- Email: [email protected]
Nuclear microprobe analysis of serpentine from the mid-Atlantic ridge
Nuclear Instruments and Methods in Physics Research B 158 (1999) 575±581
www.elsevier.nl/locate/nimb
Nuclear microprobe analysis of serpentine from the mid-Atlantic ridge Beate Orberger
a,1
, Nicole Metrich b, Michelle Mosbah Yves Fouquet d
b,*
, Catherine Mevel c,
a
Laboratoire de G eochimie, Universit e Paris Sud, F-91405 Orsay Cedex, France Laboratoire Pierre S ue, CEA-CNRS, CE-Saclay, F-91115 Gif-sur-Yvette, Cedex, France Laboratoire de P etrologie, CNRS UPRES A 7058, UPMC, Place Jussieu, F-75005 Paris, France d IFREMER-Brest, B.P. 70, F-29285 Plouzan e, France b
c
Abstract At mid-ocean ridges, ultrama®c rocks are serpentinized by interaction with seawater-derived ¯uids. Elements, dissolved in large quantities in seawater, e.g., Na, K, Cl, Br, Ca and Sr, can be, in small amounts, incorporated as traces into the crystal structure of the various serpentine minerals (Mg3 Si2 O5 (OH)4 ). These trace elements can be used to track the composition of the reacting ¯uids and to constrain physico±chemical conditions. This paper represents the ®rst application of particle-induced X- and c-ray emission (PIXE/PIGE) analysis to serpentine using the nuclear microprobe at the Laboratoire Pierre S ue (CEA-CNRS). Three types of serpentine, belonging to two dierent serpentinization generations, have been analysed in samples collected from the Mid-Atlantic Ridge (14°450 N/45°W) that exposes serpentinized peridotites on which the Logachev black smoker is placed. The trace elements Cl, F, S, Cu, Zn, Ca, K, Ni, Cr and Mn were detected from several tens to several thousands of ppm. Bromine, As and Sr are close to the detection limit of about 5 ppm. The trace element concentrations and interelement relationships in serpentines vary (a) with the serpentine type and (b) with the geographic location to the black smoker. Chlorine and in part S originated from seawater, whereas Cu, Zn, Ca, K, Ni, Cr and Fe and the major amount of S were mobilized from the unaltered host rock and partitioned between the serpentine and the aqueous solution. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Serpentine often represents hydration products of primary Mg±Fe silicates, such as olivine and
*
Corresponding author. Tel.: +33-1-69-08-57-86; fax: +331-69-08-69-23; e-mail: [email protected] 1 E-mail: [email protected]
pyroxenes. During serpentinization, some elements (Fe, Al, Ni, Cr) are mobilized and removed from the serpentine structure, whereas H2 O is added. Electron microprobe analysis on serpentines has been used since 30 yr for determination of major and minor element concentrations: Mg, Fe, Si, Al, Cr, Fe, Mn and Ni data were published by for example ([1] and refs. therein). Cr2 O3 can, thus, be detected from 0.02 to about 1.2 wt% and NiO
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 4 2 - 0
576
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581
from 0.02 to 0.5 wt%, CaO ranges from 0.02 to 0.24 wt% and TiO2 contents of 0.02 wt% [1,2]. However, most trace element concentrations in serpentines are below the detection limit of the electron microprobe. In addition, Cl contents of 0.03 up to 0.33 wt% and F contents of 0.02 up to 0.16 wt% were analysed e.g. Refs. [3,4]. The objective of this study was to analyse trace elements (e.g., Cl, Br, F, K, Na, Ca, Ni, Cr, Mn, Ti) which might give information about the seawater±peridotite interaction and element mobility during serpentinization, so that the physico± chemical conditions at serpentinization can be deduced. A nuclear microprobe technique was chosen, because it permits to analyse trace elements at levels ranging from several tens to several thousands ppm. Furthermore, it allows to perform non-destructive point analyses, preserving the primary texture of the sample. 2. Analytical method The rock samples were mounted on a trace element free ``SUPRASIL I'' (Heraeus) glass and were cut to 27 lm thickness to ensure that only serpentine was analysed. Sample surfaces were polished. Particle-induced X-ray emission (PIXE) analysis was performed using the nuclear microprobe at the Laboratoire Pierre S ue (CEA-CNRS, Saclay, France), with a beam-diameter of 10 ´ 10 lm2 . Elements such as S, Cl, K, Ca, Ti, Fe were analysed at 1.7 MeV incident proton energy (Ei ) with a 30 lm mylar absorber in front of the detector; elements with Z > 28 (e.g., Cu, Zn, Ga, Br, Rb, Sr) were analysed at 3.5 MeV Ei and with a 200 lm Al absorber. At this energy the cphotons produced by the reaction 19 F(p,p,c)19 F were also detected. The accumulated charge was 1 lC corresponding to a counting time of about 20 min per point for analyses at 1.7 MeV and 1.5±2 lC corresponding to 40±45 min for analyses at 3.5 MeV. The natural volcanic glass standards KE12 and VNM50, were analysed before and after the experiments in order to evaluate the F data ([3] and refs therein). The penetration depth of the proton beam in serpentine is 22 lm at 1.7 MeV and 84 lm at 3.5 MeV. Trace element concentrations were
calculated using the GUPIX software [5]. The accuracy and precision of the software are described in Ref. [6]. Data for Mn are of limited use; in Cr-rich samples Ka of Mn superposes Kb of Cr, thus leading to a signi®cant error. Mn data were accepted only for samples with <100 ppm Cr. Detection limits are for: S: 39 ppm, Cl: 27 ppm, K: 16 ppm, Ca: 12 ppm, Ti: 13 ppm, V: 16 ppm, Cr: 16 ppm, Fe: 28 ppm, Ni: 19 ppm, Cu: 19 ppm, Zn: 8 ppm, As: 4 ppm, Br: 4 ppm, Sr: 5 ppm, F: 90 ppm. Major elements (Mg, Si, Al, Fe) were analysed by electron microprobe at the Centre Camparis Universite 6, Paris. Scanning electron microscopy was performed on polished thin sections at the Laboratory of Geochemistry of Sedimentary Rocks at the University Paris XI to ensure that no sulphides were dispersed in the serpentine matrix. The Na contents analysed by PIGE are not presented here because, as for F, the cdetection is not a linear function of the interaction depth [7], but strongly decreases with the sample thickness. 2.1. Sample description The studied rocks represent serpentinized peridotites from the Mid Atlantic Ridge, sampled at 14°450 N/45°W along a 3 km long pro®le including the hydrothermal ®eld Logachev [8]. Logachev is the ®rst black smoker on top of ultrama®c rocks (Fig. 1). The samples were dragged from 2700 to 3300 m depth during the campaign Microsmoke 1995. All rocks are almost completely altered. The mineral assemblage consists of various serpentine minerals, amphiboles, talc, calcite as well as chromite and magnetite. Three dierent serpentine types were analysed: (1) bastite, the complete replacement of orthopyroxene [(Mg,Fe)2 Si2 O6 ] and minor clinopyroxene ([Ca(Mg,Fe)Si2 O6 ], Fig. 2a); (2) mesh serpentine replacing olivine [(Mg,Fe)2 SiO4 ] in MS 21±1, (3) serpentine veins, developing in cleavages of bastites or crosscutting bastite (Fig. 2b). 3. Results Twenty spot analyses were obtained on each sample (10 points on vein or mesh serpentine, 10
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581
577
Fig. 1. E±W cross-section A±B at 14°450 north along the Mid-Atlantic Ridge after Fouquet, 1994. *: sample numbers.
points on bastite). Representative analyses and average values are shown in Table 1a and b. The results are shown in Fig. 3a±d. Primary olivine and orthopyroxene from an unaltered sample were analysed under the same conditions as those for serpentine. Direct comparison between the trace element contents of these primary minerals and those in serpentines (especially for Ca, Ni and Fe) requires caution, as they are in¯uenced by the geological history (e.g., formation temperature, degree of partial melting, and mantle metasomatism). Bastite are characterized by, on average, higher Fe, Ca, Cr, Ti and Cl contents than vein serpentine (Table 1a). No signi®cant dierence has been observed between mesh and vein serpentine. The Fe content in bastite ranges from 3.5% to 4.2% and from 2.02% to 4.2% in vein serpentine.
The Cr content varies between 600 and 9000 ppm in bastites and between 18 and 670 ppm in vein serpentine. The Ca content in bastites range from 295 to about 2000 ppm. This is about a factor 2±4 higher compared to those from vein serpentines containing 122±450 ppm Ca. The Ti concentrations in bastites vary from 25 to 137 ppm attaining high values of on average 908 ppm. The Cl-content is a factor 1.6±3 higher in bastites (380±3500 ppm), than in vein serpentines (340±1350 ppm). In one sample, vein serpentine (MS 19±6) shows more likely bastite characteristics, except for Ca. Copper, Cl, Zn and, in part, Ti are enriched in serpentine from near the black smoker, whereas S and F are enriched in the more distal samples (Table 1a and b). Close to Logachev, Cl, Zn, Ti and Cu attain 3500, 200, 780 and 660 ppm,
Fig. 2. Serpentine from altered peridotites (14°450 N/45°W on the Mid Atlantic Ridge). (a) Bastite after pyroxene, (b) vein and mesh serpentine.
578
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581
Table 1 Representative nuclear microprobe analyses of serpentines (Laboratoire Pierre S ue, CEA-CNRS, Saclay, France) Vein serpentine (a)
Average
Bastite
Average
S (ppm) Cl K Ca Ti Cr Mn Ni Fe
MS 18-16 373 (26) 363 (13) 94 (9) 170 (8) <16 <16 509 (13) 3150 (38) 2.03 (61 ppm)
405 (28) 685 (24) 74 (7) 206 (9) <20 665 (13) * 2184 (37) 3.09 ( 77 ppm)
370 502 126 168 <16 573 n.p. 2024 2.08
282 (23) 694 (14) 84 (8) 570 (11) 152 (8) 7535 (38) * 1815 (36) 3.15 ( 95 ppm)
246 (25) 1428 (20) 97 (10) 626 (13) 150 (9) 7229 (29) * 716 (6) 4.55 (95 ppm)
267 1090 104 546 137 6485 * 1335 3.75
S (ppm) Cl K Ca Ti Cr Mn Ni Fe
MS 19-4 248 (22) 391 (16) <33 207 (17) 76 (6) 794 (16) * 2245 (34) 2.61 (78 ppm)
652 (26) 352 (15) <47 77 (6) <12 <17 752 (15) 3006 (30) 1.54 (62 ppm)
582 376 58 122 <15 <18 * 3002 2.02
357 (21) 475 (14) <51 (9) 246 (7) 103 (7) 6560 (26) * 2174 (44) 3.4 (85 ppm)
424 (25) 616 (18) 52 (9) 318 (9) 87 (8) 8190 (33) * 2312 (39) 3.79 (87 ppm)
416 594 72 295 85 6803 * 2197 3.5
S (ppm) Cl K Ca Ti Cr Mn Ni Fe (%)
MS 19-6 246 (22) 490 (15) 133 (9) 860 (9) <13 <18 937 (22) 1641 (33) 4.5 (90 ppm)
191 (12) 1024 (20) 146 (10) 935 (9) <13 <52 1005 (20) 1706 (34) 4.31 (86 ppm)
189 460 110 730 <19 27 843 1509 4.16
203 (11) 996 (20) 145 (10) 1000 (10) <20 <44 1020 (20) 1615 (32) 4.41 (88 ppm)
178 (21) 383 (15) <30 370 (7,4) <14 251 (10) 496 (15) 2010 (40) 4.4 (90 ppm)
210 818 130 757 <15 39 872 1560 4.25
Mesh serpentine +vein serpentine (olivine replacement)
Average
Bastite
MS 21-1 220 (5) 1117 (25) 72 (8) 399 (8) <25 62 (13) 286 (11) 2020 (30) 2.67 (80 ppm)
231 920 97 327 23 97 364 2367 3.02
307 (27) 2214 (22) 134 (11) 1210 (1) 854 (12) 5966 (25) * 1734 (35) 4.59 (92 ppm)
S (ppm) Cl K Ca Ti Cr Mn Ni Fe (%)
287 (26) 1353 (20) 75 (21) 385 (8) <34 119 (7) 359 (14) 2553 (36) 3.46 (70 ppm)
Average 289 (30) 2933 (30) 94 (9) 1008 (11) 787 (12) 6033 (30) * 1078 (40) 6.18 (120 ppm)
286 1991 107 767 (a) 908; (b) 25 (a) 6002; (b) 122 * 2013 4.12
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581
579
Table 1 (Continued) Vein serpentine (b)
Average
Bastite
Average
Ni Cu Zn F*
MS 18-16 2345 (47) <38 <19 404 (60)
3301 (53) <31 <32 505 (76)
2024 <30 <31 340
2281 (39) <28 <23 290 (43)
626 (13) <8 <14 454 (60)
1335 <23 <24 284
Ni (ppm) Cu Zn F*
MS 19-4 2577 (44) <24 37 (7) 265 (40)
2392 (41) <26 <34 227 (34)
3002 <32 <32 184
2666 (35) 59 (14) 48 (6) 516 (77)
2495 (25) 46 (11) 68 (4) 470 (70)
2197 45 56 470
Ni (ppm) Cu Zn F*
MS 19-6 1738 (35) <21 36 (6) 219 (33)
1499 (30) <29 61 (6) 237 (12)
1509 <18 53 160
1434 (30) <19 55 (7) 225 (34)
991 (25) <15 41 (5) <90
1560 <23 53 120
Average
Bastite
1650 210 240 <100
2820 (56) 38 (2) 81 (11) 140 (22)
Mesh/Vein serpentine Ni (ppm) Cu Zn F*
MS 21-1 1837 (46) 237 (24) 252 (13) 144 (22)
1869 (45) 172 (21) 218 (13) <90
Average* 1019 (40) 70 (16) 122 (12) <90
2013 (a) 209; (b) 46 197 <100
**
Average values of about 10 analyses. Errors in parenthesis. c-photons.
respectively (Fig. 3a±d); away from the smoker 380 ppm Cl, 31±68 ppm Zn, < 16 to 140 ppm Ti and 19 ppm to 68 ppm Cu are observed. Sulphur and F show the inverse behavior: up to 900 ppm S and 520 ppm F away from Logachev and 189±582 ppm S as well as <100 ppm F near the hydrothermal ®eld. Primary olivine and pyroxene also indicate S contents up to 160 ppm. The Ni (1509±3002 ppm) and K (58±130 ppm) contents are approximately the same for all three types of serpentines and do not change with the geographic locations (Table 1a). Most Br, Sr and As data are below the detection limits (Br: 4 ppm; As: 4 ppm, Sr: 5 ppm). A few analyses mainly in bastite, yielded values above detection limits, but with errors >10%: Br-8 and 15 ppm close to Logachev; As-6±17 ppm in the sample most distal from Logachev; Sr ± 24 ppm.
4. Discussion and conclusion This study demonstrates that the nuclear microprobe is a useful tool for analysing trace elements in serpentine at concentrations between several tens and several thousands ppm. The variation of trace element content and interelement relationships contributes to the understanding of the serpentinization process and to constrain the physico±chemical conditions. The results provide also a database for comparison with results of experimental studies realised to the seawater±rock interaction. The Cl content in serpentines suggests in®ltration of seawater into peridotite. Iron, Ni, Cr, Cu, Zn, Ca and K are inherited from primary olivine and pyroxene, and ± in part ± from Cr-spinels. The positive correlations for Fe/Cl, Zn/Cl and Ca/Cl (Fig. 3a,b,c), mainly in vein serpentine, indicate that all these
580
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581
Fig. 3. Interelement relationships of selected major and trace elements in serpentines. (a) Cl f(Fe); (b) Cl f(Zn); (c) Cl f(Ca); (d) S f(Ca). BS: Black smoker Logachev.
elements were incorporated during serpentinization through transport of metal-chlorine complexes in aqueous solution. The S content of the serpentines relates to the physico±chemical conditions at the dierent localities: At Logachev, relatively low S contents can be explained by partition of most of the S into sulphide minerals. The positive correlation of Ca and S in vein serpentines (Fig. 2d) from Logachev, indicates CaSO4 (anhydrite) precipitation and, thus, a higher oxygen fugacity than at sites away from the hydrothermal ®eld. Acknowledgements This work was possible thanks to the ®nancial support from the INSU-CNRS (Projet: Dorsale).
The samples were recovered during the Mid-Atlantic Ridge. cruise Microsmoke in 1995. The authors thank the technical sta of the van de Graa accelerator at the Laboratoire Pierre S ue (CEACNRS, Saclay), in particular L. Daudin and J.P. Gallien, for their help with analyses. References [1] M.A. Dungan, Canadian Mineralogist 17 (1979) 771. [2] F.J. Wicks, A.G. Plant, Canadian Mineralogist 17 (1979) 785. [3] T. Labotka, A.L. Albee, Canadian Mineralogist 17 (1979) 831. [4] B. Orberger, G. Friedrich, E. Woermann, J. Geoch. Exploration 37 (1) (1990) 147. [5] J.A. Maxwell, J.L. Campbell, W.J. Teedale, Nucl. Instr. and Meth. B 43 (1989) 218.
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581 [6] G.K. Czamanske, T.W. Sisson, J.L. Campbell, W.L. Teesdale, Amer. Mineralogist 78 (1993) 893. [7] M. Mosbah, N. Metrich, P. Massiot, Nucl. Instr. and Meth. B 58 (1991) 227.
581
[8] Y. Bogdanov, A. Sagalevitch, E. Chernyaev, A. Ashadze, E. Gurvich, V. Lukashin, G. Ivanov, V. Peresypkin, Bridge Newsletter 9 (1995) 9.
www.elsevier.nl/locate/nimb
Nuclear microprobe analysis of serpentine from the mid-Atlantic ridge Beate Orberger
a,1
, Nicole Metrich b, Michelle Mosbah Yves Fouquet d
b,*
, Catherine Mevel c,
a
Laboratoire de G eochimie, Universit e Paris Sud, F-91405 Orsay Cedex, France Laboratoire Pierre S ue, CEA-CNRS, CE-Saclay, F-91115 Gif-sur-Yvette, Cedex, France Laboratoire de P etrologie, CNRS UPRES A 7058, UPMC, Place Jussieu, F-75005 Paris, France d IFREMER-Brest, B.P. 70, F-29285 Plouzan e, France b
c
Abstract At mid-ocean ridges, ultrama®c rocks are serpentinized by interaction with seawater-derived ¯uids. Elements, dissolved in large quantities in seawater, e.g., Na, K, Cl, Br, Ca and Sr, can be, in small amounts, incorporated as traces into the crystal structure of the various serpentine minerals (Mg3 Si2 O5 (OH)4 ). These trace elements can be used to track the composition of the reacting ¯uids and to constrain physico±chemical conditions. This paper represents the ®rst application of particle-induced X- and c-ray emission (PIXE/PIGE) analysis to serpentine using the nuclear microprobe at the Laboratoire Pierre S ue (CEA-CNRS). Three types of serpentine, belonging to two dierent serpentinization generations, have been analysed in samples collected from the Mid-Atlantic Ridge (14°450 N/45°W) that exposes serpentinized peridotites on which the Logachev black smoker is placed. The trace elements Cl, F, S, Cu, Zn, Ca, K, Ni, Cr and Mn were detected from several tens to several thousands of ppm. Bromine, As and Sr are close to the detection limit of about 5 ppm. The trace element concentrations and interelement relationships in serpentines vary (a) with the serpentine type and (b) with the geographic location to the black smoker. Chlorine and in part S originated from seawater, whereas Cu, Zn, Ca, K, Ni, Cr and Fe and the major amount of S were mobilized from the unaltered host rock and partitioned between the serpentine and the aqueous solution. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Serpentine often represents hydration products of primary Mg±Fe silicates, such as olivine and
*
Corresponding author. Tel.: +33-1-69-08-57-86; fax: +331-69-08-69-23; e-mail: [email protected] 1 E-mail: [email protected]
pyroxenes. During serpentinization, some elements (Fe, Al, Ni, Cr) are mobilized and removed from the serpentine structure, whereas H2 O is added. Electron microprobe analysis on serpentines has been used since 30 yr for determination of major and minor element concentrations: Mg, Fe, Si, Al, Cr, Fe, Mn and Ni data were published by for example ([1] and refs. therein). Cr2 O3 can, thus, be detected from 0.02 to about 1.2 wt% and NiO
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 4 2 - 0
576
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581
from 0.02 to 0.5 wt%, CaO ranges from 0.02 to 0.24 wt% and TiO2 contents of 0.02 wt% [1,2]. However, most trace element concentrations in serpentines are below the detection limit of the electron microprobe. In addition, Cl contents of 0.03 up to 0.33 wt% and F contents of 0.02 up to 0.16 wt% were analysed e.g. Refs. [3,4]. The objective of this study was to analyse trace elements (e.g., Cl, Br, F, K, Na, Ca, Ni, Cr, Mn, Ti) which might give information about the seawater±peridotite interaction and element mobility during serpentinization, so that the physico± chemical conditions at serpentinization can be deduced. A nuclear microprobe technique was chosen, because it permits to analyse trace elements at levels ranging from several tens to several thousands ppm. Furthermore, it allows to perform non-destructive point analyses, preserving the primary texture of the sample. 2. Analytical method The rock samples were mounted on a trace element free ``SUPRASIL I'' (Heraeus) glass and were cut to 27 lm thickness to ensure that only serpentine was analysed. Sample surfaces were polished. Particle-induced X-ray emission (PIXE) analysis was performed using the nuclear microprobe at the Laboratoire Pierre S ue (CEA-CNRS, Saclay, France), with a beam-diameter of 10 ´ 10 lm2 . Elements such as S, Cl, K, Ca, Ti, Fe were analysed at 1.7 MeV incident proton energy (Ei ) with a 30 lm mylar absorber in front of the detector; elements with Z > 28 (e.g., Cu, Zn, Ga, Br, Rb, Sr) were analysed at 3.5 MeV Ei and with a 200 lm Al absorber. At this energy the cphotons produced by the reaction 19 F(p,p,c)19 F were also detected. The accumulated charge was 1 lC corresponding to a counting time of about 20 min per point for analyses at 1.7 MeV and 1.5±2 lC corresponding to 40±45 min for analyses at 3.5 MeV. The natural volcanic glass standards KE12 and VNM50, were analysed before and after the experiments in order to evaluate the F data ([3] and refs therein). The penetration depth of the proton beam in serpentine is 22 lm at 1.7 MeV and 84 lm at 3.5 MeV. Trace element concentrations were
calculated using the GUPIX software [5]. The accuracy and precision of the software are described in Ref. [6]. Data for Mn are of limited use; in Cr-rich samples Ka of Mn superposes Kb of Cr, thus leading to a signi®cant error. Mn data were accepted only for samples with <100 ppm Cr. Detection limits are for: S: 39 ppm, Cl: 27 ppm, K: 16 ppm, Ca: 12 ppm, Ti: 13 ppm, V: 16 ppm, Cr: 16 ppm, Fe: 28 ppm, Ni: 19 ppm, Cu: 19 ppm, Zn: 8 ppm, As: 4 ppm, Br: 4 ppm, Sr: 5 ppm, F: 90 ppm. Major elements (Mg, Si, Al, Fe) were analysed by electron microprobe at the Centre Camparis Universite 6, Paris. Scanning electron microscopy was performed on polished thin sections at the Laboratory of Geochemistry of Sedimentary Rocks at the University Paris XI to ensure that no sulphides were dispersed in the serpentine matrix. The Na contents analysed by PIGE are not presented here because, as for F, the cdetection is not a linear function of the interaction depth [7], but strongly decreases with the sample thickness. 2.1. Sample description The studied rocks represent serpentinized peridotites from the Mid Atlantic Ridge, sampled at 14°450 N/45°W along a 3 km long pro®le including the hydrothermal ®eld Logachev [8]. Logachev is the ®rst black smoker on top of ultrama®c rocks (Fig. 1). The samples were dragged from 2700 to 3300 m depth during the campaign Microsmoke 1995. All rocks are almost completely altered. The mineral assemblage consists of various serpentine minerals, amphiboles, talc, calcite as well as chromite and magnetite. Three dierent serpentine types were analysed: (1) bastite, the complete replacement of orthopyroxene [(Mg,Fe)2 Si2 O6 ] and minor clinopyroxene ([Ca(Mg,Fe)Si2 O6 ], Fig. 2a); (2) mesh serpentine replacing olivine [(Mg,Fe)2 SiO4 ] in MS 21±1, (3) serpentine veins, developing in cleavages of bastites or crosscutting bastite (Fig. 2b). 3. Results Twenty spot analyses were obtained on each sample (10 points on vein or mesh serpentine, 10
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581
577
Fig. 1. E±W cross-section A±B at 14°450 north along the Mid-Atlantic Ridge after Fouquet, 1994. *: sample numbers.
points on bastite). Representative analyses and average values are shown in Table 1a and b. The results are shown in Fig. 3a±d. Primary olivine and orthopyroxene from an unaltered sample were analysed under the same conditions as those for serpentine. Direct comparison between the trace element contents of these primary minerals and those in serpentines (especially for Ca, Ni and Fe) requires caution, as they are in¯uenced by the geological history (e.g., formation temperature, degree of partial melting, and mantle metasomatism). Bastite are characterized by, on average, higher Fe, Ca, Cr, Ti and Cl contents than vein serpentine (Table 1a). No signi®cant dierence has been observed between mesh and vein serpentine. The Fe content in bastite ranges from 3.5% to 4.2% and from 2.02% to 4.2% in vein serpentine.
The Cr content varies between 600 and 9000 ppm in bastites and between 18 and 670 ppm in vein serpentine. The Ca content in bastites range from 295 to about 2000 ppm. This is about a factor 2±4 higher compared to those from vein serpentines containing 122±450 ppm Ca. The Ti concentrations in bastites vary from 25 to 137 ppm attaining high values of on average 908 ppm. The Cl-content is a factor 1.6±3 higher in bastites (380±3500 ppm), than in vein serpentines (340±1350 ppm). In one sample, vein serpentine (MS 19±6) shows more likely bastite characteristics, except for Ca. Copper, Cl, Zn and, in part, Ti are enriched in serpentine from near the black smoker, whereas S and F are enriched in the more distal samples (Table 1a and b). Close to Logachev, Cl, Zn, Ti and Cu attain 3500, 200, 780 and 660 ppm,
Fig. 2. Serpentine from altered peridotites (14°450 N/45°W on the Mid Atlantic Ridge). (a) Bastite after pyroxene, (b) vein and mesh serpentine.
578
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581
Table 1 Representative nuclear microprobe analyses of serpentines (Laboratoire Pierre S ue, CEA-CNRS, Saclay, France) Vein serpentine (a)
Average
Bastite
Average
S (ppm) Cl K Ca Ti Cr Mn Ni Fe
MS 18-16 373 (26) 363 (13) 94 (9) 170 (8) <16 <16 509 (13) 3150 (38) 2.03 (61 ppm)
405 (28) 685 (24) 74 (7) 206 (9) <20 665 (13) * 2184 (37) 3.09 ( 77 ppm)
370 502 126 168 <16 573 n.p. 2024 2.08
282 (23) 694 (14) 84 (8) 570 (11) 152 (8) 7535 (38) * 1815 (36) 3.15 ( 95 ppm)
246 (25) 1428 (20) 97 (10) 626 (13) 150 (9) 7229 (29) * 716 (6) 4.55 (95 ppm)
267 1090 104 546 137 6485 * 1335 3.75
S (ppm) Cl K Ca Ti Cr Mn Ni Fe
MS 19-4 248 (22) 391 (16) <33 207 (17) 76 (6) 794 (16) * 2245 (34) 2.61 (78 ppm)
652 (26) 352 (15) <47 77 (6) <12 <17 752 (15) 3006 (30) 1.54 (62 ppm)
582 376 58 122 <15 <18 * 3002 2.02
357 (21) 475 (14) <51 (9) 246 (7) 103 (7) 6560 (26) * 2174 (44) 3.4 (85 ppm)
424 (25) 616 (18) 52 (9) 318 (9) 87 (8) 8190 (33) * 2312 (39) 3.79 (87 ppm)
416 594 72 295 85 6803 * 2197 3.5
S (ppm) Cl K Ca Ti Cr Mn Ni Fe (%)
MS 19-6 246 (22) 490 (15) 133 (9) 860 (9) <13 <18 937 (22) 1641 (33) 4.5 (90 ppm)
191 (12) 1024 (20) 146 (10) 935 (9) <13 <52 1005 (20) 1706 (34) 4.31 (86 ppm)
189 460 110 730 <19 27 843 1509 4.16
203 (11) 996 (20) 145 (10) 1000 (10) <20 <44 1020 (20) 1615 (32) 4.41 (88 ppm)
178 (21) 383 (15) <30 370 (7,4) <14 251 (10) 496 (15) 2010 (40) 4.4 (90 ppm)
210 818 130 757 <15 39 872 1560 4.25
Mesh serpentine +vein serpentine (olivine replacement)
Average
Bastite
MS 21-1 220 (5) 1117 (25) 72 (8) 399 (8) <25 62 (13) 286 (11) 2020 (30) 2.67 (80 ppm)
231 920 97 327 23 97 364 2367 3.02
307 (27) 2214 (22) 134 (11) 1210 (1) 854 (12) 5966 (25) * 1734 (35) 4.59 (92 ppm)
S (ppm) Cl K Ca Ti Cr Mn Ni Fe (%)
287 (26) 1353 (20) 75 (21) 385 (8) <34 119 (7) 359 (14) 2553 (36) 3.46 (70 ppm)
Average 289 (30) 2933 (30) 94 (9) 1008 (11) 787 (12) 6033 (30) * 1078 (40) 6.18 (120 ppm)
286 1991 107 767 (a) 908; (b) 25 (a) 6002; (b) 122 * 2013 4.12
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581
579
Table 1 (Continued) Vein serpentine (b)
Average
Bastite
Average
Ni Cu Zn F*
MS 18-16 2345 (47) <38 <19 404 (60)
3301 (53) <31 <32 505 (76)
2024 <30 <31 340
2281 (39) <28 <23 290 (43)
626 (13) <8 <14 454 (60)
1335 <23 <24 284
Ni (ppm) Cu Zn F*
MS 19-4 2577 (44) <24 37 (7) 265 (40)
2392 (41) <26 <34 227 (34)
3002 <32 <32 184
2666 (35) 59 (14) 48 (6) 516 (77)
2495 (25) 46 (11) 68 (4) 470 (70)
2197 45 56 470
Ni (ppm) Cu Zn F*
MS 19-6 1738 (35) <21 36 (6) 219 (33)
1499 (30) <29 61 (6) 237 (12)
1509 <18 53 160
1434 (30) <19 55 (7) 225 (34)
991 (25) <15 41 (5) <90
1560 <23 53 120
Average
Bastite
1650 210 240 <100
2820 (56) 38 (2) 81 (11) 140 (22)
Mesh/Vein serpentine Ni (ppm) Cu Zn F*
MS 21-1 1837 (46) 237 (24) 252 (13) 144 (22)
1869 (45) 172 (21) 218 (13) <90
Average* 1019 (40) 70 (16) 122 (12) <90
2013 (a) 209; (b) 46 197 <100
**
Average values of about 10 analyses. Errors in parenthesis. c-photons.
respectively (Fig. 3a±d); away from the smoker 380 ppm Cl, 31±68 ppm Zn, < 16 to 140 ppm Ti and 19 ppm to 68 ppm Cu are observed. Sulphur and F show the inverse behavior: up to 900 ppm S and 520 ppm F away from Logachev and 189±582 ppm S as well as <100 ppm F near the hydrothermal ®eld. Primary olivine and pyroxene also indicate S contents up to 160 ppm. The Ni (1509±3002 ppm) and K (58±130 ppm) contents are approximately the same for all three types of serpentines and do not change with the geographic locations (Table 1a). Most Br, Sr and As data are below the detection limits (Br: 4 ppm; As: 4 ppm, Sr: 5 ppm). A few analyses mainly in bastite, yielded values above detection limits, but with errors >10%: Br-8 and 15 ppm close to Logachev; As-6±17 ppm in the sample most distal from Logachev; Sr ± 24 ppm.
4. Discussion and conclusion This study demonstrates that the nuclear microprobe is a useful tool for analysing trace elements in serpentine at concentrations between several tens and several thousands ppm. The variation of trace element content and interelement relationships contributes to the understanding of the serpentinization process and to constrain the physico±chemical conditions. The results provide also a database for comparison with results of experimental studies realised to the seawater±rock interaction. The Cl content in serpentines suggests in®ltration of seawater into peridotite. Iron, Ni, Cr, Cu, Zn, Ca and K are inherited from primary olivine and pyroxene, and ± in part ± from Cr-spinels. The positive correlations for Fe/Cl, Zn/Cl and Ca/Cl (Fig. 3a,b,c), mainly in vein serpentine, indicate that all these
580
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581
Fig. 3. Interelement relationships of selected major and trace elements in serpentines. (a) Cl f(Fe); (b) Cl f(Zn); (c) Cl f(Ca); (d) S f(Ca). BS: Black smoker Logachev.
elements were incorporated during serpentinization through transport of metal-chlorine complexes in aqueous solution. The S content of the serpentines relates to the physico±chemical conditions at the dierent localities: At Logachev, relatively low S contents can be explained by partition of most of the S into sulphide minerals. The positive correlation of Ca and S in vein serpentines (Fig. 2d) from Logachev, indicates CaSO4 (anhydrite) precipitation and, thus, a higher oxygen fugacity than at sites away from the hydrothermal ®eld. Acknowledgements This work was possible thanks to the ®nancial support from the INSU-CNRS (Projet: Dorsale).
The samples were recovered during the Mid-Atlantic Ridge. cruise Microsmoke in 1995. The authors thank the technical sta of the van de Graa accelerator at the Laboratoire Pierre S ue (CEACNRS, Saclay), in particular L. Daudin and J.P. Gallien, for their help with analyses. References [1] M.A. Dungan, Canadian Mineralogist 17 (1979) 771. [2] F.J. Wicks, A.G. Plant, Canadian Mineralogist 17 (1979) 785. [3] T. Labotka, A.L. Albee, Canadian Mineralogist 17 (1979) 831. [4] B. Orberger, G. Friedrich, E. Woermann, J. Geoch. Exploration 37 (1) (1990) 147. [5] J.A. Maxwell, J.L. Campbell, W.J. Teedale, Nucl. Instr. and Meth. B 43 (1989) 218.
B. Orberger et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 575±581 [6] G.K. Czamanske, T.W. Sisson, J.L. Campbell, W.L. Teesdale, Amer. Mineralogist 78 (1993) 893. [7] M. Mosbah, N. Metrich, P. Massiot, Nucl. Instr. and Meth. B 58 (1991) 227.
581
[8] Y. Bogdanov, A. Sagalevitch, E. Chernyaev, A. Ashadze, E. Gurvich, V. Lukashin, G. Ivanov, V. Peresypkin, Bridge Newsletter 9 (1995) 9.