A geochemical investigation of trace elements in well RN-17 at Reykjanes geothermal system, SW- Iceland

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1 IOP Conference Series: Materials Science and Engineering A geochemical investigation of trace elements in well RN-17 at Reykjanes geothermal system, SW- Iceland To cite this article: L P Ottolini et al 2012 IOP Conf. Ser.: Mater. Sci. Eng Related content - An investigation of trace and isotope light elements in mineral phases from well RN- 17 (Reykjanes Peninsula, SW Iceland) N Raffone, L P Ottolini, S Tonarini et al. - The role of SIMS in the investigation of the complex crystal chemistry of mica minerals L P Ottolini, E Schingaro, F Scordari et al. - Trace analysis in EPMA M J Jercinovic, M L Williams, J Allaz et al. View the article online for updates and enhancements. Recent citations - Geochemical bias in drill cutting samples versus drill core samples returned from the Reykjanes Geothermal System, Iceland Andrew P.G. Fowler and Robert A. Zierenberg - The concept of the Iceland deep drilling project G.Ó. Friðleifsson et al This content was downloaded from IP address on 11/07/2018 at 01:58

2 A geochemical investigation of trace elements in well RN-17 at Reykjanes geothermal system, SW-Iceland L P Ottolini 1,6, N Raffone 2, G Ó Fridleifsson 3, S Tonarini 4, M D Orazio 4,5 and G Gianelli 4 1 Consiglio Nazionale delle Ricerche (CNR), Istituto di Geoscienze e Georisorse (IGG), Unità di Pavia, via A. Ferrata 1, IT Pavia, Italy 2 Università di Pavia, Dipartimento di Scienze della Terra e dell Ambiente, via A. Ferrata, 1, IT Pavia, Italy 3 HS Orka hf, Brekkustigur 36, IS-260 Reykjanesbaer, Iceland 4 Consiglio Nazionale delle Ricerche (CNR), Istituto di Geoscienze e Georisorse (IGG), Unità di Pisa, via G. Moruzzi 1, IT Pisa, Italy 5 Università di Pisa, Dipartimento di Scienze della Terra, via S. Maria 53, IT Pisa, Italy ottolini@crystal.unipv.it Abstract. Here we present the results of an investigation of well RN-17 at Reykjanes geothermal system (Reykjanes Peninsula, SW-Iceland). We show that the adoption of micro-spot secondary ion mass spectrometry (SIMS) together with bulk-rock techniques, such as X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), and thermal ionisation mass spectrometry (TIMS) carried out on 10 basaltic drill-cuttings over a depth of 3000 m gives us insights into the geochemical characteristics of the well. Whole rock and mineral phases, i.e., plagioclase, clinopyroxene, epidote and amphibole, indicate that the rocks pertaining to the cuttings were affected by hydrothermal alteration that has been able to modify significantly the Sr isotope ratios and the concentration of K, Rb, Cs, Ba and B. Seawater/rock ratio, evaluated for 87 Sr/ 86 Sr, achieved its maximum for the shallow cutting RN , decreasing although not systematically with depth, with the minimum value for the deep cutting RN Moreover, the occurrence of Na-rich plagioclase, epidote and amphibole down to the well bottom are evidence of the persistence of fluid/rock interaction at depth. 1. Introduction Hydrothermal processes in the oceanic crust produce supercritical fluids that form a major pathway connecting the earth s mantle with the oceans and the atmosphere. These fluids thus have a profound impact on the dynamics of mid-oceanic-ridges and global geochemical cycles [1]. The hydrothermal processes may be investigated by: i) direct sampling of fluids on and off the ridge axis; ii) petrological studies of rocks from deep ocean drilling and ophiolite exposures [2, 3]. In this context, the position of Iceland as a sub-aerial sector of the Mid-Atlantic Ridge, with a wealth of geothermal activity, makes it a particularly favourable location for these studies since the repeated seismicity and volcanic activity within this active rift create high permeability and high temperatures at drillable depths. 6 To whom any correspondence should be addressed. Published under licence by IOP Publishing Ltd 1

3 Significant geothermal activity occurs along the Reykjanes Peninsula with specific sites controlled by the complex local tectonics and the recent and ongoing volcanic activity. The Reykjanes field is associated with the southernmost of five volcanic fissure swarms, which are located within the active tholeiite volcanic zone of the peninsula [4-7]. The stratigraphy of the Reykjanes field is divided into two formations, the uppermost 1000 m of the thermal area is characterized by hyaloclastite tuff, breccias, tuffaceous and marine sediments; the deeper part is dominated by submarine basaltic pillow lavas and intrusive rocks [8]. The rock permeability is the key physical parameter controlling the mechanisms of water/rock interaction and the evolution of submarine alteration. Permeability results from the combination of primary volcanic features (explosivity, brecciation, porosity), fracturing upon thermal contraction of the basaltic rocks due to cooling, and subsequent tectonic fracturing and faulting in the seismically active dilation zone of the plate boundary. Recent studies have shown that the intensity of alteration and secondary mineralisation is directly related to the texture of the protolith [9, 10]. In well RN-17 a new high-temperature amphibole zone was defined below 2400 m depth, transitional into amphibolite-grade alteration, and 87 Sr/ 86 Sr ratios within alteration minerals were observed to significantly shift towards seawater values with increasing depth, and, therefore, confirming deep penetration of seawater into the Reykjanes system [9, 10]. The peak hydrothermal alteration at shallow depth occurred during late Pleistocene, when an ice-sheet covered the peninsula. In late glacial time, hydrostatic pressures (P) and temperatures (T) were elevated at shallow depth resulting in higher temperature alteration than current P-T conditions allow [8, 11-13]. Fresh water fluid inclusions observed in quartz [8] were interpreted to imply that meteoric water had dominated the geothermal system in Pleistocene time, confirmed by isotopic constraints on ice age fluids by [14]. In the present work we have used bulk-rock (XRF, ICP-MS, and TIMS) analyses and SIMS investigations on 10 drill cuttings (depth from 400 to 3000 m) from the well RN-17. The possibility to measure at a small scale the concentration of trace elements with high precision and accuracy, minimum material consumption, limited sample preparation and virtually null contamination is a formidable tool offered by SIMS in the study of mineral phases of the well RN-17, i.e., plagioclase, clinopyroxene, epidote and amphibole. They were separated from the 10 drill cuttings because no core was available. For comparative purposes, we have included some additional samples: a surface lava (GIOV-2), lacking any sign of hydrothermal alteration, collected at the top of well RN-17, and a core-sample (2246 m dolerite dyke) from well RN-19, which is located 1.1 km NE of well RN-17 [13]. The dolerite was selected since it was the only core sample available from Reykjanes at the time this research was carried out. The results of the present investigation on the geochemical characteristics of the well RN-17 follow those obtained previously by SIMS on light (Li, Be and B) and volatile elements (F, Cl) in the most abundant mineral phases of the same drill cuttings of the well [15, 16]. As a result of this research, SIMS appears a fundamental complement to bulk-rock techniques in the study of complex geological areas. 2. Analytical methods for trace and isotope analysis An overview of all analyzed rock samples is shown in table 1. All samples are of basaltic composition and olivine-free except the dolerite intrusion from RN-19. As seen in the table 1, half of the samples are of submarine extrusive origin (tuffs, pillow lavas or breccias and erosion products of the same origin); the other half of the samples are intrusive dykes, ranging in granularity from fine-grained to doleritic. Mineralogical assemblage consists of magmatic (plagioclase, clinopyroxene and opaque) and hydrothermal phases (amphibole, quartz, epidote, calcite and clay minerals). Plagioclase is the most abundant phenocryst, but additionally clinopyroxene is also found as phenocryst in the coarser grained samples below 1100 m depth. Epidote always occurs from 650 m down to 3000 m. The sample list and information summary about the analytical techniques used, the mineral phases analyzed for the different cuttings are shown in table 2. As said before, since the rock thin sections were not available for this study except for the dolerite dyke from well RN-19, fragments of cuttings were embedded in epoxy-resin mounts, and analyzed by EPMA for major/minor elements. Mineralogical observations made by [17] on drill 2

4 Table 1. Analyzed samples, rock types and rock origin for the wells RN-17 and RN-19 at Reykjanes geothermal field. Sample Rock type Origin RN Marine sediment Eroded hyaloclastite RN Basaltic tuff Submarine tuff RN Basaltic pillows Submarine pillow basalt RN Basaltic pillows Submarine pillow basalt RN Basaltic intrusion fine-medium grained Dyke RN Basaltic intrusion coarse grained Dyke RN Basaltic intrusion coarse grained Dyke RN Dolerite intrusion Dyke RN Basaltic intrusion coarse grained Dyke RN Pillow basalt Submarine pillow basalt RN (core) Dolerite intrusion Dyke or sill Table 2. Sample list and information summary for the cuttings from well RN-17 (Reykjanes geothermal system, Reykjanes Peninsula, SW-Iceland). Sample Drill-cuttings Rock type Analytical methods Analyzed minerals 87 Sr/ 86 Sr RN Basalt XRF, EPMA, ICP-MS, TIMS, SIMS Plg, Cpx WR WR RN Basalt XRF, EPMA, ICP-MS, TIMS, SIMS Plg, Cpx, Ep WR WR RN Basalt XRF, EPMA, ICP-MS, TIMS, SIMS Plg, Cpx, Ep WR WR RN Basalt XRF, EPMA, ICP-MS, TIMS, SIMS Plg, Cpx, Ep RN Basalt XRF, EPMA, ICP-MS, TIMS, SIMS Plg, Cpx, Ep, Amph WR, Plg, Cpx, Ep RN Basalt XRF, EPMA, ICP-MS, TIMS, SIMS Plg, Cpx, Ep WR WR RN Basalt XRF, EPMA, ICP-MS, TIMS, SIMS Plg, Cpx, Ep WR WR RN Basalt XRF, EPMA, ICP-MS, SIMS, TIMS Plg, Cpx, Ep, WR, Plg, Amph Cpx, Ep WR RN Basalt XRF, EPMA, ICP-MS, TIMS, SIMS Plg, Cpx, Ep, Amph WR WR RN Basalt XRF, EPMA, ICP-MS, TIMS, SIMS Plg, Cpx, Ep, Amph WR, Plg, Cpx WR Unaltered lava RN-17-GIOV-2 Basalt XRF, EPMA, ICP-MS, TIMS, SIMS Plg, Cpx WR, Plg, Cpx WR Core-sample RN Dolerite XRF, EPMA, ICP-MS, TIMS, SIMS Plg, Cpx, Amph WR 143 Nd/ 144 Nd WR WR WR, Plg, Cpx WR RN = Reykjanes; XRF = X-ray fluorescence; EPMA = electron probe microanalysis; ICP-MS = inductively coupled plasma mass spectrometry; TIMS = thermal ionisation mass spectrometry; SIMS = secondary ion mass spectrometry; WR = whole rock; Plg = plagioclase; Cpx = clinopyroxene; Ep = epidote; Amph = amphibole. 3

5 cuttings of wells RN-9, RN-10 and RN-17 showed that they typically range in size from <0.5 to 2 mm at most (and generally decrease in size with increasing drilling depth), thus precluding detailed evaluation of paragenetic sequences of hydrothermal minerals, characterisation of coexisting phase relations and analysis of the time sequence of mineral filled cross cutting fracture systems (vein). Our samples were ran by Cameca SX-50 electron microprobe (CNR-IGG, Padova) and Jeol 8200 SuperProbe (Dipartimento di Scienze della Terra, Università di Milano). The crystals were analyzed with a beam current of 15 na and acceleration voltage of 15 kv, using the following standards: albite (Na); orthoclase (K); wollastonite or diopside for plagioclase analysis (Si and Ca); MnTiO 3 (Mn and Ti); Al 2 O 3 (Al); Fe 2 O 3 (Fe 3+ ); Cr 2 O 3 (Cr); and MgO (Mg) for the Cameca SX-50 microprobe; omphacite (Na); orthoclase (K); wollastonite (Si and Ca); rodonite (Mn); ilmenite (Ti); anorthite (Al); Fe 2 O 3 (Fe 3+ ); pure chromium (Cr); and olivine (Mg and Fe 2+ ) for the Jeol 8200 SuperProbe. Trace elements were investigated by SIMS in selected clinopyroxene, plagioclase, epidote and amphibole minerals, using a Cameca IMS 4f ion microprobe installed at CNR-IGG, Pavia. Due to a significant small-scale intergrowth (in some cases at a few-micrometres scale) of the mineral assemblage, we took special care to get data representative of the trace-element concentration in single mineral phases. Moreover, the different thickness in the sample from which the X-rays (EPMA) and the secondary ions (SIMS) are produced during electron and ion microprobe analysis, and the different beam size adopted for major and trace elements (typically, 1-2 µm Ø for the electron beam and 10 µm Ø for the ion beam) sometimes caused a mismatch between major- and trace-element compositions. When necessary, we carried out our SIMS analysis with a very small 16 O - ion beam (down to ~ 5 µm Ø), monitoring the ion signals for Ti in clinopyroxene, Na and/or K in plagioclase, and comparing the SIMS data of these test elements with those obtained by EMPA for the same elements at the same microspot areas. For all the ion probe measurements, the primary current intensity ranged between 4 and 20 na, with a beam spot of ~ 5-25 µm in diameter. The width of the energy slit was 50 ev and the voltage offset applied to the sample accelerating voltage ( V) was -100 V. This corresponded to the analysis of secondary ions with emission kinetic energies in the range: ~ ev. The filtered positive secondary ions were extracted and focussed under an ion image field of 25 µm. We used the largest contrast diaphragm and field aperture (400 and 1800 µm, respectively). We analyzed rare earth elements (REE) with a procedure similar to that developed by [18] for silicates. La, Ce, Nd, Sm, Er and Yb were analyzed by measuring the signals from one isotope, i.e., 139La, 140Ce, 146Nd, 149Sm, 167Er and 174Yb. For Eu we used a deconvolution procedure to remove BaO interferences from Eu isotopes. Gd isotopes were monitored at mass 154, 160 and 162 in order to discriminate the Gd (and Dy) signal from CeO and NdO interferences. Dy was monitored also through the isotope 163. Moreover, in the case of Er, we improved the reliability of analysis by evaluating the 151 Eu 16 O contribution at mass 167 from the Eu + signal and from the oxidation ratio (i.e., EuO + /Eu + ) as determined on suitable standards under the adopted analytical conditions. Similar procedure was followed to correct Yb from GdO interference at mass 174. For the other trace elements, we detected signals from the following isotopes 45 Sc, 47 Ti, 51 V, 52 Cr, 85 Rb, 88 Sr, 89 Y, 90 Zr, 93 Nb, 133 Cs, 138 Ba, 178 Hf and 232 Th (similar analytical procedures were recently used by L. Ottolini in [19]). All of them were monitored in the same analytical run together with the REE ion signals. 45 Sc + and 52 Cr + were further corrected for 29 Si 16 O + and 24 Mg 28 Si + interferences at mass 45 and 52, respectively. 30 Si + was selected as the isotope of the reference element (Si) for these matrices. International reference materials, i.e., NIST SRM 610 and R315 [20], in addition to CNR-IGG standards, i.e., kaersutite Soda Springs (KSS) and alkali-olivine-basalt BB (in-house standard of the Geochemical Institute, Göttingen University) were used in the calibration procedure. Accuracy for REE and other trace elements is estimated to be of the order of 10 % rel. at ppm (wt) level. Below 1 ppm, precision is constrained by counting statistics in the range %. Accuracy is typically of the order of 15 % for Th; % for Cs and Rb. Whole-rock major and trace element analyses were performed at the Earth Sciences Department of Pisa University. Major elements were determined by X-ray fluorescence on an ARL 9400 XP+ spectrometer using Li 2 B 4 O 7 glass disks (sample/flux ratio = 1:7). Estimated precision (relative 4

6 standard deviation, RSD) is about 1 % for SiO 2 and about 2 % for the other major elements except those with low concentrations (~ < 0.50 wt%), for which the standard deviation is about ± 0.05 wt%. Loss on ignition (L.O.I.) was determined by gravimetry at 1000 C after pre-heating at 110 C. The XRF results are shown in table 3. Table 3. Whole-rock major element concentrations (wt%) of the studied cuttings by XRF. sample RN-17- RN-17- RN-17- RN-17- RN-17- RN-17- RN-17- RN-17- RN-17- RN-17- RN-17- RN-19- GIOV SiO TiO Al 2 O FeO T MnO MgO CaO Na 2 O K 2 O P 2 O Total L.O.I The concentrations of the bulk-rock trace elements were determined by ICP-MS (VG PQ II Plus). Sample powders were dissolved in PFA vessels on a hot plate using HNO 3 + HF. The sample solutions, spiked with Rh, Re and Bi as internal standards, were measured by external calibration using international reference materials of basaltic composition. Analytical precision, assessed by repeated analysis of the in-house standard HE-1 (Mt Etna hawaiite, n = 94), is between 2 and 5 % RSD, except for Gd, Tm, Be, Sc, Pb (6-8 % RSD). Detection limits, corresponding to 3σ of the blank concentrations, vary between 2 and 60 ppb for all elements, except Cr, Zr and Sc ( ppb). All the experimental data are listed in table 4. Sr and Nd isotope ratios were determined by TIMS at IGG-CNR, Pisa, using a Finnigan MAT 262 multicollector mass spectrometer running in dynamic mode after conventional procedures for Sr and Nd separation from the matrix. Measured 87 Sr/ 86 Sr ratios were normalized to 86 Sr/ 88 Sr = ; 143 Nd/ 144 Nd ratios to 146 Nd/ 144 Nd = During the collection of the isotopic data for this study, the mean measured value of 87 Sr/ 86 Sr for the standard NIST-SRM 987 was ± (2σ, n = 25) and that of 143 Nd/ 144 Nd for the standard La Jolla was ± (2σ, n = 25). The JNdi-1 standard [21] was additionally analyzed giving a 143 Nd/ 144 Nd value of ± (2σ, n = 25). The Sr and Nd blank levels were about 0.3 ng during the period of chemistry processing. The analyses were carried out on about 100 mg of whole-rock powders washed ultrasonically with deionized milli-q water and on 5 to 20 mg of mineral handpicked under a binocular microscope and washed with deionized milli-q water. The pertaining results for bulk rock-analysis and mineral separates are summarized in table 5. 5

7 Table 4. Whole rock trace element concentrations (ppm) by ICP-MS. sample RN-17- RN- RN- RN- RN-17- RN-17- RN-17- RN-17- RN-17- RN-17- RN-17- RN-19- GIOV La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sc V Cr Rb Sr Y Zr Nb Cs Ba Hf Ta Pb Th U Bulk-rock geochemistry The cuttings exhibit a large range in REE, Cs, Rb, Th, U, Ba, Nb and Ta contents, as well as in the light-to-heavy rare earth elements (LREE/HREE) fractionation. The C1-chondrite normalized REE concentrations are plotted in figure 1, from which we can see that all samples fall in the range of Icelandic tholeiites [22-31]. The recent work by [9] on RN-17 has revealed the cuttings to consist of hydrothermally altered basalt with a composition that is transitional between Mid-Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB); the basalts are primarily tholeiites and are in good agreement with the range of published Icelandic and Reykjanes Ridge data. As for the other trace elements analyzed in the present study (whose C1-chondrite plot is not reported here), the concentration of some of them increase from depth to surface: for Rb N, for Cs N, for Ba N from the deeper to intermediate cuttings, and up to 150 and 390 (Ba N ) in the shallower ones (RN and RN ). Also, the U/Th fractionations show significant increase, i.e., from the bottom to the surface. Differently from the cuttings, the dolerite dyke (RN ) is LREE-depleted (La N /Yb N = 0.59) and is characterized by the lowest REE (figure 1) and other trace element concentrations. 6

8 Table 5. Sr and Nd isotope data by TIMS. Sample 87 Sr/ 86 Sr 2σ mean 143 Nd/ 144 Nd 2σ mean RN whole rock RN whole rock ± ± RN whole rock RN whole rock ± ± RN plg ± RN epidote ± RN cpx ± RN whole rock RN whole rock RN whole rock RN whole rock ± ± RN plg ± RN epidote ± RN cpx ± RN whole rock RN whole rock ± ± RN plg ± RN cpx ± RN whole rock ± ± RN plg ± RN cpx ± RN-17-GIOV-2 whole rock ± ± RN-17-GIOV-2 plg ± RN-17-GIOV-2 cpx ± Figure 1. C1-chondrite-normalized REE patterns for selected wholerock (WR) cuttings and core-sample from well RN-17 and RN-19 (Reykjanes geothermal system), respectively. Normalisation values from [59]. All the REE data for our samples fall in the range of Icelandic tholeiites (grey field, from literature). 7

9 The whole rock concentration of the highly-mobile elements Li and B investigated by [15] show a wide range of B concentrations ( ppm), whereas the variability of Li is narrower ( ppm): in particular, the highest B values occur in the cuttings from RN while the highest Li concentration was measured in the unaltered surface lava RN-17-GIOV-2 and in the K-rich sample RN (figure 2a). Li contents are generally lower than those of young basalts from Mid Ocean Ridges ( ppm) and Ocean Islands ( ppm) ([32] and references therein). On the contrary, B abundances are within the literature values for young basalts ([33] and references therein). In order to avoid compositional differences related to the evolution degree of the samples, we have compared B/Ce ratios versus Ce (figure 2b), and Li/Yb ratios versus Yb (figure 2c) of our samples with those from the literature. Figure 2b shows that the cuttings are enriched in B (B/Ce = ) with respect to young basalts from MOR (B/Ce = : [34-36]) and OIB (B/Ce = : [33, 36]) whereas Li/Yb ratios (1-2.7) are similar or slightly lower than MORBs (Li/Yb = 1.6-3: [32, 34-35, 37]) and OIBs (Li/Yb = : [32, 38]). Figure 2. Plots of whole-rock data for: (a) Li versus B (ppm); (b) B/Ce versus Ce (ppm); and (c) Li/Yb versus Yb (ppm) of selected cuttings from Reykjanes geothermal field (data from [15]). The partial fields for Mid Ocean Ridges and Ocean Islands young basalts [32, 33] are shown for comparison. The extrapolated trend for Li (29.9 ppm) and B (17.2 ppm) in Mid Ocean Ridge and Ocean Island basalts (from [33]) is also reported for comparison. Measured 87 Sr/ 86 Sr ratios of bulk rocks vary between and in the cuttings, whereas lower values characterize the unaltered lava and the RN dolerite dyke ( 87 Sr/ 86 Sr = and , respectively) (see table 5). The Sr isotope ratio is generally higher for the more K-enriched and shallower cuttings (e.g., RN , RN ), and progressively decreases with depth (figure 3). In contrast, the 143 Nd/ 144 Nd values vary within a narrower range ( ) without any correlation with depth (see table 5). Thus, the 143 Nd/ 144 Nd variations of the studied cuttings are probably due to magmatic processes since hydrothermal alteration is expected to have no 8

10 effect on 143 Nd/ 144 Nd [39]. The first process investigated is the increasing of 143 Nd due to radioactive decay of 147 Sm. The 143 Nd/ 144 Nd versus Sm/Nd plot of figure 4 shows that our samples fall close to a linear array, formed by Icelandic rocks, with a slope corresponding to an age of about 170 Ma [28]. Since the crust beneath the Reykjanes Peninsula has near-zero age, it has not had sufficient time to develop a distinctive Nd isotope ratio [28, 40]. On the other hand, this linear array suggests a mixing process between melts derived from a depleted MORB-like source with melts derived from a more enriched mantle source. In this frame, the negative correlations between 143 Nd/ 144 Nd ratios and some of the compositional parameters which are relatively insensitive to low-pressure crystal fractionation, such as La/Yb (figure 5a) and Nb/Zr (figure 5b) [41] also require at least two different mantle sources: one depleted (D) and one enriched (E). The D component, which includes cuttings RN , the unaltered lava RN-17-GIOV-2 and the dolerite dyke RN , has 143 Nd/ 144 Nd ( ), LREE/HREE and Nb/Zr ratios (La/Yb = ; Nb/Zr = ) comparable to those of the source of depleted Reykjanes Peninsula lavas [39]; the E component, which includes cuttings RN , -650, -750, -1100, -1700, -2150, and -3000, has 143 Nd/ 144 Nd ( ), LREE/HREE and Nb/Zr ratios (La/Yb = ; Nb/Zr = ) that approach those of the source of enriched Reykjanes lavas [39]. The high La/Yb ratio (4.2) of cuttings RN may require the existence of a third source, similar to that of Iceland mildly alkaline lavas [42]. However, the Nb/Zr range of mildly alkaline lavas (figure 5b) does not seem to confirm such a hypothesis. As an alternative, the geochemical features of sample RN could be ascribed to a slightly lower degree of partial melting of the enriched source. Figure 3. Whole-rock 87 Sr/ 86 Sr versus depth (m) for the studied cuttings. Figure 4. Plots of 143 Nd/ 144 Nd versus Sm/Nd for the studied cuttings. The field of Icelandic rocks, along with a linear array with a slope corresponding to an age of about 170 Ma, is reported for comparison [28]. 4. Mineral geochemistry EPMA and SIMS data for all analyzed plagioclase are listed in tables 6 and 7, respectively. The occurrence of both igneous and secondary plagioclase is confirmed by the diagrams of K 2 O (wt%) and FeO T (wt%) versus the anorthite (An) index (see for instance that for FeO T in figure 6). Feldspar with An < 30 % (albite or oligoclase) occurs as replacing magmatic plagioclase [13]. Plagioclase with An > 30 % from intermediate to deep cuttings (e.g., RN , -1700, and -3000) and the 9

11 Figure 5. Plots of 143 Nd/ 144 Nd versus (a) La/Yb and (b) Nb/Zr for the studied samples. The negative correlations require at least two different sources in the mantle beneath Reykjanes geothermal field: enriched (E) and depleted (D) sources. The fields for Iceland rocks [41], Iceland mildly alkaline lavas [42], Reykjanes Peninsula depleted and enriched lavas [39] are shown for comparison. unaltered lava (RN-17-GIOV-2) are characterized by higher LREE and lower HREE concentrations (LaN/YbN = 21-33) than those from the shallower ones (e.g., RN , -750, and -1450: LaN/YbN = 5-12). The chondrite-normalized REE concentrations, obtained by SIMS, are shown in figure 7. All plg crystals have marked positive Eu anomaly (Eu N /Eu* N = 3-17, where Eu* N = (Sm N x Gd N ) 1/2 ) (figure 7) as well as positive Sr anomalies (Sr N /Sr* N = 14-44, where Sr* N = (Ce N x Nd N ) 1/2 ) together with negative Zr anomaly (Zr N /Zr* N = ), where Zr* N = (Nd N x Sm N ) 1/2. In contrast, secondary feldspar (An < 30 %) from RN cutting shows very low LREE (La N /Yb N = 0.92) contents, and marked negative Ce (Ce N /Ce* N = 0.2) (figure 7) and Ti (Ti N /Ti* N = 0.05, where Ti* N = (Gd N x Dy N ) 1/2 ) anomalies, thus suggesting a hydrothermal growth on previous Ca-plagioclase with loss of LREE during the albitisation process, as reported in [43]. The negative Ce anomaly could be related to preferential loss of Ce 4+ during hydrothermal alteration of primary plagioclase. Distinct from cuttings, plagioclase from the dolerite sample (RN m) shows very low REE contents and the most marked positive Eu (Eu N /Eu* N ~ 40) (figure 7) and Sr (Sr N /Sr* N ~ 47) anomalies, thus suggesting a cumulus origin. The results of EPMA and SIMS investigations of cpx are shown in tables 8 and 9, respectively. Cpx shows a great variability in terms of major constituents, such as FeO T (range wt%), MgO ( wt%) and CaO ( wt%), as well as in almost all trace elements analyzed (see table 9). In particular, the REE concentrations (figure 8) span from 2.3 to 10 (La N ), and from 8.4 to 34 (Yb N ), with the highest REE contents occurring in cuttings RN and RN Common geochemical features are the following: i) LREE-depletion (La N /Yb N = ) (figure 8); ii) negative Zr (Zr N /Zr* N = ), Sr (Sr N /Sr* N = ) and Eu (Eu N /Eu* N = 0.7-1) anomalies. The clinopyroxene from the dolerite (RN m) shows very marked LREE/HREE fractionation (La N /Yb N ~ 0.1), no Eu anomaly (figure 8) and depletions in almost all other trace elements. Epidote is an important index mineral in hydrothermal systems and its occurrence provides unequivocal evidence for temperature in excess of 240 C at some stage during hydrothermal activity [44, 45, 46 and references therein). Epidote commonly forms in active geothermal systems as the product of reactions between hydrothermal solutions and Ca-bearing phases, as plagioclase, in the host rocks, and typically occurs as xenoblastic crystals and radiating crystal aggregates, and filling vesicles and veins as a secondary alteration mineral. In the Reykjanes geothermal field epidote has been found 10

12 Table 6. Electron probe data (wt%) for plagioclase. sample RN-17-GIOV-2 RN RN RN grain plg 1A plg 1B plg 2B plg 3A plg 1D plg 2B plg 3E plg 3B plg 4C plg 1I plg 1N plg 1S plg 1D plg 1B plg 1C plg 2D plg 2B plg 2'D plg 3C SiO TiO Al2O Cr2O FeOT MnO MgO CaO Na2O K2O Total An sample RN RN RN grain plg 2C plg 7A plg 7C plg 8C plg 5C plg-1b plg-x plg-1ay plg-1by plg 3B plg 4D plg 4E plg 6C plg 6G plg-1a-qz plg-1b-op plg 1B plg 1C SiO TiO Al2O Cr2O FeOT MnO MgO CaO Na2O K2O Total An An = anorthite index of plg = 100 x [Ca/(Ca+Na+K)]. 11

13 Table 6. continued. sample RN RN grain plg 1G plg 1H plg-2b-op plg-2c plg 2G plg 2F plg 3C plg 3H plg 4I plg 4B plg 5D plg-2a-cc plg-2b-cc plg 6A plg 6C plg 6F plg 4B plg 3A SiO TiO Al2O Cr2O FeOT MnO MgO CaO Na2O K2O Total An sample RN RN grain plg 3C plg 5C plg-1b plg 2C plg 2F plg 2G plg 2H plg 2L plg 3A plg 3A plg 3C plg 3E plg 3L plg 3M plg 3M plg 1C plg 1D plg 1F plg 2C plg 2G plg 2I SiO TiO Al2O Cr2O FeOT MnO MgO CaO Na2O K2O Total An

14 Table 6. continued. sample RN grain plg 3B plg 3F plg 3G plg 3I plg 3M plg 3N plg 4N plg 5C plg 5H plg 5I plg 5M plg 5N plg 5P plg 5Q plg 5R plg-2c-ar plg-3a-ar plg 1P plg 4A plg 3A SiO TiO Al2O Cr2O FeOT MnO MgO CaO Na2O K2O Total An

15 Table 7. Trace element concentrations (ppm) for plagioclase by SIMS. Sample RN-17-GIOV-2 RN RN RN RN Grain plg 1A plg 1n plg 1L average (2) 1σ plg 1D2 plg 2B plg 3C average (2) 1σ plg 3A plg 7C plg 5D plg 1b plg x plg 1ay Sc Ti V Cr Rb b.d.l Sr Y Zr b.d.l Nb Cs Ba La Ce Nd Sm Eu Gd Dy Er Yb Hf Th La N /Yb N Eu N /Eu N * Ti N /Ti* N Sr N /Sr* N Zr N /Zr* N n.a n.a n.a n.a. n.a. n.a. n.a. n.a. n.a EuN/Eu*N = Eu anomaly, with Eu*N = (SmN x GdN) 1/2 ; SrN/Sr*N = Sr anomaly, with Sr*N = (CeN x NdN) 1/2 ; ZrN/Zr*N = Zr anomaly, with Zr*N = (NdN x SmN) 1/2 ; n.a. = not analyzed. Italic data represent both the average when more data are available and a single analysis when sample heterogeneity did not allow more investigations. 14

16 Table 7. continued. sample RN RN RN Grain plg 1by average (7) 1σ plg-1a-qz plg 1B plg 1B plg 2b average (3) 1σ plg 1D plg 3D plg 3C plg 2a plg 2b average (5) 1σ Sc Ti V Cr Rb Sr Y Zr Nb Cs Ba La Ce Nd Sm Eu Gd Dy Er Yb Hf n.a. n.a n.a. n.a n.a. n.a n.a. n.a Th La N /Yb N Eu N /Eu N * Ti N /Ti* N Sr N /Sr* N Zr N /Zr* N

17 Table 7. continued. Sc Ti V Sample RN RN RN RN Grain plg 6A plg 6F(2) average (2) 1σ plg 3A plg 3M average (2) 1σ plg 5E plg 5M average 82) 1σ plg 4A Cr Rb b.d.l. Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf 0.08 Ta Pb Th LaN/Yb N Eu N /Eu N * TiN/Ti* N Sr N /Sr* N Zr N /Zr* N

18 Table 8. Electron probe data (wt%) for clinopyroxene. sample RN-17-GIOV-2 RN RN RN grain cpx 3B cpx 1C cpx 2D cpx 2F cpx 1M cpx 3M cpx 3B cpx 3M cpx 3B cpx 1A cpx 1F cpx 3E cpx 2B cpx 7B cpx 7G cpx 8B cpx 5D cpx 4A cpx-1a cpx-1c cpx-3a SiO TiO Al2O Cr2O FeOT MnO MgO CaO Na2O K2O Total Wo sample RN RN grain Cpx-3b cpx 2F cpx 5D cpx 4A cpx 4B cpx 4F cpx 4G cpx 4H cpx 6A cpx 6D cpx 6E cpx 6F cpx 5G cpx-1a cpx-2a cpx-2b cpx 1A cpx 1I cpx-1a cpx-2a SiO TiO Al2O Cr2O FeOT MnO MgO CaO Na2O K2O Total Wo Wo = wollastonite index, molar [(100 x Ca/(Ca+Mg+Fe 2+ +Fe 3+ +Mn)]. 17

19 Table 8. continued. sample RN RN RN grain cpx 2D cpx 2L cpx 2B cpx 2A cpx 3F cpx 3B cpx 3A cpx 4F cpx 4L cpx 6B cpx 6E cpx 4E cpx 5A cpx 5B cpx 5G cpx-1a cpx-2a cpx-2b cpx 1a cpx 1C SiO TiO Al2O Cr2O FeOT MnO MgO CaO Na2O K2O Total Wo sample RN grain cpx 1D cpx 1I cpx 2E cpx 2I cpx 3F cpx 3G cpx 3H cpx 3I cpx-1a cpx-2a cpx-2b cpx-1a cpx 1A cpx 2E cpx 3D cpx 5A cpx 5B cpx 5F cpx 5L cpx 5O cpx-2b SiO TiO Al2O Cr2O FeOT MnO MgO CaO Na2O K2O Total Wo