The origin of the Recent non-marine evaporite deposit of Saline Valley, Inyo County, California

Geochimica et Cosmochimica Acta,1968, Vol.32,pp.1279 to1301. Pegamon PM.

Printed InNorthern Ireland

The origin of the Recent non-marine evaporite deposit of Saline Valley, Inyo County, California LAWRENCE A. HARDIE Department of Earth and Planetary Sciences, The John Hopkins University,

(Received 6 Febmaq

1968;

Baltimore, Maryland,

21218

accepted in retiaed fomn 29 July 1908)

Abstract_--A study of the mineralogy and hydrochemistry of the Recent evrtporite deposit in the Saline Valley playa has been carried out. Halite, thenerdite mirabilite, glauberite, gypsum, calcite, dolomite, ulexite, anelcime and sepiolite have been identified in effloresoences and/or in sands and muds. Inflow is from springs and mountain streams which disappear on reaching the alluvial fans so that the playa is fed by groundwater flow only. These inflow waters aze sodium-calcium sulfate-bicarbonate waters while the playa brines (shallow sub-surface, brine table 0’ to -15’) are sodium sulfate-chloride waters which progressively increase in total ion concentration from the margin in toward the center of the playa. Two initial steps in the development of the plays brines are (1) calcite precipitation within the alluvial fans causing a drastic decrease in Ca*+ and HCOs- proportions in the waters reaching the playa, and (2) precipitation of gypsum at the playa edge, which controls the initial SO,” concentration of the brines. Further evaporation simply leads to waters dominated by chloride and alkalis. The distribution of evaporite minerals in the sands and muds of the playa is zonal. From the periphery to the center of the playa, roughly concentric zones carry gypsum; gypsum + glauberite; glauberite; gleuberite + halite. This sequence would be produced in proper order by progressive evaporation under equilibrium conditions of certain solutions in the experimental system CeS04-Na&104-N&l-H20 at temperatures between about 10” and 50°C and 1 atm total pressure. Quantitative agreement in composition of experimental and natural brines co-existing with the same assemblages confirms this equilibrium model for the playa evaporites. The model implies that chemical evolution of the natural brines follows a predictable course, one controlled mainly by the bulk composition of the parent water and by the extent of evaporation. The present zonal configuration represents only a stage in the long-range geochemical evolution of the evaporite deposit; outward migration of the existing zones, and addition of a new zone, with time is postulated.

INTRODUCTION MANY closed basins in the Basin and Range province

of the western United States are floored by salt-bearing, brine-saturated lake or playa sediments of Pleistocene to Recent age. These basins, as isolated hydrologic units, provide excellent “field laboratories” for studies of geochemical and hydrological processes operating in evaporite formation. Only a few such studies have been carried out, notably those of EARDLEY (1938) on Great Salt Lake, Utah, JONES (1963, 1965) on Deep Spring Lake, California, and EUMTER and SMITH (1965) on Searles Lake, California. The present study on Saline Valley, California, is one further attempt to understand the interaction of atmosphere, waters and precipitate minerals in a modern, active evaporite environment. Early work on the evaporite deposit of the Saline Valley plays was limited to analyses of a few grab-samples of brine or salt-crust (GALE, 1914; KING, 1948, p. 190) and to two exploratory bores (TUCKER, 1926, p. 527; VER PUNCK, 1958, p. 25). Recently, LOMBARDI (1963) made a very detailed study of the chemical compositions 1279

of spring waters and brines collected from Saline Valley. He l)aid special attention to the trace element geochemist,ry of the playa waters. In t)hc present work, t,he approach has been to unravel the minera,l-brine relations existing in the active playa. To do this, the evaporite mineralogy of the upper 6-15 ft of playa sediments has been determined and major ion analyses of the co-existing brines carried out. These field data have then been interpreted using available experimental data on synthetic, water-salt systems. PHYSICAL SETTING Saline Valley is a small, closed, intermontane basin in Inyo County, California, about 15 miles northeast of Owens Lake (Fig. 1). The basin, some 35 miles long and 20 miles wide, is bounded to the west by the Inyo Mountains, which rise to over 10,000 ft above sea level, and to the east by the northern end of the Panamint Range, which reaches an altitude of about 6000 feet.

B ,--\ :, ‘\_ -.

+

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119"

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present l&es

1

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(

-

25

Sodo Lake 81

50

75

MILES

@J ployos

-_I_,

(,_~‘QuOrternorY

lakes

Fig. 1. Map showing the location of Saline Valley in relation to neighboring desert basins of southern California.

The lowest part of Saline Valley is a salt-encrusted playa which covers an approximately circular area of about 16 square miles (in Figs. 2 and 3 roughly the A very shallow body of open brine a few area enclosed by the lOSO-ft contour). inches deep, saturated with respect to NaCl, covers less than one square mile of the salt surface at the southwestern corner of the playa. Outward from this salt lake, brine is found in sand or mud 1-15 ft below the efflorescent salt crust which varies in thickness from 1 or 2 in. to about 36 in. This sub-surface brine is dilute at the

The origin of the Recent non-marine evaporite deposit

1281

playa margins (about l-3 % dissolved salts) but rapidly increases in concentration toward the center of playa (about 27-29% dissolved salts) setting up a steep concentration gradient (see LOMBARDI, 1963, Fig. 15). Drainage in Saline Valley is entirely sub-surface except when rare flash floods temporarily inundate a large part of the playa. Springs rise in the deep, narrow, winding canyons of the faulted eastern slopes of the Inyo Mountains, at the base of these scarps, and in the canyons of the Panamint Range. These waters flow on a rock-bed in the canyons, only to disappear into the ill-sorted alluvium of the huge coalescing fans which apron the very steep valley walls. On the north-eastern edge of the valley are a series of hot springs which well up through travertine, but flow from these, too, disappears downslope into alluvium. No records of rainfall in Saline Valley are available but annual precipitation probably averages less than 3 in. on the valley floor and most of this may fall during a single storm; periods of more than a year have probably passed without rain [cf. rainfall pattern in nearby Death Valley given by HUNT et al. (1966, p. 5)]. The bedrock of the Saline Valley drainage area is basically a thick (max. 15,000 ft) sequence of Paleozoic limestones and dolomites with small amounts of quartzite, sandstone, shale and a few conglomeratic horizons. Into these sediments were intruded large masses of Mesozoic quartz monzonite which, in places, has produced by contact metamorphism abundant marble, talc-silicate rocks and hornfelses. Poorly exposed fanglomerates and lake mudstone underlie Pleistocene basalt flows which cover large areas at the northern end of the valley. The youngest deposits consist of large alluvial fans and the sands, muds and salts of the present playa. MCALLISTER (1956), who mapped the Ubehebe Peak area at the southeastern corner of the valley, gives a more detailed description of the bedrock stratigraphy and petrography. The general geological features of the whole area are summarized in the Geologic Map of California, Death Valley Sheet (1: 250,000 ; published in 1958 by the California Division of Mines, San Francisco). WATER COMPOSITIONSAND EVAPORITE MINERALOGY Field methods The area of the playa surface covered by efflorescent salt crust is about 16 square miles. Sampling points on the playa were located at roughly one mile intervals; in certain parts of the playa collection points were more closely spaced. Altogether samples were collected from 50 shallow holes dug into the playa deposits. The locations of these holes, shown in Fig. 2, were by resection using a plane table and telescopic alidade. Both aerial photographs and U.S.G.S. topographic maps (Ubehebe Peak, New York Butte and Waucoba Wash quadrangles) covering the valley area were used as base-maps. At each location holes were dug by pick and shovel through the ef?lorescent crust into the underlying sand and/or mud until brine was reaohed. For the deeper brines a 2-in. dia. handauger was used. The stratigraphy observed in each hole was logged and samples of each salt and sediment layer were collected. The unconsolidated sediments were channel-sampled, wrapped in plastic bags and sealed in 10-0~ cans using a portable hand-operated canner. Sub-surface brines in sandy sediment generally flowed into a hole as soon ss it was dug but several hours were required to obtain a brine-pool in boreholes drilled into clayey sediments. When the suspended sediment had settled samples of brine were collected by ladling the clear liquid into polyethylene bottles. The temperature of the brine was measured in the borehole as soon as brine was reaohed.

1282

Water

comp08itiorw

The water samples collected in Saline Valley were analyzed by Miss Shirley Rettig, Water Resources Division, U.S. Geological Survey, Washington, D.C. The methods used have been set forth by RAINWATER and THATCHER(1960). The special problems encountered in analysis of a concentrated brine, and the techniques used to overcome these problems have been discussed by Jones (1963, pp. 97-106: 1965, pp. 25-27). N

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0

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symbols of

+

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MIRABILITE t

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the

Lombordt’r

(I%31

boreholes

Fig. 2. Map of the Saline Valley playa showing the zonal distribution of evaporite minerals within the upper 15 feet of playa sediments below the efflorescent crust.

The major-ion analyses of 24 samples of brine from Saline Valley are presented in Table 1. The sample numbers are those of the boreholes from which the brines were collected (Fig. 2). Water samples were also taken from the hot springs at the northeastern corner of the valley, from the artesian well at the northwestern edge of the playa, and from the streams in Hunter and Beveridge Canyons on the eastern slopes of the Inyo Mountains. The chemical analyses of these waters, together with available analyses by other workers, are given in Table 2. The localities of the springs and inflow streams from which these waters were collected are shown in Fig. 3. To show the relationships in the chemistry of the waters, the analyses in Tables 1

SL2 SL3 RBR.502: 8L4 81-S BLSWg IL7 SL8 SL13 SLIO RL18.5 SL22 SL23 SL24 SL24.t SL2R SL2011 SL20.2 SLPD-3 SL33 81.38 81.39 SL30.17 YL48 SLSO 2” 4 Ii 8 10 21 40 46 48 40 6,

2-0-64 1-22-03 5-27-67 1-22-03 I-22-03 “-1.114 2-0-04 2-8-04 t-25-03 2.2.64 2-8-04 2-1-04 1-23-03 1-23-83 2-l-04 2-2.64 Jan. 03 2-1-04 2-1-64 2-2-84 2-71-04 2-7.04 2-1-04 2-0.04 2-0-04

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23 74 74 4, 48 3.0 24 I.6 30 I.8 lx.8 1.2 4.1 30 6.1 13 43 8.8 14 I6 2” 9 16 “2 54 380 438 303 ,OB 018 322 332 IDS 662 ,010

Mg 334 1010 1220 127 Ii20 is(l 233 3060 4830 4660 5340 3140 610” 4890 4160 RI8 R4W 2810 1060 414 014 2300 2900 30 57 34W 1330 8400 390 5600 3”” 2000 SOW 483 1320 634

Ii 175 534 504 ,,A ““i 380 134 003 014 211 0 0 BP”0 134” l3SO 30 7700 91 1610 388 1m 159 100 Da 123

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of brinoa from Saline Valley,

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Na

snalyaos

638 468 ROQ 410 1)34 044 149 280 477 %B 0 8.6 II. tr. I”2 25 107 39 t303 109 102 137 30 13 0.4 3.6 232 ,st 362 134 410 999 378 ,210 B34 IOR “02 107 170" 1500 67” 280 &I 20 3 280 A400 12 12 87 20 15 26 10 3 1320 132 380 100

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I00.000

152.000 83.UW 31.000 147,wo 150,ow 31.60

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(concentrations

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60 160 21 328 62 4u 190 15 27 281 351 10, 276 299 69 53” 182 90 04 2.2 0.9 8.1 13 1” 204 28 14” 160

u 11.840 207.030 211,OOO 30.170 27O.O40 ,08.“3” 75.2 IO 208.760 282.300 1,0;700 233.100 178,590 287.(13” 284.33” 275.3iO 81.63” PlA,RiO 143.340 80.210 48.720 28.840 74,660 74.870 1.5.100 14,u‘so 170,“w:~ ID.00” 28O.W” 2B.000 280,000 12,““” 20”,000 2G0.000 4200 44,wo 03.400

I)iaaolved solids (calculetai)

in ppm)

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and 2 were recalculated to weight percent of major cations and major anions and plotted on trilinear diagrams (Figs. 4 and 5). It is clear from Fig. 4 that the brines of the playa are e~~nti~y sodium s~a~-c~o~~s waters, whereas the spring and stream flows are sulfate-bicarbonate waters in which the ratio of alkalis to alkaline earths varies from one spring to the next. JONES(1966) has discussed the factors influencing the primary aolute com~sitio~ of closed basin waters of the Great Basin. He finds that these composition are basicaily set by the gross lithology of the materials with which these waters have heen in contact. This would certainly seem to ae~~t, in broad terms, for the vacation in inflow water eorn~~t~~~ in Saline Valley. For example, on the west side of the valley Humer and Beveridge Canyon streams and &SineCamp [email protected] rise in, and flow through, a dominantly

LA~REKCE ,4. HARDIE

1286

NO n

inflow

HC03+C0,

Cl

woters

,P : A

Co+Mg

K (bl

(0)

Fig. 4. Trilinear diagrams showing the compositions of Saline Valley waters in terms of major ionic species. Numbers refer to: l-Lower Warm springs, 2Palm Spring, 3-Willow Creek spring, 4-Hunter Canyon stream, 6--Beveridge Canyon stream, &--artesian well, 7---Mine Camp spring, 8--McElvoy Canyon stream, S-Jaokass Spring, lo-Grapevine Canyon stream, 1I-Badwater Spring, 12-freshwater pond, 13-Many springs, 14-Upper springs. total dissolved solids

HCO, + CO,

Cl

(a)

Ca

Mg (b)

Fig. 5. Trilinear diagrams of Saline Valley waters showing the relation between composition and total ion concentration. With increase in total concentration (1) bicarbonate proportions drastically decrease, then (2) sulfate decreases. The trends are indicated by the numbered arrows in (a). A dramatic decrease in calcium and magnesium proportions with increase in total concentration is demonstrated in (b).

limestone-dolomite bedrock and are consequently enriched in alkaline earths (Fig. 4b), while the waters of McElvoy Canyon, B&water Spring and Willow Creek, which rise and flow in a quartz monzonite terrane, are far more alkali-rich. The sulfatebicrtrbonate proportions also show a rough correlation with gross lithology of the bed-rook: high sulfate waters drain from the west and southwest divides where extensive zones of sulfide mineralization are concentrated (the Cerro Gordo mining district lies across the south-west divide). In the southea&, however, the drainage zwcea is of “sulfide-free” granite and the waters (JacksrJa Spring and Grapevine Canyon stream) are predominantly bicarbonate (Fig. 4). (A more serious analysis

The origin of the Recent non-marine evaporite deposit

1287

of primary solute sources was not attempted in this study. As GARRELS (1967) points out, such an analysis requires, among other things, quantitative data on the chemistry of the minerals of the parent rocks.) Evapwite

mineralogy

The evaporite minerals found in the upper 15 ft of sediment in the Saline Valley playa are listed in Table 3. With the exception of the immediate area of the salt lake, the entire playa surface is covered by a rough, blocky salt crust which is formed by capillary efflorescence. This efflorescent crust is dominantly of halite and varies in thickness from a few inches at the playa margins to about 36 in, in the central northeast area. The surfaces of the crusts are ragged and fluted by rainwash channels, but internally, these crusts are hard, porous, sponge-like masses. The fine-grained nature of the Table 3. The evaporite minerals found in the Saline Valley playa Mineral

Occurrence

Halite (N&l) Thenwdita (Na,SO,) Mirabilite (Na,SO,.lOH,O) Glauberite (CaSO,~Na,SO,) Gypsum (CaS0,*2H,O) Ulexite (NaCaB,O,-SH,O)

Effloresoent crusts; crystalline layers; aand and mud Efflorescent cruets; crystalline layers; sand and mud Crystalline layers; mud Effloraacent orusts; crystal pockets; sand and mud Efflorescent crusta; crystal aanda; sand and mud Sand (borehole 40; scattered “aottonballs” a few millimeters

Calcite (CaCO,) Dolomite (CaCO,-MgCO,) Anal&me (NaAlSi,O,-H,O)

Travertine, sand and mud Saud and mud

Sepiolite (Mg,Si,O,.nHaO)

across)

Mud (identified by X-rays only; boreholes 18, 25, 27) Mud (identified by X-rays only; boreholes 18, 19, 26)

crust material made it difhcult to identify the constituent minerals in the field, but samples examined in the laboratory by optical and X-ray techniques (JONES, 1965, p. 43) showed that in addition to halite, small amounts of thenardite, gypsum and glauberite were generally present. Sand-sized grains of quartz, plagioclase and calcite (and, in places, dolomite) are common both within the crustal material and caught in small depressions on the crust surface. They are likely of wind-blown origin. Beneath the efflorescent crust are unconsolidated elastic lacustrine deposits. On the margins of the playa thin (2-12 in.) interbedded sands and muds overlie coarse, ill-sorted gravel of the alluvial fans. The sandy layers are composed chiefly of angular quartz and plagioclase, with varying amounts of calcite and dolomite. The mud layers are essentially composed of quartz, plagioclase, 14 A chlorite, 10 A mica and montmorillonite with some calcite and a little dolomite invariably present. Stratigraphic sequence varies markedly over short distances but argillaceous sediments become dominant in the center of the playa. Within these sands and muds, evaporite minerals have precipitated (Fig. 2). Around the edges of the playa, pinkor tan- colored pods and irregular patches of gypsum (small crystalline aggregates) occur within the sandy layers below the efflorescent crust (boreholes 46,40,43,45,44, 38, 39, 33, 32, 26). At the southern margin, gypsiferous sands 6-12 in. thick, were encountered (boreholes 1, 50, 2, 4, 7, 48 and 49). Close to the salt lake these layers lie immediately below a thick, porous halite-rich efflorescence and are brine-soaked (boreholes 2, 4 and 7), euhedral gypsum grains making a soft mush with the brine. 4

Tiny (3 mm) gypsum rosettes with delicately shaped discoidal “leaves” were found in sticky clay in borehole 26.5. In the firm muds of the central part of the playa, glauberite is the dominant mineral (Fig. 2). It occurs as scattered euhedral crystals, mainly basal tablets, up to 5-7 mm in longest diameter and may make up as much as 40 per cent of the total volume of the sediment. An unusual occurrence of glauberite was observed iu borehole 5 at the southern margin of the salt lake. Here glauberite tablets as large as 20 mm in length occur in “pockets” 2 ft below the surface in wet mud. The crystals are loose plates and can be collected by the handful. Halite euhedra up to 5 mm across are found in similar “pockets” in the wet subsurface muds in the northeastern corner of the playa.The floor of, and near, the salt lake is a crystalline layer (3 in.) of halite. This layer has a columnar structure in which multi-crystal “blades” of fine-grained halite stand perpendicular to the bedding. Underlying this surface layer is a thin (2-3 in.), black, brine-soaked mud layer carrying cubes of halite a few millimeters in length. This sequence, in holes 3 and 9, is repeated downward at least four times. In the central playa halite occurs scattered through the mud as small (< 3 mm) grains accompanying glauberite euhedra. In winter, mirabilite crystals are found in the wet muds where brines are sulfaterich, but thenardite is the stable sodium sulfate phase in the warmer seasons of the year. Such thenardite may persist indefinitely at sub-surface where the brine body has seeped down to lower levels. For example, in boreholes 25, 27, 30 and 16: magnificent pyramidal or bladed crystals of thenardite (as large as 50 mm) project downward from the base of the efflorescent crust into the now dry sandy sediments. GEOCHEMICALINTERPRETATION Development of the brines All of the spring and stream waters, on reaching the fans, sink into the alluvium within a few yards so that the playa is fed by sub-surface flow only. Evaporation through the fans must occur during passage of these groundwaters to the playa because the waters at the playa edge are considerably more concentrated than the inflow waters (compare Tables 1 and 2). This increase in concentration during transport is accompanied by considerable changes in chemistry: the relative compositions of the playa waters are significantly different from those of the inflow waters (Fig. 4). The most striking change is the drastic reduction in bicarbonate ( + carbonate) proportion in the playa brines (trend 1 of Fig. 5a). This could be accounted for by precipitation of calcite (or any other carbonate) from the groundwaters before they reach the playa. In the hot spring region of the valley, travertine deposits provide ample evidence of calcite precipitation. However, for the remaining inflow areas no surficial carbonate deposits were observed and hence precipitation of calcite, if it does occur, must take place at subsurface within the voids of the sediment of the alluvial fans. Support for this is twofold. First, MC&LUSTER (1956) reports that in the southeast corner of the valley “the fanglomerate in traces of places is well-cemented with calcite,” and, second, the brines at the margin of the playa are either just saturated or somewhat supersaturated with respect to calcite. This last line of evidence is applied cautiously because the uncertainties involved in testing brines for saturation could be large. The calcite

The origin of the Recent non-marine evaporite deposit

equilibrium

in bicarbonate

1289

waters can be represented by the reaction

CaCO$pite + H+lasjs CaF&, + HCQ-,,,~ for which (&),,, = aCa~+.aHCoI-/aH+.aCaCOI. At 15°C (about the average temperature of the brines when collected) and one atmosphere total pressure &(calcite) is 10zea (computed from the data given by GARRELS and CHRIST, 1965, p. 89). For saturation or supersaturation with respect to pure calcite (aCaCo3= l), the ion activity product (IAP = aCaa+.aHCo8-.a=+-‘) of the co-existing solution should be h 10a.2. This IAP calculation was made for each of the brines at the playa margin and the results are given in Table 4. The reliability to be placed on these numbers is difficult to assess. Likely sources of error are : (1) the pH values were measured in the laboratory months after collection of the samples. However, comparison of field and laboratory measurements made on other Saline Valley brines showed relatively small differences, no more than 0.3 pH units; (2) for waters more concentrated than seawater, determinations of effective ionic strength and individual ion activity coefficients are fraught with uncertainties due to the large degree of ion interaction (GARRELS and CHRIST,1965, p. 93 and p. 108). The values of these parameters used in Table 4 could carry uncertainties that exceed &25 per cent; and (3) normal analytical errors may introduce significant uncertainties (see JONES, 1965, pp. 25-27). If an overall uncertainty in the calculations of around half an order of magnitude is admitted, then the marginal brines have IAP (calcite) values reasonably consistent with that predicted for calcite equilibrium. This, coupled with the field observations, indicates that the first event in the chemical evolution of the Saline Valley waters is the precipitation of calcite during groundwater flow through the alluvial fans. This precipitation would buffer the calcium and bicarbonate ion activity product (aCaa+.auCo8-)at 1O-6-1O-6 for pH values in the range 7-8. Now the main supply of water to the playa seems to be from the western and northwestern divides. These inflow waters have HCO,-/Ca2+ mole ratios of about 1 (Table 2). Consequently, precipitation of calcite, which takes out calcium and bicarbonate in equal molar proportions (reaction l), will maintain both Caa+ and HCO,- at low concentration, in the order of 10-2-10-3, during evaporation. In this way, then, S0,2- and Clbecome the major anions in the playa brines, while HCO,-, once dominant, becomes virtually an “accessory” ion species. Those inflow waters draining the north and east divides have bicarbonate to calcium mole ratios much greater than 1 (Table 2) and so should become progressively richer in HCO,- and depleted in Ca2+ by calcite precipitation (see, for example, GARRELS and MACKENZIE, 1967, Fig. 4). This indeed does happen (see p. 1295). However, the end-product is not a bicarbonate brine because these waters start with a significant proportion of sulfate ions (Fig. 4a), one that is maintained, and even enhanced, by evaporative concentration. With sulfate now an important species and calcium in small but significant concentration, the next step expected in the evolution of the waters is the precipitation of a CaSO, solid phase. For this step there is direct evidence : a zone of gypsumbearing sands fringing the playa was mapped during the present study (see Fig. 2). The precipitation of gypsum is governed by the reaction CaSO,.2H,Or”

+ CaF’,f + SO&,

+ 2H,OEy

(2)

Sota:

-

SL SL SL SL SL SL SL SL

0.0194 0.0202 0.0236 0.0058 0.0088 0~0104 0.0133 0.0173

mca2+

0.27 0.27 0.27 0.37 0.28 0.27 0.32 0.32

ycaz+ 0.0029 0.0019 0.0022 0.0068 0.0026 0.0026 0.0016 0.0020

10-1.3 10-0.3 10-m 10-2.8 10-e.* 10-2.5 10-2.4

10-2.3

mic03-

%%3+

0.50 0.56 0.50 0.55 0.60 0.50 oai 0.65

YHCO3-

calcite and gypsum

10-2.9

10-3.0

10-2.8 10-8.0 lO-s.0 10-a.4 10-2.8 10-8.8

aHCO,-

0.1260 0.0734 0.1032 0.0827 0.0682 0.0818 0.0351 0.0406

~SOp*-

(1) (2) (3) (4)

2 4 7 33 38 39 48 50

10-2.3

lO-e.e 10-2.8 10-2.6 10-2.5 10-e.*

104.3

10-2.3

a&+

%tg2 ’

______~ 0.0156 0.0043 0.0157 0.0066 0.0055 00411 0.0046 0.0044 0.38 0.34 0.38 0.34 0.35 0.38 0.38 0.38

ymge+

~.. 10-2.2 10-1.8 10-2.2 10-2.6 10-2.1 10-1.8 10-2.8 10-a.* lO-B.0

10-3.0

10-“4 10-2.8 10-2.0

10-3.0

10-3.0

lO-e.s

arco30.00038 0+0068 0~00040 0.00025 0.00033 0~00016 0.00103 0~00090

fWO3

-___-.~

10-3.05

lO-?..a* lO-.¶.BB lO-2.99

10-3.30

lO-a.to

lo-a.‘,

--

0.96 0.98 0.96 0.98 0.98 0.96 0.99 0.99

W30

10-4.7

10-a.*

103.35

102.2 103.3

10-4.4

10-4.6

~04.8

103’3

103.3

10-4.5 10-4.3 lO-a.5

IAP (upsl~~~)

1OZ.M 10=-s loa,

IAP (calcite)

IAP (dolomite)

brines

105.8 104’6 105’2 106.2 104.6 105.1 104.0 10-7.5 104” - ._..-_---.--.--_._._~_-.10-7.8 10-?.S lO-s.z 10-1.75 10-7’8 10-7.6

10-7.35

l2I-p

Saline Valley

10-3.42

WI03

and sepiolite ionic equilibria for marginal npg2+

lO-7.86 10-7.8 IO-T.8 lO-8.2 lo-‘.,” 10-T.” 10-1.6 10-7.5

ws+

brines

IAP (sepiolitc) ____10’1.2 10’6.1 101.5.6 10x.‘” 10’5.? 1015.3 1015.* 10’6.5

Ionic strength has not been corrected for possiblo complexing. Magnesium ion activity ro&icient WBSdetermined by the mean salt rwthorl using the KC1 and RlgCI, data of 11~RSED :~ncl OWZ:H (1950, pl>. 562 a~,:! .xi). Silica activity coeficient assumed to be unity. IAP (dolomite) = a~~~+.al~~+.a~~~.-a~+-~; IAP (sepiolite) = a&gz+.a&Oa~a~+-4.

Ionic strength ~~_____~~~_ 1.34 0.69 1.39 0.71 0.54 1.38 0.30 0.30

dolomite

taken from GARRELS and CHRIST (1965, Fig. 4.5, curve C). IAP (gypsum) = aCa2+.agOp2-.a&0.

Table 5. Calculated __I-

(3) Bicarbonate ion activity coeffXents (4) IAP (calcite) = a~s2+.a~~03-.~H+-1;

_

10-m 10-1”

10-3.3

,()-e.1 lo-“’ 10-2.2 10-a.0 10-8”

wop3-

Saline Valley

using thtl I<( ‘I, c’aClz ~md KISO1 dntn of H \LL&II_I a~ttxl I )\I ES

0.06 0.12 0.06 0.11 0.13 0.06 0.18 0.18

yso,3-

ionic equilibria for marginal

Ionic strength has not been corrected for possible complexing. (2) Calcium and sulfate ion activity cwfficients were det,errrrinrd by the me&n salt method (1960, pp. 562, 567 and 415).

(I)

Brine

Notes:

--

1.34 0.69 1.39 0.71 0.64 1.38 0.30 0.30

SL SL SL SL SL SL SL SL

2 4 7 33 38 39 48 50

Ionic strength

Brine

Table 4. Calculated

I!

?

$ B

4

F

The origin of the Recent non-marine evaporite deposit

1291

The equilibrium constant, (Q,,, = aca~+.~sol~-.a$o/ac,sol.,,40, is in the order of 4 x 10-s (about, 10-4.4) for temperatures about lo”-25°C at 1 atm total pressure (MARSEAU and SLUSHER, 1966,p. 4017). The marginal playa brines will have inherited from the earlier (and continuing?) calcite precipitation reaction calcium ion activities in the order of 10-8-10-3. Therefore, precipitation of gypsum would not take place until evaporative concentration had produced sulfate ion activities of about, 10-S. This is indeed the order of magnitude of sulfate ion activity calculated for fhe brines found in contact with gypsum at the playa margins (Table 4), which This strongly suggests control of sulfate concentration by the gypsum equilibrium. can be tested more precisely. The appropriate IAP (gypsum) values, calculated for a CaS0,.2H,O activity of unity, are given in Table 4. If the equilibrium constant K, = 10-4*4 is used as a criterion of equilibrium, then the marginal brines are either just saturated or else slightly undersaturated with respect to gypsum (Table 4). For three of the brines, SL2, SL4, and SL7, the predicted state of equilibrium is confirmed by the field observations. At these sites, gypsum has remained unchanged (aggregates of beautifully euhedral crystals) in intimate contact with these brines for at least twelve months (Jan. 1963-Feb. 1964). (For the other brines listed in Table 4 only single visits were made to the sampling sites so no assessment of gypsum persistence can be made). This would suggest that the calculated IAP (gypsum) values are reliable and sensitive indicators of gypsum-brine equilibrium. However, it should be noted that the combined uncertainties in the assigned ionic strength, the ion activity coefficients, and the analytical values may be as high as half an order of magnitude. The equilibrium constant itself is by no means firmly established (see HARDIE, 1967, pp. l&5-186), although the error is not likely to be more than a third of an order of magnitude. If such uncertainties are taken to count, then all the calculated IAP (gypsum) values of Table 4 could be considered to be in satisfactory agreement with the experimental value. It is therefore probably safe to conclude that equilibrium with gypsum has been attained by all of the marginal playa brines found in contact with gypsum. The second event. in the development of the Saline Valley brines, then, is the precipitation of gypsum at the playa margins. This precipitation reaction keeps a tight control on the sulfate concentration in the playa brines and further evaporation will lead simply to waters dominated by chloride. This trend is shown in Fig. 5a (trend 2). Marked changes in cation composition accompany the anion changes. Figures 4(b) and 5(b) clearly show that the alkaline earths are prominent constituents of all the inflow waters but are very much subordinate to the alkali metals in the concentrated playa brines. Reduction (and control) of the calcium proportion during evaporative concentration is easily accounted for by the precipitation of both calcite and gypsum as discussed above. The mechanism for early loss of magnesium (Fig. 5b) and its control at low concentrations during subsequent, evaporation of the brines is not certain. However, two very reasonable possibilities are (1) precipitation of dolomite, and/or (2) precipitation of sepiolite. Both of these phases have been identified in muds of the playa. For the precipitation of dolomite from bicarbonate waters we may write CaCO,.MgCOpk;“‘te

+ 2H$,,, + Caf& + MgT&?,+,, + 2HCO;(,,,

1292

.I,AWREVCE

A.

HAWIF:

The equilibrium constant,, K, (dolomite) = ccca2+.a~1112+.n~,,3-:1LC;tCo,.Mr(:OB.aai Z for. this reaction at 25°C tend 1 atm. total pressure is ~10~.~, assuming a dolomit,r solubility product of ~lO-~~(Hsu, 1967, p. 179). The IAP (u~~“,.~~~?~.(~~~~~,-.ccH’ ?) values calculated for the marginal brines range from 104.0 to 1(,F2 (Table 5), supersaturated by about a half to two and a half orders of magnitude. This result is difficult to interpret. First, the equilibrium constant for dolomite solubility is still a matter of some controversy (Hsu, 1967,p. 170). Second, the spread of TAP values is a little greater than the expected uncertainty, which is probably about an order of magnitude. A random spread of high supersaturation values could mean that nucleation has not been achieved. Third, the dolomite in the playa muds could quite possibly be detrital rather than precipitated in situ. This was suggested bJ the X-ray patterns of the few samples in which dolomite occurs in more than tract amounts in the clay fractions (particularly less than 2 ill sizes). Even in these samples dolomite is a minor constituent so that only the main (211) peak could be readily identified; a sharp trace at 2.89 & 0.01 L\pointed to stoichiometric dolomite. Such stoichiometry is characteristic of most ancient dolomites, particular from evaporites (GOLDSMITH and GRAF, 1958), whereas reported occurrences of Recent, authigenic dolomite from a number of sedimentary environments are of non-stoichiometric. calcium-rich phases (e.g. JONES, 1965, p. 44; PETERSON et al., 1963; SKINNER, 1963: SHINN, 1964; DEFFEYES et aE., 1965; SHINN et al., 1965; ILLING et al., 1965) as are the dolomites synthesized in low temperature experiments (e.g. GRAF a,nd GOLDSMITH, 1956; SIEGEL, 1961). WOLLAST et al. (1968) have precipitated a sepiolite phase from sea water. For the reaction Mg,Si,O,.nH,O$f’O’ite

+ 4H,+,,, + 2MgF& + 3SiO,,,,,

+ (n + 2)H@~$’

they obtained an equilibrium constant K,(25”C, 1 atm) = 101s*a*l*o,assuming both the activity of the solid and the activity of HZ0 to be unity. Making the same assumptions, appropriate IAP (sepiolite) values have been calculated for the marginal Saline Valley brines (Table 5). These values range from 1016.2 to 1017.2, indicating undersaturation by about one and a half to three orders of magnitude. Uncertainties in these calculations are difficult to assess but may be expected to amount to between one and two orders of magnitude (this represents an error of about 10 per cent and would be due in large part to the uncertainty in pH values: see p. 1289). If such uncertainties do obtain, most of the IAP (sepiolite) values would become compatible with the experimental value. This suggests sepiolite precipitation as a most attractive possibility for control of magnesium in the playa brines. In summary, the brines at the playa margins have developed by progressive evaporative concentration of dilute ground water inflow, leading to the precipitation of calcite, gypsum and sepiolite (and/or dolomite). These precipitates control the C&2+, Mgzf, HCO,-( +CO,2-), SO,2- and SiO, contents of the brines at relatively low values so that further evaporation during subsurface flow in toward the playa center leads t#o brines dominated by Cl- and Na+( +K+). The evolution of these central playa brines is taken up below.

The origin of the Recent non-marine evaporite deposit

1293

The eflorescent crust8 The efflorescent crusts, which cover the entire playa surface, are essentislly all halite, reflecting the dominance of sodium and chloride ions in the playe brines (Fig. 4). There are significant differences in mineralogy between the crust material and the precipitates in the underlying sands and muds. This is particularly noticeable, for example, at the southern margin of the playa where an efflorescent crust composed of halite + thenardite + gypsum + glauberite overlies brine-soaked sands which carry gypsum as the only evaporite mineral. Such differences follow from the differences in the conditions of precipitation. The efflorescent crusts are the products of complete dehydration of brine drawn to the surface by capillary action. The minerals making up the crust have been precipitated over an extreme range of brine concentration during the evaporation process and are now totally dry, in contact with the atmosphere only. They do not reflect the physico-chemical conditions existing in the underlying brine. In contrast, the evaporite mineral assemblages in the sediments below the efflorescent crust are not evaporation residues. They have been precipitated at sub-surface below the brine-air interface and remain in direct contact with brine. Their character and distribution depend on the existing state of the brine body. It is, therefore, these sub-crustal assemblages that are of greatest interest. The sub-crustal parageneses Below the efflorescent crust the evaporite minerals in the sands and muds of the playa occur as a sequence of assemblages systematically arranged in laterally concentric zones. The sequence from the periphery in to the center of the playa is gypsum -+ gypsum + glauberite + glauberite + glauberite + halite. This zonal structure is shown in Fig. 2, which is a map of the evaporite mineral distribution within the upper 15 ft of playa sediments. Nothing is known of the actual depth to which the zones might reach, but it does not seem likely to be more thsn a few tens of feet. The steep, lateral concentration gradient in the present brine body suggests that some impermeable layer at not too great a depth effectively seals the bottom of the present evaporating psn. The record of marked temporal variations in activity of H,O in the Searles Lake evaporite deposit (EUQSTERand SMITH, 1965, see particularly their Fig. 22) testsea to this idea: the Searles Lake deposit seems simply to be a pack of such water-tight evaporating pans piled one atop the other. The concentric zonal system of evaporite minerals in the Saline Valley playa applies rigorously only to those parts soaked with brine. In some holes it was found that the uppermost 2 or 3 ft of bone-dry sediments between the efflorescent crust and the brine surface carries minerals such as thenardite and/or halite not consistent with the zones. These are in some instances old, buried crusts (both layered and efflorescent) but others seem to be isolated precipitates in the capillary fringe. Even below the brine-table, however, the zonal structure is not everywhere uniform. Three areas stand in strong contrast to the main sequence of zones. Each appears to have had a separate and different history. The first of these areas is at the southwest corner of the playa (area labelled “N&l rich” in Fig. 2). This s,rea, which encompasses the perennial salt lake, is floored by a smooth layer of halite. It is the

final receptacle for all the floodwaters which inundate the playa after sporadic cloud bursts. These ~oodwaters differentially dissolve the halite from the surfaccb effilorescent crusts, producing an NaC1-rich brine. Subsequent evaporation of this pool of brine leads to the precipitation of a layer of exceptionally pure halite, a crystal accumulate of both ‘Lhoppers” and solid, simple cubes. Shallow boreholes (numbers 3 and 9) put through the present crust revealed a repetitive sequence of thin (2-3 in.) mud layers alternating with thin halite layers. Each mud-halite pair presumably records a single flood: suspended mud settles first from the basked floodwaters before slow evaporation finally induces halite precipitation from a shallow pool of clear brine. The second area which breaks the continuity of zones is at the western edge of the playa (area labelled “SO, rich” in Fig. 2). This area is characterized by the assemblage glauberite + mirabilite (or thenardite), an assemblage not found anywhere else in the playa. The composition of the coexisting brine is also unusual ; by comparison with brines of similar ~oneentratio~~ from other par& of the playa it is exceedingly rich in sulfate and in magnesium (brine SL 16, Table 1). The singular chemistry of this brine demands an equally Hydrologically, the waters of Hunter and Beveridge singular parent inflow-water. canyons are obvious candidates but chemically the Mine (iamp spring-flow would perhaps be a more suitable parent (Table 2 and Fig. 3). No satisfactory explanation can be of&red here. The third area is in the central north-east: of the playa (area labelled “C&free” in Fig. 2). Here the sub-crustal saline mineral assemblages carry neither gypsum nor glauberite, being either halite or halite -I- mirabilite. The brines reflect this, they carry little or no calcium (e.g. SL 23). As a consequence, where the brines are dilute {below halite or mirabilite saturation) no saline minerals are precipitated (e.g. SL 18.5, SL 2Z), The explanation for the very low calcium content of the brines of this area is best sought in chemical changes that accompany early precipitation of calcite. Although the calcite equilibrium demands that the IAP (u+~~+.~,~,,~~,-Jx~$) be fixed, the mole ratio of HCO;- to Ca2+ will, in general, vary The course will be set by the inihial systematically during calcite precipitation. As discussed earlier (p. 1289) precipitation of calcite will remove bulk composition. HCO,- and Ca2+ from solution in equal molar proportions (see reaction 1, p. 1289). This will cause a relative enrichment in HCO,- and depletion in (!a2+ for those waters with an initial HCO,-/Ca 2+ mole ratio greater than 1 (see GABRELS and MACKENXE, 1967, Fig. 4) and just the reverse for those waters in whioh the initial ratio was less than 1. With this mechanism, then, a low calcium brine reflects deri~atio~l from a parent water with a high WCO,-/(9a2+ mole ratio. This seems 2% likely explanation for the low calcium brines of the Ya-free” area, because l&r* hydrologically most probable parent waters, Upper, Palm, Lower Warm and Man;, Springs (see Fig. 3), all have relatively high bicarbonate/caloium mole ratios (4 to 17: see Table 2). They are significantly higher than the ratios of the rest of the inflow waters, which range from less than 1 to up to 3 (see Table 3). The ~~~~e~ CaSO~-lasso*-~aCl-H~O

a-s a ~~~e~

The main zonal sequence of evaporite mineral assemblages in the sub-crust can be readily accounted for using the available experimental data in the quaternary system CaSO,-Na,SOhP;laCI-H,O. The phase relations in this system at 25°C and 1

The origin of the Recent ~un-rn~~e ev~~~~te deposit

1296

atm total pressure (D’AN~, 1933, pp. 221-222) are shown in Fig. 6. This is a Jiinecke p~j~ction (RICCI, 1951, p. 395) of the saturated solution scams from the Ha0 This gives a ternary apex unto the ~~y~o~ base of the com~tio~ ~tr~h~o~. plot of the stability fields of the solid phase assemblages in terms of the anbydrous composition of the co-existing solutions. The weakness of this type of projection is that the solution concentrations are not directly represented. Its strength, however, lies in the fact that the rules for tracing any particular oryst~~~ation path arc the same as those for ternary sterns without solid solution (see ~s~~~, 1969, Chap. II and IV; RICEI, 1951, p. 450 ff). In Fig. 6 the stability fields have been

CQSO,

GL

N#&33,

Fig- 8. Sch~~t~c pha~ rel&on~ in the system ~~~~~-~~~~~-~~~~-~~~ e,t 25°C and f atm total pressure, compiled from ape&mental data reported by D’Aws (1933, pp. 221422). This ia a Jiineckeprojection of the saturated solution surfaces from the H&I apex onto the anhydrous base of the composition tetrahedron. The fieldahave been distortedto showmore clearlytheirinterrelationships. Although the existence of EPstability fit4d fora&ydrik in this system S&25°C is not recognizedby D’.&NS ~~~33)~it has been added to conform to the new data of A-anhydrite, T-thenardite, G---gypsum, M-mirabilite, -DE (1%7). EC-halite.

distorted to show more clearly their in~r-relationships ; in the qua~t~t~t~veplot, the gypsum field crowds the other fields against the Na,SO,-NaCI join (cf. Fig. 7). The ~s~~ion does not affect the c~sta~zation paths outlined in Fig. 6: they are faiths replicas of the actual paths followed in the qua~ti~tive projection. The point of the comparison between the natural system and the experimental system can now be made, In Fig. 6 the dotted line beginning at the composition X traces a crystallization path (under isothermal, isobaric evaporation) that produces, &z power ~~~, the exact tonal sequence of mineral assemblages observed in the Saline Valley playa, i.e. gypsum 4 gypsum -t_glauberite -+ glauberite -+ glaube~te + halite. This particular crysta~zatio~ path, which must strategy end at the eutectic glauberite + halite + thenardite + solution (point d in Fig. 6), would be precisely that followed on evaporation of all bulk impositions like X that fall within the composition triangle abc (Fig. 6). For solutions with bulk compositions

outside this field, evaporation would, of course, result in other, cliflererzt crystallization sequences. For example, a bulk composition such as Y (Fig. 6), only slightI) different from X but’ in the field beg, would produce on evaporation the sequellu;’ gypsum + gypsum -1 glaubcritc + glauberite ---+ glauberitc + thenardit8c ---* glallberite + thenardite _I- halite. These paths have been outlined assuming a temperature of 25°C; this does not place any fundamental restrictions on its application to the Saline Valley system. The phase relations in the quaternary system will remain the same over the temperature range 52-18”(!. The upper limit is imposed by the conversion of gypsum to anhydritc in the presence of glauberite (HARDIE. 1965, p. 166) and the lower limit by the conversion of thenardite to mirabilite in the presence of halite (IYAM, 1933, p. 96). At temperatures below 18”(‘, down to at; least 14°C (about average temperature for the Saline Valley brines in mid-winter), the sequences of assemblages not involving thenardite are unchanged. If a solution of composition X were to be evaporated in a platinum crucible (slowly enough to ensure equilibrium and stirred to ensure solution homogeneity), the crystallization path would be that shown in Fig. 6, but one would observe only a single assemblage at a time. The crystallization sequence would bc a temporal one. In the Saline Valley playa, however, the sequence is a spatial one. X11 crystallization steps take place at, the same time and the whole sequence is laid out, in proper order, in concentric zones much like a metamorphic aureole. Unlike a metamorphic aureole, though, the playa zones are the result of the existence of a concentration gradient rather than a thermal gradient. This concentration gradient was established (and is maintained) by continuous sub-surface flow of brine in toward the center of the pan coupled with continuous evaporation of H,O. The dilute brines of the playa edge are at the head of this gradient. They are precipitating gypsum (reaction 2, p. 1289) and so play the role of the ‘rparent” solutions in the experimental model (such as X of Fig. 6). As these “parent” brines flow on into the playa they are Precipitation of gypsum, which buffers the IAP continuously evaporated. ,+.aS,) at about 4 ZK 1fP5 (p. 1291), will continue until the activity of Nat- is (%a high enough and (or) the activity of H,O low enough to initiate the reaction: 2CaS0,.2H,0~,YPs””

+ 2Na,rj,,,,+ CaSO,.Na,SO,~~~“““““” + Caf&::,,+ 4H,O$’

(3)

This reaction relation demands a This is the conversion of gypsum to glauberite. gypsum precursor. This poses no problem if the brine be evaporated in an open model outlined above) where the early-formed lake (the “platinum crucible” gypsum would remain in contact with the evaporating solution. For the playa brine body, however, this is not the case. The marked concentration gradient separates brine precipitating gypsum from brine capable of converting gypsum to glauberite. The existence of the gypsum + glauberite zone, then, points to an earlier stage in playa history when the brine concentration gradient was less steep than now so that gypsum occurred well into the central parts of the playa. With the development of the present gradient, this gypsum was converted to glauberite by reaction 3. The boundary between the gypsum and the gypsum + glauberite zones (Fig. 2) marks the onset of this reaction. The boundary between the gypsum + glauberite and the glauberite zones is that point along the gradient at which all the gypsum is,

The originof the Recent non-marineevaporitedeposit

1297

in effect, used up. Beyond this point only glauberite precipitates:

caz+ (aQ)+ 2Na& + ZSO&,, + CaSO,.Na,SO~~be”t”.

(4)

The brines, as they move onward, will continue to precipitate glauberite alone until evaporative concentration increases a,, (aNs+ will be controlled, though not buffered, by reaction 4) sufficiently to bring down halite. The onset of this reaction Na$,, + Cl;-,,, + NaCIEte

(5)

marks the boundary between the glauberite and glauberite + halite zones. Quantitative test of the model The correspondence between the natural and experimental crystallization sequences is too striking to be merely fortuitous. Yet it remains to be proven that the experimental model is, indeed, a valid (and viable) one. A simple quantitative test is possible. For the experimental system it was shown that only those solutions whose bulk compositions fall into the triangle abc of Fig. 6 would produce on evaporation the desired crystallization sequence. The critical quantitative test, then, is simply that the natural waters also have initial bulk compositions that fall within the composition triangle abc of Fig. 6. This test can be readily made if the chemical analyses of the natural brines are expressed in terms of the system CaSO,-Na,SO,-NaCl-H,O. The compositions of the brines (Table 1) closely approximate this quaternary system: Na+, Cl- and SO,a- account for over 90 per cent of the total dissolved species (Fig. 4). Of the remaining major ions only calcium is an essential constituent of the co-existing evaporite minerals. The small quantities of K+, Mga+, CO,s- and HCO,- may be safely neglected because (1) they will not have any appreciable effect on the activity of Ha0 or the activities of the major ions of the brines, and (2) they do not enter into solid solution with any of the crystalline phases under the conditions considered. With this proviso, then, the natural brine analyses were recalculated to the system CaSO,-Na,SO,-NaCl-H,O. For most of the brines considered, the dissolved solids content so calculated differed by less than 5 per cent from the original complete analysis value ; all differed by less than 10 per cent. These analyses are plotted on the appropriate part of the quantitative Janecke projection shown in Fig. 7. The phase boundaries in this diagram are based on the experimental data at 25°C and 1 atm total pressure given by D’ANs (1933, pp. 221-222). The result is unequivocal: those brines which are precipitating gypsum and so play the role of the “parent” waters of the model, all do plot within the composition triangle a&c. The validity of the experimental system as a quantitative model is thus confirmed. The Janecke projection does not show the experimental phase relations as a function of solution concentration. This relationship can be adequately demonstrated in a plot of Na,SO, concentration against NaCl concentration, as is shown in Fig. 8. The natural brine analyses, presented in the same terms, are plotted as open or patterned circles on this diagram. The square symbols represent brine analyses selected from those made by LOMBARDI (1963, Table 1). (Partial analyses, analyses of surface brines, where contamination with efflorescent crust material is likely, and analyses which did not sum to 100% were not used.)

1298 NaCl

THENARDITE

No2S0,

coso,

Fig. 7. Quantitative plot of part of the phase diagram given in Fig. 6. Field boundaries are shown to a first approximation aa straight lines. Solid circles are the anhydrous compositions of Saline Valley playa brines found co-existing with gypsum. Values are in wt. %.

LOMBARDI did not determine the co-existing minerals, so each of his brines has been assigned a co-existing mineral assemblage appropriate to its location within the evaporite zones (see Fig. 2). As Fig. 8 shows, for both LOMBARDI’S analyses and those of the present study, the fit with the experimental data is exceedingly good. The agreement between natural and experimental data demonstrated in Figs. 7 and 8 must be accepted with some caution because (1) the number of brine analyses used in the comparison is small; (2) the experimental data, too, are far from extensive; (3) the experimental values were determined at 25°C whereas the natural brine temperatures ranged from 10 to 20°C with an average at about 14°C. Fortunately, in this regard, the equilibrium solution compositions seem not to be sharply temperature dependent ; (4) the natural brines only approximate to quaternary compositions. Geochemical implications

of the model

For the active sub-crustal evaporite sytem the experimental model has these implications : (I) Simple evaporation is the major process in concentrating the waters, leading to precipitation of saline minerals. (2) At least local, stable chemical equilibrium prevails between the playa brines and their co-existing evaporite mineral assemblages. (3) The chemical evolution of each brine under isothermal, isobaric evaporation follows a predictable course, one controlled completely by the initial bulk composition of the parent water and by the extent of evaporation.* * The evaporite system acts thermodynamically as a “closed” system even though physically open to the component HsO. That is, the equilibria are controlled by P, T, and X and are not externally controlled by the chemical potential of Ha0 of the surroundings (the immediate atmosphere). This of course must be true for any evaporating system because net evaporation mill only cease when pHs0 (system) = pHzO (surroundings).

The origin of the Recent non-marine evaporite deposit I

40

,

I

,

I

,

,

I,,

SALINE

VALLEY

1299

I

BRINES

-

GY+L

0

GL*L

I”

H+GL*L colculoted

cn

from

W

GY+L GL*L

d 5

g

0-

25

-

5 0

0

20 -

40

MOLES

60

NaCl per

00

1000

I00

MOLES

120

H,O

Fig. 8. Phase relations in the system CaSOd-Na,SOhNaCl-HI0 at 25°C and 1 atm pressure expressed as a function of NaCl and Na,SO, concentrations. The phase boundaries are based on data reported by D’ANs (1933, pp. 221-222). The field of anhydrite (A + L) has been added to conform to the stability data of &DIE (1967): the boundaries of this field are tentative only. The circles and squares represent compositionsof SalineValley brinesco-existing with different evaporite mineral assemblages. GY-gypsum, GL-glauberite, H-halite, A-anhydrite, MF&-mirabilite, T-thenardite, L-liquid. (4) The observed mineral assemblages are all stable in contact with brine over the temperature range ca. 1040°C (at 1 atm total pressure). Therefore, normal seasonal temperature fluctuations should not give rise to phase changes in the zonal sequence. Bulk composition remains the single major equilibrium factor. (5) The present zonal structure represents only a stage in the long-range geochemical evolution of the evaporite deposit. The development of the zone characterized by the reaction pair gypsum + glauberite requires an earlier stage in which the gypsum zone extended right into the center of the playa. This means a brine concentration gradient less steep in the past than exists today. A progressively steepening concentration gradient seems, in fact, inevitable. This follows from the fact that with continued net evaporation, the concentration of the brines in the

central pan must continually increase simply by accumulation of ionic mass. of course, this trend could easily be reversed (or accelerated) by climatic or t,ectonic accidents since the concentration gradient will be sensitive to any factor which alIters the rate of evaporation of the waters or the rate of movement of water into, and in, the playa. However, while the existing climatic and tectonic environments remain relatively unchanged, it is postulated that the concentration gradient will steepen and the zone boundaries will move outward with time. Eventually tlliR would lead to the development of a new central zone defined by the assemblag:’ glauberite + halite + thenardite (or mirabilite) co-existing with a brine of essentially uniform composition (e.g. the ultimate brine from the evaporation of all Na,SO,-rich solutions in the system CaSO,-Na,SO,-NaCl-H,O: point d in Fig. 6). Yet this cannot be the final fate of the natural system, because the concentration of .l-. for example, would eventually become sufficiently high to producth potash salts. The quaternary model, of course, would by then be inva’lid. Acknowledgments-The problem was suggestod by Professor HANS L'. E,ucsr~n. of The John Hopkins University and Dr. BLUSH F. JONES of the U.S. Geological Survoy. These men stimulated my interest in evaporites and generally guided me through the study. They critically reviewed the manuscript and their comments led to considerable improvement in both substance and presentation. To them I extend my deepest thanks. Mr. JAMES M~JNDORFFof the U.S. Geological Survey also read the manuscript and suggested many useful changes. My thanks are also extended to Dr. REINHOLT ELLERT of the University of Sao Paula, Brazil, and Dr. JOHN NOLAN of Imperial College, London, for their sterling help and sound advice in the field. Without their efforts the sampling would have been woefully inadequate. Mr. ORESTE LOMBAI~DI of the U.S. Naval Ordnance Test Station, China T,ake, California, was a most helpful guide in those confusing first days on the playa. The Water Resources Division of the U.S. Geological Survey provided material and financial assistance in the field work. The study could not have been carried out without this help. Support in part was also provided by Grant 680-A from the Petroleum Research Fund. REFERENCES D’ANs J. (1933) Die LFaungsgleichgewichte der Systeme de+ Salze ozeanischer Salzablugerungen. Kaliforschungsantalt. DEFFEYES K. S. LUCIA F. J. and WEYL P. K. (1965) Dolomitization of Recent and PlioPleistocene sediments by marine evaporite waters on Bonaire, Netherlands Antilles. S;-,c. Econ. Paleon. Mineral. Spec. Publ. 18,71-88. EARDLEY A. J. (1938) Sediments of Great Salt Lake, Utah. Bull. -4mer. Assoc. Petrol. Geol. 22,1305-1411. EUUSTER H. P. and SMITH G. I. (1965) Mineral equilibria in the Searles Lake evaporites, California. J. Petrol. 6, 473-522. GALE H. S. (1914) Salt, borax and potash in Saline Valley, California. Bull. U.S. Geol. Sure. 540,416-421. G~RRELS R. M. (1967) Genesis of some ground waters from igneous rocks. In Researches in Geochemistry. Vol. 2 (editor H. Abelson). p. 405-420. Wiley. GARRELS R. M. and CHRIST C. L. (1965) Solutions, MineraL and Equilibria. Harper and Row. G~RELS R. M. and MACJZIZNZIEF. T. (1967) Origin of the chemical compositions of some springs and lakes. In Equilibrium Concepts in Natural Water Systems. Amer. Chem. Sot. Advances in Chemistry Series 67, pp. 222-242. GOLDSMITH J. R. and GRAF D. L. (1958) Structural and compositional variations in some natural dolomites. J. Geol. 66, 678-693. GRAF D. L. and GOLDSMITH J. R. (1956) Some hydrothermal syntheses of dolomite and protodolomite. J. Geol. 64, 173-186.

The origin of the Recent non-marine evaporite deposit

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HARDIE L. A. (1965) Phase equilibria involving minerals of the system CaSOI-Ns@O&&O. Ph.D. Dissertation, The John Hopkins University, Baltimore, Md. HARDIEL. A. (1907) The gypsum-anhydrite equilibrium at one atmosphere pressure. Amer. Mineral. 62, 171-200. HARNEDH. 5. snd OWEN B. B. (1950) The Physical Chemistry of Electrolytic Solutione. Amer. Chem. Sot. Monograph Series 95, 2nd edition. Reinhold. HSIJK. J. (1967) Chemistry of dolomite formation. In Carbonate Rocks, Part B, pp. 169-191. Elsevier. HUNT C. B. ROBINSONT. W. BO~LES W. A. and WASHBURNA. L. (1966) Hydrologic basin, Death Valley, California. U.S. Geol.Surv. Prof. Paper 494-B. ILLINUL. V., WELLS A. J. and TAYLORJ. C. M. (1965) Penecontemporary dolomite in the Persian Gulf. Sot. Econ. Paleon. Mineral. Spec. Publ. 13, 89-111. JONESB. F. (1963) The hydrology and mineralogyof Deep SpringsLake Inyo County, California. Ph.D. Dissertation The John Hopkins University, Baltimore, Md. JONESB. F. (1965) The hydrology and mineralogyof Deep SpringsLake, Inyo County, California. U.S. Geol.Surv. Prof. Paper 502-A, 56 pp. JONESB. F. (1966) Geochemical evolution of closed basin water in the western Great Basin. SecondSFpodium on Salt, Vol. 1, pp. 181-200. Northern Ohio Geol. Sot., Cleveland, Ohio. KINQ C. R. (1948) Soda ash and salt cake in California. Calif. J. Mines Geol. 44, 189-200. LOMBARDI0. (1963) Observations on the distribution of chemical elements in the terrestrial saline deposits of Saline Valley California. U.S. Naval OrdnanceTeat Station, China Lake, Tech. Publ. 2916, 42 pp. MARSHALLW. L. and SLUSHERR. (1966) Thermodynamics of calcium sulfate dihydrate in aqueous sodium chloride solutions, O-110”. J. Phye. Chem. 70, 40154027. MASIN~ G. (1960) Ternav Syetema. Dover. MCALLISTER J. F. (1956) Geology of the Ubehebe Peak quadrangle. U.S. Geol. Sure. Map GQ 95. PETERSON M. N. A., BIEN G. S. end BERNERR. A. (1963) Radiocarbon studies of recent dolomite from Deep Springs Lake, California. J. Geophye. Res. 88, 6493-6505. RAINWATERF. H. and THATCHER L. L. (1960) Methods for collection and analysis of water samples. U.S. Geol.Surv. Water-Supply Paper 1454, 301 pp. RICCIJ. E. (1951) The Ptie Rule and Heterogeneow Equilibrium. Van Nostrand. SHINNE. A. (1964) Recent dolomite, SugerloafKey. In Guidebook for G.S.A. Field Ttip No. 1, South Florida Carboante Sediments, pp. 62-67. Geol Sot. Amer. SHINN E. A., GINSBURUR. N. and LLOYD R. M. (1965) Recent supretidal dolomite from Andros Island, Bshamss. Sot. Econ. Paleon. Mineral. Spec. Publ. 13, 112-123. SIEQELF. R. (1961) Factors influencingthe precipitation of dolomitic carbonates. State Geol. Surv. Kansas Bull. 152, 127-158.

SKINNERH. C. W. (1963) Precipitation of oalcisn dolomites end msgnesien calcites in the southeast of South Australia. Amer. J. Sci. 281,449472. TUCKERW. B. (1926) Inyo County. C&f. Min. Bur. Rep. %,463-530. VER PLANCKW. E. (1958) Salt in California. Calif Dept. Nat. Res., Div. Minea, Bull. 175, 168 pp. WOLLASTR., MACI(ENZIEF. T. and BRIC~ER 0. P (1968) Experimental precipitation and genesis of sepiolite at earth-surfaceconditions. Amer. Mineral. in press.