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4He measured ratio
Geochimica et Cosmochimica Acta, Vol. 64, No. 21, pp. 3751–3762, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 ⫹ .00
Pergamon
PII S0016-7037(00)00419-1
Fractional extraction of helium by crushing of olivine and clinopyroxene phenocrysts: Effects on the 3He/4He measured ratio PAOLO SCARSI*,† Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0220, USA (Received August 19, 1999; accepted in revised form April 4, 2000)
Abstract—A critical review is reported of the crushing technique used for the extraction of the gas content from magmatic crystals. The main objective is the evaluation of the validity and accuracy of the method for obtaining data on the helium concentration in the sample and the value of the 3He/4He isotopic ratio; particular attention is given to the analysis of the effects of prolonged crushing. The technique used is that of crushing by steps; the series of experiments reported covers a wide range of conditions representative of the nature and concentration of the helium content in phenocrysts. Data on d[4He]/dt, 3He/4He, [3He], and [4He] are reported for seven experiments on phenocrysts coming from Kauai, Maui, Mangaia (ol and cpx), Tubuai, and Raivavae. The analytical function describing the release by crushing of the gas content in the reservoir hosted by the phenocrysts as a function of time of crushing has been found to be well represented by an exponential form. In some cases the release is described by a linear combination of two exponentials, suggesting the presence of two distinct gas reservoirs in the phenocrysts; the observation of a coherent variation on the isotopic ratio points out in the same direction. The exponential time constants 1 and 2, which reflect physical characteristics of the sample examined and are overall determined by the crusher and the crushing procedure, cluster in two distinct groups: 1 ranging between ⬃0.5 to ⬃3 minutes, assumed to characterize the fluid inclusions, and 2 from ⬃7 to ⬃13 minutes for the secondary He reservoir. The results from Maui and Mangaia are examined with some detail to investigate the nature of the second reservoir rich in cosmogenic 3He for Maui and radiogenic 4He for the Mangaia cpx. The result of the research reported confirms that crushing under vacuum is at present the appropriate method to resolve the “primary” helium trapped in the fluid inclusions of the phenocrysts from the in situ produced radiogenic and cosmogenic helium, while melting represents the appropriate methodology to obtain a complete extraction of the helium. Protracted crushing of the phenocrysts can eventually release, together with the He of the fluid inclusions also helium contained in secondary inclusions or in the matrix. The gas extracted in later steps is actually a mixture of inclusion-trapped and “matrix-sited helium” (see text). To minimize this effect, the crushing phase considered should be limited to the initial steps. Copyright © 2000 Elsevier Science Ltd Another source of helium is the production of radiogenic 4He generated both in the lattice of olivine/pyroxene phenocrysts and in the host lava by radioactive decay of U and Th. Recently, attention has been paid to the study of different techniques to extract helium from minerals for isotopic analysis, (Hilton et al., 1993; Stuart et al., 1994; Graham et al., 1992; Graham et al., 1998). Mineral crushing, step heating, crystal melting, and laser extraction, all under UHV, are methodologies commonly adopted in the laboratory. Each of these methods has different advantages depending upon specific applications. It is widely accepted that crushing of phenocrysts of olivine and clinopyroxene would efficiently release the helium trapped inside the fluid inclusions. However, it has been pointed out that prolonged crushing might also release helium sited in the mineral lattice (Hilton et al., 1993); if this is the case, this helium component can interfere with the correct determination of the 3He/4He ratio of the inclusions. The purpose of this paper is to describe and review critically the crushing technique used for the measurement of the helium isotope ratio on volcanic phenocrysts. The emphasis is on examining the significance and accuracy of the data, and in particular, to determine the effects of prolonged crushing. Experiments were carried out on both olivine and clinopyroxenes in order to compare the results obtained by crushing different mineral phases of the same samples.
1. INTRODUCTION
The analysis of the helium isotope ratio (3He/4He) in igneous rocks has been shown to be a powerful tool in geology, giving insight into the characteristics of magma sources (e.g., see review by Farley and Neroda, 1988) and making it possible to evaluate the effects on samples due to exposure at the Earth’s surface and their subsequent erosion history through the measurement of cosmogenic helium-3. Different components can contribute to the helium budget of a basalt, each of them with a distinctive isotopic signature so that it is possible to identify the sources and the processes active at the formation of the basalt. It is well known that minerals like olivine and clinopyroxene may retain helium in fluid inclusions trapped during the growth of the crystals within the magma. This helium shows a distinctive magmatic 3He/4He source signature that the minerals may preserve after the eruption. It is also known that if a rock is exposed to cosmic rays, after it is erupted on the surface, olivines and pyroxenes have detectable amounts of cosmogenic 3He produced by nuclear spallation interactions between cosmic ray particles and the elements present in the lattice. * Author to whom correspondence should be addressed (paolo@ifcai. pa.cnr.it). † Present address: Istituto Di Fisica Cosmica e Informatica, CNR, 90146, Palermo, Italy. 3751
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The analysis presented is the product of an extended study of noble gases in basalts from oceanic hotspots. For experiments reported here, phenocrysts from basalts covering a wide range of 3He/4He isotopic ratios were selected: Kauai (Hawaii) olivine with a 3He/4He ⬃ 25 R A , Maui (Hawaii) olivine rich in cosmogenic 3He (Craig and Poreda, 1986), phenocrysts from Mangaia and Tubuai (South Pacific) known as HIMU islands (Chauvel et al., 1992; Kogiso et al., 1997) and relatively enriched in radiogenic helium; the set is completed by a sample from Raivavae (French Polynesia) characterized by a large content of helium. 2. ANALYTICAL METHODS Each experiment carried out on phenocrysts of five basalts for a total of seven experiments consisted of a number of crushing steps performed on the same sample under vacuum, under the same experimental conditions and for different durations, using an on line crusher (see description below). The gas extracted from each step was transferred into the inlet system of the mass spectrometer and the Ne removed cryogenically in a charcoal trap at 34 K; the 3He/4He ratio and He concentrations were measured statically on the double-collecting splittube mass spectrometer GAD using the methodology described by Rison and Craig (1983). 3He was measured by ion counting with a Johnston electron multiplier and 4He by collection in a Faraday cup. The measured isotope ratios were normalized to aliquots of a high 3 He/4He (R/R A ⫽ 16.45) standard gas (MM). A measure of the blank has been carried out systematically before each experiment, with the assumption that this value remains constant in the successive steps of the same experiment (see discussion below). After each step the system was pumped out for sufficient time to minimize possible memory effects.
Fig. 1. Crusher used at SIO: schematic view.
2.1. Crushing The term “vacuum crushing” refers to the procedure by which the phenocrysts are mechanically fragmented at an operating pressure of about 10⫺7 torr to avoid atmospheric contamination. Figure 1 shows a schematic drawing of the crusher device used at SIO based on a unit built by K. Farley at Caltech. The device has been designed to optimize the gas extraction from the sample and to maintain a low blank level during the measurement. About 1–3 grams of olivine/clinopyroxene phenocrysts, 0.4 –2 mm in diameter, were hand picked from each lava sample, cleaned ultrasonically in distilled water and ethanol and loaded into the holder attached to a rotary motion feedthrough. The system was then baked and pumped out long enough to release the helium adsorbed on the surface of the grains and the crusher. After a residual pressure of about 10⫺7 torr was achieved, the device was placed inside the solenoid connected to a frequency generator which pulses and rises the sledge up letting it then fall down by gravity onto the tungsten-carbide base thereby crushing the grains and releasing the gases included. The frequency was of one stroke every 2 seconds, with an impact energy release of about 0.4 Joule/stroke. The number of steps and duration of the crushing cycle have been determined case by case, following the requirements fixed for the experiments. 2.2. Blanks Because of the small amount of gas usually contained in the phenocrysts, typically some ncc/g, a critical point in the experiments was the accurate measurement of the blank. The blank is usually expressed as the amount of He (assumed ⬇4He) present in the instrument as a background under normal operating conditions. The blank is determined by several contributions besides possible air leakage into the line, such as gas release by the crusher elements (sledge and container) as residuals from previous measurements (memory effects): an apparent blank component could also be contributed by purely instrumental characteristics, not always completely cancelled out by the “off-peak baseline” subtraction. The combined effect can simulate the presence of
both 4He and 3He with a resulting 3He/4He for the blank, in general, different from that typical of air (Fig. 2a). In our experiments the 4He and 3He blank concentrations were measured independently. Figure 2b shows the histogram of 123 blanks (expressed in 4He ncc units) obtained during more than 200 crushing experiments; the distribution can be assumed Poissonian, with a mean value of 0.17 ncc of He for the overall blank. This value represents the limit for the threshold in the analyses: for amounts of helium of this order the measurement is essentially controlled by the blank— both in terms of the concentration and isotopic ratio. The large dispersion around a median value ( ⬇ 2.5) of the 3He/4He ratio (expressed as R/R A , R A ⫽ 3He/4He in air ⫽ 1.4 ⫻ 10⫺6) of the blanks, makes it difficult to reliably correct the analyses of samples with very small amounts of helium. In our experiments we assume the value of blank measured before the first step to remain constant during all the sequence of the crushing steps. This assumption is justified by the observation that, for conditions substantially above threshold, the concentrations calculated using the 4He blank values determined before each experiment are consistent within about 7% with those obtained by using a standard overall blank value of 0.17 ncc, even though this difference could change somewhat for the last steps of each experiment. 3. RESULTS AND DISCUSSION
Table 1 summarizes the results of the stepwise crushing experiments carried out on the phenocryst samples from Kauai, Maui, Mangaia, Tubuai, and Raivavae. The parameter t (time of crushing) has been chosen as the independent variable; a device-independent parameter is represented by the energy rate released by the sledge of the crusher (⌬W ⫽ 12 J/min), for a banging frequency of one every 2 seconds. The fitting curves to the experimental values of d[4He]/dt vs.
Effects on the 3He/4He measured ratio
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following [(3.1)–(3.5)] a detailed description is given for the set of experiments listed in Table 1. For most of the cases (Maui, Mangaia cpx, Tubuai and Raivavae) both A 1 and A 2 are different from zero, with the two exponentials characterized by very distinct values 1 and 2 (ratio 2/1 ranging from 7 to 21). The absence of the term A 0 verifies that the blank correction is accurate enough to give a negligible contribution. For Kauai, a single exponential ( A 2 ⫽ 0) seems to adequately describe the observed d[4He]/dt vs. t; in this case the term A 0 , although very small, is present. The appearance of a single exponential describing the observed helium differential flux is suggestive of the predominance of a single reservoir releasing under crushing the gas content with a characteristic “relaxation time” irrespective of the likely wide range distribution in size and structure of the fluid inclusions; the 3He/4He isotopic ratio is observed to remain unchanged during all the crushing process: a second reservoir, if present, does not give an appreciable effect. When the two exponentials are present, the second reservoir, showing a higher “relaxation time,” is indicative of a gas component more difficult to extract. In the cases of Maui and Mangaia cpx, in correspondence of the crossover of the two exponentials during the sequential crushing, the observed 3He/ 4 He isotopic ratio shows a change switching from low to high (Maui) and from high to low (Mangaia cpx), coherently with the interpretation of two different gas sources characterized by a different isotopic ratio. 3.1. Maui Olivine Fig. 2. Frequency distributions for 1, 2, 3 blank determinations. (a) Blank R/R A . (b) Blank [4He] (ncc).
the integrated crushing time for the various experiments are shown in the set of Figs. 3–7 labeled (a). The experiments systematically show fitting curves represented by an expression of the type Y ⫽ A 0 ⫹ Y 1 ⫹ Y 2 ⫽ A 0 ⫹ A 1e 共⫺t/1兲 ⫹ A 2e 共⫺t/2兲, where Y ⫽ d[4He]/dt; t ⫽ progressive crushing time. The expression Y describes the presence of two distinct components Y 1 and Y 2 , each characterized by the constants A 1 , 1 and A 2 , 2, respectively. In an “ideal experiment,” with no measurement errors, we expect Y ⫽ Y 1 ⫹ Y 2 ; by fitting data in a real experiment, however, a term A 0 can appear, representing the residual deriving from an incomplete subtraction of the blank. A 0 does not represent therefore a real flux value and it must be excluded from any integration with time to obtain the cumulative amount of gas extracted; this, evidently, to avoid the “nonphysical” divergence in the integration for t ⫽ ⬁ introduced by its constant value. The function Y appears to well represent the appropriate fitting curve to our experimental data. The residuals, expressed in terms of standard deviations , show absence of any systematic trend and lie within an acceptable range of values. Fittings with other possible functions, both for the differential flux and the integrated amount of gas, all give higher residual values and show strong systematic deviations. In the
Sample M-06 is a picritic basalt from a lava flow in the upper part of the late Pleistocene Kula formation of White Hill (⬃0.5 million years old) on Maui. Previous measurements on this rock (Craig and Poreda, 1986) gave a 3He/4He ratio of 8.5 䡠 R A for the crushed olivine, while a value of 42.2 䡠 R A was obtained when the resulting powder was melted under vacuum. This clearly points to the presence of 3He not attributable to fluid inclusions and it is explained by the authors as the result of cosmic ray interaction with the exposed lava flows after eruption and cooling. This secondary component has been tentatively attributed to be hosted by the matrix (e.g., Craig and Poreda, 1986; Hilton et al., 1993). We will refer in the following to this component with the conventional name of “matrixsited” without aiming to endorse or not endorse the validity of this assumption made in the references quoted above. The aim of this experiment was to resolve the gas contained in the fluid inclusions and to test whether the component present in the matrix, presumably richer in cosmogenic 3He, can be partly released from the matrix by prolonged crushing. An ⬃3-gram sample of handpicked olivines [grains (0.9 –2 mm in diameter)] was crushed in six steps, for a total time of 40 minutes. The fitting curve to the experimental data1 of the d[4He]/dt flux measured in the different crushing steps is shown in Fig. 3b and appears to be a linear combination of two exponentials:
1
The fitting procedure to the Maui data as well as for all the other experiments (Figs. 3–7), has been made using SigmaPlot by Jandel Scientific. G represents the goodness of fit as defined by the expression. G ⫽ [¥(observed– expected)2]1/2/¥(observed).
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P. Scarsi Table 1. Data from the stepwise crushing experiments on phenocrysts. bc ⫽ corrected for blank.
Experiment Maui-6b (olivine)
Step
⌬t (min)
Time (min) cumulative
1 2 3 4 5 6
0.50 1.00 3.00 5.00 15.50 15.00
0.50 1.50 4.50 9.50 25.00 40.00
¥⫽ Mangaia (cpx)
40.00 1 2 3 4 5 6 7
¥⫽ Mangaia (fresh olivine)
2.00 3.00 5.00 10.00 15.00 10.00 15.00
¥⫽ Mangaia (altered olivine)
1 2 3
¥⫽
0.25 0.25 0.50 1.00 3.00 5.00 10.00 15.00 10.00 10.00
¥⫽
1.00 1.00 3.00 5.00 10.00 15.00
Y 2 ⫽ 0.24e ⫺t/13.11
46.819 1.926 1.503 10.687 3.207 1.238
0.25 0.50 1.00 2.00 5.00 10.00 20.00 35.00 45.00 55.00
20.040 7.538 13.814 8.907 10.777 9.719 7.820 2.257 0.815 0.483
(R/R A ) bc
1
[4He]bc (% cum. of extracted)
5.02 5.60 4.07 8.75 6.40 15.31
3.02 1.34 0.61 0.15 0.07 0.02
8.77 8.59 8.45 8.89 9.57 10.55
0.18 0.15 0.14 0.23 0.18 0.31
22 41 68 78 95 100
1.10 2.93 4.36 5.12 7.33 18.72 13.29
4.31 1.05 0.41 0.18 0.08 0.04 0.04
4.95 4.31 3.90 3.94 3.54 3.69 3.58
0.05 0.08 0.12 0.13 0.14 0.39 0.31
48 66 78 88 94 97 100
1.21 5.37 6.78
23.41 0.64 0.30
6.18 6.22 6.52
0.06 0.14 0.17
93 97 100
2.83 8.85 20.09
5.34 0.64 0.18
6.29 6.59 6.02
0.09 0.16 0.34
71 92 100
2.81 7.14 4.03 6.11 5.11 5.63 6.90 13.90 30.89 43.01
80.16 30.15 27.63 8.91 3.59 1.94 0.78 0.15 0.08 0.05
7.10 6.94 7.21 7.05 6.79 6.76 7.12 6.62 6.19 4.50
0.08 0.13 0.10 0.14 0.11 0.15 0.12 0.31 0.78 1.03
24 34 50 61 74 86 96 98 99 100
0.25 0.50 1.00 2.00 4.00 9.00 14.00
30.745 9.721 14.014 23.942 14.235 13.942 6.367
2.32 6.98 4.95 2.96 4.88 4.97 10.28
122.98 38.89 28.03 23.94 7.12 2.79 1.27
6.53 6.55 6.46 6.46 6.62 6.34 6.21
0.07 0.11 0.11 0.07 0.10 0.10 0.19
27 36 48 69 82 94 100
1.26 2.29 1.29 3.61 5.76 12.49
2.45 1.21 0.80 0.15 0.05 0.01
24.47 25.94 24.72 25.36 24.97 24.12
0.38 0.74 0.38 0.77 0.97 0.82
33 49 81 91 97 100
15.132
82.171
112.966 1.00 2.00 5.00 10.00 20.00 35.00
35.00
Y 1 ⫽ 3.02e ⫺t/1.33
8.612 3.136 2.075 1.753 1.195 0.410 0.617
2.00 7.00 14.00
14.00 1 2 3 4 5 6
¥⫽
0.25 0.25 0.50 1.00 2.00 5.00 5.00
d[4He]bc/dt (ncc/g)/min (average)
50.248
55.00 1 2 3 4 5 6 7
Kauai-6 (olivine)
2.00 5.00 10.00
14.00 1 2 3 4 5 6 7 8 9 10
Raivavae-22 (olivine)
2.00 5.00 7.00
1.510 1.342 1.842 0.726 1.135 0.351
He blk (%)
17.797
10.00
¥⫽ Tubuai-6b (olivine)
2.00 3.00 5.00
4
6.905 2.00 5.00 10.00 20.00 35.00 45.00 60.00
60.00 1 2 3
[4He]bc (ncc/g)
2.452 1.205 2.393 0.753 0.462 0.198 7.464
共ncc/g)/min, 共mcc/g)/min),
共G ⫽ 0.08, see note). The two exponential functions are resolved in Fig. 3b: Y 1 dominates over the first three crushing steps, during which
about 70% of the total He extracted is obtained; with the crossover point at t ⬃ 4 min. Figure 3c–I illustrates the ratio Y 1 /Y vs. the integrated crushing time compared with the measured R/R A . In the first three steps of crushing the measured 3He/4He remains constant within errors at R/R A ⬃ 8.5, while in the successive extractions it increases systematically up to a value (10.55 ⫾ 0.31) in
Effects on the 3He/4He measured ratio
the last step. If we assume that the gas released during the steps for which the fast component Y 1 is dominant represents essentially the helium contained in the fluid inclusions, the value of (8.58 ⫾ 0.01) 䡠 R A (calculated as the mean of R/R A measured in steps 1, 2, 3, weighted inversely by their own variances) can be used for the helium trapped in the fluid inclusions by the basalt of Maui. This value is in good agreement with that of 8.5 䡠 R A found previously by Craig and Poreda (1986) using a different crushing device and averaging over several samples. The data relative to the last three steps show that a significant “excess 3He” is extracted; the abrupt change in the slope can be considered to indicate the limiting extent at which crushing can resolve the helium trapped in the magma by the fluid inclusions from cosmogenic He. In the final steps (# ⬎3) of the experiment the helium extracted is a mixture of this helium with a different component, characterized by a higher 3He/4He ratio. Vacuum fusion experiments by Craig and Poreda (1986) suggest that the high 3He/4He ratios, up to 47.2 R/R A , measured by melting the olivine inclusions after crushing, are due to 3He stored in the matrix structure. The authors attribute this 3He to production by cosmic rays in the exposed lava flows after eruption. Hilton et al. (1993), in a crushing experiment of phenocrysts of Andean lavas rich in cosmogenic helium, noted that a release of matrix-sited helium is possible if the sample is crushed for an extended time; this component, on the other hand, cannot be completely extracted by simple vacuum crushing but only by fusion. Table 2 shows the results of the crushing experiment for the sample MAUI-06. We assume (R/R A ) mean ⫽ (8.58 ⫾ 0.01) for the helium trapped in fluid inclusions (as measured in steps #1, 2, 3) and a total concentration of 1.06 pcc/g for the cosmogenic 3 He sited in the matrix given by melting of sample powder after crushing (Craig and Poreda, 1986). The effect of prolonged crushing is evident after 40 minutes of extraction (six steps), when almost all of the helium retained in the fluid inclusions has been liberated; the increase of 23% of the value of 3He/4He can be accounted for the release of only 0.1% of the total cosmogenic 3He present in the matrix. 3.2. Mangaia Phenocrysts Mangaia is the second largest island of the Cook Islands in Polynesia and it is at present representing the most extreme
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example of the HIMU mantle component, with the most radiogenic lead measured to date (Chauvel et al., 1992). The basalts are rich in both olivine and clinopyroxene so that they are suitable for helium isotopic analyses of both minerals. The best estimate of the age of eruption is about 19.5 Ma (Kogiso et al., 1997) and it is reasonable to expect that the phenocrysts could carry detectable amount of radiogenic 4He produced by U and Th decay. 3.2.1. Clinopyroxene Approximately 2 grams of handpicked clinopyroxene (0.9 –2 mm grain size) were crushed under vacuum in seven successive steps for a total crushing time of 60 minutes. The fitting curve of the d[4He]/dt flux obtained for each crushing step as a function of the integrated crushing time is incompatible with a single exponential and it is well represented, as in the case of the Maui sample, by the combination of two exponentials (Fig. 4a) (G ⫽ 0.01; residuals shown in the figure). The two components Y 1 and Y 2 are resolved in Fig. 4b: Y 1 ⫽ 7.95e ⫺t/1.24
共ncc/g)/min,
Y 2 ⫽ 0.81e ⫺t/8.95
共ncc/g)/min.
Y 1 dominates for the first and a part of the second crushing step, while Y 2 takes over for the remaining part of the experiment. The different role of Y 1 and Y 2 is evident in Fig. 4c, where the isotopic ratio is plotted versus the integrated crushing time and compared with the value Y 1 /Y in the same intervals. The 3 He/4He measured in the first step (dominated by Y 1 , but with a Y 2 contribution of ⬃25%) is (4.95 ⫾ 0.05) 䡠 R A and changes to (4.31 ⫾ 0.08) 䡠 R A in the second, where the contributions of Y 1 and Y 2 are comparable, to reach afterwards the value of ⬃3.8 䡠 R A which can be considered, within the errors, representative of the final steps in which Y 2 is dominant. In analogy with the Maui case, the two components Y 1 and Y 2 can be considered as representing the release from two He reservoirs: Y 1 for the fluid inclusions and Y 2 for the second reservoir. We can estimate the value of R/R A for the Y 1 component alone to be of the order of 6, comparable to that obtained for the olivine samples (see Sect. 3.2.2) in which only Y 1 is relevant.
Table 2. Results of the stepwise experiments on Maui-06b olivine.* Experiment Maui-06b (olivine)
Step #
Time cum. (min)
1, 2, 3 4 5 6
4.5 9.5 25.0 40.0
(R/R A ) bc
[3He]meas. (ncc/g)
[3He]meas (pcc/g)
[3He]incl (pcc/g)
⌬(R/R A ) (%)
[3He]exec (pcc/g)
[3He]cosm (% total)
8.58 8.89 9.57 10.55
4.694 0.726 1.135 0.351
0.056 0.009 0.015 0.005
0.0560 0.0087 0.0135 0.0042
3.6 11.5 23.0
0.0004 0.0017 0.0010
0.04 0.16 0.09
* Notes. Column 4: (R/R A ) measured, corrected for blank. Column 5: [4He] measured, corrected for blank. Column 6: [3He] measured, corrected for blank. Column 7: [3He] expected from a constant isotopic ratio R/R A ⫽ (8.576 ⫾ 0.008). See text. Column 8: Deviation from R/R A ⫽ (8.576). Expressed in percent. Column 9: Excess of [3He] measured over the concentration value listed in column 7. Column 10: [3He]exc expressed as % of the total concentration of cosmogenic 3He as given by Craig and Poreda, 1986 (1.06 pcc/g). The values listed in line 1 represent the mean over steps 1, 2, 3, weighted inversely by their own variances.
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Fig. 3. Maui olivine. (a) [4He] flux rate vs. integrated crushing time. Residual ⫽ (observed-expected)/observed. (b) Components Y 1 and Y 2 of Y representing the d[4He]/dt vs. integrated crushing time. (c) I: Y 1 /Y vs. integrated crushing time. II: 3He/4He vs. integrated crushing time.
3.2.2. Fresh olivine and altered olivine About 1 gram of fresh olivine from the same specimen as the cpx was crushed in three steps for a total time of 10 minutes. Approximately 2 grams of partially altered olivine, again coming from the same specimen as the cpx and with evident presence of iddingsite, were run in the same way for 15 minutes. Both the crystals were in the (0.9 –2) mm size. The results of the measurements are listed in Table 1; in both the experiments the value of the helium concentration released after the initial two steps of crushing reaches values very close to the blank level indicating a complete extraction from the helium reservoir. For the fresh olivine, the measured isotopic ratio is constant within the error limits for all three successive extraction steps: 3 He/4He ⫽ (6.29 ⫾ 0.09) 䡠 R A ; an almost identical constant value is obtained also in the case of the altered olivine: 3He/ 4 He ⫽ (6.18 ⫾ 0.06) 䡠 R A .
The comparison of the data obtained from the olivines and those of the clinopyroxene of Mangaia for the first crushing step of 2 minutes shows important clues. The 4He concentration measured for the fresh olivine, (47 ncc/g), is approximately 5 times greater than that obtained by crushing the altered olivine for the same time, suggesting that the altered crystals of olivine suffered a partial loss of the gas originally trapped in the fluid inclusions after having been erupted to the surface. This loss, on the other hand, did not affect the 3He/4He ratio which remains unchanged within the error limits of the measurements. The 3He/4He ratio obtained during the first crushing step shows a different value for the clinopyroxene (4.95 ⫾ 0.05) 䡠 R A against the mean value 6.2 䡠 R A common to the olivine samples; this is apparently inconsistent with the assumption that both, fresh/altered olivine and the clinopyroxene were formed in the same helium environment. An explanation is however suggested by Fig. 4d, which shows that in the clinopyroxene
Effects on the 3He/4He measured ratio
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Fig. 4. Mangaia cpx. (a) [4He] flux rate vs. integrated crushing time. (b) Components Y 1 and Y 2 of Y representing the d[4He]/dt vs integrated crushing time. (c) I: Y 1 /Y vs integrated crushing time. II: 3He/4He vs. integrated crushing time.
sample the gas extracted in the first 2 minute crushing step contains a mixture of the two components: about 80% of Y1 attributed to the fluid inclusions, with the remaining 20% of Y2 enriched in 4 He. In the successive steps of crushing of the clinopyroxene sample an increasing percentage of the Y2 component is released. While in the case of Maui basalt, information on the existence, extent and location of an important cosmogenic reservoir connected to Y 2 is given by the melting experiment of Craig and Poreda (1986), due to the lack of an equivalent melting experiment for the Mangaia cpx the description of the Y 2 reservoir can only be inferred by general data. A possible explanation of the origin and the extent of the reservoir responsible for the component Y 2 can be offered by considering the dilution of Y 1 (R/R A ⬃ 6.2) with radiogenic 4 He built up after the eruption. By analogy with the case of the Maui olivine, we adopt the convention of the “matrix-sited” component, assuming that the physical location of the reservoir in principle could be sited in the matrix. Again, we stress that
this is simply a matter of naming this secondary component without aiming to endorse or not the validity of the assumption about its location. If we assume that originally the helium isotopic composition of the pyroxenes had the 3He/4He ratio typical of Y 1 (6.22 ⫾ 0.01) 䡠 R A (mean of R/R A of the fresh olivine—weighted inversely by the variances), we can estimate the 4He release at each crushing step to justify the corresponding measured value for R/R A . These values are reported in column 7 of Table 3. The total amount of excess 4He extracted in the seven steps of the experiment is about 5.08 ncc/g. It is of some interest to compare this value with the concentration of radiogenic 4He produced by radioactive decay in the cpx phenocrysts. To evaluate the expected amount of 4He, it is necessary to know the amount of U and Th and the age of the sample. No measurements as yet have been carried out on these minerals, so we can only derive an approximated value based on the following assumptions.
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Fig. 5. Tubuai olivine. (a) [4He] flux rate vs. integrated crushing time. (b) Components Y 1 and Y 2 of Y representing the d[4He]/dt vs. integrated crushing time. (c) I: Y 1 /Y vs. integrated crushing time. II: 3He/4He vs. integrated crushing time.
Table 3. Results of the stepwise experiments on Mangaia cpx.* Experiment
Step #
Time cum. (min)
(R/R A ) bc
[4He]meas (ncc/g)
[3He]meas (pcc/g)
[4He]incl (ncc/g)
[4He]exc (ncc/g)
Mangaia (cpx)
1 2 3 4 5 6 7
2 5 10 20 35 45 60
4.95 4.31 3.90 3.94 3.54 3.69 3.58
8.61 3.14 2.07 1.75 1.19 0.41 0.62
0.060 0.019 0.011 0.010 0.006 0.002 0.003
6.853 2.175 1.301 1.110 0.680 0.244 0.356
1.759 0.961 0.774 0.643 0.514 0.167 0.261
¥⫽ * Notes: Column 4: Column 5: Column 6: Column 7: Column 8:
(R/R A ) measured, corrected for blank. [4He] measured, corrected for blank. [3He] measured, corrected for blank. [4He] expected from a constant isotopic ratio R/R A ⫽ (6.29 ⫾ 0.09) (fresh olivine) and [3He] ⫽ [3He]meas. See text. Excess of [4He] measured over the concentration value listed in column 7.
5.078
Effects on the 3He/4He measured ratio
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Fig. 6. Raivavae olivine. (a) [4He] flux rate vs. integrated crushing time. (b) Components Y 1 and Y 2 of Y representing the d[4He]/dt vs. integrated crushing time. (c) I: Y 1 /Y vs. integrated crushing time. II: 3He/4He vs. integrated crushing time.
(1) We estimate, for the whole sample analyzed, a uranium and thorium concentration of 1.2 and 4.6 ppm, respectively, based on the average of the data of Woodhead (1996). (2) We assume that the U and Th content of the sample is equal to that of the melt which produced the lava.
(3) We use a Th/U ratio of 3.6 for our phenocrysts, assuming no fractionation between Th and U during the formation from the magma. (4) We use a clinopyroxene/melt partition coefficient for U between 0.002 and 0.03 (Graham et al., 1992) which gives a concentration of 2.5–35 ppb.
Table 4. Crushing experiment. Sample
Type
# of steps
Tot. crush. time (min)
[4He]released (ncc/g)
1 (min)
2 (min)
Notes
Maui Mangaia
ol cpx ol ol ol ol ol
6 7 3 3 10 7 6
40 60 10 14 55 14 35
6.905 17.797 50.248 15.132 82.171 112.966 7.464
1.33 1.24 0.63 1.52 0.47 0.08 2.83
13.11 8.95
Cosmogenic 3He HIMU-radiogenic 4He Fresh crystals Altered crystals HIMU-radiogenic 4He Large content of 4He High 3He/4He
Tubuai Raivavae Kauai
6.95 1.71
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not strong enough to liberate the helium of the matrix. It should be noted that most of the helium is extracted in the first steps of the experiments, as can be seen in Figs. 5(a) and 6a. (b) Olivine have a different crystallographic structure from the cpx. Moreover, it could contain a lower amount of U and Th and therefore a lower amount of 4He produced by radiogenic decay. 3.3. Tubuai Olivine Tubuai is a small volcanic island belonging to the Southern Cook island chain, about 1200 km from MacDonald Seamount. Its volcanism began about 10.7 Ma ago (Diraison, 1991) and the products erupted have, like for Mangaia, the isotopic composition of a HIMU endmember (Zindler and Hart, 1986). Ten crushing steps were carried out on 0.8 grams of pure olivine (10 –20 mesh in size) from Tubuai-06b basalt for a total of 55 minutes, a time longer than in the previous experiment on the olivine of Mangaia and approximately equal to that of the experiment on the pyroxene of the same sample. The results of this experiment are plotted in Fig. 7. As in the case of Maui and the Mangaia cpx experiments, the data of d[4He]/dt flux plotted vs. the integrated crushing time, cannot be fitted by a single exponential, but require the combination of two components indicating the contribution of two distinct He reservoirs. Figure 5a shows the fitting curve (G ⫽ 0.12; residuals plotted in the figure). In Fig. 5b, Y 1 and Y 2 are resolved:
Fig. 7. Kauai olivine. (a) [4He] flux rate vs. integrated crushing time. (b) 3He/4He ratio vs. integrated crushing time.
In addition, for the estimate of the radiogenic 4He of the phenocrysts, we do not take into account the effect of ␣-implantation from the host rock and we assume that the presence of the U and Th contained in the fluid inclusions of the crystal does not affect the helium concentration of the lattice. For an estimated age of 19.5 Ma, the expected 4He content in the clinopyroxene due to radiogenic decay of U and Th is of the order of (10 –100) ncc/g. The excess radiogenic helium extracted from the pyroxene amount to 5–50% of the estimated content of the radiogenic reservoir, value which although rather high, is not inconsistent with the calculated content. These values must be considered only approximate estimates, due to the large number of assumptions made in the calculations. Again, only melting the cpx powder after crushing could provide a direct information of the matrix sited helium. Finally, to explain why the 3He/4He of the olivines does not decrease substantially as compared to the decrease in the pyroxenes, two possibilities can be considered: (a) The total crushing time in the experiments on the olivines were probably too short and the stress of the sledge banging
Y 1 ⫽ 80.16e ⫺t/0.47
共ncc/g)/min,
Y 2 ⫽ 5.86e ⫺t/6.95
共ncc/g)/min.
Y 1 dominates in the first three steps, with a crossover with Y 2 at step #4 (Figs. 5b and 5c–I). The measured 3He/4He remains, for all the steps, consistent with a unique signature at ⬃7 䡠 R A , with a possible decrease shown in step #10, however compatible within the error with the common trend. A straightforward interpretation is offered by considering the presence of two classes of fluid inclusions with different structural characteristics for the reservoirs but representative of the same magma environment. 3.4. Raivavae Olivine Raivavae is a volcanic island, about 7.3 Ma old (Diraison, 1991) in the Austral Island Chain, south east of the Austral Fracture Zone. In Fig. 6 are reported the results of the experiment carried out on 0.8 grams of pure olivine (0.9 –2 mm in size) handpicked from a basalt of Raivavae. The crushing has been carried out for a total of 14 minutes in seven successive steps; after these extractions, the amount of helium was still significantly higher than the blank of the machine, and the extraction must be considered incomplete after the last run. This of course affects the calculation of the total concentration: the aim of this experiment, on the other hand, was to look for a changing trend, if any, in the isotopic ratio as consequence of crushing. Figure 6a shows that also in this case the gas flux release for each step, d[4He]/dt, plotted versus the integrated crushing
Effects on the 3He/4He measured ratio
time is well described by the presence of two reservoirs. Figure 6b shows the two components Y 1 and Y 2 (G ⫽ 0.02; residuals shown in the figure): Y 1 ⫽ 380.46e ⫺t/0.08
共ncc/g)/min,
Y 2 ⫽ 41.89e ⫺t/1.71
共ncc/g)/min.
Figure 6c–I shows that the two components invert the relative role of dominance after the first step, while the measured isotopic ratio remains constant during all of the experiment, with a possible decrease in the last step to a value, however, still compatible with the general average. The interpretation is similar to that reported for the sample for Tubuai. 3.5. Kauai Olivine Sample Kauai-6 (Hawaiian Chain) is an olivine from a basalt characterized by an high value for the isotopic ratio (3He/ 4 He ⬃ 25 R/R A ). Almost the total amount is extracted in the first five steps, corresponding to an integrated crushing time of 20 minutes, with the sixth step providing an amount of helium close to the blank value. Figure 7a plots the d[4He]/dt flux released against the integrated time of crushing. The experimental values are well fitted by the single exponential: Y ⫽ 0.01 ⫹ 2.45e ⫺t/ 2.83
共ncc/g)/min
(G ⫽ 0.10兲.
The figure also shows the individual residuals. Figure 7b gives the isotopic ratio 3He/4He (R/R A ) for the various steps as a function of the integrated crushing time: the ratio shows a constant value at 25 䡠 R/R A . The evidence clearly points to a single He reservoir which can be ascribed to the fluid inclusions. 4. CONCLUSIONS
The primary aim of this work was a quantitative analysis of the crushing technique as a mean for releasing the gas content of magmatic phenocrysts, addressed specifically to evaluate the isotopic ratio 3He/4He. The measurement of this value for the fluid inclusions in the crystals characterizes the source of the magma at eruption; it could however be affected by processes active between the time of eruption and sampling, because of gas losses or of acquisition of radiogenic and cosmogenic components. The technique used has been of crushing by steps, with time of crushing long enough, in some cases, to obtain an almost complete release of the gas content of the fluid inclusions reservoirs. The series of experiments carried out cover a wide range of conditions representative of the various classes of cases ranging from phenocrysts with high contents of cosmogenic 3He or radiogenic 4He to samples characterized by a high He content to others with high 3He/4He ratios. Considerable care has been put in the evaluation of the level and nature of the blank, to assure the absence of interference of systematic errors in the measured ratios. An exponential trend has been found to characterize the d[4He]/dt vs. crushing time in all the experiments; a single exponential (time constant 1) or a linear combination of two
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exponentials (time constants 1 and 2) have been identified. The time constants which are determined overall by the crusher and the crushing procedure, should reflect physical characteristics of the sample examined such as, e.g., the size of the bubbles, the site and hardness of the reservoir(s). The values obtained clusters in two groups: 1 ranging between ⬃0.1 to 3 minutes, 2 from ⬃7 to 13 minutes; an extreme behavior is shown by the Raivavae olivine with 1 ⫽ 0.08 min and 2 ⫽ 1.71 min depending possibly on the high content of He in the sample. The shorter 1 can be connected with the reservoir containing the helium trapped in the fluid inclusions; its content is liberated in the first steps in each experiment. The heterogeneous size and distribution of the fluid inclusions can be assumed to be responsible for the scattered values of 1 between the different samples. From the presence of 2 we can infer the existence of a second reservoir. The value of 2 can give some clue about its nature and location, but only a thin section inspection, together with the melting of the powder after the extraction of the helium contained in the bubbles can provide a definite identification (“matrix sited” or secondary inclusions) and the correct isotopic signature of the cosmogenic spallation/radiogenic decay effects on the crystals. For the Maui olivine, the data from the crushing experiment show in the last steps a progressive increase of the value of 3 He/4He with the progression of crushing, indicating that the second reservoir (2) contains the excess of 3He (cosmogenic); the melting experiment (Craig and Poreda, 1986) suggests its existence and points to a location in the matrix. The experiments on the phenocrysts with a large content of 4 He (radiogenic) raise important questions associated with the geological meaning connected with the identification of the second reservoir. Different conclusions can be drawn if we attribute the radiogenic helium to the “matrix” or to the magma. If we associate it to the magma we should find this component also in the fluid inclusions; if absent we can conclude that the crystals “sealed” their inclusions before the 4He production and that the radiogenic helium was formed after eruption. The experiments on minerals of the same basalt sample from Mangaia show that the lower value of 3He/4He measured in the clinopyroxene can be attributed to the extraction of radiogenic helium sited in the matrix of the minerals and not in the inclusions. This is supported by the fact that the inclusions of the coexisting olivine do not show the same isotopic signature. Naturally a fusion step could provide a definite conclusion. If aging is responsible for the lower value of the isotopic ratio, as observed in the stage of prolonged crushing (case of Tubuai and Raivavae), it must also be concluded that the radiogenic 4He is released by the second reservoir and it originated in the crystal lattice after the rock was erupted to the surface, excluding therefore an origin in the magma chamber before eruption, a possible interaction (assimilation) of the magma with the surrounding rock on the way to the surface, and a post eruptive radiogenic decay of the U and Th trapped in the fluid inclusions of the phenocrysts. As a general conclusion this work confirms that crushing under vacuum is at present an appropriate method to resolve the “primary” helium trapped in the fluid inclusions of the phenocrysts from the in situ produced radiogenic/cosmogenic helium;
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melting is the appropriate methodology to obtain a complete extraction of the helium. The results of the experiments carried out on olivine and clinopyroxene show that protracted crushing of these minerals can release helium contained in secondary inclusions or in their lattices so that the gas extracted in later steps is actually a mixture of inclusion-trapped and “matrixsited” helium. To minimize this effect, the crushing steps considered should be limited to short time durations; it does not exist as a general rule to optimize the crushing times to get maximum yield of mantle gas with minimum yield of matrix gas if not to proceed with a contextual analysis of the isotopic ratio behavior vs. the integrated crushing time to check possible variations. For the set of experiments reported in this paper (crusher with an impact energy release of 0.4 Joule/stroke and frequency of one stroke every 2 seconds), this optimum time appears to be about 5 minutes, but occasionally (Tubuai and Raivavae olivines) as low as 2 minutes. Samples with low amounts of helium in the fluid inclusions must be carefully handled when measuring the 3He/4He; in addition, due to the generally low amount of gas in the phenocrysts, particular attention has to be paid to the blank of the extraction system, as its value can be different from air for instrumental reasons. Acknowledgments—This work has been carried out at SIO as a part of an extended study of noble gas in basalts from oceanic hotspots. I acknowledge the suggestions and criticisms of Harmon Craig, who has made available the samples for analysis and the facilities of the Isotope Laboratory. Dr. E. Winterer kindly supplied the sample from Mangaia. I am thankful to D. Hilton for comments. Thanks to K. A. Farley for providing the design of the crusher and to D. Brewton for assistance in the laboratory. I am deeply indebted to the staff of IFCAI/CNR in Palermo for continuous support in the data analysis and helpful discussions in the interpretation of the results. This work was supported by NSF Grant No. EAR96-28481 (Mantle Geochemistry) to H. Craig.
REFERENCES Chauvel C., Hofmann A.W., and Vidal P. (1992) HIMU-EM: The French Polynesian connection. Earth Planet. Sci. Lett. 110, 99 –119. Craig H. and Poreda R.J. (1986) Cosmogenic 3He in Terrestrial rocks: The Summit Lavas of Maui. Proc. Natl. Acad. Sci. USA 83, 1970 –1974. Diraison C. (1991) Le Volcanisme aerien des archipels polynesiens de la Societe, des Marquises et des Australes-Cook; tephrostratigraphie, datation isotopique et geochimie comparees; contribution a l’etude des origines du volcanisme intraplaque du Pacifique Central. These de Doctorat d’Etat, Universite de Brest. Farley K.A. and Neroda E. (1988) Noble gases in the earth’s mantle. Annu. Rev. Earth. Planet. Sci. 26, 189 –218. Graham D.W., Humphris S.E., Jenkins W.J., and Kurz M.D. (1992) Helium isotope geochemistry of some volcanic rocks from SaintHelena. Earth Planet. Sci. Lett. 110, 121–131. Graham D.W., Larsen L.M., Hanan B.B., Storey M., Pedersen A.K., and Lupton J.E. (1998) Helium isotope composition of the early Iceland mantle plume inferred from the Teritary picrites of West Greenland. Earth Plant. Sci. Lett. 160, 241–255. Hilton D.R., Hammerschmidt K., Teufel S., and Friedrichsen H. (1993) Helium isotope characteristics of Andean geothermal fluids and lavas. Earth Planet. Sci. Lett. 120, 265–282. Kogiso T., Tatsumi Y., Shimoda G., and Barsczus H.G. (1997) High mu (HIMU) ocean island basalts in southern Polynesia; new evidence for whole mantle scale recycling of subducted oceanic crust. J. Geophys. Res. 102, 8085– 8103. Rison W. and Craig H. (1983) Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Islands basalts and xenoliths. Earth Planet. Sci. Lett. 66, 407– 426. Stuart F., Turner G., and Taylor R. (1994) He–Ar isotope systematics of fluid inclusions: Resolving mantle and crustal contributions to hydrothermal fluids. In Noble Gas Geochemistry and Cosmochemistry (ed. J. Matsuda), pp. 261–277. Terra Scientific Publishing Company. Zindler A. and Hart S. (1986) Chemical geodynamics. Ann. Rev. Earth Planet. Sci. 14, 493–571. Woodhead, J.D. (1996) Extreme HIMU in an oceanic setting; the geochemistry of Mangaia Island (Polynesia), and temporal evolution of the Cook-Austral hotspot. J. Volc. Geothermal Res. 72, 1–19.
Pergamon
PII S0016-7037(00)00419-1
Fractional extraction of helium by crushing of olivine and clinopyroxene phenocrysts: Effects on the 3He/4He measured ratio PAOLO SCARSI*,† Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0220, USA (Received August 19, 1999; accepted in revised form April 4, 2000)
Abstract—A critical review is reported of the crushing technique used for the extraction of the gas content from magmatic crystals. The main objective is the evaluation of the validity and accuracy of the method for obtaining data on the helium concentration in the sample and the value of the 3He/4He isotopic ratio; particular attention is given to the analysis of the effects of prolonged crushing. The technique used is that of crushing by steps; the series of experiments reported covers a wide range of conditions representative of the nature and concentration of the helium content in phenocrysts. Data on d[4He]/dt, 3He/4He, [3He], and [4He] are reported for seven experiments on phenocrysts coming from Kauai, Maui, Mangaia (ol and cpx), Tubuai, and Raivavae. The analytical function describing the release by crushing of the gas content in the reservoir hosted by the phenocrysts as a function of time of crushing has been found to be well represented by an exponential form. In some cases the release is described by a linear combination of two exponentials, suggesting the presence of two distinct gas reservoirs in the phenocrysts; the observation of a coherent variation on the isotopic ratio points out in the same direction. The exponential time constants 1 and 2, which reflect physical characteristics of the sample examined and are overall determined by the crusher and the crushing procedure, cluster in two distinct groups: 1 ranging between ⬃0.5 to ⬃3 minutes, assumed to characterize the fluid inclusions, and 2 from ⬃7 to ⬃13 minutes for the secondary He reservoir. The results from Maui and Mangaia are examined with some detail to investigate the nature of the second reservoir rich in cosmogenic 3He for Maui and radiogenic 4He for the Mangaia cpx. The result of the research reported confirms that crushing under vacuum is at present the appropriate method to resolve the “primary” helium trapped in the fluid inclusions of the phenocrysts from the in situ produced radiogenic and cosmogenic helium, while melting represents the appropriate methodology to obtain a complete extraction of the helium. Protracted crushing of the phenocrysts can eventually release, together with the He of the fluid inclusions also helium contained in secondary inclusions or in the matrix. The gas extracted in later steps is actually a mixture of inclusion-trapped and “matrix-sited helium” (see text). To minimize this effect, the crushing phase considered should be limited to the initial steps. Copyright © 2000 Elsevier Science Ltd Another source of helium is the production of radiogenic 4He generated both in the lattice of olivine/pyroxene phenocrysts and in the host lava by radioactive decay of U and Th. Recently, attention has been paid to the study of different techniques to extract helium from minerals for isotopic analysis, (Hilton et al., 1993; Stuart et al., 1994; Graham et al., 1992; Graham et al., 1998). Mineral crushing, step heating, crystal melting, and laser extraction, all under UHV, are methodologies commonly adopted in the laboratory. Each of these methods has different advantages depending upon specific applications. It is widely accepted that crushing of phenocrysts of olivine and clinopyroxene would efficiently release the helium trapped inside the fluid inclusions. However, it has been pointed out that prolonged crushing might also release helium sited in the mineral lattice (Hilton et al., 1993); if this is the case, this helium component can interfere with the correct determination of the 3He/4He ratio of the inclusions. The purpose of this paper is to describe and review critically the crushing technique used for the measurement of the helium isotope ratio on volcanic phenocrysts. The emphasis is on examining the significance and accuracy of the data, and in particular, to determine the effects of prolonged crushing. Experiments were carried out on both olivine and clinopyroxenes in order to compare the results obtained by crushing different mineral phases of the same samples.
1. INTRODUCTION
The analysis of the helium isotope ratio (3He/4He) in igneous rocks has been shown to be a powerful tool in geology, giving insight into the characteristics of magma sources (e.g., see review by Farley and Neroda, 1988) and making it possible to evaluate the effects on samples due to exposure at the Earth’s surface and their subsequent erosion history through the measurement of cosmogenic helium-3. Different components can contribute to the helium budget of a basalt, each of them with a distinctive isotopic signature so that it is possible to identify the sources and the processes active at the formation of the basalt. It is well known that minerals like olivine and clinopyroxene may retain helium in fluid inclusions trapped during the growth of the crystals within the magma. This helium shows a distinctive magmatic 3He/4He source signature that the minerals may preserve after the eruption. It is also known that if a rock is exposed to cosmic rays, after it is erupted on the surface, olivines and pyroxenes have detectable amounts of cosmogenic 3He produced by nuclear spallation interactions between cosmic ray particles and the elements present in the lattice. * Author to whom correspondence should be addressed (paolo@ifcai. pa.cnr.it). † Present address: Istituto Di Fisica Cosmica e Informatica, CNR, 90146, Palermo, Italy. 3751
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The analysis presented is the product of an extended study of noble gases in basalts from oceanic hotspots. For experiments reported here, phenocrysts from basalts covering a wide range of 3He/4He isotopic ratios were selected: Kauai (Hawaii) olivine with a 3He/4He ⬃ 25 R A , Maui (Hawaii) olivine rich in cosmogenic 3He (Craig and Poreda, 1986), phenocrysts from Mangaia and Tubuai (South Pacific) known as HIMU islands (Chauvel et al., 1992; Kogiso et al., 1997) and relatively enriched in radiogenic helium; the set is completed by a sample from Raivavae (French Polynesia) characterized by a large content of helium. 2. ANALYTICAL METHODS Each experiment carried out on phenocrysts of five basalts for a total of seven experiments consisted of a number of crushing steps performed on the same sample under vacuum, under the same experimental conditions and for different durations, using an on line crusher (see description below). The gas extracted from each step was transferred into the inlet system of the mass spectrometer and the Ne removed cryogenically in a charcoal trap at 34 K; the 3He/4He ratio and He concentrations were measured statically on the double-collecting splittube mass spectrometer GAD using the methodology described by Rison and Craig (1983). 3He was measured by ion counting with a Johnston electron multiplier and 4He by collection in a Faraday cup. The measured isotope ratios were normalized to aliquots of a high 3 He/4He (R/R A ⫽ 16.45) standard gas (MM). A measure of the blank has been carried out systematically before each experiment, with the assumption that this value remains constant in the successive steps of the same experiment (see discussion below). After each step the system was pumped out for sufficient time to minimize possible memory effects.
Fig. 1. Crusher used at SIO: schematic view.
2.1. Crushing The term “vacuum crushing” refers to the procedure by which the phenocrysts are mechanically fragmented at an operating pressure of about 10⫺7 torr to avoid atmospheric contamination. Figure 1 shows a schematic drawing of the crusher device used at SIO based on a unit built by K. Farley at Caltech. The device has been designed to optimize the gas extraction from the sample and to maintain a low blank level during the measurement. About 1–3 grams of olivine/clinopyroxene phenocrysts, 0.4 –2 mm in diameter, were hand picked from each lava sample, cleaned ultrasonically in distilled water and ethanol and loaded into the holder attached to a rotary motion feedthrough. The system was then baked and pumped out long enough to release the helium adsorbed on the surface of the grains and the crusher. After a residual pressure of about 10⫺7 torr was achieved, the device was placed inside the solenoid connected to a frequency generator which pulses and rises the sledge up letting it then fall down by gravity onto the tungsten-carbide base thereby crushing the grains and releasing the gases included. The frequency was of one stroke every 2 seconds, with an impact energy release of about 0.4 Joule/stroke. The number of steps and duration of the crushing cycle have been determined case by case, following the requirements fixed for the experiments. 2.2. Blanks Because of the small amount of gas usually contained in the phenocrysts, typically some ncc/g, a critical point in the experiments was the accurate measurement of the blank. The blank is usually expressed as the amount of He (assumed ⬇4He) present in the instrument as a background under normal operating conditions. The blank is determined by several contributions besides possible air leakage into the line, such as gas release by the crusher elements (sledge and container) as residuals from previous measurements (memory effects): an apparent blank component could also be contributed by purely instrumental characteristics, not always completely cancelled out by the “off-peak baseline” subtraction. The combined effect can simulate the presence of
both 4He and 3He with a resulting 3He/4He for the blank, in general, different from that typical of air (Fig. 2a). In our experiments the 4He and 3He blank concentrations were measured independently. Figure 2b shows the histogram of 123 blanks (expressed in 4He ncc units) obtained during more than 200 crushing experiments; the distribution can be assumed Poissonian, with a mean value of 0.17 ncc of He for the overall blank. This value represents the limit for the threshold in the analyses: for amounts of helium of this order the measurement is essentially controlled by the blank— both in terms of the concentration and isotopic ratio. The large dispersion around a median value ( ⬇ 2.5) of the 3He/4He ratio (expressed as R/R A , R A ⫽ 3He/4He in air ⫽ 1.4 ⫻ 10⫺6) of the blanks, makes it difficult to reliably correct the analyses of samples with very small amounts of helium. In our experiments we assume the value of blank measured before the first step to remain constant during all the sequence of the crushing steps. This assumption is justified by the observation that, for conditions substantially above threshold, the concentrations calculated using the 4He blank values determined before each experiment are consistent within about 7% with those obtained by using a standard overall blank value of 0.17 ncc, even though this difference could change somewhat for the last steps of each experiment. 3. RESULTS AND DISCUSSION
Table 1 summarizes the results of the stepwise crushing experiments carried out on the phenocryst samples from Kauai, Maui, Mangaia, Tubuai, and Raivavae. The parameter t (time of crushing) has been chosen as the independent variable; a device-independent parameter is represented by the energy rate released by the sledge of the crusher (⌬W ⫽ 12 J/min), for a banging frequency of one every 2 seconds. The fitting curves to the experimental values of d[4He]/dt vs.
Effects on the 3He/4He measured ratio
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following [(3.1)–(3.5)] a detailed description is given for the set of experiments listed in Table 1. For most of the cases (Maui, Mangaia cpx, Tubuai and Raivavae) both A 1 and A 2 are different from zero, with the two exponentials characterized by very distinct values 1 and 2 (ratio 2/1 ranging from 7 to 21). The absence of the term A 0 verifies that the blank correction is accurate enough to give a negligible contribution. For Kauai, a single exponential ( A 2 ⫽ 0) seems to adequately describe the observed d[4He]/dt vs. t; in this case the term A 0 , although very small, is present. The appearance of a single exponential describing the observed helium differential flux is suggestive of the predominance of a single reservoir releasing under crushing the gas content with a characteristic “relaxation time” irrespective of the likely wide range distribution in size and structure of the fluid inclusions; the 3He/4He isotopic ratio is observed to remain unchanged during all the crushing process: a second reservoir, if present, does not give an appreciable effect. When the two exponentials are present, the second reservoir, showing a higher “relaxation time,” is indicative of a gas component more difficult to extract. In the cases of Maui and Mangaia cpx, in correspondence of the crossover of the two exponentials during the sequential crushing, the observed 3He/ 4 He isotopic ratio shows a change switching from low to high (Maui) and from high to low (Mangaia cpx), coherently with the interpretation of two different gas sources characterized by a different isotopic ratio. 3.1. Maui Olivine Fig. 2. Frequency distributions for 1, 2, 3 blank determinations. (a) Blank R/R A . (b) Blank [4He] (ncc).
the integrated crushing time for the various experiments are shown in the set of Figs. 3–7 labeled (a). The experiments systematically show fitting curves represented by an expression of the type Y ⫽ A 0 ⫹ Y 1 ⫹ Y 2 ⫽ A 0 ⫹ A 1e 共⫺t/1兲 ⫹ A 2e 共⫺t/2兲, where Y ⫽ d[4He]/dt; t ⫽ progressive crushing time. The expression Y describes the presence of two distinct components Y 1 and Y 2 , each characterized by the constants A 1 , 1 and A 2 , 2, respectively. In an “ideal experiment,” with no measurement errors, we expect Y ⫽ Y 1 ⫹ Y 2 ; by fitting data in a real experiment, however, a term A 0 can appear, representing the residual deriving from an incomplete subtraction of the blank. A 0 does not represent therefore a real flux value and it must be excluded from any integration with time to obtain the cumulative amount of gas extracted; this, evidently, to avoid the “nonphysical” divergence in the integration for t ⫽ ⬁ introduced by its constant value. The function Y appears to well represent the appropriate fitting curve to our experimental data. The residuals, expressed in terms of standard deviations , show absence of any systematic trend and lie within an acceptable range of values. Fittings with other possible functions, both for the differential flux and the integrated amount of gas, all give higher residual values and show strong systematic deviations. In the
Sample M-06 is a picritic basalt from a lava flow in the upper part of the late Pleistocene Kula formation of White Hill (⬃0.5 million years old) on Maui. Previous measurements on this rock (Craig and Poreda, 1986) gave a 3He/4He ratio of 8.5 䡠 R A for the crushed olivine, while a value of 42.2 䡠 R A was obtained when the resulting powder was melted under vacuum. This clearly points to the presence of 3He not attributable to fluid inclusions and it is explained by the authors as the result of cosmic ray interaction with the exposed lava flows after eruption and cooling. This secondary component has been tentatively attributed to be hosted by the matrix (e.g., Craig and Poreda, 1986; Hilton et al., 1993). We will refer in the following to this component with the conventional name of “matrixsited” without aiming to endorse or not endorse the validity of this assumption made in the references quoted above. The aim of this experiment was to resolve the gas contained in the fluid inclusions and to test whether the component present in the matrix, presumably richer in cosmogenic 3He, can be partly released from the matrix by prolonged crushing. An ⬃3-gram sample of handpicked olivines [grains (0.9 –2 mm in diameter)] was crushed in six steps, for a total time of 40 minutes. The fitting curve to the experimental data1 of the d[4He]/dt flux measured in the different crushing steps is shown in Fig. 3b and appears to be a linear combination of two exponentials:
1
The fitting procedure to the Maui data as well as for all the other experiments (Figs. 3–7), has been made using SigmaPlot by Jandel Scientific. G represents the goodness of fit as defined by the expression. G ⫽ [¥(observed– expected)2]1/2/¥(observed).
3754
P. Scarsi Table 1. Data from the stepwise crushing experiments on phenocrysts. bc ⫽ corrected for blank.
Experiment Maui-6b (olivine)
Step
⌬t (min)
Time (min) cumulative
1 2 3 4 5 6
0.50 1.00 3.00 5.00 15.50 15.00
0.50 1.50 4.50 9.50 25.00 40.00
¥⫽ Mangaia (cpx)
40.00 1 2 3 4 5 6 7
¥⫽ Mangaia (fresh olivine)
2.00 3.00 5.00 10.00 15.00 10.00 15.00
¥⫽ Mangaia (altered olivine)
1 2 3
¥⫽
0.25 0.25 0.50 1.00 3.00 5.00 10.00 15.00 10.00 10.00
¥⫽
1.00 1.00 3.00 5.00 10.00 15.00
Y 2 ⫽ 0.24e ⫺t/13.11
46.819 1.926 1.503 10.687 3.207 1.238
0.25 0.50 1.00 2.00 5.00 10.00 20.00 35.00 45.00 55.00
20.040 7.538 13.814 8.907 10.777 9.719 7.820 2.257 0.815 0.483
(R/R A ) bc
1
[4He]bc (% cum. of extracted)
5.02 5.60 4.07 8.75 6.40 15.31
3.02 1.34 0.61 0.15 0.07 0.02
8.77 8.59 8.45 8.89 9.57 10.55
0.18 0.15 0.14 0.23 0.18 0.31
22 41 68 78 95 100
1.10 2.93 4.36 5.12 7.33 18.72 13.29
4.31 1.05 0.41 0.18 0.08 0.04 0.04
4.95 4.31 3.90 3.94 3.54 3.69 3.58
0.05 0.08 0.12 0.13 0.14 0.39 0.31
48 66 78 88 94 97 100
1.21 5.37 6.78
23.41 0.64 0.30
6.18 6.22 6.52
0.06 0.14 0.17
93 97 100
2.83 8.85 20.09
5.34 0.64 0.18
6.29 6.59 6.02
0.09 0.16 0.34
71 92 100
2.81 7.14 4.03 6.11 5.11 5.63 6.90 13.90 30.89 43.01
80.16 30.15 27.63 8.91 3.59 1.94 0.78 0.15 0.08 0.05
7.10 6.94 7.21 7.05 6.79 6.76 7.12 6.62 6.19 4.50
0.08 0.13 0.10 0.14 0.11 0.15 0.12 0.31 0.78 1.03
24 34 50 61 74 86 96 98 99 100
0.25 0.50 1.00 2.00 4.00 9.00 14.00
30.745 9.721 14.014 23.942 14.235 13.942 6.367
2.32 6.98 4.95 2.96 4.88 4.97 10.28
122.98 38.89 28.03 23.94 7.12 2.79 1.27
6.53 6.55 6.46 6.46 6.62 6.34 6.21
0.07 0.11 0.11 0.07 0.10 0.10 0.19
27 36 48 69 82 94 100
1.26 2.29 1.29 3.61 5.76 12.49
2.45 1.21 0.80 0.15 0.05 0.01
24.47 25.94 24.72 25.36 24.97 24.12
0.38 0.74 0.38 0.77 0.97 0.82
33 49 81 91 97 100
15.132
82.171
112.966 1.00 2.00 5.00 10.00 20.00 35.00
35.00
Y 1 ⫽ 3.02e ⫺t/1.33
8.612 3.136 2.075 1.753 1.195 0.410 0.617
2.00 7.00 14.00
14.00 1 2 3 4 5 6
¥⫽
0.25 0.25 0.50 1.00 2.00 5.00 5.00
d[4He]bc/dt (ncc/g)/min (average)
50.248
55.00 1 2 3 4 5 6 7
Kauai-6 (olivine)
2.00 5.00 10.00
14.00 1 2 3 4 5 6 7 8 9 10
Raivavae-22 (olivine)
2.00 5.00 7.00
1.510 1.342 1.842 0.726 1.135 0.351
He blk (%)
17.797
10.00
¥⫽ Tubuai-6b (olivine)
2.00 3.00 5.00
4
6.905 2.00 5.00 10.00 20.00 35.00 45.00 60.00
60.00 1 2 3
[4He]bc (ncc/g)
2.452 1.205 2.393 0.753 0.462 0.198 7.464
共ncc/g)/min, 共mcc/g)/min),
共G ⫽ 0.08, see note). The two exponential functions are resolved in Fig. 3b: Y 1 dominates over the first three crushing steps, during which
about 70% of the total He extracted is obtained; with the crossover point at t ⬃ 4 min. Figure 3c–I illustrates the ratio Y 1 /Y vs. the integrated crushing time compared with the measured R/R A . In the first three steps of crushing the measured 3He/4He remains constant within errors at R/R A ⬃ 8.5, while in the successive extractions it increases systematically up to a value (10.55 ⫾ 0.31) in
Effects on the 3He/4He measured ratio
the last step. If we assume that the gas released during the steps for which the fast component Y 1 is dominant represents essentially the helium contained in the fluid inclusions, the value of (8.58 ⫾ 0.01) 䡠 R A (calculated as the mean of R/R A measured in steps 1, 2, 3, weighted inversely by their own variances) can be used for the helium trapped in the fluid inclusions by the basalt of Maui. This value is in good agreement with that of 8.5 䡠 R A found previously by Craig and Poreda (1986) using a different crushing device and averaging over several samples. The data relative to the last three steps show that a significant “excess 3He” is extracted; the abrupt change in the slope can be considered to indicate the limiting extent at which crushing can resolve the helium trapped in the magma by the fluid inclusions from cosmogenic He. In the final steps (# ⬎3) of the experiment the helium extracted is a mixture of this helium with a different component, characterized by a higher 3He/4He ratio. Vacuum fusion experiments by Craig and Poreda (1986) suggest that the high 3He/4He ratios, up to 47.2 R/R A , measured by melting the olivine inclusions after crushing, are due to 3He stored in the matrix structure. The authors attribute this 3He to production by cosmic rays in the exposed lava flows after eruption. Hilton et al. (1993), in a crushing experiment of phenocrysts of Andean lavas rich in cosmogenic helium, noted that a release of matrix-sited helium is possible if the sample is crushed for an extended time; this component, on the other hand, cannot be completely extracted by simple vacuum crushing but only by fusion. Table 2 shows the results of the crushing experiment for the sample MAUI-06. We assume (R/R A ) mean ⫽ (8.58 ⫾ 0.01) for the helium trapped in fluid inclusions (as measured in steps #1, 2, 3) and a total concentration of 1.06 pcc/g for the cosmogenic 3 He sited in the matrix given by melting of sample powder after crushing (Craig and Poreda, 1986). The effect of prolonged crushing is evident after 40 minutes of extraction (six steps), when almost all of the helium retained in the fluid inclusions has been liberated; the increase of 23% of the value of 3He/4He can be accounted for the release of only 0.1% of the total cosmogenic 3He present in the matrix. 3.2. Mangaia Phenocrysts Mangaia is the second largest island of the Cook Islands in Polynesia and it is at present representing the most extreme
3755
example of the HIMU mantle component, with the most radiogenic lead measured to date (Chauvel et al., 1992). The basalts are rich in both olivine and clinopyroxene so that they are suitable for helium isotopic analyses of both minerals. The best estimate of the age of eruption is about 19.5 Ma (Kogiso et al., 1997) and it is reasonable to expect that the phenocrysts could carry detectable amount of radiogenic 4He produced by U and Th decay. 3.2.1. Clinopyroxene Approximately 2 grams of handpicked clinopyroxene (0.9 –2 mm grain size) were crushed under vacuum in seven successive steps for a total crushing time of 60 minutes. The fitting curve of the d[4He]/dt flux obtained for each crushing step as a function of the integrated crushing time is incompatible with a single exponential and it is well represented, as in the case of the Maui sample, by the combination of two exponentials (Fig. 4a) (G ⫽ 0.01; residuals shown in the figure). The two components Y 1 and Y 2 are resolved in Fig. 4b: Y 1 ⫽ 7.95e ⫺t/1.24
共ncc/g)/min,
Y 2 ⫽ 0.81e ⫺t/8.95
共ncc/g)/min.
Y 1 dominates for the first and a part of the second crushing step, while Y 2 takes over for the remaining part of the experiment. The different role of Y 1 and Y 2 is evident in Fig. 4c, where the isotopic ratio is plotted versus the integrated crushing time and compared with the value Y 1 /Y in the same intervals. The 3 He/4He measured in the first step (dominated by Y 1 , but with a Y 2 contribution of ⬃25%) is (4.95 ⫾ 0.05) 䡠 R A and changes to (4.31 ⫾ 0.08) 䡠 R A in the second, where the contributions of Y 1 and Y 2 are comparable, to reach afterwards the value of ⬃3.8 䡠 R A which can be considered, within the errors, representative of the final steps in which Y 2 is dominant. In analogy with the Maui case, the two components Y 1 and Y 2 can be considered as representing the release from two He reservoirs: Y 1 for the fluid inclusions and Y 2 for the second reservoir. We can estimate the value of R/R A for the Y 1 component alone to be of the order of 6, comparable to that obtained for the olivine samples (see Sect. 3.2.2) in which only Y 1 is relevant.
Table 2. Results of the stepwise experiments on Maui-06b olivine.* Experiment Maui-06b (olivine)
Step #
Time cum. (min)
1, 2, 3 4 5 6
4.5 9.5 25.0 40.0
(R/R A ) bc
[3He]meas. (ncc/g)
[3He]meas (pcc/g)
[3He]incl (pcc/g)
⌬(R/R A ) (%)
[3He]exec (pcc/g)
[3He]cosm (% total)
8.58 8.89 9.57 10.55
4.694 0.726 1.135 0.351
0.056 0.009 0.015 0.005
0.0560 0.0087 0.0135 0.0042
3.6 11.5 23.0
0.0004 0.0017 0.0010
0.04 0.16 0.09
* Notes. Column 4: (R/R A ) measured, corrected for blank. Column 5: [4He] measured, corrected for blank. Column 6: [3He] measured, corrected for blank. Column 7: [3He] expected from a constant isotopic ratio R/R A ⫽ (8.576 ⫾ 0.008). See text. Column 8: Deviation from R/R A ⫽ (8.576). Expressed in percent. Column 9: Excess of [3He] measured over the concentration value listed in column 7. Column 10: [3He]exc expressed as % of the total concentration of cosmogenic 3He as given by Craig and Poreda, 1986 (1.06 pcc/g). The values listed in line 1 represent the mean over steps 1, 2, 3, weighted inversely by their own variances.
3756
P. Scarsi
Fig. 3. Maui olivine. (a) [4He] flux rate vs. integrated crushing time. Residual ⫽ (observed-expected)/observed. (b) Components Y 1 and Y 2 of Y representing the d[4He]/dt vs. integrated crushing time. (c) I: Y 1 /Y vs. integrated crushing time. II: 3He/4He vs. integrated crushing time.
3.2.2. Fresh olivine and altered olivine About 1 gram of fresh olivine from the same specimen as the cpx was crushed in three steps for a total time of 10 minutes. Approximately 2 grams of partially altered olivine, again coming from the same specimen as the cpx and with evident presence of iddingsite, were run in the same way for 15 minutes. Both the crystals were in the (0.9 –2) mm size. The results of the measurements are listed in Table 1; in both the experiments the value of the helium concentration released after the initial two steps of crushing reaches values very close to the blank level indicating a complete extraction from the helium reservoir. For the fresh olivine, the measured isotopic ratio is constant within the error limits for all three successive extraction steps: 3 He/4He ⫽ (6.29 ⫾ 0.09) 䡠 R A ; an almost identical constant value is obtained also in the case of the altered olivine: 3He/ 4 He ⫽ (6.18 ⫾ 0.06) 䡠 R A .
The comparison of the data obtained from the olivines and those of the clinopyroxene of Mangaia for the first crushing step of 2 minutes shows important clues. The 4He concentration measured for the fresh olivine, (47 ncc/g), is approximately 5 times greater than that obtained by crushing the altered olivine for the same time, suggesting that the altered crystals of olivine suffered a partial loss of the gas originally trapped in the fluid inclusions after having been erupted to the surface. This loss, on the other hand, did not affect the 3He/4He ratio which remains unchanged within the error limits of the measurements. The 3He/4He ratio obtained during the first crushing step shows a different value for the clinopyroxene (4.95 ⫾ 0.05) 䡠 R A against the mean value 6.2 䡠 R A common to the olivine samples; this is apparently inconsistent with the assumption that both, fresh/altered olivine and the clinopyroxene were formed in the same helium environment. An explanation is however suggested by Fig. 4d, which shows that in the clinopyroxene
Effects on the 3He/4He measured ratio
3757
Fig. 4. Mangaia cpx. (a) [4He] flux rate vs. integrated crushing time. (b) Components Y 1 and Y 2 of Y representing the d[4He]/dt vs integrated crushing time. (c) I: Y 1 /Y vs integrated crushing time. II: 3He/4He vs. integrated crushing time.
sample the gas extracted in the first 2 minute crushing step contains a mixture of the two components: about 80% of Y1 attributed to the fluid inclusions, with the remaining 20% of Y2 enriched in 4 He. In the successive steps of crushing of the clinopyroxene sample an increasing percentage of the Y2 component is released. While in the case of Maui basalt, information on the existence, extent and location of an important cosmogenic reservoir connected to Y 2 is given by the melting experiment of Craig and Poreda (1986), due to the lack of an equivalent melting experiment for the Mangaia cpx the description of the Y 2 reservoir can only be inferred by general data. A possible explanation of the origin and the extent of the reservoir responsible for the component Y 2 can be offered by considering the dilution of Y 1 (R/R A ⬃ 6.2) with radiogenic 4 He built up after the eruption. By analogy with the case of the Maui olivine, we adopt the convention of the “matrix-sited” component, assuming that the physical location of the reservoir in principle could be sited in the matrix. Again, we stress that
this is simply a matter of naming this secondary component without aiming to endorse or not the validity of the assumption about its location. If we assume that originally the helium isotopic composition of the pyroxenes had the 3He/4He ratio typical of Y 1 (6.22 ⫾ 0.01) 䡠 R A (mean of R/R A of the fresh olivine—weighted inversely by the variances), we can estimate the 4He release at each crushing step to justify the corresponding measured value for R/R A . These values are reported in column 7 of Table 3. The total amount of excess 4He extracted in the seven steps of the experiment is about 5.08 ncc/g. It is of some interest to compare this value with the concentration of radiogenic 4He produced by radioactive decay in the cpx phenocrysts. To evaluate the expected amount of 4He, it is necessary to know the amount of U and Th and the age of the sample. No measurements as yet have been carried out on these minerals, so we can only derive an approximated value based on the following assumptions.
3758
P. Scarsi
Fig. 5. Tubuai olivine. (a) [4He] flux rate vs. integrated crushing time. (b) Components Y 1 and Y 2 of Y representing the d[4He]/dt vs. integrated crushing time. (c) I: Y 1 /Y vs. integrated crushing time. II: 3He/4He vs. integrated crushing time.
Table 3. Results of the stepwise experiments on Mangaia cpx.* Experiment
Step #
Time cum. (min)
(R/R A ) bc
[4He]meas (ncc/g)
[3He]meas (pcc/g)
[4He]incl (ncc/g)
[4He]exc (ncc/g)
Mangaia (cpx)
1 2 3 4 5 6 7
2 5 10 20 35 45 60
4.95 4.31 3.90 3.94 3.54 3.69 3.58
8.61 3.14 2.07 1.75 1.19 0.41 0.62
0.060 0.019 0.011 0.010 0.006 0.002 0.003
6.853 2.175 1.301 1.110 0.680 0.244 0.356
1.759 0.961 0.774 0.643 0.514 0.167 0.261
¥⫽ * Notes: Column 4: Column 5: Column 6: Column 7: Column 8:
(R/R A ) measured, corrected for blank. [4He] measured, corrected for blank. [3He] measured, corrected for blank. [4He] expected from a constant isotopic ratio R/R A ⫽ (6.29 ⫾ 0.09) (fresh olivine) and [3He] ⫽ [3He]meas. See text. Excess of [4He] measured over the concentration value listed in column 7.
5.078
Effects on the 3He/4He measured ratio
3759
Fig. 6. Raivavae olivine. (a) [4He] flux rate vs. integrated crushing time. (b) Components Y 1 and Y 2 of Y representing the d[4He]/dt vs. integrated crushing time. (c) I: Y 1 /Y vs. integrated crushing time. II: 3He/4He vs. integrated crushing time.
(1) We estimate, for the whole sample analyzed, a uranium and thorium concentration of 1.2 and 4.6 ppm, respectively, based on the average of the data of Woodhead (1996). (2) We assume that the U and Th content of the sample is equal to that of the melt which produced the lava.
(3) We use a Th/U ratio of 3.6 for our phenocrysts, assuming no fractionation between Th and U during the formation from the magma. (4) We use a clinopyroxene/melt partition coefficient for U between 0.002 and 0.03 (Graham et al., 1992) which gives a concentration of 2.5–35 ppb.
Table 4. Crushing experiment. Sample
Type
# of steps
Tot. crush. time (min)
[4He]released (ncc/g)
1 (min)
2 (min)
Notes
Maui Mangaia
ol cpx ol ol ol ol ol
6 7 3 3 10 7 6
40 60 10 14 55 14 35
6.905 17.797 50.248 15.132 82.171 112.966 7.464
1.33 1.24 0.63 1.52 0.47 0.08 2.83
13.11 8.95
Cosmogenic 3He HIMU-radiogenic 4He Fresh crystals Altered crystals HIMU-radiogenic 4He Large content of 4He High 3He/4He
Tubuai Raivavae Kauai
6.95 1.71
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P. Scarsi
not strong enough to liberate the helium of the matrix. It should be noted that most of the helium is extracted in the first steps of the experiments, as can be seen in Figs. 5(a) and 6a. (b) Olivine have a different crystallographic structure from the cpx. Moreover, it could contain a lower amount of U and Th and therefore a lower amount of 4He produced by radiogenic decay. 3.3. Tubuai Olivine Tubuai is a small volcanic island belonging to the Southern Cook island chain, about 1200 km from MacDonald Seamount. Its volcanism began about 10.7 Ma ago (Diraison, 1991) and the products erupted have, like for Mangaia, the isotopic composition of a HIMU endmember (Zindler and Hart, 1986). Ten crushing steps were carried out on 0.8 grams of pure olivine (10 –20 mesh in size) from Tubuai-06b basalt for a total of 55 minutes, a time longer than in the previous experiment on the olivine of Mangaia and approximately equal to that of the experiment on the pyroxene of the same sample. The results of this experiment are plotted in Fig. 7. As in the case of Maui and the Mangaia cpx experiments, the data of d[4He]/dt flux plotted vs. the integrated crushing time, cannot be fitted by a single exponential, but require the combination of two components indicating the contribution of two distinct He reservoirs. Figure 5a shows the fitting curve (G ⫽ 0.12; residuals plotted in the figure). In Fig. 5b, Y 1 and Y 2 are resolved:
Fig. 7. Kauai olivine. (a) [4He] flux rate vs. integrated crushing time. (b) 3He/4He ratio vs. integrated crushing time.
In addition, for the estimate of the radiogenic 4He of the phenocrysts, we do not take into account the effect of ␣-implantation from the host rock and we assume that the presence of the U and Th contained in the fluid inclusions of the crystal does not affect the helium concentration of the lattice. For an estimated age of 19.5 Ma, the expected 4He content in the clinopyroxene due to radiogenic decay of U and Th is of the order of (10 –100) ncc/g. The excess radiogenic helium extracted from the pyroxene amount to 5–50% of the estimated content of the radiogenic reservoir, value which although rather high, is not inconsistent with the calculated content. These values must be considered only approximate estimates, due to the large number of assumptions made in the calculations. Again, only melting the cpx powder after crushing could provide a direct information of the matrix sited helium. Finally, to explain why the 3He/4He of the olivines does not decrease substantially as compared to the decrease in the pyroxenes, two possibilities can be considered: (a) The total crushing time in the experiments on the olivines were probably too short and the stress of the sledge banging
Y 1 ⫽ 80.16e ⫺t/0.47
共ncc/g)/min,
Y 2 ⫽ 5.86e ⫺t/6.95
共ncc/g)/min.
Y 1 dominates in the first three steps, with a crossover with Y 2 at step #4 (Figs. 5b and 5c–I). The measured 3He/4He remains, for all the steps, consistent with a unique signature at ⬃7 䡠 R A , with a possible decrease shown in step #10, however compatible within the error with the common trend. A straightforward interpretation is offered by considering the presence of two classes of fluid inclusions with different structural characteristics for the reservoirs but representative of the same magma environment. 3.4. Raivavae Olivine Raivavae is a volcanic island, about 7.3 Ma old (Diraison, 1991) in the Austral Island Chain, south east of the Austral Fracture Zone. In Fig. 6 are reported the results of the experiment carried out on 0.8 grams of pure olivine (0.9 –2 mm in size) handpicked from a basalt of Raivavae. The crushing has been carried out for a total of 14 minutes in seven successive steps; after these extractions, the amount of helium was still significantly higher than the blank of the machine, and the extraction must be considered incomplete after the last run. This of course affects the calculation of the total concentration: the aim of this experiment, on the other hand, was to look for a changing trend, if any, in the isotopic ratio as consequence of crushing. Figure 6a shows that also in this case the gas flux release for each step, d[4He]/dt, plotted versus the integrated crushing
Effects on the 3He/4He measured ratio
time is well described by the presence of two reservoirs. Figure 6b shows the two components Y 1 and Y 2 (G ⫽ 0.02; residuals shown in the figure): Y 1 ⫽ 380.46e ⫺t/0.08
共ncc/g)/min,
Y 2 ⫽ 41.89e ⫺t/1.71
共ncc/g)/min.
Figure 6c–I shows that the two components invert the relative role of dominance after the first step, while the measured isotopic ratio remains constant during all of the experiment, with a possible decrease in the last step to a value, however, still compatible with the general average. The interpretation is similar to that reported for the sample for Tubuai. 3.5. Kauai Olivine Sample Kauai-6 (Hawaiian Chain) is an olivine from a basalt characterized by an high value for the isotopic ratio (3He/ 4 He ⬃ 25 R/R A ). Almost the total amount is extracted in the first five steps, corresponding to an integrated crushing time of 20 minutes, with the sixth step providing an amount of helium close to the blank value. Figure 7a plots the d[4He]/dt flux released against the integrated time of crushing. The experimental values are well fitted by the single exponential: Y ⫽ 0.01 ⫹ 2.45e ⫺t/ 2.83
共ncc/g)/min
(G ⫽ 0.10兲.
The figure also shows the individual residuals. Figure 7b gives the isotopic ratio 3He/4He (R/R A ) for the various steps as a function of the integrated crushing time: the ratio shows a constant value at 25 䡠 R/R A . The evidence clearly points to a single He reservoir which can be ascribed to the fluid inclusions. 4. CONCLUSIONS
The primary aim of this work was a quantitative analysis of the crushing technique as a mean for releasing the gas content of magmatic phenocrysts, addressed specifically to evaluate the isotopic ratio 3He/4He. The measurement of this value for the fluid inclusions in the crystals characterizes the source of the magma at eruption; it could however be affected by processes active between the time of eruption and sampling, because of gas losses or of acquisition of radiogenic and cosmogenic components. The technique used has been of crushing by steps, with time of crushing long enough, in some cases, to obtain an almost complete release of the gas content of the fluid inclusions reservoirs. The series of experiments carried out cover a wide range of conditions representative of the various classes of cases ranging from phenocrysts with high contents of cosmogenic 3He or radiogenic 4He to samples characterized by a high He content to others with high 3He/4He ratios. Considerable care has been put in the evaluation of the level and nature of the blank, to assure the absence of interference of systematic errors in the measured ratios. An exponential trend has been found to characterize the d[4He]/dt vs. crushing time in all the experiments; a single exponential (time constant 1) or a linear combination of two
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exponentials (time constants 1 and 2) have been identified. The time constants which are determined overall by the crusher and the crushing procedure, should reflect physical characteristics of the sample examined such as, e.g., the size of the bubbles, the site and hardness of the reservoir(s). The values obtained clusters in two groups: 1 ranging between ⬃0.1 to 3 minutes, 2 from ⬃7 to 13 minutes; an extreme behavior is shown by the Raivavae olivine with 1 ⫽ 0.08 min and 2 ⫽ 1.71 min depending possibly on the high content of He in the sample. The shorter 1 can be connected with the reservoir containing the helium trapped in the fluid inclusions; its content is liberated in the first steps in each experiment. The heterogeneous size and distribution of the fluid inclusions can be assumed to be responsible for the scattered values of 1 between the different samples. From the presence of 2 we can infer the existence of a second reservoir. The value of 2 can give some clue about its nature and location, but only a thin section inspection, together with the melting of the powder after the extraction of the helium contained in the bubbles can provide a definite identification (“matrix sited” or secondary inclusions) and the correct isotopic signature of the cosmogenic spallation/radiogenic decay effects on the crystals. For the Maui olivine, the data from the crushing experiment show in the last steps a progressive increase of the value of 3 He/4He with the progression of crushing, indicating that the second reservoir (2) contains the excess of 3He (cosmogenic); the melting experiment (Craig and Poreda, 1986) suggests its existence and points to a location in the matrix. The experiments on the phenocrysts with a large content of 4 He (radiogenic) raise important questions associated with the geological meaning connected with the identification of the second reservoir. Different conclusions can be drawn if we attribute the radiogenic helium to the “matrix” or to the magma. If we associate it to the magma we should find this component also in the fluid inclusions; if absent we can conclude that the crystals “sealed” their inclusions before the 4He production and that the radiogenic helium was formed after eruption. The experiments on minerals of the same basalt sample from Mangaia show that the lower value of 3He/4He measured in the clinopyroxene can be attributed to the extraction of radiogenic helium sited in the matrix of the minerals and not in the inclusions. This is supported by the fact that the inclusions of the coexisting olivine do not show the same isotopic signature. Naturally a fusion step could provide a definite conclusion. If aging is responsible for the lower value of the isotopic ratio, as observed in the stage of prolonged crushing (case of Tubuai and Raivavae), it must also be concluded that the radiogenic 4He is released by the second reservoir and it originated in the crystal lattice after the rock was erupted to the surface, excluding therefore an origin in the magma chamber before eruption, a possible interaction (assimilation) of the magma with the surrounding rock on the way to the surface, and a post eruptive radiogenic decay of the U and Th trapped in the fluid inclusions of the phenocrysts. As a general conclusion this work confirms that crushing under vacuum is at present an appropriate method to resolve the “primary” helium trapped in the fluid inclusions of the phenocrysts from the in situ produced radiogenic/cosmogenic helium;
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melting is the appropriate methodology to obtain a complete extraction of the helium. The results of the experiments carried out on olivine and clinopyroxene show that protracted crushing of these minerals can release helium contained in secondary inclusions or in their lattices so that the gas extracted in later steps is actually a mixture of inclusion-trapped and “matrixsited” helium. To minimize this effect, the crushing steps considered should be limited to short time durations; it does not exist as a general rule to optimize the crushing times to get maximum yield of mantle gas with minimum yield of matrix gas if not to proceed with a contextual analysis of the isotopic ratio behavior vs. the integrated crushing time to check possible variations. For the set of experiments reported in this paper (crusher with an impact energy release of 0.4 Joule/stroke and frequency of one stroke every 2 seconds), this optimum time appears to be about 5 minutes, but occasionally (Tubuai and Raivavae olivines) as low as 2 minutes. Samples with low amounts of helium in the fluid inclusions must be carefully handled when measuring the 3He/4He; in addition, due to the generally low amount of gas in the phenocrysts, particular attention has to be paid to the blank of the extraction system, as its value can be different from air for instrumental reasons. Acknowledgments—This work has been carried out at SIO as a part of an extended study of noble gas in basalts from oceanic hotspots. I acknowledge the suggestions and criticisms of Harmon Craig, who has made available the samples for analysis and the facilities of the Isotope Laboratory. Dr. E. Winterer kindly supplied the sample from Mangaia. I am thankful to D. Hilton for comments. Thanks to K. A. Farley for providing the design of the crusher and to D. Brewton for assistance in the laboratory. I am deeply indebted to the staff of IFCAI/CNR in Palermo for continuous support in the data analysis and helpful discussions in the interpretation of the results. This work was supported by NSF Grant No. EAR96-28481 (Mantle Geochemistry) to H. Craig.
REFERENCES Chauvel C., Hofmann A.W., and Vidal P. (1992) HIMU-EM: The French Polynesian connection. Earth Planet. Sci. Lett. 110, 99 –119. Craig H. and Poreda R.J. (1986) Cosmogenic 3He in Terrestrial rocks: The Summit Lavas of Maui. Proc. Natl. Acad. Sci. USA 83, 1970 –1974. Diraison C. (1991) Le Volcanisme aerien des archipels polynesiens de la Societe, des Marquises et des Australes-Cook; tephrostratigraphie, datation isotopique et geochimie comparees; contribution a l’etude des origines du volcanisme intraplaque du Pacifique Central. These de Doctorat d’Etat, Universite de Brest. Farley K.A. and Neroda E. (1988) Noble gases in the earth’s mantle. Annu. Rev. Earth. Planet. Sci. 26, 189 –218. Graham D.W., Humphris S.E., Jenkins W.J., and Kurz M.D. (1992) Helium isotope geochemistry of some volcanic rocks from SaintHelena. Earth Planet. Sci. Lett. 110, 121–131. Graham D.W., Larsen L.M., Hanan B.B., Storey M., Pedersen A.K., and Lupton J.E. (1998) Helium isotope composition of the early Iceland mantle plume inferred from the Teritary picrites of West Greenland. Earth Plant. Sci. Lett. 160, 241–255. Hilton D.R., Hammerschmidt K., Teufel S., and Friedrichsen H. (1993) Helium isotope characteristics of Andean geothermal fluids and lavas. Earth Planet. Sci. Lett. 120, 265–282. Kogiso T., Tatsumi Y., Shimoda G., and Barsczus H.G. (1997) High mu (HIMU) ocean island basalts in southern Polynesia; new evidence for whole mantle scale recycling of subducted oceanic crust. J. Geophys. Res. 102, 8085– 8103. Rison W. and Craig H. (1983) Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Islands basalts and xenoliths. Earth Planet. Sci. Lett. 66, 407– 426. Stuart F., Turner G., and Taylor R. (1994) He–Ar isotope systematics of fluid inclusions: Resolving mantle and crustal contributions to hydrothermal fluids. In Noble Gas Geochemistry and Cosmochemistry (ed. J. Matsuda), pp. 261–277. Terra Scientific Publishing Company. Zindler A. and Hart S. (1986) Chemical geodynamics. Ann. Rev. Earth Planet. Sci. 14, 493–571. Woodhead, J.D. (1996) Extreme HIMU in an oceanic setting; the geochemistry of Mangaia Island (Polynesia), and temporal evolution of the Cook-Austral hotspot. J. Volc. Geothermal Res. 72, 1–19.