Sr Isotopes and REEs Geochemistry of Anhydrites from L Vent Black Smoker Chimney, East Pacific Rise 9ºN 10ºN

Transcription

1 Journal of Earth Science, Vol. 26, No. 6, p , December 2015 ISSN X Printed in China DOI: /s Sr Isotopes and REEs Geochemistry of Anhydrites from L Vent Black Smoker Chimney, East Pacific Rise 9ºN 10ºN Huiqiang Yao* 1, Huaiyang Zhou* 2, 3, Xiaotong Peng 2, 3, Gaowen He 1 1. Guangzhou Marine Geological Survey, Guangzhou , China 2. National Key Laboratory of Marine Geology, Tongji University, Shanghai , China 3. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou , China ABSTRACT: High resolution sampling, for Sr isotope and REE analyses, was carried out along a transaction of L vent chimney collected from East Pacific Rise 9ºN 10ºN. Sr isotopes show these anhydrites are precipitated from a mixture between hydrothermal fluid and seawater. The calculated relative proportion of seawater and hydrothermal fluid shows that the mixing is heterogeneous on the transection of the L vent chimney. Anhydrites from the chimney show uniform chondrite-normalized REE pattern with enrichment of LREE and positive Eu anomaly. While normalized to the REE of end-member hydrothermal fluid, anhydrites also show uniform REE pattern but with negative Eu anomaly and enrichment of HREE. Combining previous studies on REEs of hydrothermal fluids from different hydrothermal systems and the hydrothermal fluid data from this region, we suggested that REE-anion complexing, rather than crystallography controlling, is the main factor that controls the REE partition behavior in the anhydrite during its precipitation from the mixture of hydrothermal fluid and seawater. KEY WORDS: East Pacific Rise 9ºN 10ºN, black smoker chimney, anhydrite, Sr isotope, REE. 0 INTRODUCTION Anhydrite is a unique mineral existing in the hydrothermal system for its retrograde solubility with temperature increasing and has been the focus of many researchers (Chiba et al., 1998; Mills et al., 1998; Teagle et al., 1998a, b, c; Bischoff and Seyfried, 1978) when studying on modern and ancient hydrothermal deposits. However the genesis of anhydrite in modern hydrothermal deposits and ancient hydrothermal deposits is still open to debate. Studies on modern seafloor hydrothermal deposits show that anhydrite was formed by the mixing of ascending hydrothermal solution with seawater (Humphris and Bach, 2005; Teagle et al., 1998c; Humphris, 1998; Shikazono et al., 1983; Sato, 1973), while studies on ancient hydrothermal deposits show that anhydrites can be also formed by the simple heating of seawater due to a decrease in anhydrite solubility with increasing temperature (Kusakabe and Chiba, 1979; Farrell et al., 1978). Previous studies on anhydrites show that Sr isotopes can be used to determine the relative proportion of seawater and hydrothermal fluid, trace the evolution of hydrothermal fluid, and identify the origin of anhydrite in different hydrothermal systems, including ancient massive sulfide-sulfate deposits and *Corresponding author: hqyao@163.com zhouhy@tongji.edu.cn China University of Geosciences and Springer-Verlag Berlin Heidelberg 2015 Manuscript received February 17, Manuscript accepted June 3, modern seafloor hydrothermal deposits (Teagle et al., 1998a, b, c; Kusakabe and Chiba, 1979; Farrell et al., 1978, and so on). Thus, Sr isotopes can provide key evidence for the origin of anhydrite in different hydrothermal deposits and improve our understanding of the relative role of seawater and hydrothermal fluid in the formation of modern and ancient massive sulfide deposits. REE is a powerful tracer to study the evolution of geochemical systems and has been widely used as a tracer of sub-seafloor processes in hydrothermal systems (Craddock et al., 2010; Lin et al., 2010; Bao et al., 2008; Humphris, 1998; Elderfield, 1988; Owen and Oliverez, 1988). Different seafloor hydrothermal systems show various chondrite-normalized REE patterns. In sediment-starving hydrothermal system, hydrothermal fluid, sulfides and sulfate are generally characterized by positive Eu anomaly and enrichment of LREE (Humphris, 1998; Mills and Elderfield, 1995). By comparison, at Pacmanus hydrothermal field, where magmatic input may exist, hydrothermal fluid and its precipitates are characterized by different REE patterns, with variable Eu anomalies and LREE-depleted and/or enriched (Bach et al., 2003). Anhydrite contains more REE content than sulfide and was considered as the main phase of removing REE from the mixed fluids (Mills and Elderfield, 1995; Mitra et al., 1994). Therefore, anhydrite may be a useful carrier to study the REE geochemistry in different hydrothermal systems. Many studies on anhydrite have been concentrated in seafloor hydrothermal systems, however these studies are mainly focused on the anhydrites of stockzone from drilling-cores. Studies on the anhydrites from active black smoker chimney Yao, H. Q., Zhou, H. Y., Peng, X. T., et al., Sr Isotopes and REEs Geochemistry of Anhydrites from L Vent Black Smoker Chimney, East Pacific Rise 9ºN 10ºN. Journal of Earth Science, 26(6): doi: /s

2 Sr Isotopes and REEs Geochemistry of Anhydrites from L Vent Black Smoker Chimney, East Pacific Rise 9ºN 10ºN 921 have rarely been reported (Mills and Elderfield, 1995). Anhydrite is prevalent along the transection of L vent black smoker chimney, East Pacific Rise (EPR) 9ºN 10ºN, which offers a good opportunity to study Sr isotope and REE geochemistry within a sulfide-sulfate chimney. This paper reports high resolution Sr isotope and REE data along a transection of an active black smoker chimney for the first time, and such information may improve our understanding on: (1) the genesis of anhydrite, (2) the mixing condition, and (3) the REE behavior of anhydrites in the active black smoker chimney. 1 GEOLOGICAL SETTING Along EPR from north to south, there are lots of hydrothermal vents, with high or low temperature. EPR 9ºN 10ºN lies to the south of Clipperton transform fault and in a fast-spreading environment. Full spreading rate of this zone is 11 cm/a and water depth along the axial area varies from to m (Haymon et al., 1993). According to the data from the near-bottom 100 khz ARGO survey and the thickness of the sediments, the region is divided into 10 geological segments and their relative ages are also determined (Fig. 1). Segments B1 and B2 were judged being the two youngest segments. The ages show an increasing trend from segment B to Segment F with the oldest age at 9º17 N, and then become younger again (Fornari et al., 1998; Haymon et al., 1993, 1991). In B segment, there are many high-temperature vents from north to south, such as N, Q, G, Bio9, P, V, T, J, A, L, H, B, C, R, D, E, K, F and so on. L vent is approximately 500 m south of the A vent site, and located in the narrow axial summit collapse trough. In addition, there exist many low-temperature diffuse flow vents, of which temperature are generally lower than 35 o C, in this segment (Von Damm and Lilley, 2004; Von Damm, 2000). In contrast to the large sulfide mounds that may typify deposits on the Mid-Atlantic Ridge, hydrothermal chimneys at the fast-spreading East Pacific Rise in the EPR 9ºN 10ºN commonly occur as small, discrete individual structures with height rarely exceeding 15 m (Haymon et al., 1991; Bluth and Ohmoto, 1988; Graham et al., 1988; Thompson et al., 1988). 2 SAMPLES AND METHODS The black smoker chimney sample was collected by Alvin submersible in Voyage AT-07 in The location of the chimney sample is 09º46.24 N, 104º16.83 W, with a water depth of m. Based on the information of dive log ( we believed that the sample was collected from L vent, which belongs to segment B2. There are three channels on the transection, channel A, channel B and channel C (Fig. 2a). The direction of channel A skews to the transaction, but the direction of channel B and channel C are perpendicular to the transection. Figure 1. (a) General geologic map showing the location of the East Pacific Rise 9ºN 10ºN; (b) detailed geologic map of East Pacific Rise 9ºN 10ºN (modified from Haymon et al., 1991).

3 922 Huiqiang Yao, Huaiyang Zhou, Xiaotong Peng and Gaowen He Figure 2. Fragment of L vent chimney collected from East Pacific Rise 9ºN 10ºN. (a) The top-view of the L vent chimney sample, there are only two channels that can be observed in the transection, channel A and channel B; (b) the transection for anhydrite collection, three channels were observed along the transection. Eighteen subsamples were taken along these three channels (Fig. 2b). The subsample was crudely crushed to meshes, then repeatedly cleaned by Millipore water and dried at 30 o C. Approximately 20 mg of anhydrite was hand-picked under a binocular, then dissolved with 1 ml 2.5 N HCl at C for about 24 h in Teflon vessels with an 84 Sr spike added. The solutions were then evaporated to dryness at 100 C and were dissolved again with 2 ml 2.5 N HCl, and then were volumetrically splitted with half being used for the Sr isotopic composition, and half being used for the REE determination. Sr was separated in a 5 ml AG 50W-X12 ( ) anionic resin exchange column. Strontium isotopic composition was determined by MAT 262 at the laboratory for radiogenic isotope geochemistry of Institute of Geology and Geophysics, Chinese Academy of Sciences (CAS). The repeated analysis of NIST SRM 987 gave 87 Sr/ 86 Sr= ± (2σ, n=30). The measurements of REE were carried out by inductively-coupled-plasma mass spectrometer (ICP-MS), PQ-Excell at National Research Center of Geoanalysis, Chinese Academy of Geological Science. Based on the repeated analysis of samples, the analytical precision was considered as less than 10%. 3 RESULTS Table 1 presents the 87 Sr/ 86 Sr ratios of anhydrites from the transection of the active black smoker chimney in EPR 9ºN 10ºN, and 87 Sr/ 86 Sr ratios of anhydrites from other hydrothermal systems were also included for comparison. The 87 Sr/ 86 Sr ratios of anhydrites from L vent chimney vary from to and fall within the overall range between and for anhydrites from both drill core and submersible collection at the TAG mound (Mills et al., 1998; Teagle et al., 1998a, b, c; Mills and Elderfield, 1995). Contrasting to Sr isotopic compositions of anhydrites from other hydrothermal systems, the Sr isotopic compositions of L vent anhydrites are similar to those of anhydrites from Grimsey hydrothermal field ( , mean= , n=10) (Kuhn et al., 2003), and slightly higher than those of anhydrites from TAG hydrothermal mound, including black smoker chimney and white smoker chimney ( , mean= , n=3) (Mills and Elderfield, 1995). While contrasting to Sr isotopic compositions of anhydrites from stockzone, the Sr isotopic compositions of L vent anhydrites are lower than those of anhydrites from the inner of TAG hydrothermal mound (Humphris and Bach, 2005; Mills et al., 1998) and Kuroko massive sulfide-sulfate deposits (Ogawa et al., 2007), but higher than that from ODP 504B core (Teagle et al., 1998c). Assuming conservative mixing of Sr between seawater and hydrothermal fluid, the relative proportions of seawater and hydrothermal fluid can be calculated from mass balance equations. The calculated seawater proportions (SW%) were also listed in Table 1, too. The seawater proportions for anhydrites in this study with the range of 47% to 82% and with an average of 67.5%, which is higher than the range for anhydrites from the TAG chimney (36% 62%, with an average value of 45.7%), and similar to that of anhydrites from the inner of TAG mound (40% 99.5%, with an average value of 69.1%), and lower than that of anhydrites from Kuroko deposit (64% 95%, with an average value of 79.8%). REE contents of the studied anhydrites are presented in Table 2, together with REE data of seawater (Mitra et al., 1994) for comparison. The mean end-member REE contents of hydrothermal fluids from EPR 13 o N (Douville et al., 1999) was chosen and also shown in Table 2. REE data on hydrothermal fluid from this area are not available. REE contents in anhydrites of this study are in the range of 1.71 ppm 5.88 ppm, with an average of 3.43 ppm). These contents are about three orders of magnitude greater than those found in end-member black hydrothermal fluids and about six orders of magnitude greater than the REE concentrations in seawater (Bao et al., 2008; Douville et al., 1999; Klinkhammer et al., 1995, 1994, 1983). For comparative purpose, the content data have been normalized to the average REE concentrations of chondrites (Sun and McDonough, 1989) (Fig. 3). La N /Yb N is used to compare the slope of the chondrite-normalized REE patterns (where subscript N indicates normalized to the chondrite) and indicates the fractionation extent of light rare earth element (LREE) to heavy rare earth element (HREE). All of the samples show LREE(La to Eu) enrichment as demonstrated by high La N /Yb N values ranging from 7.2 to In addition, all patterns of samples show remarkable positive Eu anomalies, with values of δeu (where δeu is defined as Eu N /(Sm N Gd N ) 0.5 ) in the range of 1.5

4 Sr Isotopes and REEs Geochemistry of Anhydrites from L Vent Black Smoker Chimney, East Pacific Rise 9ºN 10ºN 923 Table 1 The Sr isotopic composition of anhydrites from L vent black smoker chimney East Pacific Rise 9ºN 10ºN and related Sr isotopic composition data from different hydrothermal systems Sample* 87 Sr/ 86 Sr ±2σ SW% $ Sample 87 Sr/ 86 Sr±2σ ±2σ SW% $ S ± S ± S ± S ± S ± S ± S ± S ± S ± S ± S ± S ± S ± S ± S ± S ± Grimsey chimneys a 87 Sr/ 86 Sr = (average= ), (n=10) # TAG chimneys b 87 Sr/ 86 Sr = (average= ), SW%=36% 62% (45.7%), (n=3) Inner of TAG mound c 87 Sr/ 86 Sr = (average= ), SW%=40% 99.5% (69.1%), (n=35) ODP 504B hole d 87 Sr/ 86 Sr = (average= ), (n=19) # Kuroko deposit e 87 Sr/ 86 Sr = (average= ), SW%=64% 95% (79.8%), (n=12) *. The absence of samples S0 and S1-1 is for that they were contaminated during the collection of anhydrite. $. The seawater and hydrothermal fluid with respective Sr concentrations of 85 and 150 μmol/kg, and 87 Sr/ 86 Sr compositions of and (Ravizza et al., 2001). #. the seawater proportion was not calculated for the lacking of the Sr concentration for end-member hydrothermal fluid in the area. Data for a from Kuhn et al., Data for b from Mills and Elderfried, Data for c from Humphris and Bach, 2005 and Mills et al., Data for d from Teagle et al., 1998c. Data for e from Ogawa et al., Table 2 REE geochemistry of anhydrites from L vent chimney East Pacific Rise, 9 o N 10 o N Sample La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La N / (ppm) Yb N δeu REE (ppm) S S S S S S S S S S S S S S S S HF/pm SW/pm Note: subscript N indicates normalized to the chondrite; LREE- La to Eu, HREE- Gd to Lu; HF indicates end-member hydrothermal fluid (the average value of two EPR 13ºN end-member hydrothermal fluid, data from (Douville et al., 1999)); SW indicates seawater, data from (Mitra et al., 1994); pm=p mol/kg.

5 924 Huiqiang Yao, Huaiyang Zhou, Xiaotong Peng and Gaowen He Figure 5. The relationship between total REEs and Sr isotopic compositions of anhydrites from L vent chimney, East Pacific Rise 9ºN 10ºN. Figure 3. Chondrite-normalized REE patterns for anhydrites from the black smoker chimney in EPR 9ºN 10ºN. For comparative purpose, chondrite-normalized REE pattern for seawater and end-member hydrothermal fluid are also presented. The Tm data of seawater and end-member hydrothermal fluid is interpolated between Er and Yb, respectively. Chondrite data are from Sun and McDonough (1989). Seawater data are from Mitra et al. (1994). Figure 4. Total REE concentration (a) and La N /Yb N (b) vs. the Eu anomalies for anhydrites from L vent chimney, East Pacific Rise 9ºN 10ºN. to 5.0. Weak negative correlation between δeu and total REEs and weak positive correlation between δeu and La N /Yb N were observed (Fig. 4), similar to those of the anhydrites from TAG mound (Humphris, 1998). Negative correlation between REE and 87 Sr/ 86 Sr ratios (R 2 =0.61, n=16) was observed (Fig. 5). 4 DISCUSSION 4.1 Genesis of Anhydrite in the L Vent Chimney Sr isotopic composition was suggested as a sensitive indicator for genesis of anhydrite (Humphris and Bach, 2005; Humphris, 1998; Teagle et al., 1998c; Shikazono et al., 1983; Kusakabe and Chiba, 1979; Farrell et al., 1978; Sato, 1973). The Sr isotopes within the range of end-member hydrothermal fluid in the area and seawater, indicating the anhydrites in L vent chimney are formed by the mixing of hydrothermal fluid and seawater, other than by simple heating of seawater. Generally, end-member hydrothermal fluid is considered as being with zero Mg content and has little or no SO 4 (Ravizza et al., 2001; Von Damm, 2000), so the formation of anhydrite needs SO 4 from seawater. This can be evidenced by the sulfur isotopic compositions of anhydrite. Sulfur isotopic composition of anhydrites from different hydrothermal systems, except for those having sulfur contribution from magmatic input (Kim et al., 2004), are near or equal to that of seawater value (Zhou et al., 2013; Chiba et al., 1998; Herzig et al., 1998; Bowers, 1989; Bluth and Ohmoto, 1988; Woodruff and Shanks III, 1988; Styrt et al., 1981). 4.2 Mixing on the Transection of the L Vent Chimney In previous studies, Sr isotopic composition of anhydrite was used in calculating proportion of end-member hydrothermal fluid and seawater (Humphris and Bach, 2005; Mills et al., 1998; Mills and Tivey, 1999; Teagle et al., 1998c). Previous studies show that hydrothermal fluids are variable, rather than constant (Ding et al., 2005; Von Damm, 2004; Von Damm et al. 1995). For example, Von Damm et al. (1997) observed the transition from vapor phase to brine phase in the F vent (Von Damm et al., 1997). Variation of hydrothermal fluid (especially the variation of Sr content of hydrothermal fluid) will have profound influence on the proportion calculating of seawater and hydrothermal fluid. Therefore it should be dealt with care when carrying out the calculation. For understanding the mixing condition on the transection of the L vent chimney, it is assumed that the Sr concentration of end-member hydrothermal fluids is constant during the mixing calculation. The constant Sr concentration of end-member hydrothermal fluid about 150 μmol/kg was chosen during the calculation because it is the only published data from the L vent. It is expected that the out-

6 Sr Isotopes and REEs Geochemistry of Anhydrites from L Vent Black Smoker Chimney, East Pacific Rise 9ºN 10ºN 925 er anhydrites should have a higher seawater proportion and the inner and/or the near fluid channel anhydrites should have a lower seawater proportion. However the results show there seems no regular relationship between the location and the seawater proportion. Possible explanations may include (1) these anhydrites were formed by hydrothermal fluid with variable Sr concentration other than constant Sr concentration; (2) the mixing was heterogeneous during the growth of an active chimney, especially for multi-channel chimney. We believed that it could be the co-result of the Sr concentration variation of hydrothermal fluid and the heterogeneous mixing process. Studies on individual anhydrite crystals will be required for improving the understanding of the mixing on the transection of the L vent chimney. 4.3 REE Geochemistry of Anhydrites in L Vent Chimney As shown above, the anhydrites are formed from the mixture of hydrothermal fluid and seawater, the REE content is controlled by the mixing extent of seawater and hydrothermal fluid. This is evidenced by an observed negative correlation between Sr isotopes and total REEs content. This indicates that the mixing extent between hydrothermal fluid and seawater is the main factor controlling the REE content in anhydrite (Fig. 5). Generally, the concentrations of REE in the seawater are one to four orders of magnitude lower than those in end-member black smoker hydrothermal fluid (Ravizza et al., 2001), the REE pattern of a mixed fluid is most influenced by the latter. The hydrothermal fluids sampled from the L vent are characterized by low ph, high temperature, low F - and low SO 4 (Ravizza et al., 2001; Von Damm, 2000). The L vent fluid is different from the fluids, which are characterized by high temperature, high F - and low SO 4, sampled from magmatic input hydrothermal systems (Craddock et al., 2010; Bach et al., 2003). The L vent fluid is also different from the fluids, which are characterized by low temperature, high SO 4 and variable F -, sampled from the low temperature hydrothermal systems (Bach et al., 2003). Previous studies show that different kinds of hydrothermal fluids mix with seawater will have different REE geochemistry for mixtures, and result in different REE geochemistry in anhydrites (Craddock et al., 2010; Humphris and Bach, 2005; Bach et al., 2003; Mills and Elderfield, 1995). The hydrothermal fluid with the low ph, high temperature, low F - and low SO 4 mixed with seawater, the REEs in the mixture are generally complexed with anion, like Cl -, SO 4, F -, CO 3 and so on (Humphris and Bach, 2005; Bach et al., 2003), and the anhydrites precipitated from the mixed fluid are characteristic of in the ordinary uniform REE pattern, with the LREE enrichment and positive Eu anomaly (Humphris and Bach, 2005; Mills and Elderfield, 1995). For the fluids with high temperature, high F - and low SO 4, the LREE and Eu are dominantly complexed with Cl - and HREE are dominantly complexed with F - (Craddock et al., 2010; Bach et al., 2003), and the anhydrites formed from the mixed fluid have a wide variable REE patterns, including variable enrichment in MREE and HREE and variable Eu anomalies. For the hydrothermal fluid with low temperature, high SO 4 and variable F -, sulfate complexs, free REE 3+ ions (including Eu) dominate the REE species (Bach et al., 2003) in the mixed fluid, and the anhydrites formed from such mixed fluid are characterized by flat REE pattern, slight or no Eu anomaly and slightly HREE enrichment (Bach et al., 2003). The anhydrites from the L vent chimney are characterized by uniform REE pattern, with LREE enrichment and positive Eu anomaly, and are similar to anhydrites from TAG (Humphris and Bach, 2005; Mills and Elderfield, 1995). It may be inferred that the REE geochemistry of the mixture of hydrothermal fluid is mainly controlled by chemistry of the end-member hydrothermal fluid. There exist different explanations on factors that influence the REE pattern of anhydrite precipitation from a mixture of hydrothermal fluid and seawater. Mills and Elderfield (1995) calculated the distribution of REE in anhydrite and suggested that REE partitioning into anhydrite was controlled by crystallography. But they could not explain the notably low partition of La, Ce and Eu in anhydrite. This can be also observed in the L vent anhydrites (Fig. 6). Both of TAG anhydrites and L vent anhydrites show that the other factors except crystallographic process may contribute to the distribution of REE in anhydrite. Another explanation suggested that REE-anion complexing is the main factor that influences the partitioning behavior into anhydrite (Bao et al., 2008; Humphris and Bach, 2005; Bach et al., 2003; Douville et al., 1999). For the absence of REE data in end-member hydrothermal fluid from this area, we normalized the anhydrite REE data to the mean REE concentration in end-member hydrothermal fluid from EPR 13 o N (Douville et al., 1999). The normalized REE patterns are uniform, but are characterized by distinct negative Eu anomaly (overall range= ) and slight enrichment of HREE (La H /Yb H = , where subscript H indicates normalized to end-member hydrothermal fluid) (Fig. 7). These uniform patterns were also observed by Humphris (1998), and were explained as REE-anion complexing. Species calculation shows that LREE complexes are more stable than that of HREE complexes under a given temperature and pressure (Humphris and Bach, 2005), leading to the depletion of LREE and the enrichment of HREE when normalized to end-member hydrothermal fluids. For Eu, Humphris and Bach (2005) suggested that the discrimination against Eu during the precipitation of anhydrite Figure 6. Apparent distribution coefficients for L vent anhydrites plotted against the square of the difference between the ionic radii of the REE and Ca 2+. Ionic radii are from Shannon (1976); REE are assumed to be eight-fold coordination, and all are assumed to be trivalent ions, except Eu 2+ ; the calculation is the same as Mills and Elderfield (1995).

7 926 Huiqiang Yao, Huaiyang Zhou, Xiaotong Peng and Gaowen He from EPR 9ºN 10ºN, it is suggested that REE-anion complexing, rather than crystallography, is the main factor that controls the REE partition behavior during the precipitation of anhydrite from the mixture of hydrothermal fluid and seawater. Figure 7. End-member black smoker fluids-normalized REE patterns for anhydrites from L vent chimney, East Pacific Rise 9ºN 10ºN, the Tm data of end-member hydrothermal fluid is interpolated between Er and Yb. is likely due to two factors: (1) the ionic radius of Eu 2+ is significantly larger that of Ca 2+ (125 pm compared with 112 pm for Ca 2+ ); (2) the strong chloro-complexation. Species calculation mode shows that under the conditions for anhydrite precipitation with different mixing hypothesis, free Eu 2+ ion is rare (Humphris and Bach, 2005). Therefore, the Eu-anion complexing may be the key factor influencing the Eu partition behavior in anhydrite. In a closed environment, with REE-anion complexing and precipitation of anhydrite, the REE content of the mixture in hydrothermal fluid and seawater will decrease, while the Eu anomaly and the fraction between LREE and HREE will increase (Fig. 4). Open environment for precipitation of anhydrites may be the reason for the weak negative correlation between δeu and total REEs and the weak positive relationship between δeu and La N /Yb N. Therefore we suggest that REE-anion complex, rather than crystallography, is the main factor influencing the partitioning behavior of REE into anhydrite of L vent chimney. 5 SUMMARY High resolution Sr isotopic composition and REE analyses were carried out along the transaction of L vent chimney collected from East Pacific Rise 9ºN 10ºN. Sr isotopes show these anhydrites are the product of the mixing between hydrothermal fluid and seawater. The negative correlation between Sr isotopes and total REEs contents indicates that the mixing extent of hydrothermal fluid and seawater is the key factor that influences the REE content in anhydrite. Using a constant Sr concentration of hydrothermal fluid, the calculation on relative proportion of seawater and hydrothermal show that there seems no regular relationship between the location and the seawater proportion and indicates heterogeneous mixing on the transection of the L vent chimney. REE data show uniform chondrite-normalized REE patterns with enrichment of LREE and positive Eu anomalies, which is similar to that of most of anhydrites from TAG hydrothermal mound. When normalized to the REE of end-member hydrothermal fluid, these data show similar REE pattern while with negative Eu anomalies and enrichment of HREE. Together with the previous studies on REEs of hydrothermal fluid from different hydrothermal systems and hydrothermal fluids data ACKNOWLEDGMENTS This manuscript has benefited greatly through and helpful reviews by two anonymous reviewers and Ge Yao is also thanked for uploading the manuscript to the review system when the authors are on board. This study was supported by the China Ocean Mineral Resource R & D Association (COMRA) Projects (Nos. DY R-08, DY R-01, and DY R-05). REFERENCES CITED Bach, W., Roberts, S, Vanko, D. A., et al., Controls of Fluid Chemistry and Complexation on the Rare-Earth Element Contents of Anhydrite from the Pacmanus Subseafloor Hydrothermal System, Manus Basin, Papua New Guinea. Mineralium Deposita, 38: , doi: /s Bao, X. S., Zhou, H. Y., Peng, X. T., et al., Geochemistry of REE and Yttrium in Hydrothermal Fluids from the Endeavour Segment, Juan de Fuca Ridge. Geochemical Journal, 42: Bischoff, J. L., Seyfried, W. E., Hydrothermal Chemistry of Seawater from 25 o C to 350 o C. American Journal of Science, 278: Bluth, G. J., Ohmoto, H., Sulfide-Sulfate Chimneys on the EPR 11 o and 13 o N Latitudes. Part II: Sulfur Isotopes. Canadian Mineralogist, 26: Bowers, T. S., Stable Isotope Signatures of Water-Rock Interaction in Mid-Ocean Ridge Hydrothermal Systems: Sulfur, Oxygen and Hydrogen. Journal of Geophysical Research, 94: Chiba, H., Uchiyama, N., Teagle, D. A. H., Stable Isotope Study of Anhydrite and Sulfide Minerals at the TAG Hydrothermal Mound, Mid-Atlantic Ridge, 26 N. Proceedings of the Ocean Drilling Program, Scientific Results, 158: Craddock, P. R., Bach, W., Seewald, J. S., et al., Rare Earth Element Abundances in Hydrothermal Fluids from the Manus Basin, Papua New Guinea: Indicators of Sub-Seafloor Hydrothermal Processes in Back-Arc Basins. Geochimica et Cosmochimica Acta, 74: Ding, K., Seyfried, J. W. E., Zhang, Z., et al., The in situ ph of Hydrothermal Fluids at Mid-Ocean Ridges. Earth and Planetary Science Letters, 237(1 2): Douville, E., Bienvenu, P., Charlou, J. L., et al., Yttrium and Rare Earth Elements in Fluids from Various Deep-Sea Hydrothermal Systems. Geochimica et Cosmochimica Acta, 63(5): Elderfield, H., The Oceanic Chemistry of the Rare-Earth Elements. Philosophical Transactions of the Royal Society of London, Series A: Mathematical and Physical Sciences, 325: Farrell, C. W., Holland, H. D., Petersen, U., The Isotopic Composition of Strontium in Barites and Anhydrites from

8 Sr Isotopes and REEs Geochemistry of Anhydrites from L Vent Black Smoker Chimney, East Pacific Rise 9ºN 10ºN 927 Kuroko Deposits. Mining Geology, 28: Fornari, D. J., Shank, T., Von Damm, K. L., et al., Time-Series Temperature Measurements at High-Temperature Hydrothermal Vents, East Pacific Rise N: Evidence for Monitoring a Crustal Cracking Event. Earth and Planetary Science Letters, 160: Graham, U. M., Bluth, G. J., Ohmoto, H., Sulfide-Sulfate Chimneys on the East Pacific Rise 11 N and 13 N, Part I: Mineralogy and Paragenesis. Canadian Mineralogist, 26: Haymon, R. M., Fornari, D. J., Damm, K. L. V., et al., Volcanic Eruption of the Mid-Ocean Ridge along the East Pacific Rise Crest at 9 45' 52'N: Direct Submersible Observations of Seafloor Phenomena Associated with an Eruption Event in April, Earth and Planetary Science Letters, 119: Haymon, R. M., Fornari, D. J., Edwards, M. H., et al., Hydrothermal Vent Distribution along the East Pacific Rise Crest (9 09' 54'N) and Its Relationship to Magmatic and Tectonic Processes on Fast-Spreading Mid-Ocean Ridges. Earth and Planetary Science Letters, 104(2 4): Herzig, P. M., Petersen, S., Hannington, M. D., Geochemistry and Sulfur-Isotopic Composition of the TAG Hydrothermal Mound, Mid-Atlantic Ridge, 26 o N. Proceedings of the Ocean Drilling Program, Scientific Results, 158: Humphris, S. E., Rare Earth Element Composition of Anhydrite: Implications for Deposition and Mobility within the Active TAG Hydrothermal Mound. Proceedings of the Ocean Drilling Program, Scientific Results, 158: Humphris, S. E., Bach, W., On the Sr Isotope and REE Compositions of Anhydrites from the TAG Seafloor Hydrothermal System. Geochimica et Cosmochimica Acta, 69(6): Kim, J., Lee, I., Lee, K.-Y., S, Sr, and Pb Isotopic Systematics of Hydrothermal Chimney Precipitates from the Eastern Manus Basin, Western Pacific: Evaluation of Magmatic Contribution to Hydrothermal System. Journal of Geophysical Research: Solid Earth, 109(B12): doi: /2003jb Klinkhammer, G. P., Chin, C. S., Wilson, C., et al., Venting from the Mid-Atlantic Ridge at : The Lucky Strike Hydrothermal Site. In: Parson, L. M., Walker, C. L., Dixon, D. R., eds., Hydrothermal Vents and Processes. Geological Society, London, Special Publication, 87: Klinkhammer, G. P., Elderfield, H., Edmond, J. M., et al., Geochemical Implications of Rare Earth Element Patterns in Hydrothermal Fluids from Mid-Ocean Ridges. Geochimica et Cosmochimica Acta, 58(23): Klinkhammer, G. P., Elderfield, H., Hudson, A., Rare Earth Elements in Seawater near Hydrothermal Vents. Nature, 305: Kuhn, T., Herzig, P. M., Hannington, M. D., et al., Origin of Fluids and Anhydrite Precipitation in the Sediment-Hosted Grimsey Hydrothermal Field North of Iceland. Chemical Geology, 202: 5 21 Kusakabe, M., Chiba, H., Oxygen Isotope Geothermometry Applied to Sulfate Minerals from the Kuroko Deposits. Mining Geology, 29: Lin, L., Pang, Y. C., Ma, L. Y., et al., Submarine Hydrothermal/Hot Spring Deposition of Early Cambrian Niutitang Formation in South China. Journal of Earth Science, 21(1): Mills, R. A., Elderfried, H., Rare Earth Element Geochemistry of Hydrothermal Deposits from the Active TAG Mound, 26 N Mid-Atlantic Ridge. Geochimica et Cosmochimica Acta, 59(17): Mills, R. A., Teagle, D. A. H., Tivey, M. K., Fluid Mixing and Anhydrite Precipitation within the TAG Mound. Proceedings of the Ocean Drilling Program, Scientific Results, 158: Mills, R. A., Tivey, M. K., Seawater Entrainment and Fluid Evolution with TAG Hydrothermal Mound: Evidence from Analysis of Anhydrite. In: Cann, J. R., Elderfield, H., Laughton, A., eds., Mid-Ocean Ridge. Cambridge University Press, Cambridge Mitra, A., Elderfield, H., Greaves, M. J., Rare Earth Elements in Submarine Hydrothermal Fluids and Plumes from the Mid-Atlantic Ridge. Marine Chemistry, 46: Ogawa, Y., Shikazono, N., Ishiyama, D., et al., Mechanisms for Anhydrite and Gypsum Formation in the Kuroko Massive Sulfide-Sulfate Deposits, North Japan. Mineralium Deposita, 42: Owen, R. M., Oliverez, A. M., Geochemistry of Rare Earth Elements in Pacific Hydrothermal Sediments. Marine Chemistry, 25: Ravizza, G., Blusztajn, J., Damm, K. L. V., et al., Sr Isotope Variations in Vent Fluids from N East Pacific Rise: Evidence of a Non-Zero-Mg Fluid Component. Geochimica et Cosmochimica Acta, 65(5): Sato, T., A Chloride Complex Model for Kuroko Mineralization. Geochemical Journal, 7: Shannon, R. D., Revised Effective Ionic Radii and Systematic Studies of Interatomic Distance in Halides and Chalcogenides. Acta Crystallographica Section A, 32: Shikazono, N., Holland, H. D., Quirk, R. F., Anhydrite in Kuroko Deposits: Mode of Occurrence and Depositional Mechanisms. Economic Geology Monograph, 5: Styrt, M. M., Brackmann, A. J., Holland, H. D., et al., The Mineralogy and the Isotopic Composition of Sulfur in Hydrothermal Sulphide/Sulfate Deposits on the East Pacific Rise, 21 o N Latitude. Earth and Planetary Science Letters, 53: Sun, S. S., McDonough, W. F., Chemical and Isotopic Systematics of Oceanic Basalts: Implications for Mantle Composition and Processes. Geological Society, London, Special Publications, 42: Teagle, D. A. H., Alt, J. C., Chiba, H., et al., 1998a. Dissecting an Active Hydrothermal Deposit: The Strontium and Oxygen Isotopic Anatomy of the TAG Hydrothermal Mound Anhydrite. Proceedings of the Ocean Drilling

9 928 Huiqiang Yao, Huaiyang Zhou, Xiaotong Peng and Gaowen He Program, Scientific Results, 158: Teagle, D. A. H., Alt, J. C., Chiba, H., et al., 1998b. Strontium and Oxygen Isotopic Constraints on Fluid Mixing Alteration and Mineralization in the TAG Hydrothermal Deposit. Chemical Geology, 149: 1 24 Teagle, D. A. H., Alt, J. C., Halliday, A. N., 1998c. Tracing the Chemical Evolution of Fluids during Hydrothermal Recharge: Constraints from Anhydrite Recovered in ODP Hole 504B. Earth and Planetary Science Letters, 155: Thompson, G., Humphris, S. E., Shroeder, B., et al., Hydrothermal Mineralization on the Mid-Atlantic Ridge. Canadian Mineralogist, 26: Von Damm, K. L., Chemistry of Hydrothermal Vent Fluids from 9-10 N, East Pacific Rise: Time Zero, the Immediate Posteruptive Period. Journal of Geophysical Research, 105(B5): Von Damm, K. L., Evolution of the Hydrothermal System at East Pacific Rise 9º54 N: Geochemical Evidence for Changes in the Upper Oceanic Crust. In: German, C. R., Lin, J., Parson, L. M., eds., Mid-Ocean Ridges: Hydrothermal Interactions Between the Lithosphere and Oceans. American Geophysical Union, Washington DC Von Damm, K. L., Buttermore, L. G., Oosting, S. E., et al., Direct Observation of the Evolution of a Seafloor Black Smoker from Vapor to Brine. Earth and Planetary Science Letters, 149(1 4): Von Damm, K. L., Lilley, M. D., Diffuse Flow Hydrothermal Fluids from 9 50 N East Pacific Rise: Origin, Evolution and Biogeochemical Controls. In: Wilcock, W. S. D., Delong, E. F., Kelley, D. S., et al., eds., The Subseafloor Biosphere at Mid-Ocean Ridges. AGU, Washington DC Von Damm, K. L., Oosting, S. E., Kozlowskl, R., et al., Evolution of East Pacific Rise Hydrothermal Vent Fluids Following a Volcanic Eruption. Nature, 375: Woodruff, L. G., Shanks III, W. C., Sulfur Isotope Study of Chimney Minerals and Vent Fluids from 21ºN, East Pacific Rise: Hydrothermal Sulfur Sources and Disequilibrium Sulfate Reduction. Journal of Geophysical Research, 93(B5): Zhou, J. X., Huang, Z. L., Bao, G. P., et al., Sources and Thermo-Chemical Sulfate Reduction for Reduced Sulfur in the Hydrothermal Fluids, Southeastern SYG Pb-Zn Metallogenic Province, SW China. Journal of Earth Science, 24(5):