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Wasserburg, G. J.

Publications and source records attributed to Wasserburg, G. J..

At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Rb-Sr and Sm-Nd chronology and genealogy of mare basalts from the Sea of Tranquility

Rb-Sr and Sm-Nd ages of two Apollo 11 mare basalts, high-K basalt 10072 and low-K basalt 10062, are reported. Rb-Sr, Sm-Nd, and Ar-40-Ar-39 ages are in good agreement and indicate an extensive time interval for filling of the Sea of Tranquility, presumably by thin lava flows, in agreement with similar observations for the Ocean of Storms. Initial Sr and Nd isotopic compositions on Apollo 11 basalts reveal at least two parent sources producing basalts. The Sm-Nd isotopic data demonstrate that low-K and high-Ti basalts from Apollo 11 and 17 derived from distinct reservoirs, while low-Ti Apollo 15 mare basalt sources have Sm/Nd similar to the sources of Apollo 11 basalts. Groupings of mare basalt based on Ti content and on isotopic data do not coincide.

Papanastassiou, D. A.↗
NASA NTRS1976

Inferences about magma sources and mantle structure from variations of Nd-143/Nd-144

Continental flood basalts and mid-ocean ridge (MOR) tholeiitic basalts have distinctly different Nd-143/Nd-144 which may permit a priori distinction between continental and oceanic igneous rocks. Initial Nd-143/Nd-144 of continental igneous rocks through time fall on a Sm/Nd evolution curve with chondritic REE abundance ratio. These observations indicate that many continental igneous rocks are derived from a reservoir with chondritic REE pattern which may represent primary material remaining since the formation of the earth. Oceanic igneous rocks are derived from a different ancient reservoir which has Sm/Nd higher than chondritic. Initial Nd-143/Nd-144 and Sr-87/Sr-86 in young basalts from both oceans and continents show a strong correlation suggesting that Sm-Nd and Rb-Sr fractionation events in the mantle may be correlative and caused by the same process. From this correlation Rb/Sr for the earth is inferred to be 0.029.

Depaolo, D. J.↗
NASA NTRS1976

Petrography of isotopically-dated clasts in the Kapoeta howardite and petrologic constraints on the evolution of its parent body

Results are presented for petrographic and electron microprobe studies of the isotopically dated A, B, C, and rho basaltic rock fragments separated from the howardite Kapoeta. Other lithic clasts and numerous mineral fragments in thin sections of Kapoeta are investigated in order to outline the range in lithology and chemistry of the source materials from which the Kapoeta meteorite is derived. The data obtained are compared to those from other meteorite and lunar samples, with particular reference to the observational consequences for the evolution of the Kapoeta parent body. A major conclusion is that there is no clearcut evidence for young magmatism on the Kapoeta parent body. The observations preclude the interpretation that the Kapoeta is a simple mixture of eucrites and diogenites. In contrast to the moon, a source of anorthositic rocks does not appear to have been present on the Kapoeta parent body which involved chiefly pyroxene. The FeO-MnO relationships suggest that the source of the materials in the Kapoeta parent planet are fundamentally related.

Dymek, R. F.↗
NASA NTRS1976

Demonstration of Mg-26 excess in Allende and evidence for Al-26

The observation of a large anomaly in the isotopic composition of Mg in a Ca-Al rich chondrule from the Allende meteorite is discussed. A strong correlation is found in this chondrule between the Mg-26 excess and the Al/Mg ratio. The most plausible cause of the anomaly is attributed to the in situ decay of now extinct Al-26. However, the general pattern of Mg isotopic anomalies is not yet shown to be fully consistent with the Al-26 hypothesis, since an apparently negative Mg-26 anomaly is obtained which cannot be explained by Al-26. A large natural fractionation effect is observed for Mg isotopes. The resolution of the O anomaly and its relation with Mg effects require further investigation.

Lee, T.↗
NASA NTRS1976

Rb-Sr age of troctolite 76535

Rb-Sr systematics is studied for the lunar troctolite 76535 in phases covering a wide range of Rb and Sr concentrations. A line is obtained corresponding to an age of 4.61 plus or minus 0.07 AE and an initial Sr-87/Sr-86 of 0.69900 plus or minus 0.00003. The Rb-Sr age of 4.55 plus or minus 0.10 AE for the dunite (72417) and that of the troctolite are in approximate agreement and permit an interpretation that these rocks formed during a single major lunar differentiation which presumbly produced an anorthositic-gabbroic lunar crust and layered upper mantle.

Papanastassiou, D. A.↗
NASA NTRS1975

Cratering and cosmogenic nuclides

A simple probabilistic model was constructed for the average value of a cosmogenic nuclide as a function of depth in a regolith. An arbitrary function was chosen for the size distribution of craters. The resulting integro-differential equation was found to reduce in limiting cases to the marching equation with a characteristic residence time and to the diffusion equation. The regolith diffusion constant is shown to be a simple integral of the cratering rate weighted by geometrical terms. This formal treatment provides a direct and general connection between cosmogenic nuclides and cratering rates and crater population in a simple analytical form. The validity of this model remains to be tested.

Blake, M. L.↗
NASA NTRS1975

Apollo 17 neutron stratigraphy - Sedimentation and mixing in the lunar regolith

A report is presented concerning the Gd isotopic ratios in ten samples, representing nearly the entire depth of the drill stem which was recovered during the Apollo 17 mission. The lunar neutron energy spectrum is discussed along with questions regarding neutron fluence and neutron flux. The significance of the Apollo 17 deep drill stem is considered and attention is given to the statistical nature of the fluence in the lunar regolith. Vertical mixing models are also described.

Curtis, D. B.↗
NASA NTRS1975

Rb-Sr study of a lunar dunite and evidence for early lunar differentiates

An Rb-Sr study of Boulder 3 from Station 2 sampled during the Apollo 17 mission is reported. The investigated boulder is a metaclastic rock, inferred to have come from the South Massif, and contains a variety of lithic clasts. Since the rock type may be critical in understanding lunar differentiation processes, an extensive investigation was carried out of the dunite clast and other clasts as well as of the fine-grained matrix. The investigation had the purpose to define the age of crystallization of the dunite and to determine the extent to which later metamorphism of the host rock may have effected the isotopic systematics of this clast. The isochron determined yielded an age of 4.55 plus or minus 0.10 AE for the dunite. This result demonstrates that differentiation began close to the time of formation of the moon and that relics of this process are preserved.

Papanastassiou, D. A.↗
NASA NTRS1974

Isotopic evidence for a terminal lunar cataclysm

Most highland total rock samples define a single U-Pb isochron which corresponds to a metamorphism age of about 3.9 aeons. This age is also obtained for internal U-Pb isochrons for some of these samples. The data on 18 rock samples range from concordant samples with U-238/Pb-206 ratio of about 1.2 to discordant ones with U-238/Pb-206 ratio of about 0.02. This feature, coupled with a correlated pattern of U-238/Pb-204 ratios, indicates that Pb was extensively mobilized at about 3.9 aeons. The observed Pb-U 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. Rb-Sr internal isochrons for the same rocks determine distinct metamorphic events in the interval 3.85 to 4.00 aeons. It is concluded 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 about 3.9 aeons, although diffrentiation by internal magma generation cannot be excluded.

Tera, F.↗
NASA NTRS1974

Evidence for an approximately 4.5 aeon age of plagioclase clasts in a lunar highland breccia

Argon from neutron-irradiated mineral separates and whole rock samples of a metamorphosed breccia (65015) from Apollo 16 has been analyzed with a large number of gas extraction steps in order to obtain a high resolution in the apparent ages and to identify the gas released from different sources. The results on plagioclase show a Ar-40/Ar-39 plateau age of 3.98 b.y. which is attributed to the time of metamorphism, and an age of about 4.5 b.y. in the high-temperature fraction. Correlation of the release pattern with Ar-37 instead of Ar-39 permits the association of the approximate 4.5 b.y. age with relict plagioclasts which were demonstrated in previous petrographic and Rb-Sr studies as being unequilibrated. This result suggests that it is possible to identify lithic components which represent the early lunar crust.

Jessberger, E. K.↗
NASA NTRS1974

Mineralogy, petrology, and chemistry of basalt fragments returned by Luna 16

Mineralogical, petrological, and chemical analyses, along with Rb-Sr age and Ar-40/Ar-39 measurements, were carried out with the B-1 sample returned from the Luna 16 mission. The sample, weighing 62 mg, is a fine-grain basalt of ophitic structure. It differs from the Apollo samples in that the pyroxene and plagioclass contents are almost identical, and the ilmenite content (7%) lies between those of the Apollo-11 and Apollo-12 samples. Chemically, it is characterized by a high Sr content and a high K/U ratio.

Albee, A. L.↗
NASA NTRS1974

U-Th-Pb systematics on lunar rocks and inferences about lunar evolution and the age of the moon

The investigation reported continues a study conducted by Tera et al. (1974). An attempt is made to summarize all of the data currently available in the literature on terra materials and to discuss the implications of the upper intersection with the concordia curve. Data on total rocks and some plagioclase separates are presented in a graph. The data appear to give further support to a model of a terminal lunar cataclysm associated with intense global bombardment at about 3.9 aeons.

Tera, F.↗
NASA NTRS1973

The age and petrography of two Luna 20 fragments and inferences for widespread lunar metamorphism.

Ages were determined by the Ar-40/Ar-39 method on two metaclastic rocks returned from the lunar highlands north of Mare Fecunditatis by the Luna 20 probe. Both samples gave very well-defined argon retention ages of 3.90 plus or minus 0.04 AE which are indistinguishable from each other within a resolution of 0.02 AE. Both fragments, 22006 and 22007, are highly recrystallized polymict breccias; there is no evidence of radiogenic Ar-40, and the age almost surely dates the time of recrystallization. The cosmic ray exposure ages of these fragments are similar and high: 900 million years for 22006, 1300 million years for 22007. 22007 also contains substantial trapped argon with a high Ar-40/Ar-36. The Luna 20 results greatly extend the area of the moon's surface exhibiting a well-defined record of metamorphism at 3.9 AE. So far, lunar history in the interval from 4.6 to 3.9 AE is not preserved in the ages of surface rocks. This obliteration suggests lunar-wide metamorphic conditions occurring or terminating at this time as a result of major impacts.

Podosek, F. A.↗