Rare earth elements in waters from the albitite-bearing granodiorites of Central Sardinia, Italy

Ž .Chemical Geology 182 2002 1–14www.elsevier.comrlocaterchemgeo

Rare earth elements in waters from the albitite-bearinggranodiorites of Central Sardinia, Italy

Riccardo Biddau, Rosa Cidu, Franco Frau)

Dipartimento di Scienze della Terra UniÕersita di Cagliari, Via Trentino 51, I-09127 Cagliari, Italy`

Accepted 21 February 2001

Abstract

With the aim of contributing to the knowledge of the geochemical behaviour and mobility of the rare earth elementŽ . Ž .REE in the natural water systems, the ground and surface waters of the Ottana–Orani area Central Sardinia, Italy weresampled. The study area consists of albititic bodies included in Hercynian granodiorites. The waters have pH in the range of

Ž .6.0–8.6, total dissolved solid TDS of between 0.1 and 0.6 grl, and major cation composition dominated by Ca and Na,whereas predominant anions are Cl andror HCO .3

The pH and the major-element composition of the waters are the factors affecting the concentration of REE in solution.The concentrations ofÝREEqY in the samples filtered at 0.4mm vary between 140 and 1600 ngrl, with La of between 14and 314 ngrl, and Yb of between-6 and 12 ngrl. A negative Ce anomaly, especially marked at high pH, is observed inthe groundwaters. The surface waters show lower REE concentrations, which are independent of pH, and negligible Ceanomaly.

Speciation calculations, carried out with the EQ3NR computer program, showed that the complexes with the CO2y3

ligand are the dominant REE species at pH in the range of 6.7–8.6. The REE3q ions dominate the speciation at pH-6.7Ž .and only in the light REE LREE .

The relative concentrations of REE in water roughly reflect those in the aquifer host rocks. However, whenconcentrations of REE in water are normalised relative to the parent rocks, a preferential fractionation of heavy REEŽ .HREE into the water phase can be observed, suggesting the greater mobility and stability of HREE in aqueous solution.q2002 Elsevier Science B.V. All rights reserved.

Keywords: Rare earth elements; Hydrogeochemistry; REE speciation; Sardinia, Italy

1. Introduction

Thanks to their properties and to their consistentŽ .chemical behaviour, rare earth elements REE are

considered important trace elements in the study and

) Corresponding author. Fax:q39-70282236.Ž .E-mail address: [email protected] F. Frau .

understanding of various geological processes, par-Žticularly in the petrogenesis of magmatic rocks e.g.,

.Wilson, 1989 and in the evolution of the crust–man-Ž .tle system McLennan, 1989; Condie, 1993 , based

on the supposition that these elements areAimmo-bileB during metamorphic processes. Though a num-ber of studies have provided contrasting testsŽ .Grauch, 1989 and references therein , it emergesthat in some conditions the REE are clearly soluble

0009-2541r02r$ - see front matterq2002 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 01 00272-8

( )R. Biddau et al.rChemical Geology 182 2002 1–142

and mobile, especially in the hydrothermal environ-Žment Alderton et al., 1980; Michard, 1989; Banks et

.al., 1994; Giere, 1996 , but also in weathering pro-´Žcesses Nesbitt, 1979; Ward, 1986; Braun et al.,

.1990, 1993; Mongelli, 1993; Gouveia et al., 1993 .A number of studies have been carried out on the

Žgeochemistry of the REE in oceanic waters Elder-field and Greaves, 1982; De Baar et al., 1988; Alibo

. Žand Nozaki, 1999 , river waters Goldstein and Ja-cobsen, 1988; Elderfield et al., 1990; Tricca et al.,

. Ž1999 and groundwaters Banner et al., 1989; Fee et.al., 1992; Johannesson et al., 1997 . The studies on

groundwaters in particular have pointed out the po-tential use of REE as natural tracers in the locationof aquifers, in the study of mixing processes, and inthe study of low-temperature water–rock interaction.Groundwaters may reflect some characteristics of thedistribution of REE in the rocks that the waters have

Žinteracted with Smedley, 1991; Gosselin et al., 1992;.Banks et al., 1999 .

However, the interpretation of the geochemicalbehaviour of the REE in the natural waters is com-plicated by various processes, such as the formationof aqueous complexes, colloidal transport, ion ex-change, and adsorption, which may cause fractiona-tion of these elements in the aqueous solutions ascompared to the parent rocks. The fractionation pro-cess is mainly due to differences in the relative

Ž 3q.affinity of the trivalent ions of the REE REEtowards surface adsorption and towards complexa-tion with the potential ligands in solution, as a resultof the gradual, systematic reduction of the ionic

Ž .radius from La to Lu lanthanide contraction , whichleads to the greater stability of the heavy rare earth

Ž .elements HREE in solution and to the preferentialŽ .adsorption of the light rare earth elements LREE

on the colloidal and particulate material suspended inŽthe waters Byrne and Kim, 1990; Erel and Stolper,

.1993 . Several models have shown the importance ofthe aqueous speciation of the REE in controlling

Žtheir concentration in the dissolved fraction Cantrelland Byrne, 1987; Millero, 1992; Lee and Byrne,

.1993 . These studies have dealt with complexationprocesses in oceanic waters, and indicate that theformation of more stable complexes of carbonateions with the HREE than with the LREE is one ofthe main factors responsible for HREE enrichmentobserved in REE patterns in the oceanic waters

Žnormalised to shales e.g., German and Elderfield,.1990 . It is considered that this behaviour also causes

the HREE enrichment in the shale-normalised pat-terns in the dissolved fraction of several river watersŽGoldstein and Jacobsen, 1988; Elderfield et al.,

.1990 .The aim of this study is to give a further contribu-

tion to the knowledge of the factors that control themobility of the REE in the ground and surfacewaters, with particular reference to the processes thatcause the fractionation of these elements duringlow-temperature water–rock interaction. To this pur-pose we chose a sampling area of Central SardiniaŽ .Italy recently involved in mining prospecting wherewidespread REE mineralization, though of poor eco-nomic interest, is associated with albitite inclusionsin granodiorite.

2. Description of the study area

2.1. Geological setting

The basement of Central Sardinia consists ofmagmatic and sedimentary sequences, tectonised andmetamorphosed during the Hercynian orogeny, andintruded by late- and post-tectonic Hercynian grani-

Ž .toids Bralia et al., 1982; Carmignani et al., 1992 .The geology of the investigated area is shownschematically in Fig. 1. The Palaeozoic basementconsists of Ordovician–Silurian metamorphic rocksof medium–low grade, such as metasandstone,

Žmetaarkose, schist, marble, and calc-schist Porcu,.1983; Carmignani et al., 1986 .

The Hercynian granitoids are the dominant rocksin the investigated area, and are mainly representedby a medium-grained equigranular granodioriteŽ .Ghezzo and Orsini, 1982 . The main primary miner-

Žalogical phases are quartz, plagioclase oligoclase–. Ž .andesine , K-feldspar microcline , and biotite; the

Žmain accessory minerals are allanite generally. Žmetamictic , titanite, and zircon Bornioli et al., 1997;

.Carcangiu et al., 1998 .Hydrothermal activity was a prevalent post-mag-

matic phenomenon in the area, and the involvedfluids were responsible for important mineralizationsto chlorite, talc and especially albite. The albitites,generally vein-shaped bodies, are widespread and

( )R. Biddau et al.rChemical Geology 182 2002 1–14 3

Ž .Fig. 1. Geological sketch map of the Ottana–Orani area Central Sardinia showing the location of the water sampling points.

mainly occur at the point where faults cross. Thegeological structure is represented by a late Hercy-nian fault system, characterised by a general NE–SWtrend, that is often reactivated by Alpine tectonics.

The main mineral phases in the albitite are albiteŽ .plagioclase up to 70% in modal proportion and

Ž .quartz; K-feldspar microcline is subordinate. Theaccessory minerals include allanite, epidote, chlorite,

Žtitanite, and rare muscovite Fiori et al., 1994;.Bornioli et al., 1997; Carcangiu et al., 1998 . The

albitite bodies are exploited for the ceramics industryin some quarries close to the villages of Ottana andOrani.

In Tertiary times, the Palaeozoic basement ofSardinia was affected by important tectonic activity,and the island was characterised by massive volcanic

Žactivity of an orogenic type Coulon, 1977; Becca-.luva et al., 1987 . In the Ottana–Orani area, volcan-

ism developed between the upper Oligocene and thelower Miocene, with deposition of a thick ignimbrite

Ž .tuffaceous sequence Porcu, 1983 . The Quaternarydeposits are located near Ottana, and are representedby alluvial sediments.

ŽThe hydrology is characterised by low-flowF10. Ž .lrs streams and springs flow range:-0.1–2 lrs .

ŽThe streams drain the granitoid rocks sample nos. 1,.9, 15, 18 ; the sample no. 21 only drains the meta-

morphic rocks outcropping near Orani and Sarule.The springs are mostly seasonal and linked to themain tectonic structures. Sample nos. 5, 8, and 13are perennial springs.

2.2. REE in the albitite and granodiorite of theOttana–Orani area

In order to investigate the extent of REE fraction-ation in the waters, the distribution of REE in thegranodiorite and albitite in the Ottana–Orani areawas derived from data already available.


Fig. 2. Chondrite-normalised REE patterns of the main rockformations in the Ottana–Orani area. Data from Bornioli et al.Ž . Ž .1997 and Carcangiu et al. 1998 . Chondrite REE values are

Ž .from Haskin et al. 1966 .

TheÝREEqY concentrations in the granodioriteŽvary between 200 and 260 ppm near Orani Carcangiu

.et al., 1998 , and between 170 and 410 ppm nearŽ .Ottana Bornioli et al., 1997 . The albitite facies

show a large variation of REE concentrations, andhave been divided into two groups: REE-enriched

Ž .albitite up to 1500 ppm ofÝREEqY and REE-de-Ž .pleted albitite 14 ppm-ÝREEqY-166 ppm .

The chondrite-normalised REE patterns of theseŽlithologies are plotted in Fig. 2 data from Bornioli et

Ž . Ž .al. 1997 and Carcangiu et al. 1998 .The REE-depleted albitites are strongly chlori-

tised, whereas the REE-rich albitites are epidote-bearing. Microprobe analyses showed that epidoteand titanite are the REE-bearing minerals in thealbitites. Epidote is generally LREE-enriched, rela-tive to HREE, whereas titanite is Y- and HREE-en-riched, relative to LREE; REE were not detected inthe plagioclase, K-feldspar, and chlorite. In the gran-odiorite, the only REE-bearing mineral is magmaticallanite; titanite does not contain detectable REEŽFiori and Grillo, 1991; Carcangiu et al., 1996, 1997,

.1998; Bornioli et al., 1997 .

3. Methods

3.1. Sampling and analytical methods

During the spring of 1999, 22 water samples werecollected in the Ottana–Orani area. Seventeen weresprings and five were streams.

The temperature, pH, alkalinity, conductivity, andŽ .redox potential Eh were measured in the field. The

Eh was measured by platinum electrode and valuesŽcorrected against the ZoBell’s solution Nordstrom,

.1977 . The water samples were filtered through 0.4-mm Nucleporew membrane filters with an all-plasticfiltration assembly, and collected into acid-cleanedpolyethylene bottles. At each sampling site, twofiltered 50-ml aliquots were collected and acidifiedto 1% vrv ultrapure HNO for cation analysis; an3

unacidified filtered 100-ml aliquot was also taken foranion analysis. For eight samples, a further unfiltered50-ml aliquot, acidified to 1% vrv ultrapure HNO ,3

was collected only for the analysis of the totalamount of REE.

Unacidified aliquots were analysed by ion chro-Ž w . ymatography HPLC Dionex DX-120 for Cl ,

SO2y, NOy, Bry and Fy. Acidified aliquots were4 3

analysed for major and trace elements by inductivelyŽcoupled plasma atomic emission spectrometry ICP-

.AES and inductively coupled plasma mass spec-Ž .trometry ICP-MS , respectively.

3.2. REE analysis

REE analysis was carried out on the acidifiedŽ . Žaliquots 22 filtered, 8 unfiltered by ICP-MS Perkin

w .Elmer SCIEX-ELAN5000 equipped with a cross-flow nebulizer. No pre-concentration or pre-treat-ment was needed prior to analysis. The operatingconditions were optimised in order to minimise po-

Ž .tential interference on REE Table 1 . Under theseconditions, the levels of oxides and doubly chargedions were CeOrCes0.02 and Ba2qrBas0.01, re-spectively.

Table 1ICP-MS operating conditions for the determination of REE and Y

Ž .in waters from the Ottana–Orani area Sardinia, Italy

Plasma r.f. power 1150 WPlasma flow 15 lrminNebulizer flow 0.95 lrminAuxiliary flow 0.9 lrminSample Uptake 1 mlrminResolution High: 0.60"0.1 amuSweepsrreading 12Readingrreplicate 1Point across peak 1Scanning mode Peak hop

()

R.B

iddauet

al.rC

hemicalG

eology182

20021

–14

5

Table 2Ž .Major chemical composition of waters from the Ottana–Orani area Sardinia, Italy

Ž . Ž . Ž .TDS: total dissolved solids; Alk: alkalinity reported as HCO . Ion balance error %s100= sum cationsysum anionsr sum cationsqsum anions3

Ž .Sample Flow T 8C pH Eh Cond. TDS Ca Mg Na K Alk Si Cl SO NO Br F Ion balance4 3Ž . Ž . Ž . Ž .lrs mV mSrcm error %Ž .No. Type mgrl

1 stream 10 8 8.2 451 0.39 260 31 15 45 1.3 119 2.6 82 26 2 0.2 0.2y1.02 spring 0.01 11 7.7 446 0.83 559 106 24 70 0.8 299 7.6 145 43 25 0.4 0.4 0.13 spring 0.005 14 7.0 458 0.73 492 62 31 69 2.1 214 11.2 100 43 79 0.4 0.4 1.84 spring 0.03 14 6.8 447 0.66 413 47 20 64 15.2 151 12.7 114 46 33 0.4 0.1 0.15 spring 0.03 15 6.4 473 0.80 473 36 20 72 1.2 106 10.8 130 37 27 0.4 0.2y0.36 spring 0.03 13 6.5 464 0.36 234 27 15 32 1.9 98 13.3 50 23 38 0.2 0.5y1.57 spring 0.05 13 6.4 403 0.44 276 34 15 42 0.9 115 14.0 76 21 31 0.2 0.5y1.48 spring 2 12 6.0 491 0.16 98 10 5 18 0.9 28 8.2 29 8 13 0.1 0.1y0.99 stream 4 8 6.7 385 0.17 116 10 5 23 1.1 42 8.5 41 9 5 0.1 0.1y1.0

10 spring 0.2 11 6.4 475 0.28 176 21 10 31 1.3 64 9.1 57 15 9 0.2 0.1 1.311 spring 0.2 14 6.7 471 0.65 401 55 25 54 2.7 199 10.5 100 33 33 0.3 0.4y0.912 spring 0.008 14 7.9 453 0.79 546 69 32 62 3.5 238 8.8 83 31 148 0.3 0.4y2.113 spring 0.01 24 7.1 475 0.59 306 28 17 57 1.8 129 13.0 97 22 18 0.3 0.2y2.414 spring 0.006 15 7.4 457 1.65 996 165 50 115 4.4 470 9.4 249 105 76 0.9 0.2y2.015 stream 3 15 8.6 448 0.44 246 29 14 44 1.6 116 4.6 78 21 2 0.2 0.3y0.916 spring 0.03 15 8.0 456 0.75 438 66 27 44 1.7 241 8.4 75 24 82 0.3 0.4y2.217 spring 0.03 14 6.8 457 0.60 357 44 25 46 2.9 168 9.6 80 28 48 0.3 0.4y0.118 stream 10 11 7.7 469 0.30 170 19 9 33 0.8 70 8.4 56 15 2 0.2 0.2 0.719 spring – 11 6.8 377 0.41 233 27 16 41 0.8 150 12.0 56 14 5 0.4 0.4 0.120 spring 0.1 12 6.9 474 0.73 452 35 32 72 2.9 96 10.4 157 64 44 0.6 0.1y3.421 stream 0.05 15 7.9 484 0.70 359 59 15 56 2.5 210 10.7 84 36 3 0.3 0.3 0.522 spring 0.3 16 6.9 536 0.22 106 15 4 18 1.2 38 3.9 37 9 2 0.1 0.1y0.8


The 89Y, 139La, 140Ce,141Pr, 142Nd, 152Sm,151Eu,153Eu, 158Gd, 159Tb, 164Dy, 165Ho, 166Er, 169Tm,174Yb, and 175Lu isotopes were chosen to minimise

Ž .isobaric overlap. Following Stetzenbach et al. 1994 ,both 151Eu and 153Eu isotopes were used to verifythe potential interference with Ba oxiderhydroxide

Ž135 16 136 16 1 137 16 .species Ba O, Ba O H, Ba O . In theanalysed waters, the average concentration differencebetween151Eu and153Eu was 12% for Ba concentra-

Ž .tions of -8 to 180mgrl mean value: 40mgrl .Therefore, no correction for oxide interference wascarried out.

No internal standard was used for instrumentaldrift correction, it being inefficient for low intensity

Ž .signals Denoyer, 1994 . The precision and accuracyvalues in the REE analysis were 15% for concentra-tions of 10 ngrl, while for concentrations greater orequal to 100 ngrl, precision was 5% and accuracy9%. The detection limit for each REE has beencalculated at the 5s standard deviation of the meanintensity measured in the blank solution at regularintervals over an analytical sequence. The ICP-MSinstrumental response may vary over different ana-lytical cycles, and the corresponding detection limitsare reported accordingly.

The concentrations of REE below detection limithave been disregarded in statistical and graphicalpresentations.

Table 3Ž .Concentrations of Y and REE in waters from the Ottana–Orani area Sardinia, Italy

ufsUnfiltered samples.

Ž .Sample Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ý REEqY

Ž .No. Type ngrl

1 stream 80 76 128 20 84 19 4 17-2 11 -8 20 -2 7 -6 4662 spring 111 32 16 10 43 16 6 17-2 11 -8 18 -2 -6 -6 3093 spring 52 26 16 7 27 36 20 7-2 -6 -8 14 -2 -6 -6 2054 spring 39 32 34 8 38 17 6 11-2 -6 10 15 -2 -6 -6 2115 spring 418 218 130 73 300 80 22 91 11 72-10 39 -7 29 8 14805 uf spring 511 284 203 92 387 107 29 112 29 83-10 46 11 36 9 19306 spring 325 314 107 66 300 54 4 67 5 37 10 34-2 20 7 13507 spring 129 94 51 19 88 16-2 23 -2 14 -8 21 -2 8 -6 4627 uf spring 136 123 91 29 110 24-2 23 -2 13 -8 21 -2 9 -6 5798 spring 195 56 28 19 91 30 4 29 2 26 -8 33 -2 23 8 5248 uf spring 214 69 38 22 106 29 4 37 2 36 9 37-2 25 8 6369 stream 64 35 74 12 46 14-2 13 -2 11 -8 18 -2 7 -6 29410 spring 94 74 32 21 95 26 6 24-2 11 -8 18 -2 7 -6 38511 spring 108 178 46 37 160 31 5 27-2 12 -8 15 -2 -6 -6 58912 spring 129 137 13 26 117 19 4 17-2 10 -8 18 -2 -6 -6 49013 spring 571 145 54 47 202 61 9 69 12 69 14 51 8 50 8 137213 uf spring 771 249 172 79 329 84 12 107 19 99 15 71 13 74 13 210714 spring 129 125 30 20 75 64 47 29-6 14 -6 -7 -5 -9 -6 53315 stream 78 92 196 20 77 19-9 31 -6 17 -6 -7 -5 -9 -6 53016 spring 201 146 12 25 98 12-9 30 -6 17 -6 -7 -5 -9 -6 54117 spring 130 289 -7 55 222 32 -9 40 -6 16 -6 -7 -5 -9 -6 78418 stream 65 78 123 17 59 17-9 29 -6 16 -6 -7 -5 -9 -6 40418 uf stream 128 165 339 39 169 37-9 51 -6 30 -6 -7 -5 -9 -6 95819 spring 45 32 27 5 27 8 -9 23 -6 10 -6 7 -5 -9 -6 18420 spring 129 44 -7 -5 18 -8 -9 20 -6 11 -6 -7 -5 -9 -6 22220 uf spring 132 45 7 6 22 -8 -9 21 -6 10 -6 -7 -5 -9 -6 24321 stream 124 76 92 17 78 25 10 25-6 14 -10 -7 -5 -9 -6 46121 uf stream 139 114 152 31 114 35 11 30-6 23 -10 -7 -5 -9 -6 64922 spring 12 14 12 12 15 14 9 13-7 8 -10 9 7 11 6 14222uf spring 21 19 17 14 20 17 10 14-7 8 -10 9 7 12 5 173


Ž .Fig. 3. Box-and-whisker plot of concentration ngrl ratios be-Ž . Ž .tween filtered lower boxes and unfiltered water boxes water

samples.

The speciation of REE was calculated by theŽEQ3NR computer program version 7.2c; Wolery,

.1992 , using the given database of thermodynamicconstants for the inorganic complexes of the REE,and using the Davies approximation to calculate theactivity coefficients. The measured Eh was assumedto represent the redox state in water.

4. Results and discussion

4.1. Chemical composition of waters

Table 2 reports the main physicochemical parame-ters and major element concentrations in the watersfrom the Ottana–Orani area. The pH values are

Žgenerally higher in streams mean value and standard. Ž .deviation: 7.8"0.6 than in springs 6.9"0.5 .

Spring nos. 12 and 16 have a pH of about 8.0, whichis probably related to the dissolution of carbonateminerals occurring in the alluvial sediments, wherespring waters flow out.

Ž . Ž .Fig. 4. Box-and-whisker plots for filtered water samples divided into springs a and streams b .


Ž .The total dissolved solid TDS content rangesfrom 0.1 to 0.6 grl, with the exception of sample

Ž .No. 14 TDS: 1 grl owing to a partial circulation inthe Tertiary volcanic rocks. Major cation composi-

Ž .tion of streams and springs except sample No. 14 isŽ . Ždominated by Ca up to 5.3 meqrl and Na up to

.3.1 meqrl , whereas predominant anions in waterŽ . Žare Cl up to 4.4 meqrl andror HCO up to 4.93. Ž .meqrl , with subordinate SO up to 1.0 meqrl .4

Based on the anionic composition, the following twoŽ .types of waters can be distinguished: 1 HCO -3

Ž .dominated and 2 Cl-dominated. The former corre-sponds to waters more closely related to the meta-morphic basement, whereas the latter includesstreams and springs with a prevalent circulation ingranitoid rocks. The higher concentrations of bicar-bonate in the former waters are related to the pres-ence of carbonate formations in the metamorphicbasement.

4.2. REE hydrogeochemistry

The REE concentrations of the Ottana–Orani wa-ters are reported in Table 3. A wide range of REEconcentrations has been observed in the filtered wa-

Ž .ters. The total valuesÝREEqY range from 140 to1600 ngrl, with La varying from 14 to 314 ngrl andYb from -6 to 12 ngrl. The springs generally havehigher REE concentrations than the more dilutestream waters. The unfiltered samples show higherconcentrations than the filtered ones, with largervariations for the LREE. These data are graphicallysummarised as box-and-whisker plots in Figs. 3 and4. Fig. 4 shows that La, Ce, and Nd among the REE,and Y are the most abundant elements and that theREE concentration decreases with increasing theatomic number. Analogous trends have been gener-

Žally observed in groundwater e.g., Banks et al.,.1999 .

The pH is known to be one of the main factorsŽcontrolling the REE concentration in solution Gold-

stein and Jacobsen, 1988; Elderfield et al., 1990;.Smedley, 1991; Banks et al., 1999 . In the spring

waters of the Ottana–Orani area the highest REEconcentrations often occur at the lowest pH values.This trend is more marked for the LREE than for the

Ž .HREE Fig. 5 . At a given pH, the Cl-dominatedspring waters generally exhibit lower REE concen-

Ž .Fig. 5. Plots of Ce, Nd and Dy concentrations ngrl vs. pH forwaters from the Ottana–Orani area. Legend:`sCl-dominatedspring waters,vsHCO -dominated spring waters, and=s3

stream waters.

trations than the HCO -dominated spring waters.3ŽAmong the Cl-dominated waters, spring nos. 5 pH

. Ž .s6.4 and 13 pHs7.1 stand out for their highŽREE concentrations these samples are the outliers in

.Fig. 4a . These springs may have experienced alonger underground circulation and therefore a longerwater–rock interaction, or may be from a largercatchment with greater probability of interaction withREE-enriched facies. Since the other perennial springŽ .sample no. 8, pHs6.0 is located further from the


Fig. 6. Ce) values vs. pH for all filtered water samples indicatingthe Ce anomaly.

mineralised area, it presents a low concentration ofÝREEqY.

Unlike the springs, the streams show a similarREE concentration with varying pH. The REE con-centrations in the streams are generally lower than inthe springs, but have relatively high concentrations

Ž .of Ce Fig. 5 . The behaviour of Ce in low-tempera-ture waters differs from the other trivalent REE,since it can be present in both the trivalent and the

Ž .tetravalent state Brookins, 1989 . Tetravalent Ce ishighly insoluble and hence, under oxidising condi-tions, dissolved Ce concentrations should be very

Ž .low. Precipitation of Ce as cerianite CeO or ad-2

sorption onto particulate material are considered themain processes causing Ce depletion in sea watersŽElderfield and Greaves, 1982; Sholkovitz and

. ŽSchneider, 1991 and filtered river waters Goldstein

.and Jacobsen, 1988; Sholkovitz, 1992 . Moreover,the behaviour of Ce is pH-related, Ce oxidation

Žbeing favoured at high pH Goldstein and Jacobsen,.1988; Elderfield et al., 1990 .

To investigate the behaviour of Ce in our waters,) Ž )the values of Ce , indicating the Ce anomaly Ce

Ž . w Ž . Ž . xs3 Ce r 2 La q Nd , where the subscriptASBS S S

refers to ratios normalised to the corresponding value.of shales, Elderfield et al., 1990 , have been plotted

Ž .vs. pH Fig. 6 . The plot shows a negative CeŽ ) .anomaly Ce -1 for all the springs, and the de-

crease of Ce) values at alkaline pH. In contrast, thestreams exhibit Ce)(1. These differences are notrelated to different redox conditions between streamsand springs, they being negligible, but are probably

Ždue to a greater load of particulate matter-0.4.mm in the stream waters than in the springs. In Fig.

7, on comparing a representative sample of thestreams and one of the springs, a major differencewas observed in the concentration of the REE be-tween the sample filtered at 0.4mm and the unfil-tered sample in the stream. In Fig. 7b, in particular,the Ce anomaly is absent in the REE pattern of theunfiltered sample, whereas only a slight negative Ceanomaly is present in the filtered one. This behaviouris similar to that observed in several river watersŽ .Goldstein and Jacobsen, 1988

4.3. REE aqueous speciation

Several studies have shown that free ions andcomplexes with carbonates generally dominate the

Ž .Fig. 7. Post-Archaean Australian average shales PAAS -normalised patterns for spring no. 8 and stream no. 18. Data for PAAS are takenŽ .from McLennan 1989 .


ŽFig. 8. Plot of the % La and Er species vs. pH filtered water. Žsamples . Only 14 samples are given for La and 10 for Er out of

.22 since samples with equal pH show similar speciation and,consequently, a mean value is considered for % La and Er species.

aqueous speciation of REE in groundwaters. ForŽ 3q.example, the free ions Eu were the main dis-

Žsolved species in groundwater 4.3-pH-7.2, mean.5.7 related to granite aquifers of Cornwall, while in

Žgroundwater related to adjacent metasediments 4.7.-pH-8.0, mean 6.6 besides the free ions, com-Ž q.plexes with carbonate EuCO and subordinately3

Ž q. Žsulphate EuSO become important Smedley,4. Ž .1991 . In the groundwaters pH between 7 and 9 of

Ž .Nevada and California, Johannesson et al. 1996found that carbonate complexes account for 99% ofthe species for each REE in solution, REECOq

3Ž .ybeing prevalent for the LREE, and REE CO be-3 2

ing increasingly important with increasing the atomicŽ ynumber. The complexes with other anions OH ,

y y 3y.F , Cl , PO are generally subordinate or negligi-4

ble, but may become important in particular condi-tions: e.g., the REECl2q complexes dominated theREE speciation in high salinity, chloride-rich springs

Ž .of Texas Gosselin et al., 1992 . Complexes withphosphate may be important in some polluted waters,while complexes with fluoride and sulphate may be

Ž .important in acid waters Wood, 1990 . Moreover,high concentrations of Al and F in acid watersŽ .3.3-pH-5.2 may cause aAcompetitive effectBof Al on the REE speciation due to the formation of

Žstrong complexes between Al and F Gimeno Ser-.rano et al., 2000 .

The results of the REE speciation in the watersfrom Ottana–Orani are in agreement with those ob-served in previous studies. Fig. 8 reports the percent-ages of the aqueous species of La and Er, representa-tive of LREE and HREE, respectively, as a functionof the pH in the waters of Ottana–Orani. The car-bonate complexes dominate the REE speciation atpH between 6.7 and 8.6, REECOq being prevalent3

Ž .yfor the LREE, and REE CO being increasingly3 2

important with increasing atomic number. The freeion REE3q is the main aqueous species for theLREE at pH-6.7. The REE3q ions, and the com-plexes with SO2y, Fy, and HCOy become negligi-4 3

Ž .ble -1% at pH)7.0.Ž .The calculated saturation indexes SI indicate

that the waters of Ottana–Orani are at equilibrium orŽoversaturated with respect to chalcedony 0.06-SI

.-0.69 , and the samples taken near the metamor-phic facies are also slightly oversaturated with re-

Ž .spect to calcite 0.36-SI-0.54 . All the samplesare saturated or oversaturated with respect to cerian-

Ž .ite 0.16-SI-7.77 , with increasing saturation in-Ž .dexes on increasing the pH of the waters Fig. 9 .

Ž .Fig. 9. Saturation index SI with respect to cerianite vs. pHŽ .filtered water samples .


4.4. Normalised-REE patterns in waters

In order to investigate the REE fractionation pro-cess during water–rock interaction, the REE concen-

trations in the springs and streams were normalisedto the median values of the REE concentrations inthe main rocks of the Ottana–Orani area, differenti-ating between granodiorite, REE-enriched albitite and

Ž . Ž .Fig. 10. REE concentration in some filtered springs and all stream samples normalised to granodiorite a , REE-enriched albitite b , andŽ . Ž . Ž . ŽREE-depleted albitite c from the Ottana–Orani area. REE data of parent rocks from Bornioli et al. 1997 and Carcangiu et al. 1998 . To

.keep the plots legible, only spring samples with the most complete REE sets are shown.


Ž .REE-depleted albitite Fig. 10 . The patterns nor-malised to granodiorite and to REE-enriched albitite

Ž .are HREE-enriched Fig. 10a and b , while the pat-Ž .terns normalised to REE-depleted albitite Fig. 10c

are relatively flat. However, rock-normalised pat-terns of springs and streams differ remarkably for Ceanomaly. The former show a strong negative Ceanomaly, indicating the Ce fractionation during wa-ter–rock interaction due to the preferential oxidationof Ce3q to the least soluble Ce4q, whereas in thelatter the Ce anomaly is negligible.

Ž .According to Banks et al. 1999 , HREE-enrichedpatterns in water can be explained by the following.

Ž . Ž .1 An unusual i.e., HREE-enriched REE distri-bution in the parent rock due to the presence ofspecific HREE-enriched mineral phases;

Ž . Ž .2 A normal i.e., HREE-depleted REE distribu-tion in the parent rock, combined with aAreal frac-tionationB due to water–mineral interaction, or anAapparent fractionationB due to preferential weather-ing of specific HREE-enriched mineral phases.

The REE patterns in the magmatic rocks of thestudy area suggest that the first hypothesis may beruled out. Moreover, the poor reactivity and lowabundance of the main HREE-enriched mineralŽ .titanite indicate that anAapparent fractionationB ofthe REE in the waters is not probable. The source ofthe REE in the waters of the Ottana–Orani area isrepresented mainly by the weathering of the LREE-

Ž .enriched epidote Fig. 11 . Therefore, the HREE-en-riched patterns of Fig. 10a and b may be interpreted

Fig. 11. Epidote-normalised REE patterns for some filtered springsamples from the Ottana–Orani area. Epidote data from Bornioli

Ž . Ž .et al. 1997 and Carcangiu et al. 1998 .

on the basis of the different mobility between theLREE and the HREE during chemical alteration.Several studies have shown in fact that progressiveweathering can fractionate the REE, with weatheredproducts generally enriched in LREE rather than

ŽHREE when compared to the parent rock Nesbitt,1979; Ward, 1986; Braun et al., 1990, 1993; Mon-

.gelli, 1993; Gouveia et al., 1993 . The preferentialaqueous transport of the HREE, which form strongeraqueous complexes than the LREE, would accountfor the observed patterns in Fig. 10a and b.

5. Conclusions

The factors that mostly affect the concentration ofthe REE in the groundwaters from the Ottana–Oraniarea are the pH and the major-element compositionof the waters. The highest concentrations are gener-ally found at the lowest pH values and for waters ofa HCO -dominated type. A negative Ce anomaly is3

also observed and is especially marked at high pH.The surface waters differ from the springs be-

cause they have lower REE concentrations, the nega-tive Ce anomaly is negligible, and the concentrationof the REE does not depend on pH. The lowerconcentrations are presumably due to a limited inter-action with the rocks, while the smaller Ce anomalyand lack of correlation between REE and pH areprobably related to a larger load of particulate mat-ter, which is likely to have an important contributionof adsorbed REE, especially LREE.

Speciation calculations have shown that the com-plexes with carbonate species dominate REE specia-tion in the waters of Ottana–Orani. For pH close to

Ž . qneutral between 6.5 and 7.5 , the REECO com-3Ž .yplex is prevalent, and for pH)7.5 the REE CO3 2

species is also important. The REE3q ions dominatethe speciation at pH-6.5, and only for the LREE.

The relative concentrations of REE in waterroughly reflect those in the aquifer host rocks. How-ever, when concentrations of REE in water are nor-malised relative to the parent rocks, a preferentialfractionation of HREE into the water phase can beobserved, suggesting the greater mobility and stabil-ity of HREE in aqueous solution.


Acknowledgements

The authors wish to thank the Editor and thereviewers, D. Banks and P.L. Smedley, for theiruseful suggestions in the improvement of themanuscript. This research was partially funded bythe Italian Ministry for University and Scientific and

Ž .Technological Research MURST, ex-40% funds .

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Rare earth elements in waters from the albitite-bearing granodiorites of Central Sardinia, Italy

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