The source and significance of argon isotopes in fluid inclusions from areas of mineralization

Earth

Elsevier

ond Plonetory

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Publishers

L.effers, 79 (1986) 303-318 B.V., Amsterdam - Printed

303 in The Netherlands

131

The source and significance of argon isotopes in fluid inclusions from areasof rnineralization S. Kelley *, G. Turner ’ Physics

Received

‘, A.W. Butterfield

Deportment, ’ British

March

She/field Geological

7, 1986; revised

’ and T.J. Shepherd

University, Sheffield Survey (U.K.)

version

accepted

May

’

(U.K.)

23.1986

Argon isotopes in fluid inclusions in quartz veins associated with granite-hosted tungsten mineralization in the southwest and north of England have been investigated in detail by the 40Ar-3gAr technique. The natural argon is present as a number of discrete components which can be identified through correlations with “Ar, “Ar and “Ar induced by neutron bombardment of potassium, chlorine and calcium. The potassium-correlated component arises principally from in situ decay of potassium in solid phases in the inclusions. In the case of the Hemerdon tungsten deposit of southwest England the phases responsible are small (- 25 pm) captive authigenic micas which are shown to have been deposited from a fluid 268 * 20 Ma ago, shortly after the emplacement of the host granite. The chlorine-correlated component is present in the brines which constitute the fluid phase of the inclusions. The argon in these hydrothermal fluids is made up in part of “parentless” or “excess ” 40Ar, leached from surrounding crustal rocks, and in part of dissolved ancient atmospheric argon. Absolute concentrations of both atmospheric and excess components in the brine can be estimated from (40Ar/Cl) ratios and independent determinations of the salinity of the inclusions. The absolute concentrations of the atmospheric argon are close to those found in modern meteoric water, while those of the excess component can be interpreted in terms of the degree of interaction betwen the circulating fluids and country rock. A calcium-correlated component, with a much higher ratio of excess to atmospheric argon than that in the brine, was found to be a dominant phase in one sample from the Hemerdon deposit, indicating the presence of a solid phase (probably a CaSO, daughter mineral). Inclusions of this composition represent fluids which have had a more prolonged interaction with crustal rocks. The results obtained from this study provide a systematixation and a framework for future multi-component argon studies of fluid inclusions, together with an indication of the wide range of information which can be inferred.

1. Introduction

The study of noble gas isotopes in fluid inclusions is a subject in its infancy. An early study [l] demonstrated the presence of excess 40Ar in a number of unrelated samples and pointed to the problem this raised for K-Ar dating. Little has been published on the subject since that time. The present paper represents a preliminary study aimed at characterizing the argon content of fluid inclusions from two areas of mineralization which have been subjected to extensive study by&other methods [2-41. An early aim of the work was to investigate the possibility of dating the inclusions by the 4oAr-3gAr method. However, it soon became apparent, as in the previous work, that argon production by in situ decay of potassium is only a part, and in some cases a very minor part, of the 0012-821X/86/$03.50

0 1986 Elsevier

Science

Publishers

B.V.

argon budget of the inclusions. Argon is also present as a component in the palaeo-hydrothermal fluids trapped in the inclusions. The kind of information it contains and its possible geochemical significance is the subject of this paper. Three samples of vein quartz from tin-tungsten minera&ation associated with the Hercynian Hemerdon Ball Granite, southwest England, and one sample from the Carrock Fell tungsten mineralization, associated with the Caledonian Skiddaw Granite in northern England, were selected for analysis by the stepped heating method. A further quartz sample from Carrock Fell was crushed under vacuum to release fluid and gases for analysis. The host mineral quartz was chosen in the first instance because the production of argon isotopes from the quartz itself, by neutron activation, is minimal. Essentially all the argon which is de-

304

tected has its source in the fluid or solid inclusions occluded by the quartz during growth in the mineralizing fluids. 2. Fluid

inclusion

petrography

A characteristic feature of convectively driven hydrothermal circulation within and around granite intrusives is a change in fluid conditions with time. Depending upon the nature and magnitude of the change this may give rise to inclusions with measurably different chemical compositions within the minerals deposited (i.e. the sample may contain more than one population of inclusion). For Carrock Fell, conditions remained more or less constant throughout the main phase of quartz deposition and hence the inclusions show little variation and are predominantly two phase (liquid + vapour) types [2,3]. However, during the formation of the wolframite-bearing quartz veins at Hemerdon Ball there was vigorous mixing of highly saline magmatic fluids containing > 30 wt.% dissolved salts with low salinity local groundwaters, accompanied by periods of “boiling” [4]. This has resulted in a complex assemblage of inclusion types: 2 phase (liquid + vapour) inclusions where L 28 V 2 phase (liquid + vapour) inclusions where V > L 3 phase (liquid + vapour + solid) inclusions where L > S > V and the solid is halite only multiphase (liquid + vapour + solid) inclusions containing up IO 6 different solid phases.

The solid phases referred to above are termed “daughter minerals” and represent material precipitated within the inclusions after trapping. This distinguishes them from “captive minerals” which are microscopic mineral grains occluded during crystal growth. The latter often act as sites for fluid trapping with the result that the mineral grains become partially or totally enclosed by the liquid inclusion. Distinction is difficult but in general captive minerals are disproportionally large compared to the size of their host inclusion. Finegrained white mica (sericite) is present as captive minerals both at Carrock Fell and Hemerdon Ball. However, the trapping of fluids and captive phases was synchronous with crystal growth and thus together they preserve a record of conditions during mineral formation.

3. Experimental

methods

Prior to analysis the samples were crushed, sieved and the 0.5-1.5 mm fraction purified by electromagnetic separation and hand picking under a binocular microscope to remove contaminating mineral impurities. The samples were irradiated in position G7S of the Herald reactor in October 1983 (H79-2, H79-6, CF79-10a) and July 1984 (H79-50, CF79-lob). The earlier irradiation received a fast fluence of around 3.7 x 10” cmm2 , the second a fluence of around 2.7 X 10” cmv2. Relative fluences within each irradiated package were measured using Ni wire monitors, and J values were determined using the Hb3gr hornblende standard [5,6]. We have made use of neutron-induced reactions producing “Ar from 4oCa(n, a), 38Ar from 37Cl(n, y, p), as well as 3gAr from 3gK(n, PI, to determine abundances and abundance ratios of calcium, chlorine and potassium. Two irradiation parameters used in the abundance calculations, in addition to the conventional J value, are defined below:

(1) (2) Note that (K/Ca) and (K/Cl) are weight ratios. LY and p have been determined by measurements of the (3gAr/38Ar) and (3gAr/37Ar) ratios in Hb3gr and a comparison with the corresponding elemental ratio determined by Roddick [6]. Values for J, (Y and p and the sample weights are recorded in Table 1. It should be noted that p is particularly sensitive to the fast/ thermal ratios for the neutron flux and will vary greatly for samples irradiated in different positions within a given reactor and between different reactors. The usual interference corrections were routinely applied (see for example [6]), but were generally insignificant (< l/10%). Some samples were irradiated in evacuated breakseal ampoules made of silica, which were loaded into the UHV extraction system, broken, and the argon present in them measured. Up to 10% of the total amount of 38Ar and 3gAr were found to have been released from the samples into the ampoule. Recoil loss of argon during irradia-

305 TABLE

1

Sample

irradiation

parameters

Irradiation

a

P

Sample

Weight

SH72

0.474kO.012

5.637 f 10.013

H79-2 H79-6 CF79-10a

407 96 296

0.02321~0.00014 0.02395 *O.OOOlS 0.02370* 0.00014

SH76

0.485 f 0.026

5.849 f 0.011

H79-50 CF79-lob

145 138

0.01811 0.01799

tion has previously been noted where the grain size of the samples is very small (of the order of a few microns) [7,8]. However, the samples used in this study are quartz grains in the range 500-1500 pm diameter and even daughter minerals within the inclusions generally have sizes in excess of 10 pm. Since the recoil distances are of the order of 0.1 pm [9], it is unlikely that recoil as such is responsible for the observed release. It seems more likely that the release occurred from near surface cracks and partially opened inclusions, possibly in some cases with microcrystals within them. Temperatures of up to 200°C within the reactor may also have caused decrepitation (i.e. thermal rupture) of some inclusions, particularly those close to microfractures. For stepped heating analysis, the irradiated quartz grains, packaged in Al foil, were heated in a UHV tantalum resistance furnace (cf. [lo]). Samples were heated to successively higher temperatures, each step lasting 30 minutes. Evolved gases were gettered on Zr-Ti alloy and transferred, using charcoal at liquid nitrogen temperature, into the mass spectrometer inlet system. The gases underwent further cleanup on a second Zr-Ti getter to remove the remnants of active gases and were finally admitted into the spectrometer. The crushing experiment was carried out by loading one sample into an apparatus in which grains were crushed, by repeatedly lifting and dropping a mild steel pestle, using an external magnet. The pestle weighed 500 g and was dropped through a height of 10 cm. The gases evolved by 50 drops of the pestle were transferred and underwent cleanup in the same way as the step heated samples. Five successive crushings were undertaken, all of which gave measurable amounts of gas, although the amounts decreased as the extractions proceeded. Four samples, H79-2, H79-6, H79-50, and

(mg)

J

f 0.00011 f 0.00011

CF79-lob, were heated sequentially in steps increasing by 50°C or 100°C. Unfortunately not all of sample CF79-lob fell into the hottest part of the furnace until the 850°C temperature step. The sample was contained in three separate aluminium packages. Two of the three loaded as normal, while the third failed to do so, until being dislodged prior to the 850°C heating step. No detailed inferences concerning sample temperatures of the lower temperature steps are therefore possible; however, no gas was lost and important conclusions can still be drawn from this analysis. 4. Results-general

features

The experimental data is too extensive to publish in full and can be obtained by writing to the authors. The total release from the individual samples, obtained by adding the individual temperature steps, is given in Table 2 The release patterns from the Hemerdon Ball samples (Fig. 1) fall into two categories. One sample is rich in calcium (H79-2) while two are poor in calcium (H79-6, H79-50). In all cases the gas release occurred in three distinct portions. The first peak of release occurred between 350°C and 550°C (Fig. 1). This release is poorly represented or absent in the 37Ar (Ca-derived) and 3gAr (K-derived) release but is ubiquitous in the 38Ar (Cl-derived) release and also seen in the 4oAr* release of the Ca-poor samples ( 40Ar * is used to refer to the measured 40Ar less a standard air correction based on the 36Ar). The second peak occurred between about 700°C and 1000°C (Fig. 2). This peak of release is ubiquitous but varies in importance between isotopes and samples. The second peak is particularly prominent in all isotopes from the Ca-rich sample (H79-2). The final release peak, between 1100°C and 1500°C, is also present in

TABLE 2 Bulk concentrations of argon isotopes n and inferred chemical compositions b ‘6Ar “Ar(Ca) d ‘sAr(Cl) d 39Ar( K) d 40Ar(tot) 48Ar + = Cl’ Cl K ’ b ’ d ’ ’

H79-2 35.5 3490 2180 1190 34700 25400

H79-6 33.5 30 1900 121 12570 2650

H79-50

CF79-lob

26.3 12.8 930 348 12150 4380

72 ’ 12.6 288 21.7 26000 ’ 4500

4530 238 728

38 200 72

21 125 274

CF79-10a ’ 10.6 212 3.2 5310 2180

21 39 17

24 1.9

Concentrationsx lo-lo cm3 STP/g. Concentrations ppm in quartz. Crushed sample. Isotopes corrected for neutron induced reactions other than those derived from the element in parentheses. 40Ar* = radiogenic + excess. Large atmospheric argon blank at 850°C, caused while dislodging the sample (Fig. 3) has been removed.

the release of all isotopes. The three peak release is reproduced in all our samples. It was not the release pattern we had expected at the outset. We know from studies of other volatiles in fluid inclusions [19] that the majority of inclusions larger than 5 ym decrepitate and degas by about 600°C. This corresponds to our first release peak but cannot explain the higher temperature releases.

Moreover the proportion of gas lost in the first release peak (below 600°C) varies between samples and isotopes but is never more than 40%. Possible explanations for the higher temperature release include: (a) Contamination by minerals other than quartz, not removed during mineral separation: since the samples were all hand picked prior to irradiation, contamination can probably be ruled out as a major contribution to the argon release. (b) Daughter minerals: NaCl and other soluble alkali and alkaline earth salts probably dissolve rapidly as the temperature rises, however mineral species which do not completely dissolve before

60

Temperature I’CI

Ju

H79-2 - --H79-6 _ ..-

120

LO

80

9 20 0. t 0 P, 3 2 4

LO 0 120 BO(

H79-SO- -

-0

LOO 800 12wJ Temperature tact

1600

Fig. 1. Stepped heating release patterns for the three samples from Hemerdon Ball. “Ar, ‘sAr and 39Ar have been corrected for all contributions except neutron reactions on Ca, Cl and K respectively.

0

1

2 f.lmllim

3 4 Ho

5 400 800 1200 Tenp I*cI

1 2

3 4

t.lroclion

No

5 400 800 120016000 Pm0 I’CI

Fig. 2. In vacua crushing and stepped heating release pattern: for aliquots of the sample from Carrock Fell.

307

mass decrepitation may retain noble gases to higher temperatures. (c) Captive minerals: these respond as for insoluble daughter minerals and could be a major source of high temperature argon release. (d) Fluid inclusions too small for microscope study: inclusions < 5 pm require higher temperatures before the fluid exceeds the mechanical strength of the quartz and causes irreversible rupture of the inclusion walls. (e) Argon dissolved in the quartz structure or held in defects: this explanation could be seen as a limiting case of (d). (f) Gas-rich inclusions (V Z+ L): these decrepitate at higher temperatures than the liquid-rich inclusions and by virtue of their origin (i.e. fluid boiling) may contain a high proportion of rare gases. At Hemerdon, explanations (b), (c), (d), (e) and (f) are all possible; at Carrock Fell perhaps (c), (d) and (e) are most likely. The Carrock Fell sample, CF79-10, contains no visible daughter minerals and appeared to be an ideal sample to monitor the fluid inclusion “liquid” contribution at various temperatures. For this reason it was chosen for the in-vacua crushing experiment, which is expected to release only the gas from the liquid and vapour phases. This analysis was compared with the step heating results of the same sample (Fig. 2). “Ar levels were very low and imprecise in this sample, partly due to radioactive decay between irradiation and analysis, but also due to intrinsically low calcium concentrations in the inclusions. For this reason the individual “Ar measurements are not presented in the figure (the calcium concentration, based on the total “Ar release from the step heated sample, is estimated in Table 2 however). Firstly it must be noted that although part of the sample was not heated fully until the 850°C step, the release pattern, like that of Hemerdon Ball, shows a clear initial peak in 38Ar and 40Ar* release and not in the 39Ar release. The sample also exhibits the high temperature release peak in all isotopes, but because of the heating problems it is difficult to decide, upon the presknt evidence, whether there is a middle peak. The point to note is that a high temperature release is present even though there are no daughter minerals and only rare captive minerals visible within the fluid inclusions.

In the crushing experiment the first extraction produced by far the most gas (see Fig. 2), amounts in later extractions decreasing in an approximately exponential fashion. We were thus able to judge that the majority of inclusions had broken by the 5th extraction. By assuming the same concentration of fluid inclusions for both crushed and stepheated samples, we calculated the proportion of 38Ar, 39Ar and 4oAr* released by crushing. The crushing release shows a good correspondence with the amounts released by stepped heating up to 850°C. 38Ar and 4oAr* released by heating to 850°C was around 50-60%, while only about 10% of 39Ar from potassium was released. This pattern seems to indicate that the majority of 38Ar and 40Ar * are associated with the liquid phase whereas most of the 39Ar is associated with a solid phase (very little released by crushing). Given that visible fluid inclusions are expected to rupture by 600°C this correspondence between crushing and stepped heating release occurs at a higher temperature than expected, even allowing for the difficulties in loading the stepped heating sample. 5. Isotope and elemental correlations

The normal methods of illustrating 40Ar-39Ar data, the 40Ar/39Ar against cumulative 39Ar, “age spectrum” plot and the 40Ar/36Ar against 39Ar/ 36Ar, “isochron” plot, do not yield sensible information. The isochron plot for Hemerdon Ball illustrates the problem (Fig. 3). Most points fall above the isochron corresponding to the age of the granite intrusion, a few agree with the isochron within errors, and only one falls below the line. Thus the fluid inclusions and solids contain various amounts of excess argon. The situation for the Carrock Fell samples is far more extreme, ““Ar being completely dominated by the excess component. 4oAr/K ratios are a factor of between 2 and 80 greater than that corresponding to the time at which the Skiddaw Granite, and its associated mineralization, was emplaced into the Borrowdale Volcanic Series, the Skiddaw Slate Series and the Carrock Fell Gabbro complex [3]. The problem of distinguishing excess argon from that produced by in situ decay might appear to be an intractable one. Fortunately this is not the case. The measurement of five distinct argon isotopes provides a great deal of scope for disen-

308

ponent containing radiogenic 40Ar produced by in situ decay. Argon from the Ca-rich sample appears to contain an additional component of excess 40Ar, correlating roughly with calcium content. The basis for these statements are detailed below, first for Carrock Fell and then for Hemerdon Ball. In order to compare samples which have received different neutron fluences and different fast to thermal neutron ratios and moreover to emphasize the essential chemical nature of the correlations, all our data have been converted to molar abundances of potassium, calcium, chlorine, 40Ar and 36Ar. Molar ratios of these are used in all subsequent figures rather than the argon isotope ratios. We recommend this practice for future argon studies in that it should permit a more ready comparison between analyses carried out at different times and using different reactors.

1500 2 < 2 3 1000

500

0

20

40

60

80

100

' 120

6. Carrock Fell

%r/%r

Fig. 3. Isochron diagram showing all the data from Hemerdon Ball. The line on the figure corresponds to an age of 280 Ma, the time of intrusion of the main Cornubian granites, and a maximum age for the mineralization. Most data points lie above the line indicating the presence of excess 40Ar. (“Ar/ 36Ar) ratios have been normalised to a value of J = 0.02.

tangling multi-component mixtures provided there are five or fewer discrete components in the mixture. The components present may include mixtures of any or several of: “air” argon (40Ar, 38Ar, “jAr), argon from potassium (40Ar, 3gAr), from calcium (“Ar), from chlorine ( 38Ar) and “excess” or ‘cparentless” (40Ar). The number of discrete components present and possible end member compositions may be determined by searching for multi-component isotope correlations. The situation appears to be relatively simple for all but one of the samples, namely the Ca-rich H79-2. Natural argon in the other samples appears to approximate quite well to a three-component mixture consisting of atmospheric argon (including modern atmosphere adsorbed on the samples and as “blank” from the extraction furnace), excess 40Ar intimately mixed with ancient atmospheric 40Ar and 3bAr, which correlates very strongly with chlorine, and, a potassium-rich com-

The Carrock Fell data is presented graphically in Fig. 4, which demonstrates a clear linear correlation between (40Ar*/K) and (Cl/K). Note that 4oAr* is plotted rather than 40Ar to remove, for the time being, the added complications of the atmospheric component. Both the stepped heating data and the crushing data show very similar, though not identical, linear correlations. It is worth noting that the stepped heating data points with release temperatures below 500° C have (Cl/K) > 5 and in this respect show a correspondence with the crushing data points. This is in line with the earlier inferences that only the low temperature stepped heating data represents release from fluids. The highest (Cl/K) ratio was produced by the first in vacua crushing. The most straightforward interpretation of the correlation in Fig. 4 is that it represents a mixing line between a K-rich component, with 0 Q (Cl/K) f 0.76, and 0 < (40Ar*/K) G 7 x 10m6, and a Cl-rich component, with (Cl/K) 2 23 f 2, and (40Ar*/K) 2 (320 f 20) X lO-‘j. The (40Ar*/K) ratio corresponding to the 399 f 9 Ma age of Skiddaw granite [3] is 2.96 X 10b6. Because of the dominant effect of the Cl-rich component, the limits placed, by the mixing line, on ( 40Ar*/K), for the K-rich component are too imprecise to calculate a radiometric age. Nevertheless the data

309

0

0

10

20 Cl/K

Fig. 4. ( “eAr*/K)-(Cl/K) correlation diagram for the Carrock Fell sample. Stepped heating (open circles) and in vacua crushing data (solid squares) are plotted. The shaded area represents a range of values of (40Ar*/CI) for the chlorinecorrelated “excess” argon component from 12 x 10e6 to 15 X 10e6. Radiogenic argon is represented by the intercept, which is not sufficiently well defined to yield an age. An age of 400 Ma, that of the associated Skiddaw granite, would give rise to an intercept of 2.96X 10e6.

is clearly consistent with the 40Ar* in this component being the result of 399 Ma of in situ potassium decay. That is to say the 40Ar* in this component can sensibly be labelled “radiogenic”. On the other hand the (40Ar*/K) ratio for the Cl-rich component corresponds to a radiometric “age” greater than the age of the Earth and must therefore represent ‘Lexcess” 40Ar in the rnineralizing fluids. The (40Ar*/Cl) ratio for the “excess” component is (13.5 + 1.5) X 10T6. W$e shall return to the possible significance of this in the discussion section. We have so far identified two components, a potassium-correlated radiogenic component and a chlorine-correlated excess component. A third component, with an atmospheric ( 40Ar/36Ar) ratio,

has been implicitly assumed in the above analysis by the “air correction” which made use of 36Ar to calculate 4oAr* from 40Ar. What has not yet been established is whether this argon is adsorbed modern atmosphere or ancient atmospheric argon or a mixture of both. The 36Ar release patterns in Figs. 1 and 2 correlate in a qualitative but clear way with the chlorine-derived 3aAr, showing similar peaks in release as a function of temperature. Furthermore the amounts of 36Ar released in each temperature step are very large, and are typically 1 or 2 orders of magnitude greater than the system blanks. We see both features as strong evidence that much of the 36Ar in both groups of samples originates from the inclusions rather than the modern atmosphere. Fig. 5 represents a more quantitative illustration of the qualitative correlation between 36Ar and the Cl-rich component. If one assumes that the argon is a mixture of atmospheric argon and a Cl-correlated component, and moreover if one uses “Ar and an assumed age to eliminate that part of 40Ar which is produced by in situ decay of potassium, one can derive the following equation for a ( 36Ar/40Ar) vs. (Cl/40Ar) mixing line:

(3) where the subscript A refers to the atmospheric ratio, 3.378 X 10A3. The lines drawn in Fig. 5 are theoretical ones based on equation (3) and the range of values of ( 40Ar*/Cl) determined from Fig. 4. To a good approximation the data corresponds to a two-component mixing line between air and a chlorine-correlated component for which (40Ar/36Ar) >, 715. Note that because of the very minor proportion of 40Ar arising from decay of potassium (Fig. 4) the correction for radiogenic 40Ar in this sample is negligible. Note also that the small differences in Fig. 4 for the step heated and crushed samples are reflected by corresponding differences in Fig. 5. It is particularly interesting to note that only the very high temperature points ( > 1200 o C) cluster towards the modem atmospheric ratio while all the low and intermediate temperature points cluster around the lower values of (36Ar/40Ar). If the Cl-correlated component contained no 36Ar

310

one would expect the correlation in Fig. 5 to extend to much lower values of (36Ar/40Ar), i.e. close to zero, determined essentially by the 36Ar blank levels. The strong grouping in Fig. 5 suggests that the extreme value of (40Ar/36Ar) is a measure of the actual (40Ar/36Ar) ratio of the Cl-correlated argon in the inclusions. If this is the case the extreme ratio of 715 implies that roughly 60% of the 40Ar in the chlorine-rich fluid is the parentless 40Ar* inferred from Fig. 4, while 40% is ancient atmospheric argon, probably brought in by circulating meteoric water. We shall return to the detailed significance of this observation later. 7. Hemerdon Ball

In the first instance we shall discuss the two Ca-poor samples (H79-50 and H79-6), treating them in an extirely similar fashion to the Carrock Fell samples. They contain small amounts of

0.3

I

0

10000

20000

I

30000

I

I

40000

I

50000

CI/LOAr Fig. 5. (36Ar/40Ar)-(Cl/40Ar) correlation diagram. Stepped heating data (open circles) and in vacua crushing data (solid squares) are shown. The shaded area corresponds to the shaded area in Fig. 4.

calcium (Fig. I), most of which is associated with the middle and high temperature release peaks. The low temperature steps of H79-50 exhibit the highest Cl/K ratios in the same way as the CF7910 sample. All higher temperature steps yield lower Cl/K ratios. In contrast both the low and high temperature releases of H79-6 yield high Cl/K ratios. However, despite the differences in release patterns (Fig. 1) and abundances (Table 2) of these two samples, they fall on the same mixing line (Fig. 6a, solid points). This (40Ar*/K) VS. (Cl/K) diagram indicates mixing between a Cl40Ar*-rich end member, which is more prominent in H79-6, and a K-rich end member, which dominates H79-50. The (40Ar*/Cl) ratio of the Cl-rich end member lies between the limits 1.47 X 10e6 (slope of mixing line) and 1.7 x 10m6 (40Ar*/Cl for the right most extreme data point). These ratios are a factor of 10 lower than the corresponding values for the Carrock Fell sample. Furthermore both H79-6 and H79-50 are richer in potassium with the result that the radiogenic component in this case is not swamped by excess argon and can be used to calculate a radiometric age. (40Ar*/K) for the K-rich component is obtained by extrapolating the mixing line in Fig. 6 to the region between the axis, (Cl/K) = 0, and the left most data point, (Cl/K) = 0.05. A best fit to all the data points indicates (40Ar*/K) = (1.99 f 0.03) X 10m6 and corresponds to an age of 271 + 4 Ma (MSWD = 3.5). An alternative method of arriving at an age, which we prefer, is to use the best fit line to define the slope of the mixing line (i.e. to define the composition of the Cl-rich component) and to use this slope to extrapolate individual K-rich data points back to the region of the K-rich component 0 < (Cl/K) f 0.05 (i.e. close to the (40Ar*/K) axis). A weighted mean based on an extrapolation of the 16 data poinrs with (Cl/K) < 1 yields (40Ar*/K) = (1.96 + 0.14) x low6 and an age of 268 f 20 Ma for the Hemerdon tin-tungsten mineralization. This is indistinguishable from an Rb-Sr fluid inclusion age of 269 + 4 Ma for the main stage Hercynian mineralization in southwest England [ll]. The third Hemerdon sample, H79-2, was richer in potassium, 40Ar, and especially calcium (Table 2), than H79-6 and H79-50. The high (Cl/K) ratios exhibited by the other samples in the low

311

temperature steps were seen in only two extractions from H79-2. H79-2 does not show the simple linear two-component correlations displayed by the other samples, indicating the presence of at least one other component. This aspect can be illustrated by reference to Fig. 6d in which the data is represented as a three-dimensional plot of the ratios (40Ar*/K), (Cl/K) and (Ca/K). Three subplots, 6a-c, represent projections of the data onto the three orthogonal planes. In general three-component mixtures define a mixing plane

10

0 10 l? LL. ‘631-r 10 I6L6. ‘I*,-.

-

in such a plot. A test of whether the data points are coplanar is sufficient to indicate that the data can be described in terms of a three component mixture. If the points are not coplanar, a minimum of four components is required. H79-2 (open symbols) plots in a separate field in the three subplots of Fig. 6, demonstrating the clear distinction between it and the other samples. In the graph of (Ca/K) vs. (Cl/K) for example it plots mainly in the Ca-rich, Cl-poor field. In the graph of (40Ar*/K) vs. (Cl/K) the data points lie in a wedge above the mixing line discussed previously. This latter observation implies the presence of an additional component of “excess” 40Ar* uncorrelated with (Cl/K) but possibly associated with the high calcium component. In order to test whether the data is consistent with a three-component mixture we attempted to fit the points to a mixing plane. Fig. 7 represents an “edge on” view of the best fit plane, corresponding to a (45O) rotation about the ( 40Ar*/K)

6

I !“‘, 2

4 Cl/K

6

8 12

Fig. 6. Correlation diagrams for the Hemerdon Ball samples. The solid symbols are Ca-poor, the open symbols Ca-rich. (a) (40Ar*/K)-(Cl/K) diagram. The Ca-poor samples define a two-component mixture between Cl-correlated “excess” argon and K-correlated radiogenic argon. The intercept defined by 16 K-rich points corresponds to an age for the mineralization of 268 f20 Ma. The Ca-rich samples lie above the indicated correlation line due zo the presence of an additional Ca-correlated excess component. (b) (40Ar*/K)-(Ca/K) diagram. (c) (Ca/K)-(Cl/K) diagram, illustrating the marked difference in chemistry of the Ca-poor and Ca-rich samples. (d) Diagrammatic representation of the data as a three-dimensional correlation plane.

0 H79-2 n H79-6 l

0

2

4

H79-50

6

a

10

(A.Ca + &Cl) / K Fig. 7. “Edge on” view of the correlation plane in Fig. 6. The abscissa is a linear combination of (Ca/K) and (Cl/K) with coefficients A = B = 0.707.

312

axis. The correlation is not perfect but it does exist and indicates that the data can be described to a first approximation as a three-component mixture. Two data points have been omitted from Fig. 7, namely the data points corresponding to the 950-1250°C release peak. These points with a high (Ca/K) fall well below the mixing plane defined essentially by the major (350-9OO’C) release. Clearly then the Ca-rich component is complex and contains either two distinct components, B and D in Fig. 6d, or range of compositions. The situation with regard to 36Ar in the Hemerdon Ball inclusions is represented in Fig. 8. (N.B.: in situ radiogenic 40Ar has been subtracted from the data points. Those points where the correction is greater than 50% have been omitted

because of the increasingly large errors introduced in (36Ar/40Ar) by the correction.) The behaviour of the Ca-poor samples (solid symbols) is similar to that of the Carrock Fell samples in that much of the 36Ar appears to be present in the inclusions, for which ( 40Ar/36Ar) > 385. Thus 77% (296/385) of the 40Ar in the Cl-rich component appears to represent ancient atmospheric argon brought in by meteoric water with the other 23% being “excess” parentless 40Ar *. In contrast the Ca-rich samples (open symbols) are characterized by very low (36Ar/40Ar) ratios and correspondingly high (40Ar/36Ar) ratios, >, 9600. Parentless 4oAr* thus accounts for more than 97% of the 40Ar in the Ca-rich component and must clearly have been introduced quite separately from that associated with chlorine. 8. Discussion

0

0

0 0

0

HP-2

n

Iin-6

l

I-m-50

00 ,o,

0

50000

100000

150000

200000

CI/LOAr

Fig. 8. (36Ar/40Ar)-(Cl/40Ar)) correlation diagram for the Hemerdon Ball samples. A correction for 40Ar produced by in situ decay has been applied based on (K/40Ar). Data points where the correction is greater than 50% have large errors and are omitted. 40Ar on this figure is a mixture of atmospheric and “excess” or “parentless” argon. The absence of low values for (36Ar/40Ar) in the Ca-poor samples (solid symbols) is an indication of the presence of dissolved palaeoatmospheric argon in the inclusion fluids.

The preceding analysis has shown the presence of the following distinct components present in some or all of the samples analysed. (1) A K-rich component containing 4oAr, produced by in situ radioactive decay. This radiogenic 40Ar in the Carrock Fell samples is minor ( = 1%) in comparison to the total 40Ar and cannot be used to infer a radiometric age. In contrast for Hemerdon Ball the (40Ar/K) ratio is relatively well constrained and corresponds to an “age” of 268 f 20 Ma. (2) A Cl-rich component containing “excess” or “ parentless ” 40Ar intimately mixed with ancient atmospheric argon, all of which were presumably dissolved in the hydrothermal fluids now trapped in the inclusions. (3) A Ca-rich component dominant in one of the Hemerdon Bell samples and containing parentless 40Ar but relatively little atmospheric argon. The ratio of 40Ar to calcium in this component is variable implying that it is itself a mixture of two or more components. (4) Atmospheric argon not associated with chlorine. Modern atmospheric argon is present as a “blank” in the extraction system. We believe that this is a minor component, mainly observed in the highest temperature extractions, and cannot account for the large amounts of 36Ar associated with the chlorine-rich component. We cannot exclude the possibility that some of the (chlorine-

313

free) atmospheric argon is itself ancient argon. The compositions of these components are summarized in Table 3. Before discussing the implications of these observations it is worth making a general comment about the way in which the separation into components has been carried out. In the previous section this was done largely by reducing the problem of a multi-component mixture to a series of problems involving two-component mixtures, which could be represented by simple mixing lines. This method of presenting the analysis was chosen partly because it was the one we first adopted and partly because it provides a convenient graphical representation of the data which, though somewhat long winded, is we hope most easily accepted and understood by the reader. In future studies it seems likely that a purely analytical approach, for example making use of factor analysis, could be used to provide a more immediate, if less transparent, resolution into components. The major questions which are raised by the observations, are: (1) What do the different components represent physically? (2) What is the significance of the radiometric age of the Hemerdon Ball samples? (3) What is the significance of the relative proportions of 4oAr*, atmospheric 40Ar, chlorine and calcium? (4) What is the significance of the absolute concentrations? (5) What is the future for this kind of analysis? The crushing experiment, coupled with the as-

sociation of the low temperature release with the rupture of inclusions, leads us to interpret the chlorine correlated argon as gas dissolved in the fluid phase of the inclusions. The interpretation is consistent with the observation that the fluids are essentially NaCl brines [2-41. From the evidence of our step-heating analysis, the main potassium-bearing phase is not dissolved in the fluid but is contained within solids, either as daughter or captive mineral phases or as a trace element within the quartz lattice. The K-rich white micas seen using the SEM technique are the best candidates. The 40Ar-39Ar age obtained through geochemical correlations therefore represents cooling of the liquids (and of the suspended white micas) below the blocking temperature for that mica, most likely around 35O”C, bearing in mind the fast cooling rates involved in Sn-W mineralization [20]. The age inferred, 268-20 Ma, is indistinguishable from the Rb-Sr age of 269 f 4 Ma obtained by Darbyshire and Shepherd [ll] from fluid inclusions of the related South Crofty Sn-W deposit. Although the errors we have assigned to the age are much larger than those of the Rb-Sr age, the result may be regarded as supporting the hypothesis of Darbyshire and Shepherd, that the Sn-W mineralization in southwest England is significantly younger than the intrusion of the main Cornubian granites which were intruded ca. 290-280 Ma ago [ll]. Although much of the 36Ar and chlorine are contained in fluid inclusions the extensive release of both species at temperatures in excess of 800°C

TABLE 3 Molar composition of major components released by stepped heating Carrock

Cl

K

Ca

El Q 0.76

< 0.043 51

< 0.05 1*5

=1 Q 0.05 $0.04 < 0.02

$ 0.15 ml ( 0.16 4 0.05

40Ar

36Ar

Fell

Component M n Component K Henterdon

Component Component Component Component

3.9 x 10-s

r,

Ball

M ’ K B D

(13.5*1.5)x10-s c 7x10-6

d 0.05 Q 0.05 I1 Gl

(1.6kO.1)~10-~ (1.96f0.14)x10-6 1.3x10-6 0.25 x lO+

1.8x10-s <1.9x10-9 g1.4xlo-‘e

’ Chlorine-rich component M is discussed in the text. Apart from the presence of atmospheric argon it is identical to the “air corrected” component X.

314

(see Fig. 1) suggests to us that a significant proportion may be present within the quartz lattice itself possibly in defects or submicroscopic inclusions. In order to test this possibility further we plan to carry out measurements on vein quartz, poor in inclusions. The form of the high temperature release and the bulk concentrations of “jAr are remarkably similar to those observed for supposed “palaeoatmospheric” argon in cherts [16,17]. This observation is perhaps not too surprising in view of the fact that both are deposited from meteoric aqueous solutions. Although the deposition occurs at substantially different temperatures in the two cases, the original equilibration of the meteoric waters with atmospheric gases probably occurred at very similar temperatures. The Ca-rich component seen in H79-2 undoubtedly arises from a solid calcium salt present in this one sample, probably as a daughter mineral. The mineral responsible has not been specifically identified but is most probably CaSO, [4]. Ironically H79-2, the most complicated of all the samples, was the first one we analysed! The significance of the relative proportions of 4oAr*, atmospheric 40Ar, chlorine and potassium can be understood by reference to Fig. 9. Fig. 9, is a three-dimensional 4-isotope plot of ( 36Ar/40Ar), (K/40Ar) and (C1/40Ar). Ignoring for the moment the Ca-rich Hemerdon samples, the experimental data for both Hemerdon and Carrock Fell plot on (separate) three-component correlation planes corresponding to the shaded area AKM. Various hypothetical and actual end members are shown in Fig. 9 and represent the following: A -

Pure atmospheric argon, we have argued that some of this represents contamination of samples and/or furnace by the modern atmosphere and is of no fundamental interest. We cannot exclude the possibility that some represents trapped ancient atmospheric gas, distinct from C below. This latter interpretation appears particularly attractive for the Hemerdon samples in ‘view of the evidence referred to earlier for the admixture of low salinity local groundwaters [41* K - Radiogenic argon, equivalent to the intercept in Figs. 4 and 6a. Note that Figs. 5 and 8 are obtained by projecting from point K,

Fig. 9. Diagrammatic representation of a three-dimensional ( 36Ar/40Ar)-(K/40Ar)-(Cl/40Ar) correlation diagram showing measured and hypothetical end member components. A = atmospheric argon, E = excess of parentless argon, K = in situ radiogenic argon, C = palaeoatmospheric argon dissolved in chlorine-rich brine, M= mixture of E and C in the brine, X = composition of chlorine, correlated excess component obtained by applying a “standard air correction” to the data. Measured data lies inside the shaded area and is a mixture of A, K and M.

E

C

M

X

through individual data points onto the plane (K/40Ar) = 0. - Parentless or excess 40Ar with no associated potassium (or “Ar or Cl). - This point corresponds to the composition of brine containing dissolved (ancient) atmospheric argon. Note that the line CA is horizontal. -An intimate mixture of C and E. We have argued that M is determined (approximately) by the end points of the correlations in Figs. 5 and 8. - “Chlorine-correlated” excess 40Ar. The line KX is obtained by applying a conventional air correction to the data and is equivalent to the (40Ar*/K) vs. (Cl/K) correlation in Figs. 4 and 6a (note that most of the Car-

315 04

n HI9 - 6 . HI9 - 50

0.3

N 2 x

K/L'A,

x105

Fig. 10. Hemerdon Ball data from Ca-poor samples plotted in the form of Fig. 9. Note that the data lie inside a fairly well defined triangle. The line MK in Fig. 9 is particularly well delineated indicating the presence of substantial amounts of atmospheric argon in the brine (cf. Fig. 8). The intercept of the plane with the (K/40Ar) axis, point K, indicates (40Ar/K) = (1.94f0.09) x 10-6corresponding to an age for the mineralization of (265 + 13) Ma.

2 s \ 2 d

0.2

0.1

0

rock Fell data in Fig. 4 would plot close to X in the representation of Fig. 9). Fig. 10 is a three-dimensional view of the actual Hemerdon data showing the best fit plane. Note that there is a very clear “edge” to the data field corresponding to the line MK in Fig. 9. This is a further illustration that the composition of M is well defined and indicates that the fluids contain dissolved atmospheric argon in addition to chlorine and excess 40Ar. The quality of the fit is shown in Fig. 11, which is an “edge on” view obtained by rotating the data in Fig. 10 about the (36Ar/40Ar) axis. The 40Ar/K ratio obtained for point K from this three-dimensional fit is (1.94 f 0.09) x 10m6 and corresponds to an age of 265 + 13 Ma. This age is based on an unweighted least squares fit from which two points with very large errors were excluded. It is clearly consistent with the age of 268 f 20 Ma given earlier and the Rb-Sr age of 269 f 4 Ma [ll]. Aside from chronological information contained in point K the other key item of information from the present experiment is to be found in the estimate of the composition of M. From it two useful parameters follow: (a) the ratio of atmospheric 40Ar to chlorine in the brines (i.e. the composition of point C), and (b) the ratio of

200000 (A.K+ B.Cl)/%r

400000

Fig. 11. “Edge on” view of the correlation plane in Fig. 10. The abscissa is a linear combination of (K/40Ar) and (Cl/40Ar) with coefficients A = 0.850 and B = 0.526.

excess 4”Ar to atmospheric 40Ar in the inclusions, (CM/ME).

The composition of C and the ratio (CM/ME) can be used to estimate the absolute concentrations of atmospheric 40Ar and excess 40Ar if the chlorine concentration can be determined by independent means. NaCl concentrations expressed as wt.% NaCl equivalents have been measured for the present samples by conventional thermometric techniques using a Linkam TH 600 heating/ freezing stage [2,4,12] and are indicated in Table 4. The salinity values for Carrock Fell are well defined as might be expected for samples containing only one type of inclusion. For Hemerdon the data is very much a first-order estimate because each sample contains an unknown proportion of the four types of inclusion. However, the Hemerdon samples were selected because their bulk salinity was controlled by a predominance of halite bearing and multiphase inclusions. Hence the NaCl concentrations shown in Table 4 are quite realistic and at least 3 times greater than those of Carrock

316 TABLE 4 Argon concentrations in trapped fluids

Carrock Fell Hemerdon Ball (Ca-poor) (Ca-rich) Water o”c 2o”c Sea water 0°C 2o”c 10% NaCl o”c Solution 20°c

8zt2 30*5

3.5 3.5

325

480

0.60

506

152

0.23 0.97

520 320

-

-

408 260

10

252

10

176

Fell. The corresponding 40Ar concentrations are shown for all the samples analysed based on the estimates of composition A4 from Figs. 5 and 8. The concentration of atmospheric 40Ar within the two fluids is remarkably similar bearing in mind the different temperatures, pressures and chemistry involved in their generation as well as their spatial and temporal separation. The Carrock Fell sample contains 325 f 80 $/l of atmospheric 40Ar while the Hemerdon samples contain 506 f 80 pi/l (Table 4). The atmospheric argon within the inclusions must have originated at the surface at some time, but there may be several methods of transporting it to depth. Two major possibilities are: (1) Percolation of groundwaters, retaining all or part of their noble gas budget, into the hydrothermal system. (2) Trapping of atmospheric argon in the lattices of water-bearing minerals and their burial by sedimentation or tectonics. An alternative possibility that the 36Ar may be mantle derived is not considered likely, since the concentration of 36Ar in the inclusion fluids is very large. The similarity of atmospheric argon contents leads us to think that the mechanism was the same for all samples. The experimental work of Weiss [13] and Benson and Krause [14] makes it possible to calculate the argon concentrations in water in equilibrium

with the atmosphere for a range of temperatures and salinities. The expected atmospheric argon concentrations for meteoric water, seawater and 10% NaCl solution at 0°C and 20°C have been calculated and included in Table 4. The argon concentrations for meteoric water and seawater over the likely range of surface temperatures encompass the values found in the fluid inclusions, whereas a fluid with higher salinity is saturated at lower concentrations. Higher temperatures decrease the solubility to about 100°C then increase it but temperatures this high would be unrealistic for equilibration of surface waters with the atmosphere. The present salinity of the inclusions has of course no bearing on these calculations since the high NaCl concentration results from interactions within the hydrothermal system after the fluids have been isolated from equilibration with the atmosphere. The data are not able to distinguish between meteoric water and seawater as the source of the hydrothermal fluids. It is obvious, however, that groundwater percolating through the rocks from the surface and being entrained by the hydrothermal system is the most likely source of atmospheric argon in the inclusions. The coincidence of atmospheric argon concentrations measured within the inclusions and calculated concentrations establishes that the groundwaters retained their full surface argon budget, despite burial and heating as high as 400°C [4] in the case of the Hemerdon Ball samples. Previous studies of noble gas retention in groundwaters have established full retention in waters heated to only 60°C 1151. In the case of the Hemerdon samples the coincidence is a little surprising in view of the evidence for boiling of the fluids reported by Shepherd et al. [4]. The formation of a separate vapour phase would be expected to degas the fluids of their argon content leading to low and possibly variable concentrations. The coincidence between the observed concentrations and those expected for meteoric water suggests that if boiling did occur the vapour and liquid remained a closed system, on a sufficiently small scale, or that the fluids we have analysed are not dominated by the effects of boiling. The analysis of individual inclusions with different ratios of liquid to vapour is planned using pulsed laser probe outgassing as a way of investigating this problem [18].

317

In contrast to the dissolved atmospheric gas the concentration of excess argon within the inclusion fluids from Carrock Fell and Hemerdon Ball do not coincide (Table 4). The Carrock Fell inclusions contain three times as much excess argon as the Ca-poor Hemerdon Ball inclusions. This must presumably be related to the extent of fluid/wall rock interaction during hydrothermal circulation and the concentrations may provide a semiquantitative indication of the extent of that interaction. It is possible to calculate, for example, what product of potassium abundance and time would be required to give rise to the observed concentrations. The production rate of 40Ar from potassium was 4.8 X lo-’ ~1 g-’ a-’ at the time of the Carrock Fell mineralization. Dividing the concentrations of excess argon in Table 4 by this figure we see that the hydrothermal fluids from the Carrock Fell inclusions leached the “argon equivalent” of 10” g of a K per litre of fluid. The corresponding figure for Hemerdon, based on a production rate of 4.5 X 10eq ~1 g-’ a-‘, is 3.5 X 1O’Og a 1-r. The calculations may be taken a stage further by making assumptions about the source rocks for the excess argon. The Carrock Fell sample is surrounded by Skiddaw Slates, which were deposited around 100 Ma before the mineralization and contain around 2% potassium. If these were the source of the excess argon, approximately 48 g of country rock would be required to be totally stripped of its radiogenic 40Ar to provide the excess 40Ar in 1 g of hydrothermal fluid. Devonian sediments surrounding the Hemerdon’ Ball granite were also around 100 Ma old at the time of mineralization and contain around 2% potassium. The 40Ar from 17 g of this country rock would be needed to provide the excess 40Ar in 1 g of fluid. As a way of measuring the extent of water rock interaction argon has one important advantage over other species in the fluid and that is its inertness. Once leaching into the f$uid has occurred the argon is likely to be retamed, in contrast to the chemically reactive species which may enter and leave the fluid more readily. The excess 40Ar is thus a cumulative measure of water rock interaction. This statement needs to be qualified in situa-

tions where separate phases form into which the argon may partition by physical processes. Dissolution into daughter minerals may occur, for example, though partition coefficients for noble gases are likely to favour retention in the liquid. A more serious case of open system behaviour occurs in boiling. Here partitioning is overwhelming in favour of the vapour. However, even in situations where physical partitioning occurs the argon will of course retain its isotopic composition. In particular the ratio of excess argon to atmospheric argon will be retained in both phases at the time of partitioning. Thus in situations where absolute concentrations cannot be determined or where they appear to have been affected by open system behaviour the isotopic composition may still provide a measure of water rock interaction. The Ca-rich phase of H79-2, presumed to be CaSO, and an associated fluid, represents a situation where our present level of interpretation must be based on isotopic composition. We have no convincing way at the moment of calculating absolute concentrations of 40Ar in this phase. The only quantitative statement which can be made therefore is that expressed earlier relating to the much higher proportion of excess 40Ar in this phase (Table 4). Qualitatively this is a perfectly understandable result in that Ca-rich inclusions are ascribed by Shepherd et al. [4] to the influx of late Ca-rich fluids. These later fluids would have had a greater opportunity to interact with country rock and accumulate excess 40Ar. It is also possible that they may have lost atmospheric @Ar as a result of boiling. No more definitive statement can be made on the evidence of just a single sample at present. We have presented several graphical approaches to the problem of disentangling the argon components. In some cases the data has been reduced to simple two-dimensional graphs, in others three-dimensional graphs have been used. In the early part of the paper atmospheric argon was eliminated from the data and ratios were given relative to potassium. In the later part ratios relative to 40Ar were used. We have resisted the temptation to choose one representation over another in this preliminary paper, preferring to let the reader see how the problem appears from different viewpoints. Nevertheless of the approaches adopted we feel that the one involving

318

ratios relative to 40Ar (e.g. Figs. 9 and 10) has most to recommend it since all end members, including air, contain 40Ar and therefore plot on the figure (rather than at infinity). The extension of Fig. 9 to incorporate calcium is relatively straightforward but would add nothing to our conclusions and has therefore not been included in this paper. In this study we have clearly demonstrated that, by using the “aAr-39Ar technique and taking note of correlations arising from multi-component mixtures, it is possible to provide a simple understanding of what is at first sight a very complex and intractable problem. In doing so we have provided a framework for more detailed regional studies of argon in palaeohydrothermal fluids and for studies of individual inclusions within a rock section using the laser probe. We are actively pursuing both lines of investigation together with measurements of the other noble gases. Helium in particular may be interesting in view of the potential for detecting ancient mantle helium by way of 3He. Acknowledgements

We wish to thank David Blagburn for his great skill and help with all aspects of the mass spectrometry and Mrs. Elaine Lycett for typing and endlessly revising the manuscript. We also acknowledge support from the Natural Environment Research Council (S.K. and A.W.B.) through a research grant (GR3/5004). References 1 S.N.I. Rama, S.R. Hart and E. Roedder. Excess radiogenic argon in fluid inclusions, J. Geophys. Res. 70, 509-511, 1965. 2 T.J. Shepherd, R.D. Beckinsale, C.C. Rundle, J. Durham, Genesis of Carrock Fell Tungsten Deposits, Cumbria: fluid inclusion and isotopic study, Trans. Inst. Min. Metall. 85, B63-73,1976. 3 T.K. Ball, N.J. Forty and T.K. Shepherd, Mineralization at the Carrock Fell tungsten mine, N. England: paragenetic, fluid inclusion and geochemical study, Mineral. Deposita 20, 57-65.1985.

4 T.J. Shepherd, M.F. Miller, R.C. Scrivener and D.P.F. Darbyshire. Hydrothermal fluid evolution in relation to mineralization in southwest England with special reference to the Dartmoor-Bodmin area, in: High Heat Production (HHP) Granites, Hydrothermal Circulation and Ore genesis, Inst. Min. Metall. Symp. Proc., pp. 345-364. 1985. 5 G. Turner, J.C. Huneke, F.A. Podosek, G.J. Wasserburg, 40Ar-39Ar ages and cosmic ray exposure ages of Apollo 14 samples, Earth Planet. Sci. Len. 12, 19-35, 1971. 6 J.C. Roddick, High precision intercalibration of 40Ar-39Ar standards, Geochim. Cosmochim. Acta 47, 887-898, 1983. 7 J.C. Huneke and S.P. Smith, The realities of recoil: 39Ar recoil out of small grains and anomalous age patterns in 40Ar-39Ar dating, Proc. 7th Lunar Sci. Conf., pp. 1997-2008, 1976. 8 K.A. Foland, J.S. Linder, T.E. Laskowski and N.K. Gran, 40Ar-39Ar dating of Glauconites: measured 39Ar recoil loss from well-crystallized specimens, Isot. Geosci. 2, 241-264, 1984. 9 G. Turner and P.H. Cagodan. Possible effects of 39Ar recoil in @‘Ar-“Ar dating, Proc. 5th Lunar Sci. Conf., pp. 1601-1615, 1974. 10 T. Staudacher. E.K. Jessberger, D. Diirflinger and J. Kiko, Refined ultrahigh vacuum furnace for rare gas analysis, J. Phys. E: Sci. Instrum.. 1978. 11 D.P.F. Darbyshire and T.J. Shepherd, Chronology of granite magmatism and associated mineralization, SW England, J. Geol. Sot. London 142.1159-1178, 1985. 12 T.J. Shepherd, A.H. Rankin and D.H.M. Alderton, A Practical Guide to Fluid Inclusion Studies, 239 pp., Blackie & Son Ltd., 1985. 13 R.F. Weiss, The solubility of nitrogen, oxygen and argon in water and sea water, Deep Sea Res. 17, 721-735, 1970. 14 B.B. Benson and D. Krause, Empirical laws for dilute aqueous solutions of nonpolar gases, J. Chem. Phys. 64, 689-709, 1976. 15 E. Mazor, Palaeotemperatures and other hydrogeological parameters deduced from noble gases dissolved in groundwaters; Jordan Rift Valley, Israel, Geochim. Cosmochim. Acta 36, 1321-1336, 1972. 16 G. Turner and C.M. Jones, Atmospheric argon in cherts, Abstr. 5th Int. Conf. on Geochronology, Cosmochronology and Isotope Geology, Nikko National Park, Tokyo, pp. 372-373,1982. 17 G. Turner, C.M. Jones and A.W. Butterfield, Ancient atmospheric argon in cherts, Conference on Planetary Volatile& LPI Tech. Rep. 83-01, 188-189, 1983. 18 P.M. McConville, Development of a laser probe for argon isotope studies, Ph.D. Thesis, University of Sheffield, 1985. 19 E. Roedder, Fluid Inclusions, Reviews in Mineralogy, 12, Mineralogical Society of America, 1984. 20 J.W. Purdy, E. Jager, K-Ar ages of rock forming minerals from the Central Alps, Mem. Inst. Geol. Min. Univ. Padova 30, l-31, 1976.