History of metalliferous sedimentation at deep sea drilling site 319 in the South Eastern Pacific

Geochimica et Cosmochlmica Acta, 1977. Vol. 41, pp. 741 to 153. Pergamon Press. Printed m Great Britain

History of metalliferous sedimentation at Deep Sea Drilling Site 319. in the South Eastern Pacific JACK DYMOND and JOHN B. CORLISS School of Oceanography,

Oregon State University, Corvallis, OR 97331, U.S.A. and G. Ross HEATH

Graduate School of Oceanography,

University of Rhode Island, Kingston, RI 02881, U.S.A.

(Received 30 April 1976; accepted in revised form 6 January 1977)

Abstract-Marked variations in the chemical and mineralogical composition of sediments at Site 319 have occurred during the 15 M.y. history of sedimentation at this site. The change in composition through time parallels the variability observed in surface sediments from various parts of the Nazca Plate and can be related to variations in the proportion of hydrothermal, hydrogenous, detrital and biogenous phases reaching this site at different times. Metal accumulation rates at Site 319 reach a maximum near the basement for most elements, suggesting a strong hydrothermal contribution during the early history of this site. The hydrothermal contribution decreased rapidly as Site 319 moved away from the spreading center, although a subtle increase in this source is detectable about the time spreading began on the East Pacific Rise. The most recent sedimentation exhibits a strong detritalhydrogenous influence. Post-depositional diagenesis .of amorphous phases has converted them to ironrich smectite and well-crystallized goethite without significantly altering the bulk composition of the sediment.

INTRODUCTION

recovered at this site contain the most complete record of sedimentation in the Bauer Deep. Thus, analyses of these sediments should allow us to track any time variations in composition over a 15 M.y. period.

DEEP-SEA sediments that are anomalously enriched in transition metals relative to aluminium and other common rock-forming elements have received considerable attention in recent years. These sediments are found near active spreading centers (BOSTROMand ANALYTICAL PROCEDURES PETERSON,1966), as basal deposits in Deep Sea Drilling Project cores (CRONANet al., 1972; DYMOND et The chemical composition of the sediments was deteral., 1973) and within the Bauer Deep, a basin west mined by instrumental neutron activation (INAA) and atomic absorption spectrophotometry (AAS). Each sample of the East Pacific Rise (EPR) between 5 and 20”s (Table 1) was freeze-dried and split into fractions for AAS (BISCHOFF and SAYLES, 1972; DASCH et al., 1971; and INAA analyses. All sediments were dried at 110°C SAYLESet al., 1975). The association with active volovernight and stored in a desiccator before weighing for canism occurring at spreading centers led to the analysis. Following the general procedure of GORWN et al. (1968), suggestion that these deposits form as a result of the samples were irradiated with USGS standard rocks precipitation of metals from volcanic emanations in the rotating rack of the Oregon State University TRIGA @STROM and PETERSON, 1966; CORLISS, 1971). reactor. The-samples were counted on an So/, efficient The origin of the Bauer Deep deposits are less cerGe(Li) . detector which has 0.9 keV resolution at 122 keV tain because of their location more than 500 km from and 2.0 keV resolution at 1.333 keV. Data accumulation and reduction were done with a computerized, multichanan active spreading center, and because they have nel analyzer system. Each sample was counted at least compositional features distinct from the metalliferous twice, once 5-12 days after irradiation for La and Sm and sediments located on the East Pacific Rise (DYMOND after 18 days for the remaining elements. AAS measureet rrl., 1973). Site 319, one of three holes drilled on ments were made on samples dissolved in Teflon-lined bombs using a procedure modified from BERNAS(1968). Leg 34 of the Deep Sea Drilling Project, was located More complete analytical details can be found in DYMOND in the Bauer Deep (13”01.04’S, 101”31.46’W, 4290m) et al. (1976). approximately 1100 km from the East Pacific Rise The precision of these analyses varies from less than and 600 km from the presently extinct Galapagos 1% for the major elements and for those trace elements Rise. Since the site is on crust formed at the Galawith concentrations greater than 200ppm to approximately 20% for trace elements in the CaCO,-rich samples. pagos Rise, the sediment section should preserve any Comparisons with reported data on USGS standard rocks metalliferous components produced in conjunction suggest there are no systematic errors. We have analyzed with Galapagos Rise volcanism, thereby permitting Ba and Fe by both INAA and AAS. In most cases the comparison with the sediments presently being deFe analyses agree to within 5% and the Ba analyses agree to within 15%. posited on the East Pacific Rise. Also, the sediments 741

142

J.

DYMOND. J. B. CORLISS and

G. R.

HEATH

Table 1. Samples analyzed Sample Code

Depth Below Ocean Floor

Paleontologic' Age

Sample Name ~~___

*

319-1

0.08

0.1

Y73-3-13 NG 3, 5-10 cm

319-2

0.28

0.3

V73-3-13 MG 3, 25-30 cm*

319-3

2.11

1.5

319-1-2, 60-62 cm

319-4

3.81

3.5

319-1-3, 80-82 cm

319-5

7.74

6.0

319-1-6, 23-25 cm

319-6

10.30

6.8

319-2-1, 77-79 cm

319-7

12.11

7.2

310-2-2, 110-112 cm

319-8

13.21

7.5

319-2-3, 70-72 cm

319-9

14.41

7.8

319-2-4, 40-42 cm

319-10

21.21

9.7

319-3-2, 70-72 cm

319-11

22.81

10.2

319-3-3, 80-82 cm

319-12

30.00

12.0

319-4-1 and 2, B-10, 104106 cm

319-13

40.00

12.5

319-5-2 and 3, 6-8, 40-42 cm

319-14

53.50

12.9

319-6-4 and 5, 20-22, 122-124 cm

319-15

61.30

13.2

319-7-3, 129-131 cm

319-16

65.20

13.4

319-7-6, 69-71 cm

319-17

73.65

13.6

319-8-5, 114-116 cm

319-18

78.75

13.8

319-9-2, 124-126 cm

319.19

87.87

14.1

319-10-2, 86-88 cm

319-20

97.31

14.4

319-11-2, 80-82 cm

319-21

101.58

14.6

319-11-5, 57-59 cm

319-22

102.92

14.7

319-11-6, 40-43 cm

319-23

106.89

14.8

319-12-3, 86-93 cm

* Location is 12”59.O’S,lOl”33.OSW. 4303 m depth. + From QUILTY et al. (1976), and BERGGREN and VAN COUVERING Each analysis has been corrected to a salt and CaCO,free basis in Table 2. The salt content of each sample was determined by measuring the water content of the bulk sample and assuming a constant salinity of 35x, for the interstitial water. The CaCOs content of each sample was determined by measuring the total Ca content of the sediment and solving a mass balance equation involving seasalt calcium, carbonate calcium and non-carbonate calcium. A Ca value for the non-carbonate fraction of the sediment was determined by analysis of acetic acid leached and other non-carbonate sediments from the Nazca Plate. Entering this value, 0.73 f 0.12x, the relationship between the weight per cent of salt in the sediment (S), the total weight per cent of Ca in the sediment (Car) and the per

cent

(1974).

carbonate (C) is: C = Ca, - 0.0041S - 0.73

x loo. 39.31 As discussed in more detail in DYMOND et al. (1976) this method of determining CaCO, content is most accurate in carbonate-rich sediments, where the precision is better than 1%. Inasmuch as small errors in the CaCO, content of such carbonate-rich samples are greatly magnified in the carbonate-free correction factor, this high precision makes the technique more attractive than other methods (Leco-combustion and acid leaching) which become less precise, on an absolute basis, at high CaCO, contents.

Table 2. Chemical composition, carbonate and salt correction 319-1 Depth

in Core

Major

Elements

Fe (AASC) Fe (NAAC) Fe Leached Fe Residue Mfl Mn Leached Mn Residue Si Si Leached Si Residue Al Al Leached Al Residue Ba (AASC) Ba (NAAC)

in Meters

319-2

319-3

319-4

319-5

319-6

319-7

319-8

0.08

0.28

2.11

3.81

7.74

10.30

12.11

13.21

16.45

16.68

17.50 19.30

18.20 21.30

17.85 -

15.33

18.77

6.10 10.35 4.94 4.72

16.25 18.80

6.05 -

i.15 -

i.18

i.44

i.78

17.60 -

li.73

20.70

19.22

i.80 -

i.42

Loo

1.31

i.24 2.59

i.49 -

i.55

1.54

17.51 0.22 1.23 16.29 3.65 0.65 2.99 2.36

-

i.39

li.36 -

lT.33

i.15 -

i.35

i.34

i.27 2.31

5.97 11.52 4.14 4.14

li.67 0.67 17.00 2.94 .24 2.70 2.19 2.58

-

Metalliferous sedimentation at Deep Sea Drilling Site 319

743

Table 2. (Continued) 319-1 Trace

Elements

319-2

1259 1150 105 1360 800 560 472 270 202

111 Leached Ni Residue CU Cu Leached Cu Residue Zn 2n Leached 2n Residue

319 116;

874

116;

117;

50;

442

:: Ce Nd Sm :u,

Depth

in Core

Major

Elements

;: Fe Fe Mn Mn Mn Si 51 Si Al Al Al

Factor

in Meters

I$:; Leached Residue Leached Residue Leached Residue Leached Residue

:: [ZE] elements

319-6

319-7

319-8

396

314

280

*

11:: 460 724 450 184 266

1102

1088

1198

44;

46;

2:;

2::

22; 40 11 6.0

238 40 10.8 5.9 30.2

:Fl

249 690 1190 46;

492

12 293 236 42 11 6.3 32 i::

:?I

:::

1.53 9.31 21.70 78.71 12.94

1.28 3.32 6.43 81.49 15.41

1.23 2.70 4.88 79.40 13.49

1.21 2.51 4.39 78.92 13.10

1.35 6.39 14.29 76.61 11.46

3.32 25.06 61.81 69.83 8.10

3.68 27.42 67.83 58.64 4.95

2.98 24.50 60.41 63.50 6.09

319-9

319-10

319-11

319-12

319-13

319-14

319-15

319-16

14.41

21.21

22.81

30.00

40.00

53.50

61.30

65.20

16.86 17.30 3.37 13.48 3.46 3.31 0.15 19.33 0.37 18.96 1.14 0.09 1.05 1.62 1.39

18.68

17.90 20.30

26.00 26.00

21.00 22.00

13.50 13.90

18.50 18.50

24.00 27.00

j.63

i.06

i.60

5.00

i.60

4.24

i.70

19.40

18.00

10.20

i.00

i.40

;.oo

li.00

i.34

Los

i.63

i.20

0.95

1.35

i.90

i.46

1.25 1.12

i.16 0.98

i.20 1.10

i.18 1.03

i.16 1.03

i.90 1.60

(ppm)

SC

:; 100 261 244 16.8 1044 368 675 418 131 287 4.4 127

,": Ni Ni Leached Ni Residue

cu

Leached Residue Leached Residue

15

:; 108 225

:i 169 350

160 400

10 12 114 310

:: 139 310

2:: 370

117;

1220

1530

1500

1120

1210

1900

48;

37;

soa

600

36;

38;

540

i.6 142

i.3 174

6.2 175

119 21

1:: 27 7.2 4.9 24 3.3

4.2 167 32 113 19 6.1 3.9 22 2.9

109.8 17.7 ::i 12.2 2.4

Yb Lu Hf Th Salt, CaCO,

319-S

Corrections

Correction Ca* (X)

Cu Cu 2n 2n 2n Sb La Ce Nd Sm Eu Tb

2:: 797 779

34

Yb LU Hf Th

Trace

25

:i

CaCOl

319-4

(pm)

SC Cr co

Salt,

319-3

El 13 2.6

1; 1:: 8.0 5.0 31 4.0

13; 24 7.1 3.4 16 3.0

19

2:: 70 170 34 10 7.0 32 4.7 LO

::(: Correctlow

Correction Ca* (X) CaC03 (4) Water (X) Salt (9;)

Factor

* Uncorrected Ca.

2.78 23.42 57.66 64.63 6.40

2.86 23.46 57.74 67.46 7.26

4.92 30.32 75.24 55.86 4.43

6,17 32.60 80.92 45.05 2.87

24.08 37.80 94.27 31.04 1.58

19.40 37.39 93.24 31.45 1.61

12.42 36.15 90.09 34.59 1.85

25.04 37.70 94.12 34.97 1.88

J. DYMONI). J. B. COKLISS and G. R. HEATH

744

Table 319-18

319-19

319-20

319-21

73.65

78.75

87.87

97.31

101.58

102.92

106.89

26.00 30.00

32.30 32.10 7.70 24.57 10.20 9.60 0.57 6.60 1.25 5.34 1.21 0.16 1.05 0.64 0.57

18.80 19.60

29.00 29.00 6.30 22.41 8.90 8.50 0.39 5.50 1.07 4.42 1.00 0.14 0.86 0.52 0.45

29.00 31.10

28.00 29.00 6.40 21.38 8.50 8.20 0.35 5.40 0.99 4.44 0.89 0.09 0.81 0.74 0.64

22.00 19.00

319-17 Depth in Core in Meters

2. (Coitrinutct)

Major Elements ;: Fe Fe Mn Mn Mn Si Si Si Al Al At Ba Ba

IZZi Leached Residue

;.a0 Leached Residue i.70 Leached Residue

i.40

Leached Residue (AASC) (NAAC)

1.01 1.03

i.40

5.00 0.80

0.54 0.50

8.70 5.84

i.03 0.54 0.42

j.10 9.00 1.30 0.74 0.61

Trace Elements (ppm) :f: 180 630

,':: co Ni Ni Leached Ni Residue cu Cu Leached Cu Residue Zn Zn Leached Zn Residue Sb La Ce Nd Sm

1800

67;

2:: 1;; 32 8.5 5.8 28

:ub Yb Lu

23.: 1:7

T”; Salt,

9.6 25 136 830 790 16:: 439 1214 704 326 378 7.9 151 119 i.8 2.7 12 2.3 2.0

7.6 13 4::

1070

470 j.6 112 27 83 17 4.5 2.9 14 2.1 1.2 0.6

8.7 17 127 800 650 155 1490 350 1.140 670 330 340 7.7 137

8.2 33 133 690

141; 630

8.2 137

8.6 28 122 700 470 236 1400 300 1103 570 330 239 7.2 150 122 20

7.1 22 74 490 1300 510

;.2 84 76 12 3.4 1.5

;: 18 5.2 3.5

113 18 5.1 2.6

17 2.4

11 2.2

12 2.4

7.4 1.5

5.99 32.90 81.76 30.59 1.54

9.06 35.00 87.06 35.21 1.90

8.64 34.80 86.56 34.71 1 .R6

z::

t::

CaCO? Corrections Correction Ca* f%)

Factor

~~~~~ ($T) Salt (‘X) * Cincorrected

11.80 36.10 90.02 30.16 1.51

4.75 30.85 76.60 39.98 2.33

8.72 35.00 87.16

28.07 1.37

6.26 33.00 82.18 34.54 1.85

Ca.

Samples were also analyzed after shaking 1OOmg fractions in a 3.0 pH mixture of 0.2 M ammonium oxalate and 0.2 M oxalic acid for 2 hr in the dark (SCHWEKTMANN, 1964). This procedure removes amorphous ferric hydrox-

ides and poorly crystalline ferromanganese hydroxides, but does not attack crystalline goethite (LANDA and GAST. 1973). in contrast to other leaches (e.g. 0.1 M hydrochloric acid, hydroxylamine hydrochlo~de-ascetic acid) (CHESTLK and HUGHES, 1967), and sodium dithionite reduction in a sodium citrate~sodium bicarbonate solution (MEHKA and JACKSON, 1960), subsequent treatments by the oxalate leach remove negligible quantities of metals (HEATH and DYMOND. 1977).

AAS and INAA data for as many as 22 elements are presented in Table 2 for the 23 samples analyzed. The two shallowest samples (Table I), 319-l and 319-2 were removed by gravity coring at Site 319.

These two samples provide surface sediments which probably were not recovered by the Deep Sea Drilling Project. Inspection of the data allows us to divide the scdiments from this site into three major groups: (I) Surface sediments recovered between 0 and 8 m depth (O-6 M.y. age) which have relativeIy high concentratiolls of Al, Si. Co. Ni, Ba and ram-earth elements. (2) Intermediate sediments recovered between 8 and 25 m depth (6-I I M.y. age) which also have high concentrations of Si, but are distinguished from surface sediments by lower Al. Co. Ni, Ba and rare-earth element contents. (3) Basal sediments recovered between 25 and 107 m depth (Ii- I5 M.y. age) which are characterked by Fe, Mn Cu and Zn enrichments.

745

Metalliferous sedimentation at Deep Sea Drilling Site 319 Cu iPPM1

Fe (%)

I

16 20 , I I,,

Mn PI.1

Co (PPMI

1000 1400 ,600 24 26 32 0 50 100 150 200 250 300 I I I I I I I I I I 4oo Ni (PPd) ’ ’ ’ ’ ’ Zn (PPMI S 800 12GO 400 600

25 : ::::.2. y, 1

::

‘..‘....

..‘i

BASALT,

.‘,’

.: : .;,

..:

..:

Fig. 1. Compositional variations as a function of depth for Site 319. Concentrations are corrected to a salt and calcium carbonate-free basis. Horizontal dashed lines indicate approximate boundaries between surficial (S), intermediate (I), and basal (B) sediment types.

Two features of the data which are illustrated in Fig. 1 are an increase in Ni and Co in the near-surface sediments and large variations in the Mn, Fe, Cu and Zn contents of the basal sediments. Figures 2 and 3 show that (1) Bauer Deep surface sediments have higher Al and Ni than surface sediments from the East Pacific Rise, as shown previously by DYMOND et al. (1973); (2) all sediments from Site 319 are enriched in Fe and Mn and depleted in Al relative to normal pelagic sediments and (3) basal and intermediate sediments from Site 319 resemble East Pacific Rise surface sediments, whereas the surhcial sediments are similar to other Bauer Deep surface sediments. Mineralogy

X-ray diffraction patterns using diffracted-beam monochromatized CL&, radiation were prepared for carbonate-free samples 319-1, 4, 9, 11, 18, 20, and 22 (Table 1). The data were recorded digitally and numerically filtered to enhance the signal/noise ratio, as discussed by DASCH et al. (1971) and DYMONDet al. (1973). The surface sediments are dominated by phillipsite, iron-rich smectite (destroyed by the dithionite citrate-bicarbonate treatment of MEHRAand JACKSON

(I 960)). and barite, with minor quartz and plagioclase and trace amounts of &MnO,. Apart from a slightly higher phillipsite content, these sediments have the typical mineralogy of surface Bauer Deep deposits (HEATHand DYMOND,1977). The intermediate depth sediments are dominated by well-crystallized iron-rich smectite, with minor barite and phillipsite, and trace amounts of quartz, plagioclase, pyroxene and &MnO,. These deposits have no close analogs in the surface northwest Nazca Plate sediments examined by HEATH and DYMOND (1977), but could be reproduced by adding iron-rich smectite to modern deposits on the northwest flank of the Galapagos Rise. The basal sediments are dominated by well-crystallized goethite with lesser iron-rich smectite and minor &MnO,, phillipsite, quartz, plagioclase, cristobalite, and clinoptilolite. These deposits most closely resem-

ble East Pacific Rise crest samples from 10” to 20’S, but are distinguished by their goethite and clinoptilolite contents. Our X-ray diffraction results are in good agreement with those of ZEMMELS and &OK (1976). Their data suggest, however, that the basal unit from 25 to 77 m is richer in clinoptilolite than the interval from 77 to 100 m which we sampled. FE

AL

“ER

MN

DEEP NODULES

SI

FE

MN

Fig. 2. Al-Fe-Mn and Fe-Mn-Si ternary diagrams showing the relationships between the three sediment types of Site 319 (S, I and B), typical Pacific sediment (GOLDBERG and ARIWENIIJS,1958), Bauer Deep nodules (LYLE, 1976) and surface sediments from the Nazca Plate as indicated by the Bauer Deep, 11-15” and EPR, 11-15” (D~~~~~,unpublished data),

746

J. DYMOND.J. B. CORLISS and G. R. HEATH

NI

ZN

Fig. 3. Ca-Ni- -Zn ternary diagram showing the relationships between the three sediment types of Site 319 (S, I, and B). EPR (I l-15”) surface sediment, Bauer Deep (11-15”) surface sediment, and Bauer Deep nodules. Oxalic acid leaching experiments

DISCUSSION analysis of’ sediments

NormLltk

Six samples, representing the spectrum of sediment compositions, were treated with the oxalate leach to determine the leachable and residual components Fable 3). Only small amounts of Si (2-20x) and Al (8-18%) are removed by this procedure. However, nearly all the Mn (94100%) and Ni (67-98%) are leachable, implying that they are present as poorly crystalline oxides and hydroxide throughout the section. The leachable Fe fraction decreases from about 35% in the near-surface sediment to 20% in the middle of the core and 23% in the most basal sediments. Cu shows a similar decrease in the fraction which can be leached by the oxalate solution.

HEATH and DYMOND (1977) have attempted to distinguish the hydrothermal, detrital, hydrogenous and biogenic contributions to surface northwest Nazca Plate sediments (including those near Site 319) by assuming that: (1) the ratio of the detrital contribution of each element to detrital Al is constant; (2) the ratio of the hydrothermal contribution of each element to hydrothermal Fe is constant; and (3) the ratio of the authigenic contribution of each element to authigenic Ni is constant. Since these ratios cannot be measured directly, the normative model was ‘tuned’ to yield an internally consistent partitioning

Table 3. Percentages of each element contributed by major sources. based on HEATHand DYMOND’S (1977) normative model ?O-120m ll-15my Fe hydrothermal detrital

25-7Om 7.5-llmv

97 3

95

Mn hydrothermal detrital hydrogenous

97
,100 Cl I-0

Si hydrothermal detrital biogenous

42 43 15

4’4 29

Al hydrothermal detrital

tz 32

5

a-25m 5-7.5my 95 5

O-am 0.5my a6 14

,100 kl
86 1 13

9 17 74

53 39

91

9;

3 97

12 1 5 a2

3 a7

9

a

aa hydrothermal detrital hydrogenous biogenous

5: 13

Cu hydrothermal detrital hydrogenous biogenous

a2 1 9 a

59

69

: 38

;

29

Ni hydrothermal detrital hydrogenous

60

a4

2 38

a6

1:

25 4 71

2n hydrothermal detrital hydrogenous biogenous

77 1

69

57 5 22 16

1:

75 2 1 22

5 ;

5 9

2 1 29

4; 53

56

2: 13

Metalliferous sedimentation at Deep Sea Drilling Site 319 of the surface sediments from the northwest Nazca Plate. To assess both the model and the applicability of modern depositional patterns to the sediments cored at Site 319, we have applied the model to the data of Table 2. The results are summarized in Table 3. For the lower basal sediments (13.5-15M.y.; 7s120m) the model results are similar to those obtained for modern East Pacific Rise sediments. The chief difference is the higher detrital Al and Si contribution to the Site 319 sediments. Inasmuch as the modern EPR crest is shielded from detritus by both the Peru-Chile Trench and the Galapagos Rise, whereas the Galapagos Rise was shielded only by the trench, such a difference is not surprising. The Site 319 upper basal sediments (1 i-13.5 M.y.; 25-70 m) were deposited on the west flank of the Galapagos Rise. They have no exact analogs in HEATH and DYMOND’S(1977) sample set, but are reminiscent of northern East Pacific Rise deposits in their high biogenic Ba, Cu and Zn contents. The model appears to break down for Mn in Site 319 deposits. Either the hydrothe~l MnjFe ratio was as much as 50% less than the 0.3 value for modern East Pacific Rise sediments, or much of the original Mn has been lost during diagenesis. Intermediate Site 319 sediments deposited 6-l 1 M.y. ago (8-25 m depth) are characterized by very low detrital and hydrogenous inputs, together with high percentages of biogenous and hydrothermal components. The fact that the proportions of hydrothermal Cu and Ni, for example, in this interval are greater than in the underlying deposits supports the suggestion that the newly formed EPR supplied a sign~cant amount of detritus to the Site 319 location. Although the model assigns 74% of the Si to a biogknous source, microscopic examination reveals no unusual concentration of siliceous tests in this interval. The surficial sediments at Site 319 (Wm depth; o-6 M.y.) are typical of modem Bauer Deep deposits, except that the hydrothermal Mn values are higher than any in HEATH and DYMOND’S (1977) sample set. Accumulation rates of elements

Accumulation rates of elements can be calculated from the chemical composition and bulk density data and from sedimen~tion rates based on microfossil analyses of Site 319 (QUILTYet al., 1976). Variations in the accumulation rates of elements through time (Table 4 and Fig. 4) contrast markedly with the variations of chemical composition through time. The accumulation rates of Fe, Mn, Cu and Zn decrease sharply upward from very high values measured near the basement, a pattern that is consistent with earlier observations of high accumulation rates and enrichment of these elements near spreading centers (BENDERet al., 1971; DYMONDand VEEH,1975). Perhaps more surprising is the fact that all the other elements analyzed also have their highest accumu-

747

lation rates in the near-basement portion of the core. Thus, in spite of the fact that Si, Al and Ni have low concentrations in the basal sediment and show marked enrichments in suficial sediments, their accumulation rates are 34 times higher in basal sediments than in surficial deposits (Fig. 4). This reflects the fact that the sedimentation rate for basal sediments is 30 times higher than for surficial sediments, whereas Ni, Co, Al, Si, REE and Ba are enriched by less than a factor of 10 in suriicial sediments. The smooth decrease in accumulation rates of metals with decreasing age appears to be disturbed during the interval 611 M.y. before present, which are the intermediate type sediments characterized by high Si values and, compared to surficial sediments by low Ni, Co, Al, 3a and rare-earth element abundances. Figure 5 compares the Mn accumulation rate for Site 319 with the rate measured in surface sediments from the Nazca Plate and with the median rate measured in cores for other parts of the Pacific. The rapid decrease in Mn accumulation rate away from the spreading center (with increasing age above basement) which is clear in both Site 319 and East Pacific Rise samples, is strong evidence for a hydrothermal source of Mn. The median rate for typical Pacific sediments can be considered to be the hydrogenous rate. Some variability in this rate is likely, but is small compared to the variations shown in Fig. 5. Thus, the excess Mn a~umulation rate relative to that of typical Pacific sediments can be taken as the hydrothermal contribution. From Fig. 5 it is evident that the source of Mn is dominantly hydrothermal during the first 4 M.y. of deposition. For Site 319 sediments deposited in the last 6 M.y. (9-15 M.y. above basement), the total Mn accumulation rate can be explained entirely by hydrogenous precipitation. X-ray mineralogy

The mineralogical variations in Site 319 sediments reflect both diagenesis and changes in source materials. Modern rise-crest analogs of the basal deposits never contain clinoptilolite or cristobalite, and rarely contain goethite, implying that these minerals are diagenetic at Site 319. For example, we infer that the poorly crystallized ferric hydroxide deposited when the site was at the Galapagos Rise crest has subsequently recrystallized to goethite. Such a diagenetic tr~sfo~ation is consistent with the oxalate-leach results; about 80% of the iron in modem East Pacific Rise deposits is oxalate-leachable (HEATH and DYMOND,1975), whereas only 23% is leachable in the lower basal deposits of Site 319. If the diagenetic minerals goethite, cristobalite and clinoptilolite are ignored, the basal Site 3 19 sediment differ mineralogitally from modern East Pacific Rise only in having higher contents of quartz and plagioclase, presumably due to less protection from terrigenous sources, as discussed previously. The intermediate sediments have no exact analogs in modem northwest Nazca Plate deposits. The

J. DYMOND. J. B. COKLISS and G. R. HEATH

748

Table 4. Accumulation Sedimentation* Rate cm/103 yr

Sample

OensityX g/cm3

Fe

rate data Accumulation Rates0 ngcm2/103 yr Si Al

Ni

319-l

0.11

1.14

2.9

0.86

3.1

0.64

0.022

319-2

0.11

1.15

3.0

1.1

3.2

0.58

0.021

319.3

0.11

1.18

3.5

1.2

3.8

0.73

0.19

319-4

0.11

1.17

3.9

0.92

4.0

0.65

0.018

319-5

0.11

1.14

4.0

0.90

3.8

0.61

0.015

319-6

0.44

1.20

8.5

2.0

8.5

0.68

0.014

319-7

0.44

1.34

10

2.3

14

0.66

0.021

319-8

0.44

1.32

13

2.7

14

0.92

0.020

319-9

0.44

1.32

13

2.6

15

1.0

0.020

319-10

0.44

1.29

12

2.3

12

0.85

0.018

319.11

0.44

1.30

9.2

2.1

9.2

0.53

0.012

319-12

0.44

1.56

16

3.4

6.2

1.0

0.021

319-13

3.2

1.62

32

7.1

10

1.8

0.060

319-14

3.2

1.67

25

4.9

10

1.8

0.059

319-15

3.2

1.69

52

12

20

3.8

0.089

319-16

3.2

1.68

34

7.9

17

2.6

0.052

319-17

3.2

1.72

85

23

25

4.7

0.21

319-18

3.2

1.69

221

70

46

8.3

0.57

319-19

3.2

1.74

86

25

23

3.7

0.22

319.20

3.2

1.79

172

53

33

6.0

0.48

319.21

3.2

1.68

181

54

37

6.4

0.43

319-22

3.2

1.68

107

33

21

3.4

0.21

319-23

3.2

1.68

90

21

36

5.2

0.20

* Based on age data shown in Table I. # Wet bulk density from data in “Hole Summaries, Leg 34. YEATS 64 al. (1974). ‘fiAccumulation rate calculated from the expression: A = C’. S.p (100-W), where C = concentration of element, not corrected for salt and CaCO, contents, S = sedimentation rate, p = density, and W = water content.

SlTE 319

METAL

ACCUMULATION

lmp/cm'/1000

yr

RATE

1

AGE

Fig. 4. Variations in accumulation rates for Fc. Mn, and AI through time for Site 319.

Si

ABOVE

BASEMENT

Fig. 5. Contributions of the elements Al, Si, Mn, Fe, Ni, Cu. Zn and Ba (scaled varimax factor scores) to the three varimax factors generated by Q mode factor analysis (IMMRRIII and KIPP, 1971) for Site 319 (8 elements, 23 samples).

Metalliferous sedimentation at Deep Sea Drilling Site 319 dominance of iron-rich smectite suggests that hydrothermal and biogenous components have both influenced the present mineralogy. HEATHand DYMOND (1977) postulate that biogenous opal and hydrothermal ferric hydroxide are presently reacting in’ the northern Bauer Deep to produce the distinctive ironrich smectite. This reaction is the most plausible explanation for the smectite in Site 319 sediments, particularly as the normative model points to a dominant hydro~e~al source for the iron and biogenous source for the silicon (Table 3). The biogenic influence is confirmed by the high calcite contents (Table 2) and presence of barite in intermediate sediments. The surficial sediments at Site 319 include the authigenic mjnerals phillipsite, barite and iron-rich smectite that are typical of modern, slowly accumulating Bauer Deep deposits (HEATH and DYMOND,1977). The minor detrital component (containing quartz and plagioclase) appears to be carried to the area by bottom currents from the Peru Basin (HEATH and DYMOND,1977). The increase in detrital content from the intermediate to surficial sediments probably reflects the global increase in supply of terrigenous material to the oceans during the Late Cenozoic climatic deterioration (HEATH,1969) as well as to possible local changes in deep water circulation in the southeastern Pacific during the past Ii M.y. Fuctor arwlysis

In order to further evaluate the compositional variability through time of the sediments at DSDP Site 319, we have subjected our data to Q-mode factor analysis (IMBRIEand VAN ANDEL, 1964). We have scaled the data for each sample so that the sum of the element concentrations is 100. This procedure eliminates errors introduced by carbonate and saltfree corrections, which can become large at high carbonate values. Also, since we are dealing with a small group of elements in which both the major and minor elements are of relatively equal significance, we have scaled the data so that each element has a mean value of 100. This procedure preserves inter-sample concentration ratios for each element, but prevents major elements from dominating the statistical analysis. The elements Al, Si, Mn, Fe, Ni, Cu, Zn and Ba for which a complete sample-element matrix existed were used in the first analysis. Factor analysis showed that 3 factors account for 99% of variance in the scaled data. The elemental loadings on each factor are shown in Fig. 6. Factor 1 is dominated by Si, with lesser loadings on Fe, Cu, Zn and Ba. The high Si suggests this factor is a biogenous factor. Factor 2 has high loadings of Mn, Fe, Cu and Zn. Studies of surface sediments on the Nazca Plate (HEATR and DYMOND, 1977) have demonstrated that these elements are concentrated close to the EPR. Consequently, factor 2 is considered to be a hydrothermal factor. Factor 3 is dominated by Al, Ni and Ba. This factor appears to be a combined detail-hydrogenous factor, with

-2’

FACTOR

I

FACTOR

2

FACTOR

3

td

Al

Si

749

Mn

Fe

Ni

Cu

Zn

Ba

Fig. 6. Manganese accumulation rates for Site 319 and for surface sediments from the Nazca Plate, plotted against the difference in age between the basement and the sediment. Al being added by detrital phases, Ni by hydrogenous precipitation of Fe-Mn micro nodules and coatings, and Ba by the precipitation of barite. Figure 7 shows the re~tionship between the factor loadings and the depth in the core. Factor 1 is dominant in the intermediate sediments (1&30m; 6-l 1 M.y.). Factor 2 is important in the basal sediments (the bottom 70m of the core, 11-15 M.y. old) and shows a general increase with depth. Only the upper 10m of core FACTOR

I

FACTOR 2

FACTOR 3

Fig. 7. Variations in the S-element factor components as a function of depth. The factor scores have been normalized so that the sum of factors 1, 2 and 3 equals one. The variations in factor loadings confirm the partitioning of the core into surficial, intermediate and basal sediment types.

750

J.

DYMOND.

J. B. CORLES

and

G.

R.

HUTH

Fig. 9. Variations in IPelement factor components and variations in Ni/Co and Ba/Si as a function of age.

Cl

-

-I

-

Si

o-

+I

-

Yb

Fig. 8. Contributions of elements AI. Si, Mn, Fe, Ni, Cu, Zn, Ba, SC, Cr, Co, Sb, La, Sm, Eu, Tb, Yb, Lu, Hf to 3 varimax factors (19 elements, 15 samples).

(0-6M.y. old) are enriched in factor 3. Apparently it is im~rtant only in sediments where the accumulation rate of hydrothermal and biogenous material is low. A second factor analysis of the 16 samples for which data on 19 elements was available, was also carried out. The elemental loadings showed a pattern similar to the analysis based on 8 elements, i.e. 3 factors (biogenic. hydro~ermal, and detail-hydrogcnous) account for over 98% of the variance (Fig. 8), but some interesting new features appear. The detrital-hydrogenous factor has Co instead of Ni, and also includes SC, Sb, Ba and the rare-earth elements. Because this factor dominates the more slowly deposited suticial sediments of the core (Fig. 9), and because rare earth elements are concentrated in phosphatic fish debris in Bauer Deep surface sediments (LOPEZ, personal communication, 1976), we suspect that the composition of the Bauer Deep surface sediments simply reflects less dilution by biogenous and hydrothe~l components.

The absences of Ni from the detrital-hydrogenous factor of the second factor analysis may be at least partially due to the fact that the two most surficial samples which show the strongest Ni enrichments were not included in this analysis because they had not been analyzed by activation anaiysis. Nonetheless, Ni and Co do exhibit some difference in their behavior which is exhibited by the Ni/Co vs depth plot (Fig. 9). This ratio increases strongly towards the basalt-sediment interface, suggesting that the acculnulation rate of Ni relative to Co is enhanced by hydro~ernl~~1 deposition. Thus, Ni is contribL]ted by both hydrothermal and authigenic processes, whereas the accumulation of Co is more dependent on purely hydrogenous processes. Barium appears in both the biogenic, and hydrogenous-detritai factor. The Ba:Si ratio (Fig. 9) is low where the Si input. as shown by the Si/AI ratio in Fig. 10. is high (in the intermediate sediments). This indicates that these elements accumulate independently, at last in part. It is possible that both arc transported to the sediment as biogenous debris. but

L

I 0

1 4

*

I 12

4 16

100

I 20

I 300

I

JO0

Pi

3

TOO

&,

FelNi

Si /Al

Fig. IO. Variations in the elemental ratios %/Al and Fe/Ni over the depositional history of Site 319. These ratios emphasize the distinctive character of intermediate sediments.

Metalliferous sedimentation at Deep Sea Drilling Site 319 that the rates at which they are incorporated into the sediment are determined by processes at the sediment-water interface. The detrital-hydrogenous factor may dominate when biogenous silica is dissolved at the sea floor, leaving associated biogenous Ba as barite, to be mixed with slowly accumulating fish debris confining rare earths, and with resistant inorganic detritus that contributes Al and SC. CONCLUSIONS The changes in composition of sediments throughout the section at Site 319 cannot be viewed as local anomalies, since the variations observed in this single, core are very similar to those observed in surface sediments from a broad region of the East Pacific Rise and the Bauer Deep (Figs. 2 and 3). The following depositional history provides a reasonable explanation for the compositional variations. Approximately 15 M.y. ago, Site 319 was located near the crest of the now-extinct Galapagos Rise. An active sea water hydrothermal system on the Rise transferred much of the heat from newly emplaced lavas to the ocean, and at the same time, altered the oceanic crust and transferred some elements from the basalt to the sea water-sediment system. Direct precipitation of elements such as Fe, Mn, Si and Pb from the hydrothermal solution and co-precipitation of Co, Ni, Ba and possibly the rare-earth elements from sea water produced the deposits found just above basement in Site 319. As Site 319 moved away from the Galapagos Rise the influence of hydrother~l activity decreased. Detrital. biogenous, hydrogenous and distant hydrothermal sources became relatively more important. This history explains the compositional differences between the basal and surficial sediments at Site 319, but it does not explain the distinctive compositions

751

of the intermediate sediments deposited 611 M.y. ago. TheC sediments are characterized by high contents of Si and by transition elemental ratios resembling those in the basal sediments (Figs. 2 and 3). High Si and Si/AI (Fig. 10) values in these sediments may reflect an increase in biogenous opal deposition, as suggested by the normative analysis. Thin section analysis of the sediments from this level, however, reveal only rare opal fragments which are not sufficiently abundant to account for the excess silica. The X-ray mineralogical results show that the silica is largely in secondary iron-rich smectite, and could be of either biogenous or hydrothermal origin. From Fig. 4 it is clear that the excess silica is due to an increase in the rate of Si deposition, rather than to a relative enrichment of Si because of decreased deposition of other components of the sediment. High Fe/Ni values are also characteristic of this interval of the core (Fig. 10). In surface sediments of the Nazca Plate the Fe/Ni ratio has been used as an indicator of hydro~e~a1 vs authigenic contributions (DYMONDet al., 1973). High Fe/Ni values are typical of the crest of the EPR, whereas low values are found in the slowly accumulating Bauer Deep sediments. Taken in conjunction with a 611 M.y. best estimate of the initiation of spreading of the East Pacific Rise, this ratio suggests that the distinctive compositional variations observed in Site 319 resrlted from an increase in the supply of hydrothermal components to the sediment between 6 and 11 M.y. ago. Support for this possibility emerges from e tectonic history of the Nazca Plate. An illustra ‘on of this history, based on sea-floor magnetic ano ly data (HERRON,1972; REA, 1975) is shown in \Fig. 1 . Spreading from the Galapagos Rise persisted from 15 M.y. ago, when Site 319 was near the spreading center, until approximately 8 M.y. ago. At that time, rifting began 800-1000 km west of the Galapagos

Fig. 11. Diagrammatic history of deposition for Site 319 in relation to major tectonic events of the northwest Nazca Plate.

J. DYMONI). J. B. COKLISS and G. R. HEA-rtl

752

Rise, which became inactive shortly thereafter. The new spreading center, the East Pacific Rise, has been active until the present and has produced crust between the East Pacific Rise and the Bauer Scarp during the past 8 M.y. The Bauer Scarp marks the boundary between crust produced by the East Pacific Rise and that produced by the Galapagos Rise. It is important to note that when the East Pacific Rise rifting began, Site 319 was 300 km from the new rise and about 600 km from the Galapagos Rise. Thus, it is reasonable that hydrothermal activity associated with the new spreading center would be recorded only as subtle changes in the accumulation rate of metals and the chemical composition of the sediments at Site 319. The small increases in the rates of accumulation of metals in type 2 sediments (Figs. 4 and 5) support this hypothesis. On the other hand. it is not clear why the initiation of hydrothermal activity on the EPR would lead to increased Si deposition at Site 319. It is conceivable that the new EPR hydrothermal system yielded silica-rich fluids. However, there is no evidence for this in tither field data or experimental studies (all hydrothermal fluids are essentially at equilibrium with quartz). Thus. a biogenie origin for the excess Si, Cu, Zn and Ba as inferred from the factor and normative analyses appears the most reasonable explanation for the anomalies in these elements. Our data are not suficient to indicate whether the contemporaneity of this biogenic activity with the formation of the EPR is a coincidence or a consequence of the tectonic modification of local oceanographic conditions by the new rise. We have not seriously considered the possibility that a distinct hydrothermal source of metals exists within the Bauer Deep itself. This suggestion has been made by ANDERSONand HALIJNEN (1974) on the basis of a biomodal heatflow pattern within the Bauer Deep and by McMurtry and BURNETT(1975) on the basis of metal accumulation rate data. The persistence of hydrothermal activity in the Bauer Deep for 15-20M.y. after the formation of the crust seems unlikely, and at the present time thcrc is no evidence for the existence of mid-plate volcanism in the region. Even the bimodal heat-flow distribution is questionable. since it is created only by the presence of 2 or 3 anomalously high values within the arca. Moreover, a local hydrothermal source of sediments within the Bauer Deep would not in itself account for the systematic changes in composition through time. In addition,

as we have

demonstrated

(see also

discus-

of McMr RTRL’ and BCIRNI.T-Iresults by LYLI: and DYMONU (I 976)), the rate of accumulation of surficial Bauer Deep sediments is sufficiently slow that hydrogenous precipitation alone can account for most of the chemical properties of the sediments. sion

AcknowludgPme,lrs~This research was supported by the National Science Foundation as a contribution of the International Decade of Ocean Exploration (Narca Plate Project) under a contract with Oregon State University and the Hawaii Institute of Geophysics.

We wish to thank WILLIAM EKLIJ~II), CAKLOS LO~JI:L. MITCHELL LYLE.and JOHN TOTH for helpful comments and discussions concerning the manuscript. MACI)ALENA CATALFOMAperformed the atomic absorption analyses. CHI MI,KATLI carried out the leaching procedure and X-ray diffraction measurements. Rou STILLINGER aided in the data processing, and MARC;II- WOLSU typed the manuscript.

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