Radium isotopes, alkaline earth diagenesis, and age determination of travertine from Mammoth Hot Springs, Wyoming, U.S.A.

Applied Geochemistry. Vol. 5, pp.

631~o40, 1990

0883-2927/90 $3.00 + .00 Pergamon Press plc

Printed in Great Britain

Radium isotopes, alkaline earth diagenesis, and age determination of travertine from Mammoth Hot Springs, Wyoming, U.S.A. NEIL C. STURCHIO Argonne National Laboratory, CMT-205, Argonne, IL 60439, U.S.A. (Received 22 January 1990; accepted in revised form 20 March 1990) Abstract--Travertine from active springs, former vents, and drill core from Mammoth Hot Springs, Wyoming, was analyzed for Ra isotopes (226 Ra, 228 Ra), other alkaline earth elements (Mg, Sr, Ba), and mineralogical composition. Thermal water also was analyzed. Travertine, presently being deposited, contains 3.0-15.3 pCi/g 226Ra, and has a 228Ra/226Ra ratio identical to that in thermal water. Travertinc precipitates mostly as aragonite and experiences a complete diagenetic transformation to calcite within 9 a (estimated from 228Ra/2261Ra value of youngest completely calcite travertine). Systematic compositional changes associated with this diagenetic transformation are enrichment of Mg and depletion of Sr, Ba and Ra. Apparent mineral-waterdistribution coefficientsfor Mg, Sr and Ba in aragonite and calcite are within the range of those determined experimentally, implying near-equilibrium conditions and high water-rock ratios during travertine diagenesis. Impure travertine from near the base of the travertine section in the U.S. Geological Survey Y-10 drill hole (at 72.9 m depth) has a 23°Th/234U isochron age of 7700 + 440 a. The content of 2adRa in normal, subhorizontally layered, porous travertine decreases with depth in Y-10. The observed 226Ra vs depth relation is consistent with continuous deposition of travertine at the Y-10 site from 7700 a BP to near present at a mead rate of -1.0 cm/a, and indicates minimal exchange of Ra between travertine and pore water after the early diagenetic transformation of aragonite to calcite. These data imply that, under favorable conditions, 226Ra measurements may be useful in determining ages and deposition rates for other travertines.

INTRODUCTION TRAVERTINESare calcium carbonate deposits that may be produced at the Earth's surface where groundwater emerges in a saturated or supersaturated state with respect to the minerals calcite and aragonite (BARNES, 1965). The locations of travertinedepositing springs depend upon bedrock lithology, local hydrological factors, climate, and heat flow. BARNES et al. (1978) noted that they tend to occur in association with "tectonically and seismically active belts of supercontinental magnitude". Travertine deposits are commonly found in association with, and may preserve a record of the evolution of, major hydrothermal systems (GOEFand SHEVENELL,1987). Thermal water and travertine have various practical uses that have continually attracted humans to travertine-depositing hot springs through the ages, therefore such areas may represent important archaeological archives. The ability to date travertines by the 23°Th/234U disequilibrium method has led to important applications in studies of late Quaternary archaeology, geomorphology, and paleoclimatology (IvANOVlCH and HARMON,1982). The 23°Zh/234U disequilibrium method, using a-spectrometry techniques, has been shown to be useful for dating travertines generally in the age range from - 5 to 350 ka (reviewed by SCHWARCZ, 1982). The potential use of 226Ra (which is commonly present in excess in young travertines) for extending the datable range of travertines to

younger ages was noted by SCHWARCZ (1982), although little effort appears to have been devoted toward investigating the potential applications of this method. The 1602 a half-life of 226Ra could translate to a practical dating range of - 0 . 1 - 1 0 ka provided there is good knowledge of the initial 226Ra activity and negligible 23°Th in the travertine. Similarly, the 5.75 a half-life of 228Ra could translate to a practical dating range of - 0 . 3 - 3 0 a provided there is good knowledge of the initial 228Ra activity (or the initial 228Ra/226Ra ratio), and negligible 232Th in the travertine. The principal uncertainties in Ra dating of travertine are: (1) the initial excess activity of the Ra isotope used for dating; and (2) the integrity of the travertine with respect to gain or loss of Ra during diagenesis. Once these uncertainties have been assessed quantitatively, the ability to date travertines using Ra isotopes can be developed further and may lead to important applications in problems of Holocene time. This paper presents the results of a study of Ra isotopes, alkaline earth geochemistry, and mineralogy of travertines from the classic travertinedepositing hot spring system at Mammoth Hot Springs, Yellowstone National Park, Wyoming. For this study, the activities of Ra isotopes and the concentrations of Mg, Sr and Ba were determined in: (1) travertine precipitating presently at active hot spring vents; (2) travertine from former vents; (3) travertine from a drill core through a - 7 5 m thick

631

632

N.C. Sturchio

section of the Mammoth Hot Springs deposit, and (4) thermal water from active hot springs. Also, the mineralogical composition of the travertine samples was determined by X-ray diffraction, and the age of a sample of basal travertine from the Y-10 drill core was determined by the 23°Th/234U isochron method. These data are examined here with two main objectives: (1) understanding the influence of diagenesis on the redistribution of alkaline earth elements (including Ra isotopes) in hot-spring travertine deposits; and (2) evaluating potential applications of Ra isotopes for age determination of Holocene travertines.

B A C K G R O U N D INFORMATION

The study area, Mammoth Hot Springs, is located - 8 km south of the north entrance of Yellowstone National Park, Wyoming, near the MontanaWyoming border (Fig. 1). The geology and thermal history of Mammoth Hot Springs were summarized by BARGAR(1978). Mammoth Hot Springs consists of about 100 hot springs emerging from actively depositing travertine terraces and fissure ridges at elevations between 1725 and 2085 m. Older travertine deposits are found at higher elevations to the southwest.

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Travertines in the immediate area of Mammoth Hot Springs are younger than the Pinedale glaciation, which ended by - 1 3 ka ago (PIERCE, 1979). A research corehole (Y-10) drilled into travertine near Bath Lake during October 1967 by the U. S. Geological Survey penetrated - 7 5 m of travertine containing some clastic interlayers before reaching glacial till and Mesozoic basement rock; downhole temperature measurements at all depths >15 m were -73°C (WHITEe t al., 1975). Chemical data for thermal waters and travertine from Mammoth Hot Springs are summarized by BARGAR (1978). The hot springs discharge Ca-HCO3-SO 4 type waters. The most active hot springs have temperatures -73°C. Spring water composition and temperature have remained essentially constant for over a century (GoocH and W H I T F I E L D , 1888; ALLEN and DAY, 1935; BARGAR, 1978). The spring waters are strongly supersaturated with CaCO 3 (FRIEDMAN, 1970). Fresh travertine at Mammoth Hot Springs is nearly pure CaCO3, and is deposited in a variety of micromorphologies as either aragonite (especially ~>45°C) or calcite; diagenetic changes begin to affect the travertine immediately after precipitation (PuRSELL, 1985). Estimates of average travertine deposition rates (vertical) near active springs range from 15 to 25 cm/a (WEED,1889; ALLEN and DAY, 1935; PURSELL, 1985). Early measurements of the radioactivity of Yellowstone thermal waters, gases, and hot spring deposits (SCHLUNDTand MOORE, 1909; SCHLUNDTand BRECKENRIDGE, 1938) inspired preliminary attempts to date the Mammoth Hot Springs travertines using 226Ra (SCHLUNDT, 1933).

SAMPLES AND ANALYTICAL METHODS

Travertine

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Samples of travertine were collected from the margins of active hot springs, from vent structures of formerly active hot springs, and from the Y-10 drill core (WroTE et al,, 1975). Travertine samples from springs and vents were dried at 110°C, then gently crushed and homogenized prior to analysis. Travertine samples from the Y-10 drill core were first sliced with a rock saw using deionized water as a lubricant, the saw marks were ground away using carbide grit paper, and each core slice was then cleaned ultrasonically in deionized water several times, rinsed, and dried at 110°C before being gently crushed and homogenized prior to analysis. Thin sections were prepared from adjacent portions of the core. Determination of 226Ra in travertine was done by the :2aRn emanation method. Portions of travertine (0.8-13.6 g, depending on relative age of travertine) were dissolved in 1M HC1, then brought to volume (-200 ml) with 1M HCI. Amounts of insoluble residue were small (<<1% of sample) and were removed by passing solution through pre-acidrinsed Whatman # 1 filter paper. The sample solutions were purged with He and stored in pyrex emanation flasks for at least two weeks to allow ingrowth of 222 Rn from 226 Ra. The Rn emanation system and counting method used was that described by LtJcas (1977). The system was calibrated using

Ra isotopes in travertine, Mammoth Hot Springs, Wyoming, U.S.A. a standard solution (NBS 4951) containing a known mass of 226Ra, assuming half-lives of 3.8235 d for 222Rn and 1600 a for 226Ra. Reported 226Ra activities represent the mean values of two or three successive determinations, and have been corrected for a system blank of 0.016 pCi. Activity ratios of 228Ra/226Ra in travertine samples were determined by sealing 0.2~).3 kg portions of travertine in airtight cans, allowing ingrowth of Ra daughters, acquiring gamma spectra, and reducing gamma spectra by a least squares method (LUCAS a n d MARKUN, in prep.). Activities and activity ratios are referred to within parentheses throughout the text. Activities of Th and U isotopes in selected travertine samples were determined by a-spectrometry using 229Th and 236U yield monitors, as described by STURCHIOand BINz (1988). One impure travertine sample (Y10-239) was dated by the 23°Th/234U isochron method, using a selective dissolution method similar to that described by Ku and LIANG (1984). Elemental concentrations of Mg, Sr and Ba were determined in travertine solutions by inductively-coupled plasma atomic emission spectrometry, with relative accuracies of _+3-20%. Mineralogical determinations were performed by X-ray diffraction, using a Philips diffractometer (Cu Ka X-rays, scan rate of 20°/min). Relative amounts of aragonite and calcite in each sample were estimated from the (110) and (210) aragonite peak heights and the (104) calcite peak height in comparison to mixtures prepared from known amounts of end-member aragonite and calcite from Mammoth Hot Springs travertines.

Water Large volume water samples (20-40 1) were collected for determination of dissolved Ra isotope activities. Water was pumped directly from the spring vent through a 293 mm diameter millipore-type filter (0.45/zm effective pore size)

633

housed in a plexiglass assembly, using a peristaltic pump and silicone tubing, into an acid-washed polyethylene container. The samples were acidified to pH < 2 with concentrated HCI within 0.5 h of collection. Within 12 h of collection, each sample was weighed, its pH was adjusted to between 2 and 2.5, and it was pumped at - 1 0 0 ml/min through a saturated bed containing a mixture of equal amounts (-125 ml each) of 20-50 mesh size Dowex 50-X8 cation exchange resin and Dowex XFS-43230 Ra-selective resin. The resin was then sealed in an airtight can. Several 200-minute counts of gamma activity resulting from daughter nuclides of 2Z8Ra and 226Ra w e r e acquired between two weeks and three months following collection. The counting data were acquired with a NaI detector and were reduced by a least-squares fit of the total sample spectrum against standard spectra (LUCAS and MARKUN, in prep.). This sampling method results in nearly quantitative (>99%) extraction of Ra isotopes from at least 100 1 of water containing 2 g/1 total dissolved solids. In comparison, Mammoth Hot Springs thermal water contains - 2 g/1 total dissolved solids.

RESULTS AND DISCUSSION

Composition of presently depositing travertine T a b l e 1 contains analytical data for seven samples collected from active h o t springs. Five of these samples were collected from t r a v e r t i n e f o r m i n g presently at spring vents, w h e r e w a t e r e m e r g e s from the g r o u n d at t e m p e r a t u r e s of 70-73°C. A t each of the two springs s a m p l e d o n A n g e l T e r r a c e , one additional sample was collected n e a r the margin of the area of active t r a v e r t i n e deposition a b o u t 25 m d o w n -

Table 1. Analyses of travertine, Mammoth Hot Springs, Wyoming Sample Active hot springs N a r r o w G a u g e , 73°C A n g e l - l , 72°C A n g e l - l , 33°C Angel-2, 72°C Angel-2, 48°C Cupid, 72°C Canary, 70°C Inactive spring vents MHS-24 Liberty C a p

% Aragonite

Mg (#g/g)

Sr (~g/g)

Ba (,u,g/g)

226Ra pCi/g

91 100 87 99 80 98 97

617 43(1 1770 363 1600 320 330

21(~) 1980 1700 206(l 1930 1930 206(l

61 49 92 61 88 47 48

6.82 5.47 15.31 5.60 9.26 3.65 3.01

0 0

4070 3010

699 278

83 52

14,04 _+ 0.26 1,52 _+ 0.03

0 5 0 0 0 0 {1

1920 2050 2160 1550 1830 287(I 2360 2420 22(X1 1590 1870 1640 2060

760 1620 863 220 445 209 1430 240 95 103 96 562 89

22 20 21 18 7 1I 18 7 15 11 <5 85 7

_+ 0.13 _+ 0.10 _+ 0.25 _+ 0.10 _+ 0.14 -+ 0.06 _+ 0.05

231~Wh pCi/g

(22SRa/22~'Ra)

<0.01 <0.(12 <0.(12 0.633 ± 0.007

0.208 _+ 0.004 ~(l.023

Y-I(1 drill core

Y 1(l-2.4 Y10-6.7 Y10-17.3 Y10-27.7 Y10-39.0 Y 10-51.0 Y10-68.8 Y10-71.3 Y 10-88.8 Y10-140.4 Y10-156.5 Y 10-206.6 Y 1(I-220.8

Depth (m)

Type

0.73 2.(14 5.27 8.44 11.9 15.5 21.0 21.7 27.1 42.8 47.7 63.0 67.3

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0 0 0 0 1)

Note: Uncertainties for Ra data are + Io. based on counting statistics.

0.944 0.446 0.384 2.028 0,165 (t. 385 0.176 0.163 0.682 0.096 0.019 1.292 0.043

_+ 0.021 _+ 0.011 _+ 0.0(17 _+ 0.029 _+ 0.004 _+ 0.009 _+ 0.003 _+ 0.004 _+ 0.009 + 0.003 _+ 0.001 _+ 0.040 _+ 0.001

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stream along the outflow channel, where the water temperature was lower (33-48°C). The compositions of the near-vent travertine samples are similar. Each consists of >90% aragonite, and contains 3.01--6.82 pCi/g (226Ra), 320-617 ppm Mg, 1930-2100 ppm Sr, and 47-61 ppm Ba. The (23°Th) is <0.02 pCi/g in each of the three samples in which it was determined; <0.37% of the initial (226Ra) is supported by decay of 23°Th in these samples. The downstream travertines are more calcitic (80-87% aragonite) and contain more Ra, Ba and Mg, and less Sr, than the near-vent travertines. These chemical differences may reflect mineralogical and temperature dependences of mineral-water distribution coefficients.

Thermal water composition Compositional data for water from two of the hot springs from which travertine was sampled are given in Table 2. The compositions of these two springs are similar. These and other data (GoocH and WHrrFIELD, 1888; ALLEN and DAY, 1935; ROWE et al., 1973; THOMPSON et al., 1975; STURCHIO et al., 1989) indicate that the thermal water emerging at Mammoth Hot Springs is derived from a relatively homogeneous source. The (22SRa/226Ra) ratio in Mammoth Hot Springs water ( - 0 . 6 ) is lower than in other types of hot spring water at Yellowstone (i.e. neutral-chloride and acidsulfate types), for which (228Ra/ 226Ra) values ranging from 1.0 to 9.8 have been determined (STURCHIO e t al., 1989). For comparison, the mean value of the parent ratio (232Th/238U) in carbonate rocks is near 0.33, whereas that of common igneous rocks and shales is - 1 . 3 (CLARKe t al., 1966); at radioactive equilibrium, a condition which prevails in most rock, (228Ra/226Ra) = (232Th/23SU). The (228Ra/226Ra) value in Mammoth Hot Springs water may therefore Table 2. Analyses of thermal water Spring

Narrow Gauge (Y87-1)

Date sampled Temperature (°C) pH (field) SiO2 Na K Mg Ca Sr Ba HCO3 SOn C1 (226Ra) (228Ra/226Ra)

5-9-1987 73 6.3 50.9 117 55.3 71.5 351 1.90 0.068 412 744 153 10.3 + 0.5 0.63 + 0.04

Angel-1 (Y87-2) 5-9-1987 73 6.5 51.3 120 57.9 72.0 296 1.54 0.063 405 722 145 11.2 + 0.6 0.61 -+ 0.03

Concentrations in mg/l, (226Ra) in pCi/I (_+or).

reflect derivation of Ra through interaction of the travertine-depositing waters with Paleozoic and Mesozoic carbonate rocks that are exposed around Mammoth Hot Springs (RuPI'EL, 1972; FRASERet al., 1969).

Diagenetic changes in travertine composition A detailed petrographic study of the diagenesis of travertines at M a m m o t h Hot Springs, limited to samples from surface outcrops, was reported by PURSELL (1985), who showed that diagenesis begins immediately after precipitation, whereby the primary aragonite crystals are engulfed by calcite, and the aragonite is subsequently dissolved and replaced by calcite. Although some subsurface changes in the travertine were noted by ALLEN and DAY (1935), WHITE et al. (1975), and BARGAR(1978), no similarly detailed petrographic study of the travertine in the Y10 drill core has yet been reported. BARGAR(1978) observed that nonporous dense travertine was precipitated along fissure walls exposed in a partially collapsed fissure ridge (e.g. see Fig. 6 in BARGAR, 1978) and within pore spaces in travertine from the Y-10 drill core (e.g. see Fig. 7 in BARGAR, 1978). Most of the drill core samples analyzed for this study (9 of 13) consisted of subhorizontally layered porous travertine having only minor amounts of secondary pore-filling calcite (e.g. see Fig. 7 in BAR6AR, 1978). This appears to represent the type of travertine deposited normally around active hot springs and is referred to as Type 1 travertine in Table 1. Also analyzed were four drill core samples consisting entirely of dense, banded, nonporous travertine similar to the fissure-wall travertine described by BARGAR (e.g. see Fig. 6 in BARGAR, 1978); this is referred to as Type 2 travertine in Table 1. There are significant and systematic compositional differences at Mammoth Hot Springs between modern travertines (samples from active hot springs) and older travertines (samples from former vents and Y-10 drill core) with respect to all of the compositional parameters measured (Table 1). The principal differences between modern and older travertines are" (1) older travertines consist completely of calcite (with the exception of one fissure-wall travertine, Y10-6.7, that contains 5% aragonite), whereas modern travertine is predominantly aragonite; and (2) older travertines generally contain much less Sr, Ba and Ra, and more Mg, than modern travertines. The compositional differences are depicted clearly in Figs 2-5, that show compositional data vs depth for Mg, Sr, Ba and Ra, respectively. Assuming that the depositional conditions involved in travertine formation have not changed significantly with time at Mammoth Hot Springs, then the observed compositional differences must result from diagenetic changes that occurred in the travertine. This implies that virtually all of the travertine experienced

Ra isotopes in travertine, Mammoth Hot Springs, Wyoming, U.S.A. solution-redeposition, a process that occurs ubiquitously during the transformation of the unstable CaCO 3 polymorph, aragonite, to the stable CaCO 3 polymorph, calcite, in the presence of water (see reviews by CARLSON, 1983; MORSE, 1983, and references therein). This process alters the distribution of Ra in travertine, and thus is an important factor in determining the feasibility of using Ra isotopes for age determinations.

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Apparent distribution coefficients for alkaline earths in travertine A considerable amount of attention has been given to the effects of solution composition and precipitation rate upon the incorporation of minor and trace elements, especially Mg and Sr, in carbonate minerals (reviewed by VEIZER, 1983). Available evidence indicates that the dominant mechanism of incorporation of divalent trace elements into carbonate minerals is substitution for Ca 2+ in the mineral lattice. Apparent mineral-water distribution coefficients, Dmineral_water, have been calculated for Mg, Sr, Ba and Ra in aragonite and calcite from the Mammoth travertines (Table 3), according to O imi.... 1-water = (Xi/Sca)s/(mi2 + /mca)aq 2+

(1)

where X refers to mole fraction in the solid phase, i is a cation that substitutes for Ca in CaCO3, and m is the total molality of the dissolved ion. Activity coefficient ratios of unity for i/Ca in the solid and aqueous solutions and homogeneous distribution are assumed. The data used to calculate D values for aragonite are the mean values for the modern travertines consisting of 97-100% aragonite, and the data used for calcite are mean values for the Type 1 drill core samples from Y-10, which consist entirely of

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Depth, m FIG. 3. Diagram showing Sr concentration (~g/g) vs depth (m) for all travertine samples. Symbols as in Fig. 2. Dashed horizontal lines show mean Sr values (+o) for samples consisting of 97-100% aragonite and for Type 1 drill core samples (100% calcite).

calcite. The data used for water are the mean values of the analyses listed in Table 2. The calculated D values for Mg, Sr and Ba (Table 3) are within the range of experimentally determined values reported in the studies cited in the review by VEtZER (1983), and subsequent work of MuccI and MORSE (1983) and PINGITOREand EASTMAN(1984, 1985, 1986). The relation between the relative D values for aragonite and calcite is consistent with the control imposed by their different crystal structures (SPEER, 1983; REEDER, 1983). The D values for Ra are lower than those for Ba (by a factor of - 0 . 5 ) in both aragonite and calcite (Table 3), and are lower than the D values predicted for Ra by LANGMUIRand RINSE (1985). To first approximation, the calculated D values in Table 3 appear to be consistent with equilibrium between

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FIG. 2. Diagram showing Mg concentration (/xg/g) vs depth (m) for all travertine samples. Dashed horizontal lines show mean Mg values (_+o) for samples consisting of 97-100% aragonite and for Type 1 drill core samples (100% calcite).

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Depth, m FIG. 4. Diagram showing Ba concentration (/xg/g) vs depth (m) for all travertine samples. Symbols as in Figs 2 and 3. Dashed horizontal lines show mean Ba values (_+a) for samples consisting of 97-100% aragonite and for Type 1 drill core samples (100% calcite).

636

N.C. Sturchio

Table 3. Apparent mineral-water distribution coefficients This work

Experimental

Ara~onite Dmg Ds~ DB" ORa

0.0041 _+0.0005 0.68 + 0.02 0.63 _+0.07 0.33 + 0.08

0.00064).005 0.6 -1.2 0.04 -3.0

Calcite D Mg Ds" Dn~ DRa

0.023 _ 0.004 0.16 + 0.13 0.16 __0.07 ~0.06

0.012-0.06 0.027 -0.4 0.1 -0.4

Note: D Ra value for calcite is based on value of (226Ra) indicated by intercept of Line B in Fig. 5. Sources of experimental values cited in text.

calcite and thermal water, and imply a high effective water-rock ratio during diagenesis.

Rate o f aragonite to calcite transformation: 228Ra/226Ra constraints

ratio in the travertine depending upon the amount and timing of this precipitation. The time required for completion of the diagenetic transformation from aragonite to calcite at Mammoth Hot Springs is constrained by the 228Ra/Z26Ra activity ratio of the youngest travertine consisting completely of calcite. This is the travertine sampled from the inactive coneshaped vent known as MHS-24 (see photograph in Fig. 25 of BARGAR, 1978). This vent was active in 1974, and was discharging water at a temperature of 70°C (BARGAR, 1978). When sampled for this study in mid-September 1988, MHS-24 was dry. The top of the vent of MHS-24 is about 1 m above the surrounding ground surface, so that there was no opportunity for Ra exchange between the uppermost travertine and thermal water after the spring stopped flowing. Significant Ra exchange with meteoric precipitation is precluded because the Ra content of rain and snow is negligible. Thus, the Ra contained in this sample is limited to that deposited with the initial aragonitic travertine. The (22SRa/Z26Ra) value of travertine from the top of MHS-24, when sampled, was 0.208. The age of this travertine at the time of sampling, t, is given by (228Ra/226Ra)r = (228Ra/226Ra)0(e '~228t/e-~226~ (2)

The data for Angel-2 in Tables 1 and 2 show that the (228Ra/226Ra) value in modern travertine is identical to that in the water from which it precipitates. The value of (228Ra/ZZ6Ra) may be affected by diagenetic processes such as: (1) the aragonite to calcite transformation, during which Ra may undergo significant exchange with Ra from a reservoir having a different value of (228Ra/226Ra) than that in the travertine; or (2) subsurface precipitation of calcite in pore spaces, which at Mammoth Hot Springs could significantly increase the bulk 2ZSRaf126Ra activity

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Fro. 5. Diagram showing log (226Ra) (pCi/g) vs depth (m) for all travertine samples. Symbols as in Figs 2, 3 and 4. Line A depicts simple radioactive deca~ in closed system with the slope constrained by the (23OTb/•234 U) age of sample Y10239.3. Line B is least squares linear regression of data for Type 1 travertines (see text).

where 2228 and J'226 are the decay constants of 228Ra and 226Ra, 1.21 x 10-1/a and 4.33 x 10 a/a, respectively. Assuming that (228Ra/226Ra)0, the initial (22SRa/Z26Ra) of the MHS-24 travertine sample, was equal to that of the near-vent travertine from Angel2, then the age of MHS-24 travertine at the time of sampling was 9.0 _+ 0.2 a. This represents the time that the spring stopped flowing, and implies that the complete transformation of aragonite t o calcite occurred within 9 a. The aragonite to calcite transformation may occur very rapidly in travertines; for example, modern travertine deposited as aragonite reportedly undergoes complete transformation to calcite in 10-15 d at Bagni S. Filippo, Tuscany, Italy (MALESANI and VANNUCCI, 1975). Minimum ages of the Liberty Cap travertine and the uppermost sample of travertine from the Y-10 drill core (Y10-2.4), both completely calcitic, are constrained by their low (22SRa/Z26Ra) values to be ->28 a and >37 a, respectively. These low (22~Ra/226Ra) values imply minimal exchange of Ra with thermal water following the transformation to calcite.

Age and deposition rate of travertine from the Y-10 drill core The age of impure travertine from near the base of the travertine section in the Y-10 drill core (sample Y10-239.3) was determined by the 23°Th/234U method to provide an independent control for evaluating the significance of the (226Ra) data for the Y-10 travertine. The 23°Th-234U data for this sample

Ra isotopes in travertine, Mammoth Hot Springs, Wyoming, U.S.A.

637

Table 4. Th and U isotopic data for sample Y10-239.3 Sample WR-A WR-B LE-B RE-B

U (txg/g)

Th (/xg/g)

(234U/238U)

(23°Th/232Th)

(234U/232Th)

1.92±0.09 2.34±0.09 1.85±0.09 1.44±0.07

2.65±0.19 1.83±0.10 0.62±0.04 3.10±0.19

1.72±0.07 2.~±0.05 2.27±0.04 0.89±0.05

1.19±0.08 1.54±0.07 2.41±0.09 1.08±0.07

3.81±0.32 7.83±0.52 20.9 ±1.6 1.26±0.10

Uncertainties are ±lo, based on counting statistics. WR = whole rock, LE = dilute acid carbonate leachate, RE = residue from dilute acid leach.

are listed in Table 4 and are shown on a (23°Th/232Tb)-(234U/232Th) isochron diagram in Fig. 6, where the four data points define a straight line. The slope of this line, when excess 234U activity in the carbonate fraction of the sample is taken into account, corresponds to an age of 7700 ± 400 a. This age is consistent with geological relations of the travertine at Mammoth Hot Springs, which indicate that it formed after the Pinedale glaciation. The data presented above and in the preceding sections provide an opportunity to evaluate the excess 226Ra method for determination of the age and deposition rate of the travertine section in the Y-10 drill core. The data show clearly that travertine undergoes a significant loss of Ra during the transformation from aragonite to calcite. This transformation occurs within a short time of deposition (relative to the half-life of 226Ra). This suggests the possibility of using a corrected value for the initial excess (226Ra) in the age equation t = (1/2) In [(226Ra)o/(226Ra)t]

(3)

where t is the age of the sample in a, 2 is the decay constant of 226Ra, 4.33 x 10-4/a, (226Ra)0 is the initial excess (226Ra), and (Z26Ra), is the present excess (226Ra). In this case, the corrected value for the initial excess (226Ra) may be given by (226Ra)0, corrected {r~Ra //~Ra )(226Ra)0 ' = ~,Ucalcite'~aragonite aragonite-

(4)

The Y-10 drill core samples have good strati, , , , I , , , , I , , , , I , , , , I

J

5

10

15

20

25

(234U/232Th) FIG. 6. (23°Tb/232Th) vs (234U/232Th) isochron

diagram

showing data for sample Y10-239.9. Age indicated by slope of isochron is 7700 + 400 a.

graphic control. A logarithmic decrease in (226Ra) with depth should be observed in the travertine if calcite, once formed, remained stable and did not continue to exchange Ra with pore water. Alternatively, if continuous exchange of Ra occurred between calcite and pore water, analogous to that observed for marine barite (CHURCH, 1979), then constant (226Ra) with depth should be observed. Figure 5 shows log (226Ra) vs depth for all travertine samples and two lines labeled A and B. There is a clear trend of decreasing (226Ra) with depth, excepting Type 2 sample Y10-206.6 (63.0 m) that apparently formed more recently than the enclosing travertine. Line A represents the predicted (226Ra) vs depth for a hypothetical closed system case in which travertine having an initial (226Ra) value of 7.0 pCi/g (the mean value determined for the seven samples of travertine from active hot springs) was deposited at a constant rate of 0.95 cm/a for the past 770(I a (rate given by thickness of deposit above 7700year-old sample Y10-239.3), and experienced no depletion or continuous exchange of Ra subsequent to deposition. Line B is a least-squares linear regression of log (226Ra) vs depth for the Type 1 travertines from the Y-10 drill core. The log (226Ra) intercept for Line B at zero depth is equivalent to 0.8 pCi/g, in good agreement with the (226Ra)0....... ted value (1.1 ± 0.3) predicted from Eqn (4) by assuming DRa'~cite/D~ar~gonite Ba Ba 226 Dcalcite/Daragonit e and ( Ra)0. aragonite equal to the mean value for the 97-100% aragon±tic travertines from Table 1 (4.4 ± 1.1 pCi/g). The slope of Line B indicates an average deposition rate of - 0 . 9 8 cm/a (vs 0.95 cm/a for Line A) and an age of -7400 a when extrapolated to the depth of sample Y10-239.3 (that has a 23°Th/234U age of 7700 + 400 a). The average deposition rate and age vs depth relation represented by Line B are thus in excellent agreement with those predicted for the hypothetical closed system case represented by Line A. The limited number of data points do not allow a more rigorous evaluation of possible short-term changes in deposition rate with time. However, an average deposition rate of - 1 cm/a implies that active springs that typically deposit - 2 0 cm/a (as measured by WEED, 1889; ALLEN and DAY, 1935; and PUt,SELL, 1985) were not present at the Y-10 site for more than - 5 % of the time represented by the Y-10 drill core.

638

N.C. Sturchio

Implications for determination of other travertine ages and deposition rates The data presented here for the Mammoth Hot Springs travertines are consistent with the following model for the behavior of Ra during hot-spring travertine diagenesis: (1) travertine is precipitated primarily as aragonite; (2) the aragonitic travertine transforms to calcite within 9 a, with loss of most Ra contained initially in the aragonitic travertine; (3) the calcitic travertine is buried by younger travertine and remains closed to further Ra exchange with pore water. Travertine ages and deposition rates can be determined elsewhere (for Holocene travertines) using 226Ra if the following data are available: (1) the initial (226Ra) of the travertine; (2) a profile of (226Ra) vs depth in the travertine deposit; and (3) petrographic observations and X-ray diffraction data for each travertine sample for which (226Ra) is determined. If the travertine being investigated exhibits evidence for an early diagenetic aragonite to calcite transformation, then the initial (226Ra) in the diagenetic calcite must be known. This can be derived from the y-intercept of a linear regression of the activity vs depth data (as shown in Fig. 5) only if the deposit is actively accumulating or if accumulation is known to have stopped recently relative to the half-life of 226Ra. If the travertine is being deposited as calcite, that can be shown to remain stable following deposition, then the (226Ra) in the presently depositing travertine may be assumed as the initial activity. However, if the deposit is no longer accumulating, and there is no independent information on the age of the deposit, then the only information that may be obtained from a (226Ra) vs depth profile is the average deposition rate. If sufficient activity is present to determine the deposition rate, then the minimum age of the deposit is constrained by the thickness of the deposit. Estimation of the uncertainty of a 226Ra age determination for an individual sample of travertine requires that a statistically significant number of samples must be analyzed from the travertine deposit being investigated. Statistically valid estimates for the uncertainty of the initial excess (226Ra) and the uncertainty of the slope(s) of the log (226Ra) vs depth profile can then be obtained simply. For example, the mean (226Ra) value determined in this study for modern travertine at Mammoth Hot Springs is 7.0 + 3.9 pCi/g. The standard deviation is - 5 6 % of the mean value. This alone translates to an uncertainty of -1455 a (nearly one half-life) in the 226Ra age determination. Uncertainties associated with diagenetic effects are more difficult to evaluate. For example, if the early diagenetic loss of Ra in the Mammoth Hot Springs travertine samples had not been recognized through the additional.data obtained in this study, then through uncritical application of Eqn (3) the uppermost drill core sample would seem to have had

an age of - 5 0 0 0 a, and the lowermost sample analyzed (Y10-220.8) would seem to have had an age of -11,800 a. A reasonable estimate of the minimum practical uncertainty involved in 226Ra age determination of an individual sample of travertine, based on the Mammoth Hot Springs data, is + 1 half-life of 226Ra, or - 1 6 0 0 a. This could be reduced by considering a large sample population. The Mammoth Hot Springs travertines appear to represent an optimal locality for successful application of 226Ra age determination on the basis of the following criteria: (1) presently active travertine deposition; (2) relatively rapid deposition rate; (3) constant long-term conditions (water composition and temperature); (4) purity of travertine (>99% CaCO3, minimal 23°Th); and (5) independent age constraints (field relations, 23°Th/234U age determination of basal travertine). Certain travertine deposits may be unfavorable for 226Ra age determination, depending on the specific characteristics of the deposit (thickness, porosity, purity, deposition rate), the diagenetic processes that affect it, and the long-term stability of conditions (e.g. water temperature and composition).

SUMMARY AND CONCLUSIONS This paper presented the results of an investigation of Ra isotopes and alkaline earths (Mg, Sr, Ba) in hot spring travertine deposits of known mineralogy (from surface outcrops and Y-10 drill core) and thermal water from Mammoth Hot Springs, Wyoming. These data were acquired to better understand the process of diagenesis and its influence on (1) the distribution of alkaline earths in hot spring travertine deposits, and (2) the potential applications of Ra isotopes for the determination of ages and deposition rates of Holocene travertines. It was found that the travertine precipitates mainly in the form of aragonite, and undergoes a complete diagenetic transformation to calcite within a few years. During the transformation, Mg is enriched and Sr, Ba and Ra are depleted in the travertine. Apparent mineral-water distribution coefficients for Mg, Sr and Ba are within the range of experimentally determined values, and are consistent with equilibrium conditions and a high water-rock ratio during travertine diagenesis. The age of travertine from near the base of the deposit in Y-10 was determined by the (23°Th/234U) isochron method to be 7700 +_ 400 a. The (226Ra) vs depth relation in travertine from the Y-10 drill core is consistent with continuous deposition since - 7 7 0 0 a ago at an average rate of - 1 cm/a, and indicates that minimal exchange of Ra with thermal water occurred after the early diagenetic transformation of aragonite to calcite. These data imply that, under favorable conditions, (226Ra) measurements may be useful in

Ra isotopes in travertine, Mammoth Hot Springs, Wyoming, U.S.A. d e t e r m i n i n g ages a n d d e p o s i t i o n rates for o t h e r travertines. Acknowledgements--Work supported by Office of Basic Energy Sciences, Department of Energy, under Contract W-31-109-Eng-38 to Argonne National Laboratory (ANL). J. K. Bohlke and T. Patton assisted with field work at Mammoth Hot Springs. Members of ANL's Analytical Chemistry Laboratory assisted in obtaining much of the data reported herein: F. Markun (Ra isotope measurements), E. Huff (ICP-AES), and B. Tani (XRD). C. Binz and K. Orlandini assisted in alpha spectrometric measurements of U and Th. K. Bargar of the U.S. Geological Survey, Menlo Park, CA, and Prof. H. Chafetz of the University of Houston provided useful information at an early stage of the investigation. Reviews by J. K. Bohlke, F. Goff, Y. Kharaka, T. Kraemer, and T. Patton resulted in an improved manuscript. Editorial handling: Y. K. Kharaka.

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