Internal Isochron Research Articles

Host-rock controlled epigenetic, hydrothermal metasomatic origin of the Bayan Obo REE [sbnd]Fe [sbnd]-Nb ore deposit, Inner Mongolia, P.R.C.

Bayan Obo, a complex rare earth element (REE) Fe Nb ore deposit, located in Inner Mongolia, P.R.C. is the world's largest known REE deposit. The deposit is chiefly in a marble unit (H8), but extends into an overlying unit of black shale, slate and schist unit (H9), both of which are in the upper part of the Middle Proterozoic Bayan Obo Group. Based on sedimentary structures, the presence of detrital quartz and algal fossil remains, and the 16-km long geographic extent, the H8 marble is a sedimentary deposit, and not a carbonatite of magmatic origin, as proposed by some previous investigators. The unit was weakly regionally metamorphosed (most probably the lower part of the green schist facies) into marble and quartzite prior to mineralization. Tectonically, the deposit is located on the northern flank of the Sino-Korean craton. Many hypotheses have been proposed for the origin of the Bayan Obo deposit; the studies reported here support an epigenetic, hydrothermal, metasomatic origin. Such an origin is supported by field and laboratory textural evidence; 232Th/ 208Pb internal isochron mineral ages of selected monazite and bastnaesite samples; 40Ar/ 39Ar incremental heating minimum mineral ages of selected alkali amphiboles; chemical compositions of different generations of both REE ore minerals and alkali amphiboles; and evidence of host-rock influence on the various types of Bayan Obo ores. The internal isochron ages of the REE minerals indicate Caledonian ages for various episodes of REE and Fe mineralization. No evidence was found to indicate a genetic relation between the extensive biotite granitic rocks of Hercynian age in the mine region and the Bayan Obo are deposit, as suggested by previous workers.

Rb-Sr and U-Pb dating of the Daguzhai and Luobuli granitic massifs in South China

Rb-Sr and U-Pb isotopic studies of the two contrasting granite types of the Daguzhai and Luobuli massifs in South China provide new constraints on the interpretation of isotopic age data for plutonic igneous rocks. A Rb-Sr internal isochron age of 146±7Ma for the Luobuli adamellite is interpreted to represent the age of magma crystallization, whereas the whole rock Rb- Sr isochron yields an older apparent age of 161±10Ma which is regarded as resulting from contamination processes affecting the petrogenesis of this adamellite. In the Daguzhai granite the marked scatter of whole- rock Rb-Sr data in isochron diagram is ascribed to the open system behavior of Rb during postmagmatic autometasomatism. Uniformity of initial87Sr /86Sr ratio in this granite is indicated in a plot of87Sr versus86Sr. The autometasomatism has also affected zircon U-Pb system, resulting in a spread of data along the concordia curve between 165 and 125Ma. This spread is regarded as indicating the duration of the autometasomatism.

Samarium-neodymium evolution of meteorites

We have obtained Sm-Nd data on two differentiated meteorites, Ibitira, a eucrite with distinct basaltic texture and with evidence of crystallization; and Morristown, a group 3A mesosiderite; as well as on Acapulco, an unclassified meteorite with chondritic chemical composition and a highly recrystallized texture. We have demonstrated the presence of in situ decay of short-lived 146Sm in these meteorites with initial abundance of 146Sm 144Sm from 0.009 to 0.007 for the different meteorites. Precisely defined 147Sm- 143Nd internal isochrons were obtained yielding ages of 4.60 ± 0.03 AE for Acapulco, 4.47 ± 0.02 AE for Morristown and 4.46 ± 0.02 AE for Ibitira. The corresponding initial 143Nd 144Nd values obtained were 0.2 ± 0.9, 2.1 ± 0.7 and 1.6 ± 0.8 ϵu (epsilon units). Detailed consideration of the coupled 147Sm- 143Nd and 146Sm- 142Nd systematics and of the elevated initial 143Nd 144Nd values indicates the presence of inconsistencies in the data for the two Sm-Nd parent-daughter pairs in spite of the precise isochrons. We present a model which provides an interpretation of the paired Sm-Nd systematics in terms of an impulsive disturbance. The abundance of phosphate, with large concentrations of REEs, controls the REE economy. Plagioclase, with low REE concentrations is assumed to be susceptible to exchange. Pyroxene is considered to be undisturbed because of the low diffusion coefficients of trace elements. The experimental results can be reasonably explained by this model. The results indicate that three meteorites studied, some with very low REE concentrations, including a mesosiderite, are relatively ancient objects, formed within the first 50 to 100 m.y. of the solar system, by planetary differentiation and impact processes, and were subjected to late metamorphism. The time of late metamorphism is substantially younger than 4.4 AE and possibly as recent as ~3.9 AE. This metamorphism has resulted in partial, very limited reequilibration between plagioclase and phosphate, resulting in the observed shifts in initial 143Nd 144Nd , Sm-Nd model ages, and the 142Nd 144Nd abundances, but preserving the major temporal structures of early original formation and later disturbances. These results show the need for care in the interpretation of the parent-daughter systematics, even when they appear well behaved and the mineral phases are viewed as highly resistant to mild metamorphism.

238U [sbnd]230Th [sbnd]226Ra disequilibria in young Mount St. Helens rocks: time constraint for magma formation and crystallization

We use 238U-series nuclides and 230Th/ 232Th ratios measured by mass spectrometry to constrain processes and time scales of calc-alkaline magma genesis at Mount St. Helens, Washington. Olivine basalt, pyroxene andesites and dacites that erupted 10–2 ka ago show 3–14% ( 230Th) ( 238U) and 6–54% 226Ra 230Th disequilibria. Mineral phases exhibit robust ( 226Ra) ( 230Th) fractionation. Plagioclase has large 65–280% ( 226Ra) excesses, and magnetite has large 65% ( 226Ra) deficits relative to ( 230Th). Calculated partition coefficients for Ba, Th, and U in mineral-groundmass pairs, except Ba in plagioclase, are low (⩽ 0.04). Correlation between ( 226Ra/ 230Th ) activity ratios and rm/BaTh element ratios in the minerals suggests that 226Ra partitions similar to Ba during crystallization. Internal ( 230Th) ( 238U) isochrons for 1982 summit and East Dome dacites and Goat Rocks and Kalama andesites show that closed Th U system fractionation occurred 2–6 ka ago. Apparent internal isochrons for Castle Creek basalt (34 ka) and andesite (27 ka) suggest longer magma chamber residence times and mixing of old crystals and young melt. Mineral ( 226Ra) ( 230Th) disequilibrium on Ba-normalized internal isochron diagrams suggests average magma chamber residence times of 500–3000 years. In addition, radioactive ( 226Ra/ 230Th ) heterogeneity between minerals and groundmass or whole rock is evidence for open-system Ra Th behavior. This heterogeneity suggests there has been recent, post-crystallization, changes in melt chemical composition that affected 226Ra more than 230Th. Clearly, magma fractionation, residence and transport of crystal-melt before eruption of chemically diverse lavas at Mount St. Helens occurs over geologically short periods.

A Pan African age for the HP-HT granulite gneisses of Zabargad island: implications for the early stages of the Red Sea rifting

Up to now the age of granulite gneisses intruded by the Zabargad mantle diapir has been an unsolved problem. These gneisses may represent either a part of the adjacent continental crust primarily differentiated during the Pan African orogeny, or new crust composed of Miocene clastic sediments deposited in a developing rift, crosscut by a diabase dike swarm and gabbroic intrusions, and finally metamorphosed and deformed by the mantle diapir. Previous geochronological results obtained on Zabargad island and Al Lith and Tihama-Asir complexes (Saudi Arabia) suggest an Early Miocene age of emplacement for the Zabargad mantle diapir during the early opening of the Red Sea rift. In contrast, Sm Nd and Rb Sr internal isochrons yield Pan African dates for felsic and basic granulites collected 500–600 m from the contact zone with the peridotites. Devoid of evidence for retrograde metamorphic, minerals from a felsic granulite provide well-defined Rb Sr and Sm Nd dates of 655 ± 8 and 699 ± 34 Ma for the HP-HT metamorphic event (10 kbar, 850°C). The thermal event related to the diapir emplacement is not recorded in the Sm Nd and Rb Sr systems of the studied gneisses; in contrast, the development of a retrograde amphibolite metamorphic paragenesis strongly disturbed the Rb Sr isotopic system of the mafic granulite. The initial 143Nd/ 144Nd ratio of the felsic granulite is higher than the contemporaneous value for CHUR and is in agreement with other Nd isotopic data for samples of upper crust from the Arabian shield. This result suggests that source rocks of the felsic granulite were derived at 1.0 to 1.2 Ga from either an average MORB-type mantle or a local 2.2 Ga LREE-depleted mantle. Zabargad gneisses represent a part of the disrupted lower continental crust of the Pan African Afro-Arabian shield. During early stages of the Red Sea rifting in the Miocene, these Precambrian granulites were intruded and dragged upwards by a rising peridotite diapir.

Thermal history of the shock-melted Antarctic LL-chondrites from the Yamato-79 collection

The Sr and rare gas isotopic compositions and abundances of lithophile trace elements (K, Rb, Sr, Ba, and REEs) were determined for a series of shock-melted Yamato-79 LL-chondrites to investigate their late thermal history and the chemical features of shock processes. All meteorites show similarities in shock ages (~ 1.2 Ga) as confirmed by Rb-Sr internal isochron and K-Ar dating, rare gas compositions as well as cosmic-ray exposure ages (~28 Ma), petrographie textures, and sampling sites in Antarctica. These results indicate that all of these meteorites are part of the same fall. The 1.2 Ga shock event caused a severe (partial to total) melting followed by recrystallization of olivine and clinopyroxene, vesiculation, shock-induced alkali homogenization, and local isotopic equilibration or perturbation of the Rb-Sr system. The degrees of shock effects are variable from specimen to specimen and from portion to portion, even in a single specimen. Model calculations of Fe diffusion in olivine suggest that hot and cold materials were in close contact in the impact ejecta sheets of the parent body. From these model calculations and the evidence provided by cosmogenic rare gas compositions, it is concluded that an impact melt ejecta pile composed of hot and cold brecciated materials had formed at depth (> 2 m, shielded from cosmic rays) in an impact crater by the 1.2 Ga event. The parent body was fragmented to meter-size stones by an additional collision at ~28 Ma resulting in the formation of the parent material of the Yamato-79 shocked chondrites.

Age of a eucrite clast from the Bholghati howardite

39Ar- 40Ar, Rb-Sr, and Sm-Nd investigations of the “large” eucrite clast (,1) studied by the Bholghati Consortium show that it crystallized from a melt ~ 4.51 Ga ago. The crystallization age is given most precisely by a Sm-Nd internal isochron of 4.510 ± 0.034 Ga (2σ error limits) for λ( 147Sm) = 0.00654 (Ga) −1. 142Nd 144Nd measurements show that the clast contained live 146Sm at the time of crystallization. A solar system initial 146 Sm 144Sm 0 = 0.00431 +0.0039 −0.0026 at 4.555 Ga ago can be calculated from a 146Sm- 142Nd isochron and the age of the clast. Alternatively, using ( 146Sm 144Sm ) 0 = 0.0045 as previously reported in the literature, a 146 Sm 142 Nd formation interval of 0.05 +0.12 −0.08 Ga can be calculated, which is concordant with the 147Sm- 143Nd isochron age and in agreement with Pu-U-Xe and Pu-Nd-Xe formation intervals reported in a companion Consortium paper. The initial 143Nd 144 Nd ( I Nd ) of the clast was I Nd = 0.505958 ± 50 (2σ error limits), normalized to 146Nd/ 144Nd = 0.724140 and is consistent with the CHUR reference value for evolution in a source having chondritic relative REE abundances prior to magma generation. However, when interlaboratory bias is considered, the data are also permissive of a LREE depleted source as suggested in a companion Consortium paper. A primary Rb-Sr isochron yields an age of 4.54 ± 0.06 Ga (2σ error limits) for λ( 87Rb) = 0.01402 (Ga) −1, in agreement with the more precise Sm-Nd age. The initial 87Sr 86Sr (I Sr) of the clast was I Sr = 0.699006 ± 21 and was the same within error limits as I Sr previously determined for a pristine eucrite clast from polymict eucrite Y75011; compatible with a common parent body. The 39Ar- 40Ar data and some of the Rb-Sr data show that Bholghati was affected by a major thermal event ~ 2–3 Ga ago and also provide weak evidence for a minor thermal event ≤ 1 Ga ago. The age and isotopic systematics of the clast are consistent with those of other eucrites and support the widely held assumption of a common parent body for howardites and eucrites. The isotopic investigations of polymict eucrites and howardites point to a prolonged cratering record on the howardite-eucrite-diogenite (HED) parent body and suggest that it survived the “cataclysm” affecting the Moon ~ 3.8-4.0 Ga ago.

Young asteroid melting event indicated by Rb–Sr dating of the Point of Rocks meteorite

THE record of collisions on asteroids, as preserved in meteorite breccias, provides important constraints on impact cratering events in the inner Solar System1,2. Many young gas-retention ages (with a peak at ∼500 Myr) have been reported for meteorite breccias3,4, but except for a few cases5, they have not been substantiated by the much less susceptible clocks based on refractory parent-daughter nuclides such as 87Rb–87Sr. Furthermore, the nature and intensity of the thermal events responsible for young meteorite ages have not been well evaluated. Here we report an age of 460±11 (2σ) Myr, well defined by a Rb–Sr internal isochron for the completely impact-melted L-chondrite, Point of Rocks. This age provides the first evidence for a young intense collision leading to melting on an asteroid.

U–Pb and Rb–Sr geochronology of the Wedgeport granitoid pluton, southwestern Nova Scotia

Zircons from biotite monzogranite of the Wedgeport Pluton, intrusive into deformed metasediments of the Cambrian(?) Goldenville Formation in the southwestern Meguma Terrane of Nova Scotia, yield concordant U–Pb ages of 316 ± 5 Ma. This is interpreted as the time of intrusion and crystallization. Within the error limits, the 323 ± 12 Ma Rb–Sr whole-rock isochron age is identical and gives an initial 87Sr/86Sr ratio of 0.7137 ± 0.0056. Rb–Sr analyses of mineral separates of biotite, potassium feldspar, and quartz–plagioclase from several samples yield subparallel, internal isochrons with an average age of 257 ± 8 Ma. Initial ratios of the internal isochrons range from 0.716 to 0.759. A slow-cooling model for these latter data is discarded because the mineral data fall on straight lines. Instead, a reheating event related to plutonism ca. 257 Ma ago, which was sufficient to cause local grain-to-grain migration and reequilibration of strontium and rubidium but not large-scale redistribution, is invoked. This reheating is also inferred to be responsible for the hydrothermal alteration and Sn–U mineralization concentrated along the northwestern margin of the pluton. A dextral northeast–southeast shear zone cutting the pluton is also inferred to be ca. 257 Ma old. It may be related to the last stages of westward obduction of the Meguma Terrane.These results provide a clear example of Permo-Carboniferous plutonism in the southwestern Meguma Terrane and suggest a similar interpretation may apply to other anomalously young ages recorded in this area. In light of these results, the Permo-Carboniferous age of the large East Kemptville tin deposit and its location in a dextral shear zone suggest that the association of younger plutonism and shear zones may be a significant factor for economic mineralization.

Rb-Sr and Sm-Nd isotopic study of the Glen Mountains layered complex: Initiation of rifting within the southern Oklahoma aulacogen

Rb-Sr and Sm-Nd isotopic data for rocks and minerals of the Glen Mountains layered complex (GMLC), a midcontinent mafic layered intrusion in the Wichita Mountains of southwestern Oklahoma, constrain the time of initiation of rifting within the southern Oklahoma aulacogen and provide information on the chemistry of the early Paleozoic mantle. Four whole-rock samples define a Rb-Sr isochron corresponding to a maximum crystallization age of 577 ±165 Ma and an initial Sr isotopic composition of 0.70359 ±2. These whole-rock analyses do not define a Sm-Nd isochron; rather, they display a significant range in initial Nd isotopic composition (eNd = 3.63-5.35). A three-point Sm-Nd mineral-whole-rock (internal) isochron for an anorthositic gabbro provides a crystallization age of 528 ±29 Ma. These data suggest that the GMLC was emplaced into the southern Oklahoma aulacogen during the initial phase of rifting along the southern margin of the North American craton in the early Paleozoic. This Sm-Nd internal isochron age is within analytical uncertainty of U-Pb zircon ages for granites and rhyolites from the Wichita Mountains; therefore, mafic and felsic magmatism may have been contemporaneous within the rift during the early stages of development. Hybrid rocks and composite dikes in the Wichita Mountains provide field evidence for contemporaneous mafic and felsic magmas. Initial Sr and Nd isotopic data suggest that magmas parental to the GMLC were derived from a depleted mantle source. However, Nd isotopic data for the GMLC plot distinctly below data for the depleted mantle source cited by DePaolo and thus suggest that the parental magmas of the GMLC were either contaminated by Proterozoic crust of the southern midcontinent or were derived from a heterogeneous mantle source region that had variable initial Nd isotopic compositions.

Geochronology of high-K aluminous mare basalt clasts from Apollo 14 breccia 14304

RbSr, KAr and SmNd age determinations were undertaken for two aluminous mare basalt clasts (,127 and,128) of high potassium abundances from Apollo 14 breccia 14304. Genesis of the basalts is discussed in the context of the ages and the RbSr and SmNd isotopic evidence. The RbSr isochrons for these rocks are well-defined and yield slightly different old ages of 3.95 ± 0.04 Ga and 3.99 ± 0.02 Ga (relative to λ( 87 Rb) = 0.0139 Ga −1 ) and identical low initial 87 Sr 86 Sr , I( Sr) , of 0.69938 ± 0.00011 and 0.69938 ± 0.00007, respectively. The age and I(Sr) results suggest that these basalts probably represent two related flows derived from a common source. The SmNd internal isochron for the basalt clast 14304,128 is also well-defined but yields a less precise age of 4.04 ± 0.11 Ga and an initial ϵ Nd of 0.27 ±2.61. The average 39Ar 40Ar plateau age of the whole rock sample of 14304,127 yields a slightly younger age of 3.85 ± 0.04 Ga. The concordancy of RbSr and SmNd isotopic ages for these rocks strongly suggest that they were crystallized at 3.95−3.99 Ga ago. The younger 39Ar 40Ar age was probably reset by the Imbrium event ~3.85 Ga ago. The RbSr and SmNd age data also show that these basalts are younger than most of the Apollo 14 mare basalts but distinctly older than the very high potassium (VHK) mare basalts. There seems to be an anti-correlation between potassium abundances and ages of aluminous mare basalts at the Apollo 14 site. The age and the I(Sr) value for these basalts show an approximately ten-fold fractionation of Rb Sr for a two-stage evolution model. However, the age and the ϵ Nd values indicate no significant fractionation of Sm Nd . The large Rb Sr enrichment but minimum Sm Nd fractionation relative to their source materials can be explained by the assimilation model proposed recently for VHK basalt genesis involving granitic material and aluminous basaltic magma. Aluminous mare basalt volcanism was probably derived from shallow mantle sources underlying thick highland crust, and these magmas are more likely to be contaminated by crustal materials on ascent than are other types of mare magmas.

Lead-isotope geochronology of the Portman Lake area, Northwest Territories

An area of Archean rocks extending from northwestern Saskatchewan into the Northwest Territories is referred to as the Nolan block.Whole rocks and minerals separated from granodiorite surrounding an occurrence of amazonite at Portman Lake in the centre of this block have been studied in some detail using several dating techniques. The following ages result: [Formula: see text] on abraded zircons from the granodiorite; 2589 ± 4 Ma whole-rock Pb/Pb age of the granodiorite; 2561 ± 6 and 2475 ± 13 Ma internal Pb/Pb isochrons; and 2610 ± 10 Ma Pb/Pb model ages on galena and amazonite.Pb/Pb internal isochrons were obtained by successive leaching of whole-rock samples and apparently indicate episodes of minor resetting of the whole-rock system. The close agreement between zircon ages, whole-rock Pb/Pb isochrons, and model ages on galena with no indication of Hudsonian updating suggest that the area has been essentially undisturbed since the Archean. The internal isochrons seem similar to Rb/Sr dates in the area and may indicate similarities in thermal response of the Rb–Sr and U–Pb systems in rocks.

Time-temperature relation of mineral isochrons: a thermodynamic model, and illustrative examples for the Rb [sbnd]Sr system

We have developed a thermodynamic model for the determination of the closure temperature ( T C ) at which the minerals defining an internal isochron in Rb Sr, or similar, geochronological system were set with a geochronological clock. It is shown that the equilibrium fractionation of 87Rb and 87Sr between a pair of minerals at T C [K D( 87Rb 87Sr) C] is given by the ratio of the quantity ( 87Rb/ 86Sr ) in the two minerals as measured at the present time. K D( 87Rb- 87Sr) , which equals the element distribution coefficient K D(Rb Sr) under equilibrium condition, can be calibrated as a function of temperature, and compared with the retrieved value of K D( 87Rb 87Sr) C in a natural pair to obtain T C . The various mineral pairs defining an internal isochron will yield concordant or discordant values of T C depending on whether or not they closed simultaneously with respect to the diffusion of Rb and Sr. Both types of results are expected, and are important in the analyses of the evolutionary history of the host rocks. Preliminary analyses of the published data in the Rb Sr system suggest a fairly wide range of T C even for the same mineral pair, reflecting differences in the cooling rates and physico-chemical environments of the host rocks.

A Sm/1bNd isotopic study of heterogeneous granulites from the Archean Kasai-Lomami gabbro-norite and charnockite complex (Zaire, Africa)

Sm/1bNd model ages ( t DM ) of acid granulites from the Kasai Complex agree with previous Rb/1bSr and U/1bPb ages of ∼ 2.8 Ga for the early and main stage of granulite-facies metamorphism. These Sm/1bNd results give an upper limit of 3.1 Ga for the age of the protoliths of the charno-enderbites and metapelitic gneisses, which represent the majority of the acid rocks of the complex. A Sm/1bNd study (whole-rock and internal isochrons, and t DM ages) of the associated basic granulites (meta-gabbro-norite massif and metadolerite dykes crosscutting the acid granulites) demonstrates that the second stage of granulite metamorphism, corresponding to regional deformation of all the rocks, is nearly coeval with the emplacement of the basic rocks at ∼ 2.4 Ga.

Rb‐Sr and Sm‐Nd internal isochron ages of a subophitic basalt clast and a matrix sample from the Y75011 eucrite

The Yamato polymict eucrite Y75011 contains several coarse‐grained mesostasis‐rich basaltic clasts with subophitic texture. The preservation of extensive Mg‐Fe chemical zoning in pyroxenes and the presence of a dark mesostasis shows that these clasts are representative of pristine lava‐like basalts extruded on the surface of the Howardite‐Eucrite‐Diogenite (HED) parent body. Rubidium‐strontium analyses of a large clast, Y75011,84B, yielded a precise internal isochron age of 4.60 ± 0.05 b.y. for λ(87Rb) = 0.0139 (b.y.)−1 or 4.50 ± 0.05 b.y. for λ(87Rb) = 0.0142 (b.y.)−1. The Rb‐Sr systematics of a matrix sample are nearly identical to those of the pristine clast, showing that the matrix is dominantly comminuted mesostasis‐rich basalt. The Rb‐Sr internal isochron age of the matrix is 4.56 ± 0.06 b.y. for λ(87Rb) = 0.0139 (b.y.)−1 or 4.46 ± 0.06 b.y. for λ(87Rb) = 0.0142 (b.y.)−1. These ages are within error limits of the Rb‐Sr whole rock age of chondrites and are in agreement with 206Pb/207Pb and Sm‐Nd ages of ordinary monomict eucrites for λ(87Rb) = 0.01402 (b.y.)−1 based on a comparison of the Rb‐Sr and U‐Th‐Pb ages of chondrites (Minster et al., 1982). The good precision obtained for the Rb‐Sr ages of both clast and matrix is due to the presence of a mesostasis phase in which the Rb/Sr ratio is enriched 3–4 times above that of any eucrite phase previously analysed. The initial 87Sr/86Sr ratios of clast and matrix are 0.69894 ± 2 and 0.69896 ± 3, respectively, and are in satisfactory agreement with the BABI value (Papanastassiou and Wasserburg, 1969). There is evidence of isotopic disturbance in both the Rb‐Sr and Sm‐Nd systems of the most magnetic separate analysed. A Sm‐Nd internal isochron of 4.55 ± 0.14 b.y. for the combined data from both clast and matrix exclusive of the most magnetic separate is in agreement with the more precise Rb‐Sr age. The corresponding initial 143Nd/144Nd = 0.50587 ± 19, normalized to 148Nd/144Nd = 0.24308. The clast data alone, exclusive of the most magnetic separate, define an isochron age of 4.52 ± 0.16 b.y. and initial 143Nd/144Nd = 0.50593 ± 19. The isochron error limits quoted here for the Sm‐Nd data are 2σ from the York (1966) program and include the factor [S/(N−2)]½ used in that program to compensate for “geological error.” However, this factor is not appropriate for the Rb‐Sr isochrons and has been omitted from the quoted error limits. The initial 143Nd/144Nd of the pristine clast at 4.56 b.y. ago is 0.505877 ± 25, in agreement within error limits with the CHUR value (Jacobsen and Wasserburg, 1984). Clast Y75011,84 represents the oldest pristine, unambiguously basaltic lava as yet recovered from a solar system object. Rare earth element abundance measurements (Shimizu and Masuda, 1985) show it to be one of the most differentiated eucrites. Thus moderately evolved basaltic lavas were produced very early in the history of the HED parent body. Their preservation as clasts in the polymict eucrites suggests derivation from a different geologic setting on the parent body than that occupied by the more thermally metamorphosed monomict eucrites. We suggest that the preservation of pristine clasts in polymict eucrites may be due to their impact excavation to the surface of the parent body away from heat sources concentrated within impact craters. The monomict eucrites may be derived from the floors of craters where they were thermally metamorphosed by shock heating and by the presence of overlying impact melts and nearly molten breccias.

Geochronology and petrogenesis of Apollo 14 very high potassium mare basalts

Rb‐Sr, K‐Ar, and Sm‐Nd isotopic studies were undertaken for two Apollo 14 very high potassium (VHK) mare basaltic clasts from breccias 14305 and 14168. The samples are distinctly more radiogenic than other lunar mare basalts. Rb‐Sr data for the whole rock and mineral separates determine internal isochrons corresponding to essentially identical ages of 3.83 ± 0.08 b.y. and 3.82 ± 0.12 b.y. for samples 14305 and 14168, respectively, for λ(87Rb) = 0.0139 (b.y.)−1. Their corresponding initial 87Sr/86Sr ratios are indistinguishable and are 0.6995 ± 0.0005 and 0.6997 ± 0.0004, respectively. The 39Ar–40Ar age spectra for the two samples are very similar and give identical plateau ages of 3.85 ± 0.02 b.y. for 14305 and 3.85 ± 0.05 b.y. for 14168. The identical Rb‐Sr and K‐Ar isotopic results for the two rocks indicate that they could be derived from the same flow. The Sm‐Nd isotopic data of whole rock and mineral separates for the two VHK basalts define an internal isochron age of 3.94 ± 0.16 b.y. for λ(147Sm) = 0.00654 (b.y.)−1 and an initial 143Nd/144Nd of 0.50673 ± 21. The averaged model age, relative to the chondritic initial 143Nd/144Nd of 0.505893 for the basalts is 4.56 ± 0.04 b.y. The concordance of Rb‐Sr, K‐Ar, and Sm‐Nd ages strongly suggests that VHK basalts crystallized from a melt ∼3.85 b.y, ago. The Rb‐Sr and Sm‐Nd isotopic data indicate that VHK basalts show very large Rb/Sr fractionation but no significant Sm/Nd fractionation at the time of crystallization. The source material for these basalts had a Rb/Sr ratio similar to those of Apollo 14 high‐A1 mare basalts and a nearly chondritic Sm/Nd ratio. Extreme enrichments of Rb/Sr and K/La during the formation of VHK basalts can be adequately explained by an assimilation model that involves basalt/granite interaction. K, Rb‐rich components of granitic wall rocks in highland crust were selectively introduced into ascending hot high‐A1 mare basaltic magma upon contact. A similar crustal assimilation process has been frequently observed in terrestrial, mantle‐derived magma that ascended through the continental crust.

Pre-alpine and alpine evolution of the South-alpine basement of the Orobic Alps

New K-Ar age determinations on the gneisses and schists of the south-Alpine basement of the Orobic Alps and their separated minerals have been carried out to supplement and support the previous study ofBocchio et al. (1981). The cooling ages of biotites and muscovites range from 218 to 331 my and from 170 to 330 my respectively, according to a bimodal distribution with modes at 222 and 298 my for biotite, 182 and 322 my for muscovite. The whole rock ages spread from 43 to 402 my with no significant cluster. Only a few samples give statistically significant internal isochrones, and these suggest two periods of equilibration, at 226–245 and 312–368 my. These age determinations support the conclusion drawn from regional petrologic studies (Bocchio et al., 1980;Crespi et al., 1980), that the south-Alpine basement underwent two main phases of metamorphism, now preferentially preserved in different areas: an old, Hercynian phase under a regime of intermediate pressure (staur-ky type), and a young, post-Hercynian phase under a regime of low pressure (staur-sill type) reactivated by the cover of the thick sedimentary pile on top of the basement. Furthermore, radiometric ages also point out the widespread but irregularly distributed occurence of an alpine overprint under the conditions of a very low metamorphic regime, that is documented by the growth of post-kinematic stilpnomelane in rocks of suitable bulk composition (Crespi et al., 1981, 1982), as well as by fine-grained phengite.

The age and emplacement of obducted oceanic crust in the Urals from Sm [sbnd]Nd and Rb [sbnd]Sr systematics

The Urals contain a 2000 km belt of mafic-ultramafic bodies. The Sm Nd and Rb Sr systematics of two of these bodies, the Kempersai Massif in the South Ural Mountains and the Voykar-Syninsky Ophiolite Complex in the Polar Ural Mountains have been examined. These data confirm the hypothesis that these bodies represent fragments of pre-collision oceanic crust and establish constraints on the nature and timing of events in the Uralian Orogeny. Two Kempersai gabbros define Sm Nd internal isochrons of 397 ± 20My and 396 ± 33My with ε Nd(T) = +8.7 ∓ 0.6 and +8.4 ∓ 1.3, respectively. Whole rock samples of pillow basalt, diabase, gabbros, troctolite, and a metasediment give Sm Nd values which lie on this isochron indicating that these rocks are genetically related and have an igneous crystallization age of 397 My. Whole rock samples of Voykar-Syninsky diabase, gabbros, and clinopyroxenite give Sm Nd values which lie on or within ∼ 1 ε-unit of this isochron indicating an age and ε Nd(T) virtually identical to those of Kempersai. ε Nd(T) for the Kempersai and Voykar-Syninsky mafic samples range from +7.3 to +9.0 with an average value of +8.4. This indicates that the Urals ophiolites are derived from an ancient depleted mantle source and are most plausibly pieces of the oceanic crust and lithosphere. The fact that a metasediment has the same ε Nd(397 My) as the other samples indicates derivation from an oceanic source with negligible continental input. ε Nd(T) for the massifs is ∼ 1.5 ε-units lower than the average for modern MORBs. This may be due to the differential evolution of the MORB source over the past 397 My and in conjunction with data for other ophiolites and Mesozoic MORB suggests that over the past 750 My the source for MORB has evolved at a rate less than or equal to its rate of evolution averaged over the age of the earth. Initial 87Sr 86Sr ratios are highly variable ranging from ε Sr(T) = −25.2 for a gabbro to +70.3 for a highly serpentinized harzburgite. This reflects the effects of seawater alteration which is particularly strong on ultrabasic rocks. We conclude that the long belt of mafic-ultramafic rocks in the Urals, which includes the Kempersai and Voykar-Syninsky Massifs, represents segments of Siluro-Devonian oceanic crust. Our igneous age for Kempersai in conjunction with other age constraints suggest that these segments of oceanic crust formed at least 80 My before the collision that produced the Urals.

Isotope analysis of crystalline impact melt rocks from Apollo 16 stations 11 and 13, North Ray Crater

Rubidium‐strontium and Sm‐Nd isotopic data were obtained for lunar impact melt rocks from Apollo 16 stations 11 and 13 as part of the North Ray Crater Target Rock Consortium study. Rubidium‐strontium whole‐rock data define an isochron age, T, = 3.69 ± 0.16 b.y. [λ = 0.0142 (b.y.)−1] and initial 87Sr/86Sr, ISr, = 0.69918±5 for feldspathic microporphyritic (FM) samples. Internal Rb‐Sr isochrons of T = 3.86 ± 0.05 b.y., ISr = 0.69952 ± 10 and T = 3.76 ± 0.04 b.y., ISr = 0.69916 ± 4 were determined for very high alumina (VHA) melt rock 67747 and anorthositic noritic melt rock (ANMR) 67559, respectively. (T,ISr) for these and other lunar melt rocks were examined to infer the nature of the protolith for the rocks. ISr values for the AN MR and FM rocks are consistent with an anorthositic protolith, probably a mixture of nonpristine “anorthositic gabbros” and pristine anorthosites, anorthositic norites, norites, troctolites, and possibly dunites. The source protolith of the VHA rocks apparently had a significant KREEP component and the protolith for Apollo 16 KREEP must have had nearly the same Rb/Sr ratio as that of Apollo 14 KREEP. Two‐stage model calculations using the (T,ISr) parameters suggest that some of the melt rocks were open Rb‐Sr systems during impact melting. Rubidium‐strontium model ages for the FM and VHA rocks show a smoothly monotonic decrease with increasing Rb content. Model ages of the ANMR are uniformly low relative to 4.56 b.y. It is suggested that these effects are due to formation of these rocks by impact partial melting. Rubidium‐strontium model ages of the KREEP rocks do not vary with Rb content, suggesting that they were closed Rb‐Sr systems during impact melting. Samarium‐neodymium data for mineral separates of 67747 showed only a small variation in Sm/Nd ratio and yield an imprecisely defined isochron of 3.6 ± 0.4 b.y. Plagioclase separates of 67559 appear not to have reached Nd isotopic equilibrium during impact melting; consequently, the internal isochron age is also imprecisely defined at 4.0 ± 0.4 b.y. ϵNd values for the melt rocks are about ‐1 to ‐2 at their Rb‐Sr or 40Ar‐39Ar ages, are consistent with the protolith compositions inferred from the Rb‐Sr data, and show that lunar crustal Sm‐Nd evolution departed from chondritic evolution early in lunar history. The average chondritic uniform reservoir (CHUR) model age of those ANMR and FM rocks that apparently remained closed Rb‐Sr and Sm‐Nd systems during impact melting is 4.37 ± 0.10 b.y., consistent with earlier suggestions that the lunar crust formed at about that time. The average CHUR model age of two VHA rocks is 4.10 ± 0.15 b.y., suggesting that these rocks were open Sm‐Nd systems as well as open Rb‐Sr systems during impact (partial?) melting. Current hypotheses for the site geology, stratigraphy, and radiometric age data lead us to suggest that the Nectaris basin and the Descartes formation were formed about 4.1 b.y. ago, and the Imbrium basin and the Cayley formation were formed about 3.81–3.86 b.y. ago. The ages of some individual melt rocks probably date “local” cratering events, however.

Chronology and petrogenesis of a 1.8 g lunar granitic clast:14321,1062

A study was undertaken to determine the chronology of a pristine granite clast (1062) from Apollo 14 breccia 14321 using Rb-Sr, Sm-Nd and 39Ar- 40Ar methods. The genesis of the granite as constrained by the isotopic results and trace element characteristics is discussed. Chronology: The Rb-Sr internal isochron is slightly disturbed and yields an age of 4.09 ± 0.11 AE ( λ( 87Rb) = 0.0139 AE −1 ) and an imprecise initial I( Sr) = 0.702 ∓ .008. If two data are excluded, the age becomes 4.13 ± 0.03 AE and I( Sr) = 0.698 ∓ .003. The whole rock and mineral separates are extremely radiogenic; they yield model ages which are relatively well-defined. The average model age is 4.12 ± 0.03 AE (relative to BABI = 0.69898). The Sm-Nd internal isochron is also slightly disturbed and gives an age of 4.11 ± 0.20 AE ( λ( 147Sm) = 0.00654 AE −1). The 39Ar- 40Ar average age of the non-magnetic fraction of the sample yields a slightly younger age of 3.88 ± 0.03 AE (K-Ar constants from Steiger and >a ̈ , 1977). The concordancy of Rb-Sr and Sm-Nd internal isochrons with the Rb-Sr model age strongly suggests that the granitic clast formed at 4.1 AE ago in the shallow crust and was later excavated and brecciated about 3.88 AE ago. Petrogenesis: Isotopic and trace element data of the lunar granite show large K/La and Rb/Sr fractionations, small Sm/Nd fractionation and the distinct V-shaped REE distribution pattern at the time of crystallization. A two-stage model involving crystal fractionation followed by silicate liquid immiscibility (SLI) is proposed for lunar granite genesis. We propose that the granite can be the immiscible acidic liquid produced by SLI from a residual liquid which underwent fractionation of ca, 3% of phases with REE distribution coefficients similar to those of phosphate minerals from a highly evolved parental magma with REE contents about twice those of the 15405,85 quartz monzodiorite (QMD). The extreme scarcity of lunar granitic samples and their young formation ages suggest that they are probably not directly crystallized from the differentiation of the primordial magma ocean. Our isotopic results and trace elements data from other workers suggest that granites, QMD and probably Mggabbronorites may be genetically related and may have formed in a plutonic environment similar to gabbro-granophyre associations in terrestrial layered intrusions such as the Skaergaard Intrusions.