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Boron isotope systematics of hydrothermal fluids and tourmalines: A synthesis
Chemical Geology (Isotope Geoscience Section), 94 ( 199 1) 1 1 l- 12 1 Elsevier Science Publishers B.V., Amsterdam
111
Boron isotope systematics of hydrothermal fluids and tourmalines: A synthesis M.R. Palmer Department of Geology, Bristol University, Wills Memorial
Building,
Queen’s Road, Bristol BSS IRJ, UK
(Received January 14, 199 1; revised and accepted July 24, 199 1)
ABSTRACT Palmer, M.R., 199 1. Boron isotope systematics of hydrothermal Geosci. Sect.), 94: 111-121.
fluids and tourmalines:
A synthesis. Chem. Geol. (Isot.
Boron isotope and concentration data are presented for hydrothermal fluids from different tectonic settings that reflect derivation of boron from marine evaporites (Red Sea brines), non-marine evaporites (Salton Sea), elastic sediments (Escanaba Trough and Guaymas Basin), back-arc basalts (Mariana Trough) and mid-ocean ridge basalts (21’ and 1 I-13”N East Pacific Rise; Juan de Fuca Ridge; and 23” and 26”N, Mid-Atlantic Ridge). Boron isotope systematics of these fluids are used to constrain the G”B-values of tourmalines found in ancient massive sulfide deposits and tourmalinites that formed under conditions analogous to those of modern hydrothermal systems. The data also provide insights into the physical conditions required for the formation of tourmaline in certain geologic environments.
1. Introduction
Palmer and Slack ( 1989) carried out a survey of the boron isotope composition of tourmalines from massive sulfide deposits and tourmalinites, and concluded that the S’ ‘Bvalues of the tourmalines are largely dependent on the dominant source of boron, with non-marine and marine evaporite sourcesproducing especially distinctive B isotopic signatures. The G”B-values of the major reservoirs of boron (marine evaporites, non-marine evaporites, elastic sediments and volcanic rocks) are relatively well characterized (Palmer and Slack, 1989, and referencestherein). To more closely evaluate the tourmaline 6’ ‘Bvalues more information is required for fluids that interact with these reservoirs in modern hydrothermal systems. A compilation of new and previously published data are presented here for hydrothermal fluids from the following systems: marine evaporite-hosted (Red 016%9622/91/$03.50
0 199 1 Elsevier Science Publishers
Sea); non-marine evaporite-hosted (Salton Sea, California); elastic sediment-hosted (EscanabaTrough and Guaymas Basin) ; back-arc sediment-starved spreading center (Mariana back-arc basin); and sediment-starved midocean ridge spreading centers (2 1o and 1l13”N East Pacific Rise, Juan de Fuca Ridge, and 23 ’ and 26”N Mid-Atlantic Ridge) (Fig. 1). These data allow a comparison betweenthe B isotope systematics of modern and ancient hydrothermal systems as recorded by tourmalines within massive sulfide deposits and tourmalinites. 2. Methods
and results
Boron isotope compositions and concentra“tions of hydrothermal fluids were determined using the methods of Spivack and Edmond ( 1986) and are listed in Table I. The isotope ratios are expressedas 6 “B-values, where: B.V. All rights reserved.
M.R. PALMER
112
Fig. 1. Map showing locations
of hydrothermal
systems discussed in the text.
a”B= [ (‘lB/‘oB),,,p,e/(‘lB/loB)s~an~ar~-1I x103 The standard is NBS boric acid SRM 951 which hasa measured “B/‘OB ratio of 4.04558. The in-run precision was generally < + 0.2%0, but the reproducibility isotope ratio runs was 2 0.3%0(20). The measuredboron concentrations are precise to < + 2%. 3. Discussion
3.1. Sediment-starvedmid-ocean ridge settings In sediment-starved mid-ocean ridge hydrothermal systems seawater is drawn down through oceanic crust, and within the low-temperature limb of the convection cell some boron is removed from seawater and incorporated into low-temperature weathering products. As the circulating fluid is heated above - 150’ C, boron is quantitatively leached from
the rock. Within the resolution of the data there does not appear to be uptake of boron into high-temperature secondary minerals, but the subtleties of boron behavior are partially obscured by the high concentration of boron in seawater ( (420 prnol kg- ‘, 4.6 ppm) relative to basalt (< 100 pmol kg-‘, < 1 ppm). As a result, -75-90% of the boron in submarine vent fluids is derived from seawaterthat passes through the system (Spivack and Edmond, 1987; Campbell et al., 1988). Vent fluids from sediment-starved mid-ocean ridges have the lowest boron concentrations measured in hydrothermal solutions (Fig. 2a) and can be lower than seawater due to low-temperature boron uptake. As a consequenceno boron-rich mineral phasehas been identified in studies of these sites. As seawater is the dominant component of these vent fluids they have the highest G”B-values (Fig. 2b), even though fresh oceanic crust has a low 6 “B-value ( - 3%0) relative to seawater, + 39.5%0 (Spivack and Edmond, 1987).
B ISOTOPE
SYSTEMATICS
OF HYDROTHERMAL
FLUIDS
AND TOURMALINES
(b) ‘2
(4 ‘.’ I Sediment ridge
starved spreading
Sediment ridge
10
mid-ocean center
starved spreading
mid-ocean center
0 6
6 4 4 2 2 0
0 0.5
1.0
1.5
2.0
2.5 12
Sediment starved back arc spreading center
10
0
2 0
, 0.5
1.0
(
,
1.5
(
,
2.0
"-10
0
+20
*IO
+40
+30
2.5 I
12
14
I
I
I
I IO
12
=‘
Clastic-sediment
-
hosted
Clastic-sediment
hosted
10
5
8
z
6
I;
4 2 0 0.5
1.0
1.5
2.0
"-10
2.5
0
121,s 14
+lO I
+30
+20 I
I
Id
+40
1
II
1
Marine
1
evaporite
8 6 4
-
2
-
2
0.5
1 .s
1.0
2.0
2.5
Non-marine
t -10
1 0
'
t
*IO
c
.:.x;+.:: i :j;::..*::
+20
::'-"-2:: .&& .:;.:.:.:.:...:. >...../. .,.
+30
+40
r30
+40
l2 I
,,,~,,,,,,,,~,,,,,I~~,,,I,,,,III,II,~IIIIIIIIIII
20
evaporite
-
0‘
0
25
Marine
evaporite
Non-marine
evaporite
15 10
5
2 IIS
0 IO
20
B
30
(mmol
d0
I I I I I I I14 50
,.3'
,'
-
0 -10
0
+10
Fig 2. a. Histogram of boron concentrations for hydrothermal fluids (note that evaporite data are shown by solid bars and the data from the other hydrothermal comparison). b. Histogram of 6’ ‘B-values for hydrothermal fluids.
+20
11
6
kg-l)
B(“1.o)
bottom diagram the non-marine fluids are shown as unfilled bars for
in the
114
3.2. Sediment-starvedback-arcsettings The concentration of boron in hydrothermal vent fluids from the Mariana back-arc spreading center is almost double those from sediment-starved mid-ocean ridge systems (Fig. 2a), with the concentration of rock-derived boron in the Mariana fluids being about five times higher. There are several possible explanations for the high boron levels. There is no detectable ammonia or iodine in the hydrothermal fluids (A.C. Campbell, pers. commun., 1991) and as these tracers are strongly enriched in fluids that have a marine sedimentary input (Von Damm et al., 1985; Campbell and Edmond, 1989) this indicates that there is no sediment beneath the vent site. High Cs and Rb concentrations in vent waters can result from interaction of hydrothermal fluids with low-temperature weathering products formed earlier in the history of a ridge segment (Palmer and Edmond, 1989). Such a processwould also affect the boron concentrations as it is enriched in oceanic crust low-temperature alteration products, but the degreeof Cs and Rb enrichment in the Mariana fluids (Campbell et al., 1987) is far higher than observedat equally slow-spreadingridge sites on the Mid-Atlantic Ridge. Basaltsfrom the Mariana back-arcbasin are not highly evolved (Sinton and Freyer, 1987), so the high B, Cs and Rb concentrations are not the result of interaction of fluids with oceanic crust that has concentrated incompatible elements through fractional crystallization. High volatile abundances in Mariana back-arc basin basalts are thought to be derived from the slab subducting below the island arc to the east (Sinton and Freyer, 1987) and it is likely that the high boron concentrations in the Mariana vent waters are due to interaction of hydrothermal fluids with oceanic crust enriched by slab-derived boron. Boron concentrations of the Mariana fluids are high relative to those of sediment-starved mid-ocean ridge sites, but lower than those
M.R. PALMER
from sediment-hosted systems (Fig. 2a). Consequently tourmaline is uncommon in massive sulfide deposits from ophiolite settings. Boron isotope systematics result in the 6’ ‘B-values of tourmalines being lighter than the fluids from which they are derived (Palmer and Slack, 1989), although the precise controls over B isotope fractionation during tourmaline precipitation are complex (London and Palmer, 1990). Therefore, the G”B-values of the residual fluids rise during precipitation of tourmaline. The Mariana vent waters may, thus, have higher 6’ ‘B-values than the original fluids, but the rarity of tourmaline in ophiolite settings suggeststhat the affect is minor. Preliminary data suggest isotope fractionation factors (“B/“B) tourma~ine/ ( ’ ‘B/‘OB louid Of N 0.994~ 0.987 for the P-T conditions in hydrothermal systems (London and Palmer, 1990). However, without knowing if tourmaline is forming in Mariana hydrothermal sediments, and if so, what the original B isotope composition of the fluids was, it is not possible to specify the 6 ’ ‘B-value of any precipitated tourmaline. Nevertheless, the S”B-values of the Mariana fluids are lower than those from sedimentstarved mid-ocean ridge sites, reflecting the higher proportion of rock-derived boron, but are higher than those from sedimented ridges as seawater still a significant source of boron in the vent fluids (Fig. 2b). As a result tourmalines from ophiolite-hosted settings have relatively high 6’ ‘B-values (Fig. 3 ) . The range of 6’ ‘B-values that could be generatedin such settings depends on the nature of the slab-derived component, but it is unlikely that slabderived fluids will increase the boron concentrations of back-arc basalts from normal basaltic values ( < 1 ppm) to the levels in elastic sediments ( N 100 ppm) (Spivack and Edmond, 1987;Spivack et al., 1987). So seawater will always be a major component of back-arc hydrothermal fluids and tourmalines from this type of setting will have relatively high 6’ ‘Bvalues.
B ISOTOPE
SYSTEMATICS
OF HYDROTHERMAL
FLUIDS
AND
115
TOURMALINES
6
6
-30
-20
-10
0
;’
+lO
+20
+30
+40
5’1; B (%a)
Fig. 3. Comparison between G’iB-values of boron sources, hydrothermal fluids and tourmalines from different geologic settings: (a) sediment-starved back-arc; (b) elastic sediment-hosted; (c) marine evaporite-hosted; and (d) non-marine evaporite-hosted. In each histogram the so/id boxes show 6’ ‘B-values of tourmalines from deposits of that setting, and the unfilled boxes represent the overall range of tourmaline G”B-values. Data are from Palmer and Slack ( 1989).
116
M.R. PALMER
3.3. Clastic sediment-hostedsettings At Guaymas Basin and Escanaba Trough, hydrothermal fluids circulate through oceanic crust and rise up through the sedimentary cover, depositing massive sulfides in the sediments prior to venting of fluids into the water column. Compared to the sediment-starved systems considered above, fluids from Escanaba Trough and Guaymas Basin have elevated boron concentrations (Fig. 2a) reflecting reaction of circulating fluids with terrigenous sediments having high boron ( - 100 ppm) compared to fresh oceanic crust (< 1 ppm) (Spivack and Edmond, 1987; Spivack et al., 1987) . Unaltered sediments at Guaymas and Escanaba have G”B-values of - 1.2 and - 4.5%0and concentrations of 62 and 42 ppm, respectively (Spivack et al., 1987). In comparison sediments from Guaymas Basin that are altered to greenschist facies contain - 1 ppm B, indicating that the fixed and adsorbed boron is leached from sediments during interaction with hydrothermal fluids (Spivack et al., 1987). Consequently the fluids from these areashave significantly lower 6’ ‘B-values than those considered above (Fig. 2b). If it is assumed that fluids at Guaymas and Escanaba are mixtures of sediment-derived boron and fluids with G’ ‘B-values similar to mid-ocean ridge vent waters, the G’ ‘B-value and boron concentration of the hydrothermal fluids can be plotted as a function of water/ rock ratio (Fig. 4). The water/rock ratio is delined here as the weight of water divided by the weight of sediment required to elevate the fluid boron concentrations abovethose in midocean ridge vent waters. Following the observations in sediments from Guaymas Basin (Spivack et al., 1987) it is assumedthat boron is quantitatively leached from sediments by hydrothermal fluids. The position of the vent water data relative to the theoretical curves in Fig. 4 is compatible with removal of low 6”B boron into secondary hydrothermal minerals, such astourmaline. At a temperature of 2 15’ C
I
I
I
(b)
0
2
4 [B]
( mmol
6 kg’)
-
10
Fig. 4. Variation of 6 ’ ‘B-values with boron concentration of elastic sediment-hosted systems, assuming conservative behavior of boron: (a) Guaymas Basin; and (b) Escanaba Trough. Tick marks are water/rock ratios for reaction of the fluids with sediment. Open squares are the measured values in the vent waters.
and a pressure of 0.5-l .Okbar (the estimated conditions for the fluid-sediment reaction zone at Escanaba;Campbell et al., 1991) the B isotope fractionation factor between tourmaline and fluid (as defined on p. 114) is - 0.990-0.987 (London and Palmer, 1990). Application of Rayleigh equations (e.g., Hoefs, 1987) with the isotope fractionation factors noted above,to the Escanabadata listed in Table I, indicates that a water/rock ratio of 0.71.O and 60-70% removal of the dissolved boron into secondary phasesis required to yield the observed vent water G’‘B-values at Escanaba. At Guaymas the P-T conditions are 2903 15°C and - 0.3 kbar, yielding a B isotope fraction factor of -0.990. For the Guaymas system a water/rock ratio of 0.5-0.75 and -80% removal of dissolved boron from the
P-T
B ISOTOPE
TABLE Boron
SYSTEMATICS
OF HYDROTHERMAL
TABLE
isotope
ratios
and concentrations
MID-OCEAN
RIDGE
+32.5i-0.8 +31.5i +31.OiO.8 +32.2+ +32.6f0.8 f32.7-tO.5 +29.0? +30.0&
1 l-13”N: I 3 4 5 6 Juan de Fuca Plume TAG 2179 2187
kg-’
)
fluids
Data source’ ,
SETTINGS
0.50+0.01 0.50+0.01 0.50&0.01 0.51 kO.01 0.49+0.01 0.51 kO.01 0.54-tO.01 0.55+0.01
[II
+34.9i1.0 +36.8f 1.0 +31.7* 1.0 +36.6-t 1.0 f34.7kl.O + 34.5 rf- 1.2
0.47*0.01 0.45?0.01 0.49+0.01 0.46*0.01 0.48*0.01 0.49 t 0.01
131 131 [31 [31 I31
+29.1+ 1.1 +32.3+1.3
0.35+0.01 0.39?0.01
111 111
+26.9* +26.7?
0.56+0.01 0.52?10.01
[II
1.1 1.1
1.0 1.0
121
[II 131
111
23”N
[II [31
[II
MAR:
BACK-ARC
SEDIMENT-STARVED
1.1 1.0
[41
SETTING:
Trough: +29.8+ 1.5 +22.5? 1.1 +26.1 k 1.3
I 2 3 SEDIMENT-HOSTED
Guaymas 1177-13 1176-6 1177-6 I 173-6
Basin:
Escanaba 2040 2036 2041
Trough:
EVAPORITE
Red Sea: Atlantic II Suakin Valdivia
0.81 -to.01 0.81 kO.01 0.75?0.01
[II 111 [II
+23.2?0.6 +19.7*0.6 +16.5+0.6 +17.5f0.6
1.57kO.02 1.7350.02 1.56+0.02 1.63?0.02
131 [31 131 [31
+11.51-.4 + 10.1 +0.7 +10.4+0.5
1.71 io.02 2.16&0.04 1.90f0.04
[II
0.81 +O.Ol 0.63f0.01 0.80+0.01
[II [II [II
SYSTEMS:
111
[II
SETTING:
+29.7+0.4 +39.0+0.4 +30.2+0.4
I (continued)
Sample
6”B ( %o )
NON-MARINE
Salton Port 3 Port 4 Port 5 Port 6
EVAPORITE
[Bl (pm01 kg-r
Data source* )
SETTING:
Sea:
References: [I] [3]=Spivackand
- 1.13+0.35 - 1.53kO.27 -1.83kO.27 -2.57kO.35
38.1 kO.2 38.9 k 0.2 40.410.3 48.9kO.3
[II [II [II
[II
=this study; [2] =Spivack and Edmond Edmond (1987); [4] =Campbelletal.
(1986); (1988).
131
MAR:
MARK 1990 1986
Mariana
(wol
SEDIMENT-STARVED
21”NEPR: SW 1985 SW 1982 OBS 1985 OBS 1982 NGS 1985 NGS 1982 HG 1985 HG 1982
26”N
of hydrothermal
[Bl
6”B ( %o )
117
AND TOURMALINES
I
Sample
MARINE
FLUIDS
hydrothermal fluid into secondary phases is required to yield the observed vent water compositions. On the basis of oxygen isotope systematics Kastner ( 1982) calculated a water/ rock ratio of - 1 for reaction of the hydrothermal fluids with sediment at Guaymas. Campbell et al. ( 1991) calculated a water/rock ratio of - 5 for interaction of the sediments and hydrothermal fluids at Escanabaon the basis of the alkali element concentrations of the fluids and sediments. At both sites the water/rock ratios calculated from the B isotope systematits are lower than those estimated from oxygen isotope systematics and alkali element concentrations. Some of the discrepancy may be the result of inaccuracies causedby extrapolation of the B isotope fractionation factors outside the experimental P-T conditions (London and Palmer, 1990)) but several studies have noted that boron is the most mobile speciesyet measured during high-temperature fluid-rock interactions (Spivack and Edmond, 1987; Spivack et al., 1987; Campbell et al., 1991). Therefore, it is likely B isotope systematicswill record a higher degreeof water-rock interaction than other, less mobile, tracers. Tourmaline has not been identified in hydrothermal sediments from Guaymas Basin or EscanabaTrough, but it is commonly found in association with volcanogenic, sedimenthosted massive sulfide deposits (Slack, 1982; Slack et al., 1984). Tourmaline may not have been recognized in active hydrothermal systems due to its likely very fine grain sizein such
118
M.R. PALMER
deposits (Slack, 1982) and the lack of thorough sample coverage. It is also possible that the formation of tourmaline involves a noncrystalline, boron-rich precursor phase (Slack et al., 1984). Thus, evidence for boron mineralization may be difficult to recognize without a purposeful search. The amount of tourmaline associated with massive sulfide deposits varies from trace amounts to extensive tourmalinization (Slack, 1982; Taylor and Slack, 1984). Vent waters from Guaymas and Escanaba contain residual boron after probable formation of tourmaline, so it is not possible to make a direct comparison between the 6”Bvalues of these fluids with tourmalines from sediment-hosted settings. However, the high boron concentration in elastic sedimentsmeans that at comparatively high water/rock ratios the G’ ‘B-value of hydrothermal fluids in sedimentary settings are dominated by the 6”Bvalue of the sediments (Palmer and Slack, 1989). Hence, the G”B-values of tourmalines should be lower than the G’ ‘B-values defined by elastic sediments by the appropriate isotope fractionation factor. London and Palmer ( 1990) measuredtourmaline-fluid/vapor boron isotope fractionation at 350’ C and 0.5-2.0 kbar and obtained a fractionation factor (“B/“B) tourmaline/ ( ’ ‘B/ “13)auid % 0.9940.992, i.e. the tourmalines should be w 6-8% lighter than the sediments at 350°C. Greater isotope fractionation would be expected at lower temperatures, but most of the 6’ ‘B data for tourmalines from sediment-hosted settings fall within this offset from the 6”B-values of elastic sediments (Fig. 3 ) . 3.4. Marine
evaporite-hostedsettings
Seafloorbrine pools within the axial rift zone of the Red Sea have high salinities resulting from the interaction of circulating fluids with underlying marine evaporites. Some of the brine pools contain metalliferous sediment indicative of a hydrothermal input (Backer and Schoell, 1972). Fluid samples were analyzed
from the Valdivia, Suakin, and Atlantis II deeps. There is no correlation of the boron concentrations or isotope ratios of the brines with tracers of hydrothermal input into the basins (Backer and Schoell, 1972; Zierenberg and Shanks, 1986). This is to be expected as the degreeof interaction between the Red Sea brines and the underlying basalt is lower than at mid-ocean ridge hydrothermal sites (Zierenberg and Shanks, 1986) and the oceanic crust contains low boron concentrations. The boron concentrations of the Red Seabrines are similar to those from the Mariana site and considerably lower than those from sedimenthosted systems (Fig. 2). The high Cl levels of the Red Sea brines indicate that they have undergone considerable interaction with the evaporites (Zierenberg and Shanks, 1986)) so the low dissolved boron concentrations in the brines appear to be due to the generally low abundance of borates in marine evaporites (Holser, 1979). The G”B-value of borates from marine evaporites varies from to + 18 to + 3 1%o, reflecting varying pH conditions and local inputs from non-marine sources (Swihart et al., 1986). Tourmalines from terranes containing evidence of marine evaporites span a wide range in B isotope ratios from - 12.1 to + 18.3%0 (Palmer and Slack, 1989). A comparison of the 6’ ‘B-values of the tourmalines with those of the Red Sea brines and marine evaporite borates (Fig. 3 ) reveals that some tourmalines have much lower 6’ ‘B-values than expectedfrom the isotope fractionation factors (London and Palmer, 1990). This may be due to lower temperatures of formation of these tourmalines, but it also reflects the low concentration of boron in marine evaporites. When it is present, boron is generally restricted to evaporites concentrated beyond the halite facies (Holser, 1979). Therefore, boron in tourmalines in terranescontaining evaporite sequencesis not always derived from the evaporite. It is possible that boron concentrations may be enhancedin hydrothermal fluids that have reacted with
B ISOTOPE
SYSTEMATICS
OF HYDROTHERMAL
FLUIDS
AND
119
TOURMALINES
evaporites if the fluids then pass through clastic sediments or volcanics, as high anion concentrations in fluids promote the dissolution of rocks and transport of complexes in solution. Thus, tourmaline may be locally abundant in terranes containing marine evaporite sequences even though the boron is derived from other non-marine sources.
J----
3.5. Non-marine evaporite-hostedsettings -3.0
The Salton Sea is a restricted basin largely filled with fluvial and lacustrine elastic and evaporitic deposits derived from overflows of the Colorado River. The area is characterized by high heat production related to the northward extension of the East Pacific Rise (Elders and Sass, 1988 ). The fluids considered here were collected at sequential sampling ports at the well head during the first flow test of the Salton Sea Scientific Drilling Project in December 1985 (Elders and Sass, 1988; Thompson and Fournier, 1988). The aquifer was 1898 m deep and had an estimated formation temperature of 305 “C, leading to flashing of the fluids before the first sampling port and between each of the subsequent ports. Boron is partitioned into brine relative to vapor during phase separation, but it is sufficiently volatile for a proportion of the boron to be lost with the escaping steam (Smith et al., 1987). The effect of this phase separation on the B isotope systematics is shown in Fig. 5. The trend towards lower 6’ ‘B-values in fluids that have lost vapor through phase separation is also seen in geothermal fluids from Yellowstone National Park, Wyoming, U.S.A., and is due to the greater volatility of B (OH) 3, relative to higher complexes of boron, and the preferential partitioning of the heavy isotope, B, into the trigonally coordinated species (Palmer and Sturchio, 1990). The boron concentrations of the Salton Sea fluids are an order of magnitude higher than in other hydrothermal fluids (Fig. 2a). This is due to interaction of the circulating geothermal
-2.5
-2.0
-1.5 6
11
-1.0
-0.5
I3 PL)
Fig. 5. Variation of G”B-values with boron tions in Salton Sea well-head fluids.
concentra-
fluids with non-marine evaporites which frequently contain borates as a major mineralogic component ( Aristarian and Hurlbut, 1972). In view of the high boron levels in Salton Sea geothermal fluids it is perhaps surprising that boron-bearing minerals have not been recorded in published accounts of the mineralogy of drill core material from this area. The Salton Sea brines had a pH of 5.5 at the first surface sampling point and this fell to as low as 3.08 in the later ports (Thompson and Fournier, 1988). This suggests that the in situ pH of the fluids was higher than that measured at the surface after flashing and secondary mineral formation. Tourmaline becomes less stable at pHvalues of > 6.0 (Morgan and London, 1989) and it is possible that the Salton Sea fluids only become saturated with respect to tourmaline after they have risen to the surface. The relatively high pH of hydrothermal fluids may, therefore, limit the occurrence of tourmaline in non-marine evaporite settings, such that tourmaline is only formed if fluids are expelled into shallow sediments or into a restricted basin. It is noteworthy that some tourmalinites in the Broken Hill (New South Wales, Australia) district (where the boron is thought to have been derived from non-marine evaporites) show well-developed sedimentary features, such as graded bedding and flame struc-
120
tures (Barnes, 1980; Slack et al., 1984), that indicate production in shallow sediments. Becauseof the high boron levels characteristic of non-marine evaporites the G’ ‘B-values of hydrothermal fluids in this setting will be completely dominated by the evaporite 6”B-values. The B isotope composition of the Salton Sea fluid is towards the heavy end of the wide range in G”B-values ( - 30.1 to + 7.3%0) observed for non-marine evaporites (Swihart et al., 1986). The range of G”B-values that can be generated in tourmalines containing from boron derived from non-marine evaporites is, therefore, also large (Fig. 3). In many instances it is not possible to distinguish this source of boron on the basis of the 6’ ‘B-values of the tourmalines alone, but non-marine evaporites are the only known reservoir of boron with a light enough isotopic composition to produce the very low G’ ‘B-values observed in the Broken Hill tourmalines (Slack et al., 1989). 4. Conclusions
( 1) Hydrothermal fluids from sedimentstarvedmid-ocean ridgeshave the highest 6 ’ ‘Bvalues and lowest boron concentrations. As a consequenceof the low boron concentrations tourmaline has not been found in hydrothermal deposits from this setting. (2) Vent waters from back-arc sedimentstarved settings have higher boron concentrations than mid-ocean ridge sites due to the influence of the subducting slab, but they are low relative to most other hydrothermal sites. The G’ ‘B-value of the fluids is still high because seawateris a significant source of boron in the hydrothermal fluids. Tourmaline from this setting has heavy G”B-values, but is comparatively rare. (3) Clastic sediment-hosted hydrothermal systems have vent waters with still higher boron concentrations and lower G”B-values because of the higher concentration of boron in elastic sediments compared to basalts. The
M.R. PALMER
6’ ‘B-value of tourmalines from this setting are lower than the values in elastic sediments by the appropriate isotope fractionation factor. (4) Fluids from hydrothermal systems influenced by marine evaporites have boron concentrations similar to those from back-arc settings, but have heavier 6’ ‘B-values because the boron derived from the evaporite has a 6’ IB-value similar to that of seawater. Tourmalines from terranes containing marine evaporites have the heaviest 6’ ‘B-values, but much lighter B isotope compositions are also observed, reflecting the derivation of boron from other reservoirs due to the relatively low abundanceof boron in most marine evaporites. ( 5 ) Hydrothermal fluids from non-marine evaporite settings have the highest boron concentrations due to the high abundanceof evaporite borate minerals. There is a wide range of 6 ’ ‘B-values observedin such settings, but nonmarine evaporites contain the lowest known 6 “B-value of any boron reservoir and is the only reservoir capable of yielding the exceptionally low G”B-values seenin some tourmalines. The pH of hydrothermal fluids circulating within non-marine evaporite sequencesis at the upper limit of the tourmaline stability field and only falls after the system has been perturbed by changesin P-T conditions that cause secondary mineral precipitation. Thus, extensive tourmaline may not be formed in hydrothermal systems from this setting unless fluids are expelled into permeable strata or overlying waters. Acknowledgments
I am grateful to John Edmond for his helpful advice and frequent discussions. Rob Zierenberg kindly supplied the Red Seasamples.This work was supported by the Royal Society. I thank John Slack, George Swihart and an anonymous reviewer for their helpful comments.
B ISOTOPE
SYSTEMATIC3
OF HYDROTHERMAL
FLUIDS
AND
TOURMALINES
References Aristarian, L.F. and Hurlbut, C.S., 1972. Boron minerals and deposits, Part II. Geological environments and classification of boron deposits. Miner. Rec., 3: 2 13220. Backer, H. and Schoell, M., 1972. New deeps with brines and metalliferous sediments in the Red Sea. Nature (London), Phys. Sci., 240: 153-l 58. Barnes, R.G., 1980. A metallogenic study of the Purnamoota-Yalcowinna I : 50,000 sheet, northern Broken Hill Block. Geol Surv. New South Wales, Rep. G.S. 1980/116, 198~~. Campbell, A.C. and Edmond, J.M., 1989. Halide systematics of submarine hydrothermal vents. Nature (London), 342: 168-170. Campbell, A.C., Edmond, J.M., Colodner, D., Palmer, M.R. and Falkner, K.K., 1987. Chemistry of hydrothermal fluids from the Mariana Trough back arc basin in comparison to mid-ocean ridge fluids. Eos (Trans. Am. Geophys. Union), 68: 153 1 (abstract). Campbell, A.C., Palmer, M.R., Klinkhammer, G.P., Bowers, T.S., Edmond, J.M., Lawrence, J.R., Casey, J.F., Thompson, G., Humphris, S., Rona, P. and Karson, J.A., 1988. Chemistry of hot springs on the MidAtlantic Ridge: TAG and MARK Sites. Nature (London), 335: 514-519. Campbell, A.C., German, C.R., Palmer, M.R. and Edmond, J.M., 199 1. Chemistry of hydrothermal fluids from the Escanaba Trough, Gorda Ridge. U.S. Geol. SUN. Bull (in press). Elders, W.A. and Sass, J., 1988. The Salton Sea Scientific Drilling Project (SSSDP). J. Geophys. Res., 93: 12,953-12,968. Hoefs, J., 1987. Stable Isotope Geochemistry. Springer, Berlin, 24 1 pp. Holser, W.T., 1979. Trace elements and isotopes in evaporites. In: R.G. Burns (Editor), Marine Minerals. Mineral. Sot. Am., Rev. Mineral., 6: 295-346. Kastner, M., 1982. Evidence for two distinct hydrothermal systems in the Guaymas Basin. In: J.R. Curray, D.G. Moore, et al. (Editors), Initial Reports of the Deep Sea Drilling Project, Vol. 64, Part 2. U.S. Gov. Print. Off., Washington, D.C., pp. 1143-l 157. London, D. and Palmer, M.R., 1990. Fractionation of “B/ “B between tourmaline and aqueous vapor. Geol. Sot. Am., Abstr. Prog., 22: Al 57. Morgan, G.B. and London, D., 1989. Experimental reactions of amphibolite with boron-bearing aqueous fluids at 200 MPa: implications for tourmaline stability and partial melting in mafic rocks. Contrib. Mineral. Petrol., 102: 28 l-297. Palmer, M.R. and Edmond, J.M., 1989. Cesium and rubidium in submarine hydrothermal fluids: evidence for recycling of alkali elements. Earth Planet. Sci. Lett., 95: 8-14.
121
Palmer, M.R. and Slack, J.F., 1989. Boron isotopic composition of tourmaline from massive sulfide deposits and tourmalinites. Contrib. Mineral. Petrol., 103: 434451. Palmer, M.R. and Sturchio, N.C., 1990. The boron isotope composition of the Yellowstone National Park (Wyoming) hydrothermal system: A reconnaissance. Geochim. Cosmochim. Acta, 54: 28 1 l-28 15. Sinton, J.M. and Freyer, P., 1987. Mariana Trough lavas from 18 “N: Implications for the origin ofback arc basin basalts. J. Geophys. Res., 92: 12,782- 12,802. Slack, J.F., 1982. Tourmaline in Appalachian-caledonian massive sulphide deposits and its exploration significance. Trans. Inst. Min. Metall., 91 (B): B81-B89. Slack, J.F., Herriman, N., Barnes, R.G. and Plimer, I.R., 1984. Stratiform tourmalinites in metamorphic terranes and their geologic significance. Geology, 12: 7 13716. Slack, J.F., Palmer, M.R. and Stevens, B.P.J., 1989. Boron isotope evidence for the involvement of non-marine evaporites in the origin of the Broken Hill ore deposits. Nature (London), 342: 9 13-9 16. Smith, C.L., Ficklin, W.H. and Thompson, J.M., 1987. Concentration of arsenic, antimony, and boron in steam and steam condensate at The Geysers, California. J. Volcanol. Geotherm. Res., 32: 329-34 1. Spivack, A.J. and Edmond, J.M., 1986. Determination of boron isotope ratios by thermal ionization mass spectrometry of the dicesium metaborate cation. Anal. Chem., 58: 31-35. Spivack, A.J. and Edmond, J.M., 1987. Boron isotope exchange between seawater and the oceanic crust. Geochim. Cosmochim. Acta, 51: 1033-1043. Spivack, A.J., Palmer, M.R. and Edmond, J.M., 1987. The sedimentary cycle of the boron isotopes. Geochim. Cosmochim. Acta, 5 1: 1939-l 949. Swihart, G.H., Moore, P.B. and Callis, E.L., 1986. Boron isotopic composition of marine and nonmarine evaporite borates. Geochim. Cosmochim. Acta, 50: 12971301. Taylor, B.E. and Slack, J.F., 1984. Tourmalines from Appalachian-Caledonian massive sulfide deposits: Textural, chemical, and isotopic relationships. Econ. Geol., 79: 1703-l 726. Thompson, J.M. and Fournier, R.O., 1988. Chemistry and geothermometry of brine produced from the Salton Sea Scientific Drill Hole, Imperial Valley, California. J. Geophys. Res., 93: 13,165-13,173. Von Damm, K.L., Edmond, J.M., Measures, C.I. and Grant, B., 1985. Chemistry of submarine hydrothermal solutions at Guaymas Basin, Gulf of California. ,,r’ Geochim. Cosmochim. Acta, 49: 2221-2237. Zierenberg, R.A. and Shanks, W.C., 1986. Isotopic constraints on the origin of the Atlantis II, Suakin and Valdivia brines, Red Sea. Geochim. Cosmochim. Acta, 50: 2205-2214.
111
Boron isotope systematics of hydrothermal fluids and tourmalines: A synthesis M.R. Palmer Department of Geology, Bristol University, Wills Memorial
Building,
Queen’s Road, Bristol BSS IRJ, UK
(Received January 14, 199 1; revised and accepted July 24, 199 1)
ABSTRACT Palmer, M.R., 199 1. Boron isotope systematics of hydrothermal Geosci. Sect.), 94: 111-121.
fluids and tourmalines:
A synthesis. Chem. Geol. (Isot.
Boron isotope and concentration data are presented for hydrothermal fluids from different tectonic settings that reflect derivation of boron from marine evaporites (Red Sea brines), non-marine evaporites (Salton Sea), elastic sediments (Escanaba Trough and Guaymas Basin), back-arc basalts (Mariana Trough) and mid-ocean ridge basalts (21’ and 1 I-13”N East Pacific Rise; Juan de Fuca Ridge; and 23” and 26”N, Mid-Atlantic Ridge). Boron isotope systematics of these fluids are used to constrain the G”B-values of tourmalines found in ancient massive sulfide deposits and tourmalinites that formed under conditions analogous to those of modern hydrothermal systems. The data also provide insights into the physical conditions required for the formation of tourmaline in certain geologic environments.
1. Introduction
Palmer and Slack ( 1989) carried out a survey of the boron isotope composition of tourmalines from massive sulfide deposits and tourmalinites, and concluded that the S’ ‘Bvalues of the tourmalines are largely dependent on the dominant source of boron, with non-marine and marine evaporite sourcesproducing especially distinctive B isotopic signatures. The G”B-values of the major reservoirs of boron (marine evaporites, non-marine evaporites, elastic sediments and volcanic rocks) are relatively well characterized (Palmer and Slack, 1989, and referencestherein). To more closely evaluate the tourmaline 6’ ‘Bvalues more information is required for fluids that interact with these reservoirs in modern hydrothermal systems. A compilation of new and previously published data are presented here for hydrothermal fluids from the following systems: marine evaporite-hosted (Red 016%9622/91/$03.50
0 199 1 Elsevier Science Publishers
Sea); non-marine evaporite-hosted (Salton Sea, California); elastic sediment-hosted (EscanabaTrough and Guaymas Basin) ; back-arc sediment-starved spreading center (Mariana back-arc basin); and sediment-starved midocean ridge spreading centers (2 1o and 1l13”N East Pacific Rise, Juan de Fuca Ridge, and 23 ’ and 26”N Mid-Atlantic Ridge) (Fig. 1). These data allow a comparison betweenthe B isotope systematics of modern and ancient hydrothermal systems as recorded by tourmalines within massive sulfide deposits and tourmalinites. 2. Methods
and results
Boron isotope compositions and concentra“tions of hydrothermal fluids were determined using the methods of Spivack and Edmond ( 1986) and are listed in Table I. The isotope ratios are expressedas 6 “B-values, where: B.V. All rights reserved.
M.R. PALMER
112
Fig. 1. Map showing locations
of hydrothermal
systems discussed in the text.
a”B= [ (‘lB/‘oB),,,p,e/(‘lB/loB)s~an~ar~-1I x103 The standard is NBS boric acid SRM 951 which hasa measured “B/‘OB ratio of 4.04558. The in-run precision was generally < + 0.2%0, but the reproducibility isotope ratio runs was 2 0.3%0(20). The measuredboron concentrations are precise to < + 2%. 3. Discussion
3.1. Sediment-starvedmid-ocean ridge settings In sediment-starved mid-ocean ridge hydrothermal systems seawater is drawn down through oceanic crust, and within the low-temperature limb of the convection cell some boron is removed from seawater and incorporated into low-temperature weathering products. As the circulating fluid is heated above - 150’ C, boron is quantitatively leached from
the rock. Within the resolution of the data there does not appear to be uptake of boron into high-temperature secondary minerals, but the subtleties of boron behavior are partially obscured by the high concentration of boron in seawater ( (420 prnol kg- ‘, 4.6 ppm) relative to basalt (< 100 pmol kg-‘, < 1 ppm). As a result, -75-90% of the boron in submarine vent fluids is derived from seawaterthat passes through the system (Spivack and Edmond, 1987; Campbell et al., 1988). Vent fluids from sediment-starved mid-ocean ridges have the lowest boron concentrations measured in hydrothermal solutions (Fig. 2a) and can be lower than seawater due to low-temperature boron uptake. As a consequenceno boron-rich mineral phasehas been identified in studies of these sites. As seawater is the dominant component of these vent fluids they have the highest G”B-values (Fig. 2b), even though fresh oceanic crust has a low 6 “B-value ( - 3%0) relative to seawater, + 39.5%0 (Spivack and Edmond, 1987).
B ISOTOPE
SYSTEMATICS
OF HYDROTHERMAL
FLUIDS
AND TOURMALINES
(b) ‘2
(4 ‘.’ I Sediment ridge
starved spreading
Sediment ridge
10
mid-ocean center
starved spreading
mid-ocean center
0 6
6 4 4 2 2 0
0 0.5
1.0
1.5
2.0
2.5 12
Sediment starved back arc spreading center
10
0
2 0
, 0.5
1.0
(
,
1.5
(
,
2.0
"-10
0
+20
*IO
+40
+30
2.5 I
12
14
I
I
I
I IO
12
=‘
Clastic-sediment
-
hosted
Clastic-sediment
hosted
10
5
8
z
6
I;
4 2 0 0.5
1.0
1.5
2.0
"-10
2.5
0
121,s 14
+lO I
+30
+20 I
I
Id
+40
1
II
1
Marine
1
evaporite
8 6 4
-
2
-
2
0.5
1 .s
1.0
2.0
2.5
Non-marine
t -10
1 0
'
t
*IO
c
.:.x;+.:: i :j;::..*::
+20
::'-"-2:: .&& .:;.:.:.:.:...:. >...../. .,.
+30
+40
r30
+40
l2 I
,,,~,,,,,,,,~,,,,,I~~,,,I,,,,III,II,~IIIIIIIIIII
20
evaporite
-
0‘
0
25
Marine
evaporite
Non-marine
evaporite
15 10
5
2 IIS
0 IO
20
B
30
(mmol
d0
I I I I I I I14 50
,.3'
,'
-
0 -10
0
+10
Fig 2. a. Histogram of boron concentrations for hydrothermal fluids (note that evaporite data are shown by solid bars and the data from the other hydrothermal comparison). b. Histogram of 6’ ‘B-values for hydrothermal fluids.
+20
11
6
kg-l)
B(“1.o)
bottom diagram the non-marine fluids are shown as unfilled bars for
in the
114
3.2. Sediment-starvedback-arcsettings The concentration of boron in hydrothermal vent fluids from the Mariana back-arc spreading center is almost double those from sediment-starved mid-ocean ridge systems (Fig. 2a), with the concentration of rock-derived boron in the Mariana fluids being about five times higher. There are several possible explanations for the high boron levels. There is no detectable ammonia or iodine in the hydrothermal fluids (A.C. Campbell, pers. commun., 1991) and as these tracers are strongly enriched in fluids that have a marine sedimentary input (Von Damm et al., 1985; Campbell and Edmond, 1989) this indicates that there is no sediment beneath the vent site. High Cs and Rb concentrations in vent waters can result from interaction of hydrothermal fluids with low-temperature weathering products formed earlier in the history of a ridge segment (Palmer and Edmond, 1989). Such a processwould also affect the boron concentrations as it is enriched in oceanic crust low-temperature alteration products, but the degreeof Cs and Rb enrichment in the Mariana fluids (Campbell et al., 1987) is far higher than observedat equally slow-spreadingridge sites on the Mid-Atlantic Ridge. Basaltsfrom the Mariana back-arcbasin are not highly evolved (Sinton and Freyer, 1987), so the high B, Cs and Rb concentrations are not the result of interaction of fluids with oceanic crust that has concentrated incompatible elements through fractional crystallization. High volatile abundances in Mariana back-arc basin basalts are thought to be derived from the slab subducting below the island arc to the east (Sinton and Freyer, 1987) and it is likely that the high boron concentrations in the Mariana vent waters are due to interaction of hydrothermal fluids with oceanic crust enriched by slab-derived boron. Boron concentrations of the Mariana fluids are high relative to those of sediment-starved mid-ocean ridge sites, but lower than those
M.R. PALMER
from sediment-hosted systems (Fig. 2a). Consequently tourmaline is uncommon in massive sulfide deposits from ophiolite settings. Boron isotope systematics result in the 6’ ‘B-values of tourmalines being lighter than the fluids from which they are derived (Palmer and Slack, 1989), although the precise controls over B isotope fractionation during tourmaline precipitation are complex (London and Palmer, 1990). Therefore, the G”B-values of the residual fluids rise during precipitation of tourmaline. The Mariana vent waters may, thus, have higher 6’ ‘B-values than the original fluids, but the rarity of tourmaline in ophiolite settings suggeststhat the affect is minor. Preliminary data suggest isotope fractionation factors (“B/“B) tourma~ine/ ( ’ ‘B/‘OB louid Of N 0.994~ 0.987 for the P-T conditions in hydrothermal systems (London and Palmer, 1990). However, without knowing if tourmaline is forming in Mariana hydrothermal sediments, and if so, what the original B isotope composition of the fluids was, it is not possible to specify the 6 ’ ‘B-value of any precipitated tourmaline. Nevertheless, the S”B-values of the Mariana fluids are lower than those from sedimentstarved mid-ocean ridge sites, reflecting the higher proportion of rock-derived boron, but are higher than those from sedimented ridges as seawater still a significant source of boron in the vent fluids (Fig. 2b). As a result tourmalines from ophiolite-hosted settings have relatively high 6’ ‘B-values (Fig. 3 ) . The range of 6’ ‘B-values that could be generatedin such settings depends on the nature of the slab-derived component, but it is unlikely that slabderived fluids will increase the boron concentrations of back-arc basalts from normal basaltic values ( < 1 ppm) to the levels in elastic sediments ( N 100 ppm) (Spivack and Edmond, 1987;Spivack et al., 1987). So seawater will always be a major component of back-arc hydrothermal fluids and tourmalines from this type of setting will have relatively high 6’ ‘Bvalues.
B ISOTOPE
SYSTEMATICS
OF HYDROTHERMAL
FLUIDS
AND
115
TOURMALINES
6
6
-30
-20
-10
0
;’
+lO
+20
+30
+40
5’1; B (%a)
Fig. 3. Comparison between G’iB-values of boron sources, hydrothermal fluids and tourmalines from different geologic settings: (a) sediment-starved back-arc; (b) elastic sediment-hosted; (c) marine evaporite-hosted; and (d) non-marine evaporite-hosted. In each histogram the so/id boxes show 6’ ‘B-values of tourmalines from deposits of that setting, and the unfilled boxes represent the overall range of tourmaline G”B-values. Data are from Palmer and Slack ( 1989).
116
M.R. PALMER
3.3. Clastic sediment-hostedsettings At Guaymas Basin and Escanaba Trough, hydrothermal fluids circulate through oceanic crust and rise up through the sedimentary cover, depositing massive sulfides in the sediments prior to venting of fluids into the water column. Compared to the sediment-starved systems considered above, fluids from Escanaba Trough and Guaymas Basin have elevated boron concentrations (Fig. 2a) reflecting reaction of circulating fluids with terrigenous sediments having high boron ( - 100 ppm) compared to fresh oceanic crust (< 1 ppm) (Spivack and Edmond, 1987; Spivack et al., 1987) . Unaltered sediments at Guaymas and Escanaba have G”B-values of - 1.2 and - 4.5%0and concentrations of 62 and 42 ppm, respectively (Spivack et al., 1987). In comparison sediments from Guaymas Basin that are altered to greenschist facies contain - 1 ppm B, indicating that the fixed and adsorbed boron is leached from sediments during interaction with hydrothermal fluids (Spivack et al., 1987). Consequently the fluids from these areashave significantly lower 6’ ‘B-values than those considered above (Fig. 2b). If it is assumed that fluids at Guaymas and Escanaba are mixtures of sediment-derived boron and fluids with G’ ‘B-values similar to mid-ocean ridge vent waters, the G’ ‘B-value and boron concentration of the hydrothermal fluids can be plotted as a function of water/ rock ratio (Fig. 4). The water/rock ratio is delined here as the weight of water divided by the weight of sediment required to elevate the fluid boron concentrations abovethose in midocean ridge vent waters. Following the observations in sediments from Guaymas Basin (Spivack et al., 1987) it is assumedthat boron is quantitatively leached from sediments by hydrothermal fluids. The position of the vent water data relative to the theoretical curves in Fig. 4 is compatible with removal of low 6”B boron into secondary hydrothermal minerals, such astourmaline. At a temperature of 2 15’ C
I
I
I
(b)
0
2
4 [B]
( mmol
6 kg’)
-
10
Fig. 4. Variation of 6 ’ ‘B-values with boron concentration of elastic sediment-hosted systems, assuming conservative behavior of boron: (a) Guaymas Basin; and (b) Escanaba Trough. Tick marks are water/rock ratios for reaction of the fluids with sediment. Open squares are the measured values in the vent waters.
and a pressure of 0.5-l .Okbar (the estimated conditions for the fluid-sediment reaction zone at Escanaba;Campbell et al., 1991) the B isotope fractionation factor between tourmaline and fluid (as defined on p. 114) is - 0.990-0.987 (London and Palmer, 1990). Application of Rayleigh equations (e.g., Hoefs, 1987) with the isotope fractionation factors noted above,to the Escanabadata listed in Table I, indicates that a water/rock ratio of 0.71.O and 60-70% removal of the dissolved boron into secondary phasesis required to yield the observed vent water G’‘B-values at Escanaba. At Guaymas the P-T conditions are 2903 15°C and - 0.3 kbar, yielding a B isotope fraction factor of -0.990. For the Guaymas system a water/rock ratio of 0.5-0.75 and -80% removal of dissolved boron from the
P-T
B ISOTOPE
TABLE Boron
SYSTEMATICS
OF HYDROTHERMAL
TABLE
isotope
ratios
and concentrations
MID-OCEAN
RIDGE
+32.5i-0.8 +31.5i +31.OiO.8 +32.2+ +32.6f0.8 f32.7-tO.5 +29.0? +30.0&
1 l-13”N: I 3 4 5 6 Juan de Fuca Plume TAG 2179 2187
kg-’
)
fluids
Data source’ ,
SETTINGS
0.50+0.01 0.50+0.01 0.50&0.01 0.51 kO.01 0.49+0.01 0.51 kO.01 0.54-tO.01 0.55+0.01
[II
+34.9i1.0 +36.8f 1.0 +31.7* 1.0 +36.6-t 1.0 f34.7kl.O + 34.5 rf- 1.2
0.47*0.01 0.45?0.01 0.49+0.01 0.46*0.01 0.48*0.01 0.49 t 0.01
131 131 [31 [31 I31
+29.1+ 1.1 +32.3+1.3
0.35+0.01 0.39?0.01
111 111
+26.9* +26.7?
0.56+0.01 0.52?10.01
[II
1.1 1.1
1.0 1.0
121
[II 131
111
23”N
[II [31
[II
MAR:
BACK-ARC
SEDIMENT-STARVED
1.1 1.0
[41
SETTING:
Trough: +29.8+ 1.5 +22.5? 1.1 +26.1 k 1.3
I 2 3 SEDIMENT-HOSTED
Guaymas 1177-13 1176-6 1177-6 I 173-6
Basin:
Escanaba 2040 2036 2041
Trough:
EVAPORITE
Red Sea: Atlantic II Suakin Valdivia
0.81 -to.01 0.81 kO.01 0.75?0.01
[II 111 [II
+23.2?0.6 +19.7*0.6 +16.5+0.6 +17.5f0.6
1.57kO.02 1.7350.02 1.56+0.02 1.63?0.02
131 [31 131 [31
+11.51-.4 + 10.1 +0.7 +10.4+0.5
1.71 io.02 2.16&0.04 1.90f0.04
[II
0.81 +O.Ol 0.63f0.01 0.80+0.01
[II [II [II
SYSTEMS:
111
[II
SETTING:
+29.7+0.4 +39.0+0.4 +30.2+0.4
I (continued)
Sample
6”B ( %o )
NON-MARINE
Salton Port 3 Port 4 Port 5 Port 6
EVAPORITE
[Bl (pm01 kg-r
Data source* )
SETTING:
Sea:
References: [I] [3]=Spivackand
- 1.13+0.35 - 1.53kO.27 -1.83kO.27 -2.57kO.35
38.1 kO.2 38.9 k 0.2 40.410.3 48.9kO.3
[II [II [II
[II
=this study; [2] =Spivack and Edmond Edmond (1987); [4] =Campbelletal.
(1986); (1988).
131
MAR:
MARK 1990 1986
Mariana
(wol
SEDIMENT-STARVED
21”NEPR: SW 1985 SW 1982 OBS 1985 OBS 1982 NGS 1985 NGS 1982 HG 1985 HG 1982
26”N
of hydrothermal
[Bl
6”B ( %o )
117
AND TOURMALINES
I
Sample
MARINE
FLUIDS
hydrothermal fluid into secondary phases is required to yield the observed vent water compositions. On the basis of oxygen isotope systematics Kastner ( 1982) calculated a water/ rock ratio of - 1 for reaction of the hydrothermal fluids with sediment at Guaymas. Campbell et al. ( 1991) calculated a water/rock ratio of - 5 for interaction of the sediments and hydrothermal fluids at Escanabaon the basis of the alkali element concentrations of the fluids and sediments. At both sites the water/rock ratios calculated from the B isotope systematits are lower than those estimated from oxygen isotope systematics and alkali element concentrations. Some of the discrepancy may be the result of inaccuracies causedby extrapolation of the B isotope fractionation factors outside the experimental P-T conditions (London and Palmer, 1990)) but several studies have noted that boron is the most mobile speciesyet measured during high-temperature fluid-rock interactions (Spivack and Edmond, 1987; Spivack et al., 1987; Campbell et al., 1991). Therefore, it is likely B isotope systematicswill record a higher degreeof water-rock interaction than other, less mobile, tracers. Tourmaline has not been identified in hydrothermal sediments from Guaymas Basin or EscanabaTrough, but it is commonly found in association with volcanogenic, sedimenthosted massive sulfide deposits (Slack, 1982; Slack et al., 1984). Tourmaline may not have been recognized in active hydrothermal systems due to its likely very fine grain sizein such
118
M.R. PALMER
deposits (Slack, 1982) and the lack of thorough sample coverage. It is also possible that the formation of tourmaline involves a noncrystalline, boron-rich precursor phase (Slack et al., 1984). Thus, evidence for boron mineralization may be difficult to recognize without a purposeful search. The amount of tourmaline associated with massive sulfide deposits varies from trace amounts to extensive tourmalinization (Slack, 1982; Taylor and Slack, 1984). Vent waters from Guaymas and Escanaba contain residual boron after probable formation of tourmaline, so it is not possible to make a direct comparison between the 6”Bvalues of these fluids with tourmalines from sediment-hosted settings. However, the high boron concentration in elastic sedimentsmeans that at comparatively high water/rock ratios the G’ ‘B-value of hydrothermal fluids in sedimentary settings are dominated by the 6”Bvalue of the sediments (Palmer and Slack, 1989). Hence, the G”B-values of tourmalines should be lower than the G’ ‘B-values defined by elastic sediments by the appropriate isotope fractionation factor. London and Palmer ( 1990) measuredtourmaline-fluid/vapor boron isotope fractionation at 350’ C and 0.5-2.0 kbar and obtained a fractionation factor (“B/“B) tourmaline/ ( ’ ‘B/ “13)auid % 0.9940.992, i.e. the tourmalines should be w 6-8% lighter than the sediments at 350°C. Greater isotope fractionation would be expected at lower temperatures, but most of the 6’ ‘B data for tourmalines from sediment-hosted settings fall within this offset from the 6”B-values of elastic sediments (Fig. 3 ) . 3.4. Marine
evaporite-hostedsettings
Seafloorbrine pools within the axial rift zone of the Red Sea have high salinities resulting from the interaction of circulating fluids with underlying marine evaporites. Some of the brine pools contain metalliferous sediment indicative of a hydrothermal input (Backer and Schoell, 1972). Fluid samples were analyzed
from the Valdivia, Suakin, and Atlantis II deeps. There is no correlation of the boron concentrations or isotope ratios of the brines with tracers of hydrothermal input into the basins (Backer and Schoell, 1972; Zierenberg and Shanks, 1986). This is to be expected as the degreeof interaction between the Red Sea brines and the underlying basalt is lower than at mid-ocean ridge hydrothermal sites (Zierenberg and Shanks, 1986) and the oceanic crust contains low boron concentrations. The boron concentrations of the Red Seabrines are similar to those from the Mariana site and considerably lower than those from sedimenthosted systems (Fig. 2). The high Cl levels of the Red Sea brines indicate that they have undergone considerable interaction with the evaporites (Zierenberg and Shanks, 1986)) so the low dissolved boron concentrations in the brines appear to be due to the generally low abundance of borates in marine evaporites (Holser, 1979). The G”B-value of borates from marine evaporites varies from to + 18 to + 3 1%o, reflecting varying pH conditions and local inputs from non-marine sources (Swihart et al., 1986). Tourmalines from terranes containing evidence of marine evaporites span a wide range in B isotope ratios from - 12.1 to + 18.3%0 (Palmer and Slack, 1989). A comparison of the 6’ ‘B-values of the tourmalines with those of the Red Sea brines and marine evaporite borates (Fig. 3 ) reveals that some tourmalines have much lower 6’ ‘B-values than expectedfrom the isotope fractionation factors (London and Palmer, 1990). This may be due to lower temperatures of formation of these tourmalines, but it also reflects the low concentration of boron in marine evaporites. When it is present, boron is generally restricted to evaporites concentrated beyond the halite facies (Holser, 1979). Therefore, boron in tourmalines in terranescontaining evaporite sequencesis not always derived from the evaporite. It is possible that boron concentrations may be enhancedin hydrothermal fluids that have reacted with
B ISOTOPE
SYSTEMATICS
OF HYDROTHERMAL
FLUIDS
AND
119
TOURMALINES
evaporites if the fluids then pass through clastic sediments or volcanics, as high anion concentrations in fluids promote the dissolution of rocks and transport of complexes in solution. Thus, tourmaline may be locally abundant in terranes containing marine evaporite sequences even though the boron is derived from other non-marine sources.
J----
3.5. Non-marine evaporite-hostedsettings -3.0
The Salton Sea is a restricted basin largely filled with fluvial and lacustrine elastic and evaporitic deposits derived from overflows of the Colorado River. The area is characterized by high heat production related to the northward extension of the East Pacific Rise (Elders and Sass, 1988 ). The fluids considered here were collected at sequential sampling ports at the well head during the first flow test of the Salton Sea Scientific Drilling Project in December 1985 (Elders and Sass, 1988; Thompson and Fournier, 1988). The aquifer was 1898 m deep and had an estimated formation temperature of 305 “C, leading to flashing of the fluids before the first sampling port and between each of the subsequent ports. Boron is partitioned into brine relative to vapor during phase separation, but it is sufficiently volatile for a proportion of the boron to be lost with the escaping steam (Smith et al., 1987). The effect of this phase separation on the B isotope systematics is shown in Fig. 5. The trend towards lower 6’ ‘B-values in fluids that have lost vapor through phase separation is also seen in geothermal fluids from Yellowstone National Park, Wyoming, U.S.A., and is due to the greater volatility of B (OH) 3, relative to higher complexes of boron, and the preferential partitioning of the heavy isotope, B, into the trigonally coordinated species (Palmer and Sturchio, 1990). The boron concentrations of the Salton Sea fluids are an order of magnitude higher than in other hydrothermal fluids (Fig. 2a). This is due to interaction of the circulating geothermal
-2.5
-2.0
-1.5 6
11
-1.0
-0.5
I3 PL)
Fig. 5. Variation of G”B-values with boron tions in Salton Sea well-head fluids.
concentra-
fluids with non-marine evaporites which frequently contain borates as a major mineralogic component ( Aristarian and Hurlbut, 1972). In view of the high boron levels in Salton Sea geothermal fluids it is perhaps surprising that boron-bearing minerals have not been recorded in published accounts of the mineralogy of drill core material from this area. The Salton Sea brines had a pH of 5.5 at the first surface sampling point and this fell to as low as 3.08 in the later ports (Thompson and Fournier, 1988). This suggests that the in situ pH of the fluids was higher than that measured at the surface after flashing and secondary mineral formation. Tourmaline becomes less stable at pHvalues of > 6.0 (Morgan and London, 1989) and it is possible that the Salton Sea fluids only become saturated with respect to tourmaline after they have risen to the surface. The relatively high pH of hydrothermal fluids may, therefore, limit the occurrence of tourmaline in non-marine evaporite settings, such that tourmaline is only formed if fluids are expelled into shallow sediments or into a restricted basin. It is noteworthy that some tourmalinites in the Broken Hill (New South Wales, Australia) district (where the boron is thought to have been derived from non-marine evaporites) show well-developed sedimentary features, such as graded bedding and flame struc-
120
tures (Barnes, 1980; Slack et al., 1984), that indicate production in shallow sediments. Becauseof the high boron levels characteristic of non-marine evaporites the G’ ‘B-values of hydrothermal fluids in this setting will be completely dominated by the evaporite 6”B-values. The B isotope composition of the Salton Sea fluid is towards the heavy end of the wide range in G”B-values ( - 30.1 to + 7.3%0) observed for non-marine evaporites (Swihart et al., 1986). The range of G”B-values that can be generated in tourmalines containing from boron derived from non-marine evaporites is, therefore, also large (Fig. 3). In many instances it is not possible to distinguish this source of boron on the basis of the 6’ ‘B-values of the tourmalines alone, but non-marine evaporites are the only known reservoir of boron with a light enough isotopic composition to produce the very low G’ ‘B-values observed in the Broken Hill tourmalines (Slack et al., 1989). 4. Conclusions
( 1) Hydrothermal fluids from sedimentstarvedmid-ocean ridgeshave the highest 6 ’ ‘Bvalues and lowest boron concentrations. As a consequenceof the low boron concentrations tourmaline has not been found in hydrothermal deposits from this setting. (2) Vent waters from back-arc sedimentstarved settings have higher boron concentrations than mid-ocean ridge sites due to the influence of the subducting slab, but they are low relative to most other hydrothermal sites. The G’ ‘B-value of the fluids is still high because seawateris a significant source of boron in the hydrothermal fluids. Tourmaline from this setting has heavy G”B-values, but is comparatively rare. (3) Clastic sediment-hosted hydrothermal systems have vent waters with still higher boron concentrations and lower G”B-values because of the higher concentration of boron in elastic sediments compared to basalts. The
M.R. PALMER
6’ ‘B-value of tourmalines from this setting are lower than the values in elastic sediments by the appropriate isotope fractionation factor. (4) Fluids from hydrothermal systems influenced by marine evaporites have boron concentrations similar to those from back-arc settings, but have heavier 6’ ‘B-values because the boron derived from the evaporite has a 6’ IB-value similar to that of seawater. Tourmalines from terranes containing marine evaporites have the heaviest 6’ ‘B-values, but much lighter B isotope compositions are also observed, reflecting the derivation of boron from other reservoirs due to the relatively low abundanceof boron in most marine evaporites. ( 5 ) Hydrothermal fluids from non-marine evaporite settings have the highest boron concentrations due to the high abundanceof evaporite borate minerals. There is a wide range of 6 ’ ‘B-values observedin such settings, but nonmarine evaporites contain the lowest known 6 “B-value of any boron reservoir and is the only reservoir capable of yielding the exceptionally low G”B-values seenin some tourmalines. The pH of hydrothermal fluids circulating within non-marine evaporite sequencesis at the upper limit of the tourmaline stability field and only falls after the system has been perturbed by changesin P-T conditions that cause secondary mineral precipitation. Thus, extensive tourmaline may not be formed in hydrothermal systems from this setting unless fluids are expelled into permeable strata or overlying waters. Acknowledgments
I am grateful to John Edmond for his helpful advice and frequent discussions. Rob Zierenberg kindly supplied the Red Seasamples.This work was supported by the Royal Society. I thank John Slack, George Swihart and an anonymous reviewer for their helpful comments.
B ISOTOPE
SYSTEMATIC3
OF HYDROTHERMAL
FLUIDS
AND
TOURMALINES
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