Fouad Tera

U-Th-Pb systematics in three Apollo 14 basalts and the problem of initial Pb in lunar rocks

The isotopic composition of Pb and the elemental concentration of U, Th and Pb were measured on ‘total’ rock samples 14053, 14073 and 14310 and on mineral separates of 14310 and 14053. Sample #73 appears to be quite similar to #310. Sample #310 yields total rockmodel ages ofT(206Pb/238U) = 4.24AE,T(207Pb/235U) = 4.27AE, andT(208Pb/232Th) = 4.13AE. These arenearly concordant and distinct from the Rb-Sr and K-Ar crystallization ages of 3.88 AE. Mineral separates from 14310 show a wide spread in207Pb/206Pb ranging from 0.483 to 0.995. The data points define a reasonable linear array on the coupled Pb-U evolution diagram. Similar analyses of 14053 give high, discordant total rockmodel ages ofT(206Pb/238U) = 5.60AE,T(207Pb/235U) = 5.18AE, andT(208Pb/232Th) = 5.48AE. Mineral separates show a range of207Pb/206Pb from 0.716 to 1.209. These data also define a reasonable linear array on the coupled Pb-U evolution diagram. These are the first Pb-U isochrons obtained for lunar basalts and indicate a reasonable solution to the previous discrepancy between the different methods of ‘absolute’ age determination. The resulting U-Pb isochron ages are compatible with the Rb-Sr and K-Ar ages on the same rocks. However, it is not possible to establish aprecise time of ‘crystallization’ from the Pb-U data because of the small angle of intersection between the linear arrays and the concordia curve. These data show that total rock model ages do not in general yield crystallization ages. The data on #310 and #053 show that these rocks were formed containing a highly radiogenic initial lead (207Pb/206Pb)I0 ≈ 1.46 which accounts for the excessively high total rock model ages by the U-Th-Pb method. The only significant discrepancy in the data is the apparent variability of (208Pb/206Pb)I0in #053 which remains to be resolved. The (207Pb/206Pb)I0in these rocks corresponds to the radiogenic lead evolved between 4.51 and 3.88 AE in a U-rich environment. Such data from initial Pb may provide a new chronometer for early lunar evolution. The high207Pb/206Pb ages in some total lunar soils as well as in treated fractions may be partly explained as a consequence of the contribution of lunar basalts with radiogenic initial Pb. The data prove that at the time of extrusion of some basalts, unsupported lead with extremely high207Pb/206Pb ratios was added to the lunar surface.

Isotopic evidence for a terminal lunar cataclysm

Most highland total rock samples define a single UPb isochron which corresponds to a metamorphism age of ∼ 3.9AE. This age is also obtained for internal UPb isochrons for some of these samples. The data on 18 rock samples range from concordant samples with238U/206Pb∼ 1.2 to discordant ones with238U/206Pb∼ 0.02. This feature coupled with a correlated pattern of 238U/204Pb ratios, indicates that Pb was extensively mobilized at ∼ 3.9AE. The observed PbU fractionation is essentially due to Pb volatilization during the metamorphic events. Volatile Pb transport is not accompanied by similar effects in Rb and must therefore be attributed to a specific process. RbSr internal isochrons for the same rocks determine distinct metamorphic events in the interval 3.85–4.00 AE. We conclude that highland samples from widely separated areas bear the imprint of an event or series of events in a narrow time interval which can be identified with a cataclysmic impacting rate of the moon at ∼ 3.9AE, although differentiation by internal magma generation cannot be excluded. This cataclysm is associated with the Imbrium impact and very possibly the formation of Crisium and Orientale and possibly several other major basins in a narrow time interval (∼2 × 108yr or less). The UPb data indicate formation of the lunar crust at ∼ 4.4AE, which is distinctly younger than 4.6 AE, the time generally associated with planetary formation. If the lunar crust started being formed at ∼ 4.6AE, then this process must have continued until times significantly younger than 4.4 AE. RbSr data also indicate formation of the lunar crust around 4.5 AE with only minor additions of high Rb/Sr materials to the crust at times younger than 4.3 AE. Using the UPb systematics, K/U and the average U concentration of the moon as obtained from heat-flow measurements, we estimate the lunar concentrations: primordial Pb= 35ppb;Rb= 0.5ppm with Rb/Sr= 0.006.

Isotopic and chemical investigations on Angra dos Reis

We report on extensive isotopic studies of Pb, Sr and Xe and on chemical abundance measurements of K, Rb, Sr, Ba, Nd, Sm, U and Th for total meteorite and mineral separates of the Angra dos Reis achondrite. U-Pb, Th-Pb and Pb-Pb ages are concordant at 4.54 AE for the total meteorite and for high-purity whitlockite in Angra dos Reis. This establishes Angra dos Reis as an early planetary differentiate which has not been disturbed for these systems since 4.54 AE ago. Measured ^(87)Sr/^(86)Sr in pyroxene and whitlockite for Angra dos Reis (ADOR) are distinctly below BABI by two parts in 10^4 and only one part in 10^4 above the lowest ^(87)Sr/^(86)Sr (ALL) measured in an Allende inclusion. The difference in ADOR-ALL corresponds to an interval of condensation in the solar nebula of ∼3 m.y. If ^(26)Al was the heat source for the magmatism on the parent planets of Angra dos Reis and the basaltic achondrites (BABI) then the relatively large difference in ^(87)Sr/^(86)Sr, BABI - ALL, must be the result of planetary evolution rather than condensation over ∼10 m.y. Xe isotopic measurements confirm the presence of large amounts of ^(244)Pu-produced fission Xe and show that ^(244)Pu was enriched in the whitlockite relative to the pyroxene by a factor of ∼18. We present chemical element enrichment factors between the whitlockite and the fassaitic pyroxene in Angra dos Reis. The enrichment factors demonstrate close analogy between the rare earth elements and their actinide analogs. The enrichment factor for Pu is intermediate to the enrichment factors of Nd and Sm.

U-Th-Pb systematics in lunar highland samples from the Luna 20 and Apollo 16 missions.

Luna 20 and Apollo 16 soils which represent two highland sites contain Pb which has206Pb/204Pb of from 53 to 277 andradiogenic207Pb/206Pb of from 0.78 to 0.83. The soils yield very discordant ages which range from 4.91 up to 6.08 AE. A few of the results on soils can be discussed in terms of a simple two-stage model in which the lunar crust is generated by the addition of magmatic material enriched in Pb relative to both U and Th (without U-Th fractionation). The fractionation factor (Pb/U)magma/(Pb/U)source is commonly ∼ 2.5 or more. In no way can the soil model ages give a direct determination of the age of the Moon. The highland anorthosite 60025 gave a Pb concentration of 6.9 ppb and appears distinctly radiogenic with206Pb/204Pb= 25.4. The207Pb-206Pb age is 6.64 AE but the U concentration of 0.79 ppb is sufficient to account for the radiogenic Pb. This result is distinctly different from ancient and highly radiogenic initial Pb in 14310, 14053 and 15415. The extent to which various rock types ofdifferent ages and undetermined exotic materials contribute unsupported initial Pb to lunar soil is not yet established. The complexity of the Pb-U-Th data on the soils is remarkable in comparison with the simple behavior of the Rb-Sr results.

Lead concentration and isotopic composition in five peridotite inclusions of probable mantle origin

Abstract The lead content of five whole-rock peridotite inclusions (four lherzolites and one harzburgite) in alkali basalt ranges from 82 to 570 ppb (parts per billion). Approximately 30–60 ppb of this amount can be accounted for by analyzed major silicate minerals (olivine ≤ 10 ppb; enstatite 5–28 ppb; chrome diopside ∼400 ppb). Through a series of acid leaching experiments, the remainder of the lead is shown to be quite labile and to reside in either glassy or microcrystalline veinlets or accessory mineral phases, such as apatite and mica. The lead isotopic composition of the peridotites ( 206 Pb/ 204 Pb= 18.01–18.90; 207 Pb/ 204 Pb= 15.52–15.61; 208 Pb/ 204 Pb= 37.80–38.86) lies within the range of values defined by many modern volcanic rocks and, in particular, is essentially coextensive with the abyssal tholeiite field. In all but one instance, isotopic differences were found between the peridotite and its host alkali basalt. Two of the peridotites clearly demonstrated internal isotopic heterogeneity between leachable and residual fractions that could not simply be due to contamination by the host basalt. However, there is no evidence that these ultramafic rocks form some layer in the mantle with isotopic characteristics fundamentally different from those of the magma sources of volcanic rocks.

U-Th-Pb analyses of soil from the Sea of Fertility.

Abstract The isotopic composition of lead was determined on soil samples from two levels of the Luna 16 core. The lead was distinctly radiogenic and yielded 207 Pb/ 206 Pb model ages of 4.8 AE. This is interpreted as reflecting early lunar differentiation processes. The most reliable value of 208 Pb/ 204 Pb = 111 shows that the Luna 16 soils are significantly less radiogenic than found for other lunar soils. The elemental concentrations are Pb ∼ 0.9 ppm, U ∼ 0.3 ppm and Th ∼ 1.2 ppm with a Th/U ratio of 3.8. The U concentration is the lowest so far found for lunar soils. These data indicate that only a very small amount of magic component kreeputh into the Luna 16 soil.