Using 87 Sr / 86 Sr LA-MC-ICP-MS Transects within Modern and Ancient Calcite Crystals to Determine Fluid Flow Events in Deep Granite Fractures

: The strontium isotope signature ( 87 Sr / 86 Sr) of calcite precipitated in rock fractures and faults is a frequently used tool to trace paleoﬂuid ﬂow. However, bedrock fracture networks, such as in Precambrian cratons, have often undergone multiple fracture reactivations resulting in complex sequences of fracture mineral inﬁllings. This includes numerous discrete calcite crystal overgrowths. Conventional 87 Sr / 86 Sr analysis of dissolved bulk samples of such crystals is not feasible as they will result in mixed signatures of several growth zonations. In addition, the zonations are too ﬁne-grained for sub-sampling using micro-drilling. Here, we apply high spatial resolution 87 Sr / 86 Sr spot analysis (80 µ m) in transects through zoned calcite crystals in deep Paleoproterozoic granitoid fractures using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) to trace discrete signs of paleoﬂuid ﬂow events. We compare the outermost calcite growth zone with 87 Sr / 86 Sr values of the present-day groundwater sampled in the same boreholes to distinguish potential modern precipitates. We then connect our results to previously reported radiometric dating and C and O isotope signatures to understand the temporal history and physicochemical evolution of ﬂuid ﬂow within the fractures. Comparisons of modern calcite precipitated in a borehole over a period of 17 years with modern waters prove the concept of using 87 Sr / 86 Sr as a marker for ﬂuid origin in this environment and for how 87 Sr / 86 Sr changed during marine water inﬁltration. Intermittent calcite precipitation over very long time spans is indicated in calcite of the currently open fractures, showing an evolution of 87 Sr / 86 Sr from ~0.705–0.707—a population dated to ~1.43 billion years—to crystal overgrowth values at ~0.715–0.717 that overlap with the present-day groundwater values. This shows that high spatial resolution Sr isotope analysis of ﬁne-scaled growth zonation within single calcite crystals is applicable for tracing episodic ﬂuid ﬂow in fracture networks. and M.K.-S.; investigation, H.D.; resources, H.D.; E.K., and M.K.-S.; data curation, H.D., E.K., and M.K.-S.; writing—original draft preparation, H.D.; writing—review and editing, H.D., E.K., and M.K.-S.; visualization, H.D.; supervision, H.D.; project administration, H.D.; funding acquisition, H.D.


Introduction
Sr isotopes have been used extensively in hydrological/hydrochemical studies to trace water origin and mixing, in surficial waters [1,2], at the boundary between surficial water and groundwater [3,4], in groundwaters of sedimentary basins [5], and in groundwaters of deep crystalline rock fractures [6][7][8]. 87 Sr/ 86 Sr values have also been used for sedimentary carbonates and carbonate shells to understand ocean water evolution, such as salinity variations and freshwater input from glaciations [9][10][11][12][13][14][15]. In bedrocks such as crystalline rock settings, the bulk rock 87 Sr/ 86 Sr signature evolves over time due to the decay of 87 Rb in K-bearing minerals like biotite and K-feldspar [16]. Water-rock interaction releases Sr to fluids in the fracture network and thus influences the isotope signature of the fluid [17][18][19].
If the system stays closed to semi-closed with regard to inflowing fluids with distinctly different Sr isotopic composition, the fracture fluid 87 Sr/ 86 Sr signature will evolve to higher values, in common with the host rock [20]. Ca-carbonate minerals, such as calcite, precipitated in the fractures at fracture reactivation events that induce fluid flow and mixing, can preserve this 87 Sr/ 86 Sr signature over geological timescales if the mineral is not dissolved. This is, therefore, a commonly used tool to distinguish paleofluid origin and fluctuations, in addition to other isotope diagnostics, including stable C and O isotopes [16,[21][22][23][24][25][26][27]. In Precambrian cratons, 87 Sr/ 86 Sr signatures in calcite have been used to distinguish separate generations of mineral assemblages and hence fluid flow events, but in general, these studies focused on conventional bulk sample analysis [16,[28][29][30][31][32]. A sub-sampling of macroscopically distinguishable generations has also been used [33]. Recent applications of microscale stable C (δ 13 C) and O (δ 18 O) isotope determination using Secondary Ion Mass Spectrometry (SIMS) and imaging studies have revealed that calcite crystals in granite fractures commonly are composed of multiple growth zones that represent several discrete fracture reactivation and fluid flow events [34,35].
Here, we present a comprehensive dataset of in situ Sr-isotope analysis transects derived from laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) within ancient calcite crystals from two areas in Sweden ( Figure 1, revisiting crystals previously analyzed for δ 13 C and δ 18 O [34]) to trace fluid flow events in the mineral record. We connect the calcite Sr isotope compositions with those of modern groundwater to determine whether the youngest of these calcite growth zones could be precipitated from the present-day waters. We compare and link the distinguished generations with published geochronological constraints [34,[36][37][38][39] of the same fracture assemblage to assess the overall temporal span of the precipitation history in presently water-conducting fractures. We also investigate the 87 Sr/ 86 Sr variability of modern calcite precipitated from fracture water in a borehole at 415 m depth over the course of 17 years, and compare these 87 Sr/ 86 Sr values with the modern source water. Transects within modern calcite also enable exploration of how the 87 Sr/ 86 Sr signatures of the water have changed as the fraction of modern marine water has increased in the borehole during the borehole lifetime. Micro-scale Sr isotope analysis of calcite has been shown to give important input to studies of carbonatites [40], speleothems [41], and veins in impact craters [42,43]. Here, we constrain its feasibility for determining paleofluid flow fluctuations in crystalline bedrock fractures from analysis within single crystals and from comparison with corresponding groundwaters. The results have implications for the understanding of paleofluid flow, and hydrological as well as hydrochemical evolution in deep granite aquifers, which is of importance for the planning of nuclear waste repositories.  [34], reprinted with permission (#4900621281616) from Elsevier.

Äspö Hard Rock Laboratory-Site for Sampling of Modern Calcite and Water
The Äspö Hard Rock Laboratory (Äspö HRL) was constructed in the early 1990s on the shores of the Baltic Sea in southeastern Sweden. It consists of an underground tunnel in Proterozoic granitoids and is extending to a depth of 460 m below sea level [44]. This laboratory was established to develop and test the Swedish concept for the storage of spent nuclear fuel [45]. Boreholes extending from the tunnel wall into the bedrock intersect water-conducting fractures and fracture zones. Water from these fractures has been sampled in packed off borehole sections and analyzed for hydrochemical compositions in monitoring programs aimed to understand natural variation and influence of the tunnel construction [46]. The monitoring has produced a large amount of quality-assured chemical and isotopic data. Sr isotopes have, however, only been measured on a few occasions. At great depths, there is a spatially widespread layer of old saline to brine type water [47,48]. The shallow groundwater has shorter residence time and shows influence from the paleoclimate and in modern times also from the drawdown of marine Baltic seawater induced by the tunnel system [46,48,49]. Holocene influence on the hydrochemical state includes late glaciation meltwater infiltration, dense brackish seawater infiltration during transgressions that mixed with the dilute meltwater, and finally, meteoric water recharge after the uplift of the land surface [50]. The hydrochemical composition of the shallow water at Äspö is a result of different mixing proportions of these end-member water types. The borehole sampled in this study (KA3105A) was investigated in previous studies for bacterial sulfide production [51], Fe isotope fractionation in pyrite [52], and trace element uptake in calcite [53]. Mineral crystals that precipitated in the anoxic, packed off borehole sections that isolate water-conducting fractures were sampled when the instrumentation was extracted in 2012 (inserted 1995, i.e., precipitation occurred over 17 years). In association, hydrochemical data was collected. Calcite occurred on the steel bars of the packer connections and together with sulfide on the Al-rods ( Figure 2). Two of the borehole sections (A:2 and A:3) are investigated in the present study and have Sr concentrations in calcite between 660-830 ppm in section A:2 and 840-1000 ppm in section A:3. Waters had Sr concentrations of 8.3-8.5 mg/L in A2 and 6.2-6.4 mg/L in A:3. The Sr partitioning coefficient for calcite was estimated to be 0.09-0.16 [53], in agreement with laboratory-derived coefficients [54]. Here, we compare the 87 Sr/ 86 Sr values of these calcites and the waters from which they precipitated. Strontium isotope ratios have been used previously to identify end-member groundwater compositions at Äspö [6]. These investigations showed an increase of the Sr isotope ratios with depth and with increasing Cl concentrations, although with large scatter. The inflow of Baltic seawater with relatively low 87 Sr/ 86 Sr value (~0.7094) has also had some influence on the 87 Sr/ 86 Sr values of the deeper water (0.716-0.719). This is due to ion exchange along the flow path and much lower Sr concentrations (1.5 mg/L) of the infiltrating water than the deeper waters (6-59 mg/L). , collectively termed Laxemar-Äspö in this study) are located on the Swedish east coast (Figure 1). These sites have been investigated during the latest decades for the siting of a nuclear waste repository. These sites are located in dominantly Paleoproterozoic rocks (metamorphosed in Forsmark, not as affected in Laxemar). The Götemar borehole is located in a Mesoproterozoic granite, but only features one calcite sample and no groundwater data.

Forsmark and
The rocks in the Forsmark area have crystallization ages of 1.89-1.86 Ga [55] and dominantly consist of metamorphosed granitoids with subordinate amphibolite and volcanic rocks. The rocks at Laxemar-Äspö are generally~1.8 Ga in age and quartz monzodioritic to granodioritic in composition [56]. Several events of Proterozoic and Paleozoic fracturing and fracture mineralization have been documented at the sites. The events started with Paleoproterozoic mylonitization followed by Mesoproterozoic, Neoproterozoic, Silurian-Carboniferous, Permian, and Jurassic fracture reactivation, featuring successively lower formation temperatures of the fracture coating minerals [23,[34][35][36]39,57,58]. Fracture reactivation was related to far-field orogenic events [57,59], as confirmed by high spatial resolution Rb/Sr and/or U-Pb dating of both slickenfibre minerals [36], veins [34], and bulk sample 40 Ar/ 39 Ar dating [39,60]. Bulk sample 87 Sr/ 86 Sr analysis has shown an overall evolution from low values of Proterozoic calcite veins (from~0.7070) to much higher ratios (up to~0.7175-0.7185) in coatings of calcite in currently open and water-conducting fractures [23,32,61]. The 87 Sr/ 86 Sr scatter is, however, substantial which suggests that mixed values from several mineral generations are included in the analyses. This is shown by the multiple generations of calcite detected in single crystals using in situ SIMS analysis for C and O isotopes, in a recent study [34]. Calcite crystals in open water-conducting fractures, which are the focus of the current study, have highly variable Sr concentrations, usually in the 20-50 ppm range, with excursions up to 225 ppm [31,62]. Strontium isotope ratios ( 87 Sr/ 86 Sr) have been measured in groundwater samples from cored and percussion boreholes in the areas. The groundwaters detected at the sites show-similar to Äspö HRL-an increased salinity and Sr concentration with depth [63][64][65] and a shallower, more dynamic system influenced by Quaternary infiltration and mixing of different surficial water types with the deeper saline water in the fracture system [66,67].

Materials and Methods
Calcite crystals coating open fractures were sampled from 21 deep boreholes from sites Forsmark (11 boreholes) and Laxemar-Äspö (10 boreholes, Figures 1 and 3). Modern calcite crystals were sampled from borehole instrumentation in one borehole from the Äspö HRL (Figure 2), in two different packed off sections.
In total, 29 fractures at depths of about 20 m to 800 m below the ground surface were sampled for calcite in previous studies [34,62]. The modern calcite was sampled during instrumentation extraction in a previous experiment [53]. The fracture calcite crystals were investigated in situ on the fracture surfaces using a Hitachi S-3400N SEM (Hitachi, Tokyo, Japan) equipped with an integrated energy dispersive spectroscopy (EDS) system under low-vacuum conditions (Figure 3b). The calcite crystals were then hand-picked under the microscope and mounted in epoxy. They were then polished to expose crystal cross-sections and examined again using SEM to trace zonation. A previous study involved intra-crystal SIMS-analysis (10 µm lateral beam dimension, 1-2 µm depth dimension) of carbon and oxygen isotopes in analytical transects in the different growth zones of the polished crystals [34]. In the current study, we re-visit the same spot locations in the calcite growth zones for in situ Sr-isotope ( 87 Sr/ 86 Sr) analysis. In total 101 Sr isotope analyses are presented. The same crystal growth zones analyzed with SIMS for δ 13 C and δ 18 O were targeted, but using a different spot size. Wash-out and ablation times were both 45 s. The 87 Sr-86 Sr ratios were corrected for potential isobaric interferences such as 87 Rb or doubly charged REEs through peak-stripping. A detailed description of the correction protocol, which is identical for the analysis of Sr in apatite, can be found in Emo et al. [68]. The corrected 87 Sr/ 86 Sr values are normalized to an in-house brachiopod reference material "Ecnomiosa gerda" (linear drift and accuracy correction) using a value established by Thermal Ionization Mass Spectrometry (TIMS) of 0.709168 (2sd 0.000004, [69]). A modern oyster shell from Western Australia was used as secondary reference material and analyzed at regular intervals together with the primary reference. A total of 18 analyses on the secondary standard yielded an average 87 Sr/ 86 Sr value of 0.70910 ± 0.00021 which agrees well with the modern seawater value for 87 Sr/ 86 Sr of 0.7091712 ± 0.0000021 [70] (normalized to an NBS987 value of 0.710248). The results of the reference material measurements are listed in Supplementary Data 1. Time-resolved signals of the ablation were examined for potential changes of the isotope ratio with depth. Only "flat" parts of the signal were chosen to ensure that single growth zones were sampled. The ablation crater depth is approximately 40 µm.
The 87 Sr/ 86 Sr values of the groundwaters were extracted from the site characterization database of Swedish Nuclear Fuel and Waste Management Co (SKB). These samples were taken and analyzed with TIMS (uncertainties of~±0.00002) during previous site characterization for potential nuclear waste repository siting (Forsmark and Laxemar) and related experiments (Äspö HRL). The waters were sampled in packed off sections from depths of down to 800 m below ground surface. Tables 1-3 list groundwater 87 Sr/ 86 Sr values from the same section as the calcite samples, or for representative nearby sections or boreholes. These groundwater data have been quality checked to rule out contamination from flushing water and surface water [71].

Modern Calcite and Waters
The modern calcite crystals from the borehole equipment show values that are section-specific, with higher values in A:2 than A:3 ( Figure 4). This is in line with the groundwater values. The crystals are zoned (Figure 4a) and for a few crystals, transects were done within the crystals (Figure 4b). These show systematically higher 87 Sr/ 86 Sr values in the crystal core than the rims (although they are overlapping when errors are taken into account). The 87 Sr/ 86 Sr values of the crystal cores do not overlap with the overall lower groundwater values. Most analyses thus focus on the outermost growth zone of the calcite. This is most representative for comparison with the groundwater, as the water data are from after instrumentation was extracted, and no data exist for the early history of the borehole.

Fracture Coating Calcite
Intra-crystal 87 Sr/ 86 Sr value trends are shown together with corresponding C and O isotope data in Figure 5 (Forsmark) and Figure 6 (Laxemar-Äspö). The fracture coating calcite at Forsmark shows 87 Sr/ 86 Sr values ranging from 0.71108 ± 0.00073 to 0.71630 ± 0.00067. Note that the lower Sr concentrations of the fracture coating calcites lead to higher analytical errors compared to the modern calcites. At Forsmark, the lowest values are found in the interior crystal core parts and values at younger growth zones are as a rule successively higher, as shown by the LA-MC-ICP-MS spot transects ( Figure 5). There are several interesting temporal trends within the crystals, especially a co-variation of 87 Sr/ 86 Sr and δ 13 C, which increases in the outermost growth zones of e.g., samples KFM01B:24 m (Figure 5a) and KFM05A:110 m (Figure 5b). The O isotopes do not show any specific trend with growth ( Figure 5). For KFM01B:24 m, the 87 Sr/ 86 Sr values increase from 0.71108 ± 0.00073 to 0.71436 ± 0.00027, and δ 13 C increases from −23.6 to +21.6% from the core to the crystal rim. For KFM05A:110 m, the shift to markedly higher 87 Sr/ 86 Sr values and strongly positive δ 13 C values is not occurring in the same growth zone. This may be because the spots are not from the same part of the crystal due to space limitations as the spot sizes of the different methods (SIMS vs. LA-MC-ICP-MS) differ (Figure 5d). Other crystals from Forsmark mostly reflect only one growth zone and are thus more homogeneous.   (Figure 6b), in relation to significantly depleted δ 18 O values of around −20% and relatively heavy δ 13 C (~−5% ). There is also a more complex evolution of spikes in the δ 13 C values (from −9.8 to +1.6% ) related to a shift to higher 87 Sr/ 86 Sr (from 0.71390 ± 0.00015 to 0.71609 ± 0.00023) without significant change in δ 18 O (KLX10C:122, Figure 6e). In addition, there is increasing δ 18 O with growth (from −15.7 to −5.1% ) corresponding with increasing 87 Sr/ 86 Sr (from 0.71105 ± 0.00019 to 0.71457 ± 0.00048), but without a systematic change in δ 13 C (KLX14A:80 m, Figure 6f).

87 Sr/ 86 Sr in Calcite as a Hydrochemical Marker
The correspondence in 87 Sr/ 86 Sr values between the modern calcite in the Äspö borehole and the water it precipitated from proves the concept of using calcite as a marker for groundwater source and as a timing indicator, i.e., to verify whether fracture coating calcite theoretically can precipitate from modern or ancient groundwaters. This concept has been used frequently in paleohydrogeology studies in fractured rocks [16,[28][29][30][31][32], but the current study is to our knowledge the first to prove this concept using modern calcite precipitates in a deep granite fracture aquifer.
The higher 87 Sr/ 86 Sr values of the cores of the modern crystals compared to the outermost overgrowths (although overlapping in error) and to the groundwater suggest that a temporal variation in 87 Sr/ 86 Sr over the period from the installation of the borehole instrumentation in 1995, to 2012 when the crystals were sampled and the waters were analyzed. Unfortunately, no previous measurements of 87 Sr/ 86 Sr exist from the borehole sections. However, monitoring of the conservative tracers Cl and δ 18 O shows that Baltic seawater with lower Sr concentrations infiltrated the borehole over the course of the borehole lifetime [53]. Investigations in a nearby borehole (KA3110A, 5 m away) indicate that this inflow occurred around 1998-1999 and measured tritium values around 10 TU indicate that the portion of modern water during the recent years is large [46,73]. This suggests that inflow of Baltic seawater mixing end-member with lower 87 Sr/ 86 Sr values (~0.7094 [6]) than the pristine water (A:2: 0.71746 ± 0.00010 to 0.71756 ± 0.00015, and A:3: 0.71794 ± 0.00015 as determined by the calcite cores) has influenced the 87 Sr/ 86 Sr values of the waters after 1998-1999. Therefore, the inner parts of the crystals represent the pristine groundwater in the fracture system and the overgrowths precipitated from waters influenced by a modern Baltic seawater fraction. The groundwater 87 Sr/ 86 Sr values of the present-day modern water in the section and the small shift over time in the calcites show that ion-exchange along the flow path and mixing with a deeper more Sr rich water have almost completely erased the original Baltic seawater 87 Sr/ 86 Sr signature. Nevertheless, the temporal trend in the 87 Sr/ 86 Sr values of the calcite crystals suggests that small deviations in the evolution of 87 Sr/ 86 Sr within the crystals may be used as a marker for fluctuations in fluid origin in deep bedrock fractures.

Calcite-Water Comparisons in Water Conductive Fractures
The approach used for modern calcite overgrowths at Äspö HRL that showed good agreement with corresponding groundwater 87 Sr/ 86 Sr values is ideal for determining whether fracture-coating calcite may have precipitated from modern water in the fracture system. Previous investigations have generally ruled out modern calcite precipitation in these fracture systems [23,32,61]. However, these assumptions were made based on bulk calcite samples that, in contrast to in situ analyses, likely were not representative. This is because the 87 Sr/ 86 Sr values were, to a variable but unknown degree, a mix of several generations with different 87 Sr/ 86 Sr signatures. Panels a and b of Figure 7 show a dominance of lower 87 Sr/ 86 Sr values in calcite compared with the 87 Sr/ 86 Sr value of corresponding present-day fracture water. This suggests an ancient origin in general, in agreement with previous studies [32,61]. For relevant comparison, as shown for the modern calcite precipitates at Äspö, only the outermost calcite growth zone that is in contact with the current groundwater should be used to determine potential modern calcite precipitation from modern water. Panels c and d of Figure 7 show 87 Sr/ 86 Sr value in the outermost growth zone of calcite vs. 87 Sr/ 86 Sr value of corresponding fracture water. In these comparisons, the two areas, Forsmark and Laxemar-Äspö show different scenarios. The Forsmark calcites overall show significantly lower 87 Sr/ 86 Sr values in the youngest calcite growth zones compared to the corresponding groundwaters. This "disequilibrium" between the calcites and the present-day groundwaters can be explained by different scenarios [1,61]: (1) The calcites may be of greatly different ages from the water, with 87 Sr/ 86 Sr calcite values reflecting the temporal 87 Sr/ 86 Sr evolution in the host rock [7,16]. Radiogenic 87 Sr in deep groundwaters is continuously produced by the decay of 87 Rb (half life 4.9 × 1010a, [74,75]) through water rock interaction processes involving Rb-bearing minerals such as K feldspar and biotite [23,76]. The increase of the wall rock 87 Sr/ 86 Sr has been documented at the Swedish sites studied here, with the largest increase in the more felsic rocks at Forsmark and Götemar compared to the more monzodioritic rocks at Laxemar-Simpevarp-Äspö [23,32]. Calcite precipitated from these waters is expected to show 87 Sr/ 86 Sr values reflecting the wall rock [16].
(2) Past mixing events may have involved recharging waters (or ascending waters passing through a different wall rock) with a significantly different composition than those of local water-rock interactions. The variability of 87 Sr/ 86 Sr in the calcite would thus reflect temporal isotopic changes in the source waters due to changing hydrologic conditions. Calcites precipitated from such waters should not show uniformly increasing values with time, but rather show dips and spikes between different growth zones.
Based on the data and features presented, it can be concluded that scenario 1 is supported by the 87 Sr/ 86 Sr values in the transects within different crystal growth zones, going from low ratios to higher (Figures 5 and 6). This scenario is based on the assumption that parent waters are in isotopic equilibrium with the bulk rock, although this may not always be the case [16,18]. The scenario is also supported by the observation of high 87 Sr/ 86 Sr values of some Sr rich water samples at large depth. These waters feature high Br/Cl ratios, which indicate an influence on the 87 Sr/ 86 Sr values by prolonged water-rock interaction [76]. Based on these observations, it is proposed that the calcite formed in the Forsmark fractures at ancient fracturing events rather than from modern groundwater. Calcite formation is thus considered to have been small in the Holocene at Forsmark. The groundwaters in Laxemar-Äspö and Forsmark show a slight increase in 87 Sr/ 86 Sr values at large depth, whereas Forsmark also shows highly 87 Sr-enriched values at shallow depth (0.719-0.724) (Figure 7). However, the high 87 Sr/ 86 Sr values of relatively shallow groundwaters in Forsmark are noteworthy and have been proposed to reflect water of brackish marine type (Littorina) that infiltrated the fracture network during the Holocene and/or mixed groundwater types with a significant Littorina signature [76]. This supports the influence of scenario 2 as well but is not reflected in the calcite record due to insignificant calcite precipitation during Holocene transgressions. In addition, influence from past infiltration events of different waters (i.e., Scenario 2) can be indicated by dips and spikes within some crystals (Figures 5b  and 6a). These discrepancies are uncertain as they are within the analytical error.
For Laxemar-Äspö, the Sr isotope transects also show that a scenario 1 dominated process for the evolution of Sr isotope ratios is plausible. Thus, most calcite precipitation is ancient as most calcite analyses showed lower values than the groundwaters. However, in contrast to Forsmark, overlapping 87 Sr/ 86 Sr values of calcite and groundwater are common for the outermost overgrowths (Figure 7d).
A modern origin of calcite is thus possible, in accordance with matching O isotope values of calcites and waters [35], and may have been governed by higher transmissivities compared to Forsmark [77][78][79][80], allowing Holocene water infiltrations and mixing, increasing the calcite saturation indices [66,67].
The calcite growth zones in single-crystal grains within open fractures are proposed to reflect significantly different precipitation events. This provides the possibility to determine the fluid flow history in presently open water-conducting fractures in the studied areas, both in spatiotemporal and in physicochemical terms. The general "disequilibrium" suggests that most calcite is pre-Holocene in age or even Precambrian. Independent dating constraints are, however, required to confirm the age of the different 87 Sr/ 86 Sr calcite populations. There is an association between increasing O isotope signatures and 87 Sr/ 86 Sr values with time ( Figure 8). This is in agreement with previous observations in the areas [23,31,32,61]. Similar O isotope stratigraphy has been established for calcite in fractures from Finnish and Canadian crystalline bedrock sites of bulk vein samples, but Sr isotopes have generally not been systematically presented for these [30,[81][82][83][84]. From Sellafield, UK, Sr isotopes in calcite have also been used in addition to O isotopes to distinguish paleofluid origin [27], although high spatial resolution analysis was not applied. In New Zeeland, bulk Sr, C, and O isotope analysis has been used to constrain relative timing and sources of crustal fluids in an active orogen, e.g., by trying to correlate Sr isotopes in calcite of unknown age (and indicatively complex origin) with present-day Sr isotope values of different wall rocks [21]. For the extensively studied sites of the present study, there is a well-defined geochronological and geochemical framework that allows more advanced temporal constraints of the different calcite 87 Sr/ 86 Sr populations. In brief, according to previous studies, the evolution of the O isotope composition reflects precipitation from hydrothermal fluids in the Paleo-Mesoproterozoic with depleted δ 18 O values (~−27 to −18% PDB) and 87 Sr/ 86 Sr values (0.705-0.710), trending to successively higher values at later events [23,32]. In the Laxemar-Äspö area, in situ Rb-Sr geochronology of vein assemblages of calcite with 87 Sr/ 86 Sr values of 0.707-0.708 reveals formation ages of 1431.7 ± 3.9 Ma and 1432.3 ± 7.5 Ma [38]. These values overlap with or are slightly more evolved than the initial 87 Sr/ 86 Sr (0.7040-0.7081) of coeval granitoids in southern Sweden [85], but not as evolved as the present-day 87 Sr/ 86 Sr of the wall rock in the Laxemar-Äspö area that ranges from 0.713540 in monzodioritic parts, to 0.716009 in granodiorite [32]. At Forsmark, similar initial rock 87 Sr/ 86 Sr values as at Laxemar-Äspö are anticipated, but the present-day values are far more evolved (0.756525 [86]) owing to the generally more felsic composition. Slickenside assemblages at Forsmark with 87 Sr/ 86 Sr values of calcite at 0.709-0.710 have Rb/Sr ages of 1438 ± 7.5 Ma and 1527 ± 23 Ma [36], but this group is not detected in the calcites of the present study of water-conducting fractures. A later mineral assemblage with laumontite-calcite and adularia with slightly higher 87 Sr/ 86 Sr values (~0.710 ± 0.002) and with higher δ 18 O, detected at both sites show temporal relation to the 1.1-0.9 Ga Sveconorwegian orogeny both for 40 Ar/ 39 Ar adularia ages and in situ Rb/Sr adularia-calcite ages [36,39,60]. In the Paleozoic era, 450-350 Ma calcite-adularia-illite coatings have been dated with the 40 Ar/ 39 Ar adularia ages and in situ, Rb/Sr methods for a calcite group of δ 18 O with −15 to −9% and 87 Sr/ 86 Sr of~0.713-0.716 [34,36,39,60] and this group is one of the dominant ones in the present study (Figure 8). This and younger calcite populations show isotopic signatures of ancient microbial activity in the fracture system, including 13 C-enriched calcite that formed following methanogenesis (Figure 5a,b) as well as 13 C-depleted values (Figure 6b), formed during the decomposition of organic matter and/or anaerobic oxidation of methane [34]. In one sample the onset of a younger calcite group with higher δ 18 O (>−9% ) and 87 Sr/ 86 Sr of~0.7145-0.717 [23,32] has been dated with calcite U-Pb geochronology to the Jurassic period (173 ± 8 Ma) [34]. However, it may extend in age into modern times, at least for the Laxemar-Äspö area [35], as indicated in our new comparisons of the outermost calcite growth zones and groundwater (Figure 7d). We can thereby use the established knowledge from the above-cited paleohydorogeology and geochronology studies for our determined 87 Sr/ 86 Sr stratigraphy to roughly distinguish different fluid flow events temporally. Hence, we can establish how long time a fracture has been open to fluid flow.  [34]. As shown in Figures 5 and 6, the low 87 Sr/ 86 Sr values are from the interiors of the calcite crystals and increase towards the rims and therefore mark a temporal evolution, that in the present figure goes from lower left to upper right.
For the Forsmark area, based on the connection of the high spatial resolution Sr isotope value determinations to previously reported radiometric dating, we can assess that fluid flow in these fractures has been active at least since the Paleozoic (450-350 Ma), but that recent precipitation is not indicated (Figure 7c). The crystal morphologies with successive overgrowths in cavities that have not been completely sealed until present-day suggest that fluid flow has been theoretically possible since the crystal cores formed. The mineral formation has been intermittent with long periods of equilibrium, at stagnant conditions. For Laxemar-Äspö, the large span in 87 Sr/ 86 Sr values of the calcite crystals reflects fluid flow and precipitation over much longer time spans. Particularly, in sample KLX09:740 m, the crystal cores have values (0.705-0.707, Figures 6b and 7b) that point to the onset of calcite formation in this fracture already in the Mesoproterozoic (~1431.7 ± 3.9 Ma and 1432.3 ± 7.5 Ma [38]), whereas the younger overgrowths represent a precipitation event with Paleozoic signatures and outermost overgrowths that have overlapping values with the present-day groundwaters. Intermittent crystal growth may thus have occurred for almost 1.5 billion years in this partly open fracture. This shows that high spatial resolution Sr isotope determination within calcite crystals in open water-conducting fractures can reveal evidence of numerous ancient fluid flow events over geological timescales and that these events can be resolved temporally when connected to high spatial resolution geochronology. Consequently, bulk sample 87 Sr/ 86 Sr analysis of this kind of material should be used with caution, as mixed values of several generations are likely to be obtained, which may lead to false temporal interpretations, particularly when compared to modern groundwaters.

Conclusions
We applied high spatial resolution LA-MC-ICP-MS analysis of 87 Sr/ 86 Sr in calcite grains coating deep open fractures in granitoid rock in two areas in Sweden. We compared the calcite Sr isotope compositions with groundwater in the same, or a nearby fracture to determine whether the outermost growth zone of the calcite crystals may have precipitated from modern groundwater and to identify multiple older precipitation events. These precipitation events are tied to different time periods by correlation with previously reported radiometric ages and C and O isotope signatures. This shows that high spatial resolution Sr isotope analysis within single calcite crystals can be used to distinguish episodic fluid flow within fracture networks. This in situ analytical approach is recommended for this type of zoned mineral material, because bulk sample analysis would result in mixed values of several precipitation events. For one of the sites (Laxemar), intermittent precipitation dating back tõ 1.43 Ga is indicated in a currently open fracture. In general, calcite crystal interiors in such currently open fractures date back to the mid-Paleozoic era and have overgrowths that could be precipitated from present-day waters, at least in the Laxemar-Äspö area. In the Forsmark area, recent precipitation seems insignificant. The analysis of modern (<17 years) calcite and the waters they precipitated from proves the concept of inheritance of Sr isotope values from groundwater to calcite.