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Isotopic composition of argon in modern surface volcanic rocks
EARTH AND PLANETARY SCIENCE LETTERS 8 (1970) 109-117. NORTH-HOLLAND ?UBLISHING COMPANY
ISOTOPIC COMPOSITION OF ARSON IN MODERN SURFACE VOLCANIC ROCKS Darnel KRUMMENACHER
Department of Geology, Sarr Diego State College, San Diego, Califonzia, LISA
Received 15 January 1970 A study of the 40 Ar/36Ar and 38 Ar/36 Ar ratios in argon extracteJ from 27 modern surface volcanic rocks shows that fractionation, along with contamination by non-atmospheric rgon inherited from the parent magma, is an important cause of anomalous 40 Ar/36 Ar ratios in these rocks. 1 . Introduction Several anomalously high or low 4° Ar/36 Ar ratios in argon extracted from modern volcanic rocks have been reported (2, 4) . The cause of the anomalously high ratios has been generally attributed to "magmatic" argon trapped in the rock during its crystallization, having a value higher than the atmospheric 4° Ar/36 Ar due to excess 4o Ar . The anomalously low ratios were ascribed to argon enriched in 36 Ar, either by fractionation or by mixture with primordial argon. We have attempted to get an idea of the composition of this magmatic argon, realizing that it is impossible to measttre its exact isotopic composition due to mixing with atmospheric argon as soon as the sample reaches the surface and is manipulated in the laboratory . The lava samples selected for this work are surface modern volcanic rocks (i .e ., surface rocks in which essentially no radiogenic argon accumulation has yet taken place) in which any differences from the isotopic composition of atmospheric argon must be attributed to "magmatic" argon. To our knowledge, all previous reports of anomalies in argon isotopic composition in rocks have been given in terms of 4° Ar/36 Ar ratios only*, maims due to the fact that a mass spectrometer once spiked with "Ar no longer is able to make a precise mcasewent of mass 38 . Actually, they measurement of the ratio 4()/36 is not sttft cicnt to explain the cause
Zartman et al . have given analyses of 40/361 and 38/36 argon ratios in natural eases.
of the anomaly. If it is higher than 295.5, this may mean one of the following: an excess of 40 Ar (added to an argon of atmospheric composition), or only a fractionation effect corning from argon of atmospheric composition, or a more complex effect involving both possibilities . However, a measurement of the 38/36 ratio would determine the cause of the anomaly. In this work measurements are given in terms of 40/36 and 38/36 ratios, the analyses having been performed witf: a previously unused Reynolds-type glass mass spectrometer . 2. Experimental techniques The -xtraction and purification of the gas followed the usual method used by the Berkeley group [5] fusion of the sample in a Mo crucible with high frequency, purification on a CuO/Cu trap, transfer with liquid nitrogen, and purification on a Ti trap . The extraction lines and the mass spectrometer are independent . Extra care has been taken to lower :pie amount of atmoshh(;-ic contamination by xtsing sampl-s of solid rock, baking out samples overnight at 300--400°C in a "Christmas tree"-shaped bottle t, outgassing the Mo crucible fa)r one-half hour at maximum temperature, and uteri lumping and fusing the sairrple . To avoid any contamination of effect which would change the Ua the neck of this bottle are attached ti lateral "branches" in which the samples are located and from which they are dumped with a small iron slug operated from outside by a magnet .
Ho
D.KRUMMENACHER
isotopic ratios of the extracted argon, we assured ourselves that the extraction o±'the argon from the lava was complete by bringing the sample to a full boil (to release the gases) . It was t:ien left at a slightly higher temperature for onAalf hour, using new Vycor-lined bottles made specifically for these experiments (thereby avoiding any contamination from a aAr absorbed in the walls cf the bottle during previous runs). We carried out the extractions in a sequence . We~ avoided any contamination by other gases from pr^vaously used extraction lines by thrice introducing atmospheric argon (samples X1, X2 and :K3) as blanks in the extraction line in the presence of degassed melting lava, as a test for any possible contp.mination, fractionation, or adsorption (of whica none was recorded), by introducing all of the gas samples from a separately evacuated sample :man ifold into the mass spectrometer to avoid an orifice effect, and by checking the argon transfer time with activated charcoal at liquid nitrogen temperature with the mass spectrometer (it was considered compl,;ted when less than 0.5% of the sample was not absorbed by the charcoal finger, thus limiting a possible fractionation to this value) . 'Each sample hzis been run ,.t least twice . The anoinalous samples sl~,ow a greater dispersion than the sir samples . Thi,; is the result of mixing with variable arnounts of atmospheric argon, and possibly to variations within tf,-, sample . Becaiise this work was intemded to show tendencies rather than exact figures, the amount of gas was directly measured on the mass spectrometer by using peak h6ghts, and points have been plotted on fib;. 2 only when complying to s}rict mass spectroscop}., requirements : namely, absolu-e purity of gas (in particular, free o f masses 2, 39 and 4l), identical size of analyzed sar-iples, and less than 1% mean deviati ,r!i of peak heights about the extrapolated line. 3. Mass spectrowopy analytical notes Every five ndns have been bracketed between two air argo,i samples which were carefully prepared by air purification, A source :magnet produced a fairly high discrimination (th, average of air argon ratios is 40/36 = ~ 02 .0) . All samples were nxn on the same scale of the ,,ibrating reed. Aliquots ol'samples
were prepared by dilution before measurements, if necessary, in order to run all samples virtually at the same peak heights. A small correction (< 117/o) was applied to the ratios to correct for a slight non-linearity of both the vibrating reed and the recorder. The analyzed gases were very clean : the ratio 38/28 was usually between 2 and 10 (the mass 28 belonging almost completely to the background of the mass spectrometer itself) . Masses 35 and 37 were consistently absent in the samples presented in this work (however, at the beginning of our measurements we had a ratio 35/38 sometimes exceeding l ; but with such a background the atmospheric argon when pure would show a normal isotopic composition). When slight amounts of masses 2, 39, and 41 were present, even without masses 35 and 37, the results were not reliable: the presence of hydrocarbons can result in higher 38/36 ratios . The problem of memory in a new mass spectrometer was also studied : at the beginning only our standard air argon had been repeatedly introduced and changes in isotopic ratios were noted . The first twenty sweeps of a typical run would give a normal isotopic argon composition (with a given discrimination); sweeps twenty to fifty would show the same, with discrimination changing slightly (usually higher) ; in fifty sweeps and above, a change of composition adds to the discrimination (usually enrichment in a° Ar and '8 At / 36'Ar remaining the same) . Therefore, the memory effect is due not only to release of a previous sample of different composition from the walls of the mass spectrometer, but to other processes as well. To avoid peak decay when high voltage is applied on the plates of the instrument during introduction of the sample, all of them have been introduced with high voltage disconnected and allowed to equilibrate for ten minutes . 7wenty sweeps were made for all samples . The initial composition has been calculated back to time 0 (i.e., when high voltage was applied) using the computer of the D, partment of Physics, University of California, BetIceley, fitting peak decay and variation of ratios tc exponential lines.
4. Possible causes of mass fractionation in the argon extracted from modern volcanic rocks Two possible causes of argon fractionation from these rocks are :
ISOTOPIC COMPOSITION OF ARGON
4.1 . Experimental manipulations (a) Mass spectrometer discrimination (this effect is assumed to be linear with mass). (b) Incomplete recovery of argon from the sample leading to enrichment of light isotopes in the extracted gas. It is assumed that the three argon isotopes are similarly distributed in the volcanic rock and that concentration due to partial release is therefore proportional .to their mass. (c) Incomplete transfer by charcoal at liquid nitrogen temperature .
Therefor:, it is impossible to calculate back, or find directly, whether an anomalous 40Ar/ 36 Ar ratio is due to a mass fractionation effect only (x and y can both be reduced to atmospheric values), to an addition of, or depletion in 4° At (x is atmospheric, y different from atmospheric), or both to fractionation and addition of, or depletion in 4° At (when x is reduced to atmospheric value and y remains different from atmospheric value) .
4.2. Processes in the rock itself, before laboratory manipulations (a) Partial introduction of air argon into the magma (enrichment in light isotopes). (b) Partial release of argon from the magma (enrichment in heavy isotopes) . (b) More complex processes involving recycling of already fractionated argon by a magma (enrichment in light or heavy isotopes) .
6. Isotopic composition of argon air samples
5. Graphical representation of results In figs. 1 and 2 each sample is plotted as a single point in which 4° Ar/ 36 Ar is they coordinate and 3R Ar/ 36 Ar the x coordinate . Hence, any fractionation from an argon of a given composition will plot on a particular straight line . 5 .1 . Calculation of dic slope of tfrc straight line
Let x, y be the coordinates as defined above . Assume that any fractionation effect will be twice as much on 40/36 as on 38/36 ratios. The slope of the line is therefore m = 2y/x, m being function of both y and x, every point theoretically can belong to any line, i.e ., any argon composition can be interpreted as the result ofa different fractionation on a different initial argon composition . However, whereas 40 Ar/ 36 Ar can vary by fractionation and addition of radiogenic sQ Ar, 38Ar/ 36 At can opt}vary by fractionation (see following section) . Therefore, e, the composition or the At prior to the trac-
tionation can be found by (1) determining the value of rn by using the above relation, it which v arid x
are the observed values, and (2) using the above determined rrt value in the same equation in which x = atmospheric 3® Ar/ 3" Ar and), = 4per/"Ar prior to the fractionation .
Fig. 1 presents a plot of air argon samples, not corrected for discrimination. Their average for 4° Ar/36 At is 302.0 and for , 8 Ar/36 At is 0.1897 . An atmospheric argon sample, subjected to mass fractionation, will plot on the s, '.Id line passing through these points. This litre is taken as air argon standard fractionation line in fig. 2. Points plotting on the dotted lire: represent any fractionation from the Nier figures for atmospheric argon (40 Ar/36 Ar = 295 .5 and 38 Ar/ 36 Ar = 0.1869) . The difference between those two lines is vea j small (for example, if 40/36 = 295.5, 38/36 would be 0.1876 instead of 0.1869) . The relatively high discrimination and slight shift from Nier values are most likely due to the small size of the mass spectrometer, as well as the source magnet attachment used for these experiments . Most of the air samples were run with an emission of 2.3 mAmp, two of them with 1 .7 and 3.0 mAmp emission respectively . This was done to match the analytical condition for several volcanic rocks as closely as possible . The discrimination is higher for the 1 .7 arc. ' .0 mAmp emission setups . 7 . Isotopic composition of argon from modern volcanic rocks
rig.
is a piot of the isotopic composition of
argon extracted from these rocks . The solid line rep=. eser is the standard air argon as previoiWy determined, the two dotted lines parallel to it define the dist)crsion expected from air undergoing different fractionations . The wimples in the closed area within those lines are atmospheric without fractionation .
U .KRUMMENACHER
311 310 .49 308 3C7 3C6 305 UPPles KI-12-13
303
~- Sem0les ~u~ with 2 .3 mA-P i en~ission
302
4e .Ple run with
301
,!{~ Se-Ple
300
.. with
1 .7 P emiss10,,
3 .0 ~aA .P
0.1s.1 œ,
299 298 297 2S6 295 294 293
"-tr
0 .185
O .î87
0 .189
je" , 1
3% ,
- - V- T - I r> 193
u .191
0 19"
1 . Axgva air sanîi)ics.
The dispersion are a rta, been graphically determined from dispers:io,~i of air samples . .vveyer, before interpreting the results of the 1-11c analyses one should asic wheti, -,r the magmatic argon contained. in the magma from which the lava originates (and which might still be measured in some lavas erupting to the surface) has an atmospheric composition or not . The answer is very likely no. The isotope 4O Ar is continuously forming within the earth by radioactive decay and partially diffusinf into the atmosphere: it is therefore highly improbable that the ratio 40 Ar/ 36 Ar be exactly the same in the atmosphere and in the "magmatic" argon . However, vie have alr;~,.-ady assumed that any change in value of the ratio 38 Ar/"Ar in 'Irnagmatic" argon in volcanic rocks is due; to a mass fractionation effect only * .
The validity of tiûs assumption depends on whether 3" Ar and 36 Ar might be produced in sufficfient amounts by radioactive decay in the mantle to change this ratio significantly . I do not think so for the following reasons : (1) Fleming a ,l -td Thode [61 have shown that high 36 Ar/ 36 Ar ratios (the highest 0 .592 t 0.0035) can be measured in old l3itchblendes, while relatively low a0Ar/36Ar ratios are simultaneously produced (226.4 t 0.4 in this extreme case). If the formation * Another alternatin,e would be that the ratio 36 Ar/ 36 Ar is consistently different from atmospheric 38 Ar/ `16 At in "magmatic" argon due to diffudon procestes on a large scale since the formation of the earth . We will discuss later on other measurements regarding this possibility .
ISOTOPIC COMPOSITION OF ARGON
113
350
345 A R E A
0 F
E N R 1 C N M E N T 40A r I N
r
335
Ar/ 36Ar
40
e
330
Sample% of air At enr!c-A in heavy isotopes by fractionation Samples of sir Ar enriched in Ityht isotopes by fractionation
325
Sample enriched in he vy isotopes by fractions tien and .,,,, .had In ~OA, $mples of Ar enriched in
320
40
A-,
-1
0
A- .9,
13
a A E A
air
Ar
0 F
D E P L E i 1 0 N
1 N
'40A,
21 301,
300
m0.180
o.1p0
0. zoo
)eA , , )t., 0.21a
1`4Ü . 2 . Argon all
D.KRUMMENACHER of radiogenic 38 Ar and 36 Ar were to play a role in the variations we have broug :rt forward by our measurements . we would have regularly higher 3 8 Ar/ 36 Ar ratios than the atmospheric value, which is not the 4°Ar/36Ar case (it should be noted that the low which ac-:ompanies the high 38 Ar/ 36 Ar ratio might have been changed into atmospheric values or higher by an environment richer in potassium) . (2) Wetherhill [7j has also measûred high 38 Ar/ 36Ar ratios in uranium-rich minerals . For example, a 650 my old pitchblende from Belgian Congo con36 taining 44% uranium shows a 38 Ar/ Ar ratio of + .621 ± 0 .015 with an excesi of 2 X 10 -8 cc STP 38 Ar per g. From this figure one can very roughly estir.-fate the production of excess 38 Ar to ca . 10 - " mole per g sine-- 5 by . The minimum amount of anomalous Ar extracted from our samples is 8 X l Oi -' 2 mole, which corresponds to ca . 6 X 10 -11 mole of 38 Ar . Hence the excess of 38 Ar produced by radioactivity does not exceed 2/ 1000th of the amount of 38 Ar measured in anomalous samples, which is a negligible quantity. (3) 2:artman et al . r8l analyzing; samples of Ar from natural gases collected in wells showed while the "Ar,136 Ar ratios ranged from 312 to 34 001), the 38 Ar/ 36 !1r ratio was, within experimental error, equal to the value of 0.1869 found in atmospheric argon . If the rocks of the lithosphere, :)eing at least ten times richer in U than the rocks )f the mantle, show normal 38 Ar/ 2"s Ar ratios, a normal 38 Ar/ 36 Ar ratio must be expected in the latter rocks as well . Hence lower 38 Ar/' 3 6 Ar ratios than atmospheric are to be explained by fractionation only ; higher ratios than atmospheric cannot be explained by excess 38 Ar since the ,amount produced bas been estimated to be too low to change significantly oiir measurements, ~nd natural gases from the lithosphere, which is richer in uranium than the measured lavas from the mantle, still show a normal 38 A, r/ 36 Ar.
8 . Interpretation of results i n fig. 2 (1) In most cases, the argon c ?ntained in the voî'nanic rock is unfiactionated atmospheric argon This argon is attributed to atmospheric argon which
has entered the lava during or after the eruption, after total release of magmatic argon . (2) Three samples (33, 14, 16) show atmospheric argon fractionated by enrichment in fight isotopes This might be explained best by partial introduction of atmospheric argon into the samples after the eruption. An alternate explanation would be that the magma captured an already fractionated atmospheric argon before its eruption. This effect is therefore not due to a mixture with primordial ;.argon, i .e ., depleted 40 Ar . ill (3) Three samples (19, 26, 28) show atmospheric argon fractionated by enrichment in heuvy isotopes . A more complex process might be requited in these cases : first, an equilibration of magma with atnio spheric argon, then partial release of it . The fractionation is very high in sample 28 (40/3Eî = 350 .0) : the only explanation for it is that the rock must have had a series of long leak: gradually enriching the remaining argon in heavy isotopes . (4) Two samples (36, 39) are slightly enriched in 40A r and not fractionated. Here is some "excess" of radiogerric 40 Ar . (5) One sample ( 7) is slightly enriched in 4c'Ar and strongly fractionated by enrichment in he,. - v isotopes. Its composition can be ascribed to the process of partial escal:e of argon enriched in 40 Ar .
9 . Conclusions The K/Ar age anomalies measured on surface volcanic rocks ar~-. usually thought to !3e due only to the presence of a - magmatic" argon different in isotopic composition from the atmospheric argon still trapped in the volcanic rock after its cooling . Our experiments show that these anomalies can in many cases be explained by mass fractionation effect on argon of atmospheric isotopic,., omposition trapped in the lava . If the 27 samples analyzed in this work were statistically representative of all surface volcanic rocks, about 67% of them would contain unfractionated atmospheric argon only, 11% fractionated atmospheric argon by enrichment in light isotopes, 11% fractionated atmospheric argon by enrichment in heavy isotopes, 7% unfractionated atmospheric argon enriched in 4° Ar, and 4% atmospheric argon enriched in 4° Ar and fractionated by enrichment in heavy isotopes .
ISOTOPIC COMPOSITION OF ARGON
However, experiments are presently under way to determine the composition of argon from pillows dredged at great depth in the Pacific where such fractionation processes may not have occurred and from which very high 4° Ar/a6 Ar ratios have been reported [3] . In our opinion, it is very likely that in these cases the addition of 40 Ar is far more important than the fractionation effects, if the latter even exist . Therefore, the measurements will likely show a point accumulation tendency located in the far upl er left of fig. 2, in the area of strong enrichment in `°Ar. But still the problem of the distribution of the isotopic composition of the argon throughout the mantle remains unanswered . One can use a reasonable inodel in which the argon becomes poorer in 4O Ar toward the center of the earth (because of the decream of K content in this direction). The depth at which it will be mixed with primordial argon (i.e., depleted in 40 Ar with respect to the isotopic composition of atmospheric argon) is, of course, not known. But one can be tempted to think that the argon extracted from volc.^inic rocks chilled at great depths on oceanic rises might show lower 4p Ar/ 36 Ar ratios than argon from basalts erupting from underneath a plate, since they are supposed to coots up from greater depths in the mantle. I sutbstantiate this hypothesis from the rece it discovery of primordial helium at a particular "mid-depth" in the Pacific Ocean (1 I which, might tentatively be ascribed to gas leakage from relatively shalk.w sites of tectonic activity . Acknowledgments The author is greatly indebted to Dr . G .H.Curtis for permitting and encouraging the use of a new mass spectrometer at the Department of Geophysics and Geology of the University of California at Berkeley, Drs. J .H.Reynolds, G.H .Curtis, P.Damon and G .Gastil for helpful discussions and suggestions Drs . [11azieff and M .Rimy have kindly given several uropean and Pacific samples . We are specially thankful to Dr . G.B .Dalrymple who has provided three
samples with anomalous argon for further investigation (see samples 31, 33 and 39 of this work, and corresponding sample numbers ?4, 4, 5 in Dalrymple (21 ). This work has been supported by the "hounda-
tion Noetzlin pour études vulcan6logiques", at Geneva, Switzerland, and the grant GA 929 of the National Science Foundation to the Department of Geology at the University of California, Berkely . Appendix I Average quantity of argon extracted from each g of sample The quantity of argon extracted Born "anomalous" samples (indicated with a star) is generally low . Possibly it indicates that besides losses of gas during the eruption the contamination by greater quantities of atmospheric argon dilutes it too much to be measured.
Sample
2 5 6 7* 9 11 13 1 4* 15 16' 18 19' 20 21 23
24 25 26" 27 28 " 29 31 33 . 16 "
39'
Total extracted argon (10 -1 1mole) per g (aver.) 9 3.6 40 50 100 40 600 2.1 8 0.9 48 1 .2 6 4 80 1 .6 4.3 1 .8 0.9 0.8 0.8 68 l' ti .5 9.0 24
Argon interpreted as excess argon 40 per g (1011 mole)
K/Ar ages (my) calculated with 0.190 K
+0.58±0.13
+32 .3±7 .2
-0 .017±0 .12
-1 .0±0.7
-0.019±0 .017
-1 .1 ±1 .0
+0 .15±0.12
+8 .5±6 .8
+0 .043±0 .035
+2 .4±2
+0 .0125±0.0019
+0 .7±0 .01
0.22±0.08 0.4t0 .3 ratio 40/36 is normal
+12 .6±4 .5 +22 .8±16 .5 (l
In these calculations the rat o 4(1/36 of the anomalous samples is crinpared against the tnea
D.KRUMMENACHER
116 Appendix 2
a'Ar136Ar ratios in samples analyzed by both Dalrymple (2j and Knimrnenacher (D.K.) # ç.K .
40Ar extracted (X 10-12 mole) D.K .
40 Ar/36 Ar (this paper) D.K .
40 Ar/36 Ar true value D.K .
40 Ar/36Ar
Dalrymple (true value)
40 Ar extracted (X 10-12 mole) Dalrymple
# Dalrymple
31
120
299
292.6
291 .4
mean : 2 i8
24
33
55
288.9
282.9
283.5 290.2
93 52
4
39
24
301 .3
.294 .8
329.6 356.6
115 71
Two results are in good agreement and one is not . It indicates that the anomalies in isotopic composi-
5
tion can be irregularly distributed in the rock, as Dalrymple [21 has already detern .ined.
Appendix 3 Origin of samples 1 . Mauna Loa, lava flow, 1907, Hawaii "' . 2. Mauna Loa, Pleistocene flow, Hawaii 5 . La Reunion, volcanic bomb from the "Piton de la Fournaise", October, 1960 'k . 5' . Like 5, but different volcanic bomb * . a. La Réunion, collected from flowing lava, March, 1960 *. 7. Akaka Water Fall, Pleistocene flow, Hawaii * . 9. Mayon Volcano, Phillipine Islands (no date ; if eruption, has been collected on top of moun-* tain)'` . 11 . Ngaurulroi, New Zealand, lava flow, 1954 *. 13 . Kituro-Nyefunzi, Kenya, lava flow, 1948 * . 14 . Sakura Jima, Japan, lava flow, 1946 *. 15 . Aso, Japan, volcanic bomb, eruption, 1933 *. 16 . Sakura .lima, Japan, lava flow, 1914 * . 18 . Lopdvi, crater 4, New Hebrides, eruption, 1960, Collection Marcel Rémy . 19 . Kilauea Iki, Hawaii, lava flow 1959 *. 2:0 . Kilauea Iki, Hawaii, lava flow 1958 * . 21 . Sakura Jima, Japan. Boulder by the caldera (no date given) . 23 . Vesuvio, Italy, lava flow, 1906 ** . 2.4. Vesuvio, Italy, lava flow, 1944 ** . 2.5 . Stromboli, Italy, volcanic bomb, May 10 . 1963
26 . 27 . 28 . 29 . 31 . 33 .
36 . 39 .
~~
Stromboli, Italy, volcanic bomb, September 23, 1953 ** . Stromboli, Italy, volcanic bomb, October 6, 1963 ** . Etna, Italy, lava flow, May, 1964 ** . Etna, Italy, lava flow, September, 1963 ** . Sal:ura Jima, Japan, eruption, 1940 . C()llecti()ra R.Eliailey, given by G.B . Dalrymple 1, e number 24 in Dalrymple [2) ). Obsidian from Medicine Lake Highlands, Glass Mountains (California) . Collection 1 . Friedman . Age less than 500 yr (see number 4 ir~ Dalrymple (21 ). Olivine inclusion from the Punaru Rider, Tahiti . Parent rock has given a 0 age * . Hualalai flow, 1800-1801, Hawaii . Rock containing inclusion given an age of ca . I by . (see number 5 in Dalrymple 121) .
Collection J.Noctzlin and Collection liJaxieff.
Daniel Kninrnaanacher .
ISOTOPIC COMPOSITION OF ARGON References [ 1I W.B.Clarke, M.A.Beg and H.Craig, Excess 3He in the sea: Evidence for terrestrial primordial helium, Earth Planet . Sci. Letters (in press) . 121 G.B.Dalrymple, 4OAr/36Ar analyses of historic lave flows, Earth Planet. Sci. Letters 6 (1969) 47-55. [31 G.B.Dairymple and J.G .Moore, Argon 40: Excess in submarine pillow basalts from Kilauea Volcano, Hawaii, Science 161(1968) 113 2. (41 P.E.Damon, E.W.Laughlin and J.K .Percious, Problem of excess argon 40 in volcanic rocks, Proc. Symp . Radioactive Dating, Monaco, Intern. At. Energy (1967) 463482.
(51 J.F .Evernden and G.H .Curtis, The K/A dating of the late Cenozoic rocks in Easi Africa and Italy, Current Antropology 6 (1965) 343 [61 W.H.Fleming and H.G .Thode, Argon 38 in pitchblende minerals and nuclear processes in nature : Phys. Rear. 90/3 (1953) 879. [71 G.W.Wetherhilt Variations in the isotopic abundances of neon and argon extracted from radioactive minerals, Phys. Rev. 96/3 (1954) 879 (81 R.E.Zartman, G.J .Wasserburg and J.H .Reynolds, Helium, argon and carbon in some natural gases, J. Geophys. Res. 66/ 1 (1961) 277.
ISOTOPIC COMPOSITION OF ARSON IN MODERN SURFACE VOLCANIC ROCKS Darnel KRUMMENACHER
Department of Geology, Sarr Diego State College, San Diego, Califonzia, LISA
Received 15 January 1970 A study of the 40 Ar/36Ar and 38 Ar/36 Ar ratios in argon extracteJ from 27 modern surface volcanic rocks shows that fractionation, along with contamination by non-atmospheric rgon inherited from the parent magma, is an important cause of anomalous 40 Ar/36 Ar ratios in these rocks. 1 . Introduction Several anomalously high or low 4° Ar/36 Ar ratios in argon extracted from modern volcanic rocks have been reported (2, 4) . The cause of the anomalously high ratios has been generally attributed to "magmatic" argon trapped in the rock during its crystallization, having a value higher than the atmospheric 4° Ar/36 Ar due to excess 4o Ar . The anomalously low ratios were ascribed to argon enriched in 36 Ar, either by fractionation or by mixture with primordial argon. We have attempted to get an idea of the composition of this magmatic argon, realizing that it is impossible to measttre its exact isotopic composition due to mixing with atmospheric argon as soon as the sample reaches the surface and is manipulated in the laboratory . The lava samples selected for this work are surface modern volcanic rocks (i .e ., surface rocks in which essentially no radiogenic argon accumulation has yet taken place) in which any differences from the isotopic composition of atmospheric argon must be attributed to "magmatic" argon. To our knowledge, all previous reports of anomalies in argon isotopic composition in rocks have been given in terms of 4° Ar/36 Ar ratios only*, maims due to the fact that a mass spectrometer once spiked with "Ar no longer is able to make a precise mcasewent of mass 38 . Actually, they measurement of the ratio 4()/36 is not sttft cicnt to explain the cause
Zartman et al . have given analyses of 40/361 and 38/36 argon ratios in natural eases.
of the anomaly. If it is higher than 295.5, this may mean one of the following: an excess of 40 Ar (added to an argon of atmospheric composition), or only a fractionation effect corning from argon of atmospheric composition, or a more complex effect involving both possibilities . However, a measurement of the 38/36 ratio would determine the cause of the anomaly. In this work measurements are given in terms of 40/36 and 38/36 ratios, the analyses having been performed witf: a previously unused Reynolds-type glass mass spectrometer . 2. Experimental techniques The -xtraction and purification of the gas followed the usual method used by the Berkeley group [5] fusion of the sample in a Mo crucible with high frequency, purification on a CuO/Cu trap, transfer with liquid nitrogen, and purification on a Ti trap . The extraction lines and the mass spectrometer are independent . Extra care has been taken to lower :pie amount of atmoshh(;-ic contamination by xtsing sampl-s of solid rock, baking out samples overnight at 300--400°C in a "Christmas tree"-shaped bottle t, outgassing the Mo crucible fa)r one-half hour at maximum temperature, and uteri lumping and fusing the sairrple . To avoid any contamination of effect which would change the Ua the neck of this bottle are attached ti lateral "branches" in which the samples are located and from which they are dumped with a small iron slug operated from outside by a magnet .
Ho
D.KRUMMENACHER
isotopic ratios of the extracted argon, we assured ourselves that the extraction o±'the argon from the lava was complete by bringing the sample to a full boil (to release the gases) . It was t:ien left at a slightly higher temperature for onAalf hour, using new Vycor-lined bottles made specifically for these experiments (thereby avoiding any contamination from a aAr absorbed in the walls cf the bottle during previous runs). We carried out the extractions in a sequence . We~ avoided any contamination by other gases from pr^vaously used extraction lines by thrice introducing atmospheric argon (samples X1, X2 and :K3) as blanks in the extraction line in the presence of degassed melting lava, as a test for any possible contp.mination, fractionation, or adsorption (of whica none was recorded), by introducing all of the gas samples from a separately evacuated sample :man ifold into the mass spectrometer to avoid an orifice effect, and by checking the argon transfer time with activated charcoal at liquid nitrogen temperature with the mass spectrometer (it was considered compl,;ted when less than 0.5% of the sample was not absorbed by the charcoal finger, thus limiting a possible fractionation to this value) . 'Each sample hzis been run ,.t least twice . The anoinalous samples sl~,ow a greater dispersion than the sir samples . Thi,; is the result of mixing with variable arnounts of atmospheric argon, and possibly to variations within tf,-, sample . Becaiise this work was intemded to show tendencies rather than exact figures, the amount of gas was directly measured on the mass spectrometer by using peak h6ghts, and points have been plotted on fib;. 2 only when complying to s}rict mass spectroscop}., requirements : namely, absolu-e purity of gas (in particular, free o f masses 2, 39 and 4l), identical size of analyzed sar-iples, and less than 1% mean deviati ,r!i of peak heights about the extrapolated line. 3. Mass spectrowopy analytical notes Every five ndns have been bracketed between two air argo,i samples which were carefully prepared by air purification, A source :magnet produced a fairly high discrimination (th, average of air argon ratios is 40/36 = ~ 02 .0) . All samples were nxn on the same scale of the ,,ibrating reed. Aliquots ol'samples
were prepared by dilution before measurements, if necessary, in order to run all samples virtually at the same peak heights. A small correction (< 117/o) was applied to the ratios to correct for a slight non-linearity of both the vibrating reed and the recorder. The analyzed gases were very clean : the ratio 38/28 was usually between 2 and 10 (the mass 28 belonging almost completely to the background of the mass spectrometer itself) . Masses 35 and 37 were consistently absent in the samples presented in this work (however, at the beginning of our measurements we had a ratio 35/38 sometimes exceeding l ; but with such a background the atmospheric argon when pure would show a normal isotopic composition). When slight amounts of masses 2, 39, and 41 were present, even without masses 35 and 37, the results were not reliable: the presence of hydrocarbons can result in higher 38/36 ratios . The problem of memory in a new mass spectrometer was also studied : at the beginning only our standard air argon had been repeatedly introduced and changes in isotopic ratios were noted . The first twenty sweeps of a typical run would give a normal isotopic argon composition (with a given discrimination); sweeps twenty to fifty would show the same, with discrimination changing slightly (usually higher) ; in fifty sweeps and above, a change of composition adds to the discrimination (usually enrichment in a° Ar and '8 At / 36'Ar remaining the same) . Therefore, the memory effect is due not only to release of a previous sample of different composition from the walls of the mass spectrometer, but to other processes as well. To avoid peak decay when high voltage is applied on the plates of the instrument during introduction of the sample, all of them have been introduced with high voltage disconnected and allowed to equilibrate for ten minutes . 7wenty sweeps were made for all samples . The initial composition has been calculated back to time 0 (i.e., when high voltage was applied) using the computer of the D, partment of Physics, University of California, BetIceley, fitting peak decay and variation of ratios tc exponential lines.
4. Possible causes of mass fractionation in the argon extracted from modern volcanic rocks Two possible causes of argon fractionation from these rocks are :
ISOTOPIC COMPOSITION OF ARGON
4.1 . Experimental manipulations (a) Mass spectrometer discrimination (this effect is assumed to be linear with mass). (b) Incomplete recovery of argon from the sample leading to enrichment of light isotopes in the extracted gas. It is assumed that the three argon isotopes are similarly distributed in the volcanic rock and that concentration due to partial release is therefore proportional .to their mass. (c) Incomplete transfer by charcoal at liquid nitrogen temperature .
Therefor:, it is impossible to calculate back, or find directly, whether an anomalous 40Ar/ 36 Ar ratio is due to a mass fractionation effect only (x and y can both be reduced to atmospheric values), to an addition of, or depletion in 4° At (x is atmospheric, y different from atmospheric), or both to fractionation and addition of, or depletion in 4° At (when x is reduced to atmospheric value and y remains different from atmospheric value) .
4.2. Processes in the rock itself, before laboratory manipulations (a) Partial introduction of air argon into the magma (enrichment in light isotopes). (b) Partial release of argon from the magma (enrichment in heavy isotopes) . (b) More complex processes involving recycling of already fractionated argon by a magma (enrichment in light or heavy isotopes) .
6. Isotopic composition of argon air samples
5. Graphical representation of results In figs. 1 and 2 each sample is plotted as a single point in which 4° Ar/ 36 Ar is they coordinate and 3R Ar/ 36 Ar the x coordinate . Hence, any fractionation from an argon of a given composition will plot on a particular straight line . 5 .1 . Calculation of dic slope of tfrc straight line
Let x, y be the coordinates as defined above . Assume that any fractionation effect will be twice as much on 40/36 as on 38/36 ratios. The slope of the line is therefore m = 2y/x, m being function of both y and x, every point theoretically can belong to any line, i.e ., any argon composition can be interpreted as the result ofa different fractionation on a different initial argon composition . However, whereas 40 Ar/ 36 Ar can vary by fractionation and addition of radiogenic sQ Ar, 38Ar/ 36 At can opt}vary by fractionation (see following section) . Therefore, e, the composition or the At prior to the trac-
tionation can be found by (1) determining the value of rn by using the above relation, it which v arid x
are the observed values, and (2) using the above determined rrt value in the same equation in which x = atmospheric 3® Ar/ 3" Ar and), = 4per/"Ar prior to the fractionation .
Fig. 1 presents a plot of air argon samples, not corrected for discrimination. Their average for 4° Ar/36 At is 302.0 and for , 8 Ar/36 At is 0.1897 . An atmospheric argon sample, subjected to mass fractionation, will plot on the s, '.Id line passing through these points. This litre is taken as air argon standard fractionation line in fig. 2. Points plotting on the dotted lire: represent any fractionation from the Nier figures for atmospheric argon (40 Ar/36 Ar = 295 .5 and 38 Ar/ 36 Ar = 0.1869) . The difference between those two lines is vea j small (for example, if 40/36 = 295.5, 38/36 would be 0.1876 instead of 0.1869) . The relatively high discrimination and slight shift from Nier values are most likely due to the small size of the mass spectrometer, as well as the source magnet attachment used for these experiments . Most of the air samples were run with an emission of 2.3 mAmp, two of them with 1 .7 and 3.0 mAmp emission respectively . This was done to match the analytical condition for several volcanic rocks as closely as possible . The discrimination is higher for the 1 .7 arc. ' .0 mAmp emission setups . 7 . Isotopic composition of argon from modern volcanic rocks
rig.
is a piot of the isotopic composition of
argon extracted from these rocks . The solid line rep=. eser is the standard air argon as previoiWy determined, the two dotted lines parallel to it define the dist)crsion expected from air undergoing different fractionations . The wimples in the closed area within those lines are atmospheric without fractionation .
U .KRUMMENACHER
311 310 .49 308 3C7 3C6 305 UPPles KI-12-13
303
~- Sem0les ~u~ with 2 .3 mA-P i en~ission
302
4e .Ple run with
301
,!{~ Se-Ple
300
.. with
1 .7 P emiss10,,
3 .0 ~aA .P
0.1s.1 œ,
299 298 297 2S6 295 294 293
"-tr
0 .185
O .î87
0 .189
je" , 1
3% ,
- - V- T - I r> 193
u .191
0 19"
1 . Axgva air sanîi)ics.
The dispersion are a rta, been graphically determined from dispers:io,~i of air samples . .vveyer, before interpreting the results of the 1-11c analyses one should asic wheti, -,r the magmatic argon contained. in the magma from which the lava originates (and which might still be measured in some lavas erupting to the surface) has an atmospheric composition or not . The answer is very likely no. The isotope 4O Ar is continuously forming within the earth by radioactive decay and partially diffusinf into the atmosphere: it is therefore highly improbable that the ratio 40 Ar/ 36 Ar be exactly the same in the atmosphere and in the "magmatic" argon . However, vie have alr;~,.-ady assumed that any change in value of the ratio 38 Ar/"Ar in 'Irnagmatic" argon in volcanic rocks is due; to a mass fractionation effect only * .
The validity of tiûs assumption depends on whether 3" Ar and 36 Ar might be produced in sufficfient amounts by radioactive decay in the mantle to change this ratio significantly . I do not think so for the following reasons : (1) Fleming a ,l -td Thode [61 have shown that high 36 Ar/ 36 Ar ratios (the highest 0 .592 t 0.0035) can be measured in old l3itchblendes, while relatively low a0Ar/36Ar ratios are simultaneously produced (226.4 t 0.4 in this extreme case). If the formation * Another alternatin,e would be that the ratio 36 Ar/ 36 Ar is consistently different from atmospheric 38 Ar/ `16 At in "magmatic" argon due to diffudon procestes on a large scale since the formation of the earth . We will discuss later on other measurements regarding this possibility .
ISOTOPIC COMPOSITION OF ARGON
113
350
345 A R E A
0 F
E N R 1 C N M E N T 40A r I N
r
335
Ar/ 36Ar
40
e
330
Sample% of air At enr!c-A in heavy isotopes by fractionation Samples of sir Ar enriched in Ityht isotopes by fractionation
325
Sample enriched in he vy isotopes by fractions tien and .,,,, .had In ~OA, $mples of Ar enriched in
320
40
A-,
-1
0
A- .9,
13
a A E A
air
Ar
0 F
D E P L E i 1 0 N
1 N
'40A,
21 301,
300
m0.180
o.1p0
0. zoo
)eA , , )t., 0.21a
1`4Ü . 2 . Argon all
D.KRUMMENACHER of radiogenic 38 Ar and 36 Ar were to play a role in the variations we have broug :rt forward by our measurements . we would have regularly higher 3 8 Ar/ 36 Ar ratios than the atmospheric value, which is not the 4°Ar/36Ar case (it should be noted that the low which ac-:ompanies the high 38 Ar/ 36 Ar ratio might have been changed into atmospheric values or higher by an environment richer in potassium) . (2) Wetherhill [7j has also measûred high 38 Ar/ 36Ar ratios in uranium-rich minerals . For example, a 650 my old pitchblende from Belgian Congo con36 taining 44% uranium shows a 38 Ar/ Ar ratio of + .621 ± 0 .015 with an excesi of 2 X 10 -8 cc STP 38 Ar per g. From this figure one can very roughly estir.-fate the production of excess 38 Ar to ca . 10 - " mole per g sine-- 5 by . The minimum amount of anomalous Ar extracted from our samples is 8 X l Oi -' 2 mole, which corresponds to ca . 6 X 10 -11 mole of 38 Ar . Hence the excess of 38 Ar produced by radioactivity does not exceed 2/ 1000th of the amount of 38 Ar measured in anomalous samples, which is a negligible quantity. (3) 2:artman et al . r8l analyzing; samples of Ar from natural gases collected in wells showed while the "Ar,136 Ar ratios ranged from 312 to 34 001), the 38 Ar/ 36 !1r ratio was, within experimental error, equal to the value of 0.1869 found in atmospheric argon . If the rocks of the lithosphere, :)eing at least ten times richer in U than the rocks )f the mantle, show normal 38 Ar/ 2"s Ar ratios, a normal 38 Ar/ 36 Ar ratio must be expected in the latter rocks as well . Hence lower 38 Ar/' 3 6 Ar ratios than atmospheric are to be explained by fractionation only ; higher ratios than atmospheric cannot be explained by excess 38 Ar since the ,amount produced bas been estimated to be too low to change significantly oiir measurements, ~nd natural gases from the lithosphere, which is richer in uranium than the measured lavas from the mantle, still show a normal 38 A, r/ 36 Ar.
8 . Interpretation of results i n fig. 2 (1) In most cases, the argon c ?ntained in the voî'nanic rock is unfiactionated atmospheric argon This argon is attributed to atmospheric argon which
has entered the lava during or after the eruption, after total release of magmatic argon . (2) Three samples (33, 14, 16) show atmospheric argon fractionated by enrichment in fight isotopes This might be explained best by partial introduction of atmospheric argon into the samples after the eruption. An alternate explanation would be that the magma captured an already fractionated atmospheric argon before its eruption. This effect is therefore not due to a mixture with primordial ;.argon, i .e ., depleted 40 Ar . ill (3) Three samples (19, 26, 28) show atmospheric argon fractionated by enrichment in heuvy isotopes . A more complex process might be requited in these cases : first, an equilibration of magma with atnio spheric argon, then partial release of it . The fractionation is very high in sample 28 (40/3Eî = 350 .0) : the only explanation for it is that the rock must have had a series of long leak: gradually enriching the remaining argon in heavy isotopes . (4) Two samples (36, 39) are slightly enriched in 40A r and not fractionated. Here is some "excess" of radiogerric 40 Ar . (5) One sample ( 7) is slightly enriched in 4c'Ar and strongly fractionated by enrichment in he,. - v isotopes. Its composition can be ascribed to the process of partial escal:e of argon enriched in 40 Ar .
9 . Conclusions The K/Ar age anomalies measured on surface volcanic rocks ar~-. usually thought to !3e due only to the presence of a - magmatic" argon different in isotopic composition from the atmospheric argon still trapped in the volcanic rock after its cooling . Our experiments show that these anomalies can in many cases be explained by mass fractionation effect on argon of atmospheric isotopic,., omposition trapped in the lava . If the 27 samples analyzed in this work were statistically representative of all surface volcanic rocks, about 67% of them would contain unfractionated atmospheric argon only, 11% fractionated atmospheric argon by enrichment in light isotopes, 11% fractionated atmospheric argon by enrichment in heavy isotopes, 7% unfractionated atmospheric argon enriched in 4° Ar, and 4% atmospheric argon enriched in 4° Ar and fractionated by enrichment in heavy isotopes .
ISOTOPIC COMPOSITION OF ARGON
However, experiments are presently under way to determine the composition of argon from pillows dredged at great depth in the Pacific where such fractionation processes may not have occurred and from which very high 4° Ar/a6 Ar ratios have been reported [3] . In our opinion, it is very likely that in these cases the addition of 40 Ar is far more important than the fractionation effects, if the latter even exist . Therefore, the measurements will likely show a point accumulation tendency located in the far upl er left of fig. 2, in the area of strong enrichment in `°Ar. But still the problem of the distribution of the isotopic composition of the argon throughout the mantle remains unanswered . One can use a reasonable inodel in which the argon becomes poorer in 4O Ar toward the center of the earth (because of the decream of K content in this direction). The depth at which it will be mixed with primordial argon (i.e., depleted in 40 Ar with respect to the isotopic composition of atmospheric argon) is, of course, not known. But one can be tempted to think that the argon extracted from volc.^inic rocks chilled at great depths on oceanic rises might show lower 4p Ar/ 36 Ar ratios than argon from basalts erupting from underneath a plate, since they are supposed to coots up from greater depths in the mantle. I sutbstantiate this hypothesis from the rece it discovery of primordial helium at a particular "mid-depth" in the Pacific Ocean (1 I which, might tentatively be ascribed to gas leakage from relatively shalk.w sites of tectonic activity . Acknowledgments The author is greatly indebted to Dr . G .H.Curtis for permitting and encouraging the use of a new mass spectrometer at the Department of Geophysics and Geology of the University of California at Berkeley, Drs. J .H.Reynolds, G.H .Curtis, P.Damon and G .Gastil for helpful discussions and suggestions Drs . [11azieff and M .Rimy have kindly given several uropean and Pacific samples . We are specially thankful to Dr . G.B .Dalrymple who has provided three
samples with anomalous argon for further investigation (see samples 31, 33 and 39 of this work, and corresponding sample numbers ?4, 4, 5 in Dalrymple (21 ). This work has been supported by the "hounda-
tion Noetzlin pour études vulcan6logiques", at Geneva, Switzerland, and the grant GA 929 of the National Science Foundation to the Department of Geology at the University of California, Berkely . Appendix I Average quantity of argon extracted from each g of sample The quantity of argon extracted Born "anomalous" samples (indicated with a star) is generally low . Possibly it indicates that besides losses of gas during the eruption the contamination by greater quantities of atmospheric argon dilutes it too much to be measured.
Sample
2 5 6 7* 9 11 13 1 4* 15 16' 18 19' 20 21 23
24 25 26" 27 28 " 29 31 33 . 16 "
39'
Total extracted argon (10 -1 1mole) per g (aver.) 9 3.6 40 50 100 40 600 2.1 8 0.9 48 1 .2 6 4 80 1 .6 4.3 1 .8 0.9 0.8 0.8 68 l' ti .5 9.0 24
Argon interpreted as excess argon 40 per g (1011 mole)
K/Ar ages (my) calculated with 0.190 K
+0.58±0.13
+32 .3±7 .2
-0 .017±0 .12
-1 .0±0.7
-0.019±0 .017
-1 .1 ±1 .0
+0 .15±0.12
+8 .5±6 .8
+0 .043±0 .035
+2 .4±2
+0 .0125±0.0019
+0 .7±0 .01
0.22±0.08 0.4t0 .3 ratio 40/36 is normal
+12 .6±4 .5 +22 .8±16 .5 (l
In these calculations the rat o 4(1/36 of the anomalous samples is crinpared against the tnea
D.KRUMMENACHER
116 Appendix 2
a'Ar136Ar ratios in samples analyzed by both Dalrymple (2j and Knimrnenacher (D.K.) # ç.K .
40Ar extracted (X 10-12 mole) D.K .
40 Ar/36 Ar (this paper) D.K .
40 Ar/36 Ar true value D.K .
40 Ar/36Ar
Dalrymple (true value)
40 Ar extracted (X 10-12 mole) Dalrymple
# Dalrymple
31
120
299
292.6
291 .4
mean : 2 i8
24
33
55
288.9
282.9
283.5 290.2
93 52
4
39
24
301 .3
.294 .8
329.6 356.6
115 71
Two results are in good agreement and one is not . It indicates that the anomalies in isotopic composi-
5
tion can be irregularly distributed in the rock, as Dalrymple [21 has already detern .ined.
Appendix 3 Origin of samples 1 . Mauna Loa, lava flow, 1907, Hawaii "' . 2. Mauna Loa, Pleistocene flow, Hawaii 5 . La Reunion, volcanic bomb from the "Piton de la Fournaise", October, 1960 'k . 5' . Like 5, but different volcanic bomb * . a. La Réunion, collected from flowing lava, March, 1960 *. 7. Akaka Water Fall, Pleistocene flow, Hawaii * . 9. Mayon Volcano, Phillipine Islands (no date ; if eruption, has been collected on top of moun-* tain)'` . 11 . Ngaurulroi, New Zealand, lava flow, 1954 *. 13 . Kituro-Nyefunzi, Kenya, lava flow, 1948 * . 14 . Sakura Jima, Japan, lava flow, 1946 *. 15 . Aso, Japan, volcanic bomb, eruption, 1933 *. 16 . Sakura .lima, Japan, lava flow, 1914 * . 18 . Lopdvi, crater 4, New Hebrides, eruption, 1960, Collection Marcel Rémy . 19 . Kilauea Iki, Hawaii, lava flow 1959 *. 2:0 . Kilauea Iki, Hawaii, lava flow 1958 * . 21 . Sakura Jima, Japan. Boulder by the caldera (no date given) . 23 . Vesuvio, Italy, lava flow, 1906 ** . 2.4. Vesuvio, Italy, lava flow, 1944 ** . 2.5 . Stromboli, Italy, volcanic bomb, May 10 . 1963
26 . 27 . 28 . 29 . 31 . 33 .
36 . 39 .
~~
Stromboli, Italy, volcanic bomb, September 23, 1953 ** . Stromboli, Italy, volcanic bomb, October 6, 1963 ** . Etna, Italy, lava flow, May, 1964 ** . Etna, Italy, lava flow, September, 1963 ** . Sal:ura Jima, Japan, eruption, 1940 . C()llecti()ra R.Eliailey, given by G.B . Dalrymple 1, e number 24 in Dalrymple [2) ). Obsidian from Medicine Lake Highlands, Glass Mountains (California) . Collection 1 . Friedman . Age less than 500 yr (see number 4 ir~ Dalrymple (21 ). Olivine inclusion from the Punaru Rider, Tahiti . Parent rock has given a 0 age * . Hualalai flow, 1800-1801, Hawaii . Rock containing inclusion given an age of ca . I by . (see number 5 in Dalrymple 121) .
Collection J.Noctzlin and Collection liJaxieff.
Daniel Kninrnaanacher .
ISOTOPIC COMPOSITION OF ARGON References [ 1I W.B.Clarke, M.A.Beg and H.Craig, Excess 3He in the sea: Evidence for terrestrial primordial helium, Earth Planet . Sci. Letters (in press) . 121 G.B.Dalrymple, 4OAr/36Ar analyses of historic lave flows, Earth Planet. Sci. Letters 6 (1969) 47-55. [31 G.B.Dairymple and J.G .Moore, Argon 40: Excess in submarine pillow basalts from Kilauea Volcano, Hawaii, Science 161(1968) 113 2. (41 P.E.Damon, E.W.Laughlin and J.K .Percious, Problem of excess argon 40 in volcanic rocks, Proc. Symp . Radioactive Dating, Monaco, Intern. At. Energy (1967) 463482.
(51 J.F .Evernden and G.H .Curtis, The K/A dating of the late Cenozoic rocks in Easi Africa and Italy, Current Antropology 6 (1965) 343 [61 W.H.Fleming and H.G .Thode, Argon 38 in pitchblende minerals and nuclear processes in nature : Phys. Rear. 90/3 (1953) 879. [71 G.W.Wetherhilt Variations in the isotopic abundances of neon and argon extracted from radioactive minerals, Phys. Rev. 96/3 (1954) 879 (81 R.E.Zartman, G.J .Wasserburg and J.H .Reynolds, Helium, argon and carbon in some natural gases, J. Geophys. Res. 66/ 1 (1961) 277.