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Age of an Apollo 15 mare basalt; Lunar crust and mantle evolution
EARTH AND PLANETARY SCIENCE LETTERS 13 (1971) 97-104. NORTH-HOLLAND PUBLISHINGCOMPANY,
A G E O F A N A P O L L O 15 M A R E B A S A L T ; LUNAR CRUST AND MANTLE EVOLUTION G.J. WASSERBURG and D.A. PAPANASTASSIOU The Lunatic A sylum of the Charles Arms Laboratory of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91109, USA Received 29 October 1971 Revised version received 17 November 1971
An internal Rb-Sr isochron for the large basalt boulder 15555 returned from the edge of Hadley rille by the Apollo 15 mission yields an age 3.32 +_0.06 AE and an initial 88Sr/86Sr, I = 0.69934 ~ 5. This age and I value fall well within the range obtained for the Apollo 12 basalts from the Ocean of Storms and may indicate that extensive lava flows occurred at ~ 3.3 AE over widespread areas of the moon. The Sr composition of the anorthosite 15415 is as low as that of plagioclase extracted from the Apollo 11 low K rocks. The initial Sr composition of 15415 for an assumed age of 3.3 to 4.6 AE is extremely primitive and provides further evidence for an extremely short formation interval (3 to 1 X 1 0 6 yr) of a nonchondritic moon with respect to an origin in time defined by BABI. The initial 87Sr/86Sr for crystalline rocks returned from all lunar missions is correlated with the concentration of Rb and correspondingly K, U and Th. This correlation places distinctive constraints on the evolution of lunar magmas and the internal structure of the moon.
1. Introduction In this study we report the results of Rb-Sr analyses on a basaltic rock (15555) and on an anorthositic rock (15415) returned by the Apollo 15 mission. Samples of these rocks were distributed shortly after completion of the mission according to an early allocation plan. We also report results on basaltic rock 14276 from Apollo 14 and summarize the Rb-Sr ages reported previously on Apollo 14 samples [1 ].
2. Apollo 15 rock 15555 Rock #555 weighed 9.6 kg and was retrieved from station 9a at the edge of Hadley rille. This rock is a coarse grained basalt and is described in the PET report [2]. A 1.6 g sample was allocated to us for analysis. Due to the coarse grain size it was not possible to obtain a 'total rock' analysis on this small specimen. * Contribution no. 2087.
The mineral separates analyzed are shown in table 1. Two plagioclase separates (#1 and #2) consisted of clear coarse grains which were obtained by handpicking and by magnetic separation after the rock was crushed to - 3 0 0 / a m . In previous studies [ 1,3] it has been found that Rb and Sr in lunar basalts are greatly enriched over their values in the total rock in an interstitial glassy phase of variable composition which we have labelled quintessence. Coarse grains ( - 2 0 0 ~tm) containing a large proportion of quintessence were handpicked from the crushed rock. These grains consisted of tertiaries of pyroxene, plagioclase and ilmenite to which a dark purple or grey glassy phase was attached. An ilmenite separate (#6) was obtained by handpicking. This separate contained numerous grain aggregates including occasional quintessence, pyroxene, plagioclase and cristobalite attached to the ilmenite. The rock was then crushed to - 7 5 #m and additional ilmenite, pyroxene, and quintessence separates were prepared using heavy liquids, acetone, a Frantz magnetic separator and handpicking in an attempt to produce very
98
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
Table 1 Analytical results Sample a
Weight (mg)
Rb b (10 -8 m/g)
~SSr b (10 -8 m/g)
STRb/86Sr c (X 102)
8VSr/86Sr d
A. 15555 Plagioclase Plagioclase Pyroxene Quintessence Quintessence llmenite Ilmenite
[M1 [M] [L] [M] [L] [M] [L]
2.5 4.5 32.8 0.9 ~0.3 f 10.0 7.0
0.065 e 0.083 e 0.0610 e 13.11 61.04 4.076 2.697
267.4 277.2 8.42 341.4 1604.00 87.79 42.89
0.057 0.070 1.692 8.96 8.88 10.83 14.66
B. 15415 Chip-A Chip-B
[M] [M]
t.0 3.7
0.23 e 0.19
226.2 163.1 163.1
0.24 0.27 0.27
C. 14276 Plagioclase
[L]
5.8
3.178
Total [M] Total [M] Quintessence [L] First aliquot Second aliquot
62.2 13.2
254.9 255.0 185.6 177.6
2.907 2.907 16.63 17.02
0.70178 0.70183 0.70940 0.70962
216.2 215.5
331.3 332.2
0.88380 ± 110 0.88431 -+ 17
0.13 0.47
13.24 12.96 307.1 307.0
0.69931 0.69934 0.70018 0.70347 0.70359 0.70448 0.70623
±9 _+4 +_4 +_6 ±4 ±7 ±4
0.69926 ± 13 0.69916 _+8 0.69914 _+5 g +_6 +_8 g ±5 ±5
a Obtained by purely mechanical [M] or density [L] separations. b Rb concentrations are calculated for 85Rb/87 Rb = 2.591; Sr concentrations are calculated assuming normal abundances 86Sr/88Sr = 0.1194 and 84Sr/88Sr = 0.006748. c The estimated error of this ratio is ± 0.7% of the tabulated value and arises mostly from the uncertainties in Rb instrumental fractionation. d Error corresponds to the last figures given and represents +-2amean. e Corrected for blank. f Weight estimated from grain count. Total number of grains analyzed was 300. The concentration data are dependent upon the estimated weight of this material, which could be in error by a factor of 2. g Repeat Sr measurement.
high purity mineral separates. These procedures were in general successful, but they result in extensive handling of the sample and in exposure to reagents. A high purity sample ( # 3 ) o f p y r o x e n e o f nearly uniform composition (Wo14Ens3Fs33)was obtained and contained no observable plagioclase or quintessence. An aliquot was also used for K-At age determinations [4]. A second quintessence separate of about 300 grains ( # 5 ) was obtained by handpicking (using a dodecane-hexadecane mixture as wetting agent) from the various intermediate fractions obtained in the above procedure. This separate was of m u c h greater purity than that obtained by handpicking of the coarsely crushed material (#4). An ilmenite concentrate (#7), obtained by density and magnetic sep-
arations, consisted of equal numbers of ilmenite and p y r o x e n e grains. Binary mineral grains were rare, and no plagioclase or quintessence grains were visible. These samples were processed following the analytical m e t h o d s recently reviewed [ 1]. Due to small sample size, all samples except plagioclase were brought into solution w i t h o u t evaporation to dryness at any stage. A one percent aliquot of each solution was used for a rough d e t e r m i n a t i o n o f Rb and Sr concentrations. The major aliquots o f the solutions were then spiked accordingly and evaporated to dryness. This procedure minimizes any uncertainties involved in not totally spiking during dissolution of each sample. Corrections have been applied to samples as noted in table 1 based on measured c o n t a m i n a t i o n levels o f 0.2 X 10 -9 g Sr
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
L
/
/
o.oo
//
,, I
/
~-
0.69 L ~
80
I L
0.04
/
- YP :2t
I ~
-
T - 332 + 0.06/E
3[--
.,~
-
~;-QUINTESSENGE
/"
o~/PLAGIOCLASE
.........
o-/ "ILMENITE"
/
,,, 1 /
"
/
/
~-~Ar-+o.os~E
--;--------!~e~ SAT "-O.05~:E , Q E
i I ~ _ _ 0.08 o.~z 87Rb//B6 S r
I
o~e
1
020
Fig. 1. Rb-Sr evolution diagram for rock 15555. The best fit isochron and the [T, 1] parameters are shown as well as a 4.6 AE reference isochron through BABI. The age is calculated using ~ (87 Rb) = 1.39 × 10 -11 yr-i .The insert shows the displacement ~ (in parts in 104) of data points from the best fit line in the (aTSr/a6Sr) coordinate. The lines on either side of the ~ = 0 axis correspond to a 0.05 AE uncertainty in the age. and 0.2 X 10- l ° g Rb. These corrections have a negligible effect on the age (T) and initial aTsr/a6sr (I) for this set of data. The data obtained on rock #555 are shown in fig, 1. A total range of one percent in 87Sr/86Sr was obtained for the mineral separates of this low Rb basalt. The data determine a rather precise linear array. The best fit line yields an age T = 3.32 +-0.06 AE and I = 0.69934 ¥ 5 (the uncertainties given are 2o errors for the best fit line). The deviations of the data from the best fit line in parts in 104 are shown in the insert of fig. 1. All data except the two quintessence separates lie on this line. The latter two samples are slightly off the best fit line in opposite directions. The reason for this behavior is not known at present; however, we do not consider that these deviations cause any uncertainty in the age outside of the error limits. The IT, I] quantities for rock #555 are indistinguishable from those for Apollo 12 basalts obtained in this laboratory [ 1,3]. From comparison of the relative abundances of Rb and Sr in the two quintessence separates, we conclude
99
that other phases present in these separates do not play a significant role with respect to these elements. As found previously [3], the quintessence is enriched in Sr as well as Rb, Examination of a thin section of 15555 shows that ilmenite grains contain intimately penetrating intergrowths of quintessence. From the fact that Rb/Sr increases in going from quintessence (#5) to ilmenite (#6, with binary grains) to the purest ilmenite separate (#7) we deduce that the ilmenite was almost the last phase to crystallize and it entrapped the very last stage of alkali rich residual fluids. The K/Rb (by weight) for the plagioclase (#2), pyroxene (#3), quintessence (#5) and ilmenite (#7) separates are respectively 4300; 500; 395 and 325. This shows the successive enrichment of Rb relative to K in these phases and in particular with regard to the quintessence intergrown with the ilmenite as compared to the bulk of the quintessence. The K, Rb and Sr concentrations demonstrate that the different mineral separates analyzed are not governed by mixtures of plagioclase and of a single 'quintessence'. It follows that the linear array displayed in fig. 1 is not a two component mixing line but a true isochron.
3. Apollo 15 rock 15415 From a sample allocation of 55 mg for this rock, data were obtained on two samples each consisting of several small chips (table 1). All data on #415 are in good mutual agreement, however we will use the two most precise measurements for the following discussion. The Sr compositions for #415 are similar to those of plagioclase extracted from the Apollo 11 low K rocks but the Rb/Sr for #415 is about a factor of 2 higher. If we assume an age of 3.3 or of 4.6 AE for this rock we calculate I = 0.69902 +-4 and I = 0.69897 + 4 respectively. The latter value is identical to the Basaltic Achondrite Best Initial 87Sr/86Sr, BABI = 0.69898 -+3 [5]. It is clear that rock 15415 has an extremely low Rb/Sr at present and has in the past 4.6 AE never been exposed for any significant length of time to any environment with Rb/Sr even as high as the extremely low values observed in the low K Apollo 11 rocks. Material forming this rock must have separated from a reservoir with Rb/Sr = 0.0035 (Apollo 11 low K rocks) within 3.5 X 108 yr after formation of the moon. From these data however, no
1(lO
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
precise statement of the age of 15415 is possible. An upper limit on I for 15415 is obtained by considering the upper limit of the measured 87Sr/86Sr for this rock a n d by assuming an age as low as 3.3 AE. This yields/max = 0.69906 and corresponds to a maximum formation interval for the moon of 3 × 106 yr since the solar system (Rb/Sr ~ 0.6) had the composition BABI close to 4.6 AE ago. This interval is similar to those obtained by considering the m e a s u r e d s 7Sr/86Sr for a feldspar glass fragment in the Apollo 11 soil and the I value for low K rock 10058 [6].
4. Apollo 14 rock 14276 This sample was a small pebble weighing 10 g. This rock very closely resembles 14310 in thin section. An interior sample of 1.4 g was used for the mineral separations, following the scheme established for Apollo 14 basalts [ 1]. Two 'total' rock samples were analyzed, as well as a plagioclase and a quintessence sample. The Sr extracted from the plagioclase was analyzed twice on the Lunatic I mass spectrometer at different times and the results are in good agreement. The quintessence sample of 0.6 mg was dissolved, and the solution was then spiked and processed through the chemistry as two separate aliquots. The Rb and Sr concentrations of these aliquots are in excellent agreement. The first and smaller aliquot (#12a) yielded a low quality Sr run which is nonetheless in good agreement with the more precise Sr analysis of the second quintessence aliquot. Either quintessence aliquot would be sufficient to determine a precise age for this rock due to the extremely radiogenic nature of this separate, reflecting both the higher Rb level of Apollo 14 rocks and the extreme enrichment of Rb in the quintessence (a factor of 24 over the total rock). The data are shown as deviations from the best fit line on a ~ and f versus 87Rb/86Sr diagram [1]. The data lie precisely on the best fit line and determine an age T = 3.88 -+ 0.04 AE and I = 0.70019 T 2. The errors quoted here are 2u for the best fit program for 1, but are estimated at one percent for the age since the 20 error for the age vanishes in this case. The age for rock #276 is in exact agreement with that obtained on rocks 14310, 14073 and 14001,7, which have all similarly high Rb/Sr for the whole rocks; this age is distinct from the ages 3.95 -+0.03 AE obtained for low Rb/Sr basalt 14053
APOLLO 14 ROCK IT=5.88 +__0.01/:E # 276 [I =0.70019 T 2 +2r- ~ , j +1[--- ~----T q----n -T ~ T ~T QE
T-f co :r
_2L
P
o
~.
o'.i d2
o _
_I L. . . . . . . . I
.....
L_
2.0 3Zo
87Rb/86Sr
Fig. 2. Relative displacement of measured data points from the best fit isochron for Apollo 14 rock #276 versus STRb/ 865r. The relative deviations in the 87Sr/a6Sr coordinate (~) are shown in parts in 104; the deviations in the 87Rb/86Sr coordinate (g') are given in percent. The dashed lines on either side of the ~ = 0 and ~"= 0 axes correspond to a 1% uncertainty in age. Full circles represent data plotted on both the ~ and ~" diagrams. The error estimates given in this figure are those for the best fit program only. and a low Rb/Sr basaltic clast from breccia 14321 [1]. The data for #276 lie in the same region of the T, I diagram (see fig. 3) as the high Rb/Sr basalts of Apollo 14. This is in general agreement with the petrographic observations which indicate that this rock is essentially identical to 14310. However, the I for #276 is distinctly lower than the I for rocks #310, #73 and #01,7 from Apollo 14. The precision of 87Sr/a6Sr for the plagioclase separate of #276 is adequate to establish clearly this difference in I and in addition the Rb/Sr in this separate is low enough that no significant additional uncertainty in I is produced due to the 0.04 AE uncertainty in the age #276.
5. Discussion
The age of a basalt (15555) which appears to be a local bedrock sample from the Marsh of Decay at the Apollo 15 site is 3.32 +-0.06 AE. An 4°Ar-39Ar age of 3.31 -+0.03 AE has been obtained on a plagioclase separate from 15555 [4]. Whole rock 4°Ar-39Ar ages have also been determined [4, 7]. The latter ages for this rock are less meaningful since different mineral separates yield well-defined but distinct Ar retention ages [4]. The K-Ar and Rb-Sr ages for lunar rocks are generally in good agreement and indicate no major discrepancies. A small but significant discrepancy exists for the Apollo 11 low K rocks which as a g r o u p appear to have K-Ar ages slightly older than Rb-Sr
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
ages of two low K rocks [6, 8]. The age of basalt 15555 is in precise coincidence with the ages determined on several basaltic rocks from the Apollo 12 site in the Ocean of Storms. This result shows that lava flows occurred over widespread mare areas at about the same time. The lunar basalts from all mare sites so far studied yield ages between 3.16 and 3.71 AE. As yet, no igneous rock has been found which is younger than 3.16 AE. The total range in ages obtained from mare sites is only 0.55 AE. These results establish a period of igneous activity which is rather narrow. This narrowly defined interval appears to represent the peak of mare flooding and the termination of large scale igneous activity on the moon. The samples appear to represent the most recent surface flows or sills at the respective sites. These results mean that (1) the mare basins were filled during the interval 3 . 7 1 - 3 . 1 6 AE, or (2) that the basalts represent the youngest surface flows which were poured over and into much older mare fillings, which had been subject to such heavy bombardment and cratering that the older rocks filling the basins have become unrecognizable as lithic objects [3]. Basalts from the Fra Mauro site have yielded tight clusters of ages at 3.88 and 3.95 AE. If the Frau Mauro formation represents an ejecta blanket from the excavation of Mare Imbrium and if the breccias returned from Fra Mauro were formed during this event, then it follows that Mare Imbrium was excavated after 3.88 AE [1]. In addition, since the Sea of Tranquillity shows no evidence of this gigantic impact, this impact must have occurred prior to 3.71 AE. These strong limits on the time of excavation of Mare Imbrium are certainly compatible with the observation of much younger basalts at the Apollo 15 site at the edge of Mare Imbrium. If the Apollo 14 samples do not consist of ejecta from Mare Imbrium but are instead breccias produced in localized lava pools overlying the Fra Mauro formation, then Mare Imbrium must have been formed prior to 3.95 AE. Independently of the possible association of the Fra Mauro formation with the excavation of Mare Imbrium, the ages of 3.88 and 3.95 AE indicate the existence of magmatic activity at these times either as basaltic flows in the highlands or as basaltic flows (or sills) at the site of Mare Imbrium prior to its excavation. In addition, rock 12013 shows a melting event (probably caused by an impact) at 4.00 AE ago [9]. The age of rock 12013 and of the
101
basalts from Apollo 14 may be the result of extensive magmatic activity close to 4.0 AE. Evidence for this large scale activity may also be reflected in an 'isochron' determined by magnetic separates of the Apollo 11 soil which yielded an 'age' of 4.0 AE [6, 11 ]. These data were previously interpreted as a mixing line on the Rb-Sr evolution diagram, however, in view of the recurring ages close to 4.0 AE, data on separates from the lunar soils may in fact yield meaningful age information. No igneous rocks have as yet been returned which are older than 4.00 AE. This may be a consequence of comminution produced during the early intense bombardment of the m o o n - e a r t h system or of major assimilation processes which took place on the moon, in the period from 4.0 to 3.2 AE. No direct evidence of a more ancient lunar crust has been found. The most compelling evidence for an ancient lunar crust is that found in the soils, the model ages of basaltic rocks and the internal 'total rock' isochron for rock 12013.
6. Constraints on lunar crust and mantle evolution Important constraints on the evolution of the moon's upper mantle and on its interaction with the lunar crust can be obtained from the I values of basaltic rocks and their model ages. All the existing data are shown in fig. 3. If we consider first only the mare basalts (i.e., Apollo 11, 12 and 15)we find a gross correlation of age and I, the younger rocks having higher I. This correlation, if it were strictly true and if a single I was obtained as a function of age, could be interpreted by a process in which the basalts represent the tapping of samples of a single and uniform magma reservoir. From the small enrichment o f / f o r these basalts over BABI this magma reservoir must have an extremely low Rb/Sr. If the model ages of the basalts were essentially all constant, this constant would yield the time at which this low Rb/Sr reservoir was formed; in addition the Rb/Sr of the basalts would be representative of Rb/Sr in the reservoir. If the model ages were highly variable (due to magmatic differentiation) but the I values were still low and varying approximately linearly with the internal isochron age of the basalts, we would still determine a single, uniform and tow Rb/Sr reservoir as the source region of these basalts. The detailed observations show that the model for
102
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
TO Q7085 h
I
i
I
0.701
~ROCK 12015
A-11
A-12
A-14 C~
A-15 w
o
0.700 ROCK 4_0/15555
--I 1
I,-,o I
o o
0.699 i
5.0
I
3.2
i
I
5.4
i
I
3.6
,
BABI I
5.8
,
I
4.0
,
I
4.2
,
4.4
TIME (/:E)
Fig. 3. T I diagram for all rocks for which internal isochrons have been obtained in this laboratory. The Apollo 14 rocks define two distinct clusters. Rock 14276 is clearly associated with the upper cluster but has somewhat lower I. Apollo 15 rock 15555 is shown as a full circle and falls within the range in both T and I for the Apollo 12 basalts.
which a single I is obtained for a given internal isochron age is not correct. There is a distinct range in I for the Apollo 12 basalts, and the Apollo 11 basalts show distinct I and Rb/Sr ratios. In a gross sense only, these results may be interpreted in terms of the preceding model where we would allow inhomogeneities in Rb/Sr in the reservoirs of about a factor of two. In addition, some larger fractionation in Rb and Sr must be involved (such as would be found in some differentiates) for the high K basalts for Apollo 11, after extraction of the magma from the source region. A variety of source regions is required by the small but well-defined variations in I for both Apollo 11 and 12. If we consider the Apollo 14 basalts, it is obvious that this model is not applicable. These rocks form at least two widely different clusters in I and moreover have older ages and much higher I values than Apollo 11, 12 and 15 basalts. It follows that the Apollo 14 rocks themselves may not originate from a single reservoir, and that, in addition, they must originate from different reservoirs than do the Apollo 11, 12 and 15 basalts. Rock 12013 is an even more extreme example of this. All these rocks (with the exception of the high
K Apollo 11 rocks) have similar model age approximating the age of the solar system. The most reasonable explanation of the BABI model ages and I values for all lunar samples (including the soils) is a model of the moon involving a highly radioactive crust formed at about 4.3 4.6 AE ago and magma sources at depth (100 to 400 kin), all of which have extremely low Rb/Sr ratios and low K, U and Th. The upwelling magmas of which we have samples in the interval 4.0 to 3.2 AE would become contaminated with varying amounts of crustal material. A contamination model was first put forward as a possibility by Tera et al. [10] based on Apollo 11 data. The contamination model as presently conceived would give relatively constant model ages equal to that of the original contaminating material crustal and would permit highly variable I values. The degree of contamination would be critically dependent on the particular conduit through which the magmas flowed and on the mechanism of emplacement. The ancient crust need not be homogeneous for these relationships to hold. This model of contamination of deep magma sources of low Rb with highly radioactive and radiogenic crustal material would yield a correlation of I with Rb concentration independent of age and would permit arbitrary variations o f / o n virtually any scale (e.g. rock 12013). The correlation o f / w i t h Rb/Sr is best shown by 12013 but may also be seen for the Apollo 14 rocks. In particular, rock 14276, which is associated with other rocks (e.g. rock 14310) both isotopically and petrographically, has a distinctly lower I value. The Rb/Sr for rock 14276 is also 30 percent lower than that for rock 14310 and the other high I Apollo 14 rocks. In contradistinction the model age of 14276 is the same as that for 14310. These rocks could thus represent different parts of the same flow with different degrees of contamination. The possibility that these particular data could be explained by isotopic rehomogenization during metamorphism may not be discarded, particularly since all of the basalts obtained by the Apollo 14 mission may represent clasts from breccias which were subject to thermal metamorphism and partial melting. In the present discussion, however, we will emphasize the more general interpretation. This model of crustal contamination also leads to a direct explanation of the discrepancy between the Rb-Sr and K-Ar ages on the one hand and the U-Th-Pb ages on the other. The U-Th-Pb ages so far determined
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
on rocks are model ages since they assume no significant initial lead. The result of contaminating a magma poor in Rb, K, U, Th (and Pb) with ancient crustal material rich in these elements is to give U-Th-Pb ages which are older than the internal isochron ages. This will be particularly true for 2°TPb-2°6pb ages. If the assimilation of crustal rocks does not involve any chemical fractionation then the model U-Th-Pb ages will be concordant and all data should agree with the TBABImodel age. Some chemical fractionation (factor of ~2) will not cause a major change in TBABIfor such old rocks [3] but will seriously alter the U-Th-Pb concordance because of the short mean life of 23su. This model proposed by us leads to several problems which are not yet clarified. These are principally: (1) Where is the radioactive 'crust'? Does it underlie the mare basins or is it to be found in the highlands [9] ? It must surely be available to contribute to the soils, since each soil may not simply be formed as an average of the local basalts. (2) Is the radioactive crust of the product of differentiation of the outer few hundred kilometers of the moon in the early stages, or is it mainly the result of primary zoning during the accretion of the moon [3] with only minor modifications during early lunar magmatic activity? This question is fundamental in determining the depth to which melting has occurred on the moon [3, 11]. With respect to the location of the radioactive crust, since the Apollo 14 basalts show the largest degree of contamination (excepting 12013) it is possible that this crust existed at least in the pre Mare Imbrium basin region [ 1 ]. Several workers have pointed out that the source region for the lunar basalts cannot possibly be of chondritic composition. This conclusion is based on initial 87Sr/86Sr data, relative chemical abundances of K, U and REE, total chemical abundances and the distribution of heat sources [6, 1 0 - 1 3 ] . Some of these arguments follow the general approach presented by Gast in his classic paper on the enrichment of 'refractory' elements on the earth as compared to nominal solar (chondritic) abundances [14]. The source region for the mare basalts most reasonably must lie in the outer 300 km of the moon. This lunar mantle may, in the large, represent a primary chemical layering resulting from the accretion process. Except for the mechanical constraints and
103
the limits imposed by phase transitions [ 15], no direct chemical evidence exists for the composition of the deep lunar interior. Several workers have clearly recognized the possibility that there might be primary accretional layering [ 13]. This was particularly emphasized with regard to radioactive sources [3]. In our previous discussion of the constraints on the thermal evolution of the moon we have implicitly assumed that, if the interior of the moon were molten, extensive surface lava flows should occur, unless a sufficiently strong and self-supporting mantle may effectively prevent the magmas from reaching the surface of the moon. The absence of any lunar igneous rocks with ages less than ~ 3.1 AE therefore strongly indicates that no significant magmatic activity has occurred on the moon at times younger than ~ 3.1 AE ago and that the interior of the moon (i.e. any significant fraction of the lunar mass) has not been close to its melting temperature since 3 AE ago. It was noted that the constraint of no major lunar magmatic activity after 3 AE ago and the inference of a highly radioactive crust necessitated either a nonuniform initial temperature distribution for the moon or a non-uniform chemical composition, or both. These constraints require the primitive moon to have a U abundance and an initial temperature increasing with radius toward the surface. The enrichment in U (and Th) in the original outer layers of the moon would be a direct result of layering during the accretional process. If the enrichment in U is the result of condensation processes which enrich the refractory elements, it is likely that K and Rb as volatile elements would be depleted by the same process. It follows that a lunar differentiation close to 4.6 AE is still required to enrich K and Rb in the crust as indicated by the Rb-Sr systematics. Therefore, the crust may be formed as the combined effect of early lunar differentiation processes superimposed upon an original outer layer of distinctive chemical composition produced during accretion. The extent to which the moon is a layered object is constrained by the mean density and the moment of inertia C. For a two layer planet of fixed mean density Po and with radii and densities ri, Oi, re, Pe for the interior (i) and exterior (e) layers respectively, one obtains the relations
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
104
o1(,
c
e
=
(pi-pol (1 - r i2/ r e)2 \
Po
]
(1 -
33 re/r i)
Assuming a m a x i m u m u n c e r t a i n t y of -+ 1% in A C / C [16], we obtain a m a x i m u m limit o f 340 k m for the radius of an iron core (2% o f the mass) [17]. For COi -- Po)/Po = + 0.2 we obtain an inner layer with radius 660 k m (7% o f the mass). For an o u t e r shell o f density ( P e - P o ) / P o = + 0.1 we obtain a thickness of 85 k m (15% of the mass). F r o m the above considerations it is clear that the lunar mantle which we are discussing in our m o d e l as the source o f the basalts must be rather thin if it involves significant differences in density. A multilayered m o d e l with compensating densities in the outer layers will permit s o m e w h a t greater latitude, but not on the scale o f 200 to 400 k m for each layer. The deep interior may not have any high density contrast which extends over a large v o l u m e of the m o o n . In addition it is clear that the radioactive crust inferred from abundant evidence and which must be extremely thin ( ~ 10 km) will certainly not affect the m o m e n t of inertia of the m o o n .
Acknowledgements We thank A.L. Albee for a continuing interest and support. J. Brown, T. Wen and Lily Ray followed each sample through demanding stages of Rb and Sr evolution. This w o r k was supported by N A S A under contract NAS-9-8074.
References [ 1] D.A. Papanastassiou and G.J. Wasserburg, Rb-Sr ages of igneous rocks from the Apollo 14 mission and the age of the Fra Mauro formation, Earth Planet. Sci. Letters 12 (1971) 36. [2] Lunar Sample Preliminary Examination Team, Preliminary examination of Apollo 15 lunar samples, Science (1972) in press.
[3] D.A. Papanastassiou and G.J. Wasserburg, Lunar chronology and evolution from Rb-Sr studies of Apollo 12 and 12 samples, Earth Planet. Sci. Letters 11 (1971) 37. [4] F.A. Podosek, J.C. Huneke and G.J. Wasserburg, Gas retention and cosmic-ray exposure ages of lunar rock 15555, Science (1972) in press. [5] D.A. Papanastassiou and G.J. Wasserburg, Initial strontium isotopic abundances and the resolution of small time differences in the formation of planetary objects, Earth Planet. Sci. Letters 5 (1969) 361; D.A. Papanastassiou, Thesis, California Institute of Technology (1970). [6] D.A. Papanastassiou, G.J. Wasserburg and D.S. Burnett, Rb-Sr ages of lunar rocks from the Sea of Tranquillity, Earth Planet. Sci. Letters 8 (1970) 1. [7] E.C. Alexander, Jr., P.K. Davis and R.S. Lewis, A 4°ArsgAr age for Apollo sample 15555, Science (1972) in press. [8] G. Turner, 4°Ar-39Ar ages from the lunar maria, Earth Planet. Sci. Letters 11 (1971) 169; P. Eberhardt, J. Geiss, N. Grisgler, U. Krahenbiihl, M. Misrgeli and A. Stettler, K-Ar age of Apollo 11 rock 10003, Earth Planet. Sci. Letters 11 (1971) 245. [9] A.L. Albee, D.S. Burnett, A.A. Chodos, E.L. Haines, J.C. Huneke, D.A. Papanastassiou, F.A. Podosek, G. Price Russ, III and G.J. Wasserburg, Mineralogic and isotopic investigations of lunar rock 12013, Earth Planet. Sci. Letters 9 (1970) 137. [10] F. Tera, O. Eugster, D.S. Burnett and G.J. Wasserburg, Comparative study of Li, Na, K, Rb, Cs, Ca, Sr and Ba abundances in achondrites and in Apollo 11 lunar sampies, Geochim. Cosmochim. Acta 34, Suppl. 1 (1970) 1637. [11] A.L. Albee, D.S. Burnett, A.A. Chodos, O.J. Eugster, J.C. Huneke, D.A. Papanastassiou, F.A. Podosek, G. Price Russ, III, H.G. Sanz, F. Tera and G.J. Wasserburg, Ages, irradiation history, and chemical composition of lunar rocks from the Sea of Tranquillity, Science 167 (1970) 463. [12] L.A. Haskin, R.O. Alien, P.A. Helmke, T.P. Paster, M.R. Anderson, R.L. Korotev and K.A. Zweifel, Rare earth and other trace elements in Apollo 11 lunar samples, Geochim. Cosmochim. Acta 34, Suppl. 1 (1970) 1213. [13] P.W. Gast, N.J. Hubbard and H. Wiesmann, Chemical composition and petrogenesis of basalts from Tranquillity Base, Geochim. Cosmochim. Acta 34, Suppl. 1 (1970) 1143. [ 14] P.W. Gast, Limitations on the composition of the upper mantle, J. Geophys. Res. 65 (1960) 1287. [15] G.W. Wetherill, Lunar interior, constraint on basaltic composition, Science 160 (1968) 1256. [ 16] W.M. Kaula, The gravitational field of the moon, Science 166 (1969) 1581. [17] S.C. Solomon and M. Nafi Toksbz, On the density distribution in the moon, Phys. Earth Planet. Interiors 1 (1968) 475.
A G E O F A N A P O L L O 15 M A R E B A S A L T ; LUNAR CRUST AND MANTLE EVOLUTION G.J. WASSERBURG and D.A. PAPANASTASSIOU The Lunatic A sylum of the Charles Arms Laboratory of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91109, USA Received 29 October 1971 Revised version received 17 November 1971
An internal Rb-Sr isochron for the large basalt boulder 15555 returned from the edge of Hadley rille by the Apollo 15 mission yields an age 3.32 +_0.06 AE and an initial 88Sr/86Sr, I = 0.69934 ~ 5. This age and I value fall well within the range obtained for the Apollo 12 basalts from the Ocean of Storms and may indicate that extensive lava flows occurred at ~ 3.3 AE over widespread areas of the moon. The Sr composition of the anorthosite 15415 is as low as that of plagioclase extracted from the Apollo 11 low K rocks. The initial Sr composition of 15415 for an assumed age of 3.3 to 4.6 AE is extremely primitive and provides further evidence for an extremely short formation interval (3 to 1 X 1 0 6 yr) of a nonchondritic moon with respect to an origin in time defined by BABI. The initial 87Sr/86Sr for crystalline rocks returned from all lunar missions is correlated with the concentration of Rb and correspondingly K, U and Th. This correlation places distinctive constraints on the evolution of lunar magmas and the internal structure of the moon.
1. Introduction In this study we report the results of Rb-Sr analyses on a basaltic rock (15555) and on an anorthositic rock (15415) returned by the Apollo 15 mission. Samples of these rocks were distributed shortly after completion of the mission according to an early allocation plan. We also report results on basaltic rock 14276 from Apollo 14 and summarize the Rb-Sr ages reported previously on Apollo 14 samples [1 ].
2. Apollo 15 rock 15555 Rock #555 weighed 9.6 kg and was retrieved from station 9a at the edge of Hadley rille. This rock is a coarse grained basalt and is described in the PET report [2]. A 1.6 g sample was allocated to us for analysis. Due to the coarse grain size it was not possible to obtain a 'total rock' analysis on this small specimen. * Contribution no. 2087.
The mineral separates analyzed are shown in table 1. Two plagioclase separates (#1 and #2) consisted of clear coarse grains which were obtained by handpicking and by magnetic separation after the rock was crushed to - 3 0 0 / a m . In previous studies [ 1,3] it has been found that Rb and Sr in lunar basalts are greatly enriched over their values in the total rock in an interstitial glassy phase of variable composition which we have labelled quintessence. Coarse grains ( - 2 0 0 ~tm) containing a large proportion of quintessence were handpicked from the crushed rock. These grains consisted of tertiaries of pyroxene, plagioclase and ilmenite to which a dark purple or grey glassy phase was attached. An ilmenite separate (#6) was obtained by handpicking. This separate contained numerous grain aggregates including occasional quintessence, pyroxene, plagioclase and cristobalite attached to the ilmenite. The rock was then crushed to - 7 5 #m and additional ilmenite, pyroxene, and quintessence separates were prepared using heavy liquids, acetone, a Frantz magnetic separator and handpicking in an attempt to produce very
98
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
Table 1 Analytical results Sample a
Weight (mg)
Rb b (10 -8 m/g)
~SSr b (10 -8 m/g)
STRb/86Sr c (X 102)
8VSr/86Sr d
A. 15555 Plagioclase Plagioclase Pyroxene Quintessence Quintessence llmenite Ilmenite
[M1 [M] [L] [M] [L] [M] [L]
2.5 4.5 32.8 0.9 ~0.3 f 10.0 7.0
0.065 e 0.083 e 0.0610 e 13.11 61.04 4.076 2.697
267.4 277.2 8.42 341.4 1604.00 87.79 42.89
0.057 0.070 1.692 8.96 8.88 10.83 14.66
B. 15415 Chip-A Chip-B
[M] [M]
t.0 3.7
0.23 e 0.19
226.2 163.1 163.1
0.24 0.27 0.27
C. 14276 Plagioclase
[L]
5.8
3.178
Total [M] Total [M] Quintessence [L] First aliquot Second aliquot
62.2 13.2
254.9 255.0 185.6 177.6
2.907 2.907 16.63 17.02
0.70178 0.70183 0.70940 0.70962
216.2 215.5
331.3 332.2
0.88380 ± 110 0.88431 -+ 17
0.13 0.47
13.24 12.96 307.1 307.0
0.69931 0.69934 0.70018 0.70347 0.70359 0.70448 0.70623
±9 _+4 +_4 +_6 ±4 ±7 ±4
0.69926 ± 13 0.69916 _+8 0.69914 _+5 g +_6 +_8 g ±5 ±5
a Obtained by purely mechanical [M] or density [L] separations. b Rb concentrations are calculated for 85Rb/87 Rb = 2.591; Sr concentrations are calculated assuming normal abundances 86Sr/88Sr = 0.1194 and 84Sr/88Sr = 0.006748. c The estimated error of this ratio is ± 0.7% of the tabulated value and arises mostly from the uncertainties in Rb instrumental fractionation. d Error corresponds to the last figures given and represents +-2amean. e Corrected for blank. f Weight estimated from grain count. Total number of grains analyzed was 300. The concentration data are dependent upon the estimated weight of this material, which could be in error by a factor of 2. g Repeat Sr measurement.
high purity mineral separates. These procedures were in general successful, but they result in extensive handling of the sample and in exposure to reagents. A high purity sample ( # 3 ) o f p y r o x e n e o f nearly uniform composition (Wo14Ens3Fs33)was obtained and contained no observable plagioclase or quintessence. An aliquot was also used for K-At age determinations [4]. A second quintessence separate of about 300 grains ( # 5 ) was obtained by handpicking (using a dodecane-hexadecane mixture as wetting agent) from the various intermediate fractions obtained in the above procedure. This separate was of m u c h greater purity than that obtained by handpicking of the coarsely crushed material (#4). An ilmenite concentrate (#7), obtained by density and magnetic sep-
arations, consisted of equal numbers of ilmenite and p y r o x e n e grains. Binary mineral grains were rare, and no plagioclase or quintessence grains were visible. These samples were processed following the analytical m e t h o d s recently reviewed [ 1]. Due to small sample size, all samples except plagioclase were brought into solution w i t h o u t evaporation to dryness at any stage. A one percent aliquot of each solution was used for a rough d e t e r m i n a t i o n o f Rb and Sr concentrations. The major aliquots o f the solutions were then spiked accordingly and evaporated to dryness. This procedure minimizes any uncertainties involved in not totally spiking during dissolution of each sample. Corrections have been applied to samples as noted in table 1 based on measured c o n t a m i n a t i o n levels o f 0.2 X 10 -9 g Sr
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
L
/
/
o.oo
//
,, I
/
~-
0.69 L ~
80
I L
0.04
/
- YP :2t
I ~
-
T - 332 + 0.06/E
3[--
.,~
-
~;-QUINTESSENGE
/"
o~/PLAGIOCLASE
.........
o-/ "ILMENITE"
/
,,, 1 /
"
/
/
~-~Ar-+o.os~E
--;--------!~e~ SAT "-O.05~:E , Q E
i I ~ _ _ 0.08 o.~z 87Rb//B6 S r
I
o~e
1
020
Fig. 1. Rb-Sr evolution diagram for rock 15555. The best fit isochron and the [T, 1] parameters are shown as well as a 4.6 AE reference isochron through BABI. The age is calculated using ~ (87 Rb) = 1.39 × 10 -11 yr-i .The insert shows the displacement ~ (in parts in 104) of data points from the best fit line in the (aTSr/a6Sr) coordinate. The lines on either side of the ~ = 0 axis correspond to a 0.05 AE uncertainty in the age. and 0.2 X 10- l ° g Rb. These corrections have a negligible effect on the age (T) and initial aTsr/a6sr (I) for this set of data. The data obtained on rock #555 are shown in fig, 1. A total range of one percent in 87Sr/86Sr was obtained for the mineral separates of this low Rb basalt. The data determine a rather precise linear array. The best fit line yields an age T = 3.32 +-0.06 AE and I = 0.69934 ¥ 5 (the uncertainties given are 2o errors for the best fit line). The deviations of the data from the best fit line in parts in 104 are shown in the insert of fig. 1. All data except the two quintessence separates lie on this line. The latter two samples are slightly off the best fit line in opposite directions. The reason for this behavior is not known at present; however, we do not consider that these deviations cause any uncertainty in the age outside of the error limits. The IT, I] quantities for rock #555 are indistinguishable from those for Apollo 12 basalts obtained in this laboratory [ 1,3]. From comparison of the relative abundances of Rb and Sr in the two quintessence separates, we conclude
99
that other phases present in these separates do not play a significant role with respect to these elements. As found previously [3], the quintessence is enriched in Sr as well as Rb, Examination of a thin section of 15555 shows that ilmenite grains contain intimately penetrating intergrowths of quintessence. From the fact that Rb/Sr increases in going from quintessence (#5) to ilmenite (#6, with binary grains) to the purest ilmenite separate (#7) we deduce that the ilmenite was almost the last phase to crystallize and it entrapped the very last stage of alkali rich residual fluids. The K/Rb (by weight) for the plagioclase (#2), pyroxene (#3), quintessence (#5) and ilmenite (#7) separates are respectively 4300; 500; 395 and 325. This shows the successive enrichment of Rb relative to K in these phases and in particular with regard to the quintessence intergrown with the ilmenite as compared to the bulk of the quintessence. The K, Rb and Sr concentrations demonstrate that the different mineral separates analyzed are not governed by mixtures of plagioclase and of a single 'quintessence'. It follows that the linear array displayed in fig. 1 is not a two component mixing line but a true isochron.
3. Apollo 15 rock 15415 From a sample allocation of 55 mg for this rock, data were obtained on two samples each consisting of several small chips (table 1). All data on #415 are in good mutual agreement, however we will use the two most precise measurements for the following discussion. The Sr compositions for #415 are similar to those of plagioclase extracted from the Apollo 11 low K rocks but the Rb/Sr for #415 is about a factor of 2 higher. If we assume an age of 3.3 or of 4.6 AE for this rock we calculate I = 0.69902 +-4 and I = 0.69897 + 4 respectively. The latter value is identical to the Basaltic Achondrite Best Initial 87Sr/86Sr, BABI = 0.69898 -+3 [5]. It is clear that rock 15415 has an extremely low Rb/Sr at present and has in the past 4.6 AE never been exposed for any significant length of time to any environment with Rb/Sr even as high as the extremely low values observed in the low K Apollo 11 rocks. Material forming this rock must have separated from a reservoir with Rb/Sr = 0.0035 (Apollo 11 low K rocks) within 3.5 X 108 yr after formation of the moon. From these data however, no
1(lO
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
precise statement of the age of 15415 is possible. An upper limit on I for 15415 is obtained by considering the upper limit of the measured 87Sr/86Sr for this rock a n d by assuming an age as low as 3.3 AE. This yields/max = 0.69906 and corresponds to a maximum formation interval for the moon of 3 × 106 yr since the solar system (Rb/Sr ~ 0.6) had the composition BABI close to 4.6 AE ago. This interval is similar to those obtained by considering the m e a s u r e d s 7Sr/86Sr for a feldspar glass fragment in the Apollo 11 soil and the I value for low K rock 10058 [6].
4. Apollo 14 rock 14276 This sample was a small pebble weighing 10 g. This rock very closely resembles 14310 in thin section. An interior sample of 1.4 g was used for the mineral separations, following the scheme established for Apollo 14 basalts [ 1]. Two 'total' rock samples were analyzed, as well as a plagioclase and a quintessence sample. The Sr extracted from the plagioclase was analyzed twice on the Lunatic I mass spectrometer at different times and the results are in good agreement. The quintessence sample of 0.6 mg was dissolved, and the solution was then spiked and processed through the chemistry as two separate aliquots. The Rb and Sr concentrations of these aliquots are in excellent agreement. The first and smaller aliquot (#12a) yielded a low quality Sr run which is nonetheless in good agreement with the more precise Sr analysis of the second quintessence aliquot. Either quintessence aliquot would be sufficient to determine a precise age for this rock due to the extremely radiogenic nature of this separate, reflecting both the higher Rb level of Apollo 14 rocks and the extreme enrichment of Rb in the quintessence (a factor of 24 over the total rock). The data are shown as deviations from the best fit line on a ~ and f versus 87Rb/86Sr diagram [1]. The data lie precisely on the best fit line and determine an age T = 3.88 -+ 0.04 AE and I = 0.70019 T 2. The errors quoted here are 2u for the best fit program for 1, but are estimated at one percent for the age since the 20 error for the age vanishes in this case. The age for rock #276 is in exact agreement with that obtained on rocks 14310, 14073 and 14001,7, which have all similarly high Rb/Sr for the whole rocks; this age is distinct from the ages 3.95 -+0.03 AE obtained for low Rb/Sr basalt 14053
APOLLO 14 ROCK IT=5.88 +__0.01/:E # 276 [I =0.70019 T 2 +2r- ~ , j +1[--- ~----T q----n -T ~ T ~T QE
T-f co :r
_2L
P
o
~.
o'.i d2
o _
_I L. . . . . . . . I
.....
L_
2.0 3Zo
87Rb/86Sr
Fig. 2. Relative displacement of measured data points from the best fit isochron for Apollo 14 rock #276 versus STRb/ 865r. The relative deviations in the 87Sr/a6Sr coordinate (~) are shown in parts in 104; the deviations in the 87Rb/86Sr coordinate (g') are given in percent. The dashed lines on either side of the ~ = 0 and ~"= 0 axes correspond to a 1% uncertainty in age. Full circles represent data plotted on both the ~ and ~" diagrams. The error estimates given in this figure are those for the best fit program only. and a low Rb/Sr basaltic clast from breccia 14321 [1]. The data for #276 lie in the same region of the T, I diagram (see fig. 3) as the high Rb/Sr basalts of Apollo 14. This is in general agreement with the petrographic observations which indicate that this rock is essentially identical to 14310. However, the I for #276 is distinctly lower than the I for rocks #310, #73 and #01,7 from Apollo 14. The precision of 87Sr/a6Sr for the plagioclase separate of #276 is adequate to establish clearly this difference in I and in addition the Rb/Sr in this separate is low enough that no significant additional uncertainty in I is produced due to the 0.04 AE uncertainty in the age #276.
5. Discussion
The age of a basalt (15555) which appears to be a local bedrock sample from the Marsh of Decay at the Apollo 15 site is 3.32 +-0.06 AE. An 4°Ar-39Ar age of 3.31 -+0.03 AE has been obtained on a plagioclase separate from 15555 [4]. Whole rock 4°Ar-39Ar ages have also been determined [4, 7]. The latter ages for this rock are less meaningful since different mineral separates yield well-defined but distinct Ar retention ages [4]. The K-Ar and Rb-Sr ages for lunar rocks are generally in good agreement and indicate no major discrepancies. A small but significant discrepancy exists for the Apollo 11 low K rocks which as a g r o u p appear to have K-Ar ages slightly older than Rb-Sr
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
ages of two low K rocks [6, 8]. The age of basalt 15555 is in precise coincidence with the ages determined on several basaltic rocks from the Apollo 12 site in the Ocean of Storms. This result shows that lava flows occurred over widespread mare areas at about the same time. The lunar basalts from all mare sites so far studied yield ages between 3.16 and 3.71 AE. As yet, no igneous rock has been found which is younger than 3.16 AE. The total range in ages obtained from mare sites is only 0.55 AE. These results establish a period of igneous activity which is rather narrow. This narrowly defined interval appears to represent the peak of mare flooding and the termination of large scale igneous activity on the moon. The samples appear to represent the most recent surface flows or sills at the respective sites. These results mean that (1) the mare basins were filled during the interval 3 . 7 1 - 3 . 1 6 AE, or (2) that the basalts represent the youngest surface flows which were poured over and into much older mare fillings, which had been subject to such heavy bombardment and cratering that the older rocks filling the basins have become unrecognizable as lithic objects [3]. Basalts from the Fra Mauro site have yielded tight clusters of ages at 3.88 and 3.95 AE. If the Frau Mauro formation represents an ejecta blanket from the excavation of Mare Imbrium and if the breccias returned from Fra Mauro were formed during this event, then it follows that Mare Imbrium was excavated after 3.88 AE [1]. In addition, since the Sea of Tranquillity shows no evidence of this gigantic impact, this impact must have occurred prior to 3.71 AE. These strong limits on the time of excavation of Mare Imbrium are certainly compatible with the observation of much younger basalts at the Apollo 15 site at the edge of Mare Imbrium. If the Apollo 14 samples do not consist of ejecta from Mare Imbrium but are instead breccias produced in localized lava pools overlying the Fra Mauro formation, then Mare Imbrium must have been formed prior to 3.95 AE. Independently of the possible association of the Fra Mauro formation with the excavation of Mare Imbrium, the ages of 3.88 and 3.95 AE indicate the existence of magmatic activity at these times either as basaltic flows in the highlands or as basaltic flows (or sills) at the site of Mare Imbrium prior to its excavation. In addition, rock 12013 shows a melting event (probably caused by an impact) at 4.00 AE ago [9]. The age of rock 12013 and of the
101
basalts from Apollo 14 may be the result of extensive magmatic activity close to 4.0 AE. Evidence for this large scale activity may also be reflected in an 'isochron' determined by magnetic separates of the Apollo 11 soil which yielded an 'age' of 4.0 AE [6, 11 ]. These data were previously interpreted as a mixing line on the Rb-Sr evolution diagram, however, in view of the recurring ages close to 4.0 AE, data on separates from the lunar soils may in fact yield meaningful age information. No igneous rocks have as yet been returned which are older than 4.00 AE. This may be a consequence of comminution produced during the early intense bombardment of the m o o n - e a r t h system or of major assimilation processes which took place on the moon, in the period from 4.0 to 3.2 AE. No direct evidence of a more ancient lunar crust has been found. The most compelling evidence for an ancient lunar crust is that found in the soils, the model ages of basaltic rocks and the internal 'total rock' isochron for rock 12013.
6. Constraints on lunar crust and mantle evolution Important constraints on the evolution of the moon's upper mantle and on its interaction with the lunar crust can be obtained from the I values of basaltic rocks and their model ages. All the existing data are shown in fig. 3. If we consider first only the mare basalts (i.e., Apollo 11, 12 and 15)we find a gross correlation of age and I, the younger rocks having higher I. This correlation, if it were strictly true and if a single I was obtained as a function of age, could be interpreted by a process in which the basalts represent the tapping of samples of a single and uniform magma reservoir. From the small enrichment o f / f o r these basalts over BABI this magma reservoir must have an extremely low Rb/Sr. If the model ages of the basalts were essentially all constant, this constant would yield the time at which this low Rb/Sr reservoir was formed; in addition the Rb/Sr of the basalts would be representative of Rb/Sr in the reservoir. If the model ages were highly variable (due to magmatic differentiation) but the I values were still low and varying approximately linearly with the internal isochron age of the basalts, we would still determine a single, uniform and tow Rb/Sr reservoir as the source region of these basalts. The detailed observations show that the model for
102
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
TO Q7085 h
I
i
I
0.701
~ROCK 12015
A-11
A-12
A-14 C~
A-15 w
o
0.700 ROCK 4_0/15555
--I 1
I,-,o I
o o
0.699 i
5.0
I
3.2
i
I
5.4
i
I
3.6
,
BABI I
5.8
,
I
4.0
,
I
4.2
,
4.4
TIME (/:E)
Fig. 3. T I diagram for all rocks for which internal isochrons have been obtained in this laboratory. The Apollo 14 rocks define two distinct clusters. Rock 14276 is clearly associated with the upper cluster but has somewhat lower I. Apollo 15 rock 15555 is shown as a full circle and falls within the range in both T and I for the Apollo 12 basalts.
which a single I is obtained for a given internal isochron age is not correct. There is a distinct range in I for the Apollo 12 basalts, and the Apollo 11 basalts show distinct I and Rb/Sr ratios. In a gross sense only, these results may be interpreted in terms of the preceding model where we would allow inhomogeneities in Rb/Sr in the reservoirs of about a factor of two. In addition, some larger fractionation in Rb and Sr must be involved (such as would be found in some differentiates) for the high K basalts for Apollo 11, after extraction of the magma from the source region. A variety of source regions is required by the small but well-defined variations in I for both Apollo 11 and 12. If we consider the Apollo 14 basalts, it is obvious that this model is not applicable. These rocks form at least two widely different clusters in I and moreover have older ages and much higher I values than Apollo 11, 12 and 15 basalts. It follows that the Apollo 14 rocks themselves may not originate from a single reservoir, and that, in addition, they must originate from different reservoirs than do the Apollo 11, 12 and 15 basalts. Rock 12013 is an even more extreme example of this. All these rocks (with the exception of the high
K Apollo 11 rocks) have similar model age approximating the age of the solar system. The most reasonable explanation of the BABI model ages and I values for all lunar samples (including the soils) is a model of the moon involving a highly radioactive crust formed at about 4.3 4.6 AE ago and magma sources at depth (100 to 400 kin), all of which have extremely low Rb/Sr ratios and low K, U and Th. The upwelling magmas of which we have samples in the interval 4.0 to 3.2 AE would become contaminated with varying amounts of crustal material. A contamination model was first put forward as a possibility by Tera et al. [10] based on Apollo 11 data. The contamination model as presently conceived would give relatively constant model ages equal to that of the original contaminating material crustal and would permit highly variable I values. The degree of contamination would be critically dependent on the particular conduit through which the magmas flowed and on the mechanism of emplacement. The ancient crust need not be homogeneous for these relationships to hold. This model of contamination of deep magma sources of low Rb with highly radioactive and radiogenic crustal material would yield a correlation of I with Rb concentration independent of age and would permit arbitrary variations o f / o n virtually any scale (e.g. rock 12013). The correlation o f / w i t h Rb/Sr is best shown by 12013 but may also be seen for the Apollo 14 rocks. In particular, rock 14276, which is associated with other rocks (e.g. rock 14310) both isotopically and petrographically, has a distinctly lower I value. The Rb/Sr for rock 14276 is also 30 percent lower than that for rock 14310 and the other high I Apollo 14 rocks. In contradistinction the model age of 14276 is the same as that for 14310. These rocks could thus represent different parts of the same flow with different degrees of contamination. The possibility that these particular data could be explained by isotopic rehomogenization during metamorphism may not be discarded, particularly since all of the basalts obtained by the Apollo 14 mission may represent clasts from breccias which were subject to thermal metamorphism and partial melting. In the present discussion, however, we will emphasize the more general interpretation. This model of crustal contamination also leads to a direct explanation of the discrepancy between the Rb-Sr and K-Ar ages on the one hand and the U-Th-Pb ages on the other. The U-Th-Pb ages so far determined
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
on rocks are model ages since they assume no significant initial lead. The result of contaminating a magma poor in Rb, K, U, Th (and Pb) with ancient crustal material rich in these elements is to give U-Th-Pb ages which are older than the internal isochron ages. This will be particularly true for 2°TPb-2°6pb ages. If the assimilation of crustal rocks does not involve any chemical fractionation then the model U-Th-Pb ages will be concordant and all data should agree with the TBABImodel age. Some chemical fractionation (factor of ~2) will not cause a major change in TBABIfor such old rocks [3] but will seriously alter the U-Th-Pb concordance because of the short mean life of 23su. This model proposed by us leads to several problems which are not yet clarified. These are principally: (1) Where is the radioactive 'crust'? Does it underlie the mare basins or is it to be found in the highlands [9] ? It must surely be available to contribute to the soils, since each soil may not simply be formed as an average of the local basalts. (2) Is the radioactive crust of the product of differentiation of the outer few hundred kilometers of the moon in the early stages, or is it mainly the result of primary zoning during the accretion of the moon [3] with only minor modifications during early lunar magmatic activity? This question is fundamental in determining the depth to which melting has occurred on the moon [3, 11]. With respect to the location of the radioactive crust, since the Apollo 14 basalts show the largest degree of contamination (excepting 12013) it is possible that this crust existed at least in the pre Mare Imbrium basin region [ 1 ]. Several workers have pointed out that the source region for the lunar basalts cannot possibly be of chondritic composition. This conclusion is based on initial 87Sr/86Sr data, relative chemical abundances of K, U and REE, total chemical abundances and the distribution of heat sources [6, 1 0 - 1 3 ] . Some of these arguments follow the general approach presented by Gast in his classic paper on the enrichment of 'refractory' elements on the earth as compared to nominal solar (chondritic) abundances [14]. The source region for the mare basalts most reasonably must lie in the outer 300 km of the moon. This lunar mantle may, in the large, represent a primary chemical layering resulting from the accretion process. Except for the mechanical constraints and
103
the limits imposed by phase transitions [ 15], no direct chemical evidence exists for the composition of the deep lunar interior. Several workers have clearly recognized the possibility that there might be primary accretional layering [ 13]. This was particularly emphasized with regard to radioactive sources [3]. In our previous discussion of the constraints on the thermal evolution of the moon we have implicitly assumed that, if the interior of the moon were molten, extensive surface lava flows should occur, unless a sufficiently strong and self-supporting mantle may effectively prevent the magmas from reaching the surface of the moon. The absence of any lunar igneous rocks with ages less than ~ 3.1 AE therefore strongly indicates that no significant magmatic activity has occurred on the moon at times younger than ~ 3.1 AE ago and that the interior of the moon (i.e. any significant fraction of the lunar mass) has not been close to its melting temperature since 3 AE ago. It was noted that the constraint of no major lunar magmatic activity after 3 AE ago and the inference of a highly radioactive crust necessitated either a nonuniform initial temperature distribution for the moon or a non-uniform chemical composition, or both. These constraints require the primitive moon to have a U abundance and an initial temperature increasing with radius toward the surface. The enrichment in U (and Th) in the original outer layers of the moon would be a direct result of layering during the accretional process. If the enrichment in U is the result of condensation processes which enrich the refractory elements, it is likely that K and Rb as volatile elements would be depleted by the same process. It follows that a lunar differentiation close to 4.6 AE is still required to enrich K and Rb in the crust as indicated by the Rb-Sr systematics. Therefore, the crust may be formed as the combined effect of early lunar differentiation processes superimposed upon an original outer layer of distinctive chemical composition produced during accretion. The extent to which the moon is a layered object is constrained by the mean density and the moment of inertia C. For a two layer planet of fixed mean density Po and with radii and densities ri, Oi, re, Pe for the interior (i) and exterior (e) layers respectively, one obtains the relations
G.J. Wasserburg, D.A.Papanastassiou, Lunar crust evolution
104
o1(,
c
e
=
(pi-pol (1 - r i2/ r e)2 \
Po
]
(1 -
33 re/r i)
Assuming a m a x i m u m u n c e r t a i n t y of -+ 1% in A C / C [16], we obtain a m a x i m u m limit o f 340 k m for the radius of an iron core (2% o f the mass) [17]. For COi -- Po)/Po = + 0.2 we obtain an inner layer with radius 660 k m (7% o f the mass). For an o u t e r shell o f density ( P e - P o ) / P o = + 0.1 we obtain a thickness of 85 k m (15% of the mass). F r o m the above considerations it is clear that the lunar mantle which we are discussing in our m o d e l as the source o f the basalts must be rather thin if it involves significant differences in density. A multilayered m o d e l with compensating densities in the outer layers will permit s o m e w h a t greater latitude, but not on the scale o f 200 to 400 k m for each layer. The deep interior may not have any high density contrast which extends over a large v o l u m e of the m o o n . In addition it is clear that the radioactive crust inferred from abundant evidence and which must be extremely thin ( ~ 10 km) will certainly not affect the m o m e n t of inertia of the m o o n .
Acknowledgements We thank A.L. Albee for a continuing interest and support. J. Brown, T. Wen and Lily Ray followed each sample through demanding stages of Rb and Sr evolution. This w o r k was supported by N A S A under contract NAS-9-8074.
References [ 1] D.A. Papanastassiou and G.J. Wasserburg, Rb-Sr ages of igneous rocks from the Apollo 14 mission and the age of the Fra Mauro formation, Earth Planet. Sci. Letters 12 (1971) 36. [2] Lunar Sample Preliminary Examination Team, Preliminary examination of Apollo 15 lunar samples, Science (1972) in press.
[3] D.A. Papanastassiou and G.J. Wasserburg, Lunar chronology and evolution from Rb-Sr studies of Apollo 12 and 12 samples, Earth Planet. Sci. Letters 11 (1971) 37. [4] F.A. Podosek, J.C. Huneke and G.J. Wasserburg, Gas retention and cosmic-ray exposure ages of lunar rock 15555, Science (1972) in press. [5] D.A. Papanastassiou and G.J. Wasserburg, Initial strontium isotopic abundances and the resolution of small time differences in the formation of planetary objects, Earth Planet. Sci. Letters 5 (1969) 361; D.A. Papanastassiou, Thesis, California Institute of Technology (1970). [6] D.A. Papanastassiou, G.J. Wasserburg and D.S. Burnett, Rb-Sr ages of lunar rocks from the Sea of Tranquillity, Earth Planet. Sci. Letters 8 (1970) 1. [7] E.C. Alexander, Jr., P.K. Davis and R.S. Lewis, A 4°ArsgAr age for Apollo sample 15555, Science (1972) in press. [8] G. Turner, 4°Ar-39Ar ages from the lunar maria, Earth Planet. Sci. Letters 11 (1971) 169; P. Eberhardt, J. Geiss, N. Grisgler, U. Krahenbiihl, M. Misrgeli and A. Stettler, K-Ar age of Apollo 11 rock 10003, Earth Planet. Sci. Letters 11 (1971) 245. [9] A.L. Albee, D.S. Burnett, A.A. Chodos, E.L. Haines, J.C. Huneke, D.A. Papanastassiou, F.A. Podosek, G. Price Russ, III and G.J. Wasserburg, Mineralogic and isotopic investigations of lunar rock 12013, Earth Planet. Sci. Letters 9 (1970) 137. [10] F. Tera, O. Eugster, D.S. Burnett and G.J. Wasserburg, Comparative study of Li, Na, K, Rb, Cs, Ca, Sr and Ba abundances in achondrites and in Apollo 11 lunar sampies, Geochim. Cosmochim. Acta 34, Suppl. 1 (1970) 1637. [11] A.L. Albee, D.S. Burnett, A.A. Chodos, O.J. Eugster, J.C. Huneke, D.A. Papanastassiou, F.A. Podosek, G. Price Russ, III, H.G. Sanz, F. Tera and G.J. Wasserburg, Ages, irradiation history, and chemical composition of lunar rocks from the Sea of Tranquillity, Science 167 (1970) 463. [12] L.A. Haskin, R.O. Alien, P.A. Helmke, T.P. Paster, M.R. Anderson, R.L. Korotev and K.A. Zweifel, Rare earth and other trace elements in Apollo 11 lunar samples, Geochim. Cosmochim. Acta 34, Suppl. 1 (1970) 1213. [13] P.W. Gast, N.J. Hubbard and H. Wiesmann, Chemical composition and petrogenesis of basalts from Tranquillity Base, Geochim. Cosmochim. Acta 34, Suppl. 1 (1970) 1143. [ 14] P.W. Gast, Limitations on the composition of the upper mantle, J. Geophys. Res. 65 (1960) 1287. [15] G.W. Wetherill, Lunar interior, constraint on basaltic composition, Science 160 (1968) 1256. [ 16] W.M. Kaula, The gravitational field of the moon, Science 166 (1969) 1581. [17] S.C. Solomon and M. Nafi Toksbz, On the density distribution in the moon, Phys. Earth Planet. Interiors 1 (1968) 475.