- Email: [email protected]
An evaluation of the behavior of the rare earth elements during the weathering of sea-floor basalt
Earth and Planetary Science Letters, 43 (1979) 85-92 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
85
[61
AN EVALUATION OF THE BEHAVIOR OF THE RARE EARTH ELEMENTS DURING THE WEATHERING OF SEA-FLOOR BASALT JOHN N. LUDDEN Woods Hole OceanographicInstitute, Woods Hole, iliA 02543 {U.S.A.) and Ddpartement de G~ologie *, Universitd de MontrdaL Montreal, Que. H3C 3J7 {Canada)
and G E O F F R E Y THOMPSON Woods Hole Oceanographic Institute, Woods Hole, MA 02543 {U.S.A.]
Received June 26, 1978 Revised version received December 12, 1978 We present rare earth element (REE) data for fresh and altered tholeiitic basalts sampled during a dredging transect at 23°N in the Atlantic Ocean and covering a time span of 0 to 57 million years. These data have been used to evaluate the behavior of the REE during low-temperature weathering processes, Compositional trends from altered basalt interiors to palagonitized rinds in individual pillow samples indicate significant mobility of the light REE: some elements are enriched by four orders of magnitude in rinds relative to interiors. The heavy REE show no selective mobilization and can be used in a normalization procedure which indicates that the light REE are enriched in altered interiors relative to fresh interiors of the basalts. Cerium behaves anomalously and accords with either its abundance in seawater or its fractionation from seawater during the formation of ferromanganese deposits. These results indicate that REE data from fresh glassy or crystalline basalt samples only may be used with confidence in petrological models.
1. Introduction The rare earth elements (REE), when considered relative to their abundance in chondrites, are commonly used as a critical diagnostic tool in interpreting the petrogenesis o f sea-floor basaltic rocks and glasses (e.g. [ I - 4 ] ) . In particular, the relative depletion in the light REE (LREE) is used to discriminate between basaltic magma types. Although Frey et al. [5] and Thompson [6] demonstrated some mobility o f the LREE during low-temperature weathering, and more recently Wood et al. [7], Hellman and Henderson [8] and Hellman et al. [9] presented data that Woods Hole Oceanographic Institute Contribution No. 4240. * Address for reprints.
indicate mobility o f the LREE, in particular La, during low-grade (prehnite-pumpellyite facies) metamorphism, REE data from altered basalts are still commonly interpreted to represent a primary "petrological fingerprint". The objective o f this study was to determine whether the REE are mobilized during weathering o f basaltic rocks at ambient seawater temperatures (low-temperature weathering). The preliminary results o f this study (samples < 5 m.y.), were presented by Ludden and Thompson [10]. In addition to an evaluation o f the data for these younger samples, in this paper we discuss data obtained for altered basalts up to 57 m.y. The samples were taken from a suite o f basalts recovered during a dredging transect at 23°N, at distances o f 0 - 7 2 5 k m from the Mid-Atlantic Ridge (Fig. 1). These samples represent a time sequence o f
86 basalts erupted at a single point on the ridge axis over the past 57 m.y. (age dates are based on the sample position relative to sea-floor magnetic anomalies, see Fig. 1). Thompson and Rivers [11], Henrichs and Thompson [12] and Thompson and Humphris [13] presented major element data and some trace element data for these rocks. For this study we selected 11 basalts which span the entire 0-57-m.y. time sequence. The sampling strategy was based on the known major and trace element compositions of the basalts; we selected: fresh (0 m.y.) samples which represented the most-fractionated and least-fractionated rocks sampled at the ridge crest, and samples of 2.5-57 m.y. which displayed variable degrees of alteration. For five of the altered samples we determined the REE abundances of the crystalline interiors and palagonitized rinds of the individual basalt pillow. To compare the relative effect of low- and
45°-NOBTH'
,/~-.~'~
/
.~r~/\@0 "
////
x'~-" I,'7"
// 30 ° ~
l
/ / I
~
I/I ,',',, /
I
/~
, /
.
/
~--'--~ '-""--~_
, I
,
/
I ~
I o. AZORES
.,o'~~ o,Z" ~,.~'1." '
/ ~
,,"9 /'b
//
/~
~
/ I / 39 ~sy.,/.,;," , -" / I
I
,. , ~ • SITE 396 ,~SURFA_C_E/// / / , ,
,
75 °
/
,
/
,/
/
I
/
// // ~\3
AI
I
A
I
60 °
45 Q
30 °
15 °
TRANSECT AT 23°N SHOWING BATHYMETRY, DREDGE LOCATIONS (Q AND MAGNETIC ANOMALY AGES W
52 °
51 ° ~ - - T
I 53
50 °
49 ° 1
I my
38
48 ° 1
47 ° •
46 ° 1
t my
9
45 ° T -
-
44 ° r
E
I my
O my
B
Fig. 1. A. Location map showing the position of the surface transect relative to magnetic anomalies and the nearest DSDP sites in the North Atlantic Ocean. B. Transect at 23° N showing bathymetry, dredge locations (indicated by an arrow) and magnetic anomalies.
high-temperature alteration on the REE we also analysed a 0-m.y. hydrothermally altered basalt (metabasalt), sampled from the ridge crest at 23°N.
2. Results
REE abundances are given in Table 1. H20 ÷, H20- and F%O3/FeO ratios are given as a measure of the extent of weathering of the basalts. In natural geochemical environments the REE usually exhibit a trivalent oxidation state. The principal exceptions to this are Ce and Eu. In oxidizing environments Ce may be present as Ce 4÷, and as demonstrated later, this results in Ce showing departures from a smoothed chondrite- or shale-normalized REE profile. The ratio Ce/Ce* given in Table 1 reflects the departure of Ce from the smoothed chondrite-normalized profile between La and Nd. Europium may be present as Eu 2÷ and Eu 3÷, resulting in the commonly observed Eu anomaly in basaltic liquids. The most common explanations of an Eu anomaly are that it is caused by plagioclase fractionation or that it represents a source characteristic of the melt. However, Sun and Nesbitt [14] interpret variable Eu anomalies in Archean metabasalts as an alteration effect; this possibility is discussed in relation to our data for sea-floor basaltic rocks. The fresh (0-m.y.) basalts represent pristine tholeiitic compositions recently erupted at the spreading center at 23°N; both samples show low H20+ and F%O3/FeO. The extent of weathering of the crystalline interiors of the basalts older than 0 m.y. is reflected by increasing abundances of H20+ (0.513.91 wt.%) and increasing Fe203/FeO ratio (0.7615.35). Although all the crystalline interiors of the basalts are altered relative to the fresh basalts, the degree of alteration markedly increases with age from 0 to 10 m.y.,but is variable from 10 to 57 m.y. The degree of alteration apparently depends on the extent of fracturing (and thus access of water) and the amount of matrix glass present in an individual sample (glass being particularly susceptible to alteration [6]). The glassy rinds of the basalts are completely palagonitized by 4 m.y.: fresh glass has been found in samples several tens of m.y. old obtained by deep ocean drilling (e.g. Legs 51-53 of the DSDP). Our observation of complete palagonitization in 4 m.y.
87 TABLE 1 REE concentrations (in ppm) for fresh and altered tholeiitic basalts from the ridge crest and surface transect at 23°N, Atlantic Ocean Age (m.y.)
0
0
2.5
4.5
4.5
5.0
5.0
7.5
30.5
Sample
3-103A int.
3-106 int.
6-3 int.
7-3A mar.
7-3B int.
8-16A mar.
8-16B int.
10-3 int.
12-2 int.
La Ce Nd Sm Eu Tb Ho Yb Lu
1.67 5.86 5.33 2.23 0.90 0.56 0.83 2.02 0.34
3.75 13.68 11.42 4.23 1.52 0.95 1.38 3.82 0.62
4.46 15.82 12.96 4.67 1.58 1.08 1.42 3.73 0.63
30.08 43.97 34.48 9.37 2.58 1.58 2.33 6.25 0.99
4.50 14.65 12.21 4.83 1.60 0.96 1.47 4.15 0.68
16.35 24.78 19.70 5.30 1.63 1.06 1.49 4.14 0.72
6.23 16.58 12.28 4.27 1.32 0.75 1.14 3.09 0.52
2.29 9.20 6.90 2.57 1.04
5.63 15.02 12.79 4.58 1.55
0.72 2.34 0.34
1.52 3.99 0.64
H2 O+ Fe203/FeO
0.11 0.13 0.18
0.20 0.18 0.22
0.51 0.38 0.76
7.39 9.47 27.20
2.03 0.99 2.08
11.14 14.40 17.16
2.37 1.64 3.37
1.32 0.96 0.86
2.55 1.64 2.90
Ce/Ce*
1.08
1.04
1.09
0.62
1.06
0.65
0.95
1.17
0.91
BCR-1 b
H20-
Age (m.y,)
35
35
46
46
57
57
0
BCR-1
Sample
14-2A mar.
14-2B int.
15-2A mar.
15-2B int.
16-6A mar.
16-6B int.
1-3 a
found
La Ce Nd Sm Eu Tb Ho Yb Lu
16.48 57.68 24.92 7.34 2.09 1.22 1.71 5.17 0.86
6.85 13.32 12.76 4.49 1.53 0.94 1.30 3.58 0.59
15.79 11.88 14.64 4.13 1.33 0.74 0.98 2.87 0.49
3.47 6.94 6.49 2.28 1.01 0.64 0.88 2.68 0.45
13.67 15.92 18.19 4.97 1.90 1.16 1.54 4.16 0.69
5.35 16.91 9.57 2.87 1.10 0.75 1.16 3.46 0.69
2.44 8.33 7.38 2.97 0.95 0.68 1.02 2.72 0.46
H20 ÷ H20Fe203/FeO
11.54 14.18 28.50
1.42 1.29 1.31
5.67 3.95 13.60
2.48 1.35 2.00
10.14 13.53 16.14
3.91 2.57 15.35
1.35 0.18 0.42
1.40
0.71
0.33
0.73
0.47
1.19
1.05
Ce/Ce *
24.60 53.60 26.60 6.1 1.88 1.10 1.18 3.32 0.53
Chondrites
24.2 53.7 28.5 6.7 1.95 1.08 1.33 3.70 0.55
0.33 0.88 0.60 0.181 0.069 0.047 0.070 0.200 0.034
INAA analysis of "dry" rock powders, int. = interior of basalt pillow; mar. = basalt pillow margin. a Metabasalt from ridge crest at 23°N. b Compilation of values from Taylor and Gorton [26].
m a y o n l y b e valid for d r e d g e d samples w h i c h h a v e b e e n e x p o s e d for long p e r i o d s o f t i m e t o large v o l u m e s o f seawater. The h i g h F e 2 0 3 / F e O ratios o f s o m e s a m p l e s m a y reflect s o m e i n c o r p o r a t i o n o f ferromanganese deposit on the exposed portion of t h e pillow.
2.1. Weathered basalts and fresh basalts C h o n d r i t e - n o r m a l i z e d R E E profiles for t h e basalts are p l o t t e d in Fig. 2. T h e b e h a v i o r o f t h e R E E d u r i n g p a l a g o n i t i z a t i o n o f t h e glassy rinds o f t h e basalts is s h o w n b y c o m p a r i n g t h e R E E profiles o f t h e crystal-
88 line interiors and palagonitized rinds of the 4.5-, 5.0-, 35-, 46- and 57-m.y. basalts: (1) The LREE ( L a - S m ) in the palagonitized rinds are selectively enriched relative to the interiors of the pillow basalts. (2) Cerium behaves inconsistently, and may be either depleted or enriched relative to the basalt interior. (3) The heavy REE (HREE; E u - L u ) show a uniform increase in abundance with alteration, but do not show selective mobilization. (4) Contrary to the conclusion o f Sun and Nesbitt [14], there are no selective changes in the Eu abundance. One of the problems o f interpreting changes in the chemical composition o f altered rock samples is the determination o f the composition o f their fresh precursor. There is no selective mobilization o f the HREE. Thus, for the pillow basalt samples for which we have data for the palagonitized rinds and crystalline interiors, it is possible to quantify the alteration process by assuming a constant composition for the HREE. In Table 2 we have presented the results of such a normalization procedure. In this case we have assumed a constant Yb composition between margin and interior: these data emphasize observations 1 - 4 above. In Fig. 2 we have plotted data for two fresh tholeiites (3-103 and 3-106), these samples represent the entire compositional range for rocks sampled at the ridge crest at 23°N. Both samples show a LREE pattern characteristic o f large-ion-lithophile (LIL)-
TABLE 2
La Ce Nd Sm Eu Tb Ho Yb Lu
4.5 m.y.
5.0
m.y.
35 m.y.
46 m.y.
57 m.y.
4.44 1.99 1.87 1.29 1.07 1.09 1.05 1.00 0.97
1.96 1.12 1.20 0.92 1.05 0.97 0.97 1.00 1.03
1.67 3.00 1.35 1.13 0.96 0.90 0.92 1.00 1.00
4.25 1.83 2.01 1.69 1.22 1.07 1.03 1.00 1.02
2.13 0.78 1.58 1.44 1.43 1.28 1.10 1.00 0.83
WEATHERED BASALT 57 m.y D~D__C]
~'/~ ~'~
\16-6
i°La
t Ce
mm~mm---m--~m--
m~-----
m--m
NJd
i Ha
J ymb Lu
~ Sm
h Eu
]"b
50u
WEATHERED BASALT 46 m y
I
,
5 o ~ J m ~ '~[
L
~r"
10 ,
O I
5o -
T
,
I
-I
WEATHERED BASALT 35 m y
f III
c~_____D\.~. ° i/
~'14-2
I
I
I
L
I
I
i
i
WEATHERED BASALT 5 0 m¥
fJ O
\8.16
INTERIOR
~
~ m - - m
~00~ [~
WEATHERED BASALT 4 5 m y
i / I ~ m ~ i
// 10 I
50
Enrichment of the LREE shown by the ratio of palagonitized basalt rinds relative to interiors after normalization to constant Yb Sample
F
SO~
i
i
k
•
•
i
i
i
I
i
WEATHERED BASALT 2.5 m y /FRESH BASALT / / FRACTtONATED
L ~ ~ : ~ ~/
.~'~=~.~,~. j~ . . . .
I
')
mm '<'-~j
S~/
~ 3 103
I La
=
~7-3
l Ce
L Nd
. ~_
FRESHBASALT-
. ~ ' - - -~
i
~
I
~"~METABASALT
UNFRACTIONATED J Sm
I Eu
I Tb
i Ha
t Yb
REE ATOMIC NUMBER
Fig. 2. Chondrite-normalized REE profiles for selected basalts sampled during the dredging transect at 23° N. Data for fresh basalts from the ridge crest at 23°N are plotted in the lowest diagram, the remaining diagrams show the REE prof'fles for the palagonitized rinds (open squares) and weathered interiors (solid squares) of pillow basalt samples.
Lu
89 element-depleted tholeiites from the mid-oceanic ridges (i.e. Frey et al. [5] and Schilling [15]). In this paper we are not concerned with petrographic interpretations. However, the differences in the patterns are inferred to reflect fractionation of a REE-poor phase such as olivine. No fresh samples which are considered enriched in LIL elements have been sampled from the ridge axis or surface transect at 23°N. The total range in abundance of immobile elements, i.e. Ti, Zr and Y [16,17], for both fresh and weathered samples assuming constant volume in the altered samples, is Ti, 0.9-1.5 wt.%; Zr, 6 0 - 1 0 0 ppm; Y, 2 0 - 3 0 ppm. Thus, as indicated by Thompson and coworkers [11-13], we have assumed that the fresh basalts at 23°N are representative of the compositional range of basalts erupted from the ridge axis over the past 57 m.y. Using this assumption we performed a normalization of the weathered basalts to the fresh basalts sampled at 23°N (i.e. to an average of samples 103A and 106). The results are shown in Fig. 3. The shaded area represents the range in abundance for the fresh basalts. Any element falling outside this region is considered to have been mobilized during weathering. The constancy of the HREE favour our argument for such a normalization procedure. The normalization confirms our observations for the palagonitized rinds. In addition, the altered interiors of the basalts show LREE enrichment whilst the HREE are constant. However, not all the basalt interiors show significant departures from the pattern of their fresh precursor. In particular the relatively unweathered 2.5-, 4.5- and 7.5-m.y. interiors preserve the profile of their precursor, whilst the more weathered basalt interiors (i.e. the 5.0-, 30.5, 35-, 46- and 57-m.y. samples) show significant LREE mobilization. Recent studies of incompatible element mobility in sea-floor basalts and associated glassy and palagonitized rinds [19,20], Sites 417 and 418 of the DSDP) indicate that the palagonitized samples do not show enriched LREE patterns. We interpret this difference in terms of the environment of alteration affecting each suite of samples. The drilled samples are from relatively deep within the oceanic crust: water circulation within the crust ceases soon after crustal formation [20]. The dredged samples from 23°N have been exposed to seawater throughout their lifetime. Thus, the two environments must
5 ' 4
5 j [ 4
3 i'
,~,
21
":
i
Basah Margin 46 my Basalt Imerlor 46 my
.
Basah Ma%l~' 5 my
*
,
•
4}
gasall Margin 57 my Basal[ Interior 57 my
• ,
8asall Margin 37 my Basan line,lot 37 m~
* *
~2 3 I
i 5
•
"
15
•
Basall Marg,n 4 5 rn y •
[r
, ,
~,al:l,nnate# Thole,,le
t
, ,
~Ble4de ;
[a Ce
Nd
Sm~u
Ib
HO
Yb lu
la Ce
; [{esh Oasalfs al ~3°Q Nd
BmEu
Tb
Ho
Yb Lu
Fig. 3. REE profiles for altered basalts after normalization to
a constant Yb value (see text for explanation). The shaded region represents the compositional range for fresh basalts at 23° N. Samples which plot outside this region are inferred to have an altered REE pattern (the solid circles represent the palagonitized rinds and the open circles the weathered interiors of individual pillow samples).
differ in terms of, water chemistry, temperature and most importantly, water/rock ratio. We agree with Staudigel et al.'s [19] caution that the study of dredged samples may not be directly applicable to basalts from deep within the oceanic crust. However, as alteration environments may be completely different in adjacent DSDP holes (e.g. [21]) petrological interpretations based on REE in altered rocks may be misleading.
2.2. Metabasalt {1-3) A detailed study of alteration during high-temperature sea-floor metamorphism was not the subject of this study. However, in Fig. 2 we present REE data for a metabasalt sampled from the ridge crest at 23°N. This sample appears to have preserved the profide of its precursor. We recommend that this interpretation is not taken "ipso facto" as Wood et al. [7], Hellman and Henderson [8], Hellmann et al. [9] and Sun and Nesbitt [14] have presented data that indi-
90 k--
~-loo"
,..-.,
z o
SEA-WATER x
10 7
so
U I---
V
<
~
I0 ~
.~o,~...
I
I
Ce
Nd
I
I
Sm Eu
I
I
Tb
Ho
I
I
Yb Lu
REE A T O M I C N U M B E R
Fig. 4. The REE abundance of seawater normalized to chondrites (data from Goldberg et al. [22], solid circles, and Hogdahl et al. [23], open circles).
cate mobility of REE during prehnite-pumpellyite and greenschist facies metamorphism. Using REE data Copeland et al. [18] postulated an origin for coarse-grained marine chlorites from ridge greenstones. Thes chlorites preserve a LREE profile similar to the greenstones and exhibit igneous La/Sm values of approximately 4.0 in comparison to typical sedimentary values of 65. We tentatively suggest that the REE profile of greenschist grade metamorphic rocks may be preserved by the stabilization of the REE in chlorite.
Cerium is depleted in most samples; its enrichment in the 35-m.y. and 4.5.-m.y. rinds may reflect fractionation of Ce 4÷ from seawater during the formation of ferromanganese components (these samples have high manganese and cobalt contents relative to the other palagontie rinds [11,12]). The ternary plot of La(e.f.), Ce(e.f.) and Yb(e.f.) (Fig. 5) demonstrates the significance of these data to petrological arguments: La was selected as it is the most mobile of the LREE; Ce due to its anomalous behaviour as a result of oxidation to Ce4*; Yb as it represents the stable HREE. The tie lines joining the interiors of the altered samples with their palagonitized rinds trend towards the seawater composition for all samples except the 35-m.y. basalts. The high Fe203/FeO for the 35-m.y. sample indicate the possible incorporation of some ferromanganese components, although the 4.5-m.y. palagonitized rind, which also shows high Fe203/FeO does not show the same trend towards manganese nodule compositions. The most important implications of this diagram are
La (e.f.)
3. Interpretation and Petrographic significance One explanation for the behavior of the REE during low-temperature weathering is that the palagonitized glassy rinds and weathered basalt interiors have equilibrated with the REE abundance of seawater. The extent of equilibration will depend on seawater/rock ratio: equilibration should also be faster for the glassy rinds and glassy mesostasis of the basalts. Fig. 4 shows the chondrite-normalized REE profile for a seawater ×107 [22,23]. Relative to chondrites seawater displays a decreasing enrichment with increasing atomic number for the LREE and a relatively constant HREE pattern except that seawater also shows a depletion in Ce which may be attributed to the oxidation of Ce to its tetravalent state and its subsequent fractionation from seawater: in particular Ce fractionation occurs during the formation of ferromanganese deposits [24,25]. The observed profiles for the altered samples, in particularly the behavior of Ce in the oldest samples, support this argument.
L IL
Yb (of.}
DEPLETED THOLEIITES
I
Ce (e.f.)
Fig. 5. Ternary plot of La, Ce, Yb (enrichment factor relative to chondrites; e.f.). The open squares represent palagonitized rinds and the solid squares altered interiors of individual pillows joined by tie lines which indicate the effect of low-temperature alteration of basaltic glass. The 23 ° N samples are plotted relative to fields for depleted tholeiitic basalts (Leg 2, DSDP), LIL-element-enriched tholeiitic basalts (Leg 37, DSDP), manganese nodules, seawater and the Hawaiian alkali basaltic and tholeiitic suite. The contours are generalized fields for the extent of alteration represented by wt.% H2 O÷.
91 derived from consideration of the positions of the basalt rinds relative to the basalt interiors. The trend lines joining the two compositions indicate the behavior of the REE during palagonitization. The orientation of these trends relative to the fields for LILelement-depleted tholeiites and LIL-element-enriched tholeiites (data are taken from DSDP Leg 3 [5] ; and selected data from Initial Reports o f the Deep Sea Drilling Project, Vol. 37) indicate that the trend from fresh basalt to weathered basalt and palagonitized glass results in an original depleted abundance changing to one resembling that of enriched sea-floor tholeiitic basalts (this is also apparent in the REE profiles, Figs. 2 and 3): Frey et al. [5] reached a similar conclusion from a study of altered glasses from Legs 2 and 3 of the DSDP. In addition, relatively small amounts of weathering, in some cases for samples containing as little as 2% H20+, may result in a significant LREE enrichment (e,g. the 5-m.y. basalt interior). Another point emphasized by the diagram is that, as the HREE are not selectively mobilized, the altered REE profiles do not resemble those profiles observed for basalts from oceanic islands (i.e. Hawaii) which, generally, exhibit a greater fractionation of the HREE.
4. Condusions We have presented data that indicate the REE abundances of oceanic basalts are subject to modification during weathering processes. Interpretations of the REE abundances of weathered basalts should thus be treated with caution. Although we have taken an extreme case, i.e. samples from an environment with a high water/rock ratio, we suggest that petrogenetic models based on REE abundances be restricted to fresh glass or fresh whole rock compositions. Where fresh samples are unavailable (i.e. in Archean terrains) sampling should be very selective, and, arguments based on the REE abundance should be used only where supported by data for immobile elements, i.e. Ti, Zr, Y, Nb. Our data suggest that alteration processes will not always result in a consistency of REE profile as has been considered recently (i.e. Hellman et al. [9] and Sun and Nesbitt [14]).
Acknowledgements INAA data were obtained in the laboratory of Dr. F. Frey (Massachusetts Institute of Technology). We thank S. Roy for technical assistance. This work was completed whilst J.N. Ludden was in receipt of a post-doctoral scholarship and Ocean Industry Program research grant 28/31.00 at Woods Hole Oceanographic Institution. In addition, National Science Foundation grant 22971 is acknowledged.
References 1 C.H. Langmuir, J.E. Bender, A.E. Bence and G.N, Hanson, Petrogenesis of basalts from the Famous area: Mid-Atlantic Ridge, Earth Planet. Sci. Lett. 36 (1977) 133. 2 J.G. Schilling, Iceland mantle plume, a geochemical study of the Reykjanes Ridge, Nature 242 (1973) 565. 3 J.G. Schilling, Azores mantle blob, rare earth evidence, Earth Planet. Sci. Lett. 25 (1975) 103. 4 H. Puchelt, and R. Emmerman, REE characteristics of ocean floor basalts from the MAR 37°N (leg 37, DSDP), Contrib. Mineral. Petrol. 62 (1977) 43. 5 F.A, Frey, W.B. Bryan and G. Thompson, Atlantic Ocean floor: geochemistry and petrology of basalts from Legs 2 and 3 of the Deep Sea Drilling Project, J. Geophys. Res. 79 (1974) 5507. 6 G. Thompson. A geochemical study of the low-temperature interaction of sea-water and oceanic igneous rocks, EOS 54 (1973) 1015. 7 D.A. Wood, I.L. Gibson and R.N. Thompson, Elemental mobility during zeolite facies metamorphism of the Tertiary basalts of eastern Iceland, Contrib. Mineral. Petrol. 55 (1976) 241. 8 P.L. Hellman and P, Henderson, Are rare earth elements mobile during spilitization?, Nature 267 (1977) 38. 9 P.L. Hellman, R.E. Smith and P. Henderson, Rare earth element investigation of the Cliefden outcrop, N.S.W., Australia, Contrib. Mineral. Petrol. 65 (1977) 155. 10 J.N. Ludden and G. Thompson, The behavior of the rare earth elements during submarine weathering of tholeiitic basalt, Nature (in press). 11 G. Thompson and M.L. Rivers, The low-temperature weathering of oceanic basalts by seawater, 1. Major element fluxes, Geol. Soc. Am. Abstr. Progr. 8 (1976) 1137. 12 S. Henrichs and G. Thompson, The low-temperature weathering of oceanic basalts by seawater, 2. Trace element fluxes, Geol. Soc. Am. Abstr. Progr. 8 (1976) 1098. 13 G. Thompson and S. Humphris, Seawater-rock interactions in the oceanic basement, in : Proceedings 2nd International Symposium on Water-Rock Interaction, Vol. III, H. Parquet and Y. Tardy, eds. (Science G6ologique, Universit6 Louis Pasteur, Strasbourg, 1977) 3-8.
92 14 S.S. Sun and R.W. Nesbitt, Petrogenesis of ultrabasic and basic volcanics: evidence from rare earth elements, Contrib. Mineral. Petrol. 65, (1978) 301. 15 J.G. Schilling, Rare-earth variations across "Normal Segments" of the Reykjanes Ridge 60 ° - 5 3 ° N, Mid Atlantic Ridge, 29°S, and East Pacific Rise, 2 - 1 9 ° S, and evidence on the composition of the underlying low-velocity layer, J. Geophys. Res. 80 (1975) 1459. 16 J.A. Pearce and J.R. Cann, Tectonic setting of basic volcanic rocks determined using trace element analyses, Earth Planet. Sci. Lett. 19 (1973) 290. 17 P.A. Floyd and J.A. Winchester, Magma type and tectonic setting discrimination using immobile elements, Earth Planet. Sci. Lett. 27 (1975) 211. 18 R.A. Copeland, F.A. Frey and D.R. Wones, Origin of clay minerals in a Mid-Atlantic Ridge sediment, Earth. Planet. Sci. Lett. 10 (1971) 186. 19 H. Staudigel, F.A. Frey and S.R. Hart, Incompatible trace element geochemistry and 87 Sr/86 Sr in basalts and corresponding glasses and palagonites, in: Initial Reports of the Deep Sea Drilling Project, Vol. 53 (in press).
20 S.R. Hart and H. Staudigel, Ocean crust-seawater interaction: Site 417 and 418, in: Initial Reports of the Deep Sea Drilling Project, Vol. 53 (in press). 21 S.E. Humphris, Geochemistry of long-term seawater-basalt reactions at low temperatures DSDP Sites 4 i 7A and 418A, Geol. Soc. Am. Abstr. Progr. 10 (1978) 426. 22 E.D. Goldberg, M. Koide, R.A. Schmitt and R.H. Smith, Rare earth distributions in the Marine environment, J. Geophys. Res. 68 (1963) 4209. 23 O.T. Hogdahl, S. Melsom and V.T. Bowen, Neutron activation analysis of lanthanide elements in seawater, Adv. Chem. 73 (1968) 308. 24 D.Z. Piper, Rare earth elements in the sedimentary cycle: a summary, Chem. Geol. 14 (1974) 285. 25 G.P. Glasby, Mechanisms of enrichment of the rarer elements in marine manganese nodules, Mar. Chem. 1 (1972-73) 105. 26 S.R. Taylor and M.P. Gorton, Applications of spark source mass spectrometry, III. Element sensitivity precision and accuracy, Geochim. Cosmochim. Acta 41 (1977) 1 3 7 5 1380.
85
[61
AN EVALUATION OF THE BEHAVIOR OF THE RARE EARTH ELEMENTS DURING THE WEATHERING OF SEA-FLOOR BASALT JOHN N. LUDDEN Woods Hole OceanographicInstitute, Woods Hole, iliA 02543 {U.S.A.) and Ddpartement de G~ologie *, Universitd de MontrdaL Montreal, Que. H3C 3J7 {Canada)
and G E O F F R E Y THOMPSON Woods Hole Oceanographic Institute, Woods Hole, MA 02543 {U.S.A.]
Received June 26, 1978 Revised version received December 12, 1978 We present rare earth element (REE) data for fresh and altered tholeiitic basalts sampled during a dredging transect at 23°N in the Atlantic Ocean and covering a time span of 0 to 57 million years. These data have been used to evaluate the behavior of the REE during low-temperature weathering processes, Compositional trends from altered basalt interiors to palagonitized rinds in individual pillow samples indicate significant mobility of the light REE: some elements are enriched by four orders of magnitude in rinds relative to interiors. The heavy REE show no selective mobilization and can be used in a normalization procedure which indicates that the light REE are enriched in altered interiors relative to fresh interiors of the basalts. Cerium behaves anomalously and accords with either its abundance in seawater or its fractionation from seawater during the formation of ferromanganese deposits. These results indicate that REE data from fresh glassy or crystalline basalt samples only may be used with confidence in petrological models.
1. Introduction The rare earth elements (REE), when considered relative to their abundance in chondrites, are commonly used as a critical diagnostic tool in interpreting the petrogenesis o f sea-floor basaltic rocks and glasses (e.g. [ I - 4 ] ) . In particular, the relative depletion in the light REE (LREE) is used to discriminate between basaltic magma types. Although Frey et al. [5] and Thompson [6] demonstrated some mobility o f the LREE during low-temperature weathering, and more recently Wood et al. [7], Hellman and Henderson [8] and Hellman et al. [9] presented data that Woods Hole Oceanographic Institute Contribution No. 4240. * Address for reprints.
indicate mobility o f the LREE, in particular La, during low-grade (prehnite-pumpellyite facies) metamorphism, REE data from altered basalts are still commonly interpreted to represent a primary "petrological fingerprint". The objective o f this study was to determine whether the REE are mobilized during weathering o f basaltic rocks at ambient seawater temperatures (low-temperature weathering). The preliminary results o f this study (samples < 5 m.y.), were presented by Ludden and Thompson [10]. In addition to an evaluation o f the data for these younger samples, in this paper we discuss data obtained for altered basalts up to 57 m.y. The samples were taken from a suite o f basalts recovered during a dredging transect at 23°N, at distances o f 0 - 7 2 5 k m from the Mid-Atlantic Ridge (Fig. 1). These samples represent a time sequence o f
86 basalts erupted at a single point on the ridge axis over the past 57 m.y. (age dates are based on the sample position relative to sea-floor magnetic anomalies, see Fig. 1). Thompson and Rivers [11], Henrichs and Thompson [12] and Thompson and Humphris [13] presented major element data and some trace element data for these rocks. For this study we selected 11 basalts which span the entire 0-57-m.y. time sequence. The sampling strategy was based on the known major and trace element compositions of the basalts; we selected: fresh (0 m.y.) samples which represented the most-fractionated and least-fractionated rocks sampled at the ridge crest, and samples of 2.5-57 m.y. which displayed variable degrees of alteration. For five of the altered samples we determined the REE abundances of the crystalline interiors and palagonitized rinds of the individual basalt pillow. To compare the relative effect of low- and
45°-NOBTH'
,/~-.~'~
/
.~r~/\@0 "
////
x'~-" I,'7"
// 30 ° ~
l
/ / I
~
I/I ,',',, /
I
/~
, /
.
/
~--'--~ '-""--~_
, I
,
/
I ~
I o. AZORES
.,o'~~ o,Z" ~,.~'1." '
/ ~
,,"9 /'b
//
/~
~
/ I / 39 ~sy.,/.,;," , -" / I
I
,. , ~ • SITE 396 ,~SURFA_C_E/// / / , ,
,
75 °
/
,
/
,/
/
I
/
// // ~\3
AI
I
A
I
60 °
45 Q
30 °
15 °
TRANSECT AT 23°N SHOWING BATHYMETRY, DREDGE LOCATIONS (Q AND MAGNETIC ANOMALY AGES W
52 °
51 ° ~ - - T
I 53
50 °
49 ° 1
I my
38
48 ° 1
47 ° •
46 ° 1
t my
9
45 ° T -
-
44 ° r
E
I my
O my
B
Fig. 1. A. Location map showing the position of the surface transect relative to magnetic anomalies and the nearest DSDP sites in the North Atlantic Ocean. B. Transect at 23° N showing bathymetry, dredge locations (indicated by an arrow) and magnetic anomalies.
high-temperature alteration on the REE we also analysed a 0-m.y. hydrothermally altered basalt (metabasalt), sampled from the ridge crest at 23°N.
2. Results
REE abundances are given in Table 1. H20 ÷, H20- and F%O3/FeO ratios are given as a measure of the extent of weathering of the basalts. In natural geochemical environments the REE usually exhibit a trivalent oxidation state. The principal exceptions to this are Ce and Eu. In oxidizing environments Ce may be present as Ce 4÷, and as demonstrated later, this results in Ce showing departures from a smoothed chondrite- or shale-normalized REE profile. The ratio Ce/Ce* given in Table 1 reflects the departure of Ce from the smoothed chondrite-normalized profile between La and Nd. Europium may be present as Eu 2÷ and Eu 3÷, resulting in the commonly observed Eu anomaly in basaltic liquids. The most common explanations of an Eu anomaly are that it is caused by plagioclase fractionation or that it represents a source characteristic of the melt. However, Sun and Nesbitt [14] interpret variable Eu anomalies in Archean metabasalts as an alteration effect; this possibility is discussed in relation to our data for sea-floor basaltic rocks. The fresh (0-m.y.) basalts represent pristine tholeiitic compositions recently erupted at the spreading center at 23°N; both samples show low H20+ and F%O3/FeO. The extent of weathering of the crystalline interiors of the basalts older than 0 m.y. is reflected by increasing abundances of H20+ (0.513.91 wt.%) and increasing Fe203/FeO ratio (0.7615.35). Although all the crystalline interiors of the basalts are altered relative to the fresh basalts, the degree of alteration markedly increases with age from 0 to 10 m.y.,but is variable from 10 to 57 m.y. The degree of alteration apparently depends on the extent of fracturing (and thus access of water) and the amount of matrix glass present in an individual sample (glass being particularly susceptible to alteration [6]). The glassy rinds of the basalts are completely palagonitized by 4 m.y.: fresh glass has been found in samples several tens of m.y. old obtained by deep ocean drilling (e.g. Legs 51-53 of the DSDP). Our observation of complete palagonitization in 4 m.y.
87 TABLE 1 REE concentrations (in ppm) for fresh and altered tholeiitic basalts from the ridge crest and surface transect at 23°N, Atlantic Ocean Age (m.y.)
0
0
2.5
4.5
4.5
5.0
5.0
7.5
30.5
Sample
3-103A int.
3-106 int.
6-3 int.
7-3A mar.
7-3B int.
8-16A mar.
8-16B int.
10-3 int.
12-2 int.
La Ce Nd Sm Eu Tb Ho Yb Lu
1.67 5.86 5.33 2.23 0.90 0.56 0.83 2.02 0.34
3.75 13.68 11.42 4.23 1.52 0.95 1.38 3.82 0.62
4.46 15.82 12.96 4.67 1.58 1.08 1.42 3.73 0.63
30.08 43.97 34.48 9.37 2.58 1.58 2.33 6.25 0.99
4.50 14.65 12.21 4.83 1.60 0.96 1.47 4.15 0.68
16.35 24.78 19.70 5.30 1.63 1.06 1.49 4.14 0.72
6.23 16.58 12.28 4.27 1.32 0.75 1.14 3.09 0.52
2.29 9.20 6.90 2.57 1.04
5.63 15.02 12.79 4.58 1.55
0.72 2.34 0.34
1.52 3.99 0.64
H2 O+ Fe203/FeO
0.11 0.13 0.18
0.20 0.18 0.22
0.51 0.38 0.76
7.39 9.47 27.20
2.03 0.99 2.08
11.14 14.40 17.16
2.37 1.64 3.37
1.32 0.96 0.86
2.55 1.64 2.90
Ce/Ce*
1.08
1.04
1.09
0.62
1.06
0.65
0.95
1.17
0.91
BCR-1 b
H20-
Age (m.y,)
35
35
46
46
57
57
0
BCR-1
Sample
14-2A mar.
14-2B int.
15-2A mar.
15-2B int.
16-6A mar.
16-6B int.
1-3 a
found
La Ce Nd Sm Eu Tb Ho Yb Lu
16.48 57.68 24.92 7.34 2.09 1.22 1.71 5.17 0.86
6.85 13.32 12.76 4.49 1.53 0.94 1.30 3.58 0.59
15.79 11.88 14.64 4.13 1.33 0.74 0.98 2.87 0.49
3.47 6.94 6.49 2.28 1.01 0.64 0.88 2.68 0.45
13.67 15.92 18.19 4.97 1.90 1.16 1.54 4.16 0.69
5.35 16.91 9.57 2.87 1.10 0.75 1.16 3.46 0.69
2.44 8.33 7.38 2.97 0.95 0.68 1.02 2.72 0.46
H20 ÷ H20Fe203/FeO
11.54 14.18 28.50
1.42 1.29 1.31
5.67 3.95 13.60
2.48 1.35 2.00
10.14 13.53 16.14
3.91 2.57 15.35
1.35 0.18 0.42
1.40
0.71
0.33
0.73
0.47
1.19
1.05
Ce/Ce *
24.60 53.60 26.60 6.1 1.88 1.10 1.18 3.32 0.53
Chondrites
24.2 53.7 28.5 6.7 1.95 1.08 1.33 3.70 0.55
0.33 0.88 0.60 0.181 0.069 0.047 0.070 0.200 0.034
INAA analysis of "dry" rock powders, int. = interior of basalt pillow; mar. = basalt pillow margin. a Metabasalt from ridge crest at 23°N. b Compilation of values from Taylor and Gorton [26].
m a y o n l y b e valid for d r e d g e d samples w h i c h h a v e b e e n e x p o s e d for long p e r i o d s o f t i m e t o large v o l u m e s o f seawater. The h i g h F e 2 0 3 / F e O ratios o f s o m e s a m p l e s m a y reflect s o m e i n c o r p o r a t i o n o f ferromanganese deposit on the exposed portion of t h e pillow.
2.1. Weathered basalts and fresh basalts C h o n d r i t e - n o r m a l i z e d R E E profiles for t h e basalts are p l o t t e d in Fig. 2. T h e b e h a v i o r o f t h e R E E d u r i n g p a l a g o n i t i z a t i o n o f t h e glassy rinds o f t h e basalts is s h o w n b y c o m p a r i n g t h e R E E profiles o f t h e crystal-
88 line interiors and palagonitized rinds of the 4.5-, 5.0-, 35-, 46- and 57-m.y. basalts: (1) The LREE ( L a - S m ) in the palagonitized rinds are selectively enriched relative to the interiors of the pillow basalts. (2) Cerium behaves inconsistently, and may be either depleted or enriched relative to the basalt interior. (3) The heavy REE (HREE; E u - L u ) show a uniform increase in abundance with alteration, but do not show selective mobilization. (4) Contrary to the conclusion o f Sun and Nesbitt [14], there are no selective changes in the Eu abundance. One of the problems o f interpreting changes in the chemical composition o f altered rock samples is the determination o f the composition o f their fresh precursor. There is no selective mobilization o f the HREE. Thus, for the pillow basalt samples for which we have data for the palagonitized rinds and crystalline interiors, it is possible to quantify the alteration process by assuming a constant composition for the HREE. In Table 2 we have presented the results of such a normalization procedure. In this case we have assumed a constant Yb composition between margin and interior: these data emphasize observations 1 - 4 above. In Fig. 2 we have plotted data for two fresh tholeiites (3-103 and 3-106), these samples represent the entire compositional range for rocks sampled at the ridge crest at 23°N. Both samples show a LREE pattern characteristic o f large-ion-lithophile (LIL)-
TABLE 2
La Ce Nd Sm Eu Tb Ho Yb Lu
4.5 m.y.
5.0
m.y.
35 m.y.
46 m.y.
57 m.y.
4.44 1.99 1.87 1.29 1.07 1.09 1.05 1.00 0.97
1.96 1.12 1.20 0.92 1.05 0.97 0.97 1.00 1.03
1.67 3.00 1.35 1.13 0.96 0.90 0.92 1.00 1.00
4.25 1.83 2.01 1.69 1.22 1.07 1.03 1.00 1.02
2.13 0.78 1.58 1.44 1.43 1.28 1.10 1.00 0.83
WEATHERED BASALT 57 m.y D~D__C]
~'/~ ~'~
\16-6
i°La
t Ce
mm~mm---m--~m--
m~-----
m--m
NJd
i Ha
J ymb Lu
~ Sm
h Eu
]"b
50u
WEATHERED BASALT 46 m y
I
,
5 o ~ J m ~ '~[
L
~r"
10 ,
O I
5o -
T
,
I
-I
WEATHERED BASALT 35 m y
f III
c~_____D\.~. ° i/
~'14-2
I
I
I
L
I
I
i
i
WEATHERED BASALT 5 0 m¥
fJ O
\8.16
INTERIOR
~
~ m - - m
~00~ [~
WEATHERED BASALT 4 5 m y
i / I ~ m ~ i
// 10 I
50
Enrichment of the LREE shown by the ratio of palagonitized basalt rinds relative to interiors after normalization to constant Yb Sample
F
SO~
i
i
k
•
•
i
i
i
I
i
WEATHERED BASALT 2.5 m y /FRESH BASALT / / FRACTtONATED
L ~ ~ : ~ ~/
.~'~=~.~,~. j~ . . . .
I
')
mm '<'-~j
S~/
~ 3 103
I La
=
~7-3
l Ce
L Nd
. ~_
FRESHBASALT-
. ~ ' - - -~
i
~
I
~"~METABASALT
UNFRACTIONATED J Sm
I Eu
I Tb
i Ha
t Yb
REE ATOMIC NUMBER
Fig. 2. Chondrite-normalized REE profiles for selected basalts sampled during the dredging transect at 23° N. Data for fresh basalts from the ridge crest at 23°N are plotted in the lowest diagram, the remaining diagrams show the REE prof'fles for the palagonitized rinds (open squares) and weathered interiors (solid squares) of pillow basalt samples.
Lu
89 element-depleted tholeiites from the mid-oceanic ridges (i.e. Frey et al. [5] and Schilling [15]). In this paper we are not concerned with petrographic interpretations. However, the differences in the patterns are inferred to reflect fractionation of a REE-poor phase such as olivine. No fresh samples which are considered enriched in LIL elements have been sampled from the ridge axis or surface transect at 23°N. The total range in abundance of immobile elements, i.e. Ti, Zr and Y [16,17], for both fresh and weathered samples assuming constant volume in the altered samples, is Ti, 0.9-1.5 wt.%; Zr, 6 0 - 1 0 0 ppm; Y, 2 0 - 3 0 ppm. Thus, as indicated by Thompson and coworkers [11-13], we have assumed that the fresh basalts at 23°N are representative of the compositional range of basalts erupted from the ridge axis over the past 57 m.y. Using this assumption we performed a normalization of the weathered basalts to the fresh basalts sampled at 23°N (i.e. to an average of samples 103A and 106). The results are shown in Fig. 3. The shaded area represents the range in abundance for the fresh basalts. Any element falling outside this region is considered to have been mobilized during weathering. The constancy of the HREE favour our argument for such a normalization procedure. The normalization confirms our observations for the palagonitized rinds. In addition, the altered interiors of the basalts show LREE enrichment whilst the HREE are constant. However, not all the basalt interiors show significant departures from the pattern of their fresh precursor. In particular the relatively unweathered 2.5-, 4.5- and 7.5-m.y. interiors preserve the profile of their precursor, whilst the more weathered basalt interiors (i.e. the 5.0-, 30.5, 35-, 46- and 57-m.y. samples) show significant LREE mobilization. Recent studies of incompatible element mobility in sea-floor basalts and associated glassy and palagonitized rinds [19,20], Sites 417 and 418 of the DSDP) indicate that the palagonitized samples do not show enriched LREE patterns. We interpret this difference in terms of the environment of alteration affecting each suite of samples. The drilled samples are from relatively deep within the oceanic crust: water circulation within the crust ceases soon after crustal formation [20]. The dredged samples from 23°N have been exposed to seawater throughout their lifetime. Thus, the two environments must
5 ' 4
5 j [ 4
3 i'
,~,
21
":
i
Basah Margin 46 my Basalt Imerlor 46 my
.
Basah Ma%l~' 5 my
*
,
•
4}
gasall Margin 57 my Basal[ Interior 57 my
• ,
8asall Margin 37 my Basan line,lot 37 m~
* *
~2 3 I
i 5
•
"
15
•
Basall Marg,n 4 5 rn y •
[r
, ,
~,al:l,nnate# Thole,,le
t
, ,
~Ble4de ;
[a Ce
Nd
Sm~u
Ib
HO
Yb lu
la Ce
; [{esh Oasalfs al ~3°Q Nd
BmEu
Tb
Ho
Yb Lu
Fig. 3. REE profiles for altered basalts after normalization to
a constant Yb value (see text for explanation). The shaded region represents the compositional range for fresh basalts at 23° N. Samples which plot outside this region are inferred to have an altered REE pattern (the solid circles represent the palagonitized rinds and the open circles the weathered interiors of individual pillow samples).
differ in terms of, water chemistry, temperature and most importantly, water/rock ratio. We agree with Staudigel et al.'s [19] caution that the study of dredged samples may not be directly applicable to basalts from deep within the oceanic crust. However, as alteration environments may be completely different in adjacent DSDP holes (e.g. [21]) petrological interpretations based on REE in altered rocks may be misleading.
2.2. Metabasalt {1-3) A detailed study of alteration during high-temperature sea-floor metamorphism was not the subject of this study. However, in Fig. 2 we present REE data for a metabasalt sampled from the ridge crest at 23°N. This sample appears to have preserved the profide of its precursor. We recommend that this interpretation is not taken "ipso facto" as Wood et al. [7], Hellman and Henderson [8], Hellmann et al. [9] and Sun and Nesbitt [14] have presented data that indi-
90 k--
~-loo"
,..-.,
z o
SEA-WATER x
10 7
so
U I---
V
<
~
I0 ~
.~o,~...
I
I
Ce
Nd
I
I
Sm Eu
I
I
Tb
Ho
I
I
Yb Lu
REE A T O M I C N U M B E R
Fig. 4. The REE abundance of seawater normalized to chondrites (data from Goldberg et al. [22], solid circles, and Hogdahl et al. [23], open circles).
cate mobility of REE during prehnite-pumpellyite and greenschist facies metamorphism. Using REE data Copeland et al. [18] postulated an origin for coarse-grained marine chlorites from ridge greenstones. Thes chlorites preserve a LREE profile similar to the greenstones and exhibit igneous La/Sm values of approximately 4.0 in comparison to typical sedimentary values of 65. We tentatively suggest that the REE profile of greenschist grade metamorphic rocks may be preserved by the stabilization of the REE in chlorite.
Cerium is depleted in most samples; its enrichment in the 35-m.y. and 4.5.-m.y. rinds may reflect fractionation of Ce 4÷ from seawater during the formation of ferromanganese components (these samples have high manganese and cobalt contents relative to the other palagontie rinds [11,12]). The ternary plot of La(e.f.), Ce(e.f.) and Yb(e.f.) (Fig. 5) demonstrates the significance of these data to petrological arguments: La was selected as it is the most mobile of the LREE; Ce due to its anomalous behaviour as a result of oxidation to Ce4*; Yb as it represents the stable HREE. The tie lines joining the interiors of the altered samples with their palagonitized rinds trend towards the seawater composition for all samples except the 35-m.y. basalts. The high Fe203/FeO for the 35-m.y. sample indicate the possible incorporation of some ferromanganese components, although the 4.5-m.y. palagonitized rind, which also shows high Fe203/FeO does not show the same trend towards manganese nodule compositions. The most important implications of this diagram are
La (e.f.)
3. Interpretation and Petrographic significance One explanation for the behavior of the REE during low-temperature weathering is that the palagonitized glassy rinds and weathered basalt interiors have equilibrated with the REE abundance of seawater. The extent of equilibration will depend on seawater/rock ratio: equilibration should also be faster for the glassy rinds and glassy mesostasis of the basalts. Fig. 4 shows the chondrite-normalized REE profile for a seawater ×107 [22,23]. Relative to chondrites seawater displays a decreasing enrichment with increasing atomic number for the LREE and a relatively constant HREE pattern except that seawater also shows a depletion in Ce which may be attributed to the oxidation of Ce to its tetravalent state and its subsequent fractionation from seawater: in particular Ce fractionation occurs during the formation of ferromanganese deposits [24,25]. The observed profiles for the altered samples, in particularly the behavior of Ce in the oldest samples, support this argument.
L IL
Yb (of.}
DEPLETED THOLEIITES
I
Ce (e.f.)
Fig. 5. Ternary plot of La, Ce, Yb (enrichment factor relative to chondrites; e.f.). The open squares represent palagonitized rinds and the solid squares altered interiors of individual pillows joined by tie lines which indicate the effect of low-temperature alteration of basaltic glass. The 23 ° N samples are plotted relative to fields for depleted tholeiitic basalts (Leg 2, DSDP), LIL-element-enriched tholeiitic basalts (Leg 37, DSDP), manganese nodules, seawater and the Hawaiian alkali basaltic and tholeiitic suite. The contours are generalized fields for the extent of alteration represented by wt.% H2 O÷.
91 derived from consideration of the positions of the basalt rinds relative to the basalt interiors. The trend lines joining the two compositions indicate the behavior of the REE during palagonitization. The orientation of these trends relative to the fields for LILelement-depleted tholeiites and LIL-element-enriched tholeiites (data are taken from DSDP Leg 3 [5] ; and selected data from Initial Reports o f the Deep Sea Drilling Project, Vol. 37) indicate that the trend from fresh basalt to weathered basalt and palagonitized glass results in an original depleted abundance changing to one resembling that of enriched sea-floor tholeiitic basalts (this is also apparent in the REE profiles, Figs. 2 and 3): Frey et al. [5] reached a similar conclusion from a study of altered glasses from Legs 2 and 3 of the DSDP. In addition, relatively small amounts of weathering, in some cases for samples containing as little as 2% H20+, may result in a significant LREE enrichment (e,g. the 5-m.y. basalt interior). Another point emphasized by the diagram is that, as the HREE are not selectively mobilized, the altered REE profiles do not resemble those profiles observed for basalts from oceanic islands (i.e. Hawaii) which, generally, exhibit a greater fractionation of the HREE.
4. Condusions We have presented data that indicate the REE abundances of oceanic basalts are subject to modification during weathering processes. Interpretations of the REE abundances of weathered basalts should thus be treated with caution. Although we have taken an extreme case, i.e. samples from an environment with a high water/rock ratio, we suggest that petrogenetic models based on REE abundances be restricted to fresh glass or fresh whole rock compositions. Where fresh samples are unavailable (i.e. in Archean terrains) sampling should be very selective, and, arguments based on the REE abundance should be used only where supported by data for immobile elements, i.e. Ti, Zr, Y, Nb. Our data suggest that alteration processes will not always result in a consistency of REE profile as has been considered recently (i.e. Hellman et al. [9] and Sun and Nesbitt [14]).
Acknowledgements INAA data were obtained in the laboratory of Dr. F. Frey (Massachusetts Institute of Technology). We thank S. Roy for technical assistance. This work was completed whilst J.N. Ludden was in receipt of a post-doctoral scholarship and Ocean Industry Program research grant 28/31.00 at Woods Hole Oceanographic Institution. In addition, National Science Foundation grant 22971 is acknowledged.
References 1 C.H. Langmuir, J.E. Bender, A.E. Bence and G.N, Hanson, Petrogenesis of basalts from the Famous area: Mid-Atlantic Ridge, Earth Planet. Sci. Lett. 36 (1977) 133. 2 J.G. Schilling, Iceland mantle plume, a geochemical study of the Reykjanes Ridge, Nature 242 (1973) 565. 3 J.G. Schilling, Azores mantle blob, rare earth evidence, Earth Planet. Sci. Lett. 25 (1975) 103. 4 H. Puchelt, and R. Emmerman, REE characteristics of ocean floor basalts from the MAR 37°N (leg 37, DSDP), Contrib. Mineral. Petrol. 62 (1977) 43. 5 F.A, Frey, W.B. Bryan and G. Thompson, Atlantic Ocean floor: geochemistry and petrology of basalts from Legs 2 and 3 of the Deep Sea Drilling Project, J. Geophys. Res. 79 (1974) 5507. 6 G. Thompson. A geochemical study of the low-temperature interaction of sea-water and oceanic igneous rocks, EOS 54 (1973) 1015. 7 D.A. Wood, I.L. Gibson and R.N. Thompson, Elemental mobility during zeolite facies metamorphism of the Tertiary basalts of eastern Iceland, Contrib. Mineral. Petrol. 55 (1976) 241. 8 P.L. Hellman and P, Henderson, Are rare earth elements mobile during spilitization?, Nature 267 (1977) 38. 9 P.L. Hellman, R.E. Smith and P. Henderson, Rare earth element investigation of the Cliefden outcrop, N.S.W., Australia, Contrib. Mineral. Petrol. 65 (1977) 155. 10 J.N. Ludden and G. Thompson, The behavior of the rare earth elements during submarine weathering of tholeiitic basalt, Nature (in press). 11 G. Thompson and M.L. Rivers, The low-temperature weathering of oceanic basalts by seawater, 1. Major element fluxes, Geol. Soc. Am. Abstr. Progr. 8 (1976) 1137. 12 S. Henrichs and G. Thompson, The low-temperature weathering of oceanic basalts by seawater, 2. Trace element fluxes, Geol. Soc. Am. Abstr. Progr. 8 (1976) 1098. 13 G. Thompson and S. Humphris, Seawater-rock interactions in the oceanic basement, in : Proceedings 2nd International Symposium on Water-Rock Interaction, Vol. III, H. Parquet and Y. Tardy, eds. (Science G6ologique, Universit6 Louis Pasteur, Strasbourg, 1977) 3-8.
92 14 S.S. Sun and R.W. Nesbitt, Petrogenesis of ultrabasic and basic volcanics: evidence from rare earth elements, Contrib. Mineral. Petrol. 65, (1978) 301. 15 J.G. Schilling, Rare-earth variations across "Normal Segments" of the Reykjanes Ridge 60 ° - 5 3 ° N, Mid Atlantic Ridge, 29°S, and East Pacific Rise, 2 - 1 9 ° S, and evidence on the composition of the underlying low-velocity layer, J. Geophys. Res. 80 (1975) 1459. 16 J.A. Pearce and J.R. Cann, Tectonic setting of basic volcanic rocks determined using trace element analyses, Earth Planet. Sci. Lett. 19 (1973) 290. 17 P.A. Floyd and J.A. Winchester, Magma type and tectonic setting discrimination using immobile elements, Earth Planet. Sci. Lett. 27 (1975) 211. 18 R.A. Copeland, F.A. Frey and D.R. Wones, Origin of clay minerals in a Mid-Atlantic Ridge sediment, Earth. Planet. Sci. Lett. 10 (1971) 186. 19 H. Staudigel, F.A. Frey and S.R. Hart, Incompatible trace element geochemistry and 87 Sr/86 Sr in basalts and corresponding glasses and palagonites, in: Initial Reports of the Deep Sea Drilling Project, Vol. 53 (in press).
20 S.R. Hart and H. Staudigel, Ocean crust-seawater interaction: Site 417 and 418, in: Initial Reports of the Deep Sea Drilling Project, Vol. 53 (in press). 21 S.E. Humphris, Geochemistry of long-term seawater-basalt reactions at low temperatures DSDP Sites 4 i 7A and 418A, Geol. Soc. Am. Abstr. Progr. 10 (1978) 426. 22 E.D. Goldberg, M. Koide, R.A. Schmitt and R.H. Smith, Rare earth distributions in the Marine environment, J. Geophys. Res. 68 (1963) 4209. 23 O.T. Hogdahl, S. Melsom and V.T. Bowen, Neutron activation analysis of lanthanide elements in seawater, Adv. Chem. 73 (1968) 308. 24 D.Z. Piper, Rare earth elements in the sedimentary cycle: a summary, Chem. Geol. 14 (1974) 285. 25 G.P. Glasby, Mechanisms of enrichment of the rarer elements in marine manganese nodules, Mar. Chem. 1 (1972-73) 105. 26 S.R. Taylor and M.P. Gorton, Applications of spark source mass spectrometry, III. Element sensitivity precision and accuracy, Geochim. Cosmochim. Acta 41 (1977) 1 3 7 5 1380.