Chapter 3 Experimental insights into chemistry of vein calcite · Chapter 3 Experimental insights...

Chapter 3

Experimental insights intochemistry of vein calcite

3.1 Introduction

Calcite is an ubiquitous vein-filling mineral in many rock types. Commonly, otherchemical elements substitute for calcium when calcite precipitates from solution.The concentration of these elements will reflect a variety of environmental factors inthe fluid from which the calcite precipitated. These factors include the concentrationof substituting elements in solution, temperature, pH, other aspects of fluid compo-sition (e.g. concentrations of other species including Na, CO2−

3 ), and oxidation state.In addition, dynamics at the fluid-mineral interface related to the relatives rates oftrace element incorporation into calcite may cause variations in calcite composition(e.g. sector zoning). The precipitation of calcite from aqueous solutions is controlledby the equilibrium:

Ca2+ + 2HCO−3 CaCO3 + CO2 + H2O (3.1)

In this thesis, trace elements, and both stable and radiogenic isotope ratios areused to infer changes in fluid chemistry and fluid-rock interaction during vein for-mation. Two different approaches are taken in this study to characterise chemicalvariations in the Taemas Vein Swarm. The first examines trace element concen-tration changes within individual veins at high spatial resolution ( ∼ 100 µm), toexamine how fluid chemistry evolved over time during the growth of a single vein.The second approach examines chemical variations over an outcrop, and throughoutthe TVS, to understand fluid-rock reaction on a larger scale (metre to kilometre).

In the following sections, the use of trace elements to make inferences about fluidchemistry changes are discussed. Fundamentals of rare earth element (REE) in-corporation into calcite are outlined. In addition, the results of several key paperswhich examine compositional zoning in calcite are examined. Finally, the approach,methods and results of some preliminary experiments examining the incorporationof trace and REEs into calcite, and the influence of solution chemistry on crystalmorphology are discussed.

27

28 3. Experimental insights into chemistry of vein calcite

3.2 Trace element incorporation in calcite

Calcite commonly contains appreciable concentrations of Mg, Mn and Fe, and lesseramounts of Sr and Ba (Deer et al., 1962; Mucci and Morse, 1990; Deer et al., 1992).The Ca site in the calcite structure (sixfold coordination) may be filled by variouscations (e.g. Mn2+, Fe2+, Sr2+ or Mg2+). The major factors controlling trace elementpartitioning between crystal and liquid are the size of the cation site, and the ionicradius of the substituting element (Garrels and Christ, 1982; Mucci and Morse, 1990;Zhong and Mucci, 1995).

Many studies have examined the incorporation of Mg2+ and Sr2+ into calcite (seeMucci and Morse, 1983; Wasylenki et al., 2005, for reviews), and found a strongtemperature control on the incorporation of both Mg and Sr into calcite. In gen-eral, higher precipitation rates lead to higher concentrations of both Sr and Mg incalcite (Mucci and Morse, 1990). In general, calcite will incorporate more Fe2+ andMn2+ at higher temperatures, at slower precipitation rates, and in solutions withlower aFe2+/aCa2+ and aMn2+/aCa2+ . Hence, it is possible that Mg, Sr, Mn and Feconcentration changes in vein calcite could be used to make inferences about temper-ature variations and/or changes in precipitation rate during vein growth. However,valid inferences about temperature or precipitation rate changes during vein growthcould only be made if trace element/Ca ratios remained constant in the fluid. Al-ternatively, if there was evidence that temperature remained constant during veingrowth, then the trace element composition of a vein could be interpreted in termsof changing solution composition.

3.2.1 Rare earth element incorporation into calcite

The following section introduces the rare earth elements (REEs), and outlines spe-cific information on the incorporation of REEs into calcite. This will provide a basisfor interpreting variations in REE concentrations and patterns, which may be in-fluenced by: (1) changes in absolute REE concentrations in solution, (2) changes influid oxidation state and (3) changes in the types and concentrations of complexingspecies in the fluid (e.g. CO2−

3 ).

The rare earth elements (REEs; atomic numbers 57–71) are a series of elementswith the same number and type of valence electrons in their outermost electronshell. The REEs usually occur in nature as trivalent ions, and have similar chemicalbehaviour due to their electronic configuration. Commonly, Y, Sc and Th are treatedas part of the REEs, due to their similar properties. The REEs are subdividedinto the light (La-Eu, atomic numbers 57–63, and Th) and heavy (Gd-Lu, atomicnumbers 64–71, Y and Sc) REEs. The size of the REE trivalent ions decreasessmoothly with increasing atomic number. This systematic change is reflected inthe gradual and systematic variation in the chemical properties of the REE withincreasing atomic number.

The systematics between the REEs makes them extremely useful for understandinga variety of chemical processes in the geological environment (Henderson, 1984;Taylor and McLennan, 1988). Cerium and Eu may adopt different oxidation statesin aqueous solutions. Under reducing conditions, Eu may assume a divalent state

3.2. Trace element incorporation in calcite 29

(e.g. Eu2+), while Ce may exist as a highly insoluble tetravalent (e.g. Ce4+) ion inoxidising environments. This leads to anomalies in REE normalised patterns, due tothe change in ionic radius accompanying the change in oxidation state. In addition,Yb may become divalent in higher temperature, mildly reducing conditions (Wood,1990b; Bau and Moller, 1992).

In this study, REE concentrations were normalised to the chondrite values ofMcDonough and Sun (1995), and a chondrite normalised value is designated REECN.The deviation of normalised Ce and Eu values from those anticipated in a mineral isknown as a Ce or Eu anomaly (Towell et al., 1965). This is expressed as Ce/Ce*, orEu/Eu*, where Ce* and Eu* are the predicted chondrite normalised values, basedon the concentrations of the neighbouring REE concentrations, where:

Ce/Ce* =Ce√

LaCNxPrCN

(3.2)

and

Eu/Eu* =Eu√

SmCNxGdCN

(3.3)

Anomalies exhibited by Eu and Ce may be used to extract information concerningthe redox and pH conditions of geological environments (Brookins, 1983; 1989). Innatural aqueous systems, the following reactions are thought to be common (Bauand Moller, 1992):

2Eu3+ + 2OH− = 2Eu2+ + H2O + 0.5O2 (3.4)

and2Ce3+ + 0.5O2 + 3H2O = 2CeO2 + 6H+ (3.5)

In hydrothermal fluids, REE fractionation is a function of (a) sorption and desorp-tion of REEs during migration of fluids along particle surfaces and (b) coprecipita-tion (Bau and Moller, 1992). Carbonate and OH− ligands form stronger complexeswith HREEs than LREEs. In carbonate-dominated hydrothermal solutions (such asthose which would be expected in limestones), calcite precipitating from solutionswith relatively low CO2−

3 concentrations will have flatter chondrite normalised REEpatterns than calcite precipitating from a solution with higher CO2−

3 concentrations,which will be relatively enriched in the LREE (Fig. 3.1 Bau and Moller, 1992). Thisis because in CO2−

3 poor solutions there will be little difference in the complexing ofthe light and heavy REEs.

Wood (1990a) reviewed available low-temperature (25C) data for inorganic com-plexes and inorganic speciation of REE in natural waters. Available data suggeststhat fluoride complexes will dominate at acidic to neutral pH, with carbonate com-plexes becoming dominant at neutral to alkaline pH.

Previous experimental studies (Terakado and Masuda, 1988; Zhong and Mucci,1995) determined that partition coefficients for REEs into calcite are greater thanone, with La partition coefficient estimates ranging between 10 and 4000 (the largediscrepancy is probably due to different experimental approaches). Lakshtanov andStipp (2004) suggested that concentrations of REEs in calcite are primarily con-trolled by the availability of REEs in solution. The study of Zhong and Mucci


log(

RE

E/C

a)

La Sm Tb Lu

CO32- poor, HREE rich,

less negative slope

CO32- rich, LREE rich,

more negative slope

Figure 3.1: Schematic diagram (after Bau and Moller, 1992) showing REE patternsin calcite expected for CO2−

3 poor and CO2−3 rich solutions

(1995) found that REE incorporation in calcite varied mainly as a function of the[REE]:[Ca2+] and absolute REE concentration (particularly for the LREEs), andwas not significantly influenced by calcite precipitation rate. Chondrite normalisedREE patterns in calcite are expected to be relatively enriched in the LREEs, dueto the similarity in size of the LREE ions to Ca2+, with the smaller ionic radiusHREEs being more difficult to incorporate into calcite.

A extended x-ray absorption fine-structure spectroscopy (EXAFS) study by Elzingaet al. (2002) investigated the incorporation of Nd3+, Sm3+, Dy3+ and Yb3+ into cal-cite during controlled growth experiments. Their results confirm substitution ofREEs in the Ca site. In addition, they suggest that the LREEs have sevenfoldoxygen coordination in calcite, while the HREEs have sixfold oxygen coordinationin calcite. Elzinga et al. (2002) suggest that this difference reflects changes in themechanism of charge compensation during the substitution of trivalent REEs fordivalent Ca in the calcite structure.

It has been suggested that REE incorporation into calcite occurs via a coupledsubstitution with Na (Zhong and Mucci, 1995):

REE3+aq + Na+

aq + 2Ca2+s = REE3+

s + Na+s + 2Ca2+

aq (3.6)

However, a recent study of Eu3+ incorporation into calcite via a series of free driftcalcite precipitation experiments by Stipp et al. (2003) found no correlation betweenNa+ concentrations in solution and Eu concentrations in calcite precipitated fromthose solutions. Experiments carried out by Lakshtanov and Stipp (2004) also foundno correlation between Eu and Na concentrations in calcite precipitated from so-lution. Instead, Lakshtanov and Stipp (2004) suggested that substitution of REEsinto calcite occurs via a vacancy substitution mechanism:

2Eu3+aq + 3Ca2+

s = 2Eu3+s + x + 3Ca2+

aq (3.7)

where x represents a crystal lattice vacancy.

3.3. Compositional zoning 31

3.3 Compositional zoning

A component of this thesis explores changes in calcite chemistry (trace and rareearth element concentrations; C, O and Sr isotopic ratios) during progressive veingrowth. This approach uses observations of vein microstructures (e.g. mineral fibres,crack-seal textures, crustiform growth banding) to make inferences about the growthhistory of veins. High-spatial resolution chemical analyses are then carried out onthese veins to trace changes in calcite chemistry during progressive vein growth.This approach requires:

1. The vein growth mechanism to be well constrained i.e. the relative timing ofcalcite precipitation is known.

2. An understanding of the variation in trace element and isotopic compositionsof calcite precipitated at different locations at the same time (i.e. compositionalzoning).

In the following section, results of experimental and theoretical work on compo-sitional zoning in calcite is explored. The purpose of this summary is to examinewhat significance may be placed on variations in calcite mineral chemistry in naturalsamples.

Concentric zoning of trace elements in natural calcite cements has been widelyreported, and is commonly interpreted to be caused by fluctuations in bulk fluidcomposition (e.g. Wogelius et al., 1997). However, this interpretation fails to explainthe lack of correlation between zoning patterns for neighbouring crystals in the samesample (Mason, 1987; Wang and Merino, 1992). Various experimental crystal growthstudies (e.g. Reeder and Grams, 1987; Paquette and Reeder, 1990; 1995; Prieto et al.,1997) have demonstrated that compositional zoning may be produced from solutionswhich have approximately the same chemistry throughout crystal growth.

In a series of experiments, Paquette and Reeder (1990; 1995) explored factorsinfluencing the incorporation of Mg, Mn and Sr into synthetic calcite crystals. Theynoted both compositional sectoral zoning (i.e. compositional differences betweentime-equivalent sectors on different crystallographic faces) and intrasectoral zoning(i.e. compositional differences between time-equivalent portions of a given sectorgrown from a single crystal face). Synthesised crystals showed two rhombohedralforms, 1014 (parallel to the cleavage), and 0012. Both forms have differentaffinities for Sr and Mn, and produce sectoral zoning, while only the 1014 formdisplays intrasectoral zoning. In one example, adjacent compositional zones (acrossdifferent crystal faces) had variations in Sr and Mn concentrations on the order of500–1000 ppm (Figure 6 of Paquette and Reeder, 1995).

In a natural example of fine-scale chemical variation, Wogelius et al. (1997) investi-gated trace element and isotopic zonation in vein calcite. They noted that significant(several thousand ppm), fine-scale (1–100 µm) chemical zonations occurred in Fe,Mn, Pb, Zn and Sr. Fine-scale trace element variations could be caused by mul-tiple supersaturation-nucleation-depletion (SND) disequilibrium cycles. These areexplained in detail by Ortoleva et al. (1987). Briefly, precipitation of a mineralleads to consumption of elements which form that mineral. This inhibits precipi-tation of that mineral in the solution immediately adjacent to the growth surface.


This leads to fine-scale variability in trace element concentrations, even from fluidsin ‘equilibrium’ with the precipitating mineral.

Alternative models for varying uptake of trace elements have been presented byvarious workers. Paquette and Reeder (1990) suggested that trace-element zoningwas caused by preferential adsorption of different aqueous species at different growthsteps. Wang and Merino (1992) presented an alternative hypothesis, and suggestedthat growth-induced buildup of H+ ions at the growth surface makes the surfacemore positive, preventing positive cations from attaching to growth sites, causingaccelerated growth and additional H+ buildup, leading to a positive feedback effect.

In more recent theoretical work, Watson (1996; 2004) presented a conceptualmodel, ‘growth entrapment’, to explain sectoral zoning. Watson suggests that thegrowing surface of a crystal (the ‘near-surface layer’) will have an atomic structure,chemical composition and atom mobility that differs significantly from the ‘interior’of the bulk crystal. The ‘surface composition’, which may be relatively enriched ordepleted in a certain trace element compared to the bulk crystal, may be ‘captured’as crystal growth proceeds. If trace elements are relatively enriched in the near-surface layer compared to the bulk crystal, then the surface enrichment factor, F,will be greater than 1. If trace elements are relatively depleted in the near-surfacelayer compared to the bulk crystal, then F will be less than 1 (Fig. 3.2). The de-gree to which the surface composition will be captured is dependent on the crystalgrowth rate, and the mobility of atoms in the near-surface layer. It is suggested thatvariations in the character of surface layers between adjacent crystal faces, and/orthe rate of growth of different crystal faces lead to sector zoning.

Watson (2004) applied his numerical model to the experimental data of Lorens(1981) and Tesoriero and Pankow (1996), and found the model to be consistent withexperimental data on trace element uptake during calcite precipitation. In additionto trace element sector zoning, Watson (2004) suggested that significant (per milscale) fractionation of oxygen isotopes over sectoral zones may occur. Onasch andVennemann (1995) reported oxygen isotope values from a single 20 mm long quartzcrystal from a cavity, and found that δ18O values changed by up to 2.6 h in time-equivalent growth zones, and similar results were reported for calcite by Dickson(1991).

The experimental evidence outlined above demonstrates that spatial variations intrace element concentrations within calcite veins may be unrelated to changes inbulk fluid composition. Hence, little importance can be placed on chemical changesin calcite veins which have no textural constraints on the relative timing of calciteprecipitation. However, if the vein growth history can be determined, then variationsin vein chemistry may be interpreted more robustly.

The model of Watson (2004) raises the intriguing possibility that compositionalzoning may provide semiquantitative information about the precipitation rate ofmaterial in calcite veins. Calcite which precipitated at faster rates would be morelikely to preserve significant compositional zoning, because variations in the near-surface region of the crystal would be captured. Contrastingly, calcite precipitatedmore slowly would have less prevalent compositional zoning, because there is moretime for elements to diffuse through the near-surface layer and reach equilibriumwith the crystal lattice. In a recent paper, Gabitov and Watson (2006) suggest

3.4. Experimental approach 33

Growth directionn

Calcite

Fluid

Con

cent

ratio

n

Distance

Flu

idF

luid

Con

cent

ratio

n

Sub

stra

te C

ryst

alS

ubst

rate

Cry

stal

New Growth

New Growth

Con

cent

ratio

n

Near-surface layer

Near-surface layer

100%

50%

0%Coi

Coix F

F>1

F<1

Distance

Distance

(a)

(b)

(c)

Coi(d)

N-s layer

Flu

id

Growth surface

Figure 3.2: Schematic sketch of the growth entrapment model after Watson (2004).(a) Calcite crystal growing in a fluid (near-surface layer, labeled N-s layer) with foreignions (e.g. Sr, orange dots) which are being incorporated into the calcite lattice. (b)Concentration profile of foreign ions (e.g. Sr) being incorporated into calcite lattice.(c) and (d) concentration profile of ions being incorporated into growing crystal lattice(with an initial concentration, Coi) at positive (c) and negative (d) surface enrichmentfactors (F), with different trapping efficiencies (0 %, 50 % and 100 %).

that crystal growth rates slower than 0.5 mm/yr are required to reach equilibriumpartitioning of Sr in calcite at 25C. Presumably, if crystal growth rates are fasterthan 0.5 mm/yr, then fine-scale variation in Sr concentrations will be captured ingrowing calcite crystals. At higher temperatures, equilibrium might be achieved onshorter time-scales, due to faster diffusion rates.

3.4 Experimental approach

3.4.1 Introduction and approach

The cause of fine-scale variations in trace element concentrations in vein calcite isuncertain, and two end-member hypotheses are presented:


1. Changes in bulk fluid chemistry cause variations in vein calcite composition(extrinsic control).

2. Changes in the rate and/or style of calcite precipitation and crystal growthcause changes in the concentrations of trace elements incorporated into calcite,with bulk fluid compositions remaining constant during calcite precipitation.

To assess the influence of these two processes, calcite crystals were grown fromREE doped solutions. These simple, qualitative experiments were conducted todetermine:

1. The influence that sector zoning has on the concentrations of REEs in calcite(i.e. do different areas of a crystal formed at the same times have differentREE concentrations?).

2. Whether REEs show fine-scale oscillatory zoning in crystals grown from solu-tions of approximately constant composition.

3. The style (if any) of REE fractionation (e.g. LREE enrichment or depletion)between different crystal surfaces.

4. Whether Eu or Ce anomalies are generated during calcite precipitation (insolutions which have constant oxidation state).

5. Changes in REE patterns over time with progressive crystal growth.

3.4.2 Methods

Crystals were grown following the method of Gruzensky (1967), where solid am-monium carbonate sublimates, producing NH3 and CO2 gases, which diffuse intoan aqueous solution of calcium and ammonium chloride. This causes calcite super-saturation and precipitation. Approximately 850 mL of MilliQ water ( >18 MΩresistance) was placed into four 1L LDPE Nalgene-Nunc brand bottles. Into this,sufficient CaCl2 and NH4Cl (analytical reagent grade, 99.9 % pure) was added tocreate a solution with known Ca2+ concentrations of ∼ 0.02 mol L−1 and ∼ 0.17mol L−1. Into these solutions, a mixed REE spike (La, Ce, Nd, Sm, Eu, Gd, Dy,Er, Yb, Lu), stabilised in HNO3, was added to create solutions with known concen-trations of approximately 10 parts per billion (ppb) and 100 ppb REEs (see Table3.1).

Initial solution pH was low, as a result of the acidic REE spike. Starting pH inthe 100 ppb solutions was 1.9–2, while initial pH in the 10 ppb solutions was 5 and6.3. Bottles were immersed in a temperature-controlled water bath, at a constanttemperature of 30C. Over several days, solution pH rose to values exceeding 7,and stabilised at values of 7.4–7.8. Calcite precipitation began within a few days ascarbon dioxide diffused into the solution. Solutions were allowed to rest, and wereopened and inspected every 2 weeks to monitor calcite precipitation.

Calcite was recovered from solution by placing the bottles in an ultrasonic bath,and the calcite crystals were recovered by passing the solutions through filter pa-per. Crystal morphology was examined using light and scanning electron microscopy


Table 3.1: Starting compositions of major ion and REE concentrations in differentsolutions

Chemical Species Solution 1 Solution 2 Solution 3 Solution 4Ca2+ mol L−1 0.168 0.170 0.016 0.017NH+

4 mol L−1 0.663 0.609 0.482 0.427Cl− mol L−1 0.998 0.949 0.515 0.461

La (ppb) 94 9.4 97 9.6Ce (ppb) 100 10.0 103 10.2Nd (ppb) 93 9.4 96 9.6Sm (ppb) 94 9.5 97 9.6Eu (ppb) 104 10.4 107 10.6Gd (ppb) 99 9.9 102 10.1Dy (ppb) 94 9.4 97 9.6Er (ppb) 91 9.2 94 9.3Yb (ppb) 92 9.2 95 9.4Lu (ppb) 89 8.9 92 9.1

(SEM). Recovered crystals were mounted in epoxy and polished to expose the coreof crystals. Polished crystals were examined using cathodoluminesence (CL) to de-termine the growth history, sector zoning and oscillatory zoning within crystals. Thedistribution and concentrations of Mg, Mn, Sr, Fe and REEs were determined bylaser ablation, inductively coupled plasma mass spectrometry (LA-ICP-MS). Sam-ples were analysed using a pulsed ArF Excimer laser (λ=193 nm) and a quadrupoleICP-MS (Agilent 7500s). The sample was moved at a steady speed ( ∼ 30 µm s−1)beneath the laser beam, facilitating in situ, high spatial resolution, continuous datacollection (Eggins et al., 1998). Samples were precleaned with ethanol, and the areachosen for analysis was ‘laser cleaned’ by a laser ablation pre-scan. Two laser linescans (‘traverses’) were carried out along one polished fibre. In addition, multiplespot analyses were made on selected crystals.

Multiple major and trace elements (24Mg, 43Ca, 44Ca, 45Sc, 49Mn, 57Fe, 85Rb,88Sr, 89Y, 138Ba, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 158Gd, 163Dy, 166Er, 174Yb,208Pb, 232Th, 238U) were simultaneously profiled during laser sampling by repeated,rapid sequential peak hopping, with a mass spectrometer cycle time of 0.65 s. Datareduction followed established protocols for time-resolved analysis (Longerich et al.,1996). High spatial resolution was achieved by using 40 µm spots at 10 laser pulsesper second with a moderately low laser fluence (5 J cm−2), and by minimising meanparticulate residence times in the ablation cell volume following each laser pulse(Eggins et al., 1998). Internal standardisation is carried out using 43Ca.

During initial data reduction, data is binned into 1 second intervals, giving aspatial resolution of 40 µm. Rare earth element concentrations were normalised tothe starting REE concentrations in each solution.


3.4.3 Results

Sample description

Three of the four solutions grew a mixture of fine-grained crystal aggregates, andstubby to equant calcite crystals. In the other solution (Solution 3, Table 3.1),acicular calcite crystals grew. These crystals generally grew in clusters, and radiatedfrom a central point. The growth of calcite (rather than the calcium carbonatepolymorphs, aragonite or vaterite) was confirmed by X-ray diffraction analysis.

Crystals grown from solution 1 (high Ca, high REE) show a ‘tulip’ structure, witha narrow, ‘stalk’ base and bulbous terminus (Fig. 3.3. Small, calcite crystals arepresent at the base of the larger crystal (0.5–1 mm long), where the crystal wasattached to the bottle wall. Beyond these, a single narrow ‘stalk’ of calcite growsin a columnar crystal habit. Calcite crystals then thicken, and develop a euhedralcrystal habit, terminating in well-developed crystal faces. In the region of euhedralcrystal form, prominent steps of up to ∼20 µm are observed in crystal faces, whichare not present on the basal ‘stalk’ (Fig. 3.4). Longitudinal CL images (i.e. lookingat surfaces cut parallel to the long axis of the crystal) reveal bright, patchy coresat the base of the crystal. This patchy core changes into subtle, fine, feather-likestructures. Dull luminesence occurs at the outer extremities of these crystals, withdark non-luminescent calcite at the crystal boundaries. Latitudinal images reveal (a)bright cores with oscillatory zoning surrounding these cores and (b) sector zoning.Different oscillatory zones are around 20 µm in width.

Crystals grown from the high Ca, low REE solution (solution 2, having the lowestREE/Ca) have equant to stubby crystal habits, and grow as large individual crystals(up to ∼ 1 mm across) and as clusters of smaller crystals. Steps are not as promi-nently developed as on crystals in solution 1 or 4 (Fig. 3.5). CL imaging revealsthat these crystals have oscillatory zoning, as well as prominent sector zoning.

Crystals grown from the low Ca, high REE solution (solution 3, highest REE/Ca)have a markedly different (acicular) morphology to the other crystals. Crystals areup to several mm long, and less than 0.5 mm wide (Fig. 3.6). Scanning electronmicroscope images reveal that the needles are formed from a network of bladedsubcrystals with a scalenohedral morphology (Fig. 3.6), which have a ‘fern leaf’shape, with the ‘leaves’ oriented up the long axis of the needle. Each individual‘leaf’ is approximately 100–200 µm long (Fig. 3.7). Examination of the needles inthin section reveals that each needle is composed of one continuous calcite crystal.

Crystals grown from the low Ca, low REE solution (solution 4) are generallysmaller than those grown in solution 2 (most crystals < 0.5 mm). Crystals areequant to blocky, and rhombohedra also occur which have prominent steps developedon some faces (Fig. 3.8). Some crystals display prominent oscillatory zones, whichare around 20 µm thick.

Analytical results

The results of two laser ablation traverses along a polished needle (grown in solution3) are shown in Figure 3.9. Strontium and Mg were not intentionally added intothe REE spike, and are present due to contamination in the CaCl2 powder from


200 µm 200 µm

200 µm 200 µm

Figure 3.3: Scanning electron microscope photomicrographs of elongate euhedralcrystals grown from high Ca, high REE solution (solution 1). Note elongate bases tocrystals, with crystal tops defined by well-developed rhombohedral faces.

which solutions were prepared. Magnesium concentrations average 35 parts permillion (ppm) in one traverse, and 50 ppm in the other traverse. Significantly, thecorrelation of Mg concentrations between traverses is poor (r2=0.09). Strontiumconcentrations lie between 20 and 40 ppm, and are well correlated between traverses(in contrast to Mg; r2=0.60). Rare earth element concentrations are high (tens tohundreds of ppm). Lanthanum and Yb concentrations (Fig. 3.9), illustrate thatsignificant concentration variations occur over distances of < 0.5 mm.

Europium (Eu/Eu*) and cerium (Ce/Ce*) anomalies across the calcite needle areshown in Figure 3.9. Values for Eu/Eu* are slightly different between traverses,with average values of 0.85 and 0.95, although average Eu/Eu* values are not verydifferent at the 2σ level between the adjacent traverses (0.84 ±0.1 and 0.95±0.05).Average values for Ce/Ce* are identical for the two traverses.

Spot analyses to determine the degree of REE variation within individual crystalsare shown in Figures 3.10, 3.11 and 3.12. Results are shown for crystals grownin solutions 1 and 2. Plotted on these figures are the locations of LA-ICP-MSanalyses and results, which are presented as normalised REE plots (normalised tothe starting solution concentrations of REEs). The CL images described above wereused to guide the location and interpretation of LA-ICP-MS analyses.

Figure 3.10 shows REE concentrations along an elongate calcite crystal which


100 µm 100 µm

20 µm100 µm

Figure 3.4: Enlargements of crystals shown in Figure 3.3 showing (top photomi-crographs) the transition from columnar calcite to euhedral calcite with prominentgrowth steps and (bottom photomicrographs) the base of calcite crystals (solution 1).

grew in solution 1, from the base (where growth began) to the crystal termina-tion. Figure 3.10 shows the outline of a longitudinal slice taken down the longaxis of a crystal (similar to those shown in Figure 3.3), the location of LA-ICP-MSanalyses, and the results of these analyses. At the base of the crystal, rare earthelement concentrations are high (with MREE depletion) and decrease toward thetop of the crystal. Toward the top of the crystal (in the region of euhedral crystalgrowth), REE patterns become flatter, and REE concentrations decrease. Calcitewith brighter luminescence is coincident with high REE and Sr concentrations, andlow Mn concentrations.

Figure 3.11 shows zoning and REE patterns from analyses of latitudinal slices cutthrough the top of elongate crystals from solution 1. REE concentrations are highestat the core of crystals and generally become lower toward the outside of the crys-tals. This is consistent with preferential uptake of REE from solution, and gradualdrawdown of REE concentrations in the solution. Notably, REE concentrations ondifferent crystal faces at the same time (determined by following oscillatory zonesbetween crystal faces) have considerably different REE concentrations. For example,analyses ‘10’ and ‘19’ are at apparently time-equivalent regions on opposite sides ofthe central core of crystal ‘a’ in Figure 3.11. Concentrations vary by almost an orderof magnitude between these two analyses. Similar variability in REE concentrations


500 µm 200 µm

200 µm 200 µm

Figure 3.5: Scanning electron microscope photomicrographs of stubby and equantcrystals with well-developed rhombohedral faces grown from high Ca, low REE solu-tion (solution 2).

can be demonstrated for other time-equivalent regions on different crystal faces (Fig.3.11).

Figure 3.12 shows REE concentrations through a rhombohedral calcite crystal(grown from solution 2). Rare earth element concentrations are highest near thebase of the crystal, and decrease away from this central point toward the edge of thecrystal. Growth is inferred to have begun at the point where CL intensity is highestat the crystal base (labelled ‘α’ in Figure 3.12). Oscillatory and sector zoning isobserved within this crystal. Adjacent sectors (zone 1, analyses 15–18 and zone 2,analyses 19-20) in Figure 3.12 have different concentrations at equivalent growthstages. It is noted that trace element variation within crystal sectors is even largerthan intersectoral concentration changes between different crystal sectors.

3.4.4 Discussion

In general: (1) calcite inferred to have precipitated at the start of crystal growth hashigher REE concentrations, and MREE depleted patterns, (2) calcite precipitatedtoward the end of crystal growth has lower REE concentrations and flat to LREEenriched patterns and (3) oscillatory and sector zoning is developed within calcitecrystals. This set of preliminary experiments reveals new information on REE zon-


1.0 mm 1.0 mm

1.0 mm 500 µm

Figure 3.6: Scanning electron microscope photomicrographs of acicular calcite crys-tals grown from the low Ca, high REE solution (solution 3).

ing within calcite. The analyses outlined above suggest that partition coefficients(Csolid/Cliquid) between fluid and calcite for REEs are on the order of 102 to 103,and are broadly compatible with the partition coefficients determined for REEs byZhong and Mucci (1995). However, REE patterns observed (MREE depleted, bothLREE and HREE enriched patterns) are considerably different from patterns ob-served by Zhong and Mucci (1995), which may be related to different experimentalconditions. The cause(s) for varying REE patterns is unknown, but may be relatedto changes in REE speciation (perhaps linked to changing the pCO2 when lids wereremoved from solutions to inspect growing crystals).

The growth of calcite needles is serendipitous, as it allows the change in calcitechemistry over a more substantial time to be examined than in the rhombohedralcrystals. Several points of interest are raised by these results:

1. Trace element concentrations can vary by an order of magnitude over distancesof ca. 200 µm in the calcite needles.

2. No systematic variations in Eu/Eu* or Ce/Ce* ratios were noted during thegrowth of the calcite needles.

3. Calcite needles have consistently HREE enriched patterns. Regions withhigher REE concentrations are more enriched in HREE than regions with


200 µm 200 µm

100 µm100 µm

Figure 3.7: Scanning electron microscope photomicrographs showing enlargementsof the acicular calcite crystals grown in solution 3.

lower REE concentrations (which have flatter REE patterns).

The fine-scale variations in REE concentrations observed in these synthetic calciteneedles are similar (in distance and concentration change) to the variations in traceelement concentrations determined in natural vein carbonates by Wogelius et al.(1997) and Barker et al. (2006). Notable is that trace element concentrations canvary by up to an order of magnitude, while the bulk solution composition remainsapproximately constant. Furthermore, trace element concentrations fluctuate bothupwards and downwards with time. This is a very important observation, as itsuggests that trace element concentrations alone cannot be used to make inferencesabout extrinsic chemical changes in the fluid from which calcite precipitated.

Calcite near the edges of crystals generally had low REE concentrations, whichis inferred to be due to drawdown of REEs from the fluid reservoir. Due to theinadvertent discarding of the growth solutions, REE concentrations were not mea-sured at the end of the growth experiments, making this impossible to verify. Asdescribed above, sector zones are developed in some crystals, with differing traceelement concentrations between sector zones. However, REE patterns appear to re-main similar in different sector zones, and Eu or Ce anomalies were not developed.Wood (1990b) demonstrated that at low-temperatures (25C), trivalent REE specieswill dominate. Thus, in the experiments described in this study, the generation of


500 µm 50 µm

200 µm 200 µm

Figure 3.8: Scanning electron microscope photomicrographs of stubby, equant andrhombohedral calcite crystals grown from the low Ca, low REE solution (solution 4).

REE anomalies due to changes in oxidation state would not be anticipated. Thisis borne out as no significant Eu or Ce anomalies were noted in REE patterns. Inaddition, Eu/Eu* and Ce/Ce* show no systematic variation throughout the calciteneedles (Fig. 3.9). Hence, changes in REE patterns and/or Eu and Ce anomaliesin natural calcite appear to be more robust indicators of variations in extrinsic fluidchemistry. In particular, changes in patterns or anomalies may be most confidentlyapplied to crystals and/or veins where a growth history may be determined.

Different trace elements show contrasting behaviour in adjacent traverses alongthe same calcite needle (Fig. 3.9). As described above, Sr concentrations correlatewell between traverses on the same crystal, while Mg concentrations are poorlycorrelated. This is similar to the behaviour for different trace elements in vein calcitenoted by Barker et al. (2006), which implies that there is a fundamental differencein behaviour between different trace elements during their uptake into the calcitecrystal lattice. The lack of correlation between changes in Mg and Sr concentrationsrules out the inhibition mechanism of Wang and Merino (1992), which should affectall divalent cations in a similar manner.

The growth entrapment model developed in papers by Watson (1996; 2004) couldexplain both the variable behaviour of different trace elements, as well as the sub-millimetre variations in trace-element concentrations. It is suggested that localisedsupersaturation of CaCO3 leads to enhanced calcite precipitation, and more rapid


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Figure 3.9: Results of from 2 parallel laser ablation ICP-MS line scans (separatedby approximately 100 µm) along a polished acicular calcite needle from solution 3.Magnesium, Sr, La, and Yb concentrations are shown, as well as Eu/Eu* and Ce/Ce*ratios. Note significant concentration variations over scales of < 0.5 mm. Spatialresolution is ∼ 40 µm.


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Figure 3.10: (a) Cathodoluminescence photomicrograph of an elongate calcite crys-tal grown in Experiment 1. Red circles are the location of individual laser ablationICP-MS analyses. Numbers refer to the analysis numbers shown in (b) and (c). (b)Graphs showing REE, Sr, Mn and Fe concentrations across the upper part of the crys-tal. Numbers refer to the location of individual analyses. (c) Graphs showing REE,Sr, Mn and Fe concentrations along the length of the crystal (analyses connected withred line on (a)). Note that REE concentrations gradually decrease toward the top ofthe crystal.


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Figure 3.11: Cathodoluminescence photomicrograph of latitudinal sections throughthe top of an three elongate calcite crystals from Experiment 1. Red circles are thelocation of individual laser ablation ICP-MS analyses. Numbers refer to the analysisnumbers shown in the surrounding graphs. The graphs (from crystal ‘a’ and crystal‘b’) compare normalised REE concentrations from different crystal faces at equivalentgrowth stages (determined by following oscillatory zones between crystal faces).

calcite growth, trapping whatever elements are in the near-surface region of thecrystal. Variations in growth rate in different regions at the same time will lead tosector zoning between different crystal faces.

Watson (2004) used the data of Lorens (1981) and Tesoriero and Pankow (1996)to estimate partition coefficients at different calcite growth rates. Strontium and Bashow higher partition coefficients at higher growth rates, while Mn shows lower par-tition coefficients at higher growth rates. In the calcite needles, Sr and Mn concen-trations show a weak negative correlation (r2 = −0.30) and Sr and Ba concentrationsshow a weak positive correlation (r2 = +0.43), consistent with the incorporation ofthese trace elements being somewhat controlled by crystal growth rate.

It is interesting to note that REEs La through Eu show a consistently flat patternin the calcite needles (Experiment 3), while Gd through Lu show variable enrichment(Fig. 3.13). This could be linked to the change from sevenfold to sixfold coordinationsuggested by Elzinga et al. (2002). Alternatively, it could be caused by some changein REE speciation in solution. The lack of monitoring of solution chemistry duringcalcite precipitation makes it impossible to speculate further on this.

An important result from these experiments is the variation in crystal morphol-ogy in solutions with different compositions. Among the four solutions, those withhigher REE concentrations grew elongate and needle-like crystals with scalenohe-


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Figure 3.12: (a) Cathodoluminescence photomicrograph of a rhombohedral calcitecrystal from Experiment 2. Note oscillatory and sector zoning within crystal. Redcircles are the location of individual laser ablation ICP-MS analyses. Numbers referto the analysis numbers shown on the graphs. (b) Graphs showing REE, Sr, Mnand Fe concentrations approximately parallel to the growth direction of the crystalfrom Traverse 1. (c) Graphs showing REE, Sr, Mn and Fe across the crystal froma traverse cutting across several sector zones. Analyses 15–18 and 19–23 cross twodifferent sector zones and reveal that different crystal faces growing at approximatelythe same time have different REE concentrations.


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Figure 3.13: Rare earth element concentrations (normalised to starting solutionREE concentrations) over 20 different regions of 30 µm width along an acicular cal-cite crystal, showing the variation from low concentrations, with relatively flat REEpatterns to high concentrations, with HREE enriched patterns.

dral crystal faces, while solutions with lower REE concentrations grew equant tostubby crystals with rhombohedral and prism faces. It is likely that the crystalmorphology is controlled by the calcite supersaturation state and [REE3+]/[Ca2+]ratio. Pastero et al. (2004) carried out a series of calcite growth experiments fromsolutions doped with Li, and found that crystal morphology varied as a functionof calcite supersaturation state and [Li+]/[Ca2+]. Pastero et al. (2004) found thatincreasing Li concentrations caused the crystal habit to become increasingly platy.

Crystal morphology in hydrothermal veins (influencing vein texture) is commonlyinterpreted in terms of variable rates of vein opening and mineral precipitation(Oliver and Bons, 2001). More specifically (see §6.2), fibrous vein textures havebeen interpreted as only forming in a very narrow vein aperture (Hilgers and Urai,2002). However, this series of experiments demonstrates that acicular crystals, whichbroadly resemble the high length:width crystals observed in fibrous veins, may growin an open, fluid-filled space. Hence, fibrous crystals may grow from a variety of ma-terials, by several different mechanisms (see also Taber, 1918; Means and Li, 2001).This suggests that vein textures may be critically dependent on which crystal habitsdominate. Thus, vein texture may be influenced by the solution composition andmineral supersaturation state in solution, as well as fracture-opening and mineralprecipitation dynamics.

3.4.5 Future work

This review and preliminary experimental work reveals that sector zoning and os-cillatory zoning is developed in synthetic calcite crystals. Separate growth sectorshave different trace element concentrations at equivalent growth times. Markedfluctuations in trace element chemistry occur during growth, even though the con-centrations of trace elements in solutions was likely constant, or changed only slowly


in response to depletion of trace elements during calcite precipitation. In addition,solution chemistry has a significant influence on crystal morphology.

During these preliminary experiments, pH was measured daily for the first 7 daysof growth. Ideally, pH would be monitored continuously, allowing changes in carbon-ate speciation to be estimated. Coupled with measurements of Ca2+ concentrations,this would allow the supersaturation state of calcite to be monitored constantly.Furthermore, periodic measurement of REE concentrations in the solutions duringcalcite growth would allow drawdown of REE due to calcite precipitation to be mon-itored, and allow REE speciation and potential supersaturation of REE-carbonatephases to be estimated.

The growth entrapment model of Watson (2004), and a recently published paperby Gabitov and Watson (2006) draw attention to the critical dependence of traceelement partitioning on crystal growth rate. Ideally, growing crystals would bemonitored under a light microscope with a calibrated scale, allowing growth ratesand crystal morphology to be monitored via time-lapse photography. This opticalmonitoring could be accompanied with periodic spikes of a trace element, or, ideally,enriched isotope spike, to allow precise time ‘markers’ to be identified in crystalswhich are subsequently analysed. This would enable close integration of solutionand crystal chemistry.

3.5 Conclusions

The results of several previous studies which have examined the incorporation oftrace and rare earth elements into calcite have been summarised. These studiesdemonstrate multiple controls on trace element partitioning between mineral andfluid, including temperature, growth rate, complexing species and oxidation state.Furthermore, significant variations in trace element concentrations occur in time-equivalent growth zones within calcite crystals (‘sector zoning’).

Moderate sized ( < 1 − 5 mm) synthetic calcite crystals were successfully grownfrom solutions with variable [Ca2+] and variable [REE3+]/[Ca2+]. Needle-like andelongate crystals grew from solutions with higher concentrations of REE, and calcitecrystals whose forms were dominated by rhombohedral and prism faces grew fromsolutions with low REE concentrations. Changes in crystal morphology apparentlyare linked to variable calcite supersaturation state and [REE3+]/[Ca2+]. The influ-ence of solution chemistry on crystal morphology has important implications for theinterpretation of hydrothermal vein textures.

High-spatial resolution LA-ICP-MS analyses of synthetic calcite crystals demon-strate significant fluctuations in Mg, Fe, Sr and REE concentrations over distancesof < 200 µm in inferred growth directions. Changes in concentrations of Sr, Mn andBa relative to one another are consistent with the incorporation of these elementsbeing somewhat controlled by crystal growth rate. Rare earth element concentra-tions decrease from crystal cores to rims, indicating that REEs were progressivelytaken up from solution (leading to depleted REE concentrations in solution). In ad-dition, time-equivalent regions on different crystal faces have significantly differentREE concentrations.

3.5. Conclusions 49

These analyses indicate that such fluctuations in trace element concentrationsin natural carbonates cannot be interpreted in terms of extrinsic changes in fluidchemistry alone. However, REE patterns and Eu/Eu* and Ce/Ce* anomalies donot appear to be significantly influenced by compositional zoning, and may be arobust indicator of changes in bulk solution chemistry or oxidation state.

Further experimental work, with integrated monitoring of crystal growth and so-lution chemistry is needed to constrain variables such as REE speciation, potentialsupersaturation of REE carbonate phases, and links between changes in crystal habitand solution chemistry.

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