48. Data Report
10
Mottl, M.J., Davis, E.E., Fisher, A.T., and Slack, J.F. (Eds.), 1994 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 139 48. DATA REPORT: SULFUR CONTENT OF SEDIMENT AND SULFUR ISOTOPE VALUES OF SULFIDE AND SULFATE MINERALS FROM MIDDLE VALLEY 1 Robert A. Zierenberg 2 INTRODUCTION Sulfide sulfur content of sediment and sulfur isotope ratios of sulfide and sulfate minerals were measured as part of an effort to determine the sources and mass and isotope fluxes of sulfur in and near the Middle Valley hydrothermal system. Limited access to mass spectrometry facilities resulted in the late attainment of these data relative to the deadline for submission of manuscripts to this volume. Therefore, these data are presented as a data report without significant interpretation. METHODS Samples for sulfur isotope analyses were classified by major lith ology as sediment, diabase, basalt, massive sulfide, or clastic sulfide redeposited from massive sulfide. Sediment samples were classified as clay, silt, or sand based on a visual estimation of the dominant grain size. Total sulfide sulfur was chemically extracted from bulk samples of unlithified sediment by acid dissolution in 6N HC1 using chromous chloride as a reductant (Canfield et al., 1986). H 2 S produced in the reaction was stripped from the solution by N 2 gas and precipitated as Ag 2 S. The weight of Ag 2 S produced relative to the starting weight of sediment provides a measure of the sulfide sulfur content in the sedi ments (Table 1). Injection of HC1 into the reaction vessels prior to addition of CrCl 2 allowed detection and separation of acid volatile sulfide minerals such as pyrrhotite, mackinawite, and greigite. Acid volatile sulfide was detected only in trace amounts and only in a small number of samples. The total sulfide content and the sulfur isotope data therefore reflect pyritic sulfur (Table 1). In more lithified sediment samples and in samples of basalt, diabase, or massive sulfide, subsamples for sulfur isotope analyses were taken by drilling with diamond impregnated bits using a hand held dental drill. This often allowed sampling of individual minerals, veins, burrow fills, or nodules of sulfide and sulfate minerals. Separate subsamples are listed on separate lines in Table 1. When more than one mineral was analyzed from the same subsample the results are listed on the same line of the table. Because of the selective sampling, sulfur contents are not reported for these samples. All sulfide samples were chemically treated to extract monosulfide sulfur (sphalerite and/or pyrrhotite), by dissolution under an N 2 atmosphere in 6N HC1, fol lowed by extraction of disulfide sulfur (pyrite, marcasite, and chalco pyrite) using CrCl 2 as a reductant. Sulfate mineral bearing samples were treated with 0.5 N HC1 to dissolve anhydrite and gypsum; the dissolved sulfate was reprecipitated as BaSO 4 . Samples with barite were first treated to remove anhydrite and gypsum followed by disso lution of barite in concentrated NaCO 3 and reprecipitation as BaSO 4 , as described by Breit et al. (1985). Sulfur isotope determinations were made on SO 2 produced by combustion of Ag 2 S under vacuum using Cu 2 O as an oxidant, or com bustion of BaSO 4 with Cu 2 O plus quartz. Measurements were made on a Finnigan Delta mass spectrometer and standardized against Ag 2 S standards with values determined by SF 6 by Rees (1978). Present day seawater sulfate determined by this method gives a value of 20.99%e relative to Canyon Diablo Troilite (CDT) (Rees et al., 1978). RESULTS Site 855 Site 855 was rotary cored in an area of low heat flow near a normal fault that bounds the east side of Middle Valley. Sulfur content of sediments recovered at this site are low (0.1 to 0.3 weight percent [wt%]) and do not vary systematically with depth (Davis, Mottl, Fisher, et al., 1992). Only four samples of pyrite in sediment from Hole 855C were analyzed for sulfur isotope values (Table 1). The δ 34 S values range from 40.7‰ to 5.4‰ and there is a poorly defined trend to higher values at depth (Fig. 1). Site 856 Hole 856A 1 Mottl, M.J., Davis, E.E., Fisher, A.T., and Slack, J.F. (Eds.), 1994. Proc. ODP, Sci. Results, 139: College Station, TX (Ocean Drilling Program). 2 U.S. Geological Survey, 345 Middlefield Rd., M.S. 901, Menlo Park, CA 94025 3591, U.S.A. Hole 856A was drilled in the center of a circular hill of uplifted sediment (Bent Hill). Sediment becomes increasingly indurated and is hydrothermally altered at depth in this hole (Davis, Mottl, Fisher, et al., 1992). An olivine diabase sill was cored in the bottom 3 m of the hole. Sulfur content is generally between 0.1 and 0.3 wt% in the top 20 m of the hole and becomes more variable and tends towards higher values at depth (Davis, Mottl, Fisher, et al., 1992; Table 1). Dissemi nated pyrite has δ 34 S values between 39.8%o and 17.7‰(Fig. 2). The data indicate a general trend to higher values at depth. Hole 856B Hole 856B was drilled on the south flank of Bent Hill. In addition to the turbiditic and hemipelagic sediment that fills Middle Valley, an olivine diabase sill approximately 5 m thick occurs at 62 mbsf. Hole 856B ended after drilling approximately 1.5 m into an olivine diabase sill at 120 mbsf. The recovered sediment was generally indurated to hydrothermally altered, especially near the basaltic sills. The bottom 18 m above the basaltic sill is a zone of hydrothermal alteration and sulfide mineralization that becomes more prominent toward the con tact. Pyrite is the dominant sulfide mineral through most of the sedi ment section, but pyrrhotite, sphalerite, and chalcopyrite occur in the mineralized zone adjacent to the sill. Disseminations and veins of anhydrite and barite occur in this interval. Pyrite is visibly more abundant in Hole 856B than in Hole 856A. Sulfur content of the sediment is typically between 1 and 1.5 wt% (Davis, Mottl, Fisher, et al., 1992). An interval between 18 and 24 mbsf has much higher sulfur content due to the presence of turbidites and debris flows derived from massive sulfide. The δ 34 S values of disseminated pyrite in the sediment range from 22.8‰ to 31.3‰ (Table 1). Sulfur isotope values tend to become higher downhole at least to the depth of the first olivine diabase sill (Fig. 3). Below this depth sulfur isotope values of pyrite are fairly constant but range from 6.0‰ to 13. l‰. Sulfur isotope values for the interval containing clas 739
Transcript of 48. Data Report
Mottl, M.J., Davis, E.E., Fisher, A.T., and Slack, J.F. (Eds.), 1994Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 139
48. DATA REPORT: SULFUR CONTENT OF SEDIMENT AND SULFUR ISOTOPE VALUES OFSULFIDE AND SULFATE MINERALS FROM MIDDLE VALLEY1
Robert A. Zierenberg2
INTRODUCTION
Sulfide sulfur content of sediment and sulfur isotope ratios ofsulfide and sulfate minerals were measured as part of an effort todetermine the sources and mass and isotope fluxes of sulfur in andnear the Middle Valley hydrothermal system. Limited access to massspectrometry facilities resulted in the late attainment of these datarelative to the deadline for submission of manuscripts to this volume.Therefore, these data are presented as a data report without significantinterpretation.
METHODS
Samples for sulfur isotope analyses were classified by major lith-ology as sediment, diabase, basalt, massive sulfide, or clastic sulfideredeposited from massive sulfide. Sediment samples were classifiedas clay, silt, or sand based on a visual estimation of the dominant grainsize. Total sulfide sulfur was chemically extracted from bulk samplesof unlithified sediment by acid dissolution in 6N HC1 using chromouschloride as a reductant (Canfield et al., 1986). H2S produced in thereaction was stripped from the solution by N2 gas and precipitated asAg2S. The weight of Ag2S produced relative to the starting weight ofsediment provides a measure of the sulfide sulfur content in the sedi-ments (Table 1). Injection of HC1 into the reaction vessels prior toaddition of CrCl2 allowed detection and separation of acid-volatilesulfide minerals such as pyrrhotite, mackinawite, and greigite. Acidvolatile sulfide was detected only in trace amounts and only in a smallnumber of samples. The total sulfide content and the sulfur isotopedata therefore reflect pyritic sulfur (Table 1).
In more lithified sediment samples and in samples of basalt,diabase, or massive sulfide, subsamples for sulfur isotope analyseswere taken by drilling with diamond-impregnated bits using a hand-held dental drill. This often allowed sampling of individual minerals,veins, burrow fills, or nodules of sulfide and sulfate minerals. Separatesubsamples are listed on separate lines in Table 1. When more than onemineral was analyzed from the same subsample the results are listedon the same line of the table. Because of the selective sampling, sulfurcontents are not reported for these samples. All sulfide samples werechemically treated to extract monosulfide sulfur (sphalerite and/orpyrrhotite), by dissolution under an N2 atmosphere in 6N HC1, fol-lowed by extraction of disulfide sulfur (pyrite, marcasite, and chalco-pyrite) using CrCl2 as a reductant. Sulfate-mineral-bearing sampleswere treated with 0.5 N HC1 to dissolve anhydrite and gypsum; thedissolved sulfate was reprecipitated as BaSO4. Samples with baritewere first treated to remove anhydrite and gypsum followed by disso-lution of barite in concentrated NaCO3 and reprecipitation as BaSO4,as described by Breit et al. (1985).
Sulfur isotope determinations were made on SO2 produced bycombustion of Ag2S under vacuum using Cu2O as an oxidant, or com-bustion of BaSO4 with Cu2O plus quartz. Measurements were made
on a Finnigan Delta mass spectrometer and standardized against Ag2Sstandards with values determined by SF6 by Rees (1978). Present dayseawater sulfate determined by this method gives a value of 20.99%erelative to Canyon Diablo Troilite (CDT) (Rees et al., 1978).
RESULTS
Site 855
Site 855 was rotary cored in an area of low heat flow near a normalfault that bounds the east side of Middle Valley. Sulfur content ofsediments recovered at this site are low (0.1 to 0.3 weight percent[wt%]) and do not vary systematically with depth (Davis, Mottl,Fisher, et al., 1992). Only four samples of pyrite in sediment fromHole 855C were analyzed for sulfur isotope values (Table 1). The δ34Svalues range from -40.7‰ to -5.4‰ and there is a poorly definedtrend to higher values at depth (Fig. 1).
Site 856
Hole 856A
1 Mottl, M.J., Davis, E.E., Fisher, A.T., and Slack, J.F. (Eds.), 1994. Proc. ODP, Sci.Results, 139: College Station, TX (Ocean Drilling Program).
2 U.S. Geological Survey, 345 Middlefield Rd., M.S. 901, Menlo Park, CA 94025-3591, U.S.A.
Hole 856A was drilled in the center of a circular hill of upliftedsediment (Bent Hill). Sediment becomes increasingly indurated andis hydrothermally altered at depth in this hole (Davis, Mottl, Fisher,et al., 1992).
An olivine diabase sill was cored in the bottom 3 m of the hole.Sulfur content is generally between 0.1 and 0.3 wt% in the top 20 mof the hole and becomes more variable and tends towards highervalues at depth (Davis, Mottl, Fisher, et al., 1992; Table 1). Dissemi-nated pyrite has δ34S values between -39.8%o and 17.7‰ (Fig. 2). Thedata indicate a general trend to higher values at depth.
Hole 856B
Hole 856B was drilled on the south flank of Bent Hill. In additionto the turbiditic and hemipelagic sediment that fills Middle Valley, anolivine diabase sill approximately 5 m thick occurs at 62 mbsf. Hole856B ended after drilling approximately 1.5 m into an olivine diabasesill at 120 mbsf. The recovered sediment was generally indurated tohydrothermally altered, especially near the basaltic sills. The bottom18 m above the basaltic sill is a zone of hydrothermal alteration andsulfide mineralization that becomes more prominent toward the con-tact. Pyrite is the dominant sulfide mineral through most of the sedi-ment section, but pyrrhotite, sphalerite, and chalcopyrite occur in themineralized zone adjacent to the sill. Disseminations and veins ofanhydrite and barite occur in this interval.
Pyrite is visibly more abundant in Hole 856B than in Hole 856A.Sulfur content of the sediment is typically between 1 and 1.5 wt%(Davis, Mottl, Fisher, et al., 1992). An interval between 18 and 24mbsf has much higher sulfur content due to the presence of turbiditesand debris flows derived from massive sulfide. The δ34S values ofdisseminated pyrite in the sediment range from -22.8‰ to 31.3‰(Table 1). Sulfur isotope values tend to become higher downhole atleast to the depth of the first olivine diabase sill (Fig. 3). Below thisdepth sulfur isotope values of pyrite are fairly constant but range from6.0‰ to 13. l‰. Sulfur isotope values for the interval containing clas-
739
DATA REPORT.
Table 1. Sulfur content and sulfur isotope values of Middle Valley samples.
Core, section, Depthinterval (cm) (mbsf) Lithology Py Po An Ba Notes
139-855C-2R-4, 139-1434R-4, 91-9310R-4, 62-6411R-1, 101-106
139-856A-2H-3, 76-783H-3, 30-323H-3, 142-1464H-1,47-494H-1,95-974H-2, 147-1494H-3, 32-344H-4, 53-554H-6, 80-825H-7, 103-1055H-8, 13-155H-8, 86-886H-3, 65-676H-4, 64-666H-6, 1-36H-6, 140-1427H-1, 127-1297H-5, 3-58H-2, 131-1338H-CC, 28-3010X-1,131-1331IX-1,32-3412X-1, 139-14113X-1, 111-11313X-3, 68-70
139-856B-2H-2, 53-562H-2, 53-562H-3, 25-292H-5, 8-123H-2, 87-893H-5, 137-1393H-6, 51-533H-6, 51-533H-6, 105-1073H-6, 105-1074H-2, 25-274H-2, 25-274H-2, 25-274H-5, 77-795H-5, 8-116H-2, 82-856H-2, 134-1366H-4, 117-1197H-1,25-287H-3, 89-927H-3, 89-927H-3, 89-927H-3, 89-927H-3, 89-927H-4, 67-707H-4, 67-707H-6,44-478H-4,42-449X-1, 20-22HX-5,42^512X-5, 71-7312X-5, 107-10912X-5, 117-11912X-5, 117-11913X-1, 112-11413X-2, 79-8314X-4, 24-3114X-4, 24-3114X-4, 50-5514X-CC, 34-3815X-1, 27-2915X-1, 65-6715X-1, 137-13915X-3,42-^415X-3, 108-11015X-4, 107-10915X-4, 128-13015X-5, 34-3615X-5, 119-12115X-CC, 12-1416X-1, 17-19
14.5932.5188.5295.61
6.4615.5016.6222.1722.6524.6725.0226.7330.0040.1140.7141.4444.3545.8448.2149.6051.4756.2362.5169.6980.0186.5297.09
106.41108.98
3.833.835.057.88
13.6718.6719.3119.3119.8519.8522.5522.5522.5527.5736.3842.1242.6445.4749.5553.1953.1953.1953.1953.1954.4754.4757.2463.6063.8679.7288.3188.6788.7788.7792.2293.39
105.54105.54105.80106.51110.67111.05111.77113.82114.48115.97116.18116.74117.59118.14120.27
SiltClaySandSand
ClaySiltSandClayClayClayClayClaySandSandSiltClaySandSiltSiltSiltSiltClaySilt"SiltClaySiltSiltSiltSilt
SiltSiltSandClaySiltCSCSCSCSCSCSCSCSSiltSiltClayClayClaySiltSiltSiltSiltSiltSiltSiltSiltClayClaySilt'SiltSiltClayClayClayClayClayClayClayClaySandClayClayClayClayClayClayClayClaySiltClayClay
0.751.841.710.70
0.954.820.321.170.160.110.790.040.090.430.154.950.910.270.280.121.550.180.312.590.071.570.690.450.15
0.65
0.151.340.35
9.821.050.580.220.600.32
0.946.2 21.281.321.385.010.380.39
3.170.25
-26.6-40.7
-5.4-14.2
-39.8-22.6-18.8-20.3
-4.7-24.8
-8.9-4.9
-21.02.43.87.4
-18.16.8
-22.47.1
-7.8
-11.517.716.210.5
-22.8-17.3-10.4
-5.6-2.2
2.13.03.14.74.4
-1.1-1.2
2.3-8.1
6.0-7.0
3.78.03.28.89.5
10.4
-0.8-16.8
31.37.94.3
12.77.7
11.410.18.8
13.110.89.57.2
10.37.26.66.27.17.36.7
6.57.3
6.06.0
10.14.2
Total sulfideFresh PyWeathered Py
Total sulfidePy nodulePy nodule
Total sulfidePy filled burrow
10.2Total sulfideCo-existing Py-Sl
6.9 21.623.2
22.721.8
23.822.2
Py-Ba veinPy-Ba vein
740
• DATA REPORT
Table 1 (continued).
δ3 4S
Core, section,
interval (cm)
Depth
(mbsf) Lithology Py Po An Ba Notes
139-856D-1H-1,43-441H-1, 43-441H-3, 140-1501H-3, 140-1501H-4, 34-351H-4, 79-821H-5, 14-151H-5, 14-151H-7, 78-801H-7, 78-801H-7, 78-80
139-856G-3R-1,7-83R-1, 7-84R-1, 131-1335R-1, 44-466R-1, 36-386R-1, 36-386R-2, 140-1426R-2, 140-1426R-3, 24-266R-3, 79-816R-3, 79-817R-2, 27-297R-2, 27-297R-3, 98-1007R-3, 98-1007R4,4-6
139-856H-1R-CC, 7-91R-CC, 7-93R-1, 78-803R-1, 78-803R-2,45-473R-3, 24-264R-1,48-504R-1, 48-504R-1, 142-1444R-1, 142-1444R-1, 142-1444R-2, 12-144R-2, 50-524R-2, 50-525R-1,27-286R-1, 12-146R-1, 12-148R-1, 30-328R-1,30-328R-1, 115-1179R-1, 7-910R-1, 50-5111R-1, 107-10913R-l,2-414R-1, 132-13414R-1, 132-13414R-1, 132-13414R-1, 132-13415R-1, 127-12916R-1.4-616R-l,4-616R-1,31-3217R-1,33-3417R-1, 33-34
139-857A-4H-3, 119-1215H4, 81-836H-2, 102-1068H-3, 117-1198H-3, 117-11910H-2, 123-12513X-1,117-119
139-857C-2R-1,28-312R-1,28-312R-1, 28-313R-2, 106-10813R-1, 62-6414R-1,47-5115R-1, 1-315R-1, 61-6317R-2, 84-8822R-1, 144-14730R-2, 147-15031R-3, 14-16
0.430.434.404.404.845.296.146.148.218.218.21
17.6717.6728.3137.1446.6646.6649.2049.2049.5450.0950.0957.4757.4759.6759.6760.14
4.004.00
22.8822.8823.7124.9527.0827.0828.0228.0228.0228.2228.6028.6032.6737.5237.5248.3048.3049.1552.5757.5062.3770.9277.0277.0277.0277.0281.7785.2485.2485.5190.4390.43
26.0936.7143.4264.0764.0781.63
102.67
56.7856.7856.7869.06
153.72163.27172.51173.11194.24241.74316.07325.84
MSMSMSMSMSMSMSMSMSMSMS
MSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMS
MSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMSMS
ClayClaySillSillSiltSiltSilt
SiltSiltSiltSiltSandClay
SandSiltSiltClaySiltSilt
1.854.630.200.30
1.540.83
1.33
0.143.410.410.090.260.090.140.030.10
4.65.36.74.74.54.74.54.65.05.8
12.3
5.25.78.16.37.5
8.36.28.38.9
12.49.74.98.57.78.0
6.05.74.43.85.77.86.75.85.97.9
7.57.18.28.16.46.35.35.06.77.02.61.96.7
6.95.6
-8.07.54.9
-1.95.06.72.5 •
-30.3-21.7
-8.00.63.19.0
11.9
-12.0
-16.72.06.6
17.218.0-8.4-0.510.5-2.2
8.3
7.6
7.3
7.8
5.06.4
5.0
6.7
7.5
3.1 SI vein
5.5
Coarse PyCourse SI
Po clast
20.9
Py porphyroblastLate vug filling Py
Late vug filling Py
Chalcopyrite
Py nodule
-11.2-6.6
741
DATA REPORT-
Table 1 (continued).
δ3 4S
Core, section,interval (cm)
31R-CC,O-333R-1, 100-10233R-1, 123-12533R-2, 133-13534R-3, 37-3934R-3, 67-7035R-1, 66-6835R-2, 44-4637R-2,121-12338R-1.4-639R-1, 119-12141R-1, 144-14642R-2, 29-3143R-3, 0-449R-1,111-11450R-2, 58-6050R-2, 75-7751R-2, 18-2056R-1,0-257R-1,11-1258R-1.2-359R-1.3-459R-1.3-*59R4, 72-7460R-1,126-12861R-1,41-4362R-1, 120-13063R-1,29-3164R-2, 57-5965R-l,2-466R-1,2-368R-1, 8-10
139-857D-1R-1,42-443R-1,62-634R-1, 133-1365R-1, 16-186R-1,2-48R-1, 31-33
9R-1,25-2712R-1,75-7712R-1, 121-12415R-1,55-5716R-1, 48-5017R-1,96-9818R-1,94-9622R-1,45^624R-1, 100-10224R-2, 45^1726R-1,39-4127R-1,22-2428R-1,50-5229R-2, 40-4231R-1,20-2433R-1,92-9336R-1, 129-131
139-858A-2H-7, 34-383H-1,87-915H-5, 65-686H-3, 144-1466H-CC, 7-97H-4, 105-1089X-2, 136-1389X-3, 10-129X-3,10-129X-3, 61-639X-5, 96-9811X-CC7-812X-CC, 5-614X-CC, 3-517X-CC, 2-518X-l,6-818X-2, 127-13018X-CC, 6-819X-1, 4 3 ^ 520X-3, 135-13820X4, 15-182IX-1,67-7021X-2, 28-3121X-2,71-7422X-1, 10-1123X-1, 71-7324X-1, 61-6326X-CC, 6-8
Depth(mbsf)
326.02333.40333.63335.23339.87340.17341.96343.24353.51355.84361.59376.34381.49387.70414.81420.38420.55425.08446.50451.81461.52471.13471.13476.24482.06490.91501.20509.99521.47529.12538.82558.08
581.92599.92610.23618.76627.92647.61
657.15686.65687.11715.35724.98734.86744.54782.25802.20803.15820.69829.22839.40850.30868.70888.72918.19
11.7412.7737.5544.8444.9755.4665.3665.6065.6066.1169.4673.5481.85
101.03130.27139.66142.37143.04149.73163.28163.59169.27170.38170.81178.30188.61198.21216.96
Lithology
ClaySiltSiltClaySiltClaySiltClayClayClayClayClaySiltSandClayClayClaySandClaySandClayDiabaseDiabaseDiabaseDiabaseDiabaseDiabaseSiltDiabaseDiabaseSandDiabase
DiabaseDiabaseDiabaseSandSiltDiabase
DiabaseDiabaseDiabaseDiabaseSiltSiltDiabaseDiabaseClayDiabaseDiabaseDiabaseSiltDiabaseSandDiabaseDiabase
ClaySiltSiltSiltClaySilt'SiltClayClayClayClayClaySandSandClaySandSiltClaySiltClayClay
SandClaySiltClayClaySilt"Clay
S
(%)
0.490.210.270.290.260.230.12
0.600.710.191.501.730.790.041.27
0.220.561.20
0.37
Py Po
1.56.49.16.8
-1.6-25.4-26.5-12.0-11.3
-9.638.8-1.215.054.329.116.815.313.612.414.312.89.3
-0.28.08.43.6
13.6
7.515.55.2
8.48.19.6 8.3
8.79.4
9.38.2
8.510.213.014.67.2
11.111.412.010.09.99.0
11.210.97.97.7
3.4-5.6
7.6-2.3-5.316.05.9
25.69.8
11.2-21.0
1.322.3-9.7
28.9
25.027.014.3
1.912.014.2
-32.47.7
An Ba Notes
7.6
7.5
7.7
8.6
24.2
Burrow fill py
MS clasts in turbidite
Py veinPy nodule
Py veinCoarse euhedral PyPy veinPy-Chl veinSl-Cp-Wai veinChl-Py selvage of Epi vein
Wai-Sl vein
Sl-Cp Wai veinSelvage of Epi-Qtz-Py-Cp vein
Qtz Po veinPy-Chl vein in quenched
marginPy Wai? veinPo-Wai-Qtz veinWai-Qtz-Py vein
Py-Epi veinPy vein
Qtz-Wai-Epi-Sl-Py vein
Epi-Py vein
An-Gyp cementCoarse Py
21.326.325.021.928.3
26.425.022.427.4
Dissm bladed An
An-Gyp vein
An-Gyp vein
Dissm bladed An
742
• DATA REPORT
Table 1 (continued).
Core, section,interval (cm)
29X-1.7-1130X-1, 41^43IX-1,26-2838X-CC, 10-12
139-858B-1H-2, 27-291H-5, 3-51H-CC, 20-222H-3, 27-292H-3, 119-1212H-4, 7-9
139-858C-3H-2, 101-1043H-3, 133-1355H-1, 139-1415H-5, 27-296H-1, 143-1456H-3, 82-837H- 1, 32-347H-2, 138-1418H-1,58-5910X-CC, 6-911X-CC, 30-3212X-1, 18-2013X-1, 2-513X-1,41^614X-CC, 28-30
139-858D-2H-4, 10-134H-3, 138-1404H-4, 56-586X-1, 61-64
139-858F-11R-CC, 12-1413R-CC, 0-213R-CC, 15-1714R-1, 16-1715R-CC, 1-219R-1,26-2719R-1, 37-3821R-1, 14-1621R-1, 14-1622R-CC, 12-1323R-CC, 6-724R-CC, 8-1024R-CC, 8-1025R-1, 36-3725R-1,36-3725R-1,75-7625R-1, 107-10825R-1,111-11228R-1, 19-20
139-858G-2R-1,65-674H-1, 23-255R-1, 1-36R-1,29-311 OR-1,43^513R-1,45^716R-1,88-89
Depth(mbsß
245.97256.01265.56322.80
1.776.037.10
10.4711.3911.77
15.5117.3324.9829.7734.4336.8241.8244.3847.0849.5655.7864.1873.7274.1484.46
13.9022.9223.6029.41
114.09132.90133.05142.76152.21191.16191.27210.44210.44220.12229.66239.38239.38249.26249.26249.65249.97250.01277.69
287.15306.03315.41325.39365.33394.35423.78
Lithology
SandClayClaySand
ClaySiltClayClayClayClay
ClayClaySiltSiltSiltClayClayClaySiltSiltSiltSiltSiltClaySilt
ClayClaySiltClay
SandClaySandSiltSiltSiltSandClayClaySiltClayClayClaySiltSiltSiltBasaltBasaltBasalt
BasaltBasaltBasaltBasaltBasaltBasaltBasalt
S(%)
0.490.54
Py
8.38.29.39.2
-0.5-13.7-15.3
4.54.11.7
8.515.2-2.4-2.8
3.61.84.3
-0.4-3.4
1.59.04.88.38.0
-9.2
15.1-18.7
4.96.6
14.712.111.6
11.511.111.012.79.36.3
-12.6-13.2
11.111.412.513.313.84.0
5.8
8.19.78.87.3
δ 3 4 s
Po SI
10.3
An
26.323.7
14.7
3.(122.622.322.5
22.7
23.812.3
23.523.2
23.4
22.2
11.4
22.5
Ba
44.6
Notes
Coarse Py cubes
Semi-MSSemi-MSSemi-MS
Gyp
An vein
Coarse Py, An nodule
Qtz-Wai-Py veinQtz-Wai-Py vein
Py-Chi veinAn vein
Py-Chl vein
Notes: MS = massive sulfide, CS = clastic sulfide, Py = pyrite, Po = pyrrhotite, SI = sphalerite, Cp = chalcopyrite, An = anhydrite, Gyp •gypsum, Ba = barite, Qtz = quartz, Chi = chlorite, Wai = wairakite, Epi = epidote, Dissm = disseminated.
ticly deposited massive sulfide have a narrow range from -1.2‰ to4.7‰. Sphalerite was separated from three samples of the mineralizedzone at the base of the hole. Sphalerite δ34S values range from 6.0‰ to10.2%e and are not in isotopic equilibrium with coexisting pyrite.Disseminated anhydrite and barite from this interval are slightlyenriched in 34S relative to seawater sulfate and range in value from21.6‰ to 23.9%o; barite in veins gave values of 22.2‰ and 23.8‰Two samples of disseminated sulfate (probably gypsum) in silt from53.19 mbsf have values of lO.l‰ and 4.2‰, consistent with deriva-tion of sulfate by local oxidation of pyrite from this interval (Table 1).
Hole 856D
Hole 856D was piston cored into the weathered, rubbly top of aridge of massive sulfide that occurs on the south flank of Bent Hill.Recovered material was disaggregated and partly weathered massivesulfide composed predominately of sand- to silt-sized pyrite grainsand aggregates, with less abundant amorphous silica, sphalerite, pyr-rhotite, and chalcopyrite. Pyrite from this core has 634S values of4.6‰ to 5.7%o except for one sample fragment from near the base ofthe core at 8.21 mbsf that yields values of 12.3‰ and 7.5‰ for coex-
743
100-40 -20 40 600 20
Pyrite δ^S
Figure 1. Sulfur isotope value of pyrite vs. depth for Hole 855C.
isting pyrite and sphalerite, respectively. These values are not consis-tent with isotopic equilibrium between the two sulfide minerals.
Hole 856G
Hole 856G was rotary cored near the top of the ridge of massivesulfide on the south flank of Bent Hill. This hole penetrated 65.4 m ofmassive sulfide before the hole was abandoned due to adverse drillingconditions. Massive sulfide recovered in this hole is described inDavis, Mottl, Fisher, et al. (1992). Pyrite from Hole 856G ranges inδ34S value from 4.9%oto 12.4%o, but most values are between 5‰and 9‰ (Fig. 4). Coexisting pyrrhotite and pyrite from Sample 139-856G-6R-2, 140-142 cm have identical values of 8.3‰, which isindicative of nonequilibrium but is consistent with pyrite inheritingpyrrhotite sulfur during replacement. Coexisting pyrite and sphaleritein Sample 139-856G-7R-4, 4-6 cm have values of 8.0‰ and 5.5‰,respectively, which are also incompatible with isotopic equilibrium attemperatures above 0°C. A late-stage sphalerite vein in Sample 139-856G-6R-1,36-38 cm has a value of 3. l%c, which is 4.4%O lighter thanpyrite in the wall rock. The sulfur isotope values of massive sulfide inHole 856G do not vary systematically between the different sulfidetypes defined in Davis, Mottl, Fisher, et al. (1992).
Hole 856H
Hole 856H was rotary cored near the top of the ridge of massivesulfide on the south flank of Bent Hill adjacent to Hole 856G. Thishole penetrated 93.8 m of massive sulfide before the hole was aban-doned due to adverse drilling conditions. Massive sulfide recoveredin this hole is described in Davis, Mottl, Fisher, et al. (1992). Pyritefrom Hole 856H ranges in 634S value from -8.0‰to 8.2‰, but mostvalues are between 5‰ and 9%c, similar to samples from Hole 856G(Fig. 5). Two samples of paragenetically late, vug-filling pyrite have
120-40
Figure 2. Sulfur isotope value of pyrite vs. depth for Hole 856A.
negative sulfur isotope values (-1.9‰ and -8.0‰). Pyrrhotite valuesrange from 5.0‰ to 7.8%α With one exception, coexisting pyrite andpyrrhotite have δ34S values that are within 0.5‰; in Sample 139-856H-4R-1, 142-144 the pyrrhotite value is within 0.3‰ of coarse-grained pyrite "porphyroblasts" that replace pyrrhotite. However, noneof the sample pairs appear to represent isotopic equilibrium. They areconsistent with pyrite forming by replacement of pyrrhotite through aprocess that conserves sulfur, as is suggested by petrographic observa-tions. Two microdrilled samples of sphalerite give values of 8.2‰ and8.3%o, near the upper range of values for pyrite in this hole. One sam-ple of chalcopyrite has a relatively low sulfur isotope value of 2.5‰.One barite sample gives a value of 20.9%e, identical to seawater sul-fate. As in Hole 856G, sulfur isotope values of massive sulfide in856H do not vary systematically with sulfide type.
Site 857
Site 857 was drilled in an area of relatively high heat flow (700-800 mW) about 1.6 km south of the large hydrothermal vent field atSite 858. Samples for sulfur isotope analyses were collected fromHoles 857A, 857C, and 857D. Hole 857A was drilled by advancedpiston core-extended core barrel (APC-XCB) to 112 m depth. Holes857C and 857D were drilled about 200 m east of Hole 857A and wererotary cored. At both Holes 857C and 857D, interlayered diabase sillsand metasedimentary rock were encountered at depths greater than460 m. Hole 857D was the deepest penetration in the Middle Valleyarea with a total depth of 936.2 m.
Hole 857A
Core recovered from Hole 857 A consisted of interbedded turbidi-tic and hemipelagic sediment. Pore-water sulfate in Hole 857A de-
744
• DATA REPORT
120 "
140-40
Figure 3. Sulfur isotope value of pyrite vs. depth for Hole 856B.
creases downhole to about two-thirds of bottom-water values due tobacterial reduction of sulfate (Davis, Mottl, Fisher, et al., 1992).Sulfur content of sediment is generally less than 0.5 wt% (Davis,Mottl, Fisher, et al., 1992). Sulfur isotope values of pyrite in sedimentincrease from -30.3‰ at 26 mbsf to 11.9‰ at 103 mbsf (Fig. 6). Apyrite nodule at 64 m depth is 2.5‰ higher than disseminated pyritein the sediment.
Hole 857C
Turbiditic and hemipelagic sediment in Hole 857C becomes in-creasingly lithified with depth. Basaltic sills are interbedded withmetasedimentary rock below 471 m. Pore-water sulfate values de-crease downhole to a depth of about 200 mbsf, below which sulfatecontent is relatively constant at approximately 5 mmol/kg (Davis,Mottl, Fisher, et al., 1992). Sulfur content of the sediment is generallyless than 0.5‰. Pyrite δ34S values in Hole 857C have a wide range ofvalues, from -26.5%O to 54.3%c (Fig. 6). Pyrite δ34S values in sedi-ment are similar to those in Holes 856A, 856B, and 857Ain that theygenerally increase with depth and show less variation with depth,approaching a value near 13%e. Pyrite burrow fills, veins, and nodulesall have values similar to disseminated pyrite in nearby sediment. Theextreme value of 54.3%e is from the base of a coarse sand turbidite at387.7 mbsf that contains clasts of massive sulfide composed of pyrite,pyrrhotite, and sphalerite (Davis, Mottl, Fisher, et al., 1992). Theseclasts of massive sulfide are the only evidence of a much earlier phaseof hydrothermal activity in Middle Valley. This extremely heavy sul-fur isotope value is uncommon in seafloor hydrothermal systems.
Two samples of sulfate from 56.78 mbsf were analyzed. Aleak inthe vacuum extraction line occurred during conversion of the sample,yielding a value of -6.6%α This leak was partially fixed before thesample conversion was repeated, yielding a value of -11.2‰ This
fΦQ
10
20
30
40
50
60
• 856G
-40 -20 40 600 20
Pyrite δ 3 ^
Figure 4. Sulfur isotope value of pyrite vs. depth for Hole 856G.
lighter value is considered to be more reliable and is close to the valueof pyrite in the same sediment interval (-12.0%e). The sulfate analyzedis probably gypsum which formed by oxidation of pyrite. Pyrite indiabase from Hole 857C ranges in value from -0.2%c to 9.3‰. Petro-graphic observations indicate that all of the pyrite in the diabase is ofhydrothermal origin and is not magmatic sulfide (Stakes and Frank-lin, this volume). The value of -0.2%e is from the chlorite-pyriteselvage of an epidote vein cutting diabase and the value of 9.3% isfrom a pyrite chlorite vein in diabase. Two samples of sphalerite fromwairakite-bearing veins in diabase have values of 7.5‰ and 7.6%e.Pyrite in sediment from the section intruded by the diabase sills tendsto be several per mil heavier than pyrite in the diabase.
Hole 857D
Pyrite sampled from Hole 857D includes hydrothermal pyrite indiabase sills and pyrite in sediment between diabase sills. Pyrite insediment ranges in 634S value from 9.0%e to 14.6‰ and is generallyheavier than pyrite in adjacent diabase sills (Fig. 6). One sample ofpyrrhotite in sediment has a value of 8.3‰, 1.3‰ lighter than coex-isting pyrite. The temperature calculated by assuming isotopic equi-librium between the pair is 110°C, suggesting that the assumption ofequilibrium is not valid, as it is below the present temperature at thisdepth. Pyrrhotite from a quartz-pyrrhotite vein has a value of 8.7‰.Pyrite in diabase sills ranges from 7.2%o to 12.0‰. Pyrite in veins cut-ting diabase falls within this range. Pyrrhotite from a wairakite-quartzvein cutting diabase has a value of 8.2‰; sphalerite from wairakite-bearing veins gives values of 7.7‰and 8.6‰.
Site 858
Drilling at Site 858 consisted of a transect of holes from the marginto the center of an active hydrothermal field. Approximately 20 active
745
200 -
400 -
600 -
800 -
1000-40 -20 0 20
Pyrite δ 3 4 S
Figure 5. Sulfur isotope value of pyrite vs. depth for Hole 856H.
hydrothermal vents have been mapped in this area. Hydrothermalfluid at temperatures up to 276°C vents from anhydrite chimneyswhich contain only minor amounts of sulfide (Davis, Mottl, Fisher, etal., 1992). Pore-water sulfate profiles in all of the Site 858 holescannot be interpreted simply in terms of bacterial sulfate reductionbecause the high temperature of the sediment has resulted in precipi-tation anhydrite, some of which can redissolve during core retrievaland processing (Davis, Mottl, Fisher, et al., 1992). Similarly, sulfurcontents of sediments from Site 858 reported in Davis, Mottl, Fisher,et al. (1992) cannot be interpreted in terms of pyrite content becauseof the presence of abundant anhydrite in many sediment samples.Percent sulfur reported in Table 1 of this report does represent thesulfide-sulfur, and not sulfate-sulfur, content of the sediment samples.
Hole 858A
Hole 85 8A is located about 200 m west of the center of the activevent field and was APC-XCB cored to a depth 339.1 m in turbiditic andhemipelagic sediment and metasediment. Pyrite δ34S values rangefrom -32.4%O to 28.9‰ (Fig. 7). A downhole increase in δ34S values isnot as readily apparent in Hole 85 8A as in holes further from thehydrothermal center. However, samples from below 200 mbsf do haverelatively constant sulfur isotope values near 9‰, similar to deep sam-ples from other Middle Valley sites. Anhydrite and gypsum that occurdisseminated in sediment, cementing sediment, or as veins have δ34Svalues of 21.3‰ to 28.3‰. The lowest values are near the value of sea-water sulfate, but most samples are enriched in 34S by several per mil.
Hole 858B
Hole 858B was hydraulically piston cored 20 m away from anactive hydrothermal vent. Sediment recovered from this short hole
Figure 6. Sulfur isotope value of pyrite vs. depth for Holes 857A, 857C, and
857D.
was highly altered by magnesium metasomatism and contained abun-dant pyrite and anhydrite, including an interval containing semimas-sive sulfide at approximately 10-12 mbsf. Pyrite δ34S values rangefrom -15.3%eto 4.5%o (Fig. 8). Samples of semimassive sulfide havevalues of 11‰ to 4.5‰. Anhydrite in the altered sediment has valuesof 23.7%c and 26.3%c One sample of barite is highly enriched in 34Swith a value of 44.6‰.
Hole 858C
Hole 858C was APC-XCB cored to a depth of 93.1 m. This holeis located within 200 m of both Hole 858A and Hole 858B and isapproximately 100 m away from the Dead Dog mound hydrothermalvents. Pyrite in sediment from this hole has δ34S values of -9.2%e to15.2‰(Fig. 8). Pyrite sulfur isotope values are less variable down-hole than in Hole 858A. The deepest samples have values near 8%oexcept for the sample from near the bottom of the hole at 84.46 mbsf,which has the lowest value in the hole (-9.2‰). Anhydrite from themiddle section of this hole has values between 22.3%o and 23.8‰,similar to those in Hole 85 8A. However, three samples from the top30 m of the hole and the deepest sample in the hole have values belowthat of seawater sulfate and require input of a lighter source of sulfur,such as H2S or pyrite that has been oxidized to sulfate.
Holes 858D, 858G, and 858F
Holes 858D, 858G, and 858F were drilled in the center of the heatflow anomaly that defines the active vent field, but are not imme-diately adjacent to any active hydrothermal vents. Hole 858D wasdrilled by APC-XCB to 40.7 m, Hole 858G was rotary cored to 296.9m, and Hole 858F was rotary cored from 276.5 m to 432.6 m. Both of
746
• DATA REPORT
50
100
^ 1 5 °fΦQ 200
250
300
858A
350 * ' ' ' I i I I I i I i I i I i I I I I
-40 -20 0 20 40 60
Pyrite δ 3 ^
Figure 7. Sulfur isotope value of pyrite vs. depth for Hole 858A.
100
0 20
Pyrite
Figure 8. Sulfur isotope value of pyrite vs. depth for Holes 858B and 858C.
450
Figure 9. Sulfur isotope value of pyrite vs. depth for Holes 858D, 858F, and858G.
A1412
10
8
6
4
2
0B
C14121086420
Sediment
Basalt
Massive Sulfide
-40 -20 40 60
Pyrite δ 3 4 S
Figure 10. Histograms comparing the sulfur isotope values of pyrite in (A)sediment and metasedimentary rock, (B) basalt and diabase, and (C) massive,semimassive, and clastic sulfide.
747
DATA REPORT.
the rotary cored holes had low core recovery. Basalt was recovered inHoles 858G and 858F below 249 mbsf. Pyrite in sediment from Hole858D ranges in δ34S from -18.7‰to 15.l‰ (Fig. 9). Anhydrite hasvalues of 23.2‰ and 23.5%α The deeper sedimentary section sampledby Hole 858F contains pyrite with δ34S values ranging from -13.2‰to 14.7‰, but most samples have a narrow range, from ll%c to 12%o(Fig. 9). Two samples of anhydrite have δ34S values slightly higherthan seawater sulfate (22. l‰ and 23.4%c), but one sample has a lowvalue of 11.4‰, similar to disseminated pyrite in the same sample(11.0‰). Pyrite in quartz-wairakite veins in basalt has δ34S values of13.3‰ and 13.8‰. Disseminated pyrite in basalt has a value of 4.0‰.All of the samples from Hole 858G are basalt or metabasalt. Pyritedisseminated in basalt has values of 7.3‰ to 9.7%o; pyrite in chloriteveins cutting basalt has values of 5.8%e and 8. l‰ (Fig. 9). One sampleof pyrrhotite in basalt has a value of 10.3‰. An anhydrite vein cuttingbasalt has a δ34S value of 22.5%e, slightly higher than seawater sulfate.
SUMMARY OF DATA
Near-surface sediments generally have pyrite with isotopicallylight sulfur typical of diagenetic sulfide produced by sulfate-reducingbacteria. Some isotopically heavy pyrite with δ34S values greater thanseawater sulfate is present in the sediment, requiring high degrees ofbiogenic(?) reduction of sulfate. Earlier diagenetic pyrite formed innear-surface sediment is overprinted, either with depth or by proxim-ity to the center of the hydrothermal field, by isotopically heavy (6‰to 12‰) pyrite of hydrothermal origin. Anhydrite occurs in the hottersections of the sediment pile that underlies the hydrothermal field andhas isotopic values that tend to be several per mil higher than seawatersulfate, consistent with precipitation from recharging seawater thathas undergone isotopic fractionation by preferential extraction of 32Sduring sulfate reduction. Sulfide in altered igneous rocks is also iso-topically heavy (δ34S approximately 8‰; Fig. 10). Thus, primarybasaltic sulfide (0. l‰, Sakai et al., 1984) is not the sole source of sulfurfor the hydrothermal system; a heavier source of sulfur such as reducedseawater sulfate is required. Massive, semimassive, and clastic sul-
fides have a relatively narrow range of δ34S values near 6‰, similarto hydrothermal sulfides in both sedimentary and basaltic rock thatunderlies the deposit.
ACKNOWLEDGMENTS
This study was supported by the Gilbert Research FellowshipProgram of the U.S. Geological Survey, for which the author is deeplygrateful. Doug White of the Water Resources Division of the U.S.Geological Survey in Menlo Park is thanked for access to and assis-tance with mass spectrometry.
REFERENCES*
Breit, G.N., Simmons, E.G., and Goldhaber, M.B., 1985. Dissolution of baritefor the analysis of strontium isotopes and other chemical and isotopicvariations using aqueous sodium carbonate. Chem. Geol., 52:333-336.
Canfield, D.E., Raiswell, R., Westrich, J.T., Reaves, CM., and Berner, R.A.,1986. The use of chromium reduction in the analysis of reduced inorganicsulfur in sediments and shale. Chem. Geol., 54:149-155.
Davis, E.E., Mottl, M.J., Fisher, A.T., etal., 1992. Proc. ODP, Init. Repts., 139:College Station, TX (Ocean Drilling Program).
Rees, C.E., 1978. Sulphur isotope measurements using SO2 and SF6. Geochim.Cosmochim. Ada, 42:383-389.
Rees, C.E., Jenkins, W.J., and Monster, J., 1978. The sulphur isotopic compo-sition of ocean water sulphate. Geochim. Cosmochim. Ada, 42:377-381.
Sakai, H., Des Marais, D.J., Ueda, A., and Moore, J.G., 1984. Concentrationsand isotope ratios of carbon, nitrogen and sulfur in ocean-floor basalts.Geochim. Cosmochim. Ada, 48:2433-2441.
* Abbreviations for names of organizations and publications in ODP reference lists followthe style given in Chemical Abstracts Service Source Index (published by AmericanChemical Society).
Date of initial receipt: 14 June 1993Date of acceptance: 21 July 1993Ms 139SR-226
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