87Rb87Sr chronology of H chondrites: Constraint and speculations on the early evolution of their parent body

A precise⁸⁷Rb-⁸⁷Sr whole-rock isochron for H chondrites and an internal isochron for Tieschitz (H3) have been determined. The age and⁸⁷Sr/⁸⁶Sr initial ratio of the whole rocks are4.52 ± 0.05 b.y. and0.69876 ± 0.00040(λ(⁸⁷Rb) = 1.42 × 10^?11yr^?1). For Tieschitz, whereas handpicked separates plot on a well-defined line, heavy liquid separates scatter in the⁸⁷Rb/⁸⁶Sr vs.⁸⁷Sr/⁸⁶Sr diagram. Leaching experiments by heavy liquids indicate that they might have a sizeable effect on Tieschitz minerals. The age and⁸⁷Sr/⁸⁶Sr initial ratio as determined by handpicked separates are4.53 ± 0.06 b.y. and0.69880 ± 0.00020, indistinguishable from the whole-rock isochron.These results are interpreted as “primitive isochrons” dating the condensation of chondrites from the solar nebula. The best value of this event is given by joining both isochrons together at4.518 ± 0.026 b.y. and⁸⁷Sr/⁸⁶Sr= 0.69881 ± 0.00016. The near identity of this initial ratio with the one of Allende white inclusions argues in favor of a sharp isochronism of condensation from a⁸⁷Sr/⁸⁶Sr homogeneous nebula. Data from Guaren?a [11] and Richardton [48] are interpreted as secondary internal isochrons, 100 m.y. after the condensation of the whole rocks.The data are then used to constrain a thermal evolution model of the H chondrite parent body. This body might have a 150–175 km radius, and might have been heated by²⁶Al. An²⁶Al/²⁷Al ratio of 4–6 × 10^?6 is enough for heating such a body. Further tests for this model are proposed.     

Evaluation and application of the ion microprobe in the strontium isotope geochemistry of carbonates

A modified AEI-IM20 ion microprobe has been used to measure⁸⁷Sr/⁸⁶Sr ratios in carbonates. A suite of carbonates with varying major elements (Ca, Mg, Fe, Mn) was studied at low and high (M/ΔM ? 3000) mass resolution to determine the types and intensities of molecular species isobaric with Sr peaks; Sr data collected at low mass resolution must be corrected for Ca₂ and CaMgO species. Rb/Sr ratios are extremely low, and correction for⁸⁷Rb is not required (< 0.1‰ of⁸⁷Sr).Usable Sr isotopic data may be obtained from calcite givenSr≥ 400ppm, and for Sr > 5000 ppm a precision of ～ ± 1‰ (± 0.0007) in⁸⁷Sr/⁸⁶Sr (2σ mean) can be achieved under optimum conditions. The corrections for Ca₂ and CaMgO are smaller than the within-run precision in calcite, but in dolomite the correction for CaMgO is + 1.5%. Mass fractionation corrections to⁸⁷Sr/⁸⁶Sr (based on⁸⁶Sr/⁸⁸Sr= 0.1194) are typically +8 to +10‰. Good agreement between ion probe and solid source mass spectrometer results was found for calcites of known Sr isotopic composition: ST4a (～ 400ppm Sr), average ion probe⁸⁷Sr/⁸⁶Sr= 0.7267 ± 0.0015, solid source⁸⁷Sr/⁸⁶Sr= 0.72680 ± 0.00008 [14]; JCG36 (～ 6000ppm Sr), average ion probe⁸⁷Sr/⁸⁶Sr= 0.7056 ± 0.0009, solid source⁸⁷Sr/⁸⁶Sr= 0.70588 ± 0.00009 [16]; JCG44 (～ 6000ppm Sr), average ion probe⁸⁷Sr/⁸⁶Sr= 0.7057 ± 0.0006, solid source⁸⁷Sr/⁸⁶Sr= 0.70540 ± 0.00008 [16]. The ability of the ion microprobe to measure⁸⁷Sr/⁸⁶Sr for 10-μm spots in calcite was used: (1) to measure variation in⁸⁷Sr/⁸⁶Sr of ～ 0.01 on a centimetre scale in a hydrothermally altered basalt from the Isle of Skye, northwestern Scotland; and (2) to determine the Sr isotopic composition of tiny (< 35 μm) calcite grains in a veined mantle lherzolite from Bultfontein, South Africa. Because of calcite's ubiquitous occurrence in many parageneses this technique may offer many opportunities for the measurement of fine scale heterogeneities in⁸⁷Sr/⁸⁶Sr.     

An early precambrian age for migmatitic gneisses from Vikan i Bø, Vestera˚len,North Norway

Migmatitic gneisses of overall intermediate composition occur at Vikan i Bø, North Norway. A Pb-Pb whole-rock isochron on these Vikan Gneisses yields an age of3,460 ± 70m.y. A Rb-Sr whole-rock isochron on the same rock samples yields an age of2,300 ± 150m.y., and an initial⁸⁷Sr/⁸⁶Sr ratio of0.7126 ± 0.0011.The geochemical implications of the discordance of the Pb-Pb and Rb-Sr whole-rock ages are discussed, and a timetable of the geological events in the early history of Lofoten-Vestera?len is proposed.     

Inherited isotope systems and the source region pre-history of early Caledonian granites in the Dalradian Series of Scotland

RbSr and UPb isotope analyses are reported for two pre-metamorphic Caledonian granites which intrude Dalradian rocks in the Central Highlands of Scotland. These data indicate that the origin of the granitic magmas involved partial fusion of old crustal material.UPb systems of zircon size and magnetic fractions from the Ben Vuirich granite are strongly discordant. However, U/Pb isotopic ratios precisely define a chord which intersects concordia at 514_?7⁺⁶ m.y. and 1316_?25⁺²⁶ m.y. Geological constraints suggest that the lower intersection records the post-F₂, pre-M₃ emplacement age of the granite. The upper intersection reflects the presence of old zircon xenocrysts incorporated into the granite magma without complete isotopic resetting. The ultimate source of these xenocrysts is probably a metamorphic basement complex which formed about 1320 m.y. ago, but the immediate source region of the granites could have been Dalradian sediments derived therefrom.RbSr whole-rock systems of the Ben Vuirich granite are also strongly discordant, although 8 out of 13 data points scatter about an “errorchron” of 564 ± 24 m.y. with an initial⁸⁷Sr/⁸⁶Sr ratio of about 0.716. This is interpreted as a spurious result due to incomplete homogenization of Sr isotopes in the source region during partial fusion. Initial⁸⁷Sr/⁸⁶Sr ratios at the time of emplacement indicated by the zircon data ranged from 0.7173 to 0.7191. Whole-rock samples from the Dunfallandy Hill granite have Rb/Sr ratios 2–3 times higher than those from Ben Vuirich and define a reasonably good isochron age of 491 ± 15 m.y. with an initial⁸⁷Sr/⁸⁶Sr of 0.7185 ± 0.0008. This may date granite emplacement or subsequent resetting of the high Rb/Sr rocks during Caledonian metamorphism. RbSr systematics indicate that the crustal source regions of these and other Caledonian granites separated from the upper mantle at least ca. 800 m.y. ago and probably ca. 1300 m.y. ago, thus confirming the interpretation of the upper intersection age of the zircon UPb data.     

Application of age and Sr isotope data to the petrogenesis of the Marangudzi ring complex,Rhodesia

The Marangudzi ring complex, Rhodesia, consists essentially of a gabbro mass intruded by ring dykes of quartz syenite and cone sheets of nepheline syenite. Five intrusive units (gabbro, two quartz syenite and two nepheline syenite units) have been studied using Rb-Sr and K-Ar methods. A total of 24 whole rock samples define a Rb-Sr isochron which gives an age of 186 ± 3m.y. and an initial (⁸⁷Sr/⁸⁶Sr)₀ ratio of 0.70769 ± 0.00006 (±2sigma; based on λ = 1.42 × 10^?11yr^?1). K-Ar and Rb-Sr analyses on biotite and hastingsite separates are consistent with this age assignment. Whole rock Rb-Sr isochrons for the different units treated individually agree with the above age and initial Sr ratio within analytical uncertainties. This is believed to indicate that the different rock types are comagmatic forming by fractional crystallization of a parental, mantle-derived, K₂O-rich basaltic magma, having an initial Sr ratio of 0.7077, without appreciable assimilation of the Precambrian country rock. The entire differentiation, emplacement and crystallization processes took place over a rather short time span.     

87Rb-86Sr age of rocks from the Apollo 15 landing site and significance of internal isochrons

Internal isochrons for two Apollo 15 rocks give an age of(3.34 ± 0.09)and(3.46 ± 0.04) × 10⁹ years with an identical⁸⁷Sr/⁸⁶Sr initial ratio of 0.69928. Considering the possibility for the line obtained in a⁸⁷Sr/⁸⁶Sr,⁸⁷Rb/⁸⁶Sr diagram to be a mixing line, the significance of these results are discussed.     

87Rb87Sr dating of LL chondrites

⁸⁷Rb⁸⁷Sr analyses of LL chondrites have been made in 10 whole rock meteorites, chondrules from Chainpur (LL3) and Soko Banja (LL4), density separates and chondrules from Guidder (LL5) and density separates from Jelica (LL6) and Ensisheim (LL6). Whole rocks define an isochron of age 4.486±0.020 Ga (λ⁸⁷Rb=1.42×10^?11a^?1) and initial ratio (⁸⁷Sr/⁸⁶Sr)_I=0.69887±0.00012. This is in agreement with the results for H- and E-type chondrites. Analyses for chondrules from Soko Banja yield a very good isochron of age 4.452±0.020 Ga and strontium initial ratio 0.69954±0.00024, and give an interval for metamorphism of (37±10)×10⁶ a. A more poorly defined isochron is obtained for Jelica; the age is 4.423±0.041 Ga and the strontium initial ratio 0.69959±0.00029, indicating an interval for metamorphism of (70±60)×10⁶ a. No isochron could be obtained for Chainpur. This could be due to terrestrial alteration or to a late isotopic disturbance of the meteorite. The⁸⁷Rb-⁸⁷Sr system is also disturbed in Guidder and Ensisheim, probably as a consequence of shock. These results are discussed in comparison with our former studies, and in relation with thermal metamorphism in the LL chondrite parent body(ies).     

Chronology and chemical history of the parent body of basaltic achondrites studied by the 87Rb-87Sr method

Total-rock ⁸⁷Rb-⁸⁷Sr measurements have been carried out on ten eucrites. Internal “isochrons” were determined for Ibitira, Bereba, Pasamonte, Stannern, Sioux County and Juvinas. Ibitira as well as Juvinas (previously published) give an age of 4.52 ± 0.25 b.y. The corresponding “ages” of Bereba and Sioux County are 4.17 ± 0.26 b.y. and 4.19 ± 0.14 b.y. Stannern and Pasamonte have maximum “ages” of 3.1 and 2.6 b.y., respectively. These last four “ages” are significantly lower than 4.55 b.y.Using the total rocks and the two primitive eucrites Ibitira and Juvinas, we define as the best differentiation age for eucrites 4.57 ± 0.13 b.y. which is better than any previous determination, and (⁸⁷Sr/⁸⁶Sr)_BABI = 0.69899 ± 0.00004. We discuss the early differentiation of the basaltic achondrite parent body by assuming that the parent body started with chondritic composition and had a two-step evolution during the very early history of the solar system.     

87Rb-87Sr chronology of enstatite meteorites

A⁸⁷Rb-⁸⁷Sr analysis of some enstatite meteorites has been made. Whole rocks plot on an isochron of age 4.508 ± 0.037b.y. and strontium initial ratio 0.69880 ± 0.00044 (2σ errors; λ⁸⁷Rb= 1.42 × 10^?11yr^?1) . If the Norton County results are joined, we get an age of 4.516 ± 0.029b.y. and initial ratio of 0.69874 ± 0.00022. This result is indistinguishable from the whole rock isochron for H chondrites. It is interpreted as the age of condensation from the solar nebula. The identity of the⁸⁷Sr/⁸⁶Sr initial ratio with the ones for Allende white inclusions shows that this ratio was homogeneous in the solar nebula, and that the Rb-Sr fractionations observed between the different chondrite groups appeared only shortly before or during condensation accretion.Internal studies of the type-I enstatite chondrites Abee and Indarch and the intermediate-type Saint Mark's and Saint Sauveur have been done.Abee data scatter in the⁸⁷Rb-⁸⁷Sr diagram. For Indarch, Saint Mark's and Saint Sauveur, we obtained well-defined straight lines of “age” (T) and “initial ratio” (I): Indarch,T = 4.393 ± 0.043b.y.I = 0.7005 ± 0.0009; Saint Mark's,T = 4.335 ± 0.050b.y.I = 0.69979 ± 0.00022; Saint Sauveur,T = 4.457 ± 0.047b.y.I = 0.6993 ± 0.0014. Our result on Indarch agrees with the former result of Gopalan and Wetherill [5].A careful examination of the data shows that these straight lines are neither due to leaching effects by heavy liquids, nor result from terrestrial weathering. The “isochrons” for Indarch and Saint Sauveur can be mixing lines between enstatite and feldspar. The results are interpreted in terms of cosmochemical secondary effects: type-I and intermediate-type enstatite chondrites have been shocked 60–200 m.y. after their formation. This agrees with the idea of an early generalized bombardment of the inner solar system; this also indicates that type-I enstatite chondrites were rather situated in the outershells of their parent body and might be at the origin of the scatter of I-Xe ages of enstatite meteorites.Whole rock and enstatite from Bishopville, Cumberland Falls and Mayo Belwa have also been analysed. In these three aubrites, the⁸⁷Rb-⁸⁷Sr system is perturbed. Our Bishopsville sample might not be fresh and this makes the significance of our results uncertain. Cumberland Falls and Mayo Belwa probably suffered relatively recent shocks and open-system redistribution of Rb and Sr.     

87Rb–87Sr history of the Norton County enstatite achondrite

⁸⁷Rb⁸⁷Sr analysis of the Norton County achondrite has been achieved with special attention to the rubidium analysis. Enstatite crystals and polycrystalline material give an “age” of 4.48 ± 0.04 × 10⁹ years and an initial ratio ⁸⁷Sr/⁸⁶Sr_I= 0.7005 ± 0.0004 (λ = 1.39 × 10^?11yr^?1, maximum errors). The feldspar component of the meteorite contains about 70% of the strontium and 30% of the rubidium of the whole sample, and does not lie on the isochron. Its model age relative to the strontium initial ratio of Allende is 4.6 × 10⁹ years. The data are consistent with a complex history dealing with an incomplete isotopic reequilibration of the meteorite, 120 m.y. after its formation at 4.6 × 10⁹ years, with an initial ratio similar to that of Allende.     

Rb–Sr isochron ages of the Cretaceous granitoids in the Ryoke belt, Kinki district, Southwest Japan

Abstract Granitoids are widely distributed in the Ryoke belt and have been divided into four main igneous stages based on their field setting. In this paper, we present Rb–Sr isochron ages for the younger Ryoke granitoids (second stage to fourth stage) in the Kinki district. The Yagyu granite (second stage) gave a Rb–Sr whole‐rock isochron age of 74.6 ± 10.9 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70938 ± 0.00016, and a Rb–Sr mineral isochron age of 71.8 ± 0.1 Ma. The Narukawa granite (second stage) yielded a Rb–Sr mineral isochron age of 79.5 ± 0.4 Ma. A Rb–Sr whole‐rock isochron age of 78.3 ± 3.0 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70764 ± 0.00014 was obtained for the Takijiri adamellite (third stage). The Katsuragi quartzdiorite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 85.1 ± 18.3 Ma (initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70728 ± 0.00006), and mineral isochron ages of 76.9 ± 0.5 Ma and 74.8 ± 0.5 Ma. The Minamikawachi granite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 72.8 ± 2.0 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70891 ± 0.00021. These age data indicate that the igneous activity in younger Ryoke granitoids of Kinki district occurred between 80 and 70 Ma, except for the Katsuragi quartz diorite. The isotopic data on the various igneous stages in Kinki district correspond with the relative timing from field observations. Based on the initial ⁸⁷Sr/⁸⁶Sr ratios, the granitoids of the Ryoke belt in Kinki district are spatially divided into two groups. One is granitoids with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.707–0.708, distributed in the southern part of the Ryoke belt. The other is granitoids with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.708–0.710 distributed in the northern part of the Ryoke belt. The initial ⁸⁷Sr/⁸⁶Sr ratios of granitoids increase with decreasing (becoming younger) Rb–Sr whole‐rock isochron ages.     

RbSr radiometric age of late Precambrian fossil-bearing and associated rocks from Sinai

Six samples of metamorphic rocks from three essentially coeval stratigraphic units, two of which contain Precambrian microfossils, have been analyzed by the Rb-Sr whole-rock radiometric method. Least-squares regression of the data yields an isochron date of 934 ± 80m.y. with initial ⁸⁷Sr/⁸⁶Sr= 0.7007 ± 0.0011. This date may reflect the approximate age of formation of these rocks or, alternately, a time of major metamorphism. Regardless, the date is significant in that it (a) is in agreement with the 900-m.y. date for rocks containing similar types of fossils from Bitter Springs, Australia, and (b) coupled with structural evidence, supports the time equivalence of these rocks with 1000-m.y. old rocks (“Kibaran cycle”) of Saudi Arabia.     

87Rb/87Sr study of diogenites

The five diogenites, Johnstown, Roda, Ellemeet, Shalka and Tatahouine, give scattered data in the⁸⁷Rb/⁸⁶Sr,⁸⁷Sr/⁸⁶Sr diagram. This can result from a disturbance which occurred later than 4.45 Ga ago. However, it is shown that if samples of sufficient size were analyzed, there meteorites could plot on the eucrite isochron and are thereby in agreement with a genetic relation between eucrites, howardites and diogenites. The age of eucrite differentiation from diogenites has been computed using data from the two families yielding an age of 4.47±0.1Ga(2σ) (λ=1.42×10^?11a^?1), the initial⁸⁷Sr/⁸⁶Sr ratio being BABI.     

SmNd isotopic systematics of Enderby Land granulites and evidence for the redistribution of Sm and Nd during metamorphism

Measurements of¹⁴³Nd/¹⁴⁴Nd and¹⁴⁷Sm/¹⁴⁴Nd are reported for whole rocks and mineral separates from granulites of the Napier Complex at Fyfe Hills. Charnockites, leuconorites and gabbros yield a whole rock SmNd isochron age of3060 ± 160m.y. and an initial¹⁴³Nd/¹⁴⁴Nd ratio of0.50776 ± 10 (?_Nd(3060m.y.) = ?2.0 ± 1.8). The negative ?_Nd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of Sm and Nd and partial re-equilibration of¹⁴³Nd/¹⁴⁴Nd ratios. This probably occurred during the upper granulite facies metamorphism which has also been dated at～ 3100m.y. by RbSr and UPb zircon studies [1]. Coexisting clinopyroxene, apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 ± 300 m.y. This array indicates that redistribution of Sm and Nd and re-equilibration of¹⁴³Nd/¹⁴⁴Nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at～ 2450m.y.Due to the resetting of the SmNd system on both whole rock and mineral scales, the primary crystallization age of the igneous protolith is not well constrained by the present data, although it is clearly3100m.y. If it is assumed that the complex was derived initially from a depleted mantle reservoir(?_Nd(T) ? 2), evolution of the negative ?_Nd value of ?2.0 with the observed Sm/Nd ratios requires a prehistory of～ 380m.y. This implies a primary age of～ 3480m.y. However, substantially older primary ages can be inferred if the source reservoirs had?_Nd(T) > 2 and/or substantial reductions in the Sm/Nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m.y. Such an inferred reduction in the Sm/Nd ratio may have been the result of preferential loss of Sm relative to Nd, or introduction of a low Sm/Nd fluid with?_Nd ≥ 0 during granulite facies metamorphism.     

The evolution of early precambrian crustal rocks at Isua,West Greenland — Geochemical and isotopic evidence

Petrographic and chemical evidence suggests that boulders from a conglomeratic unit in the Isua supracrustal succession were derived by the erosion of an acid volcanogenic sediment. Six samples of the boulders and surrounding matrix yield a Rb-Sr whole rock isochron with a slope corresponding to an age of 3860 ± 240 m.y. (2 sigma error), but consideration of the initial⁸⁷Sr/⁸⁶Sr ratio constrains the possible age of formation of 3710 ± 900 m.y. This is in general agreement with a published Pb/Pb age of 3760 ± 70 m.y. on Isua banded ironstones.Pb isotope compositions as well as highly fractionated, heavy element depleted, rare earth element abundance patterns for the boulders suggest that their igneous precursors were derived from a source region with a similar geochemical history to that of some components of the 3700–3800 m.y. old Ami?tsoq gneisses, involving fractionation of garnet during their evolution.A Pb/Pb whole-rock isochron for Ami?tsoq gneisses from Isua yields an age of 3800 ± 120 m.y. (2σ), in good agreement with previously published Rb-Sr age data on the same rocks. The rock leads are highly unradiogenic and demonstrate substantial U depletion at least 3800 ± 120 m.y. ago. A two-stage model for the U-Pb system yields an average²³⁸U/²⁰⁴Pb (μ₁) value of 9.3 ± 0.2 for the source region, which is significantly different from the published value of 9.9 ± 0.1 for the Isua iron formation. This indicates the existence of U-Pb heterogeneities between the source regions of plutonic and supracrustal rocks by about 3700–3800 m.y. ago. Attempts to apply U-Pb whole-rock dating to the Ami?tsoq gneisses were unsuccessful because of geologically recent U loss, possibly due to groundwater leaching.A Rb-Sr whole-rock isochron on a suite of Ami?tsoq gneiss samples from a different locality in the Isua region has yielded an age of 3780 ± 130 m.y.In contrast to the Godthaab area, there is no geochronological evidence at Isua for major rock-producing or tectonothermal events after about 3700 m.y. ago. The entire gneiss-supracrustal system developed within the approximate interval 3900–3700 m.y. ago.     

Age relationships between greenstone belts and “granites” in the Rhodesian Archaean craton

Whole-rock Rb-Sr age measurements on rocks from the Rhodesian Archaean craton of Southern Africa demonstrate that (a) the Mashaba area gneisses, which are typical of the Rhodesian Basement Complex, are approximately 3600 m.y. old, (b) volcanic rocks from the main greenstone belts, assigned to the Bulawayan Group, were extruded approximately 2600–2700 m.y. ago, (c) a cross-cutting pluton, the Sesombi tonalite, was emplaced2690 ± 70 m.y. ago, (d) the Gwenoro Dam migmatites, which field evidence suggests could be older than the main greenstone belts, were emplaced2780 ± 30 m.y. ago.The initial⁸⁷Sr/⁸⁶Sr ratios of most of the rock units of groups b, c and d are in close agreement at about 0.701, suggesting that the later granitic (sensu lato) and andesitic rocks so far analysed were not produced by remelting of, or contamination with, ancient gneissic basement.     

Strontium and oxygen isotopic composition of some basalts from Hole 504B,Costa Rica Rift,DSDP Legs 69 and 70

Seventeen whole-rock samples, generally taken at 25–50 m intervals from 5 to 560 m sub-basement in Hole 504B, drilled in 6.2 m.y. old crust, were analysed for⁸⁷Sr/⁸⁶Sr ratios, Sr and Rb concentrations, and¹⁸O/¹⁶O ratios. Sr isotope ratios for 8 samples from the upper 260 m of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the 330–560 m interval, 5 samples have a restricted range of 0.70255–0.70279, with a mean of 0.70266, the average value for fresh mid-ocean ridge basalts (MORB). In the 260–330 m interval, approximately intermediate Sr isotopic ratios are found.δ¹⁸O values (‰) range from 6.4 to 7.8 in the upper 260 m, 6.2–6.4 in the 270–320 m interval, and 5.8–6.2 in the 320–560 m interval. The values in the upper 260 m are typical for basalts which have undergone low-temperature seawater alteration, whereas the values for the 320–560 m interval correspond to MORB which have experienced essentially no oxygen isotopic alteration.The higher⁸⁷Sr/⁸⁶Sr and¹⁸O/¹⁶O ratios in the upper part of the hole can be interpreted as the result of a greater overall water/rock ratio in the upper part of the Hole 504B crust than in the lower part. Interaction of basalt with seawater(⁸⁷Sr/⁸⁶Sr=0.7091) increased basalt⁸⁷Sr/⁸⁶Sr ratios and produced smectitic alteration products which raised whole-rock δ¹⁸O values. Seawater circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below about 230 m sub-basement. These flows may have helped to seal off lower basalts from through-flowing seawater.     

A neodymium,strontium, and oxygen isotopic study of the Cretaceous Samail ophiolite and implications for the petrogenesis and seawater-hydrothermal alteration of oceanic crust

In the Samail ophiolite,¹⁴⁷Sm-¹⁴³Nd,⁸⁷Rb-⁸⁷Sr, and¹⁸O/¹⁶O isotopic systems have been used to distinguish between sea-floor hydrothermal alteration and primary magmatic isotopic variations. The Rb-Sr and¹⁸O/¹⁶O isotopic systems clearly exhibit sensitivity to hydrothermal interactions with seawater while the Sm-Nd system appears essentially undisturbed. Internal isochrons have been determined by the¹⁴⁷Sm-¹⁴³Nd method using coexisting plagioclase and pyroxene and give crystallization ages of 130 ± 12m.y. from Ibra and 100 ± 20 m.y. from Wadi Fizh. These ages are interpreted as the time of formation of the Samail oceanic crust and are older than the inferred emplacement age of 65–85 m.y. The initial¹⁴³Nd/¹⁴⁴Nd ratios for a tectonized harzburgite, cumulate gabbros, plagiogranite, sheeted dikes and a basalt have a limited range in ε_Nd of from 7.5 to 8.6 for all lithologies, demonstrating a clear oceanic affinity and supporting earlier interpretations based on geologic observations and geochemistry. The⁸⁷Sr/⁸⁶Sr initial ratios on the same rocks have an extremely large range of from 0.70296 to 0.70650 (ε_Sr = ?19.7 to +30.5) and the δ¹⁸O values vary from 2.6 to 12.7. These large variations are clearly consistent with hydrothermal interaction of seawater with the oceanic crust. A model is presented for the closed system exchange of Sr and O, that in principle illustrates how the Sr isotopic data may be utilized to estimate the water/rock ratio and subsequently used to evaluate the temperature of equilibration between the water and silicates from the¹⁸O/¹⁶O water-rock fractionation.     

Rb-Sr whole-rock study of the precambrian rocks of the Pedernal Hills,New Mexico

Rb-Sr whole-rock investigation of the Pedernal Hills, New Mexico, indicates a 1471 ± 97-m.y. date for the granites and 1364 ± 27-m.y. date for the schists and quartzite (country rocks) with initial⁸⁷Sr/⁸⁶Sr = 0.7060 ± 0.0040 and 0.7046 ± 0.0019, respectively. The schists and quartzites are older than the granites based on field and petrographic evidence. The open-system nature of these rocks during the cataclastic metamorphism which affected both the country rocks and the granites is responsible for the apparent younger date of the schists and quartzite, and the 1471 ± 97-m.y. date may be a minimum age for the granites.     

RbSr total rock isotope studies on Precambrian charnockitic gneisses from South Norway: evidence for isochron resetting during a low-grade metamorphic-deformational event

The quartzo-feldspathic charnockitic orthogneisses within the Bamble sector of the so-called Sveconorwegian (1.2–0.9 b.y.) zone are highly fractionated in K and Rb such that they comprise two chemically contrasting zones — one highly K, Rb-deficient and the other with values of the same order as upper crustal lithologies.Eight series of samples, each collected from single outcrops, have yielded Rb-Sr total rock apparent ages in two distinct groups, at ～1540 and ～1060 m.y. Outcrops in both the K-deficient and normal-K suites have produced examples of each age. The older age relates to the high-grade charnockite event, and the younger to a superimposed low-grade event which occurred at the same time as the intrusion of undeformed granite sheets and pegmatite dikes; one of the granites has yielded an isochron age of 1063 ± 20 m.y. The low-grade event involved only the partial alteration of orthopyroxenes to chlorite and/or serpentine, coupled with some corrosion of biotite; the alterations were initiated along narrow, irregularly spaced, cracks and it was their development which facilitated open system behaviour of the total rock isotopic systems at some localities. The degree of rehomogenisation is a function of the intensity of the secondary alterations.Confirmation of resetting at ～1060 m.y. is given by four mineral + host rock isochrons all yielding ages within error of the age for the intrusive granite; two of these are from outcrops where the rocks retain the older ～1540-m.y. age.The secondary total rock isotopic homogenisation cannot be explained adequately by Rb mobility or by simple mixing with a fluid having its own initial⁸⁷Sr/⁸⁶Sr composition. The primary mineralogy may have determined whether individual localities and/or samples suffered net increases or net decreases in⁸⁷Sr/⁸⁶Sr.An important implication of the results is that in this, or any similar geological situation, there would be a very real possibility of drawing erroneous conclusions from regionally-collected samples, particularly if the full significance of the later, relatively minor P-T event remained undetected and/or the scale of isotopic (re-)homogenisation, were unknown. It is only because of the methods adopted that it can be stated that there is no isotopic evidence for a high grade Sveconorwegian (Grenvillian) event in this part of southern Norway.