A note on the chemistry of seawater in the range 350°–500°C

GPochlmica @ Pqamon

a Cosmochimica Acio Vol. 47, pp. 139-144 hss Ltd. 1983. Printed in U.S.A.

NOTE

A note on the chemistry of seawater in the range 350”-500°C JAMES L. BISCNOFF and ROBERT J. ROSENBAUER U.S. Geoiogical Survey, Menlo Park, California 94025 (Received May

If, 1983; accepted in revised.furm October 14, 1982)

Abstract-The chemistry of seawater at conditions of 350” to 5OO”C,220 to i Ooobars (22 to IO0 MPa) is controlled by reactions involving magnesium hydroxide sulfate (MHSH) and anhydrite. During progressive heating from 350” to 500°C at loo0 bars (100 MPa), MHSH with a’ Mg/S04 ratio of 1.25 is formed via precipitation from solution and via reaction of solution with pre-existinganhydrite. During

adiabatic expansion the MHSH extractsadditional SO. from seawaterand converts to a stoichiometq in which Mg.60‘ = 1,16.These reactions control and greatly change the concentrations of Ca, Mg SO, in soiution and produce ~~i~~nt ionizable hydrogen, attaining I 1.7 mmoles kg-’ at maximum cond&ions. and BISCHOFF,in press). Does seawater i&elf undergo chemical changes across this temperature interval THECHEMISTRY of seawater at elevated temperatures that may account for this observation? Ca removal is of great importance to the understanding of the from solution has been noted at the higher end of interaction of seawater with oceanic crust, and in this temperature range during basalt-seawater interparticular to the nature of sea floor oceanic systems. actions (ROSENBAUERand Btsc~o~, in press) and The m~imum fluid tem~mtur~ of these oceanic may be responsible for generating H* ion by the forgeothermal systems are not known at present. Howmation of a simple Ca(OHh or a Ca(OHh compo ever, a lower limit is provided by the vent waters at nent in a hydrated silicate. 21 ON, EPR, which are discharging at 35O’C. A previous study (BISCHOFFand SWFIUJZD, 1978) Chemical composition of these fluids suggests that showed that during heating up to 35O”C, 500 bars, subsurface temperatures am well above this temperseawater becomes increasingly acidic and progresature (BISCHCFF,1980; CHEN, 1981). Evidence from sively depleted in Ca, Mg and SO,. These changes the Samail ophioiite, Oman, suggests that seawaterare due to precipitation of anhydrite and a magnehydro~e~al circulation many reach 500°C (GREG sium-hydroxysulfate-hydmte phase (MHSH). New ORYand TAYLOR. 198 1). interest has been focused on this compound by its The P & T of the critical point of seawater has not recent identification in the hydrothermal mounds of been directly determined, but based on analogy with the 2 1“N geothermal field by HAYMONand KASTsimple NaCl solutions, it should be in the vicinity of NER (198 1) who propose “caminite” as a new mineral 400°C 250 bars (C&EN, 1981). Thus, because the name. The presence of MHSH in the hydrothermal pressure of seawater in the subsurface geothermal mounds suggests localized heating of surrounding system is likely above the critical pressure, if not the seawater near the exit vent. MCDUFF and EDMOND critical temperature. buoyancy rather #an boiing (1982) suggest that MHSH might control sulfate conbecomes the limiting factor on maximum subsurface centration in sub-seafloor hy~o~e~~ systems and temperature. Thus, pressure becomes an important might provide conditions favorable for the complete variable at the higher temperatures in determining reduction of sulfate by ferrous iron in the rocks. the chemical properties of seawater because of its There is some uncertainty regarding the stoichimarked affect on the specific volume of water, As ometry of MHSH, whether it occurs as a single comheated seawater rises through oceanic crust during pound or as a series of compounds of like structure the discharge phase of the cycle, it expands and this but with variable MgiSO, ratio. UEFER etal.(198 1) expansion affects its chemistry. determined the crystal structure of MHSH synthetWe carried out the present study to determine the ically produced from seawater having a stoichiometxy chemical properties of seawater in order to underof MgSOd - ‘fiMg(OH), - ‘/3HZ0,(Mg,‘SO4 ratio = 1.33). stand seawater-basalt systems under these supercritJANECKYand SEYFRXED(in press) performed experical conditions. An important transition appears to iments on heated seawater to determine the thertake place during basalt-seawater interaction in the modynamic stability of MHSH between 300” and 350°C. Based on changes in the fluid composition, interval between 350” and 4OO”C, above which sigthey concluded that MHSH exists in two stoichinificant metal mobilization occurs independent of water/rock ratio (MOTTL er ai, 1978; ROSENBAIJER ometries, with Mg/SOL ratios of 1.25 and 1.5. INTRODUCTION

I39

Tabla 1. Lvrdar v*smus axpcri!amtaa. CxvKllt+ons Chemical compositian. al? clvolved' s*.W.ltCa)r _________l_"~*113___"~~~~~~~____"_"~""~~_______"~""~~___""""~""~""_______~~~~"~_~_~~~~~~~~~--------.---------~~~ Sample Time PH raapp* PrOmmuse vozwarxa K TT) ca (bars) rmuaining bt 25*c (day* i *g (!Uq) ____~“~“““~“__~-___~~-___“_~~“~*~~________~____~“_~~~““““~~~_____~__~___~-~“~“~~__-_-~_____--*~~II_ IS#bariC sfM&li;ng !3 12 I ?df 5.9

0. iii

26.7

f

?

350

949

137

6.5

O*?5

26.‘

123 + ?,n

2

14

399

999

132

3.8

0.1s

26.9

?%,4 I!43

3

75

422

394

121

4.2

Q.15

25.:

4

1:

448

994

112

4.0

a.14

26.‘

"l"i

5 i

24 34

473 497

992 994

104 87

4.2 4.2

0.19 0.15

26.ri 27.Q

?I.8 I:,: 11.8

___~"~"~~~~"~~~""~~_~_"__~__~__~~~_""~~"_~_~_~~___~~~""~"~""~""_~~~"~~____"~_"~_~~~~~~~~________-_~~~~~~~~~~~ Adiabatic

CoolSng

-r

41

468

620

77

f.cy

1.14

26.7

8

44

449

460

69

3.7

0.13

2:6.ci

1'1.0

Y

45;

424

370

94

3.7

0.14

2s.i

It*?

511

Pfsl

296

42

4.x

53

375 23ft _""___"rn

10 it ___""""_-"""""_"_---______I_

NR

YK

3%

36 4.4 O.27 2ci.s """""-"~"___"_~_-"""cI""*_______"--"__*~_"_.-"s-

12.4

soiutim l SeIUrtdf ur&* &alffc&fly prepared m raeh e.8watar llt&v&u~rmxh2 xitll b8nLWJ mass at 3fWC. YU pr*us mm rrrgant gr*& chla*t* salts Of sat Mtr Cat @id K to Q.1M WUI~PB kg"* Cl- (18,~8*J,,)~ lrS r3wtqw Ln actti MA and Cl wra 1wf1 tJIaa analytical preaisloa under all experimental = 0.445 mcbM# ka".

We have found that MHSR with a Mg/S@ r&n of 1.25 progressively precipitatgs during isobaric heating to the maximum ~~ti~~~ and that during ad&&tic expazision, the MWSM coz~erts to a diC feS%ntstoich4omet.q id wbi& Mg33* = 6115,

Table 2. comgxmitian ca* n*r;uraL -tu f#tYnrrS Fc+ian&ing. caiif.i C:~1B.OZ*/*~E una*r variau6 e¶cp.rfrrawaX conditions R__""LUI""""""""""-~-""""-"~~"~"""--"""-""---"--"~"""""-~"--"-~-~-"""-"""-"----""""~~~"~~"" Frserura VoluE* Sw#e Tinte w-3 m&*s *q-l TWP" IVf i~?zsl raaa&ni.??pfft 15'C 3 raayat ras S% (W -""L"~U~~"""f--"-"-"""-."."~",.v4"~""""" ~~y-_-___;_"'_"";;,_"_-_~~8_____"_;;~4$1_"____;o~1;_________25~;_3 0 sta?x zsobaric iiwetng Chrtcrl

1

i

354

980

130

2.6

44.3

I,60

2

2

375

990

124

2.5

42.3

1,79

lOa1

3 4

4 9

39a 450

984 1006

108 100

2.4 2.2

39.6 34.5

2.14 3.09

7.20 6.51

5

22

475

1000

92

2.1

x.(1

3‘70

5"OB

12.5

___~_~~_"__"~_~~~~__"~,,,,,,,""___~~~~"_""~~"~~~"______~~~__~_~~~~~____~~~~________~~~~~_ Mirbrtic

Cwliag

7

32

470

frQa

35

2.3

26.4

5,6#

0+65

a

36

448

475

bJ

1.3

28.0

Se6Ls

.3.45

9 10

37 38

425 398

370 284

51 TM

2.4 2.7

28.2 30,s

4.59 3.49

0.46

2.a4

0.92

11 39 376 223 30 3.0 31.7 -__""~~~~Y~~~~""""~__~_~~~*_"""~~~~~""___~_~~___~~~"~~~~~~"~"""""_~~__~~~_"""_~~"~~~"~____________~__~_*~_~ The Cl

tollarSng (533),

wm@anants

x; (9.67),

totd

r-&nod dimdvad

tmchangrd Cb?

at their retrgactlvu concentrntions (wmlala kg-'):

(2.0).

0.70

~a !45?1,

Chemistry of seawater In separate experiments approximateiy 150 ml each of natural seawater and evolved seawater were placed in the gold-cell apparatus and heated to 350°C at IOOObars pressure. After sampling each fluid at these conditions, the temperatures in both experiments were progressively raised to a maximum of 500°C at a constant 1000 bars total pressure (isobaric heating); the samp&s were taken at intervals of25* to 50°C. Later the two systems were.slowly cooled and deQresSUrized at intervals down to 375” and 230 bars such that the enthalpy remained approximately constant (adiabetic expansion), as shown in Fig. 1. Samples were taken under a total of I I individual sets of temperature-pressure. Analyses ofrrpiicate sampies taken after 24 hours and again after 7 days under a given set of conditions showed no detectable diiTerences. JANECKY and !SEYFRED(in press) demonstrated that equilibrium was attained in their experiments in less than 50 hrs based on the reversibility of the fluid composition during cycling between 3Qo” and 350°C. Equiiibtation at the higher temneratntes of the present study should be even more rapid. The hzk of detectabie compositional change between I day and 7 days is taken as evidence of equihbration within t day Each sttmpfe (5-10 mls) was ana&zed for pH at 25”C, Ca, Mg, IL, Na, Cl, SO,, and dissohed CC& by techniques described previousiy (Brscllror~ and SEYRUED, 1978).

The composition of the evolved seawater changed only slightly throughout the entire experimental cycle (Table If. The only signi&nt change was in pH, which dropped to the range 3.7-4.2 for all temperatures above 350°C. The cause of this pH drop, which represents a net gain of ionizable hydrogen ion of approximately 0.2 mmolai, may be the oxidation of dissolved organic matter originally contained in the deionized water. Although the pH did not drop until the sample reached 4OO”C, that such oxidation occurred is indicated by an increase in total dissobed CO2 from 0.03 milEmoW in the initial fluid to 0.2 mmotai in the sampfe at 35O”C, 1000 bars. The origw inal c5ntent of dissolved 02, resulting from equili-

t 400

I

t

600 PRESSURE

I

600

i

1

I000

(bats)

flo. 1. Temperature-pressure relations for experimental conditions of Moss Landing seawater (dots) and altered seawater fcirclesf.

141

FIG. 2. Changes of Mg,Ca, SO*, and pH in Moss Landing seawater at various temperatures and pressures. Triangles are from BISCXFF and SEYFRIED ( 1978), &lid circles arc results hm present study at 1000 bar& open circles are from present study during adiabatic expansion.

bration with air at room temperatures was about 10 ppm, sticient to oxidize the 2.4 ppm organic carbon required to yield the observed increase in dissolved CO,. Thou& slightiy acidic, the solution was virtually unbuffered with respect to H+, which may have led to an erroneously measured pH at 350°C. Another possible explanation for the pH drop is the precipitation of a small amount of Ca(OHjZ. The observed pH drop oould be accounted for by a Ca decrease of only 0.12 mmoles kg-‘, a change which is less than our anaiytieal precision. Natural seawater, in contrast, underwent considerable change (Table 2), the kind and direction of which resembles those observed at temperatures up to 35O’C. Although Na, K, Cl and tati dissolved COZ remained unchanged, Mg and SC&progressiveiy decmased during isobaric heating to 500°C on essentially the same trend with increasing temperature as was previously observed in the temperature range 250” to 350°C (Fig. 2). The slight off& in the trend at 350°C reflects the fact that the e~lier study was conducted at 500 bars and the present study at 1000 bars. increase in pressure appears to increase the solubility of the MHSH. H+ increased systematically and proportiahately to the Mg-SOa decrease over #he same interval, and reached a maximum of I 1.5 mmoles kg-’ (PH~~-~ = 2.0) under the maximum conditions of XW’C, 1000 bars. Ca attained a minimum of 1.6 mmoles kg-’ at 350°C after progressive precipitation up to this temperature as anhydrite. During successive heating to 500°C the trend reversed and Ca actually increased with temperatures to as much as 4.5 mmoles kg-*~ The explanation for this reversa! is seen in the follow&g reaction: Mg+2

f+9 Ca

7.06 1.35

l,B? 7-M

2.43 9.36

-2.?4 -0.83

r4 3.66 0 0.39 0.35 0.96 a*26 -3.01 1.31 ___________~~~1~1""~~~~~~~~________L__~~~~~~~~~~~~~~~~~~~~~~~~~~*~~~~~ ET,cipi+a+e:

urhy&it.* - 0.83 I Pole9 .WblB - 2.18 = s~i00

XIFSK _~“*~“l”_~~_~f~~__~_*___I________________~~~~*~~

I!lm+l*e

RItlo

to so4

_____________“__^-_____-___~““~_~~-____________--

w

2.14 2.16

9%

1.26 1.00

ON 1.31 0.60 __~~__~~l"r~~~_~_~_____~__~~~~~~~~~~~~~~~~~~~~~___~~"~~ StoichimW't

"'gW4

.

1,4 ng(ou)2

+ case*= MHSH f ca+2 where the precipitation ofMHSH -vet& extractsso*until anbydrite begins to tiiaeve.scave& of the tckased 334 by

Mg then causes more tiydrite to dissolve until the precipitation of MHSH ceases. This cs~usesan increase in the Ca/Mgratiowithintemperatures During ackbatic coo&i from the maximum conditions of SOOT, t0(10bars, the trends are somewhat more compiex. InitiaBy the expansion to 640 bars at 474-X CWBsesa large decRaae in St?* and minor ~~~~M~a~H~.~~~~y~ additional MHSH is formed but pH actually increjUes. ApgWvmtlythe Mg/!304 ratia is lower in the p&i&ate famed in the mote exp&nd+I fluid. Upon kther coolin$/cxpansion, SO, remains low while Mg sli&tly and prog~&vely im and Ca and H’ decrease. The achy iower co~~ntr&ons of~~~~at~~~r~u~~at~ ~~~~~ temjXr&res at tQOo bar& indicate

~t~~oft~~SHis~~i~~i_ ubility is fge.&y lowered in this exmded

fluid.

*

1/z H~II

The composition of the precipitate and the stoichiometry of the MHSH was calculated by changes in the composition of the fluid. The calculation is made by comparing the initial amounts of each component with the amounts present in solution after each sample point, after cornzcting for the amounts

removed durin& sampling A1i the Ca d&kit and an equivalent amount of !X& is attributed to anhydrite, The remaining St& deficit and all the Mg de&it is attributed MHSH. Also, a quantity of OH- (equivalent to the amount of exc&ss ionizable hydrogen produqd) is attributed to MNSH. Ionizable H is calcuW& from pHaoc using an activity coefficient of 0.8 (BEDKOFF and SEYIWED, 1978). Table 3 shows an exam&e of this calculation for the precipitate as it appears at 500°C tOOObars. Progressive changes in the amount and composition of the precipitate are shown in Table 4. Rehtive amounts of Mg and SO4 in the MHSH in each sample

hwting

250 500 1.13 0 1.1) 300 500 1.18 Q.85 2.03 350 500 1.20 1.22 2.42 _________________~~1I_____^______________~~~~~~~~~~~"~~~~-~~~~~~~~~~~~~~~~~~~.~~_~"

350 ,000 1.14 0.m 400 ,000 0.98 1.4% 450 mo* 8.97 1.k3.s 500 ?OW 0.84 2.32 ~~____-__~___~_*-~~~~---~-~---~~~~~----~~---~----~~~~~*~~~~~~~~~~~~~__l_r

I\di&bmtic

*70

534s

0.35

2.60

2.35 '.?.I 2,113 3.20

3.35 450 476 0.75 2.62 3.31 429 370 0.91 2.55 3.35 398 284 '1.87 2.48 3.35 376 223 J.89 2.44 3.31 __1~~______________1_111_1______________~~~-~~~~~~~~"~~~~~~~~__________""~_~~~~____""

uqwmion

l

da+. from ZOO*-350-C

Zrom Bimhoff

and Syfraed

(19781.

143

Chemistry of seawater

indicates the Mg/SO, ratio to be 1.26 + -04 for all sampIes in the temperature range of 350°C to 500°C at a constant 1 kbar pressure (Fig. 3, samples I-6). During adiabatic expansion, however, the MHSH appears to undergo an abrupt change with only one transitional point (sample 7) to a Mg/SO, ratio of 1.16 + .O1 (Fig. 3, samples 8-l 1). That the uncertainty in the calculated ratios is small is indicated by the narrow range of variation in the calculated Mg/ SO, ratios, and independently by the plot of excess Mg over SO, against ~1Ii~uiv~en~ of ionizable H produced in each experimental point (Fig. 4). Thus, the calculated Mg/SOo would appear to reflect accurately the stoichiometry of the MHSH produced in our experiments. The amounts of anhydrite and MHSH present in the system are shown in Table 4. The amount of anhydrite is at a maximum at 35O”C, 500 bars. Increasing the pressure to 1000 bars decreases the amount of anhydrite slightly, showing that the pressure renders anhydrite more soluble. During progressive heating to 5OO*C, 1000 bars, the amount of anhydrite successively decreases while the amount of MHSH increases. During expansion and cooling .from this point the pm-existing MHSH is made over to a slightly different stoichiometry by extracting SO, from the fluid to form MHSH richer in SO,. The following reactions describe the process: direct precipitation

of MHSH:

‘/dMg+’ + SO,_2 + 2H20 = MgS04 - l/sMg(OH)z - LhH20 + $H+ reaction with anhydrite: CL&O., i- 5fMg+’ + 2H20 = MgSO, s ‘/4Mg(OHj2. %H20 + Ca+* + %H’

fi 1: El 2.0

-

1.0

1.4

I.6

2.2

2.6

3.0

Mq IN HHSHtic-‘r&es) PlG. 3. Mg and SO1 content in MHSH. Number next to points refers to sample numbers from Table 2. Mg content is taken as total moles of Mg lost from solution minus that removed via sampling. SO., content is total moles of SO4 lost from solution minus total moles of Ca depletion, both corrected for amount removed viasampling. Lines represent different ratios in MHSH.

I

1.4 1 I

‘:

z z -Y

I.2

.p s Y

1.0

g

-

0.8

-

0.6

-

I “, z N z 0

0.4

oy 0

’

0.2

I

I

I

0.4

0.6

0.6

EXCESS

Mq

IN MHSH

,

,

I

I.0

I.2

1.4

(10-3~q”ivol~n?r)

FIG. 4. Relationship between total excess Mg in MHSH and total amounts of ionizable hydrogen produced in seawater experiments, 350”-500°C. Excess Mg is difference in total equivalents between Mg and SOr in MHSH. back reaction

during

expansion:

%4H20 + ‘/1,,SO;* + MgS04 - ‘/!!Mg(OH)*- ‘hHzO = *%4MgSOh- y!Mg(OH), - $‘$-izO+ ‘/,OH-. DISCUSSION

From the foregoing it would appear that MHSH occurs in different stoichiometries. This is consistent with the suggestion by HOCHELLA and KEEFER(in press) that MHSH may be a solid-solution series. Based on bonding-energy calculation they suggest that a variable amount of Mg can be accommodated in a single MHSH structure. The composition can vary between the limits of MgSOp +Hz0 (kieserite) in which Mg/S04 = 1, and a hypothetical compound, 2 MgS04. Mg(OH)* in which Mg/SO, = 1.5. The general formula for the series is MgS04. XMg(OH)* ( 1-21)Hz0, where X -z ‘& The two MHSH components produced in the present experiments would correspond to (1) MgSOa- ~~Mg(OH)~ +%HzO (Mg/ SO, = 1.25); (2) MgS04 .~Mg(OH~ ssH20 (Mg/SOa = 1.16). Both compounds are ahowable within the bonding constraints. Powder X-ray diffraction patterns of compounds of slightly different stoichiometries within this series are likely to be very similar if not indistinguishable because of the similarity of structures. Thus, simiiarity of apparent identity of powder d&action patterns of MHSH from different studies does not guarantee the stoichiomet~~ to be the same. For example, the reported difhaction patterns for MHSH in the study by JANECKY and SEYFRIED (1978) appear to be the same as that reported by KEEFER ef al. (1981) yet the stoichiometries appear to be different. It is possible, therefore, that the MHSH reported by BEEHOFF and SEYFRIED (1978) and by HAYMAN and KASTNER( 198 1) both of which display

J. L. Bischoff and R. J. Rosenbauer

144

the same X-ray diffraction pattern as above, may not have the same stoichiometry. Chemical analyses and/ or single crystal difEactometry are probably necessary to sort out the uncertainties regxding she stoichiometry MHSH. REFERENCES BISCHOFFJ. L. ( 1980) Geothermal system at 2 1“N, East Pacific Rise: Physical limits on geotbcrmal kid and role of adiabatic expansion. Science 207, 1465-1469. Btsc~off J. L. and SEYF~UED W. E. JR. (1978) Hydrothermal chemistry of seawater from 25” to 350°C. Amer J. Sci. 278,838-860. C. A. (1981) Geotkrtt~al system at .!1“N. Science

CHEN

211, 298. GRF,G~RYR. T. and TAYLOR H. P. ( 1981) An oxygen isotope pro& in a section of CreWcews owuliccrust,samail opbioiite, Oman: evidence for 6’s oxygen buffering of the oceans by deep (>5 km) seawater-hydrothermal circulation at mid-ocean ridges. J. Ceophys. Res. 86, B4, 2737-2755. I-hWUON

R. M. and w M. (1981) Hot spring deposits on the East Pacific Rise at 2 I “N: Preliminary description of mineralogy and genesis. E&h Planer. Sci. Left. 53, 363-38 1.

HCXXELLA M. F. and REEFERK. R. (in press) The cryst.aJ chemistry of a new magnesium hydroxy sulfate hydrate, MgS04 * %Mg(OH)r . VrHsO.a pH controlling phase. .4cfa Crystallogr. JANECKYD. R. and SEYFRIED W. E. JR. (1982) The solubility of magnesium hydroxide sulfate hydrate in seawater at elevated temperatures and pressures. Amer f. Sci. iin press). KEEFER K. D., HOCHELLA M. F. and DE JONC 8. H. W. S. (1981) The structure of the magnesium hy droxide sulfate, MgSO, * rhMg(OH)r- %HrO. Acra CrW. 837, 1003-1006.

McDurr R. E. and EDMUNDJ. M. ( 1982) On the fate ot‘ sulfate during hydrothcrmai circulation at mid-ocean ridges. Earth Planet. Sci. Lett. 57, f 17-t 32. MOTTL M. J., HOLLANDH. D. and CORR R. F. (lQ78) Chemical exchange during hydras a&ration oftbe salt by seawater-11. Experimental nzsults for Fe. Mn and sulfur species. Geochim. Cosmwhim. Acta 43,869~884. ROZ%NBAUER R. J. and BISCHO~:J. L. (in press) Product and tmnsport of heavy metals by heated seasvaten a summary of the experimental results. NATO Contirence Series (Marine Sciences). Plenum Press. SEYFRIEZD W. E. JR., GOWN B. C. and DICKSONF. W. (1979) A new reaction cell for hy~~c~~ solution equipment. Amer, tWim?ml.64, 646449.