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Nd isotopic composition of Jurassic Tethys seawater and the genesis of Alpine Mn-deposits: Evidence from Sr-Nd isotope data
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Nd isotopic composition of Jurassic Tethys seawater and the genesis of Alpine Mn-deposits: Evidence from Sr-Nd isotope data P. STILLE’~*,N. CLAUER'
and J. ABRECH-?
‘Centre de Wmentologie
et G&chimie de la Surface, Universitk Louis Pasteur, F-67084 Strasbourg-Cedex,France %titute of Mineralogy and Petrology, University of Basel, CH-4056 Basel, Switzerland (Received April 22, 1988; accepted in revisedform February 20, 1989)
Abstract-Jurassic metabasalts, metasediments and Mn ores from the Pennine realm of the Alps were examined in order to establish the Nd isotopic composition of the Jurassic Tethys seawater and to elucidate the genesis of the Mn deposits. The highly positive initial tNdvalues (+7 to +9.8) of the metabasalts and their low s7Sr/*6Srratios (0.7028 to 0.70493 indicate that they originated from a depleted mantle. The initial cNdvalues of the cherts range between -5 and -9. The Sm-Nd isotope data indicate that they are primary mixtures of basalt and continental detritus. The smaller than 2 pm fractions of the cherts and of a marble, which probably represent the formerly authigenic material, show initial 6~ values ranging between -5.9 and -6.6. The average initial Sr isotopic composition of five Mn ores is 0.70730 f. 22, identical to that of contemporaneous Jurassic seawater. The initial Nd isotopic composition values of the Mn ores are very homogeneous. The average value of five ores, a leachate and residue of one of them, calculated for an age of 170 Ma, is 0.512082 * 19. It is suggested that the ores, together with the smaller than 2 pm fractions of the sediments. define the Jurassic Tethvs seawater isotonic composition. The resulting average 143Nd/‘UNdinitial value is 0.512089 + 17 (+qd -6.5 ? 0.6). -
sequences. The investigated metabasalts, cherts, marbles and Mn ores formed in the “southern” Piemontais belt, and based on lithostratigraphic comparisons, a Middle Jurassic age (150- 170 Ma) has been estimated for all of them (e.g. SUANA, 1984). A short description of the samples and the sampling sites is given in Table 1.
INTRODUCTION
the sedimentary
SEVERAL PREVIOUS STUDIES have attempted to define the Nd isotopic composition in ancient waters (HOOKER et aZ., 198 1; CHYI et al.,1984). These investigations were mainly concerned with metalliferous sediments and, based on investigations on recent marine sediments (O’NIONS et al., 1978; PIEPGRAS et al., 1979; GOLDSTEINand O’NIONS, 198 I), assume that their rare earth elements (REE) were precipitated from ocean water. In other approaches, the seawater Nd isotbpic ratios were determined from fossil carbonates and fish teeth (SHAWand WASSERBURG,1985; STAUDIGELet al., 1985/1986; GRAND JEAN et al., 1987). STILLE and CLAUER (1986) determined the fossil Nd isotopic composition of ihe Proterozoic Animikie seawater (Canada) using the initial ratio of a Sm-Nd isochron of authigenic argillites which were devoid of detrital material. The major aim of the present study is to determine the Nd isotopic composition of the Jurassic Tethys seawater in the Pennine realm of the Alps. During the Jurassic, the Penninic region was a vast subsiding area of complex basins, divided into two main oceanic belts: the “southern” Piemantis belt and the “northern” Valais belt (TROMPY, 1982). Today, this Penninic realm consists of mid-Jurassic to midCretaceous ophiolites and associated sediments of pelagic character, which often display the original stratigraphic sequence. Basaltic extrusives are in stratigraphic contact with radiolarian cherts and siliceous shales, which in turn are overlain by marbles (“Aptychus limestone”) and marls (TROMPY, 1979; SUANA, 1984). Numerous Mn deposits of various size and mineralogical composition, possibly of hydrothermal origin (SUANA, 1984; PETERS,1984), occur within
ANALYTICAL PROCEDURES The Sm-Nd isotope analyses were carried out at the Branch of Isotope Geology of the Riiksmuseet in Stockholm and at the Laboratory for Isotope Geochemistry and Mass-spectrometry,ETH, Ziirich. The analytical techniques used are reported in more detail by CLASSONet al. (1987) and STILLE(1987). The total blank for Nd was 0.2 to 0.3 ng. Elemental Sm and Nd were measured on double filaments using Finnigan Mat 261 mass spectrometers. The ‘43Nd/ lUNd ratios are normalized to a ‘46Nd/‘44Nd = 0.7219. The mean values and standard deviations of 14 analyses of the La Jolla standard in Stockholm and of four analyses in Ziirich were 0.5 11858 + O.OOOOO9 and 0.5 11856 + 0.000011, respectively. The isotope data are compiled in Table 2. The RbSr analyses were carried out at the Centre de S&mentologie et G&chimie de la Surface in Strasbourg using oxidized Ta single filament and a Cameca 206 S mass spectrometer. Five runs of NBS 987 isotope standard were made during the study. The mean *‘Sr/%r ratio is 0.7 1032 f 0.000 13. The data were normalized to a %r/**Sr ratio of 0.1194. The analytical errors are given at the 2 mn level. The Rb-Sr data are compiled in Table 3. RESULTS AND DISCUSSION The metabasalts
Calculated for a 170 Ma age, the highly positive tNdvalues (+6.9 to +9.8) and the low 87Sr/86Srratios (0.7028 to 0.7049) indicate that the basalts originated from mantle material depleted in LIL elements relative to a chondritic reservoir for a long period of time (Fig. 1). The Slighti)' lower CNdvalue of +6.9 and the higher Sr isotopic composition value of 0.7049 for one metabasalt sample (M08) may indicate seawater alteration and/or crustal contamination. The Jurassic metabasalts define the same depleted mantle evolution as the 1000
* Present address: Laboratory for Isotope Geochemistry, IKP, Swiss Federal Institute of Technology, Sonneggstrasse 5, CH-8092 Ziirich, Switzerland 1095
P. Stille, N. Clauer and J. Abrecht
1096 Table 1. Sample description Sample
Locality
Coordinate (a)
Microscopic description
MO 1
Falotta
158100/769300
MO 12
Lltzirirti
186100/773570
MO 2 Ma 12 MO6
Falotta Praborna Alp dig1 Plaz
158100/769300 58700/600800 158340/768840
MO9
Lake Marmorera
150760/769060
MO 8
Alp Flix
155880/768810
MO 4
Falotta
158100/769300
"Radiolarian chert": red. schistose. Recrystallized quartz matrix (70%). Sericite (30%) es sinole ore~ns or lavers. Stronolv folded. Hematite: es red or opaque'pigment; concentrated in mica laye&: "Radiolarian chert": reddish-brown. Quartz: strongly pigmented with hematite and Mn oxide. Mn ore: Braunite. + Mn silicates (3%). Mn ore: Braunite. T piemontite. Metabasalt: green.-slightly schistose. Fine grained matrix with relics of augitic pyroxene (20%): metamorphic minerals: albite. epidote. Metabsalt: dark green. Medium grained. Relictic pyroxene with secondary actinolite/chlorite along cracks. Matrix: chlorite. sericite. epidote. sphene. tourmaline. pumpellyite. opaques. Metabasalt (Pillow lava): green. Fine grained. Plagioclase laths in e matrix: epidote, chlorite, pumpellyite. actinolite. sphene. opaques. Marble ("Aptychus limestone"): light greenish. Calcite: Quartz. muscovite, chlorite.
(a) Swiss coordinate system
Ma old metabasaltic amphibolites from other localities in the Swiss Pennine nappes (STILLEand TATSUMOTO, 1985). Both the eNd-valuesof the Proterozoic amphibolites and the Jurassic metabasalts are consistent with a depleted mantle evolutionary model postulated earlier by GOLDSTEIN et al. (1984). The Nd isotope data are consistent with earlier observations which indicate that the metabasalts contain trace element characteristics typical for MOR basalts (DIETRICH, 1980; LAUL&HER and BERNOULLI, 1982; VUICHARD, 1984). The sediments The Rb-Sr isotope data of the cherts yield no age information (Table 3). This is to be expected since substantial detrital clay components (>2 pm, Table 4) have been incorporated in the cherts, and SiOz, nucleating on the clay material, may have preserved the isotopic characteristics of the detrital sources (WEIS and WASSERJXIRG,1987). Additionally,
Sm-Nd isotope data
Table 2. Sample
Sm(ppm)
Nd(ppm)
147Sm/144Nd
143Nd/144Nd
Nd
(a)
(b)
(c)
Metasedlments MO l/l MO l/l MO 13/l MO 1312 MO 1312 MO4 MO 4
MO MO MO MC MO
chert 2w chert chert 2um marble (d) 2um
3.22 5.29 0.881 1.81 2.65 1.15 7.81
1.54 1.54 12/l 10.29 12/l leach (e) 10.52 12/l residue (e) 6.91 1212 1.26 1213 0.81
15.85 22.26 3.94 7.91 12.33 5.71 51.20
0.1227 0.1435 0.1353 0.1380 0.1299 0.1217 0.0923
0.512090(26) 0.512244(34) 0.512260(40) 0.512305(10) 0.512259(10) 0.512346(24) 0.512220(10)
-9.1 -6.6 -6.1 -5.3 -6.0 -4.1 -5.9
7.63 7.95 41.02 41.20 36.59 6.89 3.94
0.1217 0.1175 0.1517 0.1543 0.1141 0.1106 0.1244
0.512222(19) 0.512163(51) 0.512291(44) 0.512230(10) 0.512205(13) 0.512234(23) 0.512220(25)
-6.5 -7.5 -5.8 -7.0 -6.7 -6.1 -6.6
14.17 6.60 19.15
0.1838 0.1900 0.1885
0.513119(13) 0.512988(28) 0.513132(19)
t9.6 +6.9 +9.8
Metabsalts MO 6 MO 8 MO9
(a) (b) (c) (d) (e)
the very high ‘rRb/%r ratios of the cherts, ranging between 10 and 20, and the resulting unreasonably low initial Sr isotopic compositions, point to a subsequent increase of the Rbf Sr ratios after the chert formation, possibly in Cretaceous time. A Cretaceous metamorphic event is supported by the study of DEUTSCH (1983) who found K-Ar ages of 62 to 84 Ma in amphiboles of Na-rich quartz-albite schists from the same region. These ages date a metamorphic event of pumpellyite-prehnite facies, correlated to the closure of the Pennine ocean and to thrusting of lower Austroalpine nappes over the Mesozoic ophiolites and sediments of the Pennine sedimentary environment. The Rb-Sr system of the one “aptythus limestone” sample (M04) also appears to be altered. Corrected for a 170 Ma depositional age, its isotopic composition is 0.70820 and thus is higher than the seawater Sr isotopic composition at that time (PETERMAN et al., 1970; VEIZER and COMPSTON, 1974; BURKEet al., 1982). Earlier investigations demonstrated that, in contrast to RbSr, the REE in sediments are characterized by only a negligible mobility (WI~DEMAN and HASKIN, 1973; N.+NCE and TAYUIR, 1976; MCLENNAN et al., 197 8). Tectonic processes occurring at converging plate boundaries and subsequent metamorphic transformations of the buried and accreted sediments are similarly not expected to cause Sm/Nd fmctionation (MICHARD et al., 1985). We therefore suggest that
Table 3. Rb-Sr isotope data Sample
'47Sm/144Nd ratios ere determined to e precision of 0.3% at the 2 omean level. Errors represent 2 omean and correspond tt last digits. cNd values ere deviations in parts per 10 from chondritic Nd. Age corrected for 170 m.y.. "aptychus limestone" Leached in concentrated HCl. The 6 molar HCl leachate failed.
87Sr186Sr
19.98 10.03 13.28 0.319
0.72419(8) 0.72027(12) 0.72397(13) 0.70902(5) 0.70897(5) (b)
174 183
0.229 0.08
0.902 919 3.99 999 22.1 880
0.003 0.012 0.072
0.70767(5) 0.70772(4) 0.70779(4) (b) 0.70764(7) 0.70725(8) 0.70711(6)
14.1
0.099 0.496 0.504
0.70452(4) 0.70609(7) 0.70480(6)
Wppm)
Metasediments MO MO MO MO
l/l 13/l 1312 4
chert 117 chert 43.6 chert 37.5 marble (a) 20.5
Iii:;: I43 12/l
4.31 2.07 5.97
87Rb/86Sr
Rb(ppm)
13.8 5.22
17.0 12.6 8.21 187
Metabsalts z: MO9
(a) "aptychus limestone" (b) duplicates (c) XRF-determination
80.8
1097
Isotope composition of Nd in the Tethys Sea c
P 3,0.5124-
p
P
0.5122_.--.
----.-
a5120 t 0.5112
t
t a208
1
Timein billionyears FIG.
lA
L2
2.2
before present
2. Sm-Nd
1. cNdvs.time diagram. The depleted mantle parameters are
from GOLDSTEIN et al. (1984). The cNd-valuesof the metabasalts, me&sediments and Mn-ores are recalculated for a depositional age of 170 Ma. The amphibolite. data point is from STILLEand TATSUMOT0(1985). the Sm-Nd system was only negligibly affected during the Cretaceous metamorphic event. The +d values of the cherts, recalculated to a depositional age of 170 Ma, range between -5 and -9 (Table 2), which together with the ‘47Sm/‘44Nd ratios may reflect mixing between basaltic and continental detritus (Fig. 2). The initial h,+-value of the “aptychus limestone” sample (M04) is -4, which is slightly higher than the values found in the cherts. The isotope. study of WEIS and WASSERBURG(1987) on siliceous deposits indicates that not only the initial Sr isotopic compositions but also the initial isotopic signatures of Nd in cherts directly reflect the nature of the depositional environment and the sources of detrital material. Deep-sea samples show typical seawater Nd and Sr isotopic compositions, whereas cherts from shallow or enclosed environments incorporate Sr and Nd from continental sources. In order to define the detrital components in these sediments, relative proportions of clay minerals in <2 pm fractions as well as in the whole rocks were determined by Xray powder difbactometry (Table 4). The <2 pm fractions of the chexts MO1 and MO1 3 represent 7.5 and 10 percent of the bulk rocks, respectively. Both are mixtures of fairly well-crystallized clay minerals and quartz. The clay minerals are mixtures of illite and chlorite. The <2 pm fraction in the “aptychus limestone” represents only 2.9 percent of the bulk rock and is a mixture of well-crystallized illite and chlorite with traces of feldspars. These different proportions of clay minerals and feldspars in the size fraction <2 pm cause their Table
4. Clay
mineraloqv
Sample
<2um vol.%(a)
m, 1 chert NO 13 chert MO 4
marble
>Zurn
clay/quartz(b)
vol.%(a)
carbonate
clay/quartz(b)
vol.%(a)
7.5 10.0
4.0 0.4
92.5 90.0
n.d. n.d.
__-_ __-_
2.9
;::
26.8
0.12
70.3
(c) (a) Volume X of total rock (b) Clay-quartz ratio determined by X-ray internal quartz-muscovite standard. (c) Chlorite-muscovite ratio
powder
.
0.10
diffraction
us,ng
.
.
0.11
a12
.
ari
.
.
.
~14 015 ani “%mP%d
for metabasalts
.
an
.
ai
.
am
-I
1098
P. Stille, N. Clauer and J. Abrecht
1984; PETERS, 1984). In a trace element discrimination triangle (Fe-Mn-(Ni + Co + Cu)) for manganese deposits of BONATTI et al. (1972; 1976) Peters observed that the Co, Ni
and Cu concentrations of the ores are low compared to modem hydrogeneous ferromanganese deposits. However, the Sm and Nd concentrations of the ores investigated in this study are rather high compared with present hydrothermal manganese deposits (PIEPGRASet al., 1979; CLAUER et al., 1984). The Nd composition values of the ores are very homogeneous and suggest that no significant detrital component has been incorporated during their formation. This is supported by the fact that the oxide leached by concentrated HCl (called leachate) and the silicate residue left after leaching (called residue) of ore MO 12/ 1 (Fig. 2) have Nd isotopic compositions which are identical to that of the untreated ores. The average composition value of all five ores, the leachate and the residue of one of them, calculated for a depositional age of 170 Ma, is0512082 + 0.000019 (tNd -6.6 f 0.4). Within error limits, this value is identical to the average composition value of 0.5 12 105 f 18 for the three <2 I.cm fractions of the sediments. The pronounced homogeneity of the Nd isotopic compositions of the Mn ores suggests that the REE originally precipitated directly from Jurassic Tethys seawater. If this is correct, then the average initial Nd isotopic composition of the Mn ores and the ~2 pm fractions together yield the best estimate of the Tethys seawater isotopic composition. The values, calculated for 170 Ma and 150 Ma depositional ages are identical within error limits and yield 0.5 12089 + 17 (tNd -6.5 + 0.6) and 0.5 12105 + 18 (eNd-6.7 + 0.6), respectively. A single measurement which we have recently performed on shell debris of 150 Ma-old belemnites from the “Tabular Jura” of northern Switzerland yields a similar Nd initial value of 0.51208 f 1 (FISCHER and GYGI, 1989). Thus, although the belemnites grew “far north” of the Piemontais belt in an epicontinental type sea, they show the same Tethys seawater Nd isotopic composition as the Mn ores and the ~2 pm fractions of the cherts and the limestone from the southern Piemontais belt. The initial tNd value of -6.5 is slightly lower than the tNd range of -3 to -5 predicted for the Jurassic Atlantic Ocean water (SHAW and WASSERBURG, 1985). These authors defined the seawater Nd isotopic composition on carbonates and phosphates from shelf sediments. However, a recent isotope study on fish debris suggests that diagenetic fluids may significantly alter the Nd isotopic composition of epicontinental seas as well as control the REE in marine phosphates (GRANDJEAN et al., 1988). Thus, according to these authors,
the diagenetic flux may regionally control the seawater Nd isotopic compositions in continental shelf and platform regions. However, since the lithologic sequence investigated in the present study was neither part of a continental shelf nor part of a platform, and since the Tethyan-Atlantic junction formed only during the middle Jurassic (TRUMPY, 1982), we suggest that the deduced seawater value is representative for the Tethys but not for the Atlantic some 150 to 170 Ma ago. In a comparable study, CHYI et al. ( 1984) described similar
Nd isotopic compositions for Jurassic Mn ores and associated sediments from the Franciscan Mn deposits. Both the alpine and the Franciscan deposits are similar in that the Mn ores were not directly deposited on the basalts, but interlayered
with the radiolarian cherts. Thus, both deposits may have a very similar origin. CHn et al. (1984) also deduced the seawater Nd isotopic composition for the Jurassic ocean directly from the sediments and the Mn ores. Because the Rb-Sr system of the sediments has been affected by the Cretaceous metamorphic event, the Rb/Sr system of the Mn ores was also presumably equally disturbed during this event. However, since the present-day Rb/Sr ratios of the ores are very low and the original Rb-Sr ratios may have also been very low (e.g. DASCH, 1969), it is probable that the age-corrected 87Sr/86Srratios are rather good estimates for the original Mn ores. Corrected for a 170 Ma depositional age, the five Mn ores have an average “Srf8%r isotopic composition of 0.70730 k 22, which is identical to Jurassic seawater values (PETERMANet al., 1970; VEIZERand COMPSTON, 1974; BURKE et al., 1982).
CONCLUSIONS The ~2 pm fractions of the cherts and of a marble have similar initial tNdvalues with a narrow range of -5.9 to -6.6. Since these fractions are assumed to represent the formerly authigenic components, their initial Nd isotopic compositions should be close to Tethys seawater values. The average initial Nd isotopic composition of 5 Mn ores and the leachate and the residue of one of them, calculated for a 170 Ma formation age, is 0.512082 + 0.000019 (CNd-6.6 f 0.4) some 170 Ma ago. Within error limits this value is identical to the average composition value of 0.5 12 105 f 18 for the three <2 pm fractions of the sediments. The seawater Sr isotopic composition and the pronounced homogeneity of the Nd isotopic composition values of the Mn ores suggest that the REE originally precipitated directly from Jurassic Tethys seawater. Thus, the average initial Nd isotopic composition of 0.5 12089 f 17 (+,d -6.5 f 0.4), calculated for a depositional age of 170 Ma for Mn ores and clays together, is the best estimate of the Tethys seawater isotopic composition. Acknowledgements-We would like to thank E. Welin and St. Claesson at the Naturhistotiska Riksmuseet in Stockholm, who kindly provided analytical facilitiesfor the chemical preparationand isotopic measurement of some of the Mn samples. We are grateful to W. B. Stem (University of Basel) for his kind support in X-my difbactometric measurements. We also thank B. Kiefel, D. Tiimnt and R. A. Wendling of the Centre de Sedimentologie et Geochimie de la Surface (Strasbourg, CNRS) for technical ass&me. We also thank S. L. Goldstein, M. Frev and F. Oberli for constructive discussions during the early stage ofthis work, F. Albarede for the helpful review and editorial handling, and G. Frtih-Green for English corrections. This work was funded by a grant from the Swiss National Science Foundation to P.S. Editorial handling: F. Albar&de REFERENCES BONA-II E., KRAEMER T. and RYDELLH. (1972) Classification and genesis of submarine iron-manganese deposits. In Ferromanganese Deposits on the Ocean-Floor. International Decade on Ocean Exploration (ed. D. HORN),pp. 149-16 1. Washington National Science Foundation. BONATTI E., ZERLII M., KAY R. and RYDELLH. (1976) Metalhferous deposits from the Apennine ophiolitea: Mesozoic equivalents of modern deposits from oceanic spreading centers. Geol. Sot. Amer. Bull. 87, 83-94.
Isotope composition of Nd in the Tethys Sea BURKE W. M., DENISONR. E., HETHERINGTONE. A., KOEPNICK R. B.. NELSON.M. F. and OTTOJ. B. f 1982) Variation of seawater *‘Sr/%r throughout Phanerozoic time. G&&y 10, 516-5 19. CHYI M. S., CRERARD. A., CARL~~NR. W. and STALLARDR. F. (1984) Hydrothermal Mndeposits of the Franciscan Assemblage: II. Isotope and trace element geochemistry and implications for hydrothermal convection at spreading centers. Earth Planet. Sci. Lett. 71, 31-45. CLAESSONST. (1987) Nd isotope data on 1.9- 1.2 Ga mafic rocks from the Bothnian basin, Central Sweden. Precamb. Rex Lett. 35, 115-126. CLAUERN., STILLEP., BONNOT-COURTOIS C. and MIRE W. S. (1984) Nd-Sr isotopic and REE constraints on the genesis of hydrothermal manganese crusts in the Galapagos. Nature 311,743745. DASCHE. J. (1969) Strontium isotopes in weathering profiles, deep sea sediments and sedimentary rocks. Geochim. Cosmochim. Acta 33, 1521-1552. DEUTSCHA. (1983) Datierungen an Alkaliamphibolen und Stilp nomelan aus der sildlichen Platta Decke (Graubiinden). Eclogae Geol. He/v. 76,295-308. DIETRICHV. J. (1980) The distribution of ophiolites in the Alps. Ojioliti# Special Issue, Terhyan ophiolites 1,7-5 1. FISCHERH. and GYGI R. A. (1989) Numerical and biochronological time scales correlated at the ammonite. subzone level; K-Ar, RbSr ages and Sr, Nd and Pb seawater isotopes in an Oxfordian (late Jurassic) succession of northern Switzerland. Geol. Sot. Amer. Bull. (submitted). GOLDSTEINS. L. and O’NIONS R. K. (1981) Nd and Sr isotopic relationships in pelagic clays and ferromanganese deposits. Nature 295324-327. GOLDSTEINS. L., O’NIONSR. K. and HAMILTONP. J. (1984) A SmNd isotopic study of atmospheric dusts and par&dates from major river systems. Earth Planet. Sci. Lett. 70,221-236. GRANDJEANP., CAPPETTAH., MICHARDA. and ALBAR&DE F. (1987) The assessment of REE patterns and 14’Nd/‘“Nd ratios in fish remains. Earth Planet. Sci. I&t. 84, 181-196. GRANDJEANP., CAPPETTAH. and ALBAIUEDE F. (1988) The REE and sNdof 40-70 Ma old fish &bris from the West-African platform. Geophys. Rex Lett 15, 389-392. HOOKERP. J., HAMILTONP. J. and O’NIONS R. K. (198 1) An estimate of the Nd isotopic composition of Iapetus seawater from ca. 490 Ma metalliferous sediments. Earth Planet. Sci. I.&. 56, 180-188. LAUBSCHERH. and BERNOULLID. (1982) History and deformation of the Alps. In Mountain Building Processes (ed. K. J. HsO), pp. 169- 180. Academic Press. MCLENNANS. M., FRYERB. J. and YOUNGG. M. (1978) Rare earth elements in Huronian (Lower Proterozoic) sedimentary rocks: Composition and evolution of the post-Kenoran upper crust. Geechim. Cosmochim. Acta 43, 375-388. MICHARDA., GURRIET P., SOUDANTM. and AL-BA&DEF. (1985) Nd isotopes in French Phaherozoic shales: External vs. internal aspects of crustal evolution. Geochim. Cosmochim. Acta 49,601610. NANCEW. B. and TAYLORS. R. (1976) Rare. earth element patterns
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and crustal evolution. l-Australian post-Archaean sedimentary rocks. Geochim. Cosmochim. Acta 40, 1539-1551. O?NIONSR. K., CARTERS. R., COHENR. S., EVENSONN. M. and HAMILTONP. J. (1978) Pb, Nd and Sr isotopes in oceanic ferromanganese deposits and ocean floor basalts. Nature 273,435-438. PETERMAN2. E., HEDGEC. E. and TOURTELOT H. A. (1970) Isotopic composition of strontium in seawater throughout Phanerozoic time. Geochim. Cosmochim. Acta 34, 105-120. PETERST. J. (1984) Geochemie der Oberhalbsteiner und einiger weiterer penninischer Manganvorkommen. Beitr. Geol. Schweiz, Geotechn. Ser. 64,82-93. PIEKXAS D. J., WAS~ERBURGG. J. and DA~CH E. J. (1979) The isotopic composition of Nd in different ocean masses. Earth Planet. Sci. Lett. 45,223-236. SHAWH. F. and WASSERFIURG G. J. (1985) Sm-Nd in marine carbonates and phosphates: Implications for Nd Isotopes in seawater and crustal ages. Geochim. Cosmochim. Acta 49,503-5 18. STAUDIGELH., DOYLEP. and ZINDLERA. (1985/1986) Sr and Nd isotope systematics in fish teeth. Earth Planet. Sci. Lett. 76, 4556. STILLE P. (1987) Geochemische Aspekte der Krustenevolution im zentral-und siidaIpinen Raum. Habilitationsschrift, ETH Zilrich. STILLEP. and BULETTIM. (1987) Nd-Sr isotopic characteristics of the Lugano porphyrites and constraints on the continental crust formation in the South Alpine domain (North Italy-Switzerland). Contrib. Mineral. Petrol. 96, 140- 150. STILLEP. and CUUER N. (1986) Sm-Nd isochron age and provenan= of the argillites of the Guntlint Iron Formation in Ontario, Canada. Geochim. Cosmochim. Acta 50, 1141-l 146. STILLEP. and TAT~UMOTOM. (1985) Precambrian tholeiiticdacitic rock suites and Cambrian ultramafic rocks in the Pennine nappe system of the Alps: Evidence from Sm-Nd isotopes and rare earth elements. Contrib. Mineral. Petrol. 89, 184- 192. SUANAM. (1984) Die Manganerzlagerstitten von Tinizong (Oberhalbstein, Graubiinden). Beitr. Geol. Schweiz. Geotech. Ser. 64, l-93. TROMPYR. (1979) Geology of Switzerland, Part 1. A Guide Book. Wepf & Co., Basel, 102~. TROMPY R. (1982) Alpine palaeogeography: A reappraisal. In Mountain Building Processes (ed. K. J. HsO), pp. 149-168. Academic Press. VEIZERJ. and COMPSTON W. ( 1974) *7Sr/86Srcomposition of seawater during the Phanerozoic. Geochim. Cosmochim. Acta 38, 14611484. VUICHARDD. (1984) The ophiolitic suite of the alp Champatsch (Lower Engadine window, Switzerland): The metamorphic and tectonic evolution of a small oceanic basin in the Penninic realm. Ofioliti 9,6 19-632. WEISD. and WASSERBURG G. J. (1987) Rb-Sr and Sm-Nd systematics of cherts and other siliceous deposits. Geochim. Cosmochim. Acta 51,959-972. WEI~SERTH. J. and BERNOULLID. (1985) A transform margin in the Mesozoic Tethys: Evidence from the Swiss Alps. Geol. Rundsch. 74,665-679. WILDEMANT. R. and HASKINL. A. (1973) Rare earths in Precambrian sediments. Geochim. Cosmochim. Acta 37,4 19-438.
Gewhimica d Cosmochimica Acfa Vol. 53, PP. 1095-1099 Press plc.F’rinted inU.S.A.
+ 00
@pyright @1989 Perpmon
Nd isotopic composition of Jurassic Tethys seawater and the genesis of Alpine Mn-deposits: Evidence from Sr-Nd isotope data P. STILLE’~*,N. CLAUER'
and J. ABRECH-?
‘Centre de Wmentologie
et G&chimie de la Surface, Universitk Louis Pasteur, F-67084 Strasbourg-Cedex,France %titute of Mineralogy and Petrology, University of Basel, CH-4056 Basel, Switzerland (Received April 22, 1988; accepted in revisedform February 20, 1989)
Abstract-Jurassic metabasalts, metasediments and Mn ores from the Pennine realm of the Alps were examined in order to establish the Nd isotopic composition of the Jurassic Tethys seawater and to elucidate the genesis of the Mn deposits. The highly positive initial tNdvalues (+7 to +9.8) of the metabasalts and their low s7Sr/*6Srratios (0.7028 to 0.70493 indicate that they originated from a depleted mantle. The initial cNdvalues of the cherts range between -5 and -9. The Sm-Nd isotope data indicate that they are primary mixtures of basalt and continental detritus. The smaller than 2 pm fractions of the cherts and of a marble, which probably represent the formerly authigenic material, show initial 6~ values ranging between -5.9 and -6.6. The average initial Sr isotopic composition of five Mn ores is 0.70730 f. 22, identical to that of contemporaneous Jurassic seawater. The initial Nd isotopic composition values of the Mn ores are very homogeneous. The average value of five ores, a leachate and residue of one of them, calculated for an age of 170 Ma, is 0.512082 * 19. It is suggested that the ores, together with the smaller than 2 pm fractions of the sediments. define the Jurassic Tethvs seawater isotonic composition. The resulting average 143Nd/‘UNdinitial value is 0.512089 + 17 (+qd -6.5 ? 0.6). -
sequences. The investigated metabasalts, cherts, marbles and Mn ores formed in the “southern” Piemontais belt, and based on lithostratigraphic comparisons, a Middle Jurassic age (150- 170 Ma) has been estimated for all of them (e.g. SUANA, 1984). A short description of the samples and the sampling sites is given in Table 1.
INTRODUCTION
the sedimentary
SEVERAL PREVIOUS STUDIES have attempted to define the Nd isotopic composition in ancient waters (HOOKER et aZ., 198 1; CHYI et al.,1984). These investigations were mainly concerned with metalliferous sediments and, based on investigations on recent marine sediments (O’NIONS et al., 1978; PIEPGRAS et al., 1979; GOLDSTEINand O’NIONS, 198 I), assume that their rare earth elements (REE) were precipitated from ocean water. In other approaches, the seawater Nd isotbpic ratios were determined from fossil carbonates and fish teeth (SHAWand WASSERBURG,1985; STAUDIGELet al., 1985/1986; GRAND JEAN et al., 1987). STILLE and CLAUER (1986) determined the fossil Nd isotopic composition of ihe Proterozoic Animikie seawater (Canada) using the initial ratio of a Sm-Nd isochron of authigenic argillites which were devoid of detrital material. The major aim of the present study is to determine the Nd isotopic composition of the Jurassic Tethys seawater in the Pennine realm of the Alps. During the Jurassic, the Penninic region was a vast subsiding area of complex basins, divided into two main oceanic belts: the “southern” Piemantis belt and the “northern” Valais belt (TROMPY, 1982). Today, this Penninic realm consists of mid-Jurassic to midCretaceous ophiolites and associated sediments of pelagic character, which often display the original stratigraphic sequence. Basaltic extrusives are in stratigraphic contact with radiolarian cherts and siliceous shales, which in turn are overlain by marbles (“Aptychus limestone”) and marls (TROMPY, 1979; SUANA, 1984). Numerous Mn deposits of various size and mineralogical composition, possibly of hydrothermal origin (SUANA, 1984; PETERS,1984), occur within
ANALYTICAL PROCEDURES The Sm-Nd isotope analyses were carried out at the Branch of Isotope Geology of the Riiksmuseet in Stockholm and at the Laboratory for Isotope Geochemistry and Mass-spectrometry,ETH, Ziirich. The analytical techniques used are reported in more detail by CLASSONet al. (1987) and STILLE(1987). The total blank for Nd was 0.2 to 0.3 ng. Elemental Sm and Nd were measured on double filaments using Finnigan Mat 261 mass spectrometers. The ‘43Nd/ lUNd ratios are normalized to a ‘46Nd/‘44Nd = 0.7219. The mean values and standard deviations of 14 analyses of the La Jolla standard in Stockholm and of four analyses in Ziirich were 0.5 11858 + O.OOOOO9 and 0.5 11856 + 0.000011, respectively. The isotope data are compiled in Table 2. The RbSr analyses were carried out at the Centre de S&mentologie et G&chimie de la Surface in Strasbourg using oxidized Ta single filament and a Cameca 206 S mass spectrometer. Five runs of NBS 987 isotope standard were made during the study. The mean *‘Sr/%r ratio is 0.7 1032 f 0.000 13. The data were normalized to a %r/**Sr ratio of 0.1194. The analytical errors are given at the 2 mn level. The Rb-Sr data are compiled in Table 3. RESULTS AND DISCUSSION The metabasalts
Calculated for a 170 Ma age, the highly positive tNdvalues (+6.9 to +9.8) and the low 87Sr/86Srratios (0.7028 to 0.7049) indicate that the basalts originated from mantle material depleted in LIL elements relative to a chondritic reservoir for a long period of time (Fig. 1). The Slighti)' lower CNdvalue of +6.9 and the higher Sr isotopic composition value of 0.7049 for one metabasalt sample (M08) may indicate seawater alteration and/or crustal contamination. The Jurassic metabasalts define the same depleted mantle evolution as the 1000
* Present address: Laboratory for Isotope Geochemistry, IKP, Swiss Federal Institute of Technology, Sonneggstrasse 5, CH-8092 Ziirich, Switzerland 1095
P. Stille, N. Clauer and J. Abrecht
1096 Table 1. Sample description Sample
Locality
Coordinate (a)
Microscopic description
MO 1
Falotta
158100/769300
MO 12
Lltzirirti
186100/773570
MO 2 Ma 12 MO6
Falotta Praborna Alp dig1 Plaz
158100/769300 58700/600800 158340/768840
MO9
Lake Marmorera
150760/769060
MO 8
Alp Flix
155880/768810
MO 4
Falotta
158100/769300
"Radiolarian chert": red. schistose. Recrystallized quartz matrix (70%). Sericite (30%) es sinole ore~ns or lavers. Stronolv folded. Hematite: es red or opaque'pigment; concentrated in mica laye&: "Radiolarian chert": reddish-brown. Quartz: strongly pigmented with hematite and Mn oxide. Mn ore: Braunite. + Mn silicates (3%). Mn ore: Braunite. T piemontite. Metabasalt: green.-slightly schistose. Fine grained matrix with relics of augitic pyroxene (20%): metamorphic minerals: albite. epidote. Metabsalt: dark green. Medium grained. Relictic pyroxene with secondary actinolite/chlorite along cracks. Matrix: chlorite. sericite. epidote. sphene. tourmaline. pumpellyite. opaques. Metabasalt (Pillow lava): green. Fine grained. Plagioclase laths in e matrix: epidote, chlorite, pumpellyite. actinolite. sphene. opaques. Marble ("Aptychus limestone"): light greenish. Calcite: Quartz. muscovite, chlorite.
(a) Swiss coordinate system
Ma old metabasaltic amphibolites from other localities in the Swiss Pennine nappes (STILLEand TATSUMOTO, 1985). Both the eNd-valuesof the Proterozoic amphibolites and the Jurassic metabasalts are consistent with a depleted mantle evolutionary model postulated earlier by GOLDSTEIN et al. (1984). The Nd isotope data are consistent with earlier observations which indicate that the metabasalts contain trace element characteristics typical for MOR basalts (DIETRICH, 1980; LAUL&HER and BERNOULLI, 1982; VUICHARD, 1984). The sediments The Rb-Sr isotope data of the cherts yield no age information (Table 3). This is to be expected since substantial detrital clay components (>2 pm, Table 4) have been incorporated in the cherts, and SiOz, nucleating on the clay material, may have preserved the isotopic characteristics of the detrital sources (WEIS and WASSERJXIRG,1987). Additionally,
Sm-Nd isotope data
Table 2. Sample
Sm(ppm)
Nd(ppm)
147Sm/144Nd
143Nd/144Nd
Nd
(a)
(b)
(c)
Metasedlments MO l/l MO l/l MO 13/l MO 1312 MO 1312 MO4 MO 4
MO MO MO MC MO
chert 2w chert chert 2um marble (d) 2um
3.22 5.29 0.881 1.81 2.65 1.15 7.81
1.54 1.54 12/l 10.29 12/l leach (e) 10.52 12/l residue (e) 6.91 1212 1.26 1213 0.81
15.85 22.26 3.94 7.91 12.33 5.71 51.20
0.1227 0.1435 0.1353 0.1380 0.1299 0.1217 0.0923
0.512090(26) 0.512244(34) 0.512260(40) 0.512305(10) 0.512259(10) 0.512346(24) 0.512220(10)
-9.1 -6.6 -6.1 -5.3 -6.0 -4.1 -5.9
7.63 7.95 41.02 41.20 36.59 6.89 3.94
0.1217 0.1175 0.1517 0.1543 0.1141 0.1106 0.1244
0.512222(19) 0.512163(51) 0.512291(44) 0.512230(10) 0.512205(13) 0.512234(23) 0.512220(25)
-6.5 -7.5 -5.8 -7.0 -6.7 -6.1 -6.6
14.17 6.60 19.15
0.1838 0.1900 0.1885
0.513119(13) 0.512988(28) 0.513132(19)
t9.6 +6.9 +9.8
Metabsalts MO 6 MO 8 MO9
(a) (b) (c) (d) (e)
the very high ‘rRb/%r ratios of the cherts, ranging between 10 and 20, and the resulting unreasonably low initial Sr isotopic compositions, point to a subsequent increase of the Rbf Sr ratios after the chert formation, possibly in Cretaceous time. A Cretaceous metamorphic event is supported by the study of DEUTSCH (1983) who found K-Ar ages of 62 to 84 Ma in amphiboles of Na-rich quartz-albite schists from the same region. These ages date a metamorphic event of pumpellyite-prehnite facies, correlated to the closure of the Pennine ocean and to thrusting of lower Austroalpine nappes over the Mesozoic ophiolites and sediments of the Pennine sedimentary environment. The Rb-Sr system of the one “aptythus limestone” sample (M04) also appears to be altered. Corrected for a 170 Ma depositional age, its isotopic composition is 0.70820 and thus is higher than the seawater Sr isotopic composition at that time (PETERMAN et al., 1970; VEIZER and COMPSTON, 1974; BURKEet al., 1982). Earlier investigations demonstrated that, in contrast to RbSr, the REE in sediments are characterized by only a negligible mobility (WI~DEMAN and HASKIN, 1973; N.+NCE and TAYUIR, 1976; MCLENNAN et al., 197 8). Tectonic processes occurring at converging plate boundaries and subsequent metamorphic transformations of the buried and accreted sediments are similarly not expected to cause Sm/Nd fmctionation (MICHARD et al., 1985). We therefore suggest that
Table 3. Rb-Sr isotope data Sample
'47Sm/144Nd ratios ere determined to e precision of 0.3% at the 2 omean level. Errors represent 2 omean and correspond tt last digits. cNd values ere deviations in parts per 10 from chondritic Nd. Age corrected for 170 m.y.. "aptychus limestone" Leached in concentrated HCl. The 6 molar HCl leachate failed.
87Sr186Sr
19.98 10.03 13.28 0.319
0.72419(8) 0.72027(12) 0.72397(13) 0.70902(5) 0.70897(5) (b)
174 183
0.229 0.08
0.902 919 3.99 999 22.1 880
0.003 0.012 0.072
0.70767(5) 0.70772(4) 0.70779(4) (b) 0.70764(7) 0.70725(8) 0.70711(6)
14.1
0.099 0.496 0.504
0.70452(4) 0.70609(7) 0.70480(6)
Wppm)
Metasediments MO MO MO MO
l/l 13/l 1312 4
chert 117 chert 43.6 chert 37.5 marble (a) 20.5
Iii:;: I43 12/l
4.31 2.07 5.97
87Rb/86Sr
Rb(ppm)
13.8 5.22
17.0 12.6 8.21 187
Metabsalts z: MO9
(a) "aptychus limestone" (b) duplicates (c) XRF-determination
80.8
1097
Isotope composition of Nd in the Tethys Sea c
P 3,0.5124-
p
P
0.5122_.--.
----.-
a5120 t 0.5112
t
t a208
1
Timein billionyears FIG.
lA
L2
2.2
before present
2. Sm-Nd
1. cNdvs.time diagram. The depleted mantle parameters are
from GOLDSTEIN et al. (1984). The cNd-valuesof the metabasalts, me&sediments and Mn-ores are recalculated for a depositional age of 170 Ma. The amphibolite. data point is from STILLEand TATSUMOT0(1985). the Sm-Nd system was only negligibly affected during the Cretaceous metamorphic event. The +d values of the cherts, recalculated to a depositional age of 170 Ma, range between -5 and -9 (Table 2), which together with the ‘47Sm/‘44Nd ratios may reflect mixing between basaltic and continental detritus (Fig. 2). The initial h,+-value of the “aptychus limestone” sample (M04) is -4, which is slightly higher than the values found in the cherts. The isotope. study of WEIS and WASSERBURG(1987) on siliceous deposits indicates that not only the initial Sr isotopic compositions but also the initial isotopic signatures of Nd in cherts directly reflect the nature of the depositional environment and the sources of detrital material. Deep-sea samples show typical seawater Nd and Sr isotopic compositions, whereas cherts from shallow or enclosed environments incorporate Sr and Nd from continental sources. In order to define the detrital components in these sediments, relative proportions of clay minerals in <2 pm fractions as well as in the whole rocks were determined by Xray powder difbactometry (Table 4). The <2 pm fractions of the chexts MO1 and MO1 3 represent 7.5 and 10 percent of the bulk rocks, respectively. Both are mixtures of fairly well-crystallized clay minerals and quartz. The clay minerals are mixtures of illite and chlorite. The <2 pm fraction in the “aptychus limestone” represents only 2.9 percent of the bulk rock and is a mixture of well-crystallized illite and chlorite with traces of feldspars. These different proportions of clay minerals and feldspars in the size fraction <2 pm cause their Table
4. Clay
mineraloqv
Sample
<2um vol.%(a)
m, 1 chert NO 13 chert MO 4
marble
>Zurn
clay/quartz(b)
vol.%(a)
carbonate
clay/quartz(b)
vol.%(a)
7.5 10.0
4.0 0.4
92.5 90.0
n.d. n.d.
__-_ __-_
2.9
;::
26.8
0.12
70.3
(c) (a) Volume X of total rock (b) Clay-quartz ratio determined by X-ray internal quartz-muscovite standard. (c) Chlorite-muscovite ratio
powder
.
0.10
diffraction
us,ng
.
.
0.11
a12
.
ari
.
.
.
~14 015 ani “%mP%d
for metabasalts
.
an
.
ai
.
am
-I
1098
P. Stille, N. Clauer and J. Abrecht
1984; PETERS, 1984). In a trace element discrimination triangle (Fe-Mn-(Ni + Co + Cu)) for manganese deposits of BONATTI et al. (1972; 1976) Peters observed that the Co, Ni
and Cu concentrations of the ores are low compared to modem hydrogeneous ferromanganese deposits. However, the Sm and Nd concentrations of the ores investigated in this study are rather high compared with present hydrothermal manganese deposits (PIEPGRASet al., 1979; CLAUER et al., 1984). The Nd composition values of the ores are very homogeneous and suggest that no significant detrital component has been incorporated during their formation. This is supported by the fact that the oxide leached by concentrated HCl (called leachate) and the silicate residue left after leaching (called residue) of ore MO 12/ 1 (Fig. 2) have Nd isotopic compositions which are identical to that of the untreated ores. The average composition value of all five ores, the leachate and the residue of one of them, calculated for a depositional age of 170 Ma, is0512082 + 0.000019 (tNd -6.6 f 0.4). Within error limits, this value is identical to the average composition value of 0.5 12 105 f 18 for the three <2 I.cm fractions of the sediments. The pronounced homogeneity of the Nd isotopic compositions of the Mn ores suggests that the REE originally precipitated directly from Jurassic Tethys seawater. If this is correct, then the average initial Nd isotopic composition of the Mn ores and the ~2 pm fractions together yield the best estimate of the Tethys seawater isotopic composition. The values, calculated for 170 Ma and 150 Ma depositional ages are identical within error limits and yield 0.5 12089 + 17 (tNd -6.5 + 0.6) and 0.5 12105 + 18 (eNd-6.7 + 0.6), respectively. A single measurement which we have recently performed on shell debris of 150 Ma-old belemnites from the “Tabular Jura” of northern Switzerland yields a similar Nd initial value of 0.51208 f 1 (FISCHER and GYGI, 1989). Thus, although the belemnites grew “far north” of the Piemontais belt in an epicontinental type sea, they show the same Tethys seawater Nd isotopic composition as the Mn ores and the ~2 pm fractions of the cherts and the limestone from the southern Piemontais belt. The initial tNd value of -6.5 is slightly lower than the tNd range of -3 to -5 predicted for the Jurassic Atlantic Ocean water (SHAW and WASSERBURG, 1985). These authors defined the seawater Nd isotopic composition on carbonates and phosphates from shelf sediments. However, a recent isotope study on fish debris suggests that diagenetic fluids may significantly alter the Nd isotopic composition of epicontinental seas as well as control the REE in marine phosphates (GRANDJEAN et al., 1988). Thus, according to these authors,
the diagenetic flux may regionally control the seawater Nd isotopic compositions in continental shelf and platform regions. However, since the lithologic sequence investigated in the present study was neither part of a continental shelf nor part of a platform, and since the Tethyan-Atlantic junction formed only during the middle Jurassic (TRUMPY, 1982), we suggest that the deduced seawater value is representative for the Tethys but not for the Atlantic some 150 to 170 Ma ago. In a comparable study, CHYI et al. ( 1984) described similar
Nd isotopic compositions for Jurassic Mn ores and associated sediments from the Franciscan Mn deposits. Both the alpine and the Franciscan deposits are similar in that the Mn ores were not directly deposited on the basalts, but interlayered
with the radiolarian cherts. Thus, both deposits may have a very similar origin. CHn et al. (1984) also deduced the seawater Nd isotopic composition for the Jurassic ocean directly from the sediments and the Mn ores. Because the Rb-Sr system of the sediments has been affected by the Cretaceous metamorphic event, the Rb/Sr system of the Mn ores was also presumably equally disturbed during this event. However, since the present-day Rb/Sr ratios of the ores are very low and the original Rb-Sr ratios may have also been very low (e.g. DASCH, 1969), it is probable that the age-corrected 87Sr/86Srratios are rather good estimates for the original Mn ores. Corrected for a 170 Ma depositional age, the five Mn ores have an average “Srf8%r isotopic composition of 0.70730 k 22, which is identical to Jurassic seawater values (PETERMANet al., 1970; VEIZERand COMPSTON, 1974; BURKE et al., 1982).
CONCLUSIONS The ~2 pm fractions of the cherts and of a marble have similar initial tNdvalues with a narrow range of -5.9 to -6.6. Since these fractions are assumed to represent the formerly authigenic components, their initial Nd isotopic compositions should be close to Tethys seawater values. The average initial Nd isotopic composition of 5 Mn ores and the leachate and the residue of one of them, calculated for a 170 Ma formation age, is 0.512082 + 0.000019 (CNd-6.6 f 0.4) some 170 Ma ago. Within error limits this value is identical to the average composition value of 0.5 12 105 f 18 for the three <2 pm fractions of the sediments. The seawater Sr isotopic composition and the pronounced homogeneity of the Nd isotopic composition values of the Mn ores suggest that the REE originally precipitated directly from Jurassic Tethys seawater. Thus, the average initial Nd isotopic composition of 0.5 12089 f 17 (+,d -6.5 f 0.4), calculated for a depositional age of 170 Ma for Mn ores and clays together, is the best estimate of the Tethys seawater isotopic composition. Acknowledgements-We would like to thank E. Welin and St. Claesson at the Naturhistotiska Riksmuseet in Stockholm, who kindly provided analytical facilitiesfor the chemical preparationand isotopic measurement of some of the Mn samples. We are grateful to W. B. Stem (University of Basel) for his kind support in X-my difbactometric measurements. We also thank B. Kiefel, D. Tiimnt and R. A. Wendling of the Centre de Sedimentologie et Geochimie de la Surface (Strasbourg, CNRS) for technical ass&me. We also thank S. L. Goldstein, M. Frev and F. Oberli for constructive discussions during the early stage ofthis work, F. Albarede for the helpful review and editorial handling, and G. Frtih-Green for English corrections. This work was funded by a grant from the Swiss National Science Foundation to P.S. Editorial handling: F. Albar&de REFERENCES BONA-II E., KRAEMER T. and RYDELLH. (1972) Classification and genesis of submarine iron-manganese deposits. In Ferromanganese Deposits on the Ocean-Floor. International Decade on Ocean Exploration (ed. D. HORN),pp. 149-16 1. Washington National Science Foundation. BONATTI E., ZERLII M., KAY R. and RYDELLH. (1976) Metalhferous deposits from the Apennine ophiolitea: Mesozoic equivalents of modern deposits from oceanic spreading centers. Geol. Sot. Amer. Bull. 87, 83-94.
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