The major-ion composition of Permian seawater

Geochimica et Cosmochimica Acta, Vol. 69, No. 7, pp. 1701–1719, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 ⫹ .00

doi:10.1016/j.gca.2004.09.015

The major-ion composition of Permian seawater TIM K. LOWENSTEIN1,*, MICHAEL N. TIMOFEEFF1, VOLODYMYR M. KOVALEVYCH2, and JUSKE HORITA3 1

2

Department of Geological Sciences and Environmental Studies, State University of New York at Binghamton, Binghamton, NY 13902, USA Institute of Geology and Geochemistry of Combustible Minerals, National Academy of Sciences of Ukraine, Naukova 3A, 79053 Lviv, Ukraine 3 Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, MS 6110, Oak Ridge, TN 37831-6110, USA (Received March 3, 2004; accepted in revised form September 15, 2004) ⫺ Abstract—The major-ion (Mg2⫹, Ca2⫹, Na⫹, K⫹, SO2⫺ 4 , and Cl ) composition of Permian seawater was determined from chemical analyses of fluid inclusions in marine halites. New data from the Upper Permian San Andres Formation of Texas (274 –272 Ma) and Salado Formation of New Mexico (251 Ma), analyzed by the environmental scanning electron microscopy (ESEM) X-ray energy-dispersive spectrometry (EDS) method, along with published chemical compositions of fluid inclusions in Permian marine halites from North America (two formations of different ages) and the Central and Eastern European basins (eight formations of four different ages) show that Permian seawater shares chemical characteristics with modern seawater, ⫺ including SO2⫺ ⬎ Ca2⫹ at the point of gypsum precipitation, evolution into Mg2⫹-Na⫹-K⫹-SO2⫺ 4 4 -Cl brines, and Mg2⫹/K⫹ ratios ⬃5. Permian seawater, however, is slightly depleted in SO2⫺ and enriched in 4 Ca2⫹, although modeling results do not rule out Ca2⫹ concentrations close to those in present-day seawater. ⫹ 2⫹ Na and Mg in Permian seawater are close to (slightly below) their concentrations in modern seawater. Permian and modern seawater are both classified as aragonite seas, with Mg2⫹/Ca2⫹ ratios ⬎2, conditions favorable for precipitation of aragonite and magnesian calcite as ooids and cements. The chemistry of Permian seawater was modeled using the chemical composition of brine inclusions for three periods: Lower Permian Asselian-Sakmarian (296 –283 Ma), Lower Permian Artinskian-Kungurian (283–274 Ma), and Upper Permian Tatarian (258 –251 Ma). Parallel changes in the chemistry of brine inclusions from equivalent age evaporites in North America, Central Europe, and Eastern Europe show that seawater underwent secular variations in chemistry over the 50 million years of the Permian. Modeled SO2⫺ 4 concentrations are 20 mmol per kg H2O (mmolal) and 19 mmolal in the Asselian-Sakmarian and ArtinskianKungurian, with higher concentrations in the Upper Permian Tatarian (23 mmolal). Modeled Ca2⫹ is at or above its concentration in modern seawater throughout the Permian. Mg2⫹ is close to (slightly below) its concentration in modern seawater (55 mmolal) in the Asselian-Sakmarian (52 mmolal), and Tatarian (52 mmolal), but slightly higher than modern seawater in the Artinskian-Kungurian (60 mmolal). Mg2⫹/Ca2⫹ ratios are 3.5 (total range ⫽ 2.7 to 5.5) in the Lower Permian and rose slightly to 3.7 (total range ⫽ 3.1 to 5.8) in the Upper Permian, primarily due to decreases in Ca2⫹. These results are consistent with models that predict oscillations in the major-ion composition of Phanerozoic seawater on the basis of changes in the midocean ridge/river water flux ratio driven by changes in the rate of midocean ridge crust production. The Permian was characterized by low sea levels, icehouse conditions, and southern hemisphere glaciation. Such conditions, analogous to the present ice age, and the similarities between Permian seawater and modern seawater, all suggest that general Phanerozoic supercycles, driven by mantle convection and global volcanicity, also control the major-ion chemistry of seawater. Copyright © 2005 Elsevier Ltd

match the predictions of Hardie (1996), who modeled the major-ion composition of seawater by mixing midocean ridge (MOR) hydrothermal brine and river water (RW) inflows. Secular changes in seawater chemistry in the Hardie model are controlled by changes in the MOR/RW flux ratio driven by changes in the rate of ocean crust production at midocean ridges. Holland and Zimmermann (2000) suggested that the Mg2⫹ and SO2⫺ concentration in seawater are also influenced 4 by global dolomitization and deposition of CaSO4 evaporites particularly during times of high sea level. Most recently, Turchyn and Schrag (2004) reported that SO2⫺ in seawater 4 over the past 10 million years may have been controlled by sea level variations that influenced the area of continental shelves and weathering of pyrite. Permian marine limestones contain ooids and submarine cements that are interpreted as primary aragonite and magnesian calcite (⬎4 mol % MgCO3) (Davies, 1977), formed in aragonite seas (Sandberg, 1983), with elevated Mg2⫹/Ca2⫹ ratios (Hardie, 1996). The MgSO4-rich mineralogy of Late Permian potash

1. INTRODUCTION 2⫹

2⫹

Secular variations in the major-ion chemistry (Mg , Ca , ⫺ Na⫹, K⫹, SO2⫺ 4 , and Cl ) of Phanerozoic seawater have been documented recently from fluid inclusions in marine halites (Kovalevich et al., 1998; Zimmermann, 2000; Lowenstein et al., 2001, 2003; Brennan and Lowenstein, 2002; Horita et al., 2002; Brennan et al., 2004), the magnesium content of fossil echinoderms (Dickson, 2002), the primary mineralogy of marine potash evaporites (Hardie, 1996), and the strontium concentration in biologic calcites (Steuber and Veizer, 2002). The Phanerozoic oceans have undergone two long-term fluctuations in major-ion composition (for example, Mg2⫹/Ca2⫹ ratios) that are in phase with 100 –200 million-year oscillations in sea level, icehouse-greenhouse climates, and global volcanicity. These fluctuations in Phanerozoic seawater chemistry generally

* Author to whom correspondence should be addressed (lowenst@ binghamton.edu). 1701

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T. K. Lowenstein et al. Table 1. Minerals discussed in this paper and their formulas. Mineral

Formula

Calcite Aragonite Dolomite Magnesite Gypsum Anhydrite Halite Glauberite Polyhalite Kieserite Sylvite Carnallite Langbeinite Kainite

CaCO3 CaCO3 CaMg(CO3)2 MgCO3 CaSO42H2O CaSO4 NaCl CaSO4Na2SO4 K2Ca2Mg(SO4)42H2O MgSO4H2O KCl KMgCl36H2O K2Mg2(SO4)3 MgSO4KCl11/4H2O

evaporites, with polyhalite (K2Ca2Mg(SO4)42H2O) and kieserite (MgSO4H2O) (see Table 1 for mineral formulas), resembles the sequence of salts precipitated from evaporation of modern seawater (Harvie et al., 1980; Hardie, 1984; Lowenstein, 1988). Fluid inclusions from the Upper Permian Salado Formation of New Mexico, USA, contain brines that are chemically similar to evaporated modern seawater (Horita et al., 1991, 2002; Lowenstein et al., 2001). These mineralogical and fluid inclusion studies suggest that Permian seawater was closer to modern seawater in major-ion composition than during any other period of the Phanerozoic. Large Permian evaporite deposits occur in the United States, Central and Eastern Europe, Arabia, and South America (Zharkov, 1984). During the last two decades, the compositions of brine inclusions have been analyzed from primary halites in Permian marine evaporites in the United States (Roedder et al., 1987; Stein and Krumhansl, 1988; Bein et al., 1991; Horita et al., 1991, 2002; Lowenstein et al., 2001), Germany (Herrmann and von Borstel, 1991; Peters-Zimmermann, 1992), Poland (Peryt and Kovalevich, 1996; Kovalevich et al., 1998; Kovalevych et al., 2002), Ukraine (Kovalevich et al., 1998; Kovalevych et al., 2002), and Russia (Kovalevych et al., 2002). Many of these formations are similar in age and geographically separated, which permits a “global” sampling of Permian seawater brines. The purpose of this study is to determine the major-ion composition of Permian seawater from chemical analyses of fluid inclusions in marine halites, in light of the recent evidence for secular variations in the major-ion chemistry of seawater (Lowenstein et al., 2001; Horita et al., 2002). New data from the Late Permian San Andres Formation of Texas, USA, and Salado Formation of New Mexico are presented along with the screened database of published chemical compositions of fluid inclusions in Permian marine halites (Table 2). This paper characterizes and interprets brine inclusions in halite from the Permian, using the largest dataset of its kind for any geological period. First, we examine the geochemical and geological evidence for the seawater origin of the halites and fluid inclusions under consideration. This culling process allows us to identify those inclusion compositions most representative of Permian seawater. Second, we show that Permian seawater was similar ⫺ to modern seawater in its Na⫹-Mg2⫹-K⫹-SO2⫺ 4 -Cl major-ion

composition and Mg2⫹/K⫹ ratio, but with quantitative differences, particularly in the concentration of SO2⫺ 4 . Third, we show that syndepositional processes, notably, recycling of previously deposited salts, can complicate the chemistry of brine inclusions in halite and alter the seawater evaporation trends. Fourth, we model the major-ion composition of Permian seawater for three intervals (Lower Permian, 296 –283 Ma and 283–274 Ma, and Upper Permian, 258 –251 Ma) using the chemical evaporation paths defined by brine inclusions and the Harvie-Møller-Weare (HMW) computer program (Harvie et al., 1984). This modeling allows evaluation of secular changes in seawater chemistry within the Permian in light of recent evidence from fluid inclusions in marine halites from the Central and Eastern European basins showing systematic “second order” temporal trends in seawater chemistry (Kovalevych et al., 2002). These changes are not part of the two Phanerozoic supercycles during which seawater chemistry alternated between “CaCl2-seas” (calcite seas) and “MgSO4” (aragonite seas) (Lowenstein et al., 2003). Rather, second order trends refer to fluctuations in seawater chemistry during the 50 million years of the Permian, all within a time of “MgSO4” seas. Finally, we compare the fluid inclusion record of Permian seawater with other Permian paleoceanographic information. 2. METHODS

2.1. Analytical Techniques Fluid inclusions in halite from the San Andres and Salado Formations, analyzed by the environmental scanning electron microscopy (ESEM) X-ray energy-dispersive spectrometry (EDS) method, are reported here (Table 3). This technique uses an ESEM with X-ray energy-dispersive analysis capabilities and can measure absolute concentrations of major cations and anions in frozen inclusions (ca. Ca, Mg, K, S in sulfate, and Cl) at concentrations greater than ⬃0.1 wt.%. For Na, quantitative analyses are obtained at concentrations above 0.5 wt.% (Timofeeff et al., 2000, 2001). The ESEM X-ray EDS method gives major element chemical analyses of frozen fluid inclusions in halite greater than ⬃30 ␮m in size with typical precisions between 2% and 7% and accuracies better than 7% (Ayora et al., 1994a,b; Timofeeff et al., 2000). Fluid inclusions in halite from the Salado Formation and from the Wellington Formation, Kansas, USA, were analyzed by Horita et al. (1991) by direct extraction and ion chromatography (IC). This technique involves drilling a hole into a halite crystal, and extracting the inclusion brine with a microsyringe. The extraction IC technique allows analysis of major and minor elements such as Br and Li, but it is limited to large fluid inclusions greater than ⬃200 ␮m in size. Fluid inclusions in halite from the Zechstein Formation of northern Poland were also analyzed by direct extraction (Peryt and Kovalevich, 1996). Major ions were then measured by the microtitration technique of Petrichenko (1973). The minimum size of inclusions needed for this method is ⬃40 ␮m. The results from the Zechstein fluid inclusions, published in mg/L (Peryt and Kovalevich, 1996), were converted to molalities using a density conversion from McCaffrey et al. (1987). The extraction-microtitration technique of Petrichenko (1973) was also used to analyze fluid inclusions from an extensive suite of Permian

Permian seawater chemistry

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Table 2. Database of Permian halites used in this study, including evaporite formations, locations, ages, and number of fluid inclusions analyzed with references. Basin/Formation

Location

Stage (Series)

Age (Ma) 251

Salado

New Mexico, USA

Tatarian (Ochoan)

Zechstein

North Poland Western Poland Kaliningrad, Russia Texas, USA Russia Russia Kansas, USA Ukraine Russia Ukraine

Tatarian

258–251

Ufimian (Guadalupian) Kungurian Kungurian Artinskian (Leonardian) Sakmarian Sakmarian Asselian

274–272 277–274

San Andres Solikamsk Basin Precaspian Basin Wellington Dnipro-Donets Basin Dvina-Sukhona Basin Dnipro-Donets

283–277 290–283 296–290

Number of fluid inclusions, reference 13, this study 31, Horita et al., 1991 20a, Peryt and Kovalevich, 1996 5a, Kovalevych et al., 2002 21a, Kovalevych et al., 2002 52, this study 6a, Kovalevych et al., 2002 21a, Kovalevych et al., 2002 12, Horita et al., 1991 11a, Kovalevych et al., 2002 7a, Kovalevych et al., 2002 38a, Kovalevych et al., 2002

a The number of samples analyzed by the extraction-microtitration technique of Petrichenko (1973). For each sample, 1 to 8 analyses were done for K⫹, Mg2⫹, and SO2⫺ 4 . The total number of brine inclusions needed for these analyses varied depending on inclusion size; typically, between 3 and 9 brine inclusions were analyzed for each sample.

halites from the Dnipro-Donets Basin, Ukraine, the DvinaSukhona Basin, Russia, the Precaspian Basin, Russia, the Solikamsk Basin, Russia, and the Zechstein Formation, western Poland and Kaliningrad, Russia, recently reported by Kovalevych et al. (2002). The density conversion from McCaffrey et al. (1987) was again used to convert all the analyses in Kovalevych et al. (2002) from g/L to molalities. 2.2. Computer Modeling The Harvie, Møller, and Weare (HMW) (1984) chemical equilibrium computer program was used to simulate evaporation of modern and Permian seawater. For equilibrium evaporation of present-day seawater at 25°C, the HMW computer program calculates the molality of each major ion and the number of moles of salts precipitated at each evaporation step. The HMW-computed evaporation paths plotted for the major ions in modern seawater (Na⫹, K⫹, Ca2⫹, Mg2⫹, Cl⫺, and SO2⫺ 4 ) display distinctive linear trends which closely match the paths defined by evaporation of present-day seawater from the solar saltwork of Great Inagua Island, Bahamas, as well as the chemical compositions of fluid inclusions in modern marine halites from Great Inagua Island and Baja California, Mexico (Timofeeff et al., 2001). The HMW computer program was also used to normalize the Na⫹ and Cl⫺ concentrations of all fluid inclusions reported here to halite saturation (Timofeeff et al., 2001). 3. GEOLOGIC BACKGROUND OF PERMIAN HALITES

Each Permian fluid inclusion analyzed along with the halite crystal host must be scrutinized to determine whether they formed from seawater or nonmarine parent waters and whether they have been altered following deposition. It is important to use all available sedimentologic, petrographic, and geochemical information to select only those brine inclusions with evaporated paleoseawater. Such inclusions are found in primary halites with sedimentary textures and fabrics diagnostic of precipitation at the brine bottom and the air–water interface of a standing body of brine (Lowenstein and Hardie, 1985). Fluid

inclusions in primary halites are numerous and aligned along crystal growth bands, which gives the halite crystals a cloudy, commonly banded appearance. In contrast, halites formed in the subsurface from diagenetic fluids by cementation or by recrystallization lack primary textures and fabrics, and are typically clear and coarsely crystalline (Hardie et al., 1985). Such diagenetically formed halites are unlikely to contain fluid inclusions with evaporated paleoseawater. Once primary surface-formed halites are distinguished from diagenetic salts, the next step is to assess the marine vs. nonmarine origin of the primary halite. The marine seawater origin of ancient halites, discussed below, can be established from fossil evidence, evaporite mineral sequences, sulfur isotopes and 87Sr/86Sr ratios in associated anhydrites, Br⫺ content of halite, and the chemical compositions of fluid inclusions in halite. 3.1. Wellington Formation, Kansas, USA The Wellington Formation is ⬃230 m thick and lies at the base of the Lower Permian Leonardian Series in Kansas and the Panhandle regions of west Texas and Oklahoma (Fig. 1) (Jones, 1965). The Wellington Formation consists of a Lower Member with shale, carbonate, and anhydrite (⬃45–55 m thick), the Hutchinson Salt Member dominated by bedded halite with shale, dolomite, magnesite, and anhydrite (⬃100 m), and an Upper Member composed of shale and carbonate (⬃75 m) (Fig. 1) (Jones, 1965). Ripple marks and salt polygons identified in mine exposures of the Hutchinson Salt Member suggest a shallow-water to subaerially exposed depositional setting (Dellwig, 1968). Cloudy chevron halite with primary fluid inclusion banding was identified by Jones (1965) and Dellwig (1968), but clear, inclusion-poor halite is apparently more abundant. The Wellington Formation belongs to the Leonardian Series, which is equivalent to the Artinskian Stage (283–277 Ma) (Mazzulo, 1995; Menning, 1995). The Hutchinson Salt Member of the Wellington Formation is considered a marine evaporite. It contains numerous depositional sequences consisting of shale-carbonate-anhydrite with gypsum pseudomorphs and halite, most likely formed by pro-

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Table 3a. The major ion chemical composition of primary fluid inclusions in halite from the Guadalupian (274 –272 Ma) San Andres Formation, Texas, from the following borehole cores: G. Friemel (GF), Rex White (RW), Detten (Det), from the Lower San Andres Formation Unit 5, and Harman (H), Lower San Andres Unit 4. All values are millimolal (mmol per kg H2O). Na⫹ and Cl⫺ concentrations were normalized to halite saturation at 25°C using the HMW computer program. Mg

K

SO4

Na

Cl

Sample

510 180 430 2010 2690 290 930 1360 1680 400 690 210 640 420 1490 1300 960 750 1220 620 3050 1500 960 490 1840 1180 800 1120 1050 820 1220 1230 830 780 1670 1660 1110 1640 1230 1630 2370 2350 1370 270 590 1700 850 1080 1220 590 400 600

300 270 220 470 140 330 370 500 410 230 240 200 440 230 420 370 340 350 350 330 350 280 430 350 440 380 330 460 390 330 310 340 490 400 370 500 440 480 470 690 540 560 490 630 570 400 590 330 530 420 540 530

90 80 110 110 270 80 100 170 150 40 40 70 130 110 250 260 150 170 200 150 500 290 210 210 240 140 150 140 160 130 200 170 180 240 290 300 160 200 170 260 360 80 270 130 130 290 210 200 160 120 120 130

5100 5720 5320 2550 1830 5490 4330 3610 3100 5290 4750 5680 4850 5330 3490 3840 4340 4720 3930 4950 1480 3570 4360 5240 2920 3930 4630 4000 4170 4570 3940 3890 4530 4720 3250 3220 4050 3180 3830 3170 2190 2030 3690 5420 4880 3190 4480 4190 3820 4940 5210 4880

6250 6190 6180 6820 6810 6240 6360 6480 6580 6240 6300 6170 6300 6180 6400 6290 6300 6240 6320 6220 6930 6270 6290 6150 6550 6390 6260 6420 6340 6290 6280 6350 6320 6210 6380 6450 6380 6550 6430 6610 6760 7140 6370 6340 6360 6410 6350 6270 6470 6290 6330 6350

GF1 Aaug05 (712.1 m) GF2 Aaug05 (712.1 m) GF1 Aaug06 (712.1 m) GF1 Aaug06 (712.1 m) GF1 Daug06 (712.1 m) GF2 Daug06 (712.1 m) GF3 Daug06 (712.1 m) GF1 Eaug06 (712.1 m) GF1 BAug06 (712.1 m) GF1 BAug05 (712.1 m) GF2 BAug05 (712.1 m) GF3 BAug05 (712.1 m) RW1 Sep02a (511.4 m) RW2 Sep02a (511.4 m) RW1 Sep02b (511.4 m) RW1 Sep02c (511.4 m) RW1 Sep02d (511.4 m) RW2 Sep02d (511.4 m) RW1 Sep02e (511.4 m) RW2 Sep02e (511.4 m) RW1 Sep02g (511.4 m) RW2 Sep02g (511.4 m) RW3 Sep02g (511.4 m) RW4 Sep02g (511.4 m) RW1 Sep02h (511.4 m) RW1 Sep03a (511.4 m) RW2 Sep03a (511.4 m) RW3 Sep03a (511.4 m) RW4 Sep03a (511.4 m) RW5 Sep03a (511.4 m) RW1 Sep03b (511.4 m) RW2 Sep03b (511.4 m) Det1 Sept10a (754.8 m) Det2 Sept10a (754.8 m) Det1 Sept10b (754.8 m) Det2 Sept10b (754.8 m) Det1 Sept10c (754.8 m) Det2 Sept10c (754.8 m) Det3 Sept10c (754.8 m) Det1 Caug13 (754.8 m) Det2 Caug13 (754.8 m) Det3 Caug13 (754.8 m) Det4 Caug13 (754.8 m) Det1 AAug13a (754.8 m) Det3 AAug13a (754.8 m) Det4 AAug13a (754.8 m) Det5 AAug13a (754.8 m) H1 Sep09a (819.9 m) H2 Sep09a (819.9 m) H1 Sep09b (819.9 m) H2 Sep09b (819.9 m) H3 Sep09b (819.9 m)

gressive evolution of seawater brines. Pelecypod and foraminifera fossils of marine or brackish-water origin are found at the base of the Hutchinson Salt Member (Jones, 1965). Sulfur isotopes from anhydrites in the Carey Salt Mine, Hutchinson Salt Member have values of ⫹ 12.2–13.4‰ (Claypool et al., 1980), typical of marine evaporites of Early Permian age. Fluid ⫺ inclusions in halite have a Na⫹-Mg2⫹-K⫹-SO2⫺ major 4 -Cl element chemistry similar to halite from other Permian marine evaporites (Horita et al., 1991). Finally, the bromide content of

halite from the Hutchinson Salt Member is generally 30 – 60 parts per million, somewhat low, but still in the range of marine halites formed from seawater (Holser, 1966). Horita et al. (1991) analyzed 12 fluid inclusions from the Carey Salt mine near Hutchinson, Kansas. The brine inclusions from the Carey Mine were large, ⬎1 mm, in clear halite, from a depth of 198 m. Although the fluid inclusions did not come from primary chevron halite, the evaporation trends observed when the brine inclusion chemistries are plotted on ion-ion

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Table 3b. The major ion chemical composition of primary fluid inclusions in halite from the Ochoan Salado Formation (251 Ma) ERDA-9 drill core (Sample WIPP-3, 860 m) and from borehole core SL9-B9 in the McNutt Potash Zone. All values are millimolal (mmol per kg H2O). Na⫹ and ⫺ Cl concentrations were normalized to halite saturation at 25°C using the HMW computer program. Mg

K

SO4

Na

Cl

Sample

1090 1050 1100 1140 1270 1700 1130 1650 1140 1630 890 430 870

420 410 490 470 470 560 370 460 430 410 470 80 90

240 300 230 270 340 340 310 300 300 270 230 200 260

4030 4130 3980 3930 3740 3020 4010 3120 3960 3160 4360 5340 4560

6450 6410 6500 6480 6490 6700 6400 6640 6440 6610 6430 6150 6200

Inc1 Dec10 SL9-B9 Inc2 Dec10 SL9-B9 Inc1 Dec10C SL9-B9 Inc2 Dec10C SL9-B9 Inc1 Dec11A SL9-B9 Inc2 Dec11A SL9-B9 Inc1 Dec11D SL9-B9 Inc2 Dec11D SL9-B9 Inc4 Dec11D SL9-B9 BSLInc1 Nov25B SL9-B9 DSLInc2 Nov25D SL9-B9 AWInc1 Jan9A Wipp3-ERDA 9 AWInc4 Jan9A Wipp3-ERDA 9

diagrams (see below) suggest that they contain evaporated Early Permian seawater. 3.2. San Andres Formation, Texas, USA The San Andres Formation covers an area of more than 10,000 km2 in the Panhandle region of Texas (Hovorka, 1987). The San Andres Formation is up to 500 m thick and is the basal unit of the Upper Permian Guadalupian Series. The carbonates and evaporites of the San Andres Formation are cyclic and were deposited on a shallow-water marine platform with open marine waters to the south (the Midland Basin) (Fig. 2) (Fracasso and Hovorka, 1986). Units 4 and 5 of the Lower San Andres Formation constitute one large carbonate-anhydritehalite megacycle, 85 m thick (Unit 4) and several thinner carbonate-anhydrite-halite sequences (6 –25 m thick, Unit 5) (Hovorka et al., 1993). Primary chevron halite with primary fluid inclusion banding is common in Lower San Andres Units 4 and 5 (Hovorka et al., 1993). The evaporites of the Lower San Andres Formation were deposited in shallow water to subaerially exposed environments (Hovorka, 1987). The San Andres Formation is lowermost Guadalupian in age, estimated to be ⬃274 Ma and equivalent to the Ufimian Stage (Mazzullo, 1995; Menning, 1995). Rb-Sr data from early diagenetic clays in Lower San Andres Unit 4 gave an age of 275 ⫾ 1 Ma (Long et al., 1997). Sedimentologic and geochemical data indicate that the Lower San Andres Units 4 and 5 were sourced predominantly by seawater. Carbonates (limestone) at the base of Unit 4 contain a diverse normal-marine fauna (Fracasso and Hovorka, 1986), and dolomites at the base of Unit 5 contain a lowdiversity molluscan fauna (Hovorka et al., 1993). 87Sr/86Sr ratios and ␦34S from anhydrites, Lower San Andres Units 4 and 5, have normal Guadalupian marine values, although some 87 Sr/86Sr ratios are slightly elevated (Hovorka et al., 1993). An extensive dataset of Br⫺ in halite shows that Lower San Andres Unit 4 halites, with Br⫺ between 50 and 150 parts per million, formed by “simple evaporation of marine water” whereas Unit 5 halites, with Br⫺ between 30 and 90 ppm, formed in shallower brines and were more influenced by recycling processes (Hovorka et al., 1993). Fluid inclusions in San Andres halites have been studied by

Roedder et al. (1987) and Bein et al. (1991). Roedder et al. (1987) analyzed 120 large fluid inclusions (⬎500 ␮m) from 10 borehole cores, mostly from the Lower San Andres Formation, by extraction-ion chromatography and extraction–inductively coupled plasma atomic emission spectroscopy. Fluid inclusions from six chevron halite samples were examined in bulk by grinding and leaching. Roedder et al. (1987) state that the inclusion fluids they analyzed came from clear recrystallized halite. These fluid inclusions are probably not pristine samples of evaporated Permian seawater because the large inclusions and the halite in which they occur are not primary; the large scatter of the major-ion data indicates that the inclusions do not contain brines trapped during simple evaporation of seawater. Bein et al. (1991) reported the major-ion chemistry of 106 fluid inclusions in halite, ⬎1 mm in size, from San Andres Units 4 and 5. Those analyses are not included here because of the uncertain origin of the large fluid inclusions and the large scatter on the chemical composition plots, which indicates that brine inclusion chemistries were not controlled strictly by evaporation. Fluid inclusions analyzed by the ESEM-X-ray-EDS method for this study are: 12 brine inclusions, G. Friemel core, 20 brine inclusions, Rex White core, 15 inclusions, Detten core, all from the Lower San Andres Formation Unit 5, and 5 fluid inclusions, Harman core, Lower San Andres Unit 4 (Fig. 2, Table 3). All these fluid inclusions were between 30 and 100 ␮m in size and came from cloudy, primary fluid inclusion bands in primary chevron halites. 3.3. Salado Formation, New Mexico and Texas, USA The Salado Formation is part of the Late Permian Ochoan Series of Texas and New Mexico, consisting of, from bottom to top, the Castile, Salado, Rustler, and Dewey Lake Formations (Fig. 3). These evaporites record an evolution of environments from the laminated calcite-anhydrite-halite “basinal” facies of the Castile Formation to the shallow-water and subaerial depositional settings of the Salado and Rustler Formations (Lowenstein, 1988; Powers and Hassinger, 1985). The Salado Formation is as thick as 700 m and underlies an area of 150,000 km2 (Fig. 3) (Cheeseman, 1978; Lowenstein, 1988). The dominant rock types in the Salado Formation are halite, muddy

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Fig. 1. (A) Location of the Wellington salt basin. (B) General stratigraphic section of Lower Permian rocks of the Wellington salt basin. Horita et al. (1991) analyzed fluid inclusions in halite from near the base of the Hutchinson Salt in the Carey Salt mine near Hutchinson, Kansas. Map and stratigraphic column modified from Jones (1965).

halite, polyhalite (K2Ca2Mg(SO4)42H2O), anhydrite, dolostone, and mudstone, which occur in distinctive meter-scale sequences interpreted to have formed by progressive drawdown and brine evolution in a shallow, marginal marine basin (Lowenstein, 1988). The Salado Formation is divided into a Lower Member and an Upper Member, separated by ⬃100 m of potash-bearing evaporites, the McNutt Potash Zone, which contains sylvite (KCl), carnallite (KMgCl36H2O), and the

MgSO4 salts kieserite (MgSO4H2O), langbeinite (K2Mg2(SO4)3), and kainite (MgSO4KCl11/4H2O) (Table 1, Fig. 3) (Lowenstein, 1988). The Salado evaporites are latest Permian in age, ⬃251 Ma ⫾ 0.2 Ma (Renne et al., 2001) or 254 Ma ⫾ 5 Ma (Long et al., 1997). The Salado Formation is considered a marine evaporite derived from the evaporation of Permian seawater. Abundant dolostone-anhydrite beds in the southwestern corner of the

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Fig. 2. (A) Location of the Guadalupian Palo Duro basin, and the samples of halite analyzed in this study from cores taken through the San Andres Formation (modified from Hovorka, 1992). “Carbonate shelf” area at southern margin of Palo Duro basin is dominated by marine carbonates. “Dissolved Salt” is area where post-Permian subsurface dissolution of halite has occurred. (B) Stratigraphic section of the San Andres Formation, Palo Duro basin (modified from Hovorka and Granger, 1988).

Salado basin have been interpreted as evidence for a marine connection, the “Hovey Channel,” to the Permian ocean. Marine fossils (bryozoa, gastropods, bivalves, and brachiopods) occur in the overlying Rustler Formation just 1 m above the top of the Salado Formation (Walter, 1953). Fluid inclusions in Salado halites have a major-ion composition similar to modern seawater (see below) (Horita et al., 1991). Distinctive meterscale depositional cycles involving carbonate (magnesite), anhydrite-polyhalite with gypsum pseudomorphs, and halite, all with preserved primary sedimentary structures and textures are common in the Salado Formation (Lowenstein, 1988). 87Sr/ 86 Sr ratios indicate precipitation from dominantly marine wa-

ters, especially samples from relatively thick anhydrite-polyhalite beds (Denison et al., 1998). Some 87Sr/86Sr values that fall off the latest Permian seawater trend may have a mixed marine-nonmarine (continental meteoric water) parentage (Lowenstein, 1988; Denison et al., 1998). Fluid inclusions in Salado halites were analyzed by Stein and Krumhansl (1988) and Horita et al. (1991) from the ERDA-9 drill core (Fig. 3). The large fluid inclusions studied by Stein and Krumhansl (1988) came from clear recrystallized halite, more than 200 m above the base of the Salado Formation. The variable fluid inclusion chemistry was interpreted to reflect postdepositional brine-evaporite reactions. The petrographic

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Fig. 3. (A) Location of the Salado Formation and the SL9-B9 core and ERDA-9 core from which samples were analyzed in this study. (B) Stratigraphic column through the Salado Formation showing position of samples from the ERDA-9 core (WIPP-1 through WIPP-5) and the halite sample from the McNutt Potash Zone from the SL9-B9 core. Figures modified from Lowenstein (1988).

characteristics of these fluid inclusions and their halite host crystals indicate that they are probably not samples of unmodified evaporated Late Permian seawater and are not discussed further. Horita et al. (1991) analyzed 49 large (⬎⬃300 ␮m) fluid inclusions from five stratigraphic horizons (WIPP-1 through WIPP-5) in the ERDA-9 drill core (Fig. 3). These inclusions were located in clear parts of halite crystals but were associated with chevron halite in areas with well preserved sedimentary bedding, which was used to argue that the brine inclusions were “syndepositional or early diagenetic” in origin (Horita et al., 1991). Therefore the halites analyzed by Horita et al. (1991) probably contain inclusions with trapped surface brines. Eighteen of the 49

inclusion brines were not used here for assessing Permian seawater compositions because they are below halite saturation, as calculated by the HMW computer program. Fluid inclusions from sample WIPP-3 are notable because they occur in the lowermost halites (860 m) of the Salado Formation. Fluid inclusions analyzed by the ESEM-X-ray-EDS method for this study include two brine inclusions from the ERDA-9 core, WIPP-3 basal halites of Horita et al. (1991), and 11 brine inclusions from the McNutt Potash Zone, Borehole core SL9-B9 (Fig. 3, Table 3). All these fluid inclusions were between 30 and 100 ␮m in size and came from cloudy, primary fluid inclusion bands in primary chevron halites.

Permian seawater chemistry

3.4. Zechstein Formation, Poland The Zechstein evaporites are the deposits of a vast 1,000,000 km2 marine basin of the Late Permian (Tatarian Stage) that extended from the British Isles to Poland and beneath the North Sea (Fig. 4) (Stewart, 1963). The Zechstein deposits are commonly divided into four or five major cycles totaling greater than 1000 m in thickness (Stewart, 1963). The cycles are composed of carbonates, anhydrite, halite, and in some cases, potash salts, which vary in thickness, texture, and sedimentary characteristics depending on whether they accumulated in a platform or basin setting. In northern Poland, a subbasin of the Zechstein Sea formed the Peribaltic Gulf and deposited a thick first cycle of Kupferschiefer-Zechstein Limestone-Lower Werra Anhydrite and Oldest Halite, up to 200 m thick (Fig. 4) (Peryt and Kovalevich, 1996). Halites from the Oldest Halite (Na1) contain abundant primary textures and fabrics including chevrons, “hoppers,” and cumulates of well-sorted fine crystals deposited in saline pan to saline lagoon settings (Peryt and Kovalevich, 1996). The Zechstein evaporites were deposited during the last 5–7 million years of the Permian, from ⬃258 – 251 Ma (Menning, 1995). A date of 251.0 Ma ⫾ 1.3 Ma was obtained from the mineral langbeinite (Table 1) of the German Zechstein (Lippolt et al., 1993). The European Zechstein deposits are interpreted as marine evaporites because interbedded carbonates are fossiliferous and range from reef to basinal facies. Fluid inclusions in halites of the Polish Zechstein Oldest Halite (Na1) have major-ion compositions similar to evaporated modern seawater (Peryt and Kovalevich, 1996) (see below). Sulfur isotopes of Zechstein carbonates and anhydrites (Kovalevych et al., 2002) have values identical to other late Permian marine sediments (Claypool et al., 1980). Numerous analyses of Br⫺ in halite have values consistent with evaporation of a large body of late Permian seawater. For example, Br⫺ concentrations in the Oldest Halite of Poland range from 40 to 100 ppm (Peryt and Kovalevich, 1996). The mineral sequence of the Zechstein 2 of Germany closely matches the evaporite minerals predicted to precipitate from the equilibrium evaporation of modern seawater at 25°C (Harvie et al., 1980). Six fluid inclusions from clear, recrystallized halite, within the Zechstein Formation, Gorleben Salt Dome, Germany, were analyzed by Herrmann and von Borstel (1991). Deformed and recrystallized halite samples from a salt dome are unlikely to contain fluid inclusions with evaporated Permian seawater so they are not discussed further. Similarly, Peters-Zimmermann (1992) analyzed 10 brine inclusions from the German Zechstein halites and concluded from their chemical compositions that they were not evaporated Permian seawater but formed by dissolution of potash salts such as polyhalite, kainite, sylvite, and carnallite (see Table 1). Peryt and Kovalevich (1996) analyzed fluid inclusions from 20 samples of primary halite of the Oldest Halite (Na1) using the extraction-microtitration technique. These halites were from three cores drilled in platform, slope of platform, and basinal facies of the Zechstein (Fig. 4). 3.5. Other Permian Halites The major-ion composition of fluid inclusions from seven other Permian halites in Poland, Ukraine, and Russia was

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recently reported by Kovalevych et al. (2002) (Table 2). All fluid inclusions analyzed came from primary halites with fluid inclusion banded chevron texture. A marine origin of each evaporite sequence was established on the basis of sulfur isotopes in associated anhydrites, measurements of Br⫺ in halite, and evaporite mineralogy (Kovalevych et al., 2002). These data from central and eastern Europe include the Asselian (296 –290 Ma) and Sakmarian (290 –283 Ma) of the Dnipro-Donets Basin, Ukraine, the Sakmarian of the Dvina-Sukhona Basin, Moscow Syncline, the Kungurian (277–274 Ma) of the Solikamsk Basin, Uralian Foredeep, the Kungurian of the Precaspian Basin, Russia, and the Late Permian (258 –251 Ma) Zechstein salts of western Poland and Kaliningrad region, Russia (Table 2). Fluid inclusion chemistries from Permian halites now include three formations of different ages and locations in North America, and eight formations of four different ages in Central and Eastern Europe (Table 2). This is by far the most extensive dataset on the chemical composition of fluid inclusions in halite for any period of the Phanerozoic. 4. RESULTS

4.1. General Major-Ion Chemistry of Permian Brine Inclusions and Permian Seawater The major-ion chemistries of 52 brine inclusions from the San Andres Formation and 13 brine inclusions from the Salado Formation are listed in Table 3 and plotted vs. Cl⫺ (Fig. 5A–D) and Mg2⫹ (Fig. 5E,F). All brine inclusions have measurable ⫹ ⫺ 2⫹ concentrations of Na⫹, Mg2⫹, SO2⫺ 4 , K , and Cl ; Ca concentrations were below the ESEM-X-ray EDS detection limit of 0.1 wt.%, which is ⬃15 millimolal (millimoles per kg H2O, here termed mmolal). One sample from the basal Salado Formation ERDA-9 drill core (WIPP-3, 860 m) was analyzed in this study by the ESEM X-ray EDS technique (two inclusions, shown as solid circles in Fig. 5) and by Horita et al. (1991) using the extraction IC method (nine inclusions, shown as crosses within circled area in Fig. 5). The compositions of those 11 brine inclusions, circled in Figure 5, show that these two analytical methods give overlapping results. The ESEMX-ray EDS technique is able to analyze relatively small brine inclusions (⬎30 microns) that occur along halite crystal growth bands and are demonstrably primary. The extraction-IC method is limited to large (⬎200 micron) fluid inclusions whose origin is commonly difficult to interpret. These results confirm that the large brine inclusions analyzed by Horita et al. (1991) contain brines with the same major-ion chemical composition as those analyzed in this study by ESEM-X-ray EDS. Previously published major-ion chemistries of brine inclusions from Permian halites (Table 2) are plotted by age in Figure 6 (Asselian-Sakmarian 296 –283 Ma), Figure 7 (Artinskian-Kungurian 283–274 Ma), and Figure 8 (Tatarian 258 – 251 Ma). Fluid inclusions in all Permian marine halites contain measurable SO2⫺ and very low concentrations of Ca2⫹, which 4 indicates that the parent Permian seawater contained molar ⫺ 1 Ca2⫹ ⬍ SO2⫺ 4 ⫹ ⁄2 HCO3 . Such waters, after evaporation and precipitation of calcite and gypsum, evolve into SO2⫺ 4 -rich, Ca2⫹-depleted brines at halite saturation, as does modern seawater. Curves tracking the brine compositions predicted for

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Fig. 4. (A) Maps showing extent of Zechstein 1 evaporites, halites, and potash deposits in Europe (modified from Stewart, 1963 and Zharkov, 1984), and locations of cores through Zechstein halites, northern Poland (modified from Peryt and Kovalevich, 1996). (B) Stratigraphic section of the Zechstein 1 and 2 cycles in cores Zdrada IG8, IG6, and IG3, northern Poland. Locations of samples analyzed by Peryt and Kovalevich (1996) are shown with arrows (modified from Peryt and Kovalevich, 1996).

evaporation of modern seawater calculated by the HMW computer program are plotted in Figures 5– 8 and show that Permian seawater and present-day seawater both fall on the

2⫹ SO2⫺ -SO42⫹ chemical divide and evolve 4 -rich side of the Ca ⫺ into Na⫹-Mg2⫹-K⫹-SO2⫺ -Cl brines. 4 Permian brine inclusions trace out paths on the ion-ion plots

Permian seawater chemistry

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Fig. 5. Plots of major-ion chemistry of Permian fluid inclusions from the Guadalupian San Andres Formation, Texas, USA (274 –272 Ma) and Ochoan Salado Formation, New Mexico, USA (251 Ma). Evaporation pathways of modern seawater, simulated by the Harvie-Møller-Weare (1984) computer program, are shown with solid lines. Evaporation pathways are functions of the major-ion composition of the parent water, the amount of evaporation, and the type and ⫹ amount of salts precipitated. The drop in Na⫹ and increase in Mg2⫹, SO2⫺ at ⬃6000 mmolal Cl⫺ indicates 4 , and K precipitation of halite and loss of Na⫹ and Cl⫺ from the brines. Circled data are from the Salado Formation ERDA-9 core, sample WIPP-3 (860 m) analyzed by Horita et al. (1991) using extraction ion-chromatography (crosses, 9 analyses) and this study, using ESEM X-ray EDS (closed circles, 2 analyses).

that define the evolution of Permian seawater during evaporation and precipitation of halite. In the halite precipitation field, Permian brine inclusions show curves of decreasing Na⫹ (Figs. 5A, 6A, 7A, and 8A), but the Permian brine evolution paths are offset to the right of the modern seawater evaporation path, which suggests that Permian seawater had lower Na⫹/Cl⫺ ratios than modern seawater. The Mg2⫹ vs. Cl⫺ plots (Figs. 5B, 6B, 7B, and 8B) show that Permian inclusions define brine evolution paths that are again offset to the right of simulated modern seawater evaporation, toward higher Cl⫺ concentrations. All Permian fluid inclusions contain SO2⫺ 4 concentrations below those predicted by simulated evaporation of modern seawater (Figs. 5C, 6C, 7C, and 8C). The plots of K⫹ vs. Cl⫺

(Figs. 5D, 6D, 7D, and 8D) show that K⫹ reaches concentrations higher than those predicted by simulated evaporation of modern seawater. One possibility is that Permian seawater had elevated concentrations of K⫹, but that does not fit the data of Horita et al. (2002) who showed from the Br⫺ and K⫹ concentrations in brine inclusions in halite that K⫹ in Phanerozoic seawater has been relatively constant, between ⬃9 –11 mmolal. A second favored interpretation is that the K⫹ concentration of evaporated Permian seawater was controlled by precipitation of the mineral polyhalite. The curves depicting evaporation of modern seawater show that K⫹ concentrations remain relatively low in the halite field because of equilibrium precipitation of polyhalite. However, with SO2⫺ concentrations in Permian seawater below 4

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Fig. 6. Plots of major-ion chemistry of Lower Permian fluid inclusions from the Asselian (296 –290 Ma) and Sakmarian (290 –283 Ma) of the Dnipro-Donets basin, Ukraine, and the Sakmarian of the Dvina-Sukhona basin, Russia. Evaporation pathways of modern seawater, simulated by the Harvie-Møller-Weare (1984) computer program, are shown with solid lines. Best-fit evaporation pathways for the Asselian-Sakmarian fluid inclusions are shown with dotted lines.

those found during evaporation of modern seawater (Figs. 5C, 6C, 7C, and 8C), polyhalite saturation occurs later in the evaporation pathway. With less SO2⫺ 4 in Permian seawater than present-day seawater, K⫹ could rise to higher concentrations before polyhalite saturation was reached. A third possibility is that the high K⫹ concentrations in some Permian fluid inclusions may be due to recycling and dissolution of previously deposited salts. Such recycling produces elevated brine concentrations of the ions found in the dissolved salts. The effects of syndepositional recycling are most common in shallow water to subaerially exposed evaporite settings, where, during each flooding event, previously deposited salts are dissolved in the flood waters, changing their chemistry. For saline pans and shallow saline lakes flooded episodically by

seawater, for example, the Na⫹/Cl⫺ and K⫹/Cl⫺ ratio of the seawater is changed by dissolution of halite and sylvite, as Na⫹ and Cl⫺ and K⫹ and Cl⫺ are added in equal molar proportions. Studies of modern saline systems, such as the Qaidam Basin of western China, have shown that K⫹ is a particularly sensitive indicator of evaporite mineral recycling (Spencer et al., 1990). Brines in the Qaidam Basin, for example, that have recycled carnallite have elevated K⫹, up to several hundred mmolal above the concentrations predicted by evaporation of parent waters, because dissolution of carnallite adds K⫹ to the recycled brines. This recycling phenomenon is a probable explanation for the high concentrations of K⫹ in the 11 fluid inclusions from the McNutt Potash Zone of the Salado Formation, where sylvite and carnallite occur and for many of the brine inclusions

Permian seawater chemistry

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Fig. 7. Plots of major-ion chemistry of Lower Permian fluid inclusions from the Leonardian (Artinskian) Wellington Formation, Kansas, USA (283–277 Ma) and the Kungurian (277–274 Ma) of the Precaspian basin and the Solikamsk basin, Russia. Evaporation pathways of modern seawater, simulated by the Harvie-Møller-Weare (1984) computer program, are shown with solid lines. Best-fit evaporation pathways for the Artinskian-Kungurian fluid inclusions are shown with dashed lines.

in the San Andres Formation halites (Fig. 5D). Recycling of K-bearing minerals in the McNutt Potash Zone is supported by petrographic textures diagnostic of syndepositional dissolution of salts (Lowenstein, 1988). Syndepositional karst features commonly found in the San Andres halites (Hovorka, 1987) also suggest that syndepositional recycling processes may have modified the San Andres parent brines. 2⫹ The plots of SO2⫺ show that SO2⫺ 4 vs. Mg 4 concentrations in the Permian inclusions are below those found during simulated evaporation of modern seawater (Figs. 5E, 6E, 7E, and 8E). The K⫹ vs. Mg2⫹ plots show scatter, but in general, the

Mg2⫹/K⫹ ratios cluster near the value of 5 found in modern seawater (Figs. 6F, 7F, and 8F). Of note is that fluid inclusions from the Salado Formation McNutt Potash Zone and from the San Andres halites have elevated K⫹ concentrations and low Mg2⫹/K⫹ ratios, again suggestive of recycling of K-bearing potash minerals (Fig. 5F). In summary, the major-ion compositions of Permian brine inclusions show that Permian seawater was notably depleted in SO2⫺ 4 (Figs. 5C and E, 6C and E, 7C and E, and 8C and E) with a lower Na⫹/Cl⫺ ratio (Figs. 5A, 6A, 7A, and 8A) compared to the brines produced during simulated evaporation of modern

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Fig. 8. Plots of major-ion chemistry of Upper Permian fluid inclusions from the Ochoan Salado Formation, New Mexico, USA (251 Ma) and the Tatarian Zechstein evaporites (258 –251 Ma) of northern Poland, western Poland, and the Kaliningrad region, Russia. Evaporation pathways of modern seawater, simulated by the Harvie-Møller-Weare (1984) computer program, are shown with solid lines. Best-fit evaporation pathways for the Tatarian fluid inclusions are shown with dashed lines. Eleven fluid inclusions in halite from the Salado Formation McNutt Potash Zone are offset from the rest of the Upper Permian data (elevated K⫹/Mg2⫹ ratios, 8F) which suggest syndepositional recycling of K-bearing bittern salts.

seawater. Brine inclusions also show systematic temporal changes in major-ion chemistry during the Permian that appear to be related to fluctuations in seawater chemistry. It is assumed that if the chemical composition of fluid inclusions in halite from geographically separated deposits of about the same age is the same, then the common parent water of each halite was evaporated Permian seawater. Lower Permian (296 –283 Ma) brine inclusions (Asselian and Sakmarian, Dnipro-Donets Basin, Ukraine and Sakmarian, Dvina-Sukhona Basin, Russia, Table 2) trace out well-defined overlapping paths on the majorion plots of Figure 6. Similarly, Lower Permian Leonardian-

Kungurian (283–274 Ma) brine inclusions (Wellington Formation, Kansas, and Kungurian halite of the Solikamsk Basin and Precaspian Basin, Russia) form overlapping evaporation pathways with the lowest SO2⫺ 4 concentrations of the Permian (Fig. 7) (Kovalevych et al., 2002). Fluid inclusions from the San Andres Formation (Guadalupian, 274 –272 Ma) mark the beginning of an Upper Permian swing of seawater chemistry toward higher SO2⫺ concentrations (Fig. 5). The Upper Perm4 ian (258 –251 Ma) Zechstein Formation (Poland, Russia) and Salado Formation have brine inclusions closest in chemical composition to modern evaporated seawater, with relatively

Permian seawater chemistry

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Table 4. Major-ion compositions of primary fluid inclusions (mmol/kg H2O) in Permian marine halites, as averages of brine inclusion data for each time period. Time

Age (Ma)

m(Mg2⫹)/m(K⫹)

m(Mg2⫹)/m(SO2⫺ 4 )

m(K⫹)/m(SO2⫺ 4 )

a m(SO2⫺ 4 ) excess

Modern seawaterb Tatarian Artinskian-Kungurian Asselian-Sakmarian

0 258–251 283–274 296–283

5.2 5.2 6.0 5.2

2.7–3.0 5.7 28 10

0.52–0.54 1.1 4.6 2.0

18 9.1 2.2 5.0

a 2⫺ 2⫹ m(SO2⫺ ) in initial seawater, in mmol/kg H2O. Modern seawater, with m(Ca2⫹) ⫽ 11 and m(SO2⫺ 4 ) excess is m(SO4 ) ⫺ m(Ca 4 ) ⫽ 29, has 2⫺ ⫹ 2⫺ m(SO2⫺ 4 ) excess of 18. Permian m(SO4 ) excess was obtained from average brine inclusion m(K )/m(SO4 ) ratios, assuming Permian seawater had m(K⫹) ⫽ 10 mmol/kg H2O (see text). b Calculated with the HMW computer program for equilibrium evaporation at 25°C from first saturation with respect to halite until precipitation of polyhalite.

high concentrations of SO2⫺ (Fig. 8). Parallel changes in the 4 chemistry of brine inclusions from equivalent age evaporites in North America, Central Europe, and Eastern Europe suggest a global control of seawater chemistry during the Permian. Other factors, such as nonmarine inflow waters and syndepositional recycling, although likely, do not appear to have influenced brine chemistry and evolution enough to obscure the Permian seawater evaporation trends. 4.2. Calculation of the Major-Ion Composition of Permian Seawater The major-ion composition (Mg2⫹, Ca2⫹, Na⫹, K⫹, SO2⫺ 4 , and Cl⫺) of Permian seawater is calculated here from brine inclusion data and the HMW computer program, which produced Permian seawater evaporation paths on the ion-ion plots of Figures 6, 7, and 8 that best fit the fluid inclusion data. Fluid inclusion chemistries and evaporation paths differ as a function of age, which suggests Permian seawater underwent secular variation in major-ion chemistry. Here we calculate the chemistry of Permian seawater for three periods: Lower Permian Asselian-Sakmarian (296 –283 Ma) (Fig. 6), Lower Permian Artinskian (Leonardian)-Kungurian (283–274 Ma) (Fig. 7), and Upper Permian Tatarian (258 –251 Ma) (Fig. 8). Each of these time intervals has brine inclusion chemistries from at least three different formations that show compositional overlap on the ion-ion plots, which strengthens the case for global evolution of Permian seawater. The parent water chemistry of the Guadalupian San Andres Formation is not modeled because the brine inclusion compositions, with elevated K⫹ concentrations and low Mg2⫹/K⫹ ratios (Fig. 5D,F), suggest significant recycling of K-bearing potash minerals, as discussed earlier in section 4.1. We assume the salinity and chlorinity of Permian seawater was close to modern seawater, although no information exists for Permian seawater. The paleosalinity of ancient seawater may be obtained from measurements of the freezing point depression of aqueous inclusions in marine calcites (Goldstein and Reynolds, 1994). Specifically, the final melting temperature of ice in seawater with a salinity of 35‰ is ⫺1.9°C; the final melting temperature of ice decreases with an increase in salinity (Lyman and Fleming, 1940; Goldstein and Reynolds, 1994). The final melting temperature of ice in primary aqueous inclusions from Cambrian marine calcite cements ranges from ⫺1.7° to ⫺2.5°C, corresponding to a seawater paleosalinity of 31– 47‰, similar to the range found in present-day surface

seawater in shallow marine environments, from open to slightly restricted (Johnson and Goldstein, 1993). Primary fluid inclusions in calcites from the Upper Devonian Canning Basin, Western Australia (Ward et al., 1993; Kwong, 1995; Ward, 1996) revealed a similar range of paleosalinities. Modern seawater contains 565 mmolal Cl⫺, and we assume the same Cl⫺ concentration for the Permian oceans. The concentration of HCO⫺ 3 in seawater (2.5 mmolal) is so small compared to Cl⫺ (565 mmolal) and the other major ions that bicarbonate is ignored in the modeling procedures described below. We also assume from the recent compilation of Phanerozoic brine inclusions in halite by Horita et al. (2002) that the K⫹ concentration of Phanerozoic seawater has been relatively constant, between ⬃ 9 and 11 mmolal. Br⫺ and K⫹ concentrations in brine inclusions in halite from the Miocene, Triassic, Permian, Devonian, and Silurian show similar ratios of K⫹/Br⫺ (Horita et al., 2002). Horita et al. (2002) assume that if Br⫺, with a residence time in seawater of ⬃100 million years, has not changed its concentration in seawater in the Phanerozoic, then the similar ratios of K⫹/Br⫺ indicate that the concentration of K⫹ in seawater also has not varied. Here we use a K⫹ concentration in Permian seawater of 10 mmolal (Horita et al., 2002). With the above assumptions, the majorion composition of Permian seawater may be calculated. The Mg2⫹/K⫹ ratio in Permian seawater is obtained directly from the fluid inclusion data (Figs. 6F, 7F, 8F). Mg2⫹ and K⫹ do not form salts in the halite precipitation field until polyhalite saturation. Therefore the ratio of Mg2⫹/K⫹ in the fluid inclusions, before polyhalite precipitation, should be the same as in Permian seawater. For the three time intervals of the Permian under consideration, Mg2⫹/K⫹ ratios were obtained from the average of the fluid inclusion data for that particular time period. The average Mg2⫹/K⫹ ratios of the Permian fluid inclusions range from 5.2 to 6.0, close to the value of 5.2 in modern seawater (Table 4). If we assume the K⫹ concentration of Permian seawater was 10 mmolal (Horita et al. 2002), we can use the average ratio of Mg2⫹/K⫹ in the brine inclusions (Table 4) to calculate the Mg2⫹ of Permian seawater. The Mg2⫹ concentrations calculated in this way are 52, 60, and 52, mmolal, for the Asselian/Sakmarian, Artinskian/Kungurian, and Tatarian, respectively, all close to the Mg2⫹ concentration of 55 mmolal in modern seawater (Table 5). Ratios of K⫹/SO2⫺ in brine inclusions are used to estimate 4 the SO2⫺ content of Permian seawater. The average K⫹/SO2⫺ 4 4

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Table 5. Major-ion chemical composition of Permian seawater (mmol/kg H2O) calculated from chemical composition of fluid inclusions in marine halite.a Time Modern seawater Tatarian Artinskian-Kungurian Asselian-Sakmarian Hardie (1996) Model

Age (Ma) 0

m(Na⫹) 485

m(K⫹) 11

m(Ca2⫹) 11

m(Mg2⫹) 55

m(Cl⫺) 565

m(SO2⫺ 4 ) 29

m(Mg2⫹)/ m(Ca2⫹) 5.2

258–251 283–274 296–283 245 255 265 275 285

469 439 461 478 477 477 481 476

10 10 10 13 15 15 12 16

14 (9–17) 17 (11–20) 15 (10–19) 14 15 15 12 17

52 60 52 47 43 43 52 40

565 565 565 565 565 565 565 565

23 (18–26) 19 (13–22) 20 (15–24) 25 23 23 27 21

3.7 (3.1–5.8) 3.5 (3.0–5.5) 3.5 (2.7–5.2) 3.5 2.8 2.8 4.3 2.4

a m(Cl⫺) is assumed to be equal to modern seawater. m(K⫹) is assumed ⫽ 10 mmol/kg H2O (Horita et al., 2002). m(Mg2⫹) is estimated from m(K⫹) 2⫺ of Permian seawater and average ratio of m(Mg2⫹)/m(K⫹) in brine inclusions (Table 4). m(Ca2⫹) and (SO2⫺ 4 ) are obtained from m(SO4 ) excess (Table 4) and the assumption that the concentration product of (Ca2⫹)(SO2⫺ ) in Permian seawater was equal to the modern seawater value of 300 4 mmolal2 or had a range between 150 and 450 mmolal2, in parentheses. m(Na⫹) in Permian seawater is calculated by charge balance, after 2⫹ concentrations of all other ions (Cl⫺, SO2⫺ , Mg2⫹, and K⫹) are estimated. 4 , Ca

ratios for each Permian time interval (Table 4), together with the K⫹ concentration of 10 mmolal give SO2⫺ values of 5.0, 4 2.2, and 9.1 mmolal for the Asselian/Sakmarian, Artinskian/ Kungurian, and Tatarian, respectively (Table 4). These SO2⫺ 4 concentrations, however, represent the “excess” SO2⫺ after 4 precipitation of CaSO4. Modern seawater, for example, contains Ca2⫹ of 11 mmolal and SO2⫺ of 29 mmolal. If precipi4 tation of CaCO3 is ignored (only ⬃10% of the Ca2⫹ in seawater is lost during carbonate precipitation), then modern seawater contains excess SO2⫺ over Ca2⫹ of 18 mmolal. It is 4 clear that all Permian seawaters had excess SO2⫺ over Ca2⫹ 4 well below the modern value of 18 mmolal (Table 4 and Figs. 6E, 7E, and 8E). To estimate the actual concentration of 2⫺ SO2⫺ 4 in Permian seawater, we can use the excess SO4 values but we need an additional assumption on the amount of Ca2⫹. Permian fluid inclusions contain no measurable Ca2⫹ so the concentrations of Ca2⫹ in Permian seawater must be estimated. Horita et al. (2002) and Lowenstein et al. (2003) calculated the Ca2⫹ and SO2⫺ of ancient seawater using the assumption that 4 the concentration product of (Ca2⫹)(SO2⫺ 4 ) in ancient seawater was between 150 and 450 mmolal2, that is, between 0.5 and 1.5 2 times the value of (Ca2⫹ ⫽ 11)(SO2⫺ 4 ⫽ 29) ⬃ 319 mmolal in modern seawater. This choice of concentration products is arbitrary, but seems reasonable, because the salinity of seawater does not appear to have varied dramatically in the Paleozoic, as discussed above for the Cambrian and Devonian, and was certainly well below gypsum saturation (concentration product 2 of [Ca2⫹][SO2⫺ 4 ] ⬎ 3000 mmolal ) during the Permian. Here we use the same constraint to estimate Ca2⫹ and SO2⫺ in 4 2⫹ Permian seawater from the excess SO2⫺ )(SO2⫺ 4 and the (Ca 4 ) ⫽ 150 to 450 millimolal2. Lower Permian (Asselian-Sakmarian) seawater, for example, with SO2⫺ excess of 5.0 mmolal 4 (Table 4), must therefore have Ca2⫹ ⫽ 10 mmolal and SO2⫺ 4 2 ⫽ 15 mmolal and a (Ca2⫹)(SO2⫺ 4 ) product of 150 mmolal as the lower limit. On the high end, Lower Permian seawater has Ca2⫹ ⫽ 19 mmolal and SO2⫺ ⫽ 24 mmolal with a product of 4 456. If the (Ca2⫹)(SO2⫺ ) product is assumed to be equal to that 4 in modern seawater (⬃300 mmolal2), Asselian-Sakmarian seawater had Ca2⫹ ⫽ 15 mmolal and SO2⫺ ⫽ 20 mmolal (Table 4 5). The same estimates of Ca2⫹ and SO2⫺ are made for Lower 4 Permian Artinskian-Kungurian seawater (Ca2⫹ ⫽ 17 mmolal

and SO2⫺ ⫽ 19 mmolal if the (Ca2⫹)(SO2⫺ 4 4 ) product is equal to that in modern seawater and a range of Ca2⫹ ⫽ 11–20 mmolal, SO2⫺ ⫽ 13–22 mmolal). Upper Permian Tatarian 4 seawater had Ca2⫹ ⫽ 14 mmolal, SO2⫺ ⫽ 23 mmolal, and a 4 range of Ca2⫹ ⫽ 9 –17 mmolal and SO2⫺ ⫽ 18 –26 mmolal 4 (Table 5). Permian seawater had higher Ca2⫹ and lower SO2⫺ than 4 modern seawater, although the ranges shown in Table 5 permit Ca2⫹ concentrations near present-day values and SO2⫺ 4 slightly below modern seawater concentrations. The relatively low SO2⫺ in Lower Permian (Asselian-Sakmarian and Artinskian4 Kungurian) seawater rose to higher values in the Upper Permian Tatarian. Secular changes in the SO2⫺ concentration of 4 Permian seawater are well illustrated on the plots of SO2⫺ vs. 4 Mg2⫹ (Figs. 6E, 7E, and 8E). The average Mg2⫹/SO2⫺ 4 ratio of Lower Permian seawater evaporated into the halite field is 10 in the Asselian-Sakmarian, 28 in the Artinskian-Kungurian, and 5.7 in the Tatarian (Table 4). In contrast, the Mg2⫹/SO2⫺ ratio 4 of modern evaporated seawater in the halite field before precipitation of polyhalite is 2.7 to 3, a reflection of its higher SO2⫺ concentration. 4 Finally, the Na⫹ in Permian seawater is calculated by charge balance, after the concentrations of all other ions (Cl⫺, SO2⫺ 4 , Ca2⫹, Mg2⫹, and K⫹) are estimated. Na⫹ concentrations of 461 mmolal in the Asselian/Sakmarian and 439 mmolal in the Artinskian/Kungurian rose to a maximum of 469 mmolal in the Tatarian (Table 5). These concentrations are slightly lower than the Na⫹ concentration in modern seawater (485 mmolal). 5. DISCUSSION AND CONCLUSIONS

Permian seawater shares chemical characteristics with modern seawater, including SO2⫺ ⬎ Ca2⫹ at the point of gypsum 4 ⫺ precipitation, evolution into a Mg2⫹-Na⫹-K⫹-SO2⫺ 4 -Cl brine, 2⫹ ⫹ and Mg /K ratios ⬃5. Permian seawater, however, is slightly depleted in SO2⫺ and enriched in Ca2⫹ compared to 4 present-day seawater, although the modeling does not rule out Ca2⫹ concentrations close to the modern value of 11 mmolal. Na⫹ and Mg2⫹ in Permian seawater are close to (slightly below) their concentrations in modern seawater. Permian and modern seawater both have Mg2⫹/Ca2⫹ ratios ⬎2 (Table 5),

Permian seawater chemistry

chemically favorable for precipitation of aragonite and magnesian calcite as ooids and cements. Within this framework are temporal changes in the major-ion chemistry of Permian seawater over periods of tens of millions of years (Table 5). Modeled SO2⫺ 4 concentrations are 20 mmolal and 19 mmolal in the Asselian-Sakmarian and Artinskian-Kungurian, which then rose to higher concentrations in the Upper Permian Tatarian (23 mmolal) (Table 5). Modeled Ca2⫹ is at or above its concentration in modern seawater throughout the Permian. Mg2⫹ is close to (slightly below) its concentration in modern seawater (55 mmolal) in the Asselian-Sakmarian (52 mmolal) and the Tatarian (52 mmolal), and slightly higher in the ArtinskianKungurian (60 mmolal). Mg2⫹/Ca2⫹ ratios are 3.5 (total range of 2.7 to 5.5) in the Lower Permian and rise to 3.7 (total range ⫽ 3.1 to 5.8) in the Upper Permian, primarily due to decreases in Ca2⫹. K⫹ concentrations in the Permian are assumed constant (10 mmolal) (Horita et al., 2002) (Table 5). Dickson (2002) obtained similar Mg2⫹/Ca2⫹ ratios for Permian seawater of 2.2 to 2.5 for the Artinskian of Australia and 3.1 for the Wordian (equivalent to Guadalupian Series) of Tunisia. These paleoseawater Mg2⫹/Ca2⫹ ratios are calculated from the % MgCO3 in the skeletal calcite of Permian echinoderms. Overall, the fluid inclusion and echinoderm data support the interpretation that the Permian oceans were aragonite seas, with Mg2⫹/Ca2⫹ ratios above 2 but below the modern seawater ratio of 5.2. Hardie (1996) made 10 million year predictions of Permian seawater that compare closely to the results found here from fluid inclusions in halite (Table 5). That model, based on changes in the midocean ridge/river water flux ratio driven by changes in the rate of ocean crust production at midocean ridges, predicts that compared to modern seawater, Permian seawater should be relatively depleted in Mg2⫹, SO2⫺ 4 , and Na⫹ and relatively enriched in Ca2⫹. Hardie’s predicted Mg2⫹/ Ca2⫹ ratios for Permian seawater (2.8 – 4.3) are close to those found here (3.5–3.7). The Hardie (1996) model also predicts second order changes in the chemistry of Permian seawater (Table 5). Although the data presented here do not cover the entire Permian, the fluid inclusions in halite suggest second order oscillations in the major-ion chemistry of seawater occurred in the Permian, as discussed above. One discrepancy is the K⫹ concentration of Permian seawater, which is predicted by Hardie (1996) to be higher than in modern seawater. It appears that the K⫹ concentration in Phanerozoic seawater was relatively constant (Horita et al., 2002) and that some processes other than riverine and midocean ridge brine input are controlling its composition in the oceans (for example, precipitation of K-bearing clays in low-temperature off-axis areas of the midocean ridges). Holland and Zimmermann (2000) proposed that changes in the global rate of dolomite formation may explain the changes in the Ca2⫹, Mg2⫹, and SO2⫺ concentrations in seawater over 4 the past 40 million years, but the relationship between the global volume of Permian dolomite and Permian seawater chemistry is not clear. That model, in contrast to the Hardie (1996) model, does not rely on changes in the rate of midocean ridge crust production to explain secular changes in the majorion chemistry of seawater. Steady-state production of ocean crust over the last 180 million years has recently been argued by Rowley (2002), but the analysis of Demicco (2004) shows

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that constant ocean crust production is not required to explain the current distribution of ocean floor. Finally, the Permian was a time of relatively low sea levels, icehouse conditions, and southern hemisphere glaciation (Crowell, 1995). Continental ice sheets were largest in aerial extent at the Pennsylvanian–Permian boundary, and by the end of the Permian, evidence of glaciation is lacking. The icehouse conditions and low sea levels of the Permian are analogous to the present ice age, as are the similarities between Permian and modern seawater. These connections all suggest control of Phanerozoic supercycles by mantle convection and global volcanicity. One unresolved issue involves the low 87Sr/86Sr ratios of Permian seawater (Denison and Koepnick, 1995), with minimum values in the latest Permian, which suggests more vigorous midocean ridge activity, greater cycling of hydrothermal seawater through the midocean ridges than today, and decreased land area. There is no evidence for the above patterns; low sea levels in the Permian suggest just the opposite, rather stagnant seafloor spreading rates. It has been suggested that the low 87Sr/86Sr values reflect Permian aridity on the supercontinent of Pangea and closed basin drainage systems that together may have decreased the flux of high ratio Sr from the continents to the oceans (Denison and Koepnick, 1995). A second question involves the relationship between Permian seawater chemistry, which reached an “extreme” with maximum SO2⫺ 4 in the Tatarian, at the very end of the Permian, whereas maximum glaciation was in the Early Permian. One interpretation for the high SO2⫺ concentrations in Permian seawater, by 4 analogy to the explanation offered by Turchyn and Schrag (2004) for the last 3 million years, is that during periods of low global sea level, pyrite was weathered over a large area of exposed continental shelf, which led to an increase in seawater SO2⫺ 4 . Acknowledgments—Susan Hovorka supplied halite samples from the San Andres Formation. We thank Bill Blackburn for help running the environmental SEM, Robert Demicco, Lawrie Hardie, and Sean Brennan for discussions about seawater chemistry, and Tim Lyons, Lee Kump, and an anonymous reviewer for suggesting many improvements in the manuscript. The work was supported by NSF grant EAR9725740. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. Associate editor: L. Kump REFERENCES Ayora C., Garcia-Veigas J., and Pueyo J.-J. (1994a) X-ray microanalysis of fluid inclusions and its application to the geochemical modeling of evaporite basins. Geochim. Cosmochim. Acta 58, 43–55. Ayora C., Garcia-Veigas J., and Pueyo J.-J. (1994b) The chemical and hydrological evolution of an ancient potash-forming evaporite basin in Spain as constrained by mineral sequence, fluid inclusion composition and numerical simulation. Geochim. Cosmochim. Acta 58, 3379 –3394. Bein A., Hovorka S. D., Fisher R. S., and Roedder E. (1991) Fluid inclusions in bedded Permian halite, Palo Duro basin, Texas: Evidence for modification of seawater in evaporite brine-pools and subsequent early diagenesis. J. Sediment. Petrol. 61, 1–14. Brennan S. T., Lowenstein T. K., and Horita J. (2004) Seawater chemistry and the advent of biocalcification. Geology 32, 473– 476. Brennan S. T. and Lowenstein T. K. (2002) The major-ion composition of Silurian seawater. Geochim. Cosmochim. Acta 66, 2683–2700.

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