A Boron Isotopic Study of a Mineralogically Zoned Lacustrine Borate Deposit: the Kramer Deposit, California, U.S.A

ISOTOPE GEOSCIENCE ELSEVIER Chemical Geology (Isotope Geoscience Section) 127 (1996) 241-250

A boron isotopic study of a mineralogically zoned lacustrine borate deposit: the Kramer deposit, California, U.S.A.

George H. Swihart a**, Eddy H. McBay b, David H. Smith b, Joseph W. Siefke c aDepartment ofGeological Sciences, University of Memphis, Memphis, TN 381.5’2. USA b Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA ’ U.S. Borax, Inc., Boron. CA 93516, USA

Received 27 July 1994; accepted 9 August 1995 after revision

Abstract

An investigation of the boron isotopic composition of hydrated borates in the Kramer lacustrine deposit of southern California was undertaken in an effort to better understand the origins of mineralogically zoned deposits of this type. Twenty-one samples from fifteen depths along a drill core through the deposit reveal that isotopic zoning accompanies mineral zoning. The range of 8’ 'B for borax through the 24.9-m-thick Na-borate core facies is + 0.1 to + 1.7%0 except for samples in and just below a clay-rich 6-m interval where 6”B varies from - 5.1 to + 2.3%0. The S”B of three cottonball ulexite samples in a 3.0- m-thick Na-Ca-borate facies above the Na-borate facies ranges from - 5.5 to - 4.6%0, whereas two samples from a 6. l-m-thick basal Na-Ca-borate facies both yield - 2.1%0. The S”B of colemanite from near the top of the upper Na-Ca-borate facies is - 8.6%0. The small range of S”B through much of the Na-borate facies indicates that the source waters of the borax-precipitating lake varied little in 6”B for a time interval of (5-7) - lo4 yr. The G”B variations within and among borax crystals in the 6-m clay- rich interval, some of which are estimated to have occurred over a period of weeks, were probably produced by a combination of pH change and Rayleigh effect during partial desiccation cycles. The distinctly different ranges of 6”B exhibited in the upper and lower Na-Ca-borate facies and the distinctive cottonball habit of the crystal aggregates indicate that ulexite originated through growth in the lake-margin muds.

1. Introduction “marsh” and playa settings (Bowser and Dickson, 1966; Xiao et al., 1992). These deposits range from Nonmarine hydrated borate deposits are the major mineralogically unzoned or poorly zoned to concentri- ores of boron. Hydrated borates are found in aprons cally or complexly zoned types. Many deposits are deposited recently around hot spring and gas vents in dominated by Na- and/or Ca-borates, but some contain South America (Muessig, 1966)) in Tertiary bedded significant proportions of Mg- and/or Mg-Ca-borates. deposits of uncertain origin (W.C. Smith, 1960; Hel- The origins of mineralogically zoned deposits are vaci and Firman, 1976)) in Tertiary to recent lacustrine unclear. The Miocene-age Kramer lacustrine deposit at deposits (Bowser and Dickson, 1966; Inan et al., 1973; Boron, California, is one of the most well-studied non- G.I. Smith, 1979), and are currently forming in marine borate deposits. It has a Na-borate core, a Na-

* Corresponding author. Ca-borate intermediate zone and a discontinuous [PDI Ca-borate outer zone. Several hypotheses have been

0009.2541/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDIOOO9-2541(95)00094-l 242 G.H. Swihclrt et al. /Chemical Geology (Isotope Geoscience Section) I27 (1996) 241-250 presented for the origin of the zoning in the Kramer Depth (m) 117 deposit based on structural and petrological studies. Oi et al. ( 1989) reported a correlation between the 6”B of hydrated borate minerals from a given deposit and the ratio of 13tB to 14’B in each crystal structure. They suggested that the observed mineral-mineral dif- Mlddle Ore borax 8 claystone ferences in 6”B can be estimated from the partition function ratios of the aqueous boron species, the proportions of these species in solution (which are pH Lower Ore borax & claystone dependent) and the i3’B/t41B ratios of the minerals. I I The basis for this proposed mechanism is spectroscopic study which shows that in dilute aqueous solution the boron isotopes fractionate between the two major species, B (OH) 3 and B (OH), , as a function of temper- Fig. I. General stratigraphy in drill hole MD-636 ature (Kakihana et al., 1977). Given that very few data have been available and the The boron extraction and isotope analysis procedu- actual original spatial associations of the analyzed sam- res are described in Swihart (1987), Leeman et al. ples are not known, the discovery by Oi et al. (1989) ( 199 1) , Nakamura et al. ( 1992)) and other references of a systematic 6”B distribution suggests that the 6”B cited below. Briefly, the procedures used were as fol- of each mineral must be relatively constant throughout lows. Samples were characterized on the basis of crystal a deposit. As they noted, this point remains to be inves- form and habit and, when necessary, by X-ray powder tigated. If so, does this distribution result by primary diffraction. Portions of unzoned crystals were extracted precipitation from a solution with a relatively constant with a Foredom@ drill, whereas portions of macroscop- 6”B or from postdepositional alteration processes such ically zoned crystals were sampled with a very fine- as deposit-wide equilibration or modification by out- tipped teasing needle. side solutions? These problems and the implications A portion of each sample sufficient to contain - 1 for the origins of mineralogically zoned hydrated mg of boron was weighed. Borax was dissolved in borate deposits are addressed in the present study with water. Ulexite and colemanite were fused with Na,C03 spatially constrained (drill core) samples from the Kra- and then dissolved in - 15 ml of low-boron distilled mer deposit. and deionized (D&D) water. Removal of Na, Ca and trace constituents from the ulexite and colemanite solutions was accomplished using batch exchange followed by column cleanup. The borax sample solutions only 2. Experimental procedures required the column exchange step. For each batch exchange, - 15 ml of fresh 100-200 mesh styrene divi- The borate-bearing portion of the Kramer deposit is nylbenzene cation exchange resin were conditioned in up to - 120 m thick and has a lateral extent of about an exchange column with two bed volumes of 1 N HCl 1.6 by 6 km (W.C. Smith, 1968; Siefke, 1991). In the followed by ten bed volumes of D&D water at a rate area of the drill core utilized in the present study (MD- of l-2 ml min-‘. The cleanup column containing - 5 636) the deposit consists of ulexite-bearing layers ml of the same type of resin was regenerated and con- (Na-Ca-borate) at the top and bottom, and A zone, ditioned after each sample solution with four bed vol- Middle Ore, B zone and Lower Ore borax (Na-borate) umes of 1 N HCl followed by ten bed volumes of D&D horizons in between the ulexite intervals (Fig. 1). water at a rate of l-2 ml min- ‘. Some dispersed colemanite crystals (Ca-borate) occur After the addition of an equimolar amount of man- near the top part of the upper ulexite interval. In the nitol to prevent boron loss during evaporation (Ishi- present study, most samples were chosen at intervals kawa and Nakamura, 1990), the boron-bearing of - 3 m. Samples were sealed in plastic storage bags solution was evaporated at - 50°C on a hot plate in an to impede dehydration of borax. enclosed box equipped with KOH-saturated quartz G.H. Swihart et al. /Chemical Geology (Isotope Geoscience Section) 127 (1996) 241-250 243

Table I NIST 95 1 boric acid standard 6’lB = [ 1(“B~ ‘“B)samp~e~(“B~‘oB)standard)

Analysis “BI’OB Internal precision, -l][lOOO] (1) U,,-l

I 4.05697 0.00043 2 4.0.5443 0.0007 1 3 4.05606 0.00067 3. Results 4 4.0568 1 0.00049 5 4.05510 0.00048 6 4.05603 0.00203 Borax samples (12= 14) from nine depths in the Na- 7 4.05628 0.00098 borate facies of the Kramer deposit (core MD-636) were analyzed (Fig. 2; Table 2). The 6”B of samples Average 4.05595 UP-1 0.00090 (n = 7) from six depths in the A, Middle Ore and Lower Ore zones ranges from + 0.1 to + 1.7%0. Samples fiber filters (Fogg and Duce, 1985; Spivack and (n = 7) from two depths in the B zone and one in the Edmond, 1986). When just dry the residue was taken upper part of the Lower Ore zone yield a G”B range of up in 1 ml of D&D water, yielding a 1 mg ml- ’ solution - 5.1 to + 2.3%0. of boron. All procedures were carried out using low- In the Middle and Lower Ore zones, where the vol- boron laboratory equipment (polyethylene beakers, ume percent borax is high (water-soluble B,03 ranges columns, etc.; platinum crucible; Teflon@ evaporation from 25.5% to 29.4%)) the range of 6”B (excluding dishes) and the final solutions were stored in capped the sample in the Lower Ore which is near the contact 1.5ml polyethylene microcentrifuge tubes sealed with with the B zone) is + 0.8 to + 1.5%0 (5 samples from paraffin. 4 depths). In the A and B zones, where the volume The prepared solutions were analyzed by dicesium percents of clay are moderate and high (water-soluble metaborate (Cs,BO:, m/z 308 and 309) thermal ion- B,O, = 16.6% and 6.5%, respectively), the ranges of 6”B are + 0.1 to + 1.7%0 (2 samples from 2 depths) ization mass spectrometry at Oak Ridge National Lab- oratory using a VG”’ Isotopes Isomass 354 instrument. Depth (m) 2 p,l of an 80% ethanol-20% water slurry containing llO_ - 80 p,g of high-purity graphite were applied at the ,,5. barren center of a degassed tantalum filament (Xiao et al., -.-----___...... ______----______.

1988). This was followed before dryness by 1 p.1 of 120- ?? . upper ulexite sample solution and 1 p,l of an aqueous solution con- ,25-______r’...______--_~_____~ taining 10 p.g of Cs,C03 (Spivack and Edmond, A zone . 130- 1986), to give a B/Cs molar ratio of - 1.5. The fila- .______-_---______._____ ment load was heated at 0.8 A for 5 min and 1.0 A for 13? Middle Ore . 10 min under a heat lamp. The instrumental analysis 140 .------______.-___~______. - B .?O”B . procedures were essentially the same as those of Lee- ,45’___._.______._____--s-______*__~~___~ man et al. (1991). 100 to 200 309/308 ratios were : Lowerore . 150:.------E------collected for each filament, averaged, and corrected for . bias and “0. The relative standard deviation for seven 155. Lower ulexlte . separate filament preparations of NIST (National Insti- 160- tute of Standards and Technology) Standard Reference - barren Material 951 boric acid (Catanzaro et al., 1970) ana- 165 -10 -6 -6 -4 -2 0 2 4 lyzed over a period of a year was 0.022% (Table 1). 6’ ‘B (permil) Isotopic compositions herein are expressed as permil Fig. 2. Boron isotopic data for samples from drill hole MD-636 deviations from the certified value of the NIST boric arranged by depth from the present surface and zone. Symbols: acid standard: circfe = borax; square = cottonball ulexite; cross = colemanite. sluauupas U! SI!SSOJuegeurLueuI Icq pale3!pu! SF uop ysish:, pauoz dIqFs!A ‘Icnp!A!pu! 30 suoglod aIdyntu -!sodap aye1 30 pua aletu!xolddc au ( 1661 ‘ayja!S) SE IlaM se pazrC[euv alaM qldap lytgled -e 1~ laylo ileseq aql 103 a% auaDo!w aIpp!m lCIlea ua pla!d qDva 30 slaiaurpua3 Ma3 e u!qly palerys sp2lsrC.1~ salr?p iv-x y3ol-aloqM .aIqti2m03u03 a.n2spaq ayr?I aqi ./(Ia~g~adsai q3y~ ~I!M ‘ip2seq y3eqaIppEs BuFlCIlapun aqi 30 saiep ‘(sqldap z ~1013 salduIes 9) O%E’Z+ 01 1’s - pue cgatuo!pel Aq paleD!pu! s! ayeI %uysodap-aleloq aql3o uoy~u.103 30 awg alEmrxoldde au IIMOU~IClas!Dald IOU SF l!sodap ayl Icq paluasaldal Ishlalu! auIg aqL .gEg-aw alo3 UIOIJ saldms aql ut paAlasqo alaM samlEa asaql30 11~ *sdol lake1 .n?au sp2ish la%lel paiuarlo pue sladel xvloq 30 aseq aqi _rvau s~elsh JaIIeurs paluarlo rl~u1opuel30 aDuasald aql pue ‘slah~ xeloq atuos 30 sdol aql II? uolsono:, 30 aDual Z’O I’Z- I’Z- -s!xa aqi ‘suogwgal Islpaqna %u!ce3-plemdn u1013 I.0 ~.IEMUMO~ lade] qxqm spz?lshr~30a3ua.un330 uou1u103 aqi sapnpu! aDuap!Aa aqi ‘Ielauas u! xeloq 1auIeq P'O 0’1 + pue 1asMoa 6q palm sv inoqv (996[) uosyD!a ‘9E9 Z’O co+ -am alo3 u! xeloq aql30 lsour 30 uop’sodap hreur!ld E'O Z‘l - 103 luepunqe st aDuap!Aa au ‘spaq aql30 uogtsodap 30 po!lad aql30 q3nzu laho pa.unDDo slalcM a3lnos aql

30 aI ,g aqi 30 uoy!.n2~ ‘due 31 ‘apiq dlah it2~ sait2!pu! Z’O E’Z-b ‘auoz 8 aq3 MoIaq lsn[ pua u! 1x1013salduIes %urpnpxa O’Z co+ ‘0%~’I+ 01 1.0 + 30 a&El arIg aql ‘salelrd!Dald ~C.IIXII Z‘O l.Z- -pd paJaqeun se uayel aq ue3 sle1sd.1~ xeloq aql31 8.0 9’1+ 1’0 I’S- z.0 P’E- sapvJalv.doq-vN ‘1.p

uopsnma -p E’l p’1+ P’O o’I+ so 5’1+

‘0%9’P - 01 S’S - uro13 sake1 saI3t23 ait?loq-eN aqi alzoqe Itc?Alaiu!tu-0 ’~ 2’0 I’o+ ou t! laho pvalds sa[duIes al!xaln aalql 30 g~,,g aqL . (2 5’1 L’I + saA .ro!Jam! aIqeL) 0~1.z - 30 silnsal 8, rg p23guapr ane% %.I 1.9 Lq laqioue auo 1.11013paleledas Qpg1laA ‘sa!Dv3 aleloq -eN aqi MoIaq say3 ai!xaIn aqi u! saidures OML E'O I’S- ‘laylo qDr?a30 uo!s!Dald Ieulalxa Z’O 9’P- Z’O S‘S- paivuqisa D 1 u!qiIM an (al0 laMo7) Iahal UI-L’~~I aqi Z’O S‘8- 1E IkWh pauoz A11c3!dogsolDeur e 30 slah[ q1Mols aql P'O 9’8 - 01 IaIIeled suoplod lua3efpe 111013sqnsal ‘~SRIJUOSUI ‘0~9’ I+ 011 ‘s - uroy 8, rg 30 aSue1 e paDnpold ‘sladc1 qlMOl2 UIUI-S.0 N Ielahas 30 %ys!suo~ uoylod qcea ‘S~eWb:, asaqi 30 au0 u! sladel qiMol8 aqi 01 laIlen?d suoglod aa.Iu ‘sa!Dv3 aleloq-t?N algua aql u! pau;rur -1aiap suoysodruo3 rsa!Aeaq pue isalq8y aql hqsq -qeisa ‘(z aIqtgJ O%E*z+ 01 1’s - uxo13 sak?uel (auoz I!sodap .rameq aql IOJ wp adolos! uo~og a> qidap w-SPPI aql 1~ s~elshr~ aalql 30 al,g aqA z alqv G.H. Swihart et al. /Chemical Geology (Isotope Geoscience Section) 127 (1996) 241-250 245

Table 3 Clay and borax proportions in the Na-borate facies

Zone Thickness Water-soluble B,03 Borax” Borax Clay (m) (%) (vol%) (m) (m)

A 5.9 16.6 45.5 2.1 3.2 Middle Ore 8.7b 29.4 80.5 7.0 1.7 B 4.3b 6.5 17.8 0.8 3.5 Lower Ore 5.2 25.5 69.8 3.6 1.6

Total 14.1 10.0

“For this calculation the densities of borax and clay are assumed to be equal. bThis zone contains a tuff layer whose thickness is not included in the value given. above the ore body, but these are also early Middle modern hot spring data. H. Kakihana et al. (1987) Miocene (Whistler, 1965). determined that the 6”B of water samples (n = 5) col- G.I. Smith (1979) used 14C data to estimate a dep- lected over a 20-yr period from a hot spring in Japan osition rate of 1 cm/46 yr for a Quaternary age clay- was constant, within the resolution of the method. The rich interval (the Bottom Mud) in the Searles Lake average internal precision of the latter measurements deposit, California. From observations of modem salt was - f 0.2%, expressed as a relative standard devi- deposition in Owens Lake, California, where the salts ation. included halite, trona and burkeite (Friedman et al., Although the general picture from core MD-636 is 1976)) G.I. Smith ( 1979) estimated a deposition rate one of precipitation of borax of rather consistent S”B, for solid salt beds of 20 cm yr-‘. In the same way, samples from in and just below the B zone show greater using appropriate solubilities at temperatures ranging isotopic variability. In crystal “a” at the 144.5-m depth from 5” to 20°C (Nies and Hulbert, 1967)) we estimate (Table 2) the 8’B of the core is lightest, an interme- a maximum deposition rate for a solid borax layer in diate portion is heaviest and another intermediate sec- the range of 24 cm yr-’ at a relevant arid climate tion still farther from the core falls between the two evaporation rate of 150 cm yr- ’ cm-*. extremes. The common occurrence in the Kramer Applying these clay and borax deposition rates to deposit of 1-3-cm-thick borax layers (Bowser and the proportions of clay and borax beds (Table 3) in the Dickson, 1966), combined with the average crystal 24.9 m of the Na-borate facies in hole MD-636 yields zone thickness of -0.5 mm, suggest that the crystal a time interval of - 5. lo4 yr. This figure probably zones could be diurnal growth layers. The question then underestimates the true time interval by as much as arises of how such isotopic variability could be formed 40% because the claystone layers in the Kramer deposit over a period on the order of a few weeks to a few have been buried more deeply and undergone more months. The differences in 611B between and within compaction than the muds at Searles Lake. According crystals in the B zone might have been formed by one to these estimates, short periods of borax precipitation or more of the following: (1) variation in the 611B of were separated by long periods of time when the lake the source waters; (2) change in the lake brine 6”B was not saturated with respect to borax. because of fractionation during adsorption of boron to The results indicate that over much of the more than varying concentrations of suspended clay; (3) temper- 5 - lo4 yr the lake existed in the vicinity of drill core ature variation of the lake brine; (4) a borax-brine MD-636, the S”B of the precipitated borax varied by fractionation factor which varied with brine pH; and/ - fO.l%relative(i.e.#‘B= +O.lto +1.7%0).Var- or (5) Rayleigh fractionation during partial desicca- ious lines of evidence indicate a hot spring origin for tion. the boron and Na-bearing solutions which gave rise to The first possibility is unlikely because the amount the Kramer deposit (Barnard and Kistler, 1966; Bowser of boron in the borax-saturatedlake water was undoubt- and Dickson, 1966). Therefore, it is pertinent to com- edly normally large compared to that delivered to the pare the observed variation in 6’lB given above with lake over short intervals of time. Major changes in the 246 G.H. Swihart et al. /Chemical Geology (Isotope Geoscience Section) 127 (1996) 241-250

12 6”B of the source water would have produced long- ,,‘. ,’ term variations in the 6”B of the lake water which are 11 _I’ ‘* ,’ _..’ not evident in the borax record. ,’ .I 10 ,,:./ The phenomenon of boron adsorption to sediments ,..’ PH ., in solution is well known. A number of experimental 9 /’ . .I. ,’ studies have shown that the adsorbed boron is isotopi- ... ‘* / g _.:’ , tally lighter than the solution (Shergina and Kamin- *.: I :’ . ;< skaya, 1967; Schwartz et al., 1969; Palmer et al., 1987; 7 i.: Spivack et al., 1987). When a significant fraction of -15 -10 -5 0 5 10 15 Abor-br (Permil) the solution boron is adsorbed the solution 6i’B increases. If this mechanism was important in the Fig. 3. A,,,,_br vs. pH at 25°C. The dashed curve is calculated with ancient Kramer lake the clay-rich intervals should con- czaq= 1.0194 and the sokd cuwe with CK~~=1.0320 for a 0.646 m boron solution [molality of a saturated borax solution at 25°C as tain borax enriched in “B compared to the borax in the calculated by Felmy and Weare ( 1986) 1, The dotted linear regres- clay-poor intervals. But in fact just the opposite is sion is from the experimental data (diamond symbol) of Oi et al. observed in the Kramer deposit (Table 2 and Table 3). ( 1991). See text for further explanation. The G’'B of precipitating borax might vary with temperature because the proportions of the polyborate ions in dilute aqueous solution at various temperatures. The in solution at constant boron concentration vary with dependence of Abor_bron solution pH is defined through temperature (e.g., Mesmer et al., 1972). A temperature borate equilibria (assuming no Na-borate complex), effect would be evident in both the clay-poor and clay- mass conservation and an equation for X, (see Oi et rich zones. However, Ore zone borax in general has al., 1989, 1991). nearly constant 6”B and two analyses of adjacent por- If we assume that: (a) S”B of the initial fluid was tions of a macroscopically zoned crystal at the 148.7- equal to the average 6i’B of borax in the Ore intervals; m level, Lower Ore, are within la estimated external (b) Abor_br can be determined using the a1 ‘B-value of precision of each other (Table 2). the lightest borax in the clay-rich intervals; and (c) The borax-brine fractionation factor, Abormbr, is precipitation did not remove a significant fraction of dependent on the proportions of ‘slB and t4’B in the boron from the brine (i.e. no Rayleigh effect) and precipitate and in solution. The latter proportions in further assume a concentration for boron at borax sat- turn are dependent on the solution pH, temperature and uration, then we can calculate the pH dependence of boron concentration. The approximate dependence of Ai,bor_brat 25°C. Fig. 3 shows this relationship estimated Abor_hr on the pH of the solution can be calculated utilizing (Y,~= 1.0194 from Kakihanaet al. (1977) and following the methods of Kakihana et al. ( 1977) and stability constants from Spessard ( 1970; no added salt Oi et al. (1989, 1991). Oi et al. give an equation solution). Note that in these estimates it is assumed that approximating the equilibrium distribution of the boron the same stability constants apply over the entire cal- isotopes between two phases, which can be expressed culated range of pH. In fact, these factors vary with for brine and borax as: temperature, boron concentration and major compo- nent composition of the brine in ways that cannot be (0.5-X&%,-I)+1 adequately addressed at present. A bor-br= 10” In (2) [ X, ( %q -l)+l 1 Another approach to determination of fractionation factors is through laboratory experiments. Palmer et al. where A bor-br is the estimated fractionation factor (1987) estimated (Y,~ to be 1.032 from boron-clay ~i’Bbo,- 6i’Bbr; X, the mole fraction of 13’B calcu- adsorption experiments. The pH dependence of Abor_br lated from all the boron species in solution; and aaq the using this fractionation factor is shown in Fig. 3. Oi et ratio of the reduced partition function ratios (RPFR’s) al. ( 199 1) determined the variation of Abor_br with pH of B (OH), and B( OH), in solution. The RPFR’s of directly by precipitation experiments. Their data are B (OH), and B( OH), have been calculated (Kaki- also shown in Fig. 3. The methods described above hana et al., 1977) from the spectroscopically-deter- indicate that a pH decrease from the crossover point mined fundamental vibration frequencies of the species ( Abor_br= Cl%,) of l-2.6 units could account for the G.H. Swihart et al. /Chemical Geology (Isotope Geoscience Section) 127 (1996) 241-250 247 lightest borax results (Fig. 3). The #‘B-values even during partial desiccation. A #‘B of the source between the lightest borax and the initial brine could waters equal to the average 6i1B of borax in the Ore have formed by smaller shifts in pH. Variations larger zones, - + 1%0, compares with the 6’lB of Salton Sea than 1 unit of pH are not common in contemporary and Coso (California) geothermal fluids at - 2 and borate-bearing saline lakes (e.g., Xiao et al., 1992)) so +3%0, respectively (Leeman et al., 1992), indicating it is likely that pH change was accompanied by another extraction from reservoir rocks of continental affinity. process affecting borax 6”B. Actual pH in the ancient The Kramer beds and rocks beneath them are indeed lake cannot be determined from borax S”B because nonmarine in origin. The lacustrine Shale member of the latter also depends on the boron concentration at the Kramer beds, within which the ore body lies, rests borax saturation and the extent of desiccation (Ray- conformably upon the 183-m-thick Saddleback basalt leigh effect). member, which lies unconformably upon a 457-m- The S”B range in the B zone could also have formed thickness of nonmarine tuffs, shales, limestone, arkose by Rayleigh fractionation during partial desiccation, and conglomerate (Siefke, 1991). The pre-Tertiary provided that A bor_br# O%o. There are many possible basement consists of granitic and metamorphic rocks. combinations of initial brine S”B and Abor_br which pH changes and a Rayleigh effect during partial des- will work. The range of 6”B in the B zone is repro- iccation cycles were necessary to produce the B zone duced best using an initial brine S”B equal to the aver- borax S”B variations. Shifts in pH could have been in age of the borax values in the Ore zones and a Abor_br part caused by changes in the concentrations of dis- determined from the lightest borax analyzed. Then the solved constituents other than boron species, such as Rayleigh calculation shows that all the borax with 6”B- HCO;, of which we have no sedimentary record. values greater than the lightest borax could have been produced when the fraction of boron remaining in the 4.2. Na-Ca-borate facies brine was still greater than 0.3. There is no textural evidence that the lake ever completely desiccated dur- As discussed above, sedimentary structures and crys- ing borax precipitation. The reversal in the #iB trend tal habits in the Na-borate facies in core MD-636 indi- seen in the outermost analysis from crystal “a” at the cate that most borax formed by precipitation directly 144.5-m depth could only have been formed over such from lake brine. A number of hypotheses have been put a short time interval by a pH shift. forward for the origin of the ulexite-dominated Na-Ca- The relatively constant and larger 6’iB of borax in borate facies which borders the Na-borate facies in the the Ore zones as compared with the clay-rich B zone Kramer deposit. particularly, must be considered in light of the controls (1) W.C. Smith (1968) theorized that some of the on A,,,,, discussed above. The relatively constant ulexite (in particular some in the Upper Na-Ca-borate borax composition in the Ore zones could only have facies) could result from the interaction of Ca-bearing been formed with a delicate balance between the pre- solutions with an already formed borax lens. Bowser cipitate and source water boron fluxes, or with a t3’B/ and Dickson (1966) also state the possibility of the t41B in the brine of I:1 as it is in borax. In the case of origin of the ulexite through postdepositional alteration a A bor_br# O%O,after the onset of saturation a transient of the borax lens margins by Ca-bearing solutions. interval would have occurred during which the 6’iB of (2) G.I. Smith ( 1979) has discussed how the major- precipitating borax decreased ( ALbor_br> C%O) or element chemistry of a saline lake depends on what increased ( Ahor_hr< O%O) . Eventually, the 6”B of the minerals have precipitated in any lakes upstream from precipitating borax would equal that of the source water it. The Lower and Upper ulexite zones in the Kramer and would change no further. deposit could represent such upstream changes before Since Na and boron species were principal chemical and after Na-borate facies formation. constituents in the ancient Kramer lake (Bowser and (3) Barnard and Kistler ( 1966) suggested that sodic Dickson, 1966)) the pH was probably often controlled lake water may have precipitated borax in the lake by their equilibria. Consequently, the lake brine may interior, but ulexite along the lake margins where mix- have often had a [31B/t41B equal to 1: 1 and the 6”B ing with Ca-bearing surface runoff and groundwater of borax and brine would have been the same, perhaps occurred. 248 G.H. Swihart et al. /Chemical Geology (Isotope Geoscience Section) 127 (1996) 241-250

(4) Finally, Barnard and Kistler ( 1966) and Bowser tiguous with or vertically segregated from borax, not and Dickson ( 1966) espoused the idea that ulexite may mixed with it. have grown in the muds surrounding the lake by reac- Given the available ulexite results, proposed origin tion of Ca-rich groundwater with Na-borate-rich brine (4)) mixing of Na-borate-rich brines and Ca-rich at the time of lake activity or thereafter before sediment groundwater in muds around the lake, is the most prob- consolidation. able mode of origin of the cottonball ulexite. In this As evidence for the last suggested mode of origin case, the difference in 6l’B of the Upper and Lower Bowser and Dickson ( 1966) observed that the cotton- ulexite zones and the differences within the Upper zone ball form of the ulexite forming today just beneath the could be due to distance from the lake and relative surface of dry lakes in Nevada and California is iden- proportions of seeped lake brine (“B-rich) and clay tical to that of cottonball ulexite in the Kramer deposit. adsorbed boron ( “B-rich) in clay deposited on the flats But, as they state, this is not conclusive evidence that beyond the saturated lake level margin. Fractionation the Kramer cottonball ulexite formed in the same man- may have accompanied brine migration through the ner as that forming in dry lakes today. In fact, the lake-margin muds as well. amount of evidence available to support or refute any The top portion of the Upper ulexite zone in core of the above hypothesized modes of origin has been MD-636 contains cottonball ulexite, massive ulexite limited, and this problem is compounded by the occur- and medium-grained colemanite crystals. An apparent rence of several distinct forms of ulexite in several equilibrium fractionation factor for ulexite*olemanite settings in the Kramer deposit. Thin layers of massive was determined from coexisting samples in the Cem- ulexite ( “television rock”) occur in the Upper ulexite etery deposit, a Tertiary bedded borate deposit in Fur- zone; cottonball ulexite occurs in well-layered clay- nace Creek, California (Swihart et al., 1993). The stone; nodular ulexite occurs in “collapsed claystone’ ’ observed A u,ex_, is 2.0%0. The estimation method of (W.C. Smith, 1968). Samples from both the Lower Oi et al. (1989) based on RPFR’s gives a calculated and Upper ulexite zones in core MD-636 are nodular A ulexX:o,of 1.3%0 at 25°C. The closest cottonball ulexite or cottonball ulexite and occur in well-layered clay- and colemanite samples analyzed in the Kramer deposit stone. are 3.1 m apart in the Upper ulexite zone (Table 2). The ulexite #‘B-values given in Section 3 are sig- Here the observed A ulex-co,is 3.0%0. The close approx- nificantly lighter than those found over most of the imation of this to the value of the apparent fractionation 24.9-m interval of the Na-borate facies in core MD-636 factor obtained from the Cemetery deposit, the mixed (Table 2). In particular, they are lighter than those in occurrence of cottonball ulexite and colemanite in this the portions of the Na-borate facies that are vertically section, and the well-established secondary nature of adjacent to the ulexite zones (Fig. 2). Given the small colemanite in many deposits around the world argue variation in 6’lB exhibited over most of the Na-borate that this colemanite formed by alteration of cottonball facies (Fig. 2), possibilities ( 1) and (2) described ulexite. above should have produced Upper and Lower ulexite In contrast to cottonball ulexite, massive ulexite in facies with similar #‘B. The equilibrium fractionation the Upper ulexite zone in core MD-636 yields virtually estimation method of Oi et al. ( 1989) based on RPFR’s the same a1 ‘B-value as the colemanite (Table 2). The gives a Abor-ulex of 1.9%0 at 25°C. The closest borax cross-cutting nature of the massive ulexite and its and cottonball ulexite samples at the top of the Na- apparent lack of equilibrium with the colemanite sug- borate facies, vertically separated by 0.9 m, yield a gest that it is vein material and that its boron has expe-

A bar-ulex of 6.0%0. The analogous value for the closest rienced a more complex fractionation history. borax and ulexite samples at the bottom of the Na- borate facies. vertically separated by 2.2 m, is 3.0%~ Textural evidence argues against process (3). Coprecipitation of cottonball ulexite and borax from a 5. Conclusions single solution, as would occur in hypothesized process (3)) would form amixture of ulexite and borax crystals. ( 1) The 6”B data set from the Na-borate facies in But the cottonball ulexite in question is laterally con- the Kramer deposit is evidence that a geothermal sys- G.H. Swihart et al. /Chemical Geology (Isotope Geoscience Section) 127 (1996) 241-2.50 249 tern can produce a relatively uniform boron isotopic Bowser, C.J. and Dickson, F.W., 1966. Chemical zonation of the signature over tens of thousands of years. borates of Kramer, California. In: J.L. Rau (Editor), Second Symposium on Salt. N. Ohio Geol. Sot., 1: 122-132. (2) The results of previous studies of hydrated Catanzaro, E.J., Champion, C.E., Gamer, E.L., Mariner&o, G., Sap- borates suggested that individual borates had a rather penfield, K.M. and Shields, W.R., 1970. Boric acid: Isotopic, and uniform S”B throughout a given deposit, but the exact assay standard reference materials. U.S. Natl. Bur. Stand., Spec. spatial relationships were either not given or not Publ. 260-17.70 pp. known. The careful sampling methods of the present Felmy, A.R. and Weare, J.H., 1986. The prediction of borate mineral equilibria in natural waters: Application to Searles Lake, Cali- study demonstrate that the S”B of borax and ulexite in fornia. Geochim. Cosmochim. Acta, 50: 2771-2783. the Kramer deposit generally varies at the f 1%0 level. Fogg, T.R. and Duce, R.A., 1985. Boron in the troposphere: distri- (3) Estimated precipitation rates indicate that the bution and fluxes. J. Geophys. Res., 90: 3781-3796. common I-3-cm-thick borax layers may each have Friedman, I., Smith, G.I. and Hardcastle, K.G., 1976. Studies of been deposited in as little as a few months. The thick- Quaternary saline lakes, II. Isotopic and compositional changes ness of zones in macroscopically zoned crystals,( N 0.5 during desiccation of the brines in Owens Lake, California, 1969-71. Geochim. Cosmochim. Acta, 40: 501-511. mm) probably correspond to diurnal precipitation of Helvaci, C. and Firman, R.J., 1976. Geological setting and mineral- borax. The larger, short-term (weeks) variation of ogy of Emet borate deposits, Turkey. Trans. Inst. Min. Metall., borax 611B in the clay-rich B zone is best accounted 85B: 142-152. for by pH changes in a partially desiccating brine. Inan, K., Dunham, A.C. and Esson, J., 1973. Mineralogy, chemistry (4) The 6i ‘B data indicate that portions of the Na- and origin of Kirka borate deposit, Eskishehir Province, Turkey. Trans. Inst. Min. Metall., 82B: 114-123. Ca-borate facies consisting of cottonball ulexite formed Ishikawa, T. and Nakamura, E., 1990. Suppression of boron volatil- by reaction of groundwater and lake brine seeping ization from a hydrofluoric acid solution using a boron-mannitol through the lake margin muds. complex. Anal. Chem., 62: 2612-2616. Kakihana, H., Kotaka, M., Satoh, S., Nomura, M. and Okamoto, M., 1977. Fundamental studies on the ion-exchange separation of Acknowledgements boron isotopes. Bull. Chem. Sot. Jpn., 50: 158-163. Kakihana, H., Ossaka, T., Oi, T., Musahi, M., Okamoto, M. and Nomura, M., 1987. Boron isotopic ratios of some hot spring The authors are grateful for comments by A. Ven- watersin the Kusatsu-Shiranearea, Japan. Geochem. J., 21: 133- gosh and an anonymous reviewer. G.H.S. thanks P.B. 137. Moore (University of Chicago) for an introduction to Leeman, W.P., Vocke, R.D., Beary, E.S. and Paulsen, P.J., 1991. former Kramer Mine Manager J. Minette, who in turn Precise boron isotopic analysis of aqueous samples: Ion exchange extraction and mass spectrometry. Geochim. Cosmochim. Acta, facilitated contact with J.W.S. J.W.S. thanks the staff 55: 3901-3907. of the U.S. Borax Research Laboratory for the bulk Leeman, W.P., Vocke, R.D. and McKibben, M.A. 1992. Boron iso- soluble boron analyses. The high-purity graphite was topic fractionation between coexisting vapor and liquid in natural obtained through the kind assistance of J. Fassett geothermal systems. In: Y.K. Kharaka and A.S. Maest (Editors), (National Institute of Standards and Technology). Water-Rock Interaction, Vol. 1. A.A. Balkema, Rotterdam, pp. G.H.S. gratefully acknowledges that portions of this 1007-1010. Mesmer, R.E., Baes, C.F. and Sweeton, F.H., 1972. Acidity mea- research were supported by a grant from U.S. Borax surements at elevated temperatures, VI. Boric acid equilibria. Co. and a faculty seed grant from the University of Inorg. Chem., 11: 537-543. Memphis. Work at Oak Ridge National Laboratory was Muessig, S., 1966. Recent South American borate deposits. In: J.L. supported by the U.S. Department of Energy, Office of Rau (Editor), Second Symposium on Salt. N. Ohio Geol. Sot., Basic Energy Sciences, under contract DE-ACOS- 1: 151-159. Nakamura, E., Isbikawa, T., Birck, J-L. and All&e, C.J., 1992. 840R21400 with Martin MariettaEnergy Systems Inc. Precise boron isotopic analysis of natural rock samples using a boron-mannitol complex. Chem. Geol. (Isot. Geosci. Sect.), 94: 193-204. References Nies, N.P. and Hulbert, R.W., 1967. Solubility isotherms in the system sodium oxide-boric oxide-water. J. Chem. Eng. Data, Barnard, R.M. and Kistler, R.B., 1966. Stratigraphic and structural 12: 303-313. evolution of the Kramer sodium borate ore body, Boron, Cali- Oi, T., Nomura, M., Musashi, M., Ossaka, T., Okamoto, M. and fornia. In: J.L. Rau (Editor), Second Symposium on Salt. N. Kakihana, H., 1989. 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