Rhenium contents of rocks

Geochimica et Cosmochinlica Artx, 1961, Vol. 25, pp. 232 to 235. Pergamon Press Ltd. Printed in Nnrthrrn Irchnd

GEO~MI~

NOTES

Rhenium contents

of mks

WE have recently developed a neutron-activation method for the analysis of rhenium in rocks and some of the results obtained are reported here. Full details of the analytical procedure have been published elsewhere (MORRIS and FIFIELD, 1961). Radioactivation has special advantages as an analytical method from the points of view of sensitivity, specificity and the avoidance of contamination. Nevertheless, it is felt that the results quoted here should be taken as maximum values, since despite extensive precautions taken in the preparation of rock samples for analysis a complete freedom from contamination cannot be guarant~d. in Table 1 are shown our results for rhenium in the standard granite G-I and in the standard diabase W-I. Table 2 records results for the rhenium content of gabbroid rocks from the Insch Xass,

Table

1. Rhenium analyses of the standard granit8e (?-1 and the standard diabase W-1

W-f

G-l (Re ._-__.-

.__-

f'.p."_)

(Re p.p.m.) ______~.__I..-.

..__~_-_._-__.

0~00052 0.00084 0~00057 0~00071 0~00076 0~00061 if\T.0~00067

-_____ . .. __

0~00070

I I

/

0~00059 0*00083 0~00056 0.00084

Kv. 0,0007 1

Table 2. Rhenium content of some igneous rocks Rook 3645 3731 3815 3818 5156 5157 6159 6160 5169 5799

Re p.p.m.

Syenogabbro, 8. slope of Crallow Hill, Aberdeenshire Syenite, S.W. slope of Gallow Hill, Aberdeenshire Troctolite, 400 yds. W. of Barra Castle, Aberdeenshire Hybridized dionite, S. of Druminor House, Chat& Aberdeenshire ~bite-riebeekite-panic, Liruei Biotite-granite, Liruei Quartz-pyroxene-fayalite-porphyry, Liruei Rhyolite, Liruei Riebeckite-aegirine-granite, Liruei Comendite, Liruei

232

0~00036 0*00018 0~00031 0~00016 0.00112 0~00022 0~00029 0~00114 0*00025 0.00033

Geochemioal notes

A~~~ns~e, Scotland, and of grsnitic rooks from Liruei, Nigeria. We are indebted to Dr. J. R. BUTLER for supplying us with samples of these rocks; the sample numbers listed are those of the Department of Geology, Imperial College, London, Portions of “Specpure” silica (Johnson M&they and Co.) were also analysed by our method and showed rhenium oontents of O*OlOand 0.012 p_p.m. The samples had presumably been slightly contaminated in the production process and this illustrates the difficulty of obtaining reliable data for such a rare element as rhenium in rooks. D. F. C. Monms F. W. EZFIELD REFERENCE MORRIS

D. F. C. and ?&FIELD F. W. (1961) The determination of rhenium in rooks by neutron-

activation anslyeis. TaZmzk, 8, 612418.

pH of a natural hydrothermal solution IN comment on the geoohemical note by A. J. ELL.IS(1960) in this journal it is desired to point out a necessary modification in oalculating the pH of hydrothermal solutions. The content of bicarbonate in samples taken at the surface from bore discharges is variable and cannot be used in injunction with the carbon dioxide content of the whole discharge to calculate the pH of the underground water. Both boric and silicic acids, although weaker acids than carbonic acid, are in such high concentration that the presence of borate and silicate ions must be considered. The best samples of water are obtained from the weir boxes of twin tower silencers (SMITH 1958,p. 363). This water is separated at atmospheric pressure from about 30 per cent of steam and most of the carbon dioxide is removed with the steam. When the sample is cold, the pH is high enough for the contents of borate and silicate ions to be appreciable. In the original water underground the higher con~ntr&tion of carbon dioxide causes the replacement of borate and silio&e ions by bicarbonate ion. To illustrate this I have taken the average composition of the water from five bores at Wairakei, New Zealand, with water of the highest silica content. The discharge of the bore No. 31 of which the composition is given by ELLIS is low in enthalpy from loss of steam underground by withdmwal in other bores. The pH caloul&ed from the average composition given here should be closer to that of the original hydrothermal water. The average composition of five samples of water separated at atmospheric pressure from bores 27, 46, 48, 55, 81 is-Ch63.5 m.p.m. (moles per million grams); HCO,-:0*15 m.p.m.; total B as HBO,:2+35 m.p.m.; total Si as SiO,:944 m.p.m.; pH at 18”C:855. The average enthalpy of the discharge was 465 B.t.u./lb (water fraction at atmos. pressure 0.702). The apparent dissociation constant for boric acid at 18% and ionic strength 0.065 was found to be 0.69 x 1O-s by in~~ol&tion from tables for sea-wster given by HU~EY (1955). The value of the constant for silioio acid (see below) was obtained from the value given for log K, by ROLLERand Eawrz (1940) at 30°C by using the formula of PITZER (1937) with correction for ionic strength on the assumption that silioic acid is very similar to boric acid. 233