K. Misawa

An extremely low UPb source in the Moon: UThPb, SmNd, RbSr, and 40Ar39Ar isotopic systematics and age of lunar meteorite Asuka 881757

Abstract We have undertaken Uue5f8Thue5f8Pb, Smue5f8Nd, Rbue5f8Sr, and 40 Ar 39 Ar isotopic studies on Asuka 881757, a coarse-grained basaltic lunar meteorite whose chemical composition is close to low-Ti and very low-Ti (VLT) mare basalts. The Pbue5f8Pb internal isochron obtained for acid leached residues of separated mineral fractions yields an age of 3940 ± 28 Ma, which is similar to the U-Pb (3850 ± 150 Ma) and Th-Pb (3820 ± 290 Ma) internal isochron ages. The Sm-Nd data for the mineral separates yield an internal isochron age of 3871 ± 57 Ma and an initial 143 Nd 144 Nd value of 0.50797 ± 10. The Rb-Sr data yield an internal isochron age of 3840 ± 32 Ma ( λ( 87 Rb) = 1.42 × 10 −11 yr −1 ) and a low initial 87 Sr 86 Sr ratio of 0.69910 ± 2. The 40 Ar 39 Ar age spectra for a glass fragment and a maskelynitized plagioclase are relatively flat and give a weighted mean plateau age of 3798 ± 12 Ma. We interpret these ages to indicate that the basalt crystallized from a melt 3.87 Ga ago (the Sm-Nd age) and an impact event disturbed the Rb-Sr system and completely reset the K-Ar system at 3.80 Ga. The slightly higher Pb-Pb age compared to the Sm-Nd age could be due to the secondary Pb (from terrestrial and/or lunar surface Pb contamination) that remained in the residues after acid leaching. Alternatively, the following interpretation is also possible; the meteorite crystallized at 3.94 Ga (the Pb-Pb age) and the Sm-Nd, Rb-Sr, and K-Ar systems were disturbed by an impact event at 3.80 Ga. The crystallization age obtained here is older than those reported for low-Ti basalts (3.2–3.5 Ga) and for VLT basalts (3.4 Ga), but similar to ages of some mare basalts, indicating that the basalt may have formed from a magma related to a basin-forming event (Imbrium?). The age span for VLT basalts from different sampling sites suggest that they were erupted over a wide area during an interval of at least ~500 million years. The impact event that thermally reset the K-Ar system of Asuka 881757 must have been post-Imbrium (perhaps Orientale) in age. The lead isotopic composition of Asuka 881757 is nonradiogenic compared with typical Apollo mare basalts and the estimated 238 U 204 Pb (μ) value for the basalt source is 10 ± 3. This source-μ value is the lowest so far measured for lunar rocks. A large positive ϵNd value (7.4 ± 0.5) and the time averaged 147 Sm 144 Nd ratio for the basalt source are similar to those for some Apollo 12, 15, and 17 basalts, suggesting a LREE-depleted mantle, which is consistent with the global magma ocean hypothesis. The U-Th-Pb, Sm-Nd, and Rb-Sr data on Asuka 881757 suggest that the basalt was derived from a low U Pb , low Rb Sr , and high Sm Nd source region, mainly composed of olivine and orthopyroxene with minor amounts of plagioclase (or clinopyroxene) and with sulfides enriched in volatile chalcophile elements. The basalt source may be deep in origin and different in chemistry from those previously estimated from studies of Apollo and Luna mare basalts, indicating heterogeneous sources for mare basalts.

UThPb and SmNd isotopic systematics of the Goalpara ureilite: Resolution of terrestrial contamination

The Uue5f8Thue5f8Pb and Smue5f8Nd isotopic systematics of mineral separates from the Goalpara ureilite were studied after applying acid leaching techniques. The separates were successively leached with dilute HBr and HNO3 in order to remove any secondary Pb components in the meteorite as well as to separate a light REE-rich component which has been known to be dissolved using a similar acid leaching procedure. The leachates contained large amounts of terrestrial Pb as well as significant amounts of U, Th, Sm, and Nd, whereas the residues were highly depleted in all five of these elements (between 0.1 and 0.001 times CI chondritic abundance). The Uue5f8Pb, Thue5f8Pb, and Smue5f8Nd isotopic systematics can be interpreted as mixtures of highly depleted ureilite and terrestrial contamination in all fractions. For this reason, precise ages could not be obtained from any of these systems. The lead isotopic compositions in the residues are considered mixtures of terrestrial Pb and primordial meteoritic Pb (Canon Diablo troilite Ph) with a small amount of radiogenic Pb. However, on a Uue5f8Pb concordia diagram the residues indicate a formation age as old as 4.55 Ga, assuming the Uue5f8Pb data represent a mixture of terrestrial and ureilite U and Pb. Similarly, Smue5f8Nd systematics of the residues form a mixing line between the composition of 4.55-Gy old material and an average terrestrial crustal composition, yielding a 3.74-Ga pseudo-isochron. Furthermore, the acid leachates show terrestrial signatures of ThU and SmNd ratios, and Pb and Nd isotopic ratios. Therefore, it is doubtful that the so-called light REE-rich component in ureilites is indigenous, and that the implied metasomatic event on the ureilite parent body is real.

Further evidence for a low U/Pb source in the moon: UThPb, SmNd, and ArAr isotopic systematics of lunar meteorite Yamato-793169

Abstract The coarse-grained lunar meteorites, Yamato-793169 and Asuka-881757, represent a new type of low-Ti mare basalt. This paper reports the results of a Uue5f8Thue5f8Pb, Smue5f8Nd, and Arue5f8Ar isotopic study of Yamato-793169 performed as part of a consortium studies of lunar basaltic meteorites. The isotopic study was carried out on a small sample (100 mg) so that only three density fractions could be separated. These fractions were leached with dilute acid in order to eliminate terrestrial Pb contamination. However, the leaching procedure did not completely remove this contamination in some fractions and also apparently caused a fractionation of elements (U, Th, Pb) due to preferential leaching effects, producing a secondary disturbance of the systematics. Furthermore, the Arue5f8Ar analyses indicate that the isotopic systematics in this meteorite might have been disturbed sometime later than 750 Ma. For these reasons, the ages obtained using different isotopic systems disagree with each other and a precise formation age could not be obtained for this meteorite. However, using what results there are, two reasonable interpretations can be made: (1) the Smue5f8Nd system yielded an age of 3.4 Ga that could be interpreted as the formation age, assuming that this system is possibly the least disturbed during the metamorphic event(s) that this meteorite experienced at least once, and that all other isotopic systems and their corresponding ages were disturbed, and (2) the U-Pb system yielded a nearly concordant age of 3.8 Ga that could be interpreted as the formation age and the Smue5f8Nd isotopic systematics were somehow disturbed. We prefer the second interpretation for the reasons discussed below. On a concordia diagram, the CDT (Canon Diablo troilite)-corrected Uue5f8Pb isotopic data yield a discordia line similar to the lunar catastrophic array, indicating that the source of the meteorite formed during early lunar differentiation (∼4.4 Ga) and that the basalt was generated near 3.9 Ga. Total Arue5f8Ar age on plagioclase is 3.26 Ga, which seems to be too old if the formation age is 3.4 Ga, because low temperature fractions lost large amounts of radiogenic 40 Ar during the late thermal event. If we assume a formation age of 3.8 Ga, the estimated source 238 U/ 204 Pb (μ) is 21.6 ± 3.5 and ϵ Nd is 3.9 ± 0.3. These results indicate that the source of Yamato-793169 is more depleted than Apollo 12 and 15 LT basalts, but less depleted than Asuka-881757. Therefore, Yamato-793169 may represent a new type of LT- or VLT-like mare basalt that is different from Asuka-881757. The wide variety of lead and neodymium isotopic characteristics among LT and VLT mare basalts indicate that the lunar mantle was very heterogeneous with respect to trace element abundances.