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Heavy element affinities in Apollo 17 samples
Earth and Planetary Science Letters, 27 (1975) 163-169 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
['~ 1_..2_3
HEAVY ELEMENT AFFINITIES IN APOLLO 17 SAMPLES* R.O. ALLEN, Jr. 1 , S, JOVANOVIC and G.W. REED, Jr. Chemistry Division, Argonne National Laboratory. Argonne, [ll. [USA) Received February 21, 1975 Revised version received June 19, 1975
2°4pb, Bi, TI and Zn in samples from Apollo 17 exhibit relationships not found in samples from other sites. ~°4pb, TI and Zn in residues remaining after dilute acid leaching are correlated with one another. Orange soil 74220, which is enriched in 204pb, TI and Zn, is included in these relationships. In addition the submicron metallic phase generally associated with agglutinate formation is correlated with all three of these elements; this relationship has already been reported for 2°aPb in other samples. Thus, orange soil and agglutinates appear to be involved in concentrating heavy volatile metals. A process other than mixing is required to account for this. As a consequence of the isolation of the landing site by the surrounding massifs, local supply and recycling of volatile trace elements in soils may account for some of the interelement relations.
1. Introduction The Apollo 17 samples represent the most diverse set of samples returned from the moon; they include rocks similar to those collected at many o f the other sites as well as samples restricted to Taurus-Littrow. The trace element interrelations, so far reported, are unique. The correlations among the halogens [ 1] and the perturbed ratios of pairs of incompatible elements, K, Zr, Nb and Ba relative to TiO2 [2] are examples. We shall examine the results for a set o f elements, 2°4Pb, Bi, T1 and Zn. These elements are intermediate in mobility between the very labile halogens and the more refractory incompatible elements.
2. The data 2°4pb, Bi, T1 and Zn have been determined by fast and slow neutron activation analyses on soils, representative of the major geologic units at the Apollo 17 site; on two fragmental rock breccias 72275 and 76315; and * Work supported by USAEC and NASA. I Permanent address: Department of Chemistry, University of Virginia, Charlottesville, Virginia, 22901.
on the orange soil 74220. A shadow 76241 and a 2-cm skim 76261 soil from the North Massif, two basaltic soils, 71501 and 75080, from the valley, and the lightcolored soils, 73141 and 72701, from the South Massif were studied. Splits of 75080, sieved at 74 ~m, were measured. All the samples were leached with hot dilute HNO3 at pH 5 - 6 for 1 5 - 2 0 minutes and the element contents in the leach and the residual samples were determined. The submicron metal present in soils was measured by a magnetic resonance technique with the assistance of J.R. Norris, Jr., ANL. We have presented evidence indicating that the lunar chemistry of these trace elements may be related to a metallic phase which may be metal or metal-metal sulfide eutectic and to P2Os and also to the halogens [ 3 - 5 ] . The data are given in Table 1. The greatest uncertainty is in the 2°4pb leach results, where some of the errors are as high as 50%. The weighted sums of the sieve fractions of 75080 agree reasonably well with the unsieved sample. The <74-/1m 75080 Zn data is clearly very high and unaccountable. The percent of leachable Bi measured in the 75080 sieved fractions is too low. We assume that the leaching solution must have been at a higher pH than 5 - 6 in which case Bi salts are insoluble. The unsieved sample contained 33% leachable Bi. The
164 TABLE 1 Primordial lead, bismuth, thallium, zinc and submicron metal in Apollo 17 samples* Sample
Soils 71501,34 72701,34 73141,20 74220,111 75080,8 75080,8 (>74 um) 75080,8 (<74 ~m) 76241,17 76261,18 Rocks 72275,66 76315,70
2°4pb (ppb)
Bi (ppb)
r
1
4.8 ± 0.9 2.6 +- 0.5 4.1 ± 1.0 36 ± 2 3.4 -+0.8 1.2 ± 0.4 6.0 ± 1.1 3.3 ± 0.8 2.2 +-0.4
0.33 0.13 0.37 3.9 0.64 0.76 1.0 0.64 0.06
1.1 ± 0.2 2.4 +-0.3
r
± 0.11 :t 0.06 ± 0.19 ± 0.4 ± 0.34 ± 0.34 +-0.5 ± 0.14 ± 0.04
0.13 + 0.6 0.6 ± 0.3
0.55 0.99 0.22 0.54 0.49 (0.25 (1.0 0.87 0.62
n.d. n.d.
Tl (ppb)
Zn (ppm)
Submicron metal (%)
1
r
1
r
1
0.13 0.20 0.15 0.18 0.24 0.06) 2 0.04) 2 0.25 0.09
1.8 (1.5 1.6 14.5 1.5 1.1 2.3 2.1 1.8
0.13 0.19) 1 0.24 5.4 0.16 0.06 0.2 0.1 0.09
15 11 13 135 24 11 174 24 19
n.d. n.d. 0.65 5.6 0.55 0.65 24 0.31 0.41
0.52 0.31 0.20 n.d. 0.34 0,13 0.60 0.38 0.36
0.19 0.12
0.003 0.007
0.35 ± .05 0.17 ± .03
1.0 1.1
0.11 0.24
2.5 2.8
* Counting statistical errors are less than 10% unless otherwise indicated. Samples were leached at pH 5-6 HNO3. r = residue after leach, 1 = leach solution. I Measured T1 values were reversed due to assumed mislabeling. 2 Measured Bi values; these should be adjusted as described in text to 75080 (<74 ~m): 0.2r, 0.11; 75080 (>74 ~m): 0.7r, 0.31.
true water-soluble concentrations in the sieved samples is probably near that of the unsieved sample and the adjusted values are given in a footnote to the table. For all Apollo 17 soils the ranges in concentrations measured for these trace elements fall within about a factor of 2, i.e., 3 . 0 - 5 . 2 ppb 2°4Pb, 0 . 7 3 - 1 . 8 ppb Bi, 1 . 0 - 2 . 3 ppb q'l and 1 1 - 2 4 ppm Zn. Although they have diverse origins, i.e., comminuted subfloor basalts to light-colored anorthositic highland breccias most of these soils have about the same maturity [6]. The cluster of our data on the submicron Fe ° may also bear on this observation;this is somewhat surprising. The uniqueness of orange soil 74220 is apparent; most striking is the lack of Bi enrichment while the other elements show an eight-fold enrichment relative to the other Apollo 17 samples. We should note that another unusual sample, Apollo 16 rusty rock 66095, appears several times in considering the data. This rock contains an order of magnitude more 2°4Pb, Bi and T1 than other Apollo 16 and 17 samples; the Zn, however, is not enriched [5].
3. Inter-element relationships An approach to understanding the chemistry of these trace metals is through correlations or lack thereof among the metals themselves, and between the metals and active agents such as the halogens, P20s and Fe°-FeS. Our data only are used in so far as possible since comparable leach data from the literature are not available. We focus primarily on the Apollo 17 samples but our other data are also included when needed in discussions. Since the quantity of data is limited we will rely on comparisons between correlated pairs of elements to help support apparent trends.
3.1. Residual 2°4Pb-Tl-Zn The residual 2°4pb (2°4pbr) is correlated with Zn r and with TIr in Apollo 17 samples (Fig. 1a, b). The trends are similar for both pairs of elements. Of interest is the fact that in both figures, trend lines through
165
4•
v
/~
20
E
40 1203o 3 ,f
3O
.f
0 2 4 6 8 @l
IO soil rock Ap 17 • 0 16 • 0 14 •
I
n,
~o
f
i
I
i
i
I
2
3
4
5
residual 204pb (ppb)
0 ~' o
I
I
I
l
I.
I
I
2
3
4
5
6
residual
204pb (ppb)
4
6
r e s i d u a l TI
8
I0
(ppb)
Fig. 1. Correlations of residual nonleachable 2°4Pb-Zn, 2°4Pb-TI and Zn-TI in lunar samples. Apollo 17 soils 1-9 are 71501,34, 72701,34, 73141,20, 74220,111, 75080,8, 75080,8 (>74 #m), 75080,8 (<74 #m), 76241,17 and 76261,18; rocks are 1 = 72275,66 and 2 = 76315,70. Apollo 16 soils 1-4 are 61221,17,63501,41, 64501,18 and 64801,41; rock, 1 = 61016,131 [5]. Apollo 14 soils are 1 = 14163,152, 2 = 14259,119 and half open diamond 2 = soil breccia 14049,35 [3]. Orange soil 74220,111 and rusty rock 66095,23 are off scale but are involved in these correlations. Apollo 14 TI data from Morgan et al. [ 10]; the limits indicate that the residual concentrations are less than the totals reported.
the orange soil 74220 and the origin include a number of the samples. Samples 75080 (<74/am), 76261 and 76241 appear to fall along a different trend. In the 2°4Pbr-Tl r comparison, Apollo 14 and 16 samples appear to define a correlation related to 66095 and unrelated to 74220 or other Apollo 17 samples (Fig. lb, insert). A test Of the 2°4pbr-Zn r and 2°4pbr-T1 r relationship for Apollo 17 samples is Znr-T1r (Fig. lc). The data fall along a curve through 74220 and the origin with a mean ratio for six samples, including 74220, of 9.6 -+ 1.2 × 10 a. The 2°4pb r and Zn r in Apollo 17 breccias 76315 and 72275 and Apollo 16 breccias 66095 and 61016 are correlated (Fig. la). The breccias contain about 2.2 ppm excess Zn r relative to 2°4pb r; when this excess is subtracted 2°4pbr/Zn r = 0.0036 + 0.0006.
3.2. 2°4pbr-Znr-Tlr-Fe° relationships The correlation between 2°4Pb r and the submicron Fe ° found in Apollo 14 and 15 soils [4] includes a num-
ber of Apollo 17 samples (Fig. 2a). The 2°4pbr-Znr-T1 r relation for soils implies that since 2°4pb r has been found to be correlated with Fe ° then Zn r and T1r should be also. The Apollo 17 samples contain correlated Zn r and Fe metal (Fig. 2b). Apollo 16 soils appear to define a somewhat different Znr-metal trend which may include 14163. Soil 71501 an'd also 14049, 14259 and 15031 fall outside of the shaded sector that includes the Apollo 16 and 17 samples. A Tlr-Fe ° metal correlation is not so obvious (Fig. 2c). The best line through the Tlr-Fe ° data would intercept the T1 ordinate at about I ppb T1r. Our Apollo 17 T1 data are systematically about 15% higher than literature data [7]. Lowering our results by 15% and using literature data where available does not change the correlation and the best fit still does not intercept at the origin. In addition, the correlations of T1r with 2°4pb r and with Zn r are not improved. More data are needed to resolve this matter. We do think that there is a Tlr-Fe ° trend but it could be partially obscured by the presence of T1 in other phases. One of these could be the glass associated
166
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~5
cZ
bJ
J
~0
5
Q7
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~1"
p 14(4 ),
2
15{4)
• 9
o
%3
05
,/ 0
6 • f/S~o2
0
"N 2
0
01
113
2
•
/
/
•6
I
Ap 17 • 16 • 15" 14 .
Ii I
I
I
I
i
I
k
0.1 0.2 0.3 0.4 0.5 0.6 0.7 metal wt %
0[
1
0
0.1
I
I
I
I
0 . 2 0.:5 0.4. 0.5
I
I
0.6
0.7
o o
metal wt %
0.1
0 . 2 0.3
0.4
0.5
0.6
0.7
metal wt %
Fig. 2. Residual 2°4pb, Zn and TI correlations with s u b m i c r o n metallic p h a s e in lunar soils. Samples are as in Fig. 1. Apollo 15 soil is 1 5 0 3 1 , 2 7 [4]. Metal data o t h e r t h a n f r o m this w o r k are referenced in [5].
with agglutinate. Another could be a component mixed in the soils which has relatively high TI content and Apollo 16 breccias such as 66095 and 61016 reported by Kr~ihenbi~hl et al. [8] and by Allen et al. [5] may represent this component. The correlation with Fe ° and the involvement of 74220 suggest that Zn r and TIr are being incorporated in Apollo 17 soils in two ways. One related to metal formation and the other to glass formation. These are not independent processes in soils. If we assume the correlation line through 74220 (Fig. lb) applies to all Apollo 17 samples and indicates the amount of the trace element dissolved in glass, then in samples 75080 (<74 ~m), 76261,76241, and 75080 about one-half of the Zn r is associated with Fe ° . Reducing the Zn r contents for these samples by one-half places them on aline through 15031, 71501 and 14259 in Fig. 2b. The process may be rationalized as follows: (1) 74220 formation was a one-step process, either volcanic or impact. The 2°4pb, T1 and Zn were probably present as volatile compounds such as halides. An advantage of a volcanic source is that the volatiles released may also end up being the sublimates coating soil grains and may supply the trace metals for incorporation during the agglutinate formation discussed below. (2) The agglutinate extraction of the trace metals
may be conceived as a continuous process: deposition of sublimates on soil grains, solar wind implantation into the soil, and finally impact heating and melting Fe ° formation could occur by reduction of oxidized Fe by solar wind hydrogen as discussed by Housley et al. [9]. The reaction: FeO+2H~Fe ° +H20 has a negative free energy of approximately 101 kcal/mole which is competitive with hydrogen atom combination to form H2 ( " 9 7 kcal/mole). The atomic Fe formed may then coalesce during impact heating to give the submicron single and multidomain Fe ° measured magnetically. In the process, Fe-FeS eutectics form and the heavy trace metals become incorporated. Thus soils contain both glass and an Fe ° phase which are related and which can extract proportionate amounts of volatile heavy metals. The total amount of these trace metals incorporated will, of course, depend on the maturity of the soil provided a source of supply of the trace metals is available.
3. 3. Bir-2°4pbr-P205 A trend involving 66095 appears to be indicated by Bir and 2°4pb r in Apollo 17 soils 75080 (<74 ~um), 76241, 76261 and 72701. The Bir/2°4Pbr ratio, in-
167 cluding 66095, is 0.28 -+ 0.04. Samples 15101 (<74 /am) and possibly 15205 and 14259 and 14049 reported by Allen et al. [3,4] fall near this trend line. The samples involved are non-mare except sieved sample 75080 (<74 Ore) which has a significant non-mare component based on its U content of 0.34 [1]. Tile repeated occurrence of 66095 in element correlations (Bir-2°4pbr and Tlr-2°4Pb r in soils, Znr-2°4Pbr in rocks) involving non-mare samples suggests that (1) a 66095-1ike component must be present in samples such as breccias 76315, 72275 and 61016, and (2) some of the volatile metals in soils could have been derived from the same or a related source which supplied volatiles for 66095. The source region must be extensive to supply material to such dispersed sites. We have noted an apparent Bir-P20 s correlation with a Bi(ppb)/P2Os (wt.%) ratio of ~2.5 for Apollo 14 and 15 samples [4]. The Apollo 16 and 17 soil data do not fall along this curve but instead apparently define a new trend including 74220 (Fig. 3). In fact, the data appears to form two curves. One group of samples 74220,
1.5
76241 and 7150 l have an average ratio (Bi, ppb/P20s, wt.%) of 9.2 + 0.6; the other group including 7626 l, 72701, 73141 and four Apollo 16 soils has an average ratio of 6.2 -+ 0.9. Although there is no enrichment of Bi in 74220, it again appears closely related to other Apollo 17 samples. 3. 4. Residual trace metals and the halogens
Of the metals reported here, Bi and Zn appear to be associated with the non-leachable halogens. Bir is weakly correlated with C1r in Apollo 16 and 17 samples, probably via the phosphate phase. In the case of Zn only Apollo 16 samples show an apparent Znr-Clr trend but there is not sufficient data to confirm this; 74220 may be included in this relationship. 3.5. Leachable
2°4pb-Bi-Tl
The dilute acid-soluble fracitons of the heavy metals represent water-soluble salts probably coating grain surfaces. Only two patterns appear; they involve three of the metals, 2°4pb, Bi, and T1. (1) Apollo 17 soils 76421, (76261), 71501,75080, 73141 and breccia 76315 contain correlated Bi and 2°4pb (Bil, 2°4pbl) giving a mean Bil/2°4Pbl ratio excluding 76261, of 0.37 -+ 0.04. (2) The Apollo 17 South Massif soils, 72701 and 73141, and the Apollo 16 soils contain a large range of T11 concentrations with a much narrower range in 2°4pb 1. The Apollo 17 valley and North Massif soils have approximately constant TI1 and a range of 2°4pb 1 concentrations. This pattern is duplicated in the non-labile fractions of these elements but the South Massif samples are excepted. This appears to be reasonable, since the trace metals extracted into the agglutinates should reflect to some extent the relative concentration of these metals deposited on surfaces.
0.5 3. 6. Leachable trace metals and the halogens
Og 0
I
0.1
I
0.2
1
0.3
,1
0.4
r
0.5
I
0.6
Pz05 wt % Fig. 3. Residual Bi and P2Os correlation in lunar samples. Samples are as in Fig. 1. Apollo 17 P2Os data are from [1,11,12]; Apollo 16 data are referenced in [13].
An additional source of information on the labile metal fractions may be their relations to the halogens. A 2°4pbl-Br1 correlation was noted for Apollo 14 and 15 samples [4] ; 74220 is colinear with this trend (Fig. 4). Other Apollo 17 and Apollo 16 soils define an entirely different trend. Both trends, including the role of 74220, are strongly supported by 2°4Pbl-I 1 data. Similar
168
soil r;ck Ap 17 • 16 • 14(2),15~5: 3
0X3
2
0 w
O6
0 5
6L221
17 samples but in a different way than in Apollo 14 and 15 samples. The striking exception is 74220 2°4Pbl-Br 1 (I1) which is colinear with the Apollo 14 and 15 samples. Rusty rock 66095 appears in a number of inter-element correlations. It must represent a significant sample type, in terms o f its labile trace element contents, and be derived from a lunar source with high concentrations of these elements. As noted in the introduction there is evidence, incompatible element trends and halogen correlations, for example, that the Apollo 17 site may be unique. The heavy metal correlations support this. The geographical isolation of the Apollo 17 site could be a factor. A local supply and possible recycling of volatiles could account in part for the general trends observed.
3
,012
o
IOO
200
300
leachable Br (ppb)
Fig. 4. Leachable 2°4pb and Br correlation in lunar samples. Samples are as in Fig.1. Br data are from [1,13].
relationships between halogens and the o t h e r heavy metals cannot be arrived at on the basis of our present data. Thus, our early assumption that the halides are the labile compounds is only partially supported.
4. Summary The heavy trace metals 2 ° 4 p b , Zn and T1 appear to be correlated with one another and with the submicron metallic phase present in the non-leachable fractions of lunar soft. This submicron metallic phase is in the agglutinate and it appears that Zn and probably TI are extracted into both the metallic phase and the glass. Consistent patterns for Apollo 17 samples occur; less data are available for other sites but Apollo 16 samples are generally distinct from Apollo 17. However, the manner in which these trace metals are incorporated in lunar soils appears not to be site dependent. Apollo 14 and 15 samples fit patterns defined by Apollo 16 and 17 samples. The 2°4pb, Zn and T1 in orange soft 74220 and in the Apollo 17 softs are related, probably, by some process other than mixing. l e a c h a b l e 2°4Pb is related to Br 1 and I1 in the Apollo
References 1 S. Jovanovic and G.W. Reed, Jr., Labile and non-labile element relationships among Apollo 17 samples, Proc. Fifth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 5, 2 (Pergamon, 1974) 1685-1701. 2 A.R. Duncan, A.J. Erlank, J.P. Willis, M.K. Sher and L.H. Ahrens, Trace element evidence for a two-stage origin of some titaniferous mare basalts, Proc. Fifth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 5, 2 (Pergamon, 1974) 1147-1157. 3 R.O. Allen, Jr., S. Jovanovic and G.W. Reed, Jr., 2°4pb in Apollo 14 samples and inferences regarding primordial Pb lunar geochemistry, Proc. Third Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 3, 2 (Mit Press, 1972) 1645-1650. 4 R.O. Allen, Jr., S. Jovanovic and G.W. Reed, Jr., Geochemistry of primordial Pb, Bi and Zn in Apollo 15 samples, Proc. Fourth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 4, 2 (Pergamon, 1973) 1169-1175. 5 R.O. Allen, Jr., S. Jovanovic and G.W. Reed, Jr., A study of 2°4pb partition in lunar samples using terrestrial and meteoritic analogues, Proc. Fifth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 5, 2 (Pergamon, 1974) 1617-1623. 6 D.S. McKay, K.M. Fruland and G.H. Heiken, Grain size and the evolution of lunar soils, Proc. Fifth Lunar Science Conference, Geochim. Cosmochim. Acta, Suppl. 5, 1 (Pergamon, 1974) 887-906. 7 J.W. Morgan, R. Ganapathy, H. Higuchi, U. Kr~ihenbiihl and E. Anders, Lunar basins: tentative characterization of projectiles from meteoritic elements in Apollo 17 boulders, Proc. Fifth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 5, 2 (Pergamon, 1974) 1703-1736. 8 U. Kr~ihenbiihl, R. Ganapathy, J.W. Morgan and E. Anders, Volatile elements in Apollo 16 samples: implications for highland volcanism and accretion history of the moon,
169 Proc. Fourth Lunar Science Conference, Geochim. Cosmochim. Acta, Suppl. 4, 2 (Pergamon, 1973) 1325-1348. 9 R.M. Housley, R.W. Grant and N.E. Paton, Origin and characteristics of excess Fe metal in lunar glass welded aggregates, Proc. Fourth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 4, 3 (Pergamon, 1973) 2737. 10 J.W. Morgan, J.C. Laul, U. Kr~ihenbiihl, R. Ganapathy and E. Anders, Major impacts on the moon: characterization from trace elements in Apollo 12 and 14 samples, Proc. Third Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 3, 2 (MIT Press, 1972) 1377-1395. 11 Apollo 17 Preliminary Science Report, NASA SP-330 (1973) 7 - 1 8 .
12 J.M. Rhodes, K.V. Rogers, C. Shih, B.M. Bansal, L.E. Nyquist, H. Wiesman and N.J. Hubbard, The relationship between geology and soil chemistry at the Apollo 17 landing site, Proc. Fifth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 5, 2 (Pergamon, 1974) 1097-1117. 13 S. Jovanovic and G.W. Reed, Jr., Volatile trace elements and the characterization of the Cayley formation and the primitive lunar crust, Proc. Fourth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 4,2 (Pergamon, 1973) 1313-1324.
['~ 1_..2_3
HEAVY ELEMENT AFFINITIES IN APOLLO 17 SAMPLES* R.O. ALLEN, Jr. 1 , S, JOVANOVIC and G.W. REED, Jr. Chemistry Division, Argonne National Laboratory. Argonne, [ll. [USA) Received February 21, 1975 Revised version received June 19, 1975
2°4pb, Bi, TI and Zn in samples from Apollo 17 exhibit relationships not found in samples from other sites. ~°4pb, TI and Zn in residues remaining after dilute acid leaching are correlated with one another. Orange soil 74220, which is enriched in 204pb, TI and Zn, is included in these relationships. In addition the submicron metallic phase generally associated with agglutinate formation is correlated with all three of these elements; this relationship has already been reported for 2°aPb in other samples. Thus, orange soil and agglutinates appear to be involved in concentrating heavy volatile metals. A process other than mixing is required to account for this. As a consequence of the isolation of the landing site by the surrounding massifs, local supply and recycling of volatile trace elements in soils may account for some of the interelement relations.
1. Introduction The Apollo 17 samples represent the most diverse set of samples returned from the moon; they include rocks similar to those collected at many o f the other sites as well as samples restricted to Taurus-Littrow. The trace element interrelations, so far reported, are unique. The correlations among the halogens [ 1] and the perturbed ratios of pairs of incompatible elements, K, Zr, Nb and Ba relative to TiO2 [2] are examples. We shall examine the results for a set o f elements, 2°4Pb, Bi, T1 and Zn. These elements are intermediate in mobility between the very labile halogens and the more refractory incompatible elements.
2. The data 2°4pb, Bi, T1 and Zn have been determined by fast and slow neutron activation analyses on soils, representative of the major geologic units at the Apollo 17 site; on two fragmental rock breccias 72275 and 76315; and * Work supported by USAEC and NASA. I Permanent address: Department of Chemistry, University of Virginia, Charlottesville, Virginia, 22901.
on the orange soil 74220. A shadow 76241 and a 2-cm skim 76261 soil from the North Massif, two basaltic soils, 71501 and 75080, from the valley, and the lightcolored soils, 73141 and 72701, from the South Massif were studied. Splits of 75080, sieved at 74 ~m, were measured. All the samples were leached with hot dilute HNO3 at pH 5 - 6 for 1 5 - 2 0 minutes and the element contents in the leach and the residual samples were determined. The submicron metal present in soils was measured by a magnetic resonance technique with the assistance of J.R. Norris, Jr., ANL. We have presented evidence indicating that the lunar chemistry of these trace elements may be related to a metallic phase which may be metal or metal-metal sulfide eutectic and to P2Os and also to the halogens [ 3 - 5 ] . The data are given in Table 1. The greatest uncertainty is in the 2°4pb leach results, where some of the errors are as high as 50%. The weighted sums of the sieve fractions of 75080 agree reasonably well with the unsieved sample. The <74-/1m 75080 Zn data is clearly very high and unaccountable. The percent of leachable Bi measured in the 75080 sieved fractions is too low. We assume that the leaching solution must have been at a higher pH than 5 - 6 in which case Bi salts are insoluble. The unsieved sample contained 33% leachable Bi. The
164 TABLE 1 Primordial lead, bismuth, thallium, zinc and submicron metal in Apollo 17 samples* Sample
Soils 71501,34 72701,34 73141,20 74220,111 75080,8 75080,8 (>74 um) 75080,8 (<74 ~m) 76241,17 76261,18 Rocks 72275,66 76315,70
2°4pb (ppb)
Bi (ppb)
r
1
4.8 ± 0.9 2.6 +- 0.5 4.1 ± 1.0 36 ± 2 3.4 -+0.8 1.2 ± 0.4 6.0 ± 1.1 3.3 ± 0.8 2.2 +-0.4
0.33 0.13 0.37 3.9 0.64 0.76 1.0 0.64 0.06
1.1 ± 0.2 2.4 +-0.3
r
± 0.11 :t 0.06 ± 0.19 ± 0.4 ± 0.34 ± 0.34 +-0.5 ± 0.14 ± 0.04
0.13 + 0.6 0.6 ± 0.3
0.55 0.99 0.22 0.54 0.49 (0.25 (1.0 0.87 0.62
n.d. n.d.
Tl (ppb)
Zn (ppm)
Submicron metal (%)
1
r
1
r
1
0.13 0.20 0.15 0.18 0.24 0.06) 2 0.04) 2 0.25 0.09
1.8 (1.5 1.6 14.5 1.5 1.1 2.3 2.1 1.8
0.13 0.19) 1 0.24 5.4 0.16 0.06 0.2 0.1 0.09
15 11 13 135 24 11 174 24 19
n.d. n.d. 0.65 5.6 0.55 0.65 24 0.31 0.41
0.52 0.31 0.20 n.d. 0.34 0,13 0.60 0.38 0.36
0.19 0.12
0.003 0.007
0.35 ± .05 0.17 ± .03
1.0 1.1
0.11 0.24
2.5 2.8
* Counting statistical errors are less than 10% unless otherwise indicated. Samples were leached at pH 5-6 HNO3. r = residue after leach, 1 = leach solution. I Measured T1 values were reversed due to assumed mislabeling. 2 Measured Bi values; these should be adjusted as described in text to 75080 (<74 ~m): 0.2r, 0.11; 75080 (>74 ~m): 0.7r, 0.31.
true water-soluble concentrations in the sieved samples is probably near that of the unsieved sample and the adjusted values are given in a footnote to the table. For all Apollo 17 soils the ranges in concentrations measured for these trace elements fall within about a factor of 2, i.e., 3 . 0 - 5 . 2 ppb 2°4Pb, 0 . 7 3 - 1 . 8 ppb Bi, 1 . 0 - 2 . 3 ppb q'l and 1 1 - 2 4 ppm Zn. Although they have diverse origins, i.e., comminuted subfloor basalts to light-colored anorthositic highland breccias most of these soils have about the same maturity [6]. The cluster of our data on the submicron Fe ° may also bear on this observation;this is somewhat surprising. The uniqueness of orange soil 74220 is apparent; most striking is the lack of Bi enrichment while the other elements show an eight-fold enrichment relative to the other Apollo 17 samples. We should note that another unusual sample, Apollo 16 rusty rock 66095, appears several times in considering the data. This rock contains an order of magnitude more 2°4Pb, Bi and T1 than other Apollo 16 and 17 samples; the Zn, however, is not enriched [5].
3. Inter-element relationships An approach to understanding the chemistry of these trace metals is through correlations or lack thereof among the metals themselves, and between the metals and active agents such as the halogens, P20s and Fe°-FeS. Our data only are used in so far as possible since comparable leach data from the literature are not available. We focus primarily on the Apollo 17 samples but our other data are also included when needed in discussions. Since the quantity of data is limited we will rely on comparisons between correlated pairs of elements to help support apparent trends.
3.1. Residual 2°4Pb-Tl-Zn The residual 2°4pb (2°4pbr) is correlated with Zn r and with TIr in Apollo 17 samples (Fig. 1a, b). The trends are similar for both pairs of elements. Of interest is the fact that in both figures, trend lines through
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Fig. 1. Correlations of residual nonleachable 2°4Pb-Zn, 2°4Pb-TI and Zn-TI in lunar samples. Apollo 17 soils 1-9 are 71501,34, 72701,34, 73141,20, 74220,111, 75080,8, 75080,8 (>74 #m), 75080,8 (<74 #m), 76241,17 and 76261,18; rocks are 1 = 72275,66 and 2 = 76315,70. Apollo 16 soils 1-4 are 61221,17,63501,41, 64501,18 and 64801,41; rock, 1 = 61016,131 [5]. Apollo 14 soils are 1 = 14163,152, 2 = 14259,119 and half open diamond 2 = soil breccia 14049,35 [3]. Orange soil 74220,111 and rusty rock 66095,23 are off scale but are involved in these correlations. Apollo 14 TI data from Morgan et al. [ 10]; the limits indicate that the residual concentrations are less than the totals reported.
the orange soil 74220 and the origin include a number of the samples. Samples 75080 (<74/am), 76261 and 76241 appear to fall along a different trend. In the 2°4Pbr-Tl r comparison, Apollo 14 and 16 samples appear to define a correlation related to 66095 and unrelated to 74220 or other Apollo 17 samples (Fig. lb, insert). A test Of the 2°4pbr-Zn r and 2°4pbr-T1 r relationship for Apollo 17 samples is Znr-T1r (Fig. lc). The data fall along a curve through 74220 and the origin with a mean ratio for six samples, including 74220, of 9.6 -+ 1.2 × 10 a. The 2°4pb r and Zn r in Apollo 17 breccias 76315 and 72275 and Apollo 16 breccias 66095 and 61016 are correlated (Fig. la). The breccias contain about 2.2 ppm excess Zn r relative to 2°4pb r; when this excess is subtracted 2°4pbr/Zn r = 0.0036 + 0.0006.
3.2. 2°4pbr-Znr-Tlr-Fe° relationships The correlation between 2°4Pb r and the submicron Fe ° found in Apollo 14 and 15 soils [4] includes a num-
ber of Apollo 17 samples (Fig. 2a). The 2°4pbr-Znr-T1 r relation for soils implies that since 2°4pb r has been found to be correlated with Fe ° then Zn r and T1r should be also. The Apollo 17 samples contain correlated Zn r and Fe metal (Fig. 2b). Apollo 16 soils appear to define a somewhat different Znr-metal trend which may include 14163. Soil 71501 an'd also 14049, 14259 and 15031 fall outside of the shaded sector that includes the Apollo 16 and 17 samples. A Tlr-Fe ° metal correlation is not so obvious (Fig. 2c). The best line through the Tlr-Fe ° data would intercept the T1 ordinate at about I ppb T1r. Our Apollo 17 T1 data are systematically about 15% higher than literature data [7]. Lowering our results by 15% and using literature data where available does not change the correlation and the best fit still does not intercept at the origin. In addition, the correlations of T1r with 2°4pb r and with Zn r are not improved. More data are needed to resolve this matter. We do think that there is a Tlr-Fe ° trend but it could be partially obscured by the presence of T1 in other phases. One of these could be the glass associated
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Fig. 2. Residual 2°4pb, Zn and TI correlations with s u b m i c r o n metallic p h a s e in lunar soils. Samples are as in Fig. 1. Apollo 15 soil is 1 5 0 3 1 , 2 7 [4]. Metal data o t h e r t h a n f r o m this w o r k are referenced in [5].
with agglutinate. Another could be a component mixed in the soils which has relatively high TI content and Apollo 16 breccias such as 66095 and 61016 reported by Kr~ihenbi~hl et al. [8] and by Allen et al. [5] may represent this component. The correlation with Fe ° and the involvement of 74220 suggest that Zn r and TIr are being incorporated in Apollo 17 soils in two ways. One related to metal formation and the other to glass formation. These are not independent processes in soils. If we assume the correlation line through 74220 (Fig. lb) applies to all Apollo 17 samples and indicates the amount of the trace element dissolved in glass, then in samples 75080 (<74 ~m), 76261,76241, and 75080 about one-half of the Zn r is associated with Fe ° . Reducing the Zn r contents for these samples by one-half places them on aline through 15031, 71501 and 14259 in Fig. 2b. The process may be rationalized as follows: (1) 74220 formation was a one-step process, either volcanic or impact. The 2°4pb, T1 and Zn were probably present as volatile compounds such as halides. An advantage of a volcanic source is that the volatiles released may also end up being the sublimates coating soil grains and may supply the trace metals for incorporation during the agglutinate formation discussed below. (2) The agglutinate extraction of the trace metals
may be conceived as a continuous process: deposition of sublimates on soil grains, solar wind implantation into the soil, and finally impact heating and melting Fe ° formation could occur by reduction of oxidized Fe by solar wind hydrogen as discussed by Housley et al. [9]. The reaction: FeO+2H~Fe ° +H20 has a negative free energy of approximately 101 kcal/mole which is competitive with hydrogen atom combination to form H2 ( " 9 7 kcal/mole). The atomic Fe formed may then coalesce during impact heating to give the submicron single and multidomain Fe ° measured magnetically. In the process, Fe-FeS eutectics form and the heavy trace metals become incorporated. Thus soils contain both glass and an Fe ° phase which are related and which can extract proportionate amounts of volatile heavy metals. The total amount of these trace metals incorporated will, of course, depend on the maturity of the soil provided a source of supply of the trace metals is available.
3. 3. Bir-2°4pbr-P205 A trend involving 66095 appears to be indicated by Bir and 2°4pb r in Apollo 17 soils 75080 (<74 ~um), 76241, 76261 and 72701. The Bir/2°4Pbr ratio, in-
167 cluding 66095, is 0.28 -+ 0.04. Samples 15101 (<74 /am) and possibly 15205 and 14259 and 14049 reported by Allen et al. [3,4] fall near this trend line. The samples involved are non-mare except sieved sample 75080 (<74 Ore) which has a significant non-mare component based on its U content of 0.34 [1]. Tile repeated occurrence of 66095 in element correlations (Bir-2°4pbr and Tlr-2°4Pb r in soils, Znr-2°4Pbr in rocks) involving non-mare samples suggests that (1) a 66095-1ike component must be present in samples such as breccias 76315, 72275 and 61016, and (2) some of the volatile metals in soils could have been derived from the same or a related source which supplied volatiles for 66095. The source region must be extensive to supply material to such dispersed sites. We have noted an apparent Bir-P20 s correlation with a Bi(ppb)/P2Os (wt.%) ratio of ~2.5 for Apollo 14 and 15 samples [4]. The Apollo 16 and 17 soil data do not fall along this curve but instead apparently define a new trend including 74220 (Fig. 3). In fact, the data appears to form two curves. One group of samples 74220,
1.5
76241 and 7150 l have an average ratio (Bi, ppb/P20s, wt.%) of 9.2 + 0.6; the other group including 7626 l, 72701, 73141 and four Apollo 16 soils has an average ratio of 6.2 -+ 0.9. Although there is no enrichment of Bi in 74220, it again appears closely related to other Apollo 17 samples. 3. 4. Residual trace metals and the halogens
Of the metals reported here, Bi and Zn appear to be associated with the non-leachable halogens. Bir is weakly correlated with C1r in Apollo 16 and 17 samples, probably via the phosphate phase. In the case of Zn only Apollo 16 samples show an apparent Znr-Clr trend but there is not sufficient data to confirm this; 74220 may be included in this relationship. 3.5. Leachable
2°4pb-Bi-Tl
The dilute acid-soluble fracitons of the heavy metals represent water-soluble salts probably coating grain surfaces. Only two patterns appear; they involve three of the metals, 2°4pb, Bi, and T1. (1) Apollo 17 soils 76421, (76261), 71501,75080, 73141 and breccia 76315 contain correlated Bi and 2°4pb (Bil, 2°4pbl) giving a mean Bil/2°4Pbl ratio excluding 76261, of 0.37 -+ 0.04. (2) The Apollo 17 South Massif soils, 72701 and 73141, and the Apollo 16 soils contain a large range of T11 concentrations with a much narrower range in 2°4pb 1. The Apollo 17 valley and North Massif soils have approximately constant TI1 and a range of 2°4pb 1 concentrations. This pattern is duplicated in the non-labile fractions of these elements but the South Massif samples are excepted. This appears to be reasonable, since the trace metals extracted into the agglutinates should reflect to some extent the relative concentration of these metals deposited on surfaces.
0.5 3. 6. Leachable trace metals and the halogens
Og 0
I
0.1
I
0.2
1
0.3
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0.4
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An additional source of information on the labile metal fractions may be their relations to the halogens. A 2°4pbl-Br1 correlation was noted for Apollo 14 and 15 samples [4] ; 74220 is colinear with this trend (Fig. 4). Other Apollo 17 and Apollo 16 soils define an entirely different trend. Both trends, including the role of 74220, are strongly supported by 2°4Pbl-I 1 data. Similar
168
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17 samples but in a different way than in Apollo 14 and 15 samples. The striking exception is 74220 2°4Pbl-Br 1 (I1) which is colinear with the Apollo 14 and 15 samples. Rusty rock 66095 appears in a number of inter-element correlations. It must represent a significant sample type, in terms o f its labile trace element contents, and be derived from a lunar source with high concentrations of these elements. As noted in the introduction there is evidence, incompatible element trends and halogen correlations, for example, that the Apollo 17 site may be unique. The heavy metal correlations support this. The geographical isolation of the Apollo 17 site could be a factor. A local supply and possible recycling of volatiles could account in part for the general trends observed.
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300
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Fig. 4. Leachable 2°4pb and Br correlation in lunar samples. Samples are as in Fig.1. Br data are from [1,13].
relationships between halogens and the o t h e r heavy metals cannot be arrived at on the basis of our present data. Thus, our early assumption that the halides are the labile compounds is only partially supported.
4. Summary The heavy trace metals 2 ° 4 p b , Zn and T1 appear to be correlated with one another and with the submicron metallic phase present in the non-leachable fractions of lunar soft. This submicron metallic phase is in the agglutinate and it appears that Zn and probably TI are extracted into both the metallic phase and the glass. Consistent patterns for Apollo 17 samples occur; less data are available for other sites but Apollo 16 samples are generally distinct from Apollo 17. However, the manner in which these trace metals are incorporated in lunar soils appears not to be site dependent. Apollo 14 and 15 samples fit patterns defined by Apollo 16 and 17 samples. The 2°4pb, Zn and T1 in orange soft 74220 and in the Apollo 17 softs are related, probably, by some process other than mixing. l e a c h a b l e 2°4Pb is related to Br 1 and I1 in the Apollo
References 1 S. Jovanovic and G.W. Reed, Jr., Labile and non-labile element relationships among Apollo 17 samples, Proc. Fifth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 5, 2 (Pergamon, 1974) 1685-1701. 2 A.R. Duncan, A.J. Erlank, J.P. Willis, M.K. Sher and L.H. Ahrens, Trace element evidence for a two-stage origin of some titaniferous mare basalts, Proc. Fifth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 5, 2 (Pergamon, 1974) 1147-1157. 3 R.O. Allen, Jr., S. Jovanovic and G.W. Reed, Jr., 2°4pb in Apollo 14 samples and inferences regarding primordial Pb lunar geochemistry, Proc. Third Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 3, 2 (Mit Press, 1972) 1645-1650. 4 R.O. Allen, Jr., S. Jovanovic and G.W. Reed, Jr., Geochemistry of primordial Pb, Bi and Zn in Apollo 15 samples, Proc. Fourth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 4, 2 (Pergamon, 1973) 1169-1175. 5 R.O. Allen, Jr., S. Jovanovic and G.W. Reed, Jr., A study of 2°4pb partition in lunar samples using terrestrial and meteoritic analogues, Proc. Fifth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 5, 2 (Pergamon, 1974) 1617-1623. 6 D.S. McKay, K.M. Fruland and G.H. Heiken, Grain size and the evolution of lunar soils, Proc. Fifth Lunar Science Conference, Geochim. Cosmochim. Acta, Suppl. 5, 1 (Pergamon, 1974) 887-906. 7 J.W. Morgan, R. Ganapathy, H. Higuchi, U. Kr~ihenbiihl and E. Anders, Lunar basins: tentative characterization of projectiles from meteoritic elements in Apollo 17 boulders, Proc. Fifth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 5, 2 (Pergamon, 1974) 1703-1736. 8 U. Kr~ihenbiihl, R. Ganapathy, J.W. Morgan and E. Anders, Volatile elements in Apollo 16 samples: implications for highland volcanism and accretion history of the moon,
169 Proc. Fourth Lunar Science Conference, Geochim. Cosmochim. Acta, Suppl. 4, 2 (Pergamon, 1973) 1325-1348. 9 R.M. Housley, R.W. Grant and N.E. Paton, Origin and characteristics of excess Fe metal in lunar glass welded aggregates, Proc. Fourth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 4, 3 (Pergamon, 1973) 2737. 10 J.W. Morgan, J.C. Laul, U. Kr~ihenbiihl, R. Ganapathy and E. Anders, Major impacts on the moon: characterization from trace elements in Apollo 12 and 14 samples, Proc. Third Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 3, 2 (MIT Press, 1972) 1377-1395. 11 Apollo 17 Preliminary Science Report, NASA SP-330 (1973) 7 - 1 8 .
12 J.M. Rhodes, K.V. Rogers, C. Shih, B.M. Bansal, L.E. Nyquist, H. Wiesman and N.J. Hubbard, The relationship between geology and soil chemistry at the Apollo 17 landing site, Proc. Fifth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 5, 2 (Pergamon, 1974) 1097-1117. 13 S. Jovanovic and G.W. Reed, Jr., Volatile trace elements and the characterization of the Cayley formation and the primitive lunar crust, Proc. Fourth Lunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 4,2 (Pergamon, 1973) 1313-1324.