Isotopic evidence for uranium exchange during low-temperature alteration of oceanic basalt

Earth and Planetary Science Letters, 42 (1979) 27-34 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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ISOTOPIC EVIDENCE FOR URANIUM EXCHANGE DURING LOW-TEMPERATURE ALTERATION OF OCEANIC BASALT J.D. MACDOUGALL, R.C. FINKEL, J. CARLSON Scripps Institution o f Oceanography, La Jolla, CA 92093 (U.S.A.) and S. KRISHNASWAMI Physical Research Laboratory Ahmedabad, 380009 (India)

Received August 8, 1978 Revised version received October 17, 1978

Measurements of uranium concentration and the 234U/238 U activity ratio in oceanic basalts which have undergone low-temperature seafloor alteration indicate that uranium uptake is a pervasive occurrence but that the various phases involved behave differently with respect to this process. Palagonite exhibits uranium contents 8-20 times higher than unaltered glass coupled with low 234U/238U, suggesting ongoing preferential leaching of 234U. Altered crystalline interiors of several old basalts have 234U/238U > 1, indicative of recent uranium exchange with seawater. The data also provide evidence for uranium sources with 234U/238Uhigher than the seawater value of 1.14. Manganese crusts on basalts of a variety of ages have isotopic ratios indicating that they either are recent deposits or also have experienced continuing uranium exchange with seawater.

1. Introduction Low-temperature alteration o f basalts on the seafloor has received considerable attention because of its importance in the overall context of element fluxes in the ocean [ 1 - 3 ] . It is possible, for example, that this process provides a quantitatively important source of calcium, magnesium and some transition elements, and a sink for potassium and uranium [ 2 - 5 ] . Although there is reasonable certainty about the compositional changes which take place in the basalts themselves for the major elements, much less is known about minor and trace elements. At least in part this is due to the much wider variation in trace element concentrations in fresh basalts, and the consequent difficulty in selecting a suitable value for a particular element with which to compare concentrations in altered samples. In this paper we investigate the case of uranium

exchange in altering oceanic basalts, with particular emphasis on the isotopic composition o f uranium and its implications for the time scale of the exchange. For most of the samples investigated it was possible to use uranium concentrations in associated unaltered glass as a measure of the whole-rock uranium at the time of eruption. Previous studies (e.g. [5,6]) have shown that uranium concentrations in altered oceanic basalts are frequently an order of magnitude or more greater than in their fresh counterparts. Aumento [6] documented a general increase in whole-rock uranium contents as a function o f distance from the Mid-Atlantic Ridge for a series of dredged tholeiites, and argued that basalts continuously gain uranium at a rate of about 1 ppm per 10 m.y. However, other studies have shown little evidence for such regular behavior. Thompson [3] found no correlation between uranium

28 concentration and degree of alteration as measured by water content. Macdougall [5] found generally lower uranium concentrations for highly altered rocks than those reported by Aumento [6] and also noted considerable variability even within single samples. He showed that uranium gained by altering basalts resides in palagonite, montmorillonite-family clay minerals, and possibly also dispersed fine-grained iron oxide alteration products. Coupled with concentration nreasurements in altered samples, the isotopic composition of uranium, specifically the activity ratio 234U/238U (hereafter written simply as 234U/238U), potentially can provide infornration about the nature and chronology of the exchange process, and the source of the added uranium. In seawater, 234U/238U = 1.14 -+ 0.03 [7]. Indigenous uranium in freshly erupted basalts is expected to be in secular equilibrium, with 234U/238U = 1.00 [8]. Thus, addition of seawater uranium will produce actvity ratios >1.0, which will return toward equilibrium with a half time of 2.48 X l0 s years, tire halflife of 234U. In principle, then, 234U/238U can be used both as a simple tracer for seawater uranium and as a chronometer to monitor the time scale over which uranium exchange occurs. In practice, as will become apparent from the data presented in this paper, the processes involved are complex and not easily deciphered. Some uranium isotopic measurements for altered oceanic basalts have already appeared in the literature [5,91. In [5], preliminary results for some of the samples reported in this paper were discussed. In [9], disequilibria were reported from several altered basalts, including 234U excesses for young (<1 m.y.) basalts, and =aaU depletions for older (>1 m.y.) basalts, relative to secular equilibrium values. Below we report further measurements for a larger suite of samples which help to clarify some aspects of uranium behavior during alteration.

ticle spectrometry. A 232U tracer was used ill order to determine concentrations: chemical yields were typically 50-80%. All whole-rock samples were sawed pieces of unifom>appearing interior portions of the basalt. Glass, palagonite and manganese crust samples were hand separated and picked under the binocular nricroscope. Both glass and palagonite gamples were at least 95% pure based on visual estimates. This is substantiated by the low uranium content measured for the AMPH 3D glass, and the consistent isotopic ratios for the palagonite samples. Powder X-ray diffraction patterns for two of the palagonite separates (D115, D113) showed only very small feldspar peaks, presumably from residual microphenocrysts. Some of the manganese oxide crust samples contain small amounts of palagonite (up to ~15c/), which is intergrown and extremely difficult to separate mechanically. However, the high uranium concentration of the manganese makes it relatively insensitive to small amounts of palagonite contamination. Chemical separation and counting of uranium for the four whole-rock Deep Sea Drilling Project (DSDP) samples were carried out at tlre Physical Research Laboratory, Ahmedabad, lndia, and the preliminary results referred to in [5] are for these samples. All others were processed at La Jolla, and the DSDP samples were also recounted at the La Jolla Mt. Soledad Radioisotope Laboratory.

3. Results The samples which were analyzed in this work span a range of ages and geographical locations. We discuss them below in three separate categories: (1) fresh young basalts; (2) a suite of samples from a traverse across the Mid-Indian Ocean Ridge (MIOR); and (3) old DSDP core basalts. 3.1. Fresh y o u n g basalts

2. Analytical procedures Uranium concentrations and isotope ratios for the samples measured in this work are given in Tables 1 and 2. For these analyses, uranium was separated by conventional ion exchange and solvent extraction techniques, electroplated and counted by alpha par-

In order to examine the assumption that secular equilibrium should obtain in young unaltered basalts, two samples dredged from the East Pacific Rise were measured. AMPH-3D is a glassy pillow basalt with an estimated age ~<106 years based on location and fission track studies (Macdougall, unpublished). Hart [1] and Hart et al. [10] analyzed a sample from the

29 same dredge haul for a series of elements and concluded that in spite of the fact that glassy portions seemed fresh based on some indicators, there was also evidence that potassium was being leached and cesium added from seawater. The uranium results (Table 1) show no evidence for uptake of seawater uranium. Although uranium removal by leaching cannot be ruled out, the fact that 234U/238U is at equilibrium suggests that it is not an ongoing process. It is well documented that leaching often preferentially removes 234U atoms which are in-situ products o f 238U decay, thus lowering 234U/23SU [11,12,17]. Basalt DS5-6 has an estimated maximum age of 1.5 m.y. [13]. The sample analyzed was cut from a fine-grained, uniformly grey region of a pillow interior. Although it has a thick manganese coating, the glassy margin of this rock is not palagonitized. An induced fission track uranium determination for the glassy margin gave the same result, within experimental errors, as the radiochemical determination for the interior sample (Table 1) suggesting negligible addition of seawater uranium to the crystalline portion of this pillow. For both these young basalts, uranium concentrations are low and 234U/238Uis at equilibrium. Because

precise ages are unavailable, it is not clear that this reflects the fact that these isotopes were in secular equilibrium at the time of eruption. However, it does imply that equilibrium is established before the onset of appreciable low-temperature alteration.

3.2. Basalts from the Mid-Indian Ocean Ridge Four basalts from the MIOR, ranging in age from 3 m.y. (D115) to 23 m.y. (D113), were analyzed. These samples fall along a traverse approximately perpendicular to the ridge axis (Fig. 1). Their ages were estimated from the magnetic anomaly data of Fisher et al. [14]. Chemical data for these samples have been reported by Engel et al. [15]. For the pillow fragments analyzed in this work, there is a visible progression in degree of alteration with age, although it is not entirely uniform. For each sample, uranium concentration and isotopic measurements were made by radiochemical methods for a crystalline interior sample, a separated palagonite sample, and a portion of the manganese crust. Concentration measurements in fragments of unaltered glass were made by the induced fission track technique Neither palagonite nor manganese crust thickness

TABLE 1 Whole-rock samples Sample

Young basalts AMPH 3D DS5-6

Location

12°52'S, 110°58'W 9°13'N, 105°12'W

Age (m.y.)

<1 <1.5

U (ppb) *

234 U/238 U (activity)

unaltered glass

crystalline interior

56 ± 2; 69 ± 1 (R) 103 ± 4

106 -+ 2

1.00 -+0.03 0.97 -+0.03

Mid-Indian Ocean Ridge Dl15 24°02'S,70°14.1'E Dl14 24°08.6'S, 72°26.6'E D143 23°43.7'S, 72°42.6'E Dl13 23°20.6'S, 74°56.5'E

3 12 13 23

52±2 58 ± 3 57 ± 3 36 -+ 2

67± 2 163 ± 3 68 ± 2 611 ± 10

0.99±0.03 1.19 ± 0.02 1.04 ± 0.04 1.14 -+0.02

Deep Sea Drilling Project 213-17-2-95 10°12.71'S, 93°53.77'E 213-19-2 238-56-1-144 11°09.21'S, 70°31.56'E 238-57-2-79

57 57 30 30

51 ± 2 47 ± 3 47 ± 2

380 -+ 15 620 ± 10 170 ± 6 570 ± 25

0.96 0.99 1.00 1.07

± 0.05 ± 0.02 ± 0.04 ± 0.02

* Interior samples were measured by radiochemical techniques and glass samples by fission track, except for one analysis of AMPH 3D glass made by radiochemistry (marked "R"). All errors are la counting errors.

30

70 °

20 °

"3, '5, '"3, '"5 0143

25*

?

BI3 •

25 °

\

"'3.

?"

"3. 70o

60*

90*

120*

I~) =

Fig. 1. Map showing the locations of the four Mid-Indian Ocean Ridge samples analyzed in this work, and magnetic anomaly pattern in the area [14] on which rock ages are based.

seems to be strictly correlated with age for these rocks; however, between the time of collection and sampling for this work they had been handled roughly and stored loose in burlap sacks inside wooden crates,

making it difficult to know whether or not original crust thicknesses are still preserved. For the crystalline interior samples, there is no clear-cut relationship between either uranium concen-

31 tration or isotopic ratio and age (Table 1). However, two of the four samples have low uranium concentrations, only marginally higher than those determined for glassy margins, and for both of these 234U/238U is at equilibrium. This implies little or no uptake or exchange of seawater uranium. It also serves to emphasize the variable nature of seawater-basalt interaction at low temperatures - the crystalline interior sample from D114, of approximately the same age as D143, has more than twice as much uranium and a distinctly non-equilibrium isotopic ratio. In contrast to D115 and D143, both D114 and D113 have high interior uranium contents relative to initial values inferred from the glass analyses, and also exhibit excesses of 234U. For D113, the uranium gain is approximately 550 ppb, and virtually all of this must have the current seawater isotopic composition. The limited data set does not permit a distinction to be made between continuous exchange and recent pickup. Sample D1 14 is particularly interesting. The increase in uranium content for this sample, using the glass analysis as the initial value, is 105 ppb. If the initial uranium is in secular equilibrium, then the added uranium component must have 234U/238U/> 1.29. Even for the unlikely case of recent complete exchange, the source must have 234U/238U/> 1.19. Both values are higher than seawater, although, considering the experimental uncertainty, only marginally so in the latter case. High ratios have also been observed for sediments from the East Pacific Rise [16].

It is conceivable that 234U-enriched source fluids are created by preferential leaching of 234U from underlying basalts, analogous to the case for continental groundwater (e.g. [ 17]). However, no such fluids have been measured from the oceanic environment. It is also rather curious that the manganese crust from this sample exhibits the lowest 234U/238U value for the MIOR rocks (Table 2). All others show evidence for recent exchange with seawater uranium. The palagonite and manganese crust measurements for these basalts provide an interesting contrast to the interior samples (Table 2). Uranium contents in the palagonite are uniformly high, ranging from 8 to 20 times that of associated unaltered glass, consistent with previous observations on the uranium distribution in altered basalts [5]. With the exception of D115, the youngest sample, all palagonite samples measured have substantial 234U depletions relative to secular equilibrium. These data imply preferential leaching of in-situ-produced 234U. They contrast with less extensive isotopic data reported by Bacon [9] who measured "margins" of two weathered pillow basalts described as "50% palagonitized" and "~50% palagonitized". One of these has an extremely high uranium content (7000 ppb), 234U/238U = 0.98 • 0.01 and a magnetic anomaly age of 5 m.y.; the other has U = 880 ppb, 234U/238U = 1.01 + 0.02 and a magnetic anomaly age of 46 m.y. The manganese crust samples also present a relatively consistent picture (Table 2). All have high uranium concentrations, in the range of those found

TABLE 2 Separated phases from Mid4ndian Ocean Ridge samples * Sample

Age (m.y.)

Manganese crust

Palagonite

Interior

D115:U (ppb) 234U/238U

3

6700 +- 100 1.09 -+ 0.02

493 +, 13 0.99 +, 0.04

67 +, 2 0.99 +, 0.03

Dll4:U (ppb) 234U/238U

12

5800 +, 100 0.98 +, 0.02

461 -+ 15 0.85 +- 0.04

163 +, 3 1.19 +, 0.02

D143:U (ppb) 234U/238U

13

5000 +- 100 1.04 -+ 0.02

470 -+ 11 0.84 +, 0.03

68 +, 2 1.04 +, 0.04

Dll3:U (ppb) 234u/Z38u

23

8300 • 100 1.05 +- 0.01

717 + 14 0.92 +, 0.03

611 +, 10 1.14 +- 0.02

* Isotopic ratios are activity ratios.

32 in manganese nodules (e.g. [18]), and other manganese crusts [19]. All but D114 have isotopic ratios greater than unity at the 1o error limit level. In each case the entire thickness of the crust was sampled, and, especially in view of the range in age of the basalts upon which the crusts are growing, it seems likely that the high ratios result from exchange with seawater rather than recent growth.

3.3. Basalt from Deep Sea Drilling Pro/ect cores Two crystalline interior samples from different depths within each of two DSDP cores were analyzed. On the basis of the seafloor magnetic pattern, both sites sampled old basalts (Table 1). All four samples measured were visibly highly altered, and all have substantially higher uranium contents than the associated unaltered glass. Three of the four have 234U/ 2aSU at equilibrium within analytical uncertainty, perhaps to be expected for old, buried samples. A fourth has 2a4U/2aSU = 1.07 -+ 0.02. This ratio implies recent interaction with a 234U-enriched source, possibly seawater, at a depth of ~0.5 km beneath the seawater-sediment interface.

4. Discussion

The work reported here indicates that low-temperature alteration of oceanic basalts is accompanied by a large increase in uranium content, in agreement with previous results [5,6]. Further, the measured isotopic ratios demonstrate that nranganese crusts, palagonite rims and the fine-grained alteration products of the crystalline interiors all behave differently with respect to uranium uptake. It is clear from the many chemical analyses of altered basalts in the literature that the degree of alteration as measured by almost any chemical parameter is far from a linear function of time or distance from a spreading center. Nevertheless, the uranium isotopic compositions for the two fresh-appearing young basalts measured in this work indicate that secular equilibrium has been attained before the onset of substantial alteration. This suggests that any deviations from equilibrium that are observed in the altered basalts result from the alteration process itself, or from subsequent exchange.

Consider first the isotopic data for palagonite (Table 2). Although DI 15 is at equilibrium within the experimental uncertainties, the three older samples all have 234U/238U considerably less than one. This suggests that preferential leaching of in-situ-produced 234U is occuring. Thus at least two processes must be involved to explain the observed uranium concentrations and isotopic compositions. Firsl, uptake of uranium occurs during and/or after palagonite formation m amounts 8 20 times the original concentration in the basaltic glass. Presumably this uranium comes from seawater and therefore has 234U/ 238U = 1.14. Secondly, preferential removal of 234U must occur. It has been demonstrated recently [12] that such removal can occur by direct ejection of tire recoiling nuclei from small grains, and by dissolution of recoil-damaged regions. Both mechanisms require the presence of intergranular water. This suggests continuous communication between seawater and palagonite, but, in view of the low Z34u/Z38Llfor most of the samples, little actual exchange of uranium. Seawater percolation through the palagonite layer, necessary to explain the low Z34U/238U, stops abruptly at the palagonite-glass interface. This can be seen from the uranium concentration data in Table 2, and also has been discussed previously [5]. However, unless they formed very recently, the manganese crusts overlying the palagonite layers in samples DI 15, D143 and DI 13 evidently experience uranium exchange with a source having 23'aU/238U ~> 1.04, most likely seawater. The absolute uraniunr concentrations in the crusts are about an order of magnitude higher than those in the underlying palagonite, and the manganese crusts on these rocks are at least as thick as the palagonite layers, ruling out the palagonite as a source for a substantial amount of the excess 234U. For the interior samples with high uranium contents from the MIOR traverse there is also evidence for ongoing uranium exchange with a source with excess 234U. The oldest sample has 234U/=rSU equal to seawater, and D114 exhibits a value substantially above that observed in seawater. As pointed out previously, this implies a source with =aau/=asu ~> 1.29, assuming the initial uranium in the sample to be at equilibrium. It is tempting to assign this source to circulating seawater which has leached 234U preferentially from some other part of layer 2, perhaps by hydrothermal alteration. However, except for the

33 palagonite analyses, there is no indication from our own data of substantial removal of 2a4U even from old basalts. In addition, the manganese crust from this rock shows no evidence for exchange with a 234Uenriched source (Table 2). Because of its small relative mass, palagonite cannot be an important 234U source except perhaps in a very localized way. There is also evidence for a 234U-enriched source in our data from DSDP Site 238, although in this case fluids with 234U/23Su greater than the seawater value are not required. The DSDP samples were selected in the same way as interior samples from the MIOR traverse: they were cut from uniform-appearing interiors of glass-rimmed pillows. Sample 57-2-76 comes from a depth of 527 m below the sedimentwater interface, and 21 m below the sediment-basalt contact. Its high 234U/238U is in contrast to the equilibrium value for 56-1-143, located only 10 m above it in the column. The two other old, deeply buried basalts from Site 213 also exhibit equilibrium ratios. Sample 57-2-76 has evidently recently gained or exchanged uranium with a 234U-enriched source. The nature of such sources, for which the data in this paper present firm evidence, is unclear, although in principle they can be produced by known mecha.nisms [12]. Our measurements show that such sources have interacted both with basalts exposed directly to seawater and those buried beneath a considerable sediment thickness. Finally, we note that there is a discrepancy between tile results reported here and those described by Bacon [9]. He analyzed two old, altered dredge basalts and found that interior samples had 234U/ 2aSU much below equilibrium, while palagonite-rich margins were at equilibrium. These findings are precisely the reverse of those reported here. The reason for this discrepancy is unknown.

5. Conclusions It is clear from the data presented here that the utility of uranium isotope measurements for studying the nature and chronology of basalt alteration at present is limited by two factors: lack of knowledge of the uranium source in individual cases, and ignorance of the nature of the process for the different phases involved. We have discussed results which bear

on both of these questions, but much more data will be required before a consistent picture emerges. The problem of source isotopic ratios is exemplified by our evidence for sources with 234U/23SU greater than seawater. At least two possibilities must be considered: hydrothermal solutions and sediment pore water. Although reliable published data do not exist, it is likely that both of these have 234U/238U ~> 1.0, and quite possibly above the seawater value. (One pore water measurement has been reported [20], but the uncertainties are extremely large: 1.19 -+0.15.) However, until precise 234U/238U values for the sources are known, interpretation of isotopic ratios in the basalts themselves in terms of the chronology of the alteration process will be difficult. Our results show that the behaviour of uranium varies depending upon its mineralogical site in the rock. In addition, for any given mineralogical site, three different "varieties" of uranium maybe present, each of which can be expected to act differently with respect to exchange processes: uranium originally present in the rock, uranium acquired during alteration, and in-situ-produced radiogenic Z34U. The potential of uranium isotopic studies for elucidating the exchange processes involved for different phases and different "varieties" of uranium is demonstrated by the palagonite results. These show both uranium uptake during growth and subsequent preferential removal of 234U. Perhaps the most interesting general conclusion regarding low-temperature basalt alteration on the seafloor which can be drawn from these results is that all three phases studied - manganese crust, palagonite and clay-rich pillow interiors - show evidence for continued communication with seawater and exchange (or pickup) of uranium for basalts spanning a wide range of ages.

Acknowledgements We thank Chris Maclsaac for assistence with some of this work. Michiko Hitchcox rapidly and accurately typed various drafts of the manuscript. DSDP samples are supplied through the assistance of the National Science Foundation. Research supported in part by NSF grant EAR77-14920.

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