Rare earth element geochemistry of hydrothermal deposits from the active TAG Mound, 26°N Mid-Atlantic Ridge

Pergamon Geochimica et Cosmochimica Acta, Vol. 59, No. 17, pp. 3511-3524, 1995

Copyright © 1995 Elsevier Science Ltd Printed in the USA. All rights reserved

0016-7037/95 $9.50 + .00

0016-7037(95)00224-3

Rare earth element geochemistry of hydrothermal deposits from the active TAG Mound, 26°N Mid-Atlantic Ridge

RACHEL A. MILLS t and HENRY ELDERFIELD 2

~ Department of Oceanography, University of Southampton, Southampton SO17 1B J, UK 2Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

(Received May 23, 1994; accepted in revised form May 17, 1995 )

Abstract--The rare earth element (REE) geochemistry of various phases from the active TAG hydrothermal mound has been examined and related to their mineralogy and fluid chemistry. The mound deposits range from black and white smoker chimneys, massive anhydrite/sulphide mixtures, oxides, and ochres. All phases, except black smoker chimney anhydrite, demonstrate a positive Eu anomaly when normalised to chondrite REE values. REE substitution into sulphide and sulphate phases appears to be strongly influ- enced by crystallographic control for all REE other than Eu. Precipitation of anhydrite within the TAG mound is the major mechanism for removal of REE during mound circulation and 0.15-0.35 g anhydrite is inferred to precipitate from every kg of fluid venting from the white smoker chimneys. Oxides from the mound fall into three different categories with distinct REE patterns: oxide rims on sulphides, atacamite- bearing oxides, and silica-rich Fe-oxides and ochres. The oxide rim phases contain sulphide and seawater derived REEs whereas the atacamite-bearing oxides and the ochreous material exhibit no seawater signature which suggests precipitation from, or alteration by, a modified hydrothermal fluid.

1. INTRODUCTION

The active TAG hydrothermal mound is one of the largest known hydrothermal deposits at a sediment-free oceanic spreading centre (Thompson et al., 1988; Tivey et ill., 1995). Studies of deposits on the East Pacific Rise (EPR) have given rise to a model of black smoker chimney formation (Haymon, 1983; Goldfarb et al., 1983) which explains the mineral zonation within black smoker chimney walls and the changes observed over time. However, studies of the origin and evolution of large mound deposits such as TAG are limited (Han- nington et al., 1988; Thompson et al., 1988; Tivey et al., 1995).

The REEs are powerful tracers in the study of evolution of geochemical systems and have been used widely to identify sources of oceanic REEs and mixing processes within the oceans (Elderfield, 1988). The REEs exist in the trivalent state under most natural conditions and behave in a chemi- cally coherent manner. The exceptions are Ce and Eu which can behave anomalously under certain redox conditions due to the formation of C e 4+ and Eu 2÷ species. The contraction of the 4f subshell with increasing atomic number leads to a decrease in ionic radius across the group. This has conse- quences for the complexing properties of the group and hence the way the elements are transported in solution. The REE geochemistry of high-temperature hydrothermal fluids is relatively well characterised (Michard et al., 1983; Michard and Albarede, 1986; Mitra et al., 1994; Klinkhammer et al., 1994), as is the geochemistry of the mid-ocean ridge basalts and seawater (Elderfield, 1988).

All of the REEs except for Eu are predicted to be in a trivalent state at the temperatures and pressures associated with high temperature venting at TAG (Wood, 1990a). Eu 2÷ is predicted to be the predominant species at temperatures in excess of 250°C, elevated pressures and low Eh (Sverjensky, 1984). Active and ancient hydrothermal deposits exhibit a

3511

variety of REE patterns. For example, Bence ( 1983 ) observed fluid-like REE patterns in sulphides from the EPR whereas Gillis et al. (1990) observed flat REE patterns in Snake Pit sulphides on the Mid-Atlantic Ridge. Two types of REE patterns have been observed in hydrothermal mineral assem- blages by Barrett et al. (1990): ( 1 ) anhydrite and barite, con- taining relatively high REE concentrations and a positive Eu anomaly; Eu 2÷ is assumed to substitute into the Ca 2÷ or Ba ~÷ sites within the mineral lattice (Guichard et al., 1979; Morgan and Wandless, 1980), and (2) pristine sulphides, with low REE abundences due to discrimination against the large ionic radii of the REEs with REE patterns derived from the parent fluid (Bence, 1983; Alt, 1988; Gillis et al., 1990). This paper describes the distribution of the REEs within various phases from the TAG mound. The REEs have been used to define the interplay between hydrothermal fluids and seawater and the resultant mound mineralogy and to identify mixing processes, modes of formation, and alteration in the TAG hydrothermal mound.

2. THE TAG HYDROTHERMAL AREA

The TAG hydrothermal area has been the focus of many studies since the early 1970's (e.g., Scott et al., 1974, 1978; Shearme et al., 1983). This area comprises a number of different components including the active mound discovered in 1985 (Rona et al., 1986). The active mound lies to the east of the median valley floor at a depth of 3680 m and is approximately 200 m in diameter and 50 m high (Thompson et al., 1988; Tivey et al., 1995). The large size of the TAG hydrothermal mound may reflect its maturity (Lalou et al., 1990, 1993). A number of inactive mounds lie to the north of the active site (Rona et al., 1993); two of these (Mir and Alvin) have been sampled using submersibles (Rona et al., 1993; Von Herzen et al., 1993).

Figure 1 shows the overall structure of the active TAG mound and the locations of samples used in this study. The

3512 R . A . Mills and H. Elderf ie ld

TAG Hydrothermal Mound

lOOM

" •

,

"/2 ~ SULPHIDE - CARBONATE ; ~ BOUNDARY

/ KEY

• Black smoker chimney

Massive anhydrite

• Mound samples

*~* White smoker chimney

Ochres

Steep slope talus

FIG. 1. M a p o f the T A G hyd ro the rma l m o u n d with s amp le types f rom Tab le 1 s h o w n (adap ted f r o m T ivey et al., 1995).

apex of the mound is shrouded in black smoke which is vig- orously emitted from a cluster of chimneys (black smoker complex in Fig. 1 ). Finely disseminated, black sulphides precipitate as fluid at temperatures of 366°C mixes with cold seawater (Edmond et al., 1995). This dense black smoke which hides the individual chimneys, rises rapidly and mixes turbulently to coalesce and form a buoyant plume. At the base of the black smoker complex and on the mound surface are blocks of massive anhydrite undergoing dissolution in the ambient seawater (massive anhydrite in Fig. 1 ; Thompson et al.,

1988; Tivey et al., 1995). This anhydrite is interspersed with plate-like layers of marcasite and chalcopyrite, some of which exhibit surfaces coated with Fe oxide (Tivey et al., 1995). Diffuse black smoke and shimmering water emanate from fis- sures in this surface and are entrained upwards into the main buoyant plume (Tivey et al., 1995).

Strongly directed flow from depth is inferred to discharge from the black smoker chimneys, with pooling and mixing of some of the fluid within the mound (Tivey et al., 1995). To the southeast of the black smoker complex is the area dubbed

Table 1. Description of TAG samples

S A M P L E D E S C R I P T I O N *

Black smoker chimney: 2179-4-1-ccp 2179-4-1-oxl 2179-4-1-ox2 2179-4-1-anh 2178-4- l -ccp 2183-7-3-ox White smoker chimney: 2187-1-5-sph 2187-1-ox 2187-1-5-ox 2187-1-2-anh Mound samples: 2179-1 - 1-sulphide 2183-7-0-anh 2186-1 - 1-sulphide 2183 -9-1 -sulphide 2190-13-1-ox 2186-1-1-ox 2183-9- l-ox Steep talus slope: 2183-6-2-sulphide 2183-6-2-ox 2190-6-1 -atac Ochres: 2190-12-1-och 2183-10-2-och 2183-5-2-och 2183-2-2-Si-ox

Chalcopyrite from interior of active chimney Orange Fe-oxide from chimney exterior Red Fe-oxide from chimney wall, intimately intergrown with anhydrite Anhydrite from chimney wall Chalcopyrite from inactive chimney Orange Fe-oxide from inactive chimney exterior

Sphalerite from interior of active chimney Orange Fe-oxide from long term marker deployed in white smoker field 1986-1990 Orange Fe-oxide from chimney exterior Anhydrite from base of white smoker chimney

Cu-Fe massive sulphide crust from base of black smoker complex; chalcopyrite, pyrite, marcasite Massive anhydrite from base of black smoker complex; anhydrite, amorphous silica, gypsum Fe sulphide rich sample from top of active mound Chalcopyrite and pyrite rich sample from top of active mound; chalcopyrite, pyrite, marcasite Outer oxide rim on pyrite and sphalerite rich mound sample; amorphous silica Outer oxide rim on 2186-1 - l-sulphide Outer oxide rim on 2183-9-1 -sulphide

Massive sulphide from atacamite bearing sample; pyrite, marcasite (sphalerite, chalcopyrite) Outer Fe-oxide layer on massive sulphide, associated with atacamite Atacamite

Red ochre from steep outer wall of mound; haematite, goethite, amorphous silica, quartz Brown ochre from steep outer wall of mound; goethite, amorphous silica Yellow/orange ochre from steep outer wall of mound; amorphous silica, haematite, goethite Mixture of amorphous Fe-oxide and silica from top of active mound

* descriptions from Tivey et al., in press, minerals are listed in order of abundance, parentheses indicate minor constituents

REE geochemistry of hydrothermal deposits 3513

the Kremlin owing to the bulbous shape of the sphalerite-rich, white smoker chimneys (Kremlin area in Fig. 1; Thompson et al., 1988). These chimneys discharge lower-temperature fluids (-----300°C) (Edmond et al., 1995). The white smoker particles are mainly Fe-coated amorphous silica with occa- sional sulphide grains (M. Cooper, pers. commun. , 1994). The white smoker outflow is probably related to the black smoker fluids because of their close proximity, but the chemistry of the two smoker types is significantly different (Ed- mond et al., 1995; Mitra et al., 1994). The white smoker fluid chemistry is attributed to mixing of b lack smoker fluid with seawater and precipitation of pyrite and chalcopyrite which lowers the pH and dissolves sphalerite within the mound (Ed- mond et al., 1995; Tivey et al., 1995). The white smoker fluids are, therefore, depleted in Fe, Cu, and H2S and enriched in Zn and exhibit pH values below 3 (Edmond et al., 1995). The presence of amorphous silica in white smoker chimneys is attributed to conduct ive cooling of the hydrothermal fluid within the mound (Tivey et al., 1995).

In addition to discrete black and white smokers, low-temperature diffuse flow covers a great area of the mound surface at TAG. The zones of diffuse flow, which are delineated by the presence of anemones, have an extremely heterogeneous spatial distribution (Becker and Von Herzen, 1993). This diffuse flow is a mixture of endmember black smoker fluid and seawater that has been modified by precipitation and dissolution within the mound (Mil ls et al., 1993a; Schultz and E1- derfield, 1994).

The steep outer slopes of the mound are covered with sulphide talus which is weathering to a variety of secondary sulphate and oxide products (steep talus slope in Fig. 1 ). Ata- camite (a secondary Cu-chloride) is frequently associated with the surfaces of altered sulphides, suggesting local alteration of Cu sulphides fol lowed by deposit ion of atacamite at the mound surface (Hannington et al., 1988; Herzig et al., 1991; Hannington, 1993). Gold enr ichment is associated with some of the weathered sulphides (Hannington et al., 1988). Deep red and yellow silica-rich ochres are present on the steep talus scarps of the inner mound (ochres in Fig. 1 ), and some- t imes are associated with regions of upwell ing of low-temperature fluids (Becker and Von Herzen, 1993 ). These oxide- rich portions of the mound are eventually transported to the surrounding sediments via mass wast ing events and form one component of the T A G metall iferous sediments (Metz et al., 1988; German et al., 1993; Mills et al., 1993b). The distinc- tion between mound and pelagic sediment is indistinct; the r ed /b rown sulphide and oxide material is gradually super- seded by tan carbonate ooze away from the mound (Rona et al., 1993). This carbonate ooze forms a thin blanket on the weathered basalt pillows which are typical of the Atlantic median valley.

3. SAMPLES

A suite of samples from the TAG mound were collected by the deep-sea submersible Alvin in 1990; the mineralogy and chemistry of these samples has been described by Tivey et al. (1995). These samples are described in Table 1 and locations shown in Fig. 1. Sam- ples include sulphides, anhydrite, and oxides from both black smoker and white smoker chimneys as well as mound samples which include (a) massive anhydrite and sulphide crust from near the smoker complex; (b) sulphides and oxides from the top of the active mound; (c)

atacamite, Fe-oxide, and sulphide from massive sulphide talus ex- posed on steep outer slopes of the mound; and (d) ochres from the steep outer slopes of the mound and silica/Fe-oxide from the top of the mound. These samples reflect the full range of material present on the TAG mound that has been sampled to date (Von Herzen et al., 1993; Tivey et al., 1995).

3.1. Black Smoker Chinmey Samples

Scrapings of the innermost sulphide dominated layer were taken from the interior of a chimney that was active when recovered (2179- 4-l-ccp), and from a relict chimney away from the black smoker complex (2178-4-1-ccp). The predominant sulphide phase in these samples was chalcopyrite. Coatings of amorphous Fe-oxyhydroxide from the exterior walls of both active (2179-4-1-oxl) and inactive (2183-7-3-ox) chimneys were analysed. A third oxide was separated from the active chimney wall where it is intimately associated with anhydrite (2179-4-1-ox2). This phase is a dendritic Fe oxide that appears red to opaque in transmitted light. Anhydrite picked from the wall of the active black smoker chimney was also analysed (2179- 4-1-anh).

3.2. White Smoker Chimney Samples

The sulphide-rich interior (dominantly sphalerite) of a white smoker chimney was sampled (2187-1-5-sph), as was the oxide-rich coating of the white smoker chimney surface (2187-1-5-ox) and the oxide coating of a syntactic foam marker deployed in the white smoker field in 1986 and recovered in 1990 (2187-1-ox). Anhydrite grains picked from the base of an active white smoker chimney were also analysed (2187- l-2-anh).

3.3. TAG Mound Sulphides and Oxides

Samples from the active mound included massive anhydrite (2183- 7-0) and a sulphide crust (2179-1-1) from the base of the black smoker complex and sulphides and their associated oxides from the top of the mound (2186-1-1, 2183-9-1 ) as well as the outer oxide rim from a sulphide sample (2190-13-1). Atacamite-bearing Fe-oxide (2190-6-1, 2183-6-2) and sulphides (2183-6-2) from the steep outer slopes were analysed. As it was impossible to separate the sulphide phase for analysis due to the extensive alteration of these atacamite-bearing samples, REE analyses represent mixtures of primary sulphide and the alteration products.

3.4. Ochres

Ochres are distinguished from orange mound oxides by their deep orange to red colour. Three samples have been studied here from the periphery of the TAG mound. Sample 2183-5-1 exhibits clasts of oxide with the morphology of sulphide grains, suggesting oxidation of sulphide. The other two ochre samples (2183-10-2 and 2190-12- 2) exhibit dendrites suggesting some primary deposition of Fe-oxide. A silica rich amorphous Fe-oxide (2183-2-2) from the surface of the mound was also analysed and has been included with the ochre samples because of the high silica content.

4. ANALYTICAL METHODS

Samples were obtained from archive at Woods Hole Oceano- graphic Institution. Scrapings were taken from areas of known mineralogy as determined by X-ray diffractometry and/or petrography (Tivey et al., 1995). Individual mineral grains were picked out and homogenised samples were digested using aqua regia (3:1, HCl:HNO3) to oxidise the sulphides. The mixture was evaporated to dryness and then further digested using a mixture of HNO3 and HF to remove any silicate material present. The samples were diluted to a 1 : 100 ratio using 6 M HCI. Anhydrite samples were leached with 1 M NaCI (Specpure) over a period of 3 - 4 days to dissolve all anhydrite; this left a residue of sulphide material.

The REEs were separated using a modification of the method of Greaves et al. (1989). This involved two passages through the cation exchange columns: the first to remove the major cations, Fe, Cu, and

3514 R.A. Mills and H. Elderfield

Zn, and the second to purify the REEs. One column passage was sufficient for anhydrite samples. Aliquots of dissolved oxides and anhydrite samples were taken to give approximately 4 ng Nd and then spiked as for 1 L seawater samples. Aliquots of sulphide samples, which have low REE concentrations, were limited by the cation exchange column capacity. Samples were taken to give the maximum REE yield without overloading the cation exchange column. Column eluates were loaded on Re/Ta triple filaments and analysed by thermal ionisation mass spectrometry (TIMS) using a V.G. Isomass 54E mass spectrometer. The precision for this method is 4% (2a) or better and the sample/blank ratio is always > 100 (Greaves et al., 1989).

Data have been normalised to the average REE concentrations for ten chondrites (Nakamura, 1974). Various components of the REE patterns can be quantified to allow comparison of the different samples; the Ce anomaly, the Eu anomaly, and the fractionation of HREE from LREE. The deviation of Ce from the rest of the REEs can be expressed as:

Ce anomaly = Ce/Ce* = 3Ce,,/(2La,, + Nd,,), ( 1 )

where the subscript n refers to chondrite normalised values and the superscript * refers to the value obtained by linear interpolation between adjacent elements. In the absence of Gd data for every sample, the chondrite normalised Eu/Sm ratio (Eu,/Sm,) has been used to approximate the Eu anomaly. The chondrite normalised Nd/Yb ratio has been used to quantify the fractionation of light REEs (LREE; La through to Eu) from heavy REEs (HREE; Gd through to Lu). All values are shown in Table 2.

Strontium isotope ratio measurements were carried out on the TAG anhydrite phases to determine the relative proportions of hydrothermal fluid and seawater Sr present. Strontium was separated for analysis by cation exchange chromatography. Strontium isotope ratios were measured by TIMS using a V.G. Sector 54 mass spectrometer in dynamic mode, the reproducibility is 15 ppm (2a).

5. RESULTS

REE data for all samples are recorded in Table 2 along with the calculated Ce anomaly, the chondrite normalised E u / S m and N d / Y b values, and the REE contents of seawater and TAG hydrothermal fluids (Mit ra et al., 1994). The REE concentrations in the TAG mound samples vary widely over several orders of magnitude, e.g., --0.001 ppm Nd in white smoker chimney sulphides and > 10 ppm Nd in some ochres.

Endmember hydrothermal fluids at black smoker vents have enriched REE concentrat ions ( 1 0 - 1 0 , 0 0 0 × seawater concentrat ions) and chondri te normalised REE patterns with a large positive Eu anomaly, no Ce anomaly, and enr ichment in the LREEs compared to the HREEs (Michard et al., 1983; Michard and Albarede, 1986; Campbel l et al., 1988; Mitra et al., 1994; Kl inkhammer et al., 1994). This pattern is common to all high-temperature fluids from sediment-free ridges measured to date, al though the degree of enr ichment and size of the Eu anomaly vary from site to site (Mitra et al., 1994; Kl inkhammer et al., 1994). Figure 2 shows that the white smoker fluid has lower REE concentrations than the black smoker fluid except for Eu, which is enriched over and above black smoker fluid levels (Mit ra et al., 1994). The white smoker fluid Eu anomaly (Eu,,/Smn = 96) is larger than that in the black smoker fluid (EuJSm, , = 7.6) and the LREE enr ichment is similar. The T A G black smoker Nd,,/Yb~ ratio is 10.1 and the white smoker Nd,,/Yb,, ratio is 13.4 (Table 2; Mitra et al., 1994).

5.2. Black Smoker Chimneys

REE patterns for all samples from black smoker chimneys are shown in Fig. 3a. REE patterns of T A G sulphides resem-

ble the fluid patterns in form, but the size of the Eu anomaly is diminished in the solid phases relative to the fluid. Chal- copyrite from both active and relict chimneys exhibit extremely low REE concentrat ions (0 .006-0 .008 ppm Nd) , whereas the oxide and sulphate phases contain elevated REE concentrations ( 0 . 7 - 3.1 ppm Nd) . The chalcopyrite REE patterns show LREE enr ichment and a positive Eu anomaly (Eu,,/Sm,, = 2 . 0 - 6 . 2 ) . The two oxides from the active chimney exhibit similar REE patterns to one another suggesting they both formed in similar chemical environments despite their different morphology and locations within the chimney. The oxidised phases f rom the active black smoker show neg- ligible negative Ce anomaly and a pronounced Eu anomaly (Eu,,/Sm~ = 4 . 6 - 5 . 5 ) . The relict chimney oxide coating (2183-7-3) exhibits a small Ce anomaly ( C e / C e * = 0.40) and some HREE enr ichment in comparison to the active chimney oxides (Nd,, /Yb, = 2.6). The black smoker anhydrite shows progressive HREE depletion (Nd,,/Yb,, = 16.0) and has a small negative Eu anomaly (Eu, /Smn = 0.78).

The black smoker chimney phases have been normalised to the black smoker fluid composi t ion in Fig. 3b to emphasise the sources of REE to the solid phase. The REE patterns are relatively fiat except for the relict chimney oxide phase (2183- 7-3 ) which exhibits HREE enr ichment and a Ce anomaly. All phases exhibit negative Eu anomalies which are larger in the sulphate and sulphide phases than the oxide phases.

5.3. White Smoker Chimneys

Chondri te-normalised patterns for white smoker chimney samples are shown in Fig. 4a. The sphalerite sample contains extremely low REE concentrat ions (0.0013 ppm Nd) ; the REE pattern exhibits some LREE enr ichment ( N d J Y b n = 4.2) with a pronounced Eu anomaly (Eun/Sm,, = 9.8). White smoker sulphides, oxides, and anhydrite exhibit larger Eu anomalies than the corresponding black smoker phases; this reflects the white smoker fluid chemistry (see Fig. 2) . The white smoker oxides exhibit enriched REE contents ( 2 . 0 - 2 . 3 ppm Nd) ; two samples have a slightly negative Ce anomaly ( C e / C e * = 0 . 4 - 0 . 5 ) and HREE enrichment compared with fluid and sulphide phases ( N d , / Y b , = 2 . 9 - 3 . 2 ) . These oxides exhibit the largest Eu anomaly observed in the TAG mound material (Eu, /Sm,, = 3 6 - 4 5 ) . The white smoker anhydrite REE pattern is similar to the fluid and sulphide pattern with a pronounced Eu anomaly (EuJSm, , = 15) and more extreme LREE enr ichment ( N d J Y b , = 26).

When normalised to the white smoker fluid data (Fig. 4b ) , the sulphide and oxide phases all exhibi t HREE enrichment over the LREE; conversely the white smoker anhydrite exhibits relative depletion in the HREE relative to the other phases present. The oxide phases exhibi t small Ce anomalies and some HREE enr ichment demonstrat ing the influence of seawater. All solid phases have negative Eu anomalies when normalised to the parent fluid.

5.4. Mound Sulphides and Oxides

All TAG mound surface samples are plotted by type in Fig. 5 a - c . The massive anhydri te sample has a slightly LREE enriched pattern (Ndn/Yb, = 7.8) with a small Eu anomaly

Tab

le 2

. R

EE

dat

a fo

r T

AG

sam

ples

La

Ce

Nd

Sm

E

u

Gd

Dy

Er

Yb

L

u

Ce/

E

un

/ p

pm

p

pm

p

pm

p

pm

p

pm

p

pm

p

pm

p

pm

p

pm

p

pm

C

e*

Sm

n

Nd

nl

Yb

n

SA

MP

LE

Bla

ck

smok

er

chim

ney

: 21

79-4

-1-c

cp

0.01

2 0.

014

0.00

85

0.00

19

0.00

15

0.00

17

0.00

037

0.66

2.

02

9.62

21

79-4

-1-o

xl

0.53

7 1.

62

0.92

6 0.

219

0.38

0 0.

194

0.15

0 0.

0718

0.

0538

0.

0652

1.

3 4.

56

7.21

21

79-4

-1-o

x2

0.45

0 0.

950

0.71

3 0.

164

0.34

3 0.

175

0.11

5 0.

0547

0.

0429

0.

89

5.50

6.

96

2179

-4-1

-anh

1.

27

2.85

3.

16

0.84

8 0.

250

0.93

3 0.

513

0.18

4 0.

0828

0.

78

0.77

6 16

.0

2178

-4-1

-ccp

0.

0130

0.

0064

0.

0014

0.

0033

0.

0007

9 0.

0007

4 6.

25

3.62

21

83-7

-3-o

x 2.

49

2.00

2.

53

0.50

6 0.

761

0.60

0 0.

594

0.44

1 0.

413

0.06

32

0.40

3.

96

2.57

W

hite

sm

oker

ch

imn

ey:

2187

-1-5

-sph

0.

0019

0.

0024

0.

0013

0.

0002

4 0.

0008

8 0.

0001

8 0.

0002

3 0.

0001

3 0.

0001

3 0.

0000

17

0.68

9.

84

4.19

21

87-1

-ox

2.54

2.

45

2.29

0.

422

5.76

0.

436

0.43

8 0.

350

0.33

1 0.

0565

0.

50

36.0

2.

90

2187

-1-5

-ox

0.89

8 0.

663

0.61

2 0.

119

2.01

0.

142

0.12

1 0.

0878

0.

0814

0.

40

44.7

3.

15

2187

-1-2

-anh

2.

10

3.85

2.

01

0.33

4 1.

90

0.21

7 0.

126

0.05

08

0.03

30

0.00

550

0.84

15

.0

25.5

M

ound

sa

mpl

es:

2179

-1-1

-sul

phid

e 0.

0059

0.

0130

0.

0067

0.

0016

0.

0017

0.

0012

0.

0006

0 0.

0005

2 0.

0000

62

1.1

2.76

5.

40

2183

-7-0

-anh

0.

384

1.31

1.

15

0.30

2 0.

254

0.26

1 0.

202

0.08

32

0.06

19

1.1

2.20

7.

78

2186

-1-1

-sul

phid

e 0.

0120

0.

0133

0.

0029

4 0.

0102

0.

0023

4 0.

0010

6 0.

0010

4 9.

16

5.36

21

83-9

-1-s

ulph

ide

0.00

98

0.01

18

0.00

81

0.00

16

0.00

34

0.00

17

0.00

17

0.00

13

0.00

12

0.63

5.

77

2.83

21

90-1

3-1-

ox

1.55

1.

10

0.22

9 0.

659

0.09

02

0.07

75

7.61

5.

95

2186

-1-1

-ox

2.96

3.

09

3.48

0.

710

1.68

0.

665

0.59

6 0.

412

0.37

2 0.

0599

0.

502

6.25

3.

92

2183

-9-1

-ox

1.74

1.

66

1.70

0.

319

0.78

9 0.

407

0.34

9 0.

273

0.26

1 0.

0458

0.

48

6.53

2.

73

Stee

p ta

lus

slop

e:

2183

-6-2

-sul

phid

e 0.

0754

0.

144

0.08

13

0.28

6 0.

0084

2 0.

0715

0.

0309

0.

0286

9.

33

2.11

21

83-6

-2-o

x 0.

155

0.18

6 0.

319

0.14

6 0.

435

0.06

36

0.06

29

0.48

7.

88

2.12

21

90-6

-1-a

tac

0.12

1 0.

275

0.58

6 0.

372

1.26

0.

346

0.25

1 0.

0945

0.

0788

0.

60

8.96

3.

12

Och

res:

21

90-1

2-1-

och

1.44

3.

88

3.63

1.

04

2.72

0.

709

0.46

3 0.

191

0.16

4 0.

0212

1.

0 6.

92

9.27

21

83-1

0-2-

och

2.74

5.

49

5.64

1.

71

3.18

0.

313

0.54

5 0.

231

0.16

9 0.

0265

0.

81

4.89

14

.0

2183

-5-2

-och

3.

73

9.25

13

.5

3.25

9.

71

1.93

1.

20

0.49

6 0.

366

0.03

46

0.77

7.

88

15.5

21

83-2

-2-S

i-ox

0.

358

0.71

8 0.

560

0A33

0.

350

0.11

3 0.

0680

0.

0347

0.

0286

0.

0040

7 0.

89

6.90

8.

20

*Bla

ck s

mok

er fl

uid

0.00

0568

0.

0013

2 0.

0008

68

0.00

0187

0.

0005

42

0.00

0170

0.

0001

23

4.93

.10

-5

3.60

"10

-5

4.41

.10-

6 0.

947

7.6

10.1

*W

hite

sm

oker

flui

d 0.

0003

40

0.00

0479

0.

0002

14

3.58

.10

-5

0.00

132

2.56

.10-

5 1.

79.1

0-5

7.96

.10-

6 6.

70,1

0-6

8.08

.10-

7 0.

691

96

13.4

*S

ea w

ater

(34

00m

) 3.

54.1

0 -6

7.

62.1

0 -7

3.

09.1

0 -6

6.

21"1

0 -7

1.

61"1

0 -7

9.83

"10

-7

1.03

"10

-6

9.15

"10

-7

9.38

"10

-7

1.54

"10

-7

0.10

0.

59

1.38

*

Dat

a fr

om M

itra

et a

l., 1

994;

bla

ck s

mok

er fl

uid

data

is r

ecal

cula

ted

to M

g=0,

whi

te s

mok

er fl

uid

is r

ecal

cula

ted

to M

g=4

mm

ol k

g -I

,.<

e~ 2


0.1

0.001

o.oooo~

,t ambient sea water (3500 m)

• average black smoker fluid

0 . 0 0 0 0 0 0 1 ~ I I I I I I I I I I I I I I

La Ce Nd Sm Eu Gd Dy E r Yb Lu

FIG. 2. Chondrite-normalised REE data for North Atlantic seawater from the TAG area (3500 m depth), and for black smoker and white smoker fluids. Data are from Mitra et al., 1994. Black smoker fluid data are the mean of 5 samples recalculated for Mg = 0, white smoker data are the mean of 4 samples recalculated for Mg = 4 mmol/kg. Seawater at TAG, as at other oceanic sites, exhibits a pronounced negative Ce anomaly (Ce/Ce* = 0.10) with HREE enrichment (Mitra et al., 1994).

(Eu,/Sm,, = 2.20) shown in Fig. 5a. The massive anhydrite is interspersed with sulphide crusts that are mainly chalcopyrite, pyrite, and marcasite. The sulphide crust (2179-1-1) sampled from the same area as the anhydrite is also plotted along with two other mound sulphides that are mixtures of pyrite, marcasite, and chalcopyrite. All three mound sulphides (Fig. 5a) contain extremely low concentrations of REEs (0.007-0.013 ppm Nd) that are comparable with black smoker chimney concentrations (Table 2). The size of the Eu anomaly is in the range exhibited by the black and white smoker chimney sulphides (Eu,/Sm, = 2.8-9.2). The Cu- Fe crust (2179-1-1) exhibits the smallest Eu anomaly and the Fe-rich sulphide (2186-1-1) the largest. All of these samples show some LREE enrichment; the sulphides to a lesser extent (Nd,/Yb~ = 2.8-5.4) than the anhydrite sample (Nd,/Yb,, =7 .8 ) .

REE patterns for the oxide coatings (Fig. 5b) are similar to oxides from the relict black smoker chimney (2183-7-3- ox; Fig. 3a). The oxides that form as coatings on mound sulphides (2183-9-1-ox; 2186-1-1-ox) exhibit the highest REE concentrations (1.1-3.5 ppm Nd) with a small Ce anomaly (Ce/Ce* = 0.48-0.50) and some HREE enrichment (Nd,,/ Yb, = 2.7-5.9).

The three atacamite-bearing samples from the steep talus slope have similar REE patterns to one another (Fig. 5c), with a small range in REE concentrations (0.14-0.58 ppm Nd) which reflects the extensive alteration of the sulphide phases. These samples display different REE patterns from the rest of the TAG mound material; they are depleted in the LREE compared with all other samples and exhibit some of the lowest Nd,,/Yb,, ratios of all TAG material (2.1-3.1 ). These samples have significant positive Eu anomalies (Eun/Sm,, = 7.9-9.3 ).

5.5. Ochres

REE patterns for TAG ochres and the silica-rich oxide (2183-2-2-Si-ox) are shown in Fig. 6. Ochres exhibit some of the highest REE contents of all TAG mound ma-

terial (3 .6-13 ppm Nd). The REE patterns are LREE enriched, with high Nd,/Yb,, ratios ( 8 . 2 - 1 6 ) , and a pronounced Eu anomaly (Eu,/Sm,, = 4 .9 -7 .9 ) . An amorphous, silica-rich Fe-oxide sample from the top of the mound (2183-2-2) contains the lowest REE concentrations (0.56 ppm Nd) but the REE pattern is similar to those of the ochres and distinct from the mound oxide REE patterns (Fig. 5b).

5.6. Strontium Isotopes in TAG Anhydrites

The composition of the fluid from which anhydrite samples precipitated can be estimated from mixing parame- ters calculated from 87Sr/86Sr isotope ratios (Table 3). The 87Sr/86Sr ratios for three anhydrite samples are intermediate between the values of seawater and endmember hydrothermal fluid, reflecting extensive fluid mixing prior to anhydrite precipitation. Assuming conservative mixing of seawater and hydrothermal fluid with respective Sr concentrations of 7.7 and 9 ppm, and 87Sr/86Sr compositions of 0.70916 and 0.70319 (Elderfield, 1986; Elderfield et al., 1993 ), the relative proportions of seawater and hydrothermal fluid can be calculated from mass balance equations. The hydrothermal component in the fluid that precipitated the anhydrite ranged from 38% to 64%, the remainder being seawater (Table 3). Anhydrite only precipitates at temperatures in excess of 150°C; apparent temperatures of precipitation have been calculated for TAG anhydrites (Table 3 ). The large seawater component in the TAG anhydrites suggests that there is entrainment of seawater into the mound coupled with some conductive heating of the circulating fluid to induce anhydrite precipitation. The REE composition of these mixtures can then be estimated using the REE composition of seawater and hydrothermal fluid (Mitra et al., 1994). The resulting fluid compositions have been used to calculate REE distribution coefficients for the TAG anhydrites in the following discussion.


Ca)

1 0 0

0 . 0 1 r...)

O . O 0 0 l I I I I I I I I I I I I I I I

La Ce Nd Sm Eu Gd Dy Er Yb Lu

(b)

1 0 0 0 0 0

• 2179-4-1-¢cp at 2179~1- I -ox l

-----O---- 2179M-1 -ox2 - - - 0 - - - 2179 -4 - I - anh ----{2]---- 2178-4-1-cep

2183-7-3-ox

.=

1 0 0 0

0 . 1 I I I I I I I I I I I I I I I


FIG. 3. (a) Chondrite-normalised and (b) black smoker fluid-normalised REE data for the active (solid symbols) and relict (open symbols) black smoker chimney phases.

6. DISCUSSION

6.1. REE Distribution Coefficients

Crystallographic control has been invoked in previous studies to explain REE patterns in hydrothermal deposits (Gui- chard et al., 1979; Morgan and Wandless, 1980). Alt (1988) invoked preferential substitution of the smaller HREEs into the sulphide lattice to explain the HREE enrichment observed in sulphides from Green Sea Mount, EPR. This can be eval- uated by calculating the distribution coefficient (Ko) of each REE between the solid phase and the associated hydrothermal fluid, defined as:

concentration in hydrothermal mineral (ppm) Ko = (2)

concentration in fluid (ppm)

Using the fluid REE concentration data, an apparent distribution coefficient can be calculated for each solid phase. This approach ignores nonequilibrium, kinetic effects, and complexation in solution and assumes that the fluid chemistry is stable over the timescale required for sulphide deposition. A plot of the Ko for each REE against ionic radius should pro- vide information as to the mode of replacement of these elements into the crystal lattice (Onuma et al., 1968). Elemental substitution can be approximated by the following expression (Morgan and Wandless, 1980):

In Ko = A(rREE -- rM) 2 + B, (3)

where rREE and rM are the ionic radii of the REE and the major cation undergoing substitution, respectively. A and B are con- stants which depend on lattice structure and charge balance. If crystallographic control is the major contributor to REE


(a)

100

,.q

°= 0.01 r..)

0 . 0 ~ 1

• $ ---@

I I I I I I I I I I I I I I I


(b)

100000

~ 10

0.1

2187- l-5-sph @ 2187-1-ox A 2187-1-5-ox

2187-1-2-anh

f ~ ....

I I I I I I I I I I I I

La Ce Nd Sm Eu Gd Dy Er I I I

Yb Lu

FIG. 4. (a) Chondrite-normalised and (b) white smoker fluid-normalised REE data for the white smoker chimney phases.

distributions, a straight-line relationship will be obtained when log Ko is plotted against the square of the difference between each individual REE and the element being substi- tuted ([rREE -- rM] 2) (Morgan and Wandless, 1980). This approach has been utilised here to identify controls of REE distribution between various phases within the TAG mound. All effective ionic radii used in this paper are from Shannon (1976) assuming the REE to be eightfold coordination. This approach has recently been successfully utilised in decon- volving the controls on hydrothermal fluid geochemistry (Klinkhammer et al., 1994).

6.2. Sulphide Formation at TAG

Figure 7a shows a plot of Ko, against [FREE - - FFe] 2 for each REE within the black smoker sulphides and Fig. 7b, KD a g a i n s t [FRE E - - r zn ] 2 in the white smoker sulphide. The black

smoker data (Fig. 7a) are very scattered whereas the white smoker data (Fig. 7b) show a more linear trend. Ko values for the white smoker sulphide, which is mainly sphalerite, decrease as the REEs become more incompatible with the sulphide lattice (Fig. 7b). The scatter in the black smoker sulphide phases may reflect noncrystallographic control of REE incorporation into the chalcopyrite (Fig. 7a). The low Ko values for Eu 2+ fall on the linear trend exhibited by the trivalent REEs (Fig. 7a and b) suggesting that the distribution coefficient of the large Eu 2÷ cation is controlled by crystallographic effects. Evidently, the HREE enrichment of the white smoker chimney sulphide with respect to the parent fluid must be due to preferential substitution of the smaller HREEs into the sulphide lattice. Thus, continued precipitation of sulphides would generate a progressively larger Eu anomaly in solution and enhance the Nd , /Yb , ratio of the residual fluid.


i 0 .01

U

[ ~ 21 ~?-O-imh 1 + 21116- l - 1 - l a ~ d ~ - - . . O - - | ll3-~-1 .Nlpl l l~ - . . . O - - 2179- I - I - i i ~ a g ~

I I [ I I I I I I I I I I f I

L a C e b id S i l l E u G d D y E r Y b L u

(b)

"o =

0 .01 U

I I I I I I I I I I I l I I I


(c)

0 .01 - - - - i i - - 2 1 9 o 4 - 1 ~ - - - o - - 21s3~-2~x

I I I I I ) I I I I I I I I


FiG. 5. Chondrite-normalised REE data for (a) massive anhydrite and mound sulphides, (b) mound Fe-oxide phases, and (c) atacamite- bearing sulphides and oxides.

= 9.2) and the Zn-rich mound sulphide (Eu,/Sm, = 5.8) samples exhibit Eu anomalies that are closer to the white smoker chimney sulphide (Eu,/Sn~ = 9.8). Thus, the REE patterns are consistent with mound sulphides precipitating from fluids that are intermediate between the black smoker and the white smoker fluids. This observation has implications for fluid circulation within the mound that will be discussed later. The inactive chalcopyrite sample has a high Eu,/Sm, ratio (6.25) which suggests that fluid evolution is not the only process controlling the Eu,/Sm, ratio of TAG sulphides.

6.3. Sulphate Formation at TAG

Anhydrite is the only sulphate phase present at TAG; no barite has been observed in any mound sample (Thompson et al., 1988; Tivey et al., 1995). Figure 8 is a plot of KD against [rREE -- rca] 2 for the REE in the three different types of anhydrite analysed in this study. The REEs other than Eu 2+, Ce 3+ , and La 3+ fall on linear arrays reflecting the crystallographic control of the REE distribution between the anhydrite and the inferred fluid composition. The fact that the plots de- viate from equilibrium, linear arrays suggests that other factors such as temperature, complexation, and kinetics are also contributing factors to REE behaviour.

There are two controls on REE distribution in anhydrite at equilibrium: ( 1 ) the similarity in the ionic radii of the LREEs, Nd 3+ and Ce 3÷ with the ionic radius of Ca 2+, and (2) the valence state of the Eu 2÷ cation which matches Ca 2÷ . If the anhydrite crystal structure deviates from the ideal lattice then the first of these two controls will predominate, since the 3+ valence state can be easily balanced by substitution of Na ÷ and K ÷ from seawater for Ca 2+ . The TAG anhydrite phases exhibit maximum Ko values for those LREEs which have similar ionic radii to that of Ca 2÷ (Fig. 8). Ce 3+ and La 3+ exhibit anomalously low Ko values which may be an artefact of the assignment of ionic radii values to the REEs in sulphate phases (Shannon, 1976). There is little or no fractionation of the LREE from the HREE during anhydrite precipitation.

The Eu distribution coefficient between anhydrite and fluid is substantially lower than those for the other REEs. Europium distribution into chimney sulphide phases at TAG appears to be controlled by crystallographic substitution whereas anhydrite precipitation appears to discriminate against Eu species. Precipitation of anhydrite represents a major sink of REEs from solution and the discrimination against the divalent Eu cation will enhance the Eu anomaly of the residual fluid.

Mound sulphide REE pattems (Fig. 5a) are similar to those of the black and white smoker chimney sulphides (Fig. 3a and 4a). The range in the black and white smoker sulphide Eu anomalies (Table 2) supports the inference that the fluid Eu anomaly increases in size as precipitation occurs within the mound, discriminating against the Eu 2+ cation. Therefore, the range of Eu anomaly observed for these mound sulphides may similarly reflect evolution of the fluid during circulation within the mound. Alternatively, the range in Eu anomaly may reflect the differing temperature of precipitation within the mound. The Cu-Fe crust from the base of the black smoker complex (2179-1-1) displays the smallest Eu anomaly (Eun/ Sm, = 2.8), whereas the Fe-rich mound sulphide (Eu,/Sm,

6.4. Oxide Formation at TAG

6.4.1. Chimney oxides

All of the oxides from the active and relict chimneys have REE concentrations that are two orders of magnitude higher than the sulphide phases associated with them. The REE patterns are LREE enriched and the Eu anomalies are pronounced and intermediate between respective sulphide and fluid phases. The oxide coating on an inactive black smoker chimney (2183-7-3-ox) exhibits HREE enrichment and a Ce anomaly, suggesting more seawater influence than for the equivalent coating on an active black smoker chimney (2179- 4-1-ox 1; Fig. 3a and b). The white smoker oxide phases also

100

2183-5-2-och 0 2183-10-2-och ---- 2190-12-1-och

- - £ ] - - - 2183-2-2-Si-ox

"d

t- O 0 .01 u

3520 R.A. Mills and H. ElderIield

0.0001 i i i i i t J i i i i i J i


FIG. 6. Chondrite-normalised REE data for TAG ochres and the silica-rich Fe oxide.

exhibit some seawater influence in the REE pattern. Oxide REE patterns can be modified by adsorption of REEs from seawater after hydrothermal activity has ceased (Koeppen- kastrop and De Carlo, 1992).

The extent of mixing between hydrothermal fluids and seawater in the chimney oxide phases was tested by generating REE patterns for various mixtures of seawater and hydrothermal fluid. However, it should noted that the resultant REE pattern only exhibits a Ce anomaly and HREE enrichment with more than 90% seawater present. This emphasises the limitations in using REE as tracers of mixing of fluid and seawater and puts lower limits on the seawater contribution to samples with no apparent seawater signature.

The active black smoker chimney oxides appear to precipitate from a mixture of about 5% fluid and 95% seawater (Fig. 9a), the white smoker chimney oxides from 1% fluid and 99% seawater (Fig. 9b) and the relict black smoker chimney oxide precipitates from 0.3% fluid and 99.7% seawater (Fig. 9c). All of these oxides show more seawater influence than the anhydrite phase from the same area (see Table 3). The in- creased seawater component of the relict chimney oxide pre- sumably reflects cessation of hydrothermal activity. These oxides must be derived from primary precipitation of Fe oxides at the chimney exterior, or from alteration of sulphides. Since sulphide REEs concentrations are too low to contribute substantially to the REE pattern of the oxides, the former mechanism is favoured. Petrologic observations of a dendritic Fe- oxide phase within the chimney wall suggest that these oxides are primary precipitates (Tivey et al., 1995).

6.4.2. Mound oxides

Oxide rims on mound sulphides exhibit a substantial seawater component which is reflected in the Ce anomaly and HREE enrichment of these phases (Fig. 5b). The REE patterns are similar to the relict black smoker chimney oxide (Fig. 3; 2183-7-3) which suggests at least 99.7% seawater in the parental fluid.

Previous studies of an atacamite-bearing oxide from TAG showed a pronounced Eu anomaly and some LREE enrich-

ment, with no evidence for a seawater signature in the REE pattern (Herzig et al., 1991 ). Atacamite is formed during supergene alteration of primary chalcopyrite and is stabilised by the high C1 content of seawater (Hannington et al., 1988). Copper, along with Au, is remobilised from the sulphide phases, transported as Cl complexes in acidic, low-temperature fluids and redeposited as atacamite at the altered surface (Hannington, 1993 ). The predominant complexing ligand for the REEs at low pH is SO4 which does not fractionate LREEs from HREEs (Wood, 1990b). Chlorine complexes are predicted to be important in SO4 and CO3 poor, reducing hydrothermal fluids during mixing with seawater within the mound (Wood, 1990b). LREEs form more stable Cl complexes than the HREEs in acidic solution (Wood, 1990b). Thus, we hy- pothesise that the atacamite-bearing samples analyzed here exhibit LREE depletion because these elements are more highly complexed in the transporting fluid and, therefore, are discriminated against during atacamite formation.

The REE compositions of TAG mound oxides are consistent with their formation through alteration and the oxidation of sulphides through either direct interaction with seawater and/or low-temperature supergene alteration. The two types of REE pattern exhibited by TAG oxides (Fig. 5b and c) have also been observed in sulphide derived sediments from this area (Mills et al., 1993b). Mineralogical studies of the sediment core have confirmed the presence of atacamite in layers with LREE depletion (Metz et al., 1988; Mills et al., 1993b). This confirms that these sediments are derived from mass- wasting of sulphide-derived material from similar mound fea- tures in the TAG area.

6.4.3. Ochres

Previous studies of ochreous material from active and ancient ocean-ridge settings all show significant Ce anomalies, HREE enrichment and some degree of Eu anomaly (Robert- son and Fleet, 1976; Alt, 1988 ). The TAG ochre REE patterns exhibit positive Eu anomalies and LREE enrichment but no Ce anomaly or HREE enrichment. These patterns cannot be generated either by oxidation of pristine sulphide REE pat-


terns or by mixing of black smoker fluid with seawater. The ochres occur at the periphery of the TAG mound; submersible observations have suggested that some of these deposits are bathed in low-temperature, diffuse fluid flow. The ochre samples contain 4.3-53 wt% SiO2 as amorphous silica or quartz and the mineralogy consists of haematite, goethite with amorphous silica, and quartz (Tivey et al., 1995). The high silica content of ochreous material could be derived from conductive cooling of circulating hydrothermal fluid; supersaturation with amorphous silica does not occur during conservative mixing of fluid with seawater (Janecky and Seyfried, 1984). Alternatively, the ochres may have formed from a late-stage silica-rich fluid when the entire hydrothermal system waned during a previous episode (Lalou et al., 1990). The high REE concentrations, the lack of Ce anomaly, the high Nd,/Yb, ratio, and the high Eu,/Sm, ratio all point to precipitation from or alteration by a modified hydrothermal fluid. The implications of this are discussed below.

6.5. Circulation Within the Mound

The white smoker fluid is modified from black smoker fluid within the mound by precipitation of anhydrite, pyrite, and chalcopyrite, dissolution of sphalerite, and mixing with seawater (Edmond et al., 1995; Tivey et al., 1995). Comparison of the REE content of the two fluids (Fig. 2) suggests extensive precipitation of the REEs with the exception of Eu, which is added to the fluids. Anhydrite is the only phase known to precipitate within the mound which contains appreciable quantities of REEs; sulphide REE concentrations are several orders of magnitude lower (Table 2). However, precipitation of anhydrite within the mound cannot account for the increase in Eu concentration in the white smoker fluids.

Excluding Eu, the black smoker anhydrite REE pattern is very similar in form to the difference between the black smoker and white smoker fluids. The amount of anhydrite precipitation required to reduce the REE content by the observed amount is 0.15-0.35 g of anhydrite per kg of fluid (i.e., 1.1-2.6 mmol anhydrite per kg fluid). This would ef- fectively reduce the Ca concentration from 30.8 mmol kg 1 in the black smoker fluids to 29.7-28.2 mmol kg ~ in the white smoker fluids which is close to the observed value of 27 mmol kg -~ (Edmond et al., 1995). Entrainment of seawater into the TAG mound and dilution of the white smoker fluids would further reduce the Ca concentration (Tivey et al., 1995 ). Tivey et al. ( 1995 ) predict that 2.9 mmol of anhydrite per kg fluid is precipitated within the mound using the major

(a)

lO0

• 2179-4-1: black smoker chimney chalcopyrit~ ] - - ~- - - 2178~4- I: relict chinmey chaleopyrit¢

I bEr Sm Nd Ce La Eu

o _ _ _ •

200 400 600 800 1000 1200 (Mp. -M ) 2 pm 2

(b)

100 I • 2187-1-5: white smoker chimney sphalerite I

0.1 i i i J 0 200 400 600 800 1000 1200 1400

2 2 (M -Mz. ) pm

FiG. 7. Ko for the TAG chimney sulphides ploued against (a) (rREE - - r F e ) 2 and (b) (raKE -- rz,) 2. Europium is plotted as Eu 2÷, all other REEs as 3 +. Ko has been calculated using the black and white smoker data from Mitra et al., 1994 (Table 2). Straight lines represent least squares fits to the data.

element data from TAG fluids. This is in close agreement with the above estimates from REE chemistry. Recent Ocean Drill- ing Program results from TAG have confirmed the hypothesis that significant anhydrite precipitation is currently occurring within the mound (S. Humphris, pers. commun., 1994).

During circulation within the mound, there is potential for fluid modification by a variety of precipitation and dissolution processes; namely enhancement of the Eu anomaly and a change in the Nd,/Yb, ratio depending on the relative amount

Table 3: Strontium isotope data for TAG

Sample 87sr/g6s r

2179-4-1: black smoker 0.705136 2183-7-0: base of black smoker 0.706669 complex 2187-1-2: white smoker 0.705308

anhydrite samples

hydrothermal sea in fe r r ed fluid§ water t tempera ture

(%) (%) (°C) Y

64 36 235 38 62 141

1"Sea water 87Sr/86Sr=0.70916 (Elderfield, 1986)

§TAG black smoker fluid 87Sr/86Sr=0.70319 (Elderfield et al., 1993)

61 39 224

¥Calculated by mixing 366°C hydmthermal fluid with 2.7°C sea water; anhydrite precipitates at temperatures >150°C.


100000

ioooo

.,-, "0 d2 m

1000 c-,

100

• 2 1 7 9 ~ - - l - a n h : b l a c k s m o k e r a n h y d r i t e

2 1 8 7 - 1 - 2 - a n h : w h i t e s m o k e r a n h y d r i t e

2 1 8 3 - 7 - 0 - a n h : m a s s i v e a n h y d r i t e

q % m Gd Dy Er Eu Yb Lu

50 100 2150 2 200 250 (MREE-Mca) pm

FIG. 8. Ko for the three types of TAG anhydrite plotted against (rREE -- rcS-. Eu is plotted as Eu 2+, other REE as 3+. The REE composition of the parental fluid was calculated using the proportion of hydrothermal fluid vs. seawater as inferred from the strontium isotope data described in the text.

of sulphide and sulphate precipitation. Assuming that the REE patterns of the ochre and atacamite bearing deposits reflect the nature of this fluid then these phases exhibit the most marked evolution. These phases show an enhanced positive Eu anomaly compared with chimney wall oxides that are inferred to be primary precipitates from mixtures of hydrothermal fluid and seawater (Fig. 9 a - c ) . Atacamite-bearing samples exhibit a low NdJYb, , ratio whereas ochres exhibit high Nd , /Yb , ratios compared with chimney oxides. REE patterns for atacamite-bearing and ochre samples are consistent with alteration in, or precipitation from, a modified hydrothermal fluid with minimal seawater influence. This is also consistent with observations of fluids of 80% seawater and 20% hydrothermal fluid upwelling through ochreous sediments to the south of the TAG mound (Mil ls et al., 1993a).

The increase in the size of the posit ive Eu anomaly during mound circulation can also be seen in the range of values observed in the different sulphides. The Eu anomaly in TAG sulphides increases in the order black smoker sulphide, Cu- Fe basal crust, mound Cu-Fe-Zn sulphides, white smoker sulphide. This may reflect evolution of the high temperature fluid during circulation within the mound, or alternatively, the temperatures at which the minerals precipitated.

7. CONCLUSIONS

Black and white smoker chimney sulphides have relatively low REE contents. All REE patterns are LREE-enriched and HREE-depleted in a similar way to the hydrothermal fluid. All sulphides exhibit a positive Eu anomaly. REE distribution into the solid phase is controlled by substitution of the dissolved REE species into the crystal lattice.

Samples representing primary precipitation of sulphide on the fiat surface of the mound exhibi t REE patterns that are intermediate between those of sulphide phases from the black and white smoker chimneys. The observed range in Eu anomaly in the sulphides may reflect the evolution of the hydro-

(a)

100

10

0.1

- 4 - - 95% sea w ~ + 5% black smoker fluid ----dr-- 2179-4-1~xl

at 21794- l-ox2

i i t

i I I I i i t t I I I i I I i

L a C e N d S m E u G d D y E r Y b L u

(b)

lO00

-m

0.1 i i I I i i ~ i i I I I i i i


(c)

100

1o

._~

r j

0.1 t I I i I I I I i I I I I I


FIG. 9. TAG chimney oxides: comparison with calculations of mixing of hydrothermal fluid and seawater. The fluid mixtures have been scaled to allow direct comparison with the solid phase data. (a) chondrite-normalised REE data for the chimney wall oxide from an active black smoker compared with the REE pattern for a mixture of 95% seawater and 5% black smoker fluid. (b) chondrite-normalised REE data for chimney wall oxide from a relict black smoker compared with the REE pattern for a mixture of 99.7% seawater and 0.3% black smoker fluid. (c) chondrite-normalised REE data for white smoker oxides compared with the REE pattern for a mixture of 99% seawater and 1% white smoker fluid.


thermal fluid circulating within the mound and/or the tem-

perature at which the sulphides precipitate. Anhydrite has relatively high REE contents. The distribu-

tion of the REE into anhydrite is crystaUographically controlled except for Eu 2÷. All of the other trivalent REE Ko values approach equilibrium in the anhydrites sampled here. Precipitation of anhydrite within the TAG mound is the major mechanism for modifying the REE content o f the white smoker fluids. 0 .15-0 .35 g of anhydrite is inferred to precipitate from every kg of fluid venting from the white smoker chimneys.

TAG oxide phases also contain relatively high concentrations of REEs. Black and white smoker chimney oxides appear to represent primary precipitates from varying mixtures of fluid and seawater. However, mound oxides associated with sulphides may instead result from oxidation by seawater and supergene alteration of sulphides.

Circulation of hydrothermal fluid mixed with seawater within the TAG hydrothermal mound appears to result in some evolution of the chemistry of that fluid which is reflected in the REE chemistry of the minerals precipitated.

Acknowledgments--This research was funded by NERC award GT4/ 89/AAPS/12 to RAM and grant GR3/6639 to HE. Thanks are due to the Captain, scientists, and crew of RV Atlantis II/DSDV Alvin leg 125, and especially to Meg Tivey and Susan Humphris for ar- chiving and supplying the samples and for many fruitful discussions and useful comments on an early version of this manuscript. This manuscript has benefited from reviews by J. Alt, T. Barrett, and an anonymous reviewer. We would like to thank Kate Davis for prep- aration of figures and Mervyn Greaves for advice on analytical tech- niques. This is Cambridge Earth Sciences Series, contribution number ES.4091.

Editorial handling: C. R. German

REFERENCES

Alt J. C. (1988) The chemistry and sulfur isotope composition of massive sulfide and associated deposits on Green Seamount, East- ern Pacific. Econ. Geol. 83, 1026-1033.

Barrett T. J., Jarvis I., and Jarvis K. E. (1990) Rare earth clement geochemistry of massive sulfides-sulfates and gossans on the Southern Explorer Ridge. Geology 18, 583-586.

Bence A. E. ( 1983 ) Volcanogenic massive sulfides: Rock/water in- teractions in basaltic systems and their effects on the distributions of the rare earth elements and selected first series transition elements. Proc. 4th Int. Sympos. Water-Rock Int., 48-49.

Becker K. and Von Herzen R. (1993) Conductive heat flow measurements using ALVIN at the TAG active hydrothermal mound, MAR at 26°N. Eos 74, 99 (abstr.).

Campbell A. C., et al. (1988) Chemistry of hot springs on the Mid- Atlantic Ridge. Nature 335, 514-519.

Edmond J. M., et al. (1995) Time-series studies of vent fluids from the TAG and MARK sites (1986, 1990), Mid-Atlantic Ridge and a mechanism for Cu/Zn zonation in massive sulphide orebodies. In Hydrothermal Vents and Processes (ed. L. M. Parson et al.); Geol. Soc. Lond. Spec. Publ. 87, 77-86.

Elderfield H. (1986) Strontium isotope stratigraphy. Palaeogeogr. Palaeoclimatol. Palaeoecol. 57, 71-90.

Elderfield H. (1988) The oceanic chemistry of the rare-earth elements. Phil. Trans. Roy. Soc. London A325, 105-126.

Elderfield H., Mills R. A., and Rudnicki M. D. (1993) Geochemical and thermal fluxes, high-temperature venting and diffuse flow from mid-ocean ridge hydrothermal systems: the TAG hydrothermal field, Mid-Atlantic Ridge 26°N. In Magmatic Processes and Plate Tectonics (ed. H. M. Pilchard et al.); Geol. Soc. Spec. Publ. 76, 295-307.

German C. R., et al. (1993) A geochemical study of metalliferous sediment from the TAG hydrothermal mound, 26°08 ' N, MAR. J. Geophys. Res. 98, 9683-9692.

Gillis K. M., Smith A. D., and Ludden J. N. (1990) Trace element and Sr isotopic contents of hydrothermal clays and sulfides from the Snakepit hydrothermal field: ODP site 649. Ocean Drilling Prog. Sci. Res. 1061109, 315-319.

Goldfarb M. S., Converse D. R., Holland H. D., and Edmond J. M. ( 1983 ) The genesis of hot spring deposits on the East Pacific Rise, 21°N. Econ. GeoL Mono. 5, 184-197.

Greaves M. J., Elderfield H., and Klinkhammer G. P. (1989) Deter- mination of the rare earth elements in natural waters by isotope- dilution mass spectrometry. Anal. Chim. Acta 218, 265-280.

Guichard F., Church T. M., Treuil M., and Jaffrezic H. (1979) Rare earths in barites: distribution and effects on aqueous partitioning. Geochim. Cosmochim. Acta 43, 983-997.

Hannington M. D. (1993) The formation of atacamite during weathering of sulfides on the modem seafloor. Canadian Mineral. 31, 945-956.

Hannington M. D., Thompson G., Rona P. A., and Scott S. D. (1988) Gold and native copper in supergene sulphides from the Mid-At- lantic Ridge. Nature 333, 64-66.

Haymon R. M. ( 1983 ) Growth history of hydrothermal black smoker chimneys. Nature 301, 695-698.

Herzig P. M., Hannington M. D., Scott S. D., Maliofis G., Rona P. A., and Thompson G. ( 1991 ) Gold-rich sea-floor gossans in the Troodos ophiol- ite and on the Mid-Atlantic Ridge. Econ. Geol. 86, 1747-1755.

Janecky D. R. and Seyfried W. E. (1984) Formation of massive sulfide deposits on oceanic ridge crests: Incremental reaction models for mixing between hydrothermal solutions and sea water. Geo- chim. Cosmochim. Acta 48, 2723-2738.

Klinkhammer G. P., Elderfield H., Edmond J. M., and Mitra A. (1994) Geochemical implications of rare earth element patterns in hydrothermal fluids from mid-ocean ridges. Geochim. Cosmochim. Acta 58, 5105-5113.

Koeppenkastrop D. and De Carlo E. H. (1992) Sorption of rare-earth elements from sea water onto synthetic mineral particles: An ex- perimental approach. Chem. Geol. 95, 251-263.

Lalou C., Thompson G., Arnold M., Brichet E., Brnffle E., and Rona P. A. (1990) Geochronology of TAG and Snakepit hydrothermal fields, Mid-Atlantic Ridge: Witness to a long and complex hydrothermal history. Earth Planet. Sci. Lett. 97, 113-128.

Lalou C., Reyss J-L., Brichet E., Aronld M., Thompson G., Fouquet Y., and Rona P. (1993) New age data for Mid-Atlantic Ridge hydrothermal sites: TAG and Snakepit chronology revisited. J. Geophys. Res. 98, 9705-9713.

Metz S., Trefry J. H., and Nelsen T. (1988) History and geochemistry of a metalliferous sediment core from the Mid-Atlantic Ridge. Geochim. Cosmochim. Acta 48, 47-62.

Michard A. and Albarede F. (1986) The REE content of some hydrothermal fluids. Chem. GeoL 55, 51-60.

Michard A., Albarede F., Michard G., Minster J. F., and Charlou J. L. (1983) Rare-earth elements and uranium in high-temperature solutions from East Pacific Rise hydrothermal vent field (13°N). Nature 303, 795-797.

Mills R. A., Thomson J., Elderfield H., and Rona P. A. (1993a) Pore- water geochemistry of metalliferous sediments from the Mid-At- lantic Ridge: diagenesis and low-temperature fluxes. Eos, Trans. Amer. Geophys. U. 74, 101.

Mills R. A., Elderfield H., and Thomson J. (1993b) A dual origin for the hydrothermal component in a metalliferous sediment core from the Mid-Atlantic Ridge. J. Geophys. Res. 98, 9671-9681.

Mitra A., Elderfield H., and Greaves M. J. (1994) Rare earth elements in submarine hydrothermal fluids and plumes from the Mid-Atlan- tic Ridge. Mar. Chem. 47, 217-236.

Morgan J. W. and Wandless G. A. (1980) Rare earth element distribution in some hydrothermal minerals: evidence for crystallographic control. Geochim. Cosmochim. Acta 44, 973-980.

Nakamura N. (1974) Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochim. Cosmochim. Acta 38, 757-775.

Onuma N., Higuchi H., Wakita H., and Nagasawa H. (1968) Trace element partition between two pyroxenes and the host lava. Earth Planet. Sci. Lett. 5, 47-51.

3524 R . A . Mills and H. Elderfield

Robertson A. H. F. and Fleet A. J. (1976) The origins of rare earths in metalliferous sediments of the Troodos Massif, Cyprus. Earth Planet. Sci. Lett. 28, 385-394.

Rona P. A., Klinkhammer G., Nelsen T. A., Trefry J. H., and Elder- field H. (1986) Black smokers, massive sulphides and vent biota at the Mid-Atlantic Ridge. Nature 321, 33-37 .

Rona P. A., Bogdanov Y. A., Gurvich E. G., Rimski-Korsakov N. A., Sagalevitch A. M., Hannington M. D., and Thompson G. (1993) Relict hydrothermal zones in the TAG hydrothermal field, Mid-Atlant ic Ridge 26°N, 45°W. J. Geophys. Res. 98, 9715 -9730 .

Schultz A. and Elderfield H. (1994) Direct measurement of the phys- ics and chemistry of diffuse and high-temperature effluent at TAG and Broken Spur vent fields. Eos 75, 308.

Scott M. R., Scott R. B., Rona P. A., Butler L. W., and Nalwark A. J. (1974) Rapidly accumulating manganese deposit from the median valley of the Mid-Atlantic Ridge. Geophys. Res. Lett. 1, 355-358.

Scott M. R., Scott R. B., Morse J. W., Betzer P. R., Butler L. W., and Rona P. A. (1978) Metal-enriched sediments from the TAG hydrothermal field. Nature 276, 811 - 813.

Shannon R. D. (1976) Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst. A32, 751-767.

Shearme S., Cronan D. S., and Rona P. A. (1983) Geochemistry of sediments from the TAG hydrothermal field, MAR at latitude 26°N. Mar. Geol. 51, 269-291.

Sverjensky D. A. (1984) Europium redox equilibria in aqueous solution. Earth Planet. Sci. Lett. 67, 70-78 .

Thompson G., Humphris S. E., Schroeder B., Sulanowska M., and Rona P. A. (1988) Active vents and massive sulfides at 26°N (TAG) and 23°N (Snakepit) on the Mid-Atlantic Ridge. Canadian Mineral. 26, 697-711.

Tivey M. K., Humphris S. E., Thompson G., Hannington M. D., and Rona P. A. (1995) Deducing patterns of fluid flow and mixing within the active TAG mound using mineralogical and geochemical data. Z Geophys. Res. (in press).

Von Herzen R., et al. (1993) Mid-Atlantic Ridge hydrothermal processes: prelude to drilling. Eos 74, 382-383.

Wood S. A. (1990a) The aqueous geochemistry of the rare-earth elements and yttrium. 2. theoretical predictions of speciation in hydrothermal solutions to 350°C at saturation water vapor pres- sure. Chem. Geol. 88, 99-125.

Wood S. A. (1990b) The aqueous geochemistry of the rare-earth elements and yttrium. 1. review of available low-temperature data for inorganic complexes and the inorganic REE speciation of natural waters. Chem. Geol. 82, 159-186.

Documents

Rare earth element geochemistry of hydrothermal deposits from the active TAG Mound, 26°N Mid-Atlantic Ridge