalt-hosted petroleum reservoir, Nuussuaq, West Greenland. *Lithos*, **92**, 55-82.

775 Rosenbauer, R. J., Thomas, B., Bischoff, J. L., and Palandri, J. (2012) Carbon sequestration via
776 reaction with basaltic rocks: Geochemical modelling and experimental results. *Geochim.
777 Cosmochim. Acta* **89**, 116-133.

778 Ryu, J-S., Vigier, N., Decarreau, A., Lee, S.-W., Lee, K.-S., Song, H., and Petit, S.
779 (2016) Experimental investigation of Mg isotope fractionation during
780 mineral dissolution and clay formation. *Chem. Geol.* **445**, 135-145.

781 Saenger, C., and Wang, Z. (2014) Magnesium isotope fractionation in biogenic
782 and abiogenic carbonates: Implications for paleoenvironmental
783 proxies. *Quaternary Sci. Rev.* **90**, 1-21.

784 Saulnier, S., Rollion-Bard, C., Vigier, N. and Chaussidon, M. (2012) Mg isotope fractionation
785 during calcite precipitation: An experimental study. *Geochim. Cosmochim. Acta* **91**, 75-91.

786 Schott, J., Mavromatis, V., Fujii, T., Pearce, C.R., and Oelkers, E.H. (2016) The control of
787 carbonate mineral Mg isotope composition by aqueous speciation: Theoretical and
788 experimental modeling. *Chem. Geol.* **445**, 120-134.
789 Shirokova, L. S., Mavromatis, V., Bundelova, I.A., Pokrovsky, O.S., Bénézeth, P.,
790 Gérard, E., Pearce, C. R., and Oelkers, E.H. (2012) Using Mg Isotopes to trace
cyanobacterially mediated magnesium carbonate precipitation in
791 alkaline lakes. *Aquat. Geochem.* **19**, 1-24.

793 Sigfusson, B., Gislason, S. R., Matter, J. M., Stute, M., Gunnlaugsson, E., Gunnarsson, I.,
794 Aradottir, E. S., Sigurdardottir, H., Mesfin, K. G., Alfredsson, H. A., Wolff-Boenisch, D.,
795 Arnarson, M. T., and Oelkers, E. H. (2015) Solving the carbon-dioxide buoyancy challenge:
The design and field testing of a dissolved CO₂ injection system. *Int. J. Greenhouse Gas Cont.*
796 37, 213-219.

798 Sigfusson, B., Arnarson, M. T., Snæbjörnsdóttir, S.Ó., Karlsdóttir, M. R., Aradottir, E. S., and
799 Gunnarsson, I. (2018) Reducing emissions of carbon dioxide and hydrogen sulphide at
800 Hellishidi power plant in 2014-2017 and the role of CarbFix in achieving the 2040 Iceland
801 climate goals. *Energy Procedia* **146**, 135-145.

802 Snæbjörnsdóttir, S.Ó., Wiese, F., Fridriksson, T., Ármannsson, H., Einarsson, G.M. and Gislason,
803 S.R. (2014) CO₂ storage potential of basaltic rocks in Iceland and the oceanic ridges. *Energy
804 Procedia* **63**, 4585-4600.

805 Snæbjörnsdóttir, S.Ó., Oelkers, E.H., Mesfin, K., Aradóttir, E.S., Dideriksen, K., Gunnarsson, I.,
806 Gunnlaugsson, E., Matter, J.M., Stute, M. and Gislason, S.R. (2017) The chemistry and
807 saturation states of subsurface fluids during the in situ mineralisation of CO₂ and H₂S at the
808 CarbFix site in SW-Iceland. *Int. J. Greenhouse Gas Cont.* **58**, 87-102.

809 Snæbjörnsdóttir, S.Ó., Gislason, S.R., Galecka, I.M., and Oelkers, E.H. (2018) Reaction path
810 modelling of in-situ mineralisation of CO₂ at the CarbFix site at Hellishedi, SW-Iceland. *Geochim. Cosmochim. Acta.* **58**, 87-102.

812 Snæbjörnsdóttir, S.Ó., Tomasdóttir, S., Sigfusson, B., Aradóttir, E.S., Gunnarsson, G., Niemi, A.,
813 Basirat, F., Gislason, S.R., Oelkers, E.H., and Franzson, H. (2018) The geology and hydrology
814 of gne CarbFix2 site, SW-Iceland. *Energy Procedia* **146**, 146-157.

815 Stefánsson, A., Arnórsson, S., Gunnarsson, I., Kaasalainen, H., and Gunnlaugsson, E., 2011. The
816 geochemistry and sequestration of H₂S into the geothermal system at Hellisheidi, Iceland. *J. Volcan. Geothermal Res.* **202**, 179-188.

818 Stockmann, G. J., Wolff-Boenisch, D., Gislason, S. R., and Oelkers, E. H. (2011) Do carbonate precipitates affect dissolution kinetics? 1: Basaltic glass. *Chem. Geol.* **284**, 306-316.

820 Teng, F.-Z., Wadhwa, M., and Helz, R. T. (2007) Investigation of magnesium
821 isotope fractionation during basalt differentiation: Implications for a
822 chondritic composition of the terrestrial mantle. *Earth Planet. Sci. Let.* **261**,
823 84-92.

824 Teng, F.-Z., Li, W., Rudnick, R. L., and Gardner, L. R. (2010a) Contrasting lithium
825 and magnesium isotope fractionation during continental
826 weathering. *Earth Planet. Sci. Let.* **300**, 63-71.

827 Teng, F.-Z. (2017) Magnesium Isotope Geochemistry. *Revs. Min. Geochem.* **82**, 219-
828 287.

829 Tipper, E., A. Galy, A., Gaillardet, J. Bickle, M., Elderfield H., and Carder, E.
830 (2006a) The magnesium isotope budget of the modern ocean: Constraints
831 from riverine magnesium isotope ratios. *Earth Planet. Sci. Let.* **250**, 241-253.
832 Tipper, E.T., Galy, A. and Bickle, M.J. (2006b) Riverine evidence for a fractionated reservoir of
833 Ca and Mg on the continents: Implications for the oceanic Ca cycle. *Earth Planet. Sci. Let.*
834 **247**, 267-279.

835 Tipper, E.T., Galy, A., and Bickle, M. (2008) Calcium and magnesium isotope systematics in
836 rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control?
837 *Geochim. Cosmochim. Acta* **72**, 1057-1075.

838 Tipper, E.T., Calmels, D., Gaillardet, J., Louvet, P., Capmas, F., and Dubacq, B. (2012a) Positive
839 correlation between Li and Mg isotope ratios in the river waters of the Mackenzie Basin
840 challenges the interpretation of apparent isotopic fractionation during weathering. *Earth
841 Planet. Sci. Let.* **333**, 35-45

842 Tipper, E.T., Lemarchand, E., Hindshaw, R.S., Reynolds, B.C. and Bourdon, B. (2012b) Seasonal
843 sensitivity of weathering processes: Hints from magnesium isotopes in a glacial stream. *Chem.
844 Geol.* **312–313**, 80–92.

845 Uhlig, D., Schuessler, J.A., Bouchez, J.L., Dixon, J., and von Blanckenburg, F. (2017)
846 Quantifying nutrient uptake as driver of rock weathering in forest ecosystems by magnesium
847 stable isotopes. *Biogeosciences* **14**, 3111-3128.

848 Van Pham, H., Aagaard, P., and Hellevang, H. (2012) On the potential for CO₂ mineral storage in
849 continental flood basalts - PHREEQC batch and 1D diffusion - reaction simulations:
850 *Geochem. Trans.* **13**, 12.

851 Wimpenny, J., Gíslason, S.R., James, R. H., Gannoun, A., Pogge Von Strandmann,
852 P.A.E., and Burton, K.W. (2010) The behaviour of Li and Mg Isotopes during
853 primary phase dissolution and secondary mineral formation in
854 basalt. *Geochim. Cosmochim. Acta* **74**, 5259-279.

855 Wimpenny, J., Colla, C.A., Yin, Q.Z., Rustad, J.R., Casey, W.H. (2014) Investigating the
856 behaviour of Mg isotopes during the formation of clay minerals. *Geochim. Cosmochim. Acta*
857 **128**, 178-194.

858 Wolff-Boenisch, D., Wenau, S., Gislason, S. R., and Oelkers, E. H. (2011) Dissolution of basalts
859 and peridotite in seawater, in the presence of ligands, and CO₂: Implications for mineral
860 sequestration of carbon dioxide: *Geochim. Cosmochim. Acta* **75**, 5510-5525.

861 Wombacher, F., Eisenhauer, A., Bohm, F., Gussone, N., Regenberg, M., Dullo, W.-Chr., and
862 Ruggeberg, A. (2011) Magnesium stable isotope fractionation in marine biogenic calcite and
863 aragonite. *Geochim. Cosmochim. Acta* **75**, 5797-5818.

864 Young, E. D., and Galy, A. (2004) The isotope geochemistry and
865 cosmochemistry of magnesium. *Revs Min. Geochem.* **55**, 197-230.
867 **Figure Captions**
868

869 Fig. 1. Maps showing location of the study area (a) Map showing active volcanic zones in Iceland: RP = Reykjanes Peninsula, WVZ = Western Volcanic Zone, SISZ = South Iceland Seismic Zone, MVZ = Mid-Iceland Volcanic Zone, EVZ = Eastern Volcanic Zone and NVZ = Northern Volcanic Zone. (b) Map showing location of central volcanoes (dark gray) on Reykjanes Peninsula and associated fissure swarms (gray). (c) Map of the CO₂ injection site in, Hellisheidi, SW-Iceland. The CarbFix wells are shown as labeled gray dots. Mapped bedrock faults are located toward east and are part of the Hengill fissure swarm - after Alfredsson et al. (2013).
870
871
872
873
874
875
876

877 Fig 2. The temporal evolution of a) pH and b) Mg concentration of fluids collected from
878 the HN-04 monitoring well, located 500 m away from the HN-02 injection well
879 before during and after the injection of gas charged waters at the CarbFix site. The
880 timing of the two injections is indicated by the two blue bars. Data from
881 Snæbjörnsdóttir et al. (2017) and Alfredsson et al. (2013). The reported
882 uncertainties on these analyses were reported to be ±0.02 units on pH and ±3% on
883 the Mg concentration analyses.
884

884 Fig 3. The temporal evolution of δ²⁶Mg of fluids collected from the CarbFix site. Open
885 diamonds illustrate the compositions of shallow wells prior to the injection, whereas
886 the filled circles represent the composition of monitoring well samples. The dashed
887 horizontal line shows the compositions of the basalts at the CarbFix site whereas the
888 timing of the two injections is indicated by the two blue bars – see text.
889

890 Fig 4. The temporal evolution of the fraction of Mg released by basalt dissolution that has
891 been incorporated into clay minerals (fMg, clay). Fractions were calculated assuming a
892 closed-system Rayleigh model (see text).
893
Table 1. Summary of measured Mg isotopic compositions of pre-injection shallow well fluids, and post-injection monitoring well fluids collected from HN-04. All isotopic compositions were measured at the HELGES lab, GFZ Potsdam. 2SD refers to twice the standard deviation of n MC-ICP-MS measurements of the same solution. The uncertainty of the presented data (relevant for geological interpretation) is estimated to be 0.10 ‰ (2SD) for δ²⁶Mg and 0.06 ‰ (2SD) for δ²⁵Mg based on long-term repeated analyses of samples and references materials – see text.

<table><thead><tr><th>sample</th><th>date<br>(day/month/year)</th><th>c<sub>Mg</sub><br>(mol kg<sup>-1</sup>)<br>x 10<sup>-6</sup></th><th>δ<sup>25</sup>Mg<sub>DSM3</sub><br>(‰)</th><th>2SD</th><th>δ<sup>26</sup>Mg<sub>DSM3</sub><br>(‰)</th><th>2SD</th><th>n</th><th>Well</th></tr></thead><tbody><tr><td colspan="10"><em>Post-injection monitoring well samples collected from HN-04</em></td></tr><tr><td>12KGM08</td><td>09/02/2012</td><td>36</td><td>-0.43</td><td>0.05</td><td>-0.84</td><td>0.09</td><td>6</td><td>HN-04</td></tr><tr><td>12KGM11</td><td>16/02/2012</td><td>51</td><td>-0.45</td><td>0.06</td><td>-0.88</td><td>0.10</td><td>4</td><td>HN-04</td></tr><tr><td>12KGM19</td><td>27/02/2012</td><td>58</td><td>-0.49</td><td>0.05</td><td>-0.94</td><td>0.09</td><td>4</td><td>HN-04</td></tr><tr><td>12KGM25</td><td>08/03/2012</td><td>86</td><td>-0.49</td><td>0.05</td><td>-0.93</td><td>0.02</td><td>4</td><td>HN-04</td></tr><tr><td>12KGM33</td><td>26/03/2012</td><td>101</td><td>-0.48</td><td>0.04</td><td>-0.94</td><td>0.01</td><td>4</td><td>HN-04</td></tr><tr><td>12KGM44</td><td>18/04/2012</td><td>74</td><td>-0.54</td><td>0.05</td><td>-1.05</td><td>0.07</td><td>4</td><td>HN-04</td></tr><tr><td>12KGM49</td><td>04/05/2012</td><td>36</td><td>-0.48</td><td>0.06</td><td>-0.93</td><td>0.12</td><td>4</td><td>HN-04</td></tr><tr><td>12KGM60</td><td>30/05/2012</td><td>54</td><td>-0.52</td><td>0.06</td><td>-1.01</td><td>0.12</td><td>4</td><td>HN-04</td></tr><tr><td>12SOS01</td><td>08/06/2012</td><td>73</td><td>-0.53</td><td>0.07</td><td>-1.02</td><td>0.08</td><td>4</td><td>HN-04</td></tr><tr><td>12SOS09</td><td>17/07/2012</td><td>64</td><td>-0.52</td><td>0.07</td><td>-0.98</td><td>0.10</td><td>3</td><td>HN-04</td></tr><tr><td>12SOS15</td><td>31/07/2012</td><td>66</td><td>-0.57</td><td>0.04</td><td>-1.1</td><td>0.08</td><td>4</td><td>HN-04</td></tr><tr><td>12SOS21</td><td>14/08/2012</td><td>86</td><td>-0.51</td><td>0.03</td><td>-0.99</td><td>0.09</td><td>4</td><td>HN-04</td></tr><tr><td>12SOS28</td><td>28/08/2012</td><td>79</td><td>-0.67</td><td>0.06</td><td>-1.26</td><td>0.07</td><td>3</td><td>HN-04</td></tr><tr><td>12SOS34</td><td>24/09/2012</td><td>69</td><td>-0.57</td><td>0.04</td><td>-1.11</td><td>0.05</td><td>3</td><td>HN-04</td></tr><tr><td>12SOS39</td><td>29/10/2012</td><td>62</td><td>-0.59</td><td>0.04</td><td>-1.14</td><td>0.07</td><td>4</td><td>HN-04</td></tr><tr><td>13SOS01</td><td>07/01/2013</td><td>42</td><td>-0.64</td><td>0.07</td><td>-1.23</td><td>0.06</td><td>4</td><td>HN-04</td></tr><tr><td>13SOS10</td><td>16/04/2013</td><td>39</td><td>-0.69</td><td>0.07</td><td>-1.31</td><td>0.04</td><td>3</td><td>HN-04</td></tr><tr><td>13SOS17</td><td>10/06/2013</td><td>40</td><td>-0.62</td><td>0.01</td><td>-1.22</td><td>0.03</td><td>4</td><td>HN-04</td></tr><tr><td>14SOS11</td><td>08/06/2014</td><td></td><td>-0.69</td><td>0.06</td><td>-1.34</td><td>0.10</td><td>4</td><td>HN-04</td></tr><tr><td colspan="10"><em>Pre-injection samples taken from indicated shallow well.</em></td></tr><tr><td>08HAA09</td><td>07/08/2008</td><td>180</td><td>-0.33</td><td>0.08</td><td>-0.64</td><td>0.11</td><td>5</td><td>HK-25</td></tr><tr><td>09HAA18</td><td>29/05/2010</td><td>175</td><td>-0.31</td><td>0.03</td><td>-0.57</td><td>0.06</td><td>3</td><td>HK-12</td></tr><tr><td>10HAA25</td><td>02/06/2010</td><td>226</td><td>-0.35</td><td>0.08</td><td>-0.69</td><td>0.13</td><td>3</td><td>HK-25</td></tr><tr><td>10HAA32</td><td>25/06/2010</td><td>169</td><td>-0.29</td><td>0.02</td><td>-0.59</td><td>0.02</td><td>2</td><td>HK-13</td></tr><tr><td colspan="10"><em>reference materials</em></td></tr><tr><td>IAPSO seawater (batch a)</td><td></td><td></td><td>-0.47</td><td>0.04</td><td>-0.90</td><td>0.08</td><td>8</td><td></td></tr><tr><td>IAPSO seawater (batch b)</td><td></td><td></td><td>-0.45</td><td>0.05</td><td>-0.88</td><td>0.08</td><td>4</td><td></td></tr><tr><td>IAPSO seawater mean (batch a & b)</td><td></td><td></td><td>-0.46</td><td>0.04</td><td>-0.90</td><td>0.08</td><td>12</td><td></td></tr><tr><td>seawater (reference value)<sup>a</sup></td><td></td><td></td><td>-0.43</td><td>0.06</td><td>-0.83</td><td>0.09</td><td></td><td></td></tr><tr><td>Cambridge-1</td><td></td><td></td><td>-1.35</td><td>0.06</td><td>-2.62</td><td>0.10</td><td>21</td><td></td></tr><tr><td>Cambridge-1 (reference value)<sup>a</sup></td><td></td><td></td><td>-1.34</td><td>0.03</td><td>-2.61</td><td>0.05</td><td></td><td></td></tr></tbody></table>

a Published reference values are from compilations by Pogge von Strandmann et al. (2011), An and Huang (2014), Foster et al. (2010), Ling et al. (2011) and references therein.
<figure><img src="image_3.png" /><figcaption>Figure 1.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

912

<figure><img src="image_5.png" /></figure>

913

Figure 2.

914

915
<figure><img src="image_6.png" /><figcaption>Figure 3.</figcaption></figure>

917

918

919
<figure><img src="image_7.png" /><figcaption>Figure 4.</figcaption></figure>

920

921

922

923

924