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Rare-gas dating, II. Attempted uranium-helium dating of young volcanic rocks from the madeira archipelago
Earth and Planetary Science Letters, 25 (1975) 142-150 © North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
RARE-GAS DATING, II. ATTEMPTED U R A N I U M - H E L I U M DATING OF YOUNG VOLCANIC ROCKS FROM THE MADEIRA ARCHIPELAGO M.P. FERREIRA, R. MACEDO, V. COSTA and J.H. REYNOLDS* Departamento de Geologia, Umversidade, Coimbra (Portugal) and
J.E. RILEY, Jr.** and M.W. ROWE Department of Chemistry, Texas A and M University, College Station, Texas (U.S.A.)
Revised version received November 13, 1974
Uranium-helium "ages" have been determined for a suite of 25 whole-rock basalts from Madeira and Porto Santo islands in the Madeira Archipelago. We include petrographic descriptions of these samples. Uranium measurements were by delayed-neutron activation analysis. Helium measurements were by isotopic dilution in an all-metal system characterized by very low argon and helium blanks. For 19 of the samples "preferred" K-Ar ages were also obtained. None of the rocks are concordant in these two ages; all have lost helium. The uranium-helium "ages" are best described, on the average, as equal to 74% of the K-Ar ages minus 0.2 m.y. Nevertheless in our suite of young, chemicaUy similar lavas the correlation between the K-Ar age and the helium content is good enough to make the helium measurement worthwhile as a check on a possibly anomalous argon age, especially in a system where the additional labor required to make the helium is minimal. The Columbia River basalt from which standard samples BCR-1 and BCR-2 were prepared was also dated. Its uranium-thorium-helium "age", 4.45 ± 0.24 m.y. shows that this rock has retained even less of its radiogenic helium (30%) than the island basalts.
1. Introduction The u r a n i u m - h e l i u m method o f radiometric dating is historically the oldest [1] but because o f the prevalence of helium loss has been of little practical importance in the very rapid development of geochronology which has taken place since World War II. The older literature was reviewed thoroughly in 1954 by Hurley [2]. In a 1963 publication, Damon and Green [3] reexamined the method using isotopic dilution. Aware that for many rocks rare-gas dating o f whole rock samples is unsatisfactory, their approach was to apply the method to zircon and magnetite separates. They found that carefully selected zircons can exhibit good * Permanent address: Department of Physics, University of California, Berkeley, California 94720. ** Chemical Research Laboratory, Bell Laboratory, Murray Hill, New Jersey 07974.
helium retention but that magnetites usually suffer from the uranium being preferentially located at the surfaces of the grains so that there is loss from the magnetite due to "outgoing" alpha particles. Our approach has been to see if whole rock samples of a suite o f young basaltic rocks could be dated successfully, at least in part, by the u r a n i u m - h e l i u m method. With whole-rock samples "outgoing" alpha particles from grain boundaries are not necessarily lost. And with very young rocks o f favorable structure there might be insignificant diffusive losses. In principle the u r a n i u m - h e l i u m method has a great advantage over the p o t a s s i u m - a r g o n method for very young rocks because o f the expected lack o f atmospheric contamination. Certainly the subject merited investigation because of the great geologic importance of successful dating of young volcanic rocks in which the paleomagnetic record has been imprinted. Since completing the work, we have seen the report o f a
143 similar study carried out concurrently by Leventhal [4] who obtained similar results. His paper also coarains a good review of the more recent literature concerning uranium-helium dating. In another paper in this series [5] we have described our new techniques for rare-gas dating. They include a metal sample system designed for low blanks, which is integrated with an AEI MS-10 mass spectrometer. Planned primarily for K-Ar dating, this system was easy to adapt to U-He dating as well. A tracer of 3He was ineluded in the gas pipette. The sample loader for the system was constructed from Coming Type 1720 glass which is essentially impervious to helium. Otherwise the system was made of metal. By these techniques the helium blank has been suppressed to the point where it is not easily measurable: a typical upper limit for blank 4He is 1.2 X 10 -9 cm 3 STP. In a context of potassium-argon dating, there were no additional complications in the system related to the helium work. Additional labor involved in making the helium measurements was minimal. It involved only brief measurements of the peaks at masses 3, 4, (and 2) in the mass spectrometer after the static measurements for argon were complete.
2. The samples Except for Berkeley interlaboratory standard sample BCR-2, which is a coarsely ground Columbia River basalt from the same quarry as the U.S.G.S. standard rock BCR-1 and which was used as a working standard, the samples for this report were all young whole-rock basalts from Madeira (MAD) and Porto Santa (PS) islands in the Madeira Archipelago. Thin sections were prepared for all the samples and only pure, unaltered rocks were included in the study. Petrographically, the samples we dated were distributed among five categories: (a) Hawaiite. Samples: MAD-6/1971, MAD-23/1971. Both rocks are massive and gray in colour. MAD23/1971 has 5% slightly corroded labradorite phenocrysts (An65 to Arts0). The groundmass has a fluidal texture and its grain size averages 200/~ in MAD6/1971 and 150/~ in MAD-23/1971. The groundmass is made of 70% andesine (An45 to Ans0), 10-15% iron oxides, 5% and 15% augite respectively in Mad-6 and MAD-23, 10% olivine in MAD-6 and 2% analcite in MAD-23.
03) Alkali olivine basalt. Samples: MAD-1A/1971, MAD, I B[1971, MAD-5/1971, MAD-8/1971, MAD9/1971, MAD-10/1971, MAD-11/1971, MAD-14/ 1971, MAD-18/1971, MAD-21/1971, MAD-24/1971, MAD-2511971, MAD-54/1971. These rocks are dark gray in colour and some of them (MAD-8/1971,9, 11,21 and 54) are slightly vesicular. Except for MAD-1A and MAD-1B, these basalts have partially resorbed olivine phenocrysts, smaller than 2 mm and never exceeding 15% of the rock volume. The groundmass has an intergranular texture, with fluidal tendencies in MAD-24. The grain sizes are: < 50/a (MAD-5/1971), < 100/l (11, 25),~ 100/a (1A, 8 ) , " 150 p (IB, 9, 10, 14, 18), < 200/a (21, 24), -- 200/a (54). The gr0undmass is made of 30-45% labradorite (An63 to An53), 3550% augite, 5-15% iron oxides and, in MAD-14, 18, 21 and 25, up to 5% olivine. Analcite amounts to less than 5% and occurs both as an interstitial phase and as an alteration product of the labradorite. Augite is always fresh and olivine shows iddingsite rims in the samples MAD-1A, 5, 9, 10 and 21. Apatite is very scarce. (c) Olivine augite basaits. Samples: MAD-2/1971, MAD-3/1971, MAD-50/1971. Dark gray, massive rocks, with phenocrysts of augite (5-15 %), olivine (10%) and plagioclase (less than 1%). The olivine phenocrysts tend to be corroded and altered to iddingsite. The groundmass has an intergranular texture and its grain size ranges from < 80/1 (MAD-2/1971) to ~--200/a (MAD-50). The groundmass is 40-60% sodic labradorite, 20-35% augite, and 10% iron oxides. Analcite is very scarce. (d) Latite-andesite. Sample: PS- 18A/1961. This massive, light gray rock has andesine (An43) and barkevicite phenocrysts that comprise 5% of the volume. The groundmass has an average grain size of 100 t~ and shows andesine and sanidine microlites, these two amounting to 70%, quartz (5%), cristobalite (8%), augite (5%), iron oxides (5%) and accessory apatite. (e) Ankaramite. Samples: PS-6/1961, MAD-53/ 1971. These rocks are vesicular, dark gray in colour and show a few small (2 mm) olivine and augite phenocrysts. The groundmass, whose grain size is 100/a is composed of au~te (60-70%), plagioclase (15%), iron oxides (5-10%) and olivine (less than 5%). In PS-6/197 the plagioclase occurs as relatively large (500 ~) crystal:
144 that host all the other phases. Analcite is a minor phase in MAD-53/1971 and calcite fills some of the vesicles in PS-6/1961. Workingstandard. Since BCR-2 is the same rock as BCR-1, we quote G.R. Himmelberg's description of the latter from reference [6]. Basalt BCR-1, "An aphanitic, hypocrystalline, basalt with an intersertal texture consisting of randomly oriented plagioclase laths with interstitial augite, brown glass, and iron oxides. The plagioclase is slightly zoned with an approximate composition of An56 (/3 = 1.560 -+ 0.002). Augite is brown with 2 V3, variable from 31 to 44 °. Individual grains of pyroxene may show zoning from augite to subcalcic augite. The glass is partially devitrified and commonly contains abundant iron oxides. Using Rittman's [7] chemical classification of volcanic rocks this sample is an andesine trachybasalt."
3. Methods The helium measurements were made by isotopic dilution, using an almost pure 3He spike. (The ratio 4He/3He, measured in a large sample of the spike gas introduced directly into the mass spectrometer without heating any of the getters in the system, was 0.0054.) The discrimination correction for the mass spectrometer and the 3He content of the pipette were determined by measurements made on unspiked and spiked samples of the helium extracted from the Berkeley interlaboratory standard "Bru" (Bruderheim meteorite) for which helium analyses have been obtained in several laboratories and averaged to give a "standard analysis". In the next paragraph we summarize the details of the procedure. The amount of spike gas delivered by the pipette is attenuated with use of the pipette according to the factor exp (-j/104) for the jth sample delivered. In addition we must define the following quantities: S = apparent (i.e. not corrected for mass discrimination of the mass spectrometer) ratio 4/3 for the spike as determined in a blank run. This quantity is assumed to be a constant for the apparatus. A blank run is one in which all steps of the procedure followed in processing a sample are carried out, except for the dropping of a fresh sample into the crucible. Ci= apparent 4/3 ratio in a calibration mixture of
the ith spike and a convenient quantity of the reference sample, Bru. M/= apparent 4/3 ratio in a mixture of the/th spike and the unknown sample. R = apparent 4/3 ratio in an analysis without spike of the reference material, Bru. A large enough quantity is taken in the unspiked run so that contamination by other sources of helium is insignificant. 4OR = amount of 4He in the reference sample mixed with spike i. 4Qx = amount of 4He in the unknown. The equation obtained f o r 4Qx is then:
F
1
4Qx =t-R(Ci~ S)e--xp(-i---/104)A (M/.- S) exp (-f/104) where the quantity in the square brackets is a calibration factor which we can call Q. It represent the amount of 3He delivered by the pipette (extrapolated to i = 0) as inferred from the calibration run on the Bruderheim standard. We carried out the calibration procedure twice and obtained values of Q which were the same within 2.5%. The average value was (11.83 +- 0.14) X 10 -8 cm 3 STP. The value of S for this work was found to be 0.014 with a standard deviation of 0.010. The discrimination correction for 4He/3He ratios during this period, while not appearing explicitly in the equa, tion, is of some interest. We obtained: (4He/3He)tru e = 0.934 (4He/3He)apparen t During the period in which the He data were obtained for this paper there were frequent runs of a standard basalt, Berkeley interlaboratory standard BCR-2. From the consistent values of the 4He content obtained for BCR-2 we could verify that the discrimination for masses 3 and 4 in the mass spectrometer was quite constant. During one brief period, when several unknowns for this paper and several samples of the BCR-2 standard were run, we experimented with unorthodox settings for the ion source of the MS-10 which ultimately proved to be unsatisfactory. During this period the helium results for BCR-2 were consistently different from before, indicating that there was a different discrimination factor for masses 3 and 4. We were able to deduce the unorthodox discrimination factor from these observations and to
145 correct the data for this effect. This correction has been applied to the data in this paper when necessary and so indicated in the tables. It was a 10% correction. Mass 2 was always monitored along with masses 3 and 4 in order to ensure that there was not sufficient H 2 present to contribute a significant HD peak. Levels of H 2 were always so low that the HD effect is negligible. The uranium content of the samples was measured by delayed-neutron counting at the Texas A and M University. The techniques have been described elsewhere [8]. The uranium contents found ranged from 0.61 to 3.18 ppm; The samples were counted in quintuplicate; the probable error in the mean of the five determinations was, on the average, 1.0% of the mean value. A small (4.5%) correction was applied for delayed neutrons from thorium, which are counted with a sensitivity relative to uranium of 0.01365. It was assumed that the Th/U ratio was the same (3.46) in all the samples as in U.S.G.S. standard rock BCR-1, for which uranium and thorium have been measured by isotope dilution [9]. The Berkeley standard BCR-2 was collected from the same quarry and the same flow as BCR-1 so that the assumed Th/U ratio was certain for that sample. The thorium content is of much more consequence in calculating the uranium-helium "ages" than it is in correcting the uranium values determined by delayed neutron counting. Again we have assumed the ratio 3.46 for all our samples. In a young rock of this composition 45% of the helium originates in the decay of thorium and its daughters so that serious departures from the assumed thorium content would lead to serious misestimates of the helium "age". If rocks with more nearly concordant argon and helium "ages" had been found in this study, thorium measurements would have been undertaken. Of the uranium values determined, only the value for BCR-2 can be compared with published values. Our value for BCR-2 is 1.608 -+ 0.012 ppm, which can be compared with Tatsumoto's value [9] for BCR-1 of 1.73 ppm. The agreement is probably satisfactory in view of the fact that the samples are of a very different grain size. BCR-2 is 2 0 - 3 5 mesh, whereas BCR-1 is a fine-grained powder. It may be that the fines which were discarded in the preparation of the Berkeley sample were richer in uranium than the material retained. There is evidence below that one of the Madeira samples (1 B) has more uranium in the fines than in a fraction of medium coarseness. On the other hand, published values
for the uranium content of BCR-1 by delayed neutron activation are 1.80 [10], 1.42 [11], and 1.61 [4] which average to 1.61 + 0.11 which is exactly our value for BCR-2 so that there may not have been fractionation of uranium between the two standards. The uranium-helium "ages" computed in this paper are based on the following half lives: 238U, 4.51 × 109 yr [12]; 235U, 7.13 X 108 yr [13]; 232Th, 1.42 × 1020yr [14]. For simplicity we assumed that in the emplacement of the lavas all the disintegration products of uranium and thorium were in radioactive equilibrium and, as previously mentioned, that the thorium/uranium ratio was uniformly 3.46 by weight. For the young rocks treated in this paper, a linear relation between 4He content and age is an adequate approximation. One obtains t = 0.0460 [4He]/[U] m.y. where [4He] is the content of 4He in 10 -8 cm 3 STP/g and [U] is the uranium content in ppm. The potassium-argon ages given in this paper are the subject of a separate article, in preparation. Dating of volcanic rocks from the Madeira Archipelago was the first large project undertaken at the new laboratory in Coimbra - more than 100 samples have been run, including exploratory work and replications. Initially, when little was known about the age of the Madeira rocks, runs with 0.3 g of sample were undertaken. In a recent evaluation of all the Madeira ages, we have decided not to report any of the potassiumargon ages obtained in these 0.3-g runs. For reasons that are not completely clear, the potassium-argon ages of the Madeira lavas are not always easy to replicate. Trouble of this kind has been encountered elsewhere by other analysts - for example in dating the Waianae and Koolau lavas from Oahu, Hawaii [ 15]. By using 1-g samples our difficulties have been largely overcome. The potassium-argon ages reported here are "preferred" in three respects. First, they are all based on work with 1-g samples. Secondly, they are from runs for which the purification of the gas samples was complete, and thirdly, they are from runs for which the mass discrimination of the mass spectrometer, as monitored internally in each run by a double (3BAr and 39Ar) spike, was part of a stable series with respect to that quantity. These criteria for acceptance of the K - A r ages are so severe that we presently lack quotable ages for 6 of the 26 samples treated in this paper. But the conclusion6 reached from the other 20 samples are clear-cut.
MAD-18 MAD-21 MAD-23 MAD-24 MAD-25 MAD-50 MAD-53 MAD-54
MAD-14
MAD-10 MAD-I 1
MAD-9
MAD-8
MAD-6-MG
MAD-6-G
MAD-2 MAD-3 MAD-5 MAD-6-M
MAD-1 B-MG
MAD-1B-G
0.344 0.307 0.300 0.337 0.331 1.043 0.328 1.019 0.317 1.003 0.313 0.327 1.005 0.274 1.042 0.345 0.287 0.320 0.303 0.333 1.009 0.276 0.291 0.339 01304 0.318 0.325 0.321 0.318 0.317 0.293
0.310 0.314 0.308 1.020 0.334 1.004 0.305 1.026
MAD-1A
MAD-IB-M
Weight (g)
Sample
TABLE 1
36 11 18 52 54 85 38" 84 39 * 83 12 57 86 13 87 58 14 59 16 41" 128 15 17 43* 21 22 23 24 25 26 27
9 30 31 82 34 81 35 80
Spike No.
14.49 49.70 11.84 28.03 28.18 33.68 31.00 28.79 29.09 29.16 22.97 22.31 22.36 25.10 25.24 18.26 21.51 22.61 11.78 12.52 12.62 27.41 18.72 18.66 53.11 54.19 13.88 0.90 0.37 0.36 0.43
14.53 16.62 16.66 17.23 17.85 18.55 19.15 19.51
± ± ± ±
0.40 0.41 0.41 0.44
[4He] (10 -8 cm 3 STP/g)
2.18 0.86 2.41 1.02 0.98 1.24 1.02 0.61
0.88
1.12 0.90
1.23
1.02
3.17
3.18
1.25 1.89 0.81 3.16
1.21
1.18
1.13
1.14
Uranium (ppm)
0.53 1.21 0.67 0.41 0.41 0.49 0.45 * 0.42 0.42* 0.42 1.04 1.01 1.01 0.94 0.95 0.75 1.10 1.16 0.62 0.65* 0.66 0.58 1.00 0.36* 2.39 2.55 0.51 (0.004 ± 0.018) (0.027 ± 0.031) (0.027 -+ 0.031) (0.032 ± 0.033)
0.59 0.67 0.68 0.70 0.70 0.72 0.73 0.74
U-He age (m.y.)
0.0029
0.024
0.040
0.0035
0.017
0.00071
0.023
0.047
0.0099
0.019
0.016
0.059
Standard deviation (m.y.)
10.0
3.7
3.5
0.4
1.7
0.2
5.3
10.8
1.4
2.7
2.3
9.4
(%)
± ± ± ± ±
0.03 0.05 0.02 0.06 0.05 0.39 ± 0.03 0.89 -+ 0.04**
0~85 1.26 1.35 3.94 3.25
0.84 ± 0.02
1.59 ± 0.03
1.27 ± 0.05
1.61 ± 0.04
0.95 ± 0.02**
0.90 ± 0.02**
2.01 ± 0.05 1.18 ± 0.03 1.14 ± 0.02**
1.21±0.03
1.22±0.04
1.20±0.06
K-Ar Age (m.y.)
He He He He
age age age age
H H H H
0.029 0.072 0.072 0.081
m.y. m.y. m.y. m.y.
MG defined above
G defined above
M defined above
M = m e d i u m sieve fraction G = "grosso" or coarse sieve fraction MG = " m u i t o grosso" or very coarse sieve fraction
Comments
0.302 0.304 0.303 0.303 0.301 0.302 0.301 0.320 0.368 0.357 0.377 0.345 0.355
PS-6 PS-18A BCR-2
28 29 2 4 5 6 7 8 44* 48* 49* 119 154
Spike No.
140.05 91.68 156.23 156.51 155.70 153.16 155.29 149.12 154.58 156.35 156.38 158.77 161.36
[4He] (10-8 cm 3 STP/g)
2.81 2.50 1.61
Uranium (ppm)
2.29 1.69 4.47 4.48 4.45 4.38 4.44 4.26 4.42* 4.47* 4.47* 4.54 4.62
U-He age (m.y.)
0.088
Standard deviation (m.y.)
2.0
(%)
-=14.79
K-Ar Age (m.y.)
Comments
* For these samples a correction was applied to the 4He concentration because of unorthodox 4/3 discrimination (see text). ** For these samples an error of 0.13 m.y. in the K - A r ages was computed on the basis of replicated samples. This error (0.13 m.y.) is thought to be a better measure of the analytical precision than the internally computed errors appearing in the body of the table.
Weight (g)
Sample
TABLE 1 (continued)
148
4. Results The results are set out in Table 1. We comment as follows: (1) For 12 of the island samples we have one or more replicate ages. The standard deviation for each sample where there are replications is listed in the table. On the average these are 0.02 m.y. or 4.3%. This average standard deviation is an adequate measure of the precision of the helium "ages". In addition a systematic error of 5.5% has to be assigned due to the fact that the spikepipette has been calibrated versus the Bruderheim standard whose 4 He content is uncertain by that amount, judging from interlaboratory analyses [16]. With a few exceptions, the degree of agreement among replications is quite satisfactory even though 0.3-g samples are involved in every case. The difficulty, to which we have alluded above, in repeating K - A r ages for small samples of Madeira lavas does not occur for U-He "ages". (2) For two samples, MAD-53/1971 and MAD54/1971 the content of 4He is so low that it is not meaningful to quote an "age" because the spread of 4He values measured in blank runs overlaps the 4He measured in the samples. The nominal "ages" are given in parentheses, but we lay more stress on upper limits to the "ages" which also appear in the table. These are calculated by attributing all 4He encountered in the sample to uranium or thorium decay and none to the blank. So calculated the upper limit for the U-He "age" of MAD-53 is 0.029 m.y. and that for MAD-54 is 0.072 m.y. These two samples, for reasons we do not understand, are extraordinarily low in 4He. And the fact has been verified: there are replications for sample MAD-54 (see Table) and runs were made (not reported in the Table) of MAD-53 without spike which verified the low helium content. (3) The U-He "ages" are invariably younger than the K - A r ages for the 20 samples where we can make the comparison. The K - A r ages require some discussion. They are all "preferred" ages in the senses discussed above. All the errors quoted in the body of the Table for the ages are internally computed, allowing every measured quantity which enters the calculation to vary appropriately. Even so it appears, because of the "refractory nature" of the Madeira basalts for K - A r age determinations, that these internally computed errors are underestimates. A better idea of the analytical errors can probably be obtained by comparing the K - A r ages determined for the three dif-
ferent grain sizes (M = medium, G = coarse, MG = very coarse) of samples MAD-1B and MAD-6. Although the ages for the three grain sizes of MAD-1B are almost the same, the ages for the three samples of MAD-6 show a standard deviation of 0.13 m.y. or 13% of the age. A similar standard deviation was observed for two determinations of the age of MAD-54. We believe that these "externally computed" errors are a more reasonable estimate of the analytical error for the Madeira basalts. tn addition to the analytical errors there is the possibility that the ratio:36Ar/40Ar for trapped argon differs in some of the samples from the atmospheric value (0.003384) we have assumed. Typically, a 1% error in this trapped ratio would lead for our samples to an error in age of 0.07 m.y. The relationship between the U-He "ages" and the K - A r ages is shown graphically in Fig. 1. We have averaged the results for the gram size fractions of MAD-1B and MAD-6 and plotted just a single point for each. (4) In discussing this relationship, one can adopt three points of view. (a) If samples MAD-53 and MAD-54, which exhibit gross helium loss, are excluded from consideration, the other Madeira samples in Fig. 1 have lost, very roughly speaking, a constant fraction of the radiogenic helium On the average (again omitting samples 53 and 54) this loss fraction is 0.37 + 0.15 for the 13 samples. 5 /
i
i i j*
L /i (..
0 0 3
/.
I
• I 4
t
I
i
I
r
I l
Same plot on contracted scale showing BCR-2
I
I 8 I
I F
I
BcR21
-"
I 12
I I
~ ] 16 I
I
/
/
~'/// /~o /"
/
J
.L~.\/~/// ~C,/
25o 24,
i I
/// I
////27o
///
188 o6
n
~ogCP..2--
o23
//
0'531
•3
• e8
NUMBERS IDENTIFY MADEIRA SAMPLES
541 1
I
I 2
I
I 3
K-Ar age, m.y.
Fig. 1. Comparison of rare-gas ages for the basalts.
I 4
149 (b) On the other hand, the points can also be described (more roughly in this case and eliminating samples 54, 23, and 24) as defining a line parallel to the concordancy line but below it by about 0.5 m.y. This would be saying that each sample lacks the amount of helium it would accumulate in 0.5 m.y. (c) A better fit for the entire array of Madeira points (omitting none) would be a combination of these two types of loss: a fractional loss of 26% plus a small offset of 0.2 m.y. We have not been able to suggest any reason, based on the petrographic examinations, why samples 53 and 54 should be so anomalously lossy for helium. (5) Our working standard BCR-2, a rock which has been very well dated (14.8 m.y.) by the K - A r method has lost a greater fraction of its radiogenic helium than any of the Madeira samples (except 23, 53 and 54), namely 70%. This effect could be reasonably attributed to the greater age of the sample or to the fact that it is a different type of rock, a theolitic basalt representing one of the great basalt "floods".
5. Discussion We had hoped we might find a subset of samples for which U - H e "ages" agreed with the K - A r ages and that such a subset might have petrographic characteristics which would enable us to predict this concordancy. But even without making thorium analyses it is clear that our samples do not include such a concordant population. Leventhal [4] similarly failed to find concordant ages for young basalts from the San Francisco Volcanic Field near Flagstaff, Arizona. Neve~rtheless, the marked correlation one finds between KZAr age and 4He content can be very useful in age determinations on young rocks so that in a system such as ours, where the helium measurement involves literally only a few extra seconds of labor per sample, recording that datum is worthwhile. In Fig. 2 we have plotted the total 4He content in those samples of Madeira basalt for which we had reliable K - A r ages but no uranium data so that they could not be included in Fig. 1. All the samples weighted 1 g, to within a few percent, so that the ordinate is essentially4He concentration. Clearly there is a
160
I
u
F
I
I
NUMBERS IDENTIFY MADEIRA SAMPLES
o ~c 42e
E ~ 80
e28
T
31o 40
55e
39 43
8
50,,h
400
"
0
iTo
I
52~34 46 I 1.0
• •
• 58
26
e30
,~
6~ I
I 2.0
K-At
age, my.
I
I 3.0
I
4.0
Fig. 2. Helium content vs. K-Ar ages for additional Madeira basalts.
good correlation between the x andy coordinates. The correlation is very useful in confirming the resuits for samples from this population with extreme K - A r ages, such as MAD-51 and MAD-58. The uranium contents of the Madeira basalts are constant to within about 50% so that for the purpose of detecting marked deviations from the correlation it is not necessary to measure the uranium.
Acknowledgements We thank J.T. Ferreira for his technical assistance. Establishment of the Geochronology Laboratory at the University of Coimbra was a joint project with the University of California, Berkeley. This work was supported in part by the Arthur L. Day Fund of the U.S. National Academy of Sciences, by the U.S. National Science Foundation, and by the Instituto de Alta Cultura, Lisbon. One of us (J.H. Reynolds) received support in Portugal as a Fulbright-Hays re~ search scholar of the U.S. Department of State. The laboratory is an ongoing project of the Instituto de Alta Cultura with occasional participation by the Mass Spectroscopy Laboratory at the Department of Physics, Berkeley, which is supported in part by the U.S. Atomic Energy Commission. In that connection this report bears number UCB-34P-32-89.
150
References 1 E. Rutherford, Radioactive Transformation (Yale University Press, 1906) 187-193. 2 P.M. Hurley, The helium age method and distribution and migration of helium in rocks, in Nuclear Geology, H. Faul, ed. (John Wiley and Sons, New York, 1954) 301-329. 3 P.E. Damon and W.D. Green, Investigation of the helium age dating method by stable isotope-dilution technique, in: Radioactive Dating, Proceedings of the 1962 Athens Symposium (International Atomic Energy Agency, Vienna, 1963) 5 5 - 7 1 . 4 J.S. Leventhal, Chronology and correlation of young basalts by uranium-thorium-helium measurements, Ph.D. Thesis, Department of Geosciences, University of Arizona, Tuscon (1972). 5 V. Costa, M.P. Ferreira, R. Macedo and J.H. Reynolds, Raregas dating, I. A demountable metal system with low blanks, Earth Planet. Sci. Lett. 25 (1975) 131-141. 6 F.J. Flanagan, U.S. Geological Survey silicate rock standards, Geochim. Cosmochim. Acta 31 (1967) 289-308. 7 A. Rittman, Nomenclature of volcanic rocks, Bull Volcanol. Ser. II, XII (1952) 76-102. 8 J.E. Riley, Jr., A system for the determination of uranium in meteorites by delayed-neutron counting, M.S. Thesis, Texas A. and M. University, College Station, Texas (1971).
9 M. Tatsumoto (1968) privately communicated to and reported by F.J. Flanagan, U.S. Geological Survey standards, II, Geochim. Cosmochim. Acta 33 (1969) 81. 10 N.H. Gale, Development of delayed-neutron technique as rapid and precise method for determination of uranium and thorium at trace levels in rocks and minerals, with applications to isotope geochronology, in: Radioactive Dating and Methods of Low Level Counting, Proceedings of the 1967 Monaco symposium (International Atomic Energy Agency, Vienna, 1967) 431. 11 E.I. Hamilton, The uranium content of some international standards, Earth Planet. Sci. Lett. 1 (1966) 317. 12 A.F. Kovarik and N.I. Adams, Redetermination of the disintegration constant of 238U, Phys. Rev. 98 (1955) 46. 13 E.H. Fleming, Jr., A. Ghiorso and B.B. Cunningham, The specific alpha-activities and half-lives of 234U, 235U, and 236U, Phys. Rev. 88 (1952) 642. 14 F.E. Senftle, T.A. Farley, and N. Lazar, Half-life of 232Th and the branching ratio of 212Bi, Phys. Rev. 104 (1956) 1629. 15 R.R. Doell and G.B. Dalrympte, Potassium-argon ages and paleo-magnetism of the Waianae and Koolau Volcani~ Series, Oahu, Hawaii, Geol. Soc. Am. Bull. 84 (1973) 1217. 16 J.H. Reynolds, Memorandum to holders of the Bruderheim Standard (January 1973 ) unpublished.
RARE-GAS DATING, II. ATTEMPTED U R A N I U M - H E L I U M DATING OF YOUNG VOLCANIC ROCKS FROM THE MADEIRA ARCHIPELAGO M.P. FERREIRA, R. MACEDO, V. COSTA and J.H. REYNOLDS* Departamento de Geologia, Umversidade, Coimbra (Portugal) and
J.E. RILEY, Jr.** and M.W. ROWE Department of Chemistry, Texas A and M University, College Station, Texas (U.S.A.)
Revised version received November 13, 1974
Uranium-helium "ages" have been determined for a suite of 25 whole-rock basalts from Madeira and Porto Santo islands in the Madeira Archipelago. We include petrographic descriptions of these samples. Uranium measurements were by delayed-neutron activation analysis. Helium measurements were by isotopic dilution in an all-metal system characterized by very low argon and helium blanks. For 19 of the samples "preferred" K-Ar ages were also obtained. None of the rocks are concordant in these two ages; all have lost helium. The uranium-helium "ages" are best described, on the average, as equal to 74% of the K-Ar ages minus 0.2 m.y. Nevertheless in our suite of young, chemicaUy similar lavas the correlation between the K-Ar age and the helium content is good enough to make the helium measurement worthwhile as a check on a possibly anomalous argon age, especially in a system where the additional labor required to make the helium is minimal. The Columbia River basalt from which standard samples BCR-1 and BCR-2 were prepared was also dated. Its uranium-thorium-helium "age", 4.45 ± 0.24 m.y. shows that this rock has retained even less of its radiogenic helium (30%) than the island basalts.
1. Introduction The u r a n i u m - h e l i u m method o f radiometric dating is historically the oldest [1] but because o f the prevalence of helium loss has been of little practical importance in the very rapid development of geochronology which has taken place since World War II. The older literature was reviewed thoroughly in 1954 by Hurley [2]. In a 1963 publication, Damon and Green [3] reexamined the method using isotopic dilution. Aware that for many rocks rare-gas dating o f whole rock samples is unsatisfactory, their approach was to apply the method to zircon and magnetite separates. They found that carefully selected zircons can exhibit good * Permanent address: Department of Physics, University of California, Berkeley, California 94720. ** Chemical Research Laboratory, Bell Laboratory, Murray Hill, New Jersey 07974.
helium retention but that magnetites usually suffer from the uranium being preferentially located at the surfaces of the grains so that there is loss from the magnetite due to "outgoing" alpha particles. Our approach has been to see if whole rock samples of a suite o f young basaltic rocks could be dated successfully, at least in part, by the u r a n i u m - h e l i u m method. With whole-rock samples "outgoing" alpha particles from grain boundaries are not necessarily lost. And with very young rocks o f favorable structure there might be insignificant diffusive losses. In principle the u r a n i u m - h e l i u m method has a great advantage over the p o t a s s i u m - a r g o n method for very young rocks because o f the expected lack o f atmospheric contamination. Certainly the subject merited investigation because of the great geologic importance of successful dating of young volcanic rocks in which the paleomagnetic record has been imprinted. Since completing the work, we have seen the report o f a
143 similar study carried out concurrently by Leventhal [4] who obtained similar results. His paper also coarains a good review of the more recent literature concerning uranium-helium dating. In another paper in this series [5] we have described our new techniques for rare-gas dating. They include a metal sample system designed for low blanks, which is integrated with an AEI MS-10 mass spectrometer. Planned primarily for K-Ar dating, this system was easy to adapt to U-He dating as well. A tracer of 3He was ineluded in the gas pipette. The sample loader for the system was constructed from Coming Type 1720 glass which is essentially impervious to helium. Otherwise the system was made of metal. By these techniques the helium blank has been suppressed to the point where it is not easily measurable: a typical upper limit for blank 4He is 1.2 X 10 -9 cm 3 STP. In a context of potassium-argon dating, there were no additional complications in the system related to the helium work. Additional labor involved in making the helium measurements was minimal. It involved only brief measurements of the peaks at masses 3, 4, (and 2) in the mass spectrometer after the static measurements for argon were complete.
2. The samples Except for Berkeley interlaboratory standard sample BCR-2, which is a coarsely ground Columbia River basalt from the same quarry as the U.S.G.S. standard rock BCR-1 and which was used as a working standard, the samples for this report were all young whole-rock basalts from Madeira (MAD) and Porto Santa (PS) islands in the Madeira Archipelago. Thin sections were prepared for all the samples and only pure, unaltered rocks were included in the study. Petrographically, the samples we dated were distributed among five categories: (a) Hawaiite. Samples: MAD-6/1971, MAD-23/1971. Both rocks are massive and gray in colour. MAD23/1971 has 5% slightly corroded labradorite phenocrysts (An65 to Arts0). The groundmass has a fluidal texture and its grain size averages 200/~ in MAD6/1971 and 150/~ in MAD-23/1971. The groundmass is made of 70% andesine (An45 to Ans0), 10-15% iron oxides, 5% and 15% augite respectively in Mad-6 and MAD-23, 10% olivine in MAD-6 and 2% analcite in MAD-23.
03) Alkali olivine basalt. Samples: MAD-1A/1971, MAD, I B[1971, MAD-5/1971, MAD-8/1971, MAD9/1971, MAD-10/1971, MAD-11/1971, MAD-14/ 1971, MAD-18/1971, MAD-21/1971, MAD-24/1971, MAD-2511971, MAD-54/1971. These rocks are dark gray in colour and some of them (MAD-8/1971,9, 11,21 and 54) are slightly vesicular. Except for MAD-1A and MAD-1B, these basalts have partially resorbed olivine phenocrysts, smaller than 2 mm and never exceeding 15% of the rock volume. The groundmass has an intergranular texture, with fluidal tendencies in MAD-24. The grain sizes are: < 50/a (MAD-5/1971), < 100/l (11, 25),~ 100/a (1A, 8 ) , " 150 p (IB, 9, 10, 14, 18), < 200/a (21, 24), -- 200/a (54). The gr0undmass is made of 30-45% labradorite (An63 to An53), 3550% augite, 5-15% iron oxides and, in MAD-14, 18, 21 and 25, up to 5% olivine. Analcite amounts to less than 5% and occurs both as an interstitial phase and as an alteration product of the labradorite. Augite is always fresh and olivine shows iddingsite rims in the samples MAD-1A, 5, 9, 10 and 21. Apatite is very scarce. (c) Olivine augite basaits. Samples: MAD-2/1971, MAD-3/1971, MAD-50/1971. Dark gray, massive rocks, with phenocrysts of augite (5-15 %), olivine (10%) and plagioclase (less than 1%). The olivine phenocrysts tend to be corroded and altered to iddingsite. The groundmass has an intergranular texture and its grain size ranges from < 80/1 (MAD-2/1971) to ~--200/a (MAD-50). The groundmass is 40-60% sodic labradorite, 20-35% augite, and 10% iron oxides. Analcite is very scarce. (d) Latite-andesite. Sample: PS- 18A/1961. This massive, light gray rock has andesine (An43) and barkevicite phenocrysts that comprise 5% of the volume. The groundmass has an average grain size of 100 t~ and shows andesine and sanidine microlites, these two amounting to 70%, quartz (5%), cristobalite (8%), augite (5%), iron oxides (5%) and accessory apatite. (e) Ankaramite. Samples: PS-6/1961, MAD-53/ 1971. These rocks are vesicular, dark gray in colour and show a few small (2 mm) olivine and augite phenocrysts. The groundmass, whose grain size is 100/a is composed of au~te (60-70%), plagioclase (15%), iron oxides (5-10%) and olivine (less than 5%). In PS-6/197 the plagioclase occurs as relatively large (500 ~) crystal:
144 that host all the other phases. Analcite is a minor phase in MAD-53/1971 and calcite fills some of the vesicles in PS-6/1961. Workingstandard. Since BCR-2 is the same rock as BCR-1, we quote G.R. Himmelberg's description of the latter from reference [6]. Basalt BCR-1, "An aphanitic, hypocrystalline, basalt with an intersertal texture consisting of randomly oriented plagioclase laths with interstitial augite, brown glass, and iron oxides. The plagioclase is slightly zoned with an approximate composition of An56 (/3 = 1.560 -+ 0.002). Augite is brown with 2 V3, variable from 31 to 44 °. Individual grains of pyroxene may show zoning from augite to subcalcic augite. The glass is partially devitrified and commonly contains abundant iron oxides. Using Rittman's [7] chemical classification of volcanic rocks this sample is an andesine trachybasalt."
3. Methods The helium measurements were made by isotopic dilution, using an almost pure 3He spike. (The ratio 4He/3He, measured in a large sample of the spike gas introduced directly into the mass spectrometer without heating any of the getters in the system, was 0.0054.) The discrimination correction for the mass spectrometer and the 3He content of the pipette were determined by measurements made on unspiked and spiked samples of the helium extracted from the Berkeley interlaboratory standard "Bru" (Bruderheim meteorite) for which helium analyses have been obtained in several laboratories and averaged to give a "standard analysis". In the next paragraph we summarize the details of the procedure. The amount of spike gas delivered by the pipette is attenuated with use of the pipette according to the factor exp (-j/104) for the jth sample delivered. In addition we must define the following quantities: S = apparent (i.e. not corrected for mass discrimination of the mass spectrometer) ratio 4/3 for the spike as determined in a blank run. This quantity is assumed to be a constant for the apparatus. A blank run is one in which all steps of the procedure followed in processing a sample are carried out, except for the dropping of a fresh sample into the crucible. Ci= apparent 4/3 ratio in a calibration mixture of
the ith spike and a convenient quantity of the reference sample, Bru. M/= apparent 4/3 ratio in a mixture of the/th spike and the unknown sample. R = apparent 4/3 ratio in an analysis without spike of the reference material, Bru. A large enough quantity is taken in the unspiked run so that contamination by other sources of helium is insignificant. 4OR = amount of 4He in the reference sample mixed with spike i. 4Qx = amount of 4He in the unknown. The equation obtained f o r 4Qx is then:
F
1
4Qx =t-R(Ci~ S)e--xp(-i---/104)A (M/.- S) exp (-f/104) where the quantity in the square brackets is a calibration factor which we can call Q. It represent the amount of 3He delivered by the pipette (extrapolated to i = 0) as inferred from the calibration run on the Bruderheim standard. We carried out the calibration procedure twice and obtained values of Q which were the same within 2.5%. The average value was (11.83 +- 0.14) X 10 -8 cm 3 STP. The value of S for this work was found to be 0.014 with a standard deviation of 0.010. The discrimination correction for 4He/3He ratios during this period, while not appearing explicitly in the equa, tion, is of some interest. We obtained: (4He/3He)tru e = 0.934 (4He/3He)apparen t During the period in which the He data were obtained for this paper there were frequent runs of a standard basalt, Berkeley interlaboratory standard BCR-2. From the consistent values of the 4He content obtained for BCR-2 we could verify that the discrimination for masses 3 and 4 in the mass spectrometer was quite constant. During one brief period, when several unknowns for this paper and several samples of the BCR-2 standard were run, we experimented with unorthodox settings for the ion source of the MS-10 which ultimately proved to be unsatisfactory. During this period the helium results for BCR-2 were consistently different from before, indicating that there was a different discrimination factor for masses 3 and 4. We were able to deduce the unorthodox discrimination factor from these observations and to
145 correct the data for this effect. This correction has been applied to the data in this paper when necessary and so indicated in the tables. It was a 10% correction. Mass 2 was always monitored along with masses 3 and 4 in order to ensure that there was not sufficient H 2 present to contribute a significant HD peak. Levels of H 2 were always so low that the HD effect is negligible. The uranium content of the samples was measured by delayed-neutron counting at the Texas A and M University. The techniques have been described elsewhere [8]. The uranium contents found ranged from 0.61 to 3.18 ppm; The samples were counted in quintuplicate; the probable error in the mean of the five determinations was, on the average, 1.0% of the mean value. A small (4.5%) correction was applied for delayed neutrons from thorium, which are counted with a sensitivity relative to uranium of 0.01365. It was assumed that the Th/U ratio was the same (3.46) in all the samples as in U.S.G.S. standard rock BCR-1, for which uranium and thorium have been measured by isotope dilution [9]. The Berkeley standard BCR-2 was collected from the same quarry and the same flow as BCR-1 so that the assumed Th/U ratio was certain for that sample. The thorium content is of much more consequence in calculating the uranium-helium "ages" than it is in correcting the uranium values determined by delayed neutron counting. Again we have assumed the ratio 3.46 for all our samples. In a young rock of this composition 45% of the helium originates in the decay of thorium and its daughters so that serious departures from the assumed thorium content would lead to serious misestimates of the helium "age". If rocks with more nearly concordant argon and helium "ages" had been found in this study, thorium measurements would have been undertaken. Of the uranium values determined, only the value for BCR-2 can be compared with published values. Our value for BCR-2 is 1.608 -+ 0.012 ppm, which can be compared with Tatsumoto's value [9] for BCR-1 of 1.73 ppm. The agreement is probably satisfactory in view of the fact that the samples are of a very different grain size. BCR-2 is 2 0 - 3 5 mesh, whereas BCR-1 is a fine-grained powder. It may be that the fines which were discarded in the preparation of the Berkeley sample were richer in uranium than the material retained. There is evidence below that one of the Madeira samples (1 B) has more uranium in the fines than in a fraction of medium coarseness. On the other hand, published values
for the uranium content of BCR-1 by delayed neutron activation are 1.80 [10], 1.42 [11], and 1.61 [4] which average to 1.61 + 0.11 which is exactly our value for BCR-2 so that there may not have been fractionation of uranium between the two standards. The uranium-helium "ages" computed in this paper are based on the following half lives: 238U, 4.51 × 109 yr [12]; 235U, 7.13 X 108 yr [13]; 232Th, 1.42 × 1020yr [14]. For simplicity we assumed that in the emplacement of the lavas all the disintegration products of uranium and thorium were in radioactive equilibrium and, as previously mentioned, that the thorium/uranium ratio was uniformly 3.46 by weight. For the young rocks treated in this paper, a linear relation between 4He content and age is an adequate approximation. One obtains t = 0.0460 [4He]/[U] m.y. where [4He] is the content of 4He in 10 -8 cm 3 STP/g and [U] is the uranium content in ppm. The potassium-argon ages given in this paper are the subject of a separate article, in preparation. Dating of volcanic rocks from the Madeira Archipelago was the first large project undertaken at the new laboratory in Coimbra - more than 100 samples have been run, including exploratory work and replications. Initially, when little was known about the age of the Madeira rocks, runs with 0.3 g of sample were undertaken. In a recent evaluation of all the Madeira ages, we have decided not to report any of the potassiumargon ages obtained in these 0.3-g runs. For reasons that are not completely clear, the potassium-argon ages of the Madeira lavas are not always easy to replicate. Trouble of this kind has been encountered elsewhere by other analysts - for example in dating the Waianae and Koolau lavas from Oahu, Hawaii [ 15]. By using 1-g samples our difficulties have been largely overcome. The potassium-argon ages reported here are "preferred" in three respects. First, they are all based on work with 1-g samples. Secondly, they are from runs for which the purification of the gas samples was complete, and thirdly, they are from runs for which the mass discrimination of the mass spectrometer, as monitored internally in each run by a double (3BAr and 39Ar) spike, was part of a stable series with respect to that quantity. These criteria for acceptance of the K - A r ages are so severe that we presently lack quotable ages for 6 of the 26 samples treated in this paper. But the conclusion6 reached from the other 20 samples are clear-cut.
MAD-18 MAD-21 MAD-23 MAD-24 MAD-25 MAD-50 MAD-53 MAD-54
MAD-14
MAD-10 MAD-I 1
MAD-9
MAD-8
MAD-6-MG
MAD-6-G
MAD-2 MAD-3 MAD-5 MAD-6-M
MAD-1 B-MG
MAD-1B-G
0.344 0.307 0.300 0.337 0.331 1.043 0.328 1.019 0.317 1.003 0.313 0.327 1.005 0.274 1.042 0.345 0.287 0.320 0.303 0.333 1.009 0.276 0.291 0.339 01304 0.318 0.325 0.321 0.318 0.317 0.293
0.310 0.314 0.308 1.020 0.334 1.004 0.305 1.026
MAD-1A
MAD-IB-M
Weight (g)
Sample
TABLE 1
36 11 18 52 54 85 38" 84 39 * 83 12 57 86 13 87 58 14 59 16 41" 128 15 17 43* 21 22 23 24 25 26 27
9 30 31 82 34 81 35 80
Spike No.
14.49 49.70 11.84 28.03 28.18 33.68 31.00 28.79 29.09 29.16 22.97 22.31 22.36 25.10 25.24 18.26 21.51 22.61 11.78 12.52 12.62 27.41 18.72 18.66 53.11 54.19 13.88 0.90 0.37 0.36 0.43
14.53 16.62 16.66 17.23 17.85 18.55 19.15 19.51
± ± ± ±
0.40 0.41 0.41 0.44
[4He] (10 -8 cm 3 STP/g)
2.18 0.86 2.41 1.02 0.98 1.24 1.02 0.61
0.88
1.12 0.90
1.23
1.02
3.17
3.18
1.25 1.89 0.81 3.16
1.21
1.18
1.13
1.14
Uranium (ppm)
0.53 1.21 0.67 0.41 0.41 0.49 0.45 * 0.42 0.42* 0.42 1.04 1.01 1.01 0.94 0.95 0.75 1.10 1.16 0.62 0.65* 0.66 0.58 1.00 0.36* 2.39 2.55 0.51 (0.004 ± 0.018) (0.027 ± 0.031) (0.027 -+ 0.031) (0.032 ± 0.033)
0.59 0.67 0.68 0.70 0.70 0.72 0.73 0.74
U-He age (m.y.)
0.0029
0.024
0.040
0.0035
0.017
0.00071
0.023
0.047
0.0099
0.019
0.016
0.059
Standard deviation (m.y.)
10.0
3.7
3.5
0.4
1.7
0.2
5.3
10.8
1.4
2.7
2.3
9.4
(%)
± ± ± ± ±
0.03 0.05 0.02 0.06 0.05 0.39 ± 0.03 0.89 -+ 0.04**
0~85 1.26 1.35 3.94 3.25
0.84 ± 0.02
1.59 ± 0.03
1.27 ± 0.05
1.61 ± 0.04
0.95 ± 0.02**
0.90 ± 0.02**
2.01 ± 0.05 1.18 ± 0.03 1.14 ± 0.02**
1.21±0.03
1.22±0.04
1.20±0.06
K-Ar Age (m.y.)
He He He He
age age age age
H H H H
0.029 0.072 0.072 0.081
m.y. m.y. m.y. m.y.
MG defined above
G defined above
M defined above
M = m e d i u m sieve fraction G = "grosso" or coarse sieve fraction MG = " m u i t o grosso" or very coarse sieve fraction
Comments
0.302 0.304 0.303 0.303 0.301 0.302 0.301 0.320 0.368 0.357 0.377 0.345 0.355
PS-6 PS-18A BCR-2
28 29 2 4 5 6 7 8 44* 48* 49* 119 154
Spike No.
140.05 91.68 156.23 156.51 155.70 153.16 155.29 149.12 154.58 156.35 156.38 158.77 161.36
[4He] (10-8 cm 3 STP/g)
2.81 2.50 1.61
Uranium (ppm)
2.29 1.69 4.47 4.48 4.45 4.38 4.44 4.26 4.42* 4.47* 4.47* 4.54 4.62
U-He age (m.y.)
0.088
Standard deviation (m.y.)
2.0
(%)
-=14.79
K-Ar Age (m.y.)
Comments
* For these samples a correction was applied to the 4He concentration because of unorthodox 4/3 discrimination (see text). ** For these samples an error of 0.13 m.y. in the K - A r ages was computed on the basis of replicated samples. This error (0.13 m.y.) is thought to be a better measure of the analytical precision than the internally computed errors appearing in the body of the table.
Weight (g)
Sample
TABLE 1 (continued)
148
4. Results The results are set out in Table 1. We comment as follows: (1) For 12 of the island samples we have one or more replicate ages. The standard deviation for each sample where there are replications is listed in the table. On the average these are 0.02 m.y. or 4.3%. This average standard deviation is an adequate measure of the precision of the helium "ages". In addition a systematic error of 5.5% has to be assigned due to the fact that the spikepipette has been calibrated versus the Bruderheim standard whose 4 He content is uncertain by that amount, judging from interlaboratory analyses [16]. With a few exceptions, the degree of agreement among replications is quite satisfactory even though 0.3-g samples are involved in every case. The difficulty, to which we have alluded above, in repeating K - A r ages for small samples of Madeira lavas does not occur for U-He "ages". (2) For two samples, MAD-53/1971 and MAD54/1971 the content of 4He is so low that it is not meaningful to quote an "age" because the spread of 4He values measured in blank runs overlaps the 4He measured in the samples. The nominal "ages" are given in parentheses, but we lay more stress on upper limits to the "ages" which also appear in the table. These are calculated by attributing all 4He encountered in the sample to uranium or thorium decay and none to the blank. So calculated the upper limit for the U-He "age" of MAD-53 is 0.029 m.y. and that for MAD-54 is 0.072 m.y. These two samples, for reasons we do not understand, are extraordinarily low in 4He. And the fact has been verified: there are replications for sample MAD-54 (see Table) and runs were made (not reported in the Table) of MAD-53 without spike which verified the low helium content. (3) The U-He "ages" are invariably younger than the K - A r ages for the 20 samples where we can make the comparison. The K - A r ages require some discussion. They are all "preferred" ages in the senses discussed above. All the errors quoted in the body of the Table for the ages are internally computed, allowing every measured quantity which enters the calculation to vary appropriately. Even so it appears, because of the "refractory nature" of the Madeira basalts for K - A r age determinations, that these internally computed errors are underestimates. A better idea of the analytical errors can probably be obtained by comparing the K - A r ages determined for the three dif-
ferent grain sizes (M = medium, G = coarse, MG = very coarse) of samples MAD-1B and MAD-6. Although the ages for the three grain sizes of MAD-1B are almost the same, the ages for the three samples of MAD-6 show a standard deviation of 0.13 m.y. or 13% of the age. A similar standard deviation was observed for two determinations of the age of MAD-54. We believe that these "externally computed" errors are a more reasonable estimate of the analytical error for the Madeira basalts. tn addition to the analytical errors there is the possibility that the ratio:36Ar/40Ar for trapped argon differs in some of the samples from the atmospheric value (0.003384) we have assumed. Typically, a 1% error in this trapped ratio would lead for our samples to an error in age of 0.07 m.y. The relationship between the U-He "ages" and the K - A r ages is shown graphically in Fig. 1. We have averaged the results for the gram size fractions of MAD-1B and MAD-6 and plotted just a single point for each. (4) In discussing this relationship, one can adopt three points of view. (a) If samples MAD-53 and MAD-54, which exhibit gross helium loss, are excluded from consideration, the other Madeira samples in Fig. 1 have lost, very roughly speaking, a constant fraction of the radiogenic helium On the average (again omitting samples 53 and 54) this loss fraction is 0.37 + 0.15 for the 13 samples. 5 /
i
i i j*
L /i (..
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I
• I 4
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I
i
I
r
I l
Same plot on contracted scale showing BCR-2
I
I 8 I
I F
I
BcR21
-"
I 12
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~ ] 16 I
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NUMBERS IDENTIFY MADEIRA SAMPLES
541 1
I
I 2
I
I 3
K-Ar age, m.y.
Fig. 1. Comparison of rare-gas ages for the basalts.
I 4
149 (b) On the other hand, the points can also be described (more roughly in this case and eliminating samples 54, 23, and 24) as defining a line parallel to the concordancy line but below it by about 0.5 m.y. This would be saying that each sample lacks the amount of helium it would accumulate in 0.5 m.y. (c) A better fit for the entire array of Madeira points (omitting none) would be a combination of these two types of loss: a fractional loss of 26% plus a small offset of 0.2 m.y. We have not been able to suggest any reason, based on the petrographic examinations, why samples 53 and 54 should be so anomalously lossy for helium. (5) Our working standard BCR-2, a rock which has been very well dated (14.8 m.y.) by the K - A r method has lost a greater fraction of its radiogenic helium than any of the Madeira samples (except 23, 53 and 54), namely 70%. This effect could be reasonably attributed to the greater age of the sample or to the fact that it is a different type of rock, a theolitic basalt representing one of the great basalt "floods".
5. Discussion We had hoped we might find a subset of samples for which U - H e "ages" agreed with the K - A r ages and that such a subset might have petrographic characteristics which would enable us to predict this concordancy. But even without making thorium analyses it is clear that our samples do not include such a concordant population. Leventhal [4] similarly failed to find concordant ages for young basalts from the San Francisco Volcanic Field near Flagstaff, Arizona. Neve~rtheless, the marked correlation one finds between KZAr age and 4He content can be very useful in age determinations on young rocks so that in a system such as ours, where the helium measurement involves literally only a few extra seconds of labor per sample, recording that datum is worthwhile. In Fig. 2 we have plotted the total 4He content in those samples of Madeira basalt for which we had reliable K - A r ages but no uranium data so that they could not be included in Fig. 1. All the samples weighted 1 g, to within a few percent, so that the ordinate is essentially4He concentration. Clearly there is a
160
I
u
F
I
I
NUMBERS IDENTIFY MADEIRA SAMPLES
o ~c 42e
E ~ 80
e28
T
31o 40
55e
39 43
8
50,,h
400
"
0
iTo
I
52~34 46 I 1.0
• •
• 58
26
e30
,~
6~ I
I 2.0
K-At
age, my.
I
I 3.0
I
4.0
Fig. 2. Helium content vs. K-Ar ages for additional Madeira basalts.
good correlation between the x andy coordinates. The correlation is very useful in confirming the resuits for samples from this population with extreme K - A r ages, such as MAD-51 and MAD-58. The uranium contents of the Madeira basalts are constant to within about 50% so that for the purpose of detecting marked deviations from the correlation it is not necessary to measure the uranium.
Acknowledgements We thank J.T. Ferreira for his technical assistance. Establishment of the Geochronology Laboratory at the University of Coimbra was a joint project with the University of California, Berkeley. This work was supported in part by the Arthur L. Day Fund of the U.S. National Academy of Sciences, by the U.S. National Science Foundation, and by the Instituto de Alta Cultura, Lisbon. One of us (J.H. Reynolds) received support in Portugal as a Fulbright-Hays re~ search scholar of the U.S. Department of State. The laboratory is an ongoing project of the Instituto de Alta Cultura with occasional participation by the Mass Spectroscopy Laboratory at the Department of Physics, Berkeley, which is supported in part by the U.S. Atomic Energy Commission. In that connection this report bears number UCB-34P-32-89.
150
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