Model Age Calculations Research Articles

Initial Hf isotope compositions in magmatic zircon from early Proterozoic rocks from the Gawler Craton, Australia: A test for zircon model ages

We evaluate the validity of age information obtained from two-stage Hf model ages in zircon. Multiple isotope investigations of minerals from the Kiana Granite of the Archean Sleafordian Complex from the southern Gawler Craton, Australia, reveal a complex geologic evolution of the suite. Rubidium–Sr mineral ages define a tectono-thermal event at 1673.4 ± 4.2 Ma, associated with the Kimban orogeny. ID-TIMS U–Pb ages from single zircons yield an upper concordia intercept at 2348 ± 21 Ma, which marks the time of magmatic emplacement. Whole-rock Nd model ages yield crustal residence ages of ∼ 2.8 Ga. For each single zircon grain, the Lu–Hf isotope systematic was determined by solution MC-ICPMS. Two-stage Hf model ages for the magmatic zircons vary by as much as 500 Myrs, depending on the choice of Lu/Hf for the source of the host rock, putting further emphasis on the need to constrain the source Lu/Hf of a rock used for model age calculations. An average Lu/Hf for mafic crust was assumed for the source reservoir of the rocks to calculate its Hf isotope evolution through time, which yields zircon model ages of ∼ 3.5 Ga that coincide with the initial growth episode postulated for the Gondwana supercontinent. It is demonstrated that two-stage zircon model ages deduced from initial Hf isotopes can provide important age information for the source of igneous rocks, and consequently on important crustal formation events, even when conventional geochronologic investigations fail, e.g., due to thermo-tectonic overprints or mixing.

How precise are U-series coral ages?

U-series dating of fossil reef corals is a well established and widely applied technique in paleoclimate research. Many fossil corals, however, show evidence for post-depositional diagenetic alteration, and it is generally accepted that the accuracy of U-series coral ages is more limited due to coral diagenesis than analytical precision. In recent years, three models have been published that try to correct the effects of diagenesis and allow the calculation of model ages [Thompson W. G., Spiegelmann M. W., Goldstein S. L., and Speed R. C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters 210, 365–381; Villemant B., and Feuillet N. (2003) Dating open systems by the 238U– 234U– 230Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters 210, 105–118; Scholz D., Mangini A., and Felis T. (2004) U-series dating of diagenetically altered fossil reef corals. Earth and Planetary Science Letters 218, 163–178]. Here, we assess the age variability of both conventional 230Th/U-dating and the three models by application to different sub-samples of individual coral specimens. The age variability, estimated as the 2 σ-standard deviation on the individual ages, is compared with the errors quoted by the different methods. Our results show that the errors of conventional 230Th/U-dating as well as those of the method of Thompson et al. (2003) do not account for the true age variability. The age variability of both methods is in the range of the errors given by the models of Villemant and Feuillet (2003) and Scholz et al. (2004). Furthermore, we show that the widely used reliability criteria are not sufficient to identify all diagenetically altered corals. In contrast, analysis of different sub-samples of one coral specimen allows (i) to estimate the real age variability, (ii) to test if the assumptions of the models are fulfilled, and (iii) to investigate the diagenetic processes in more detail. Thus, this method should generally be applied to obtain more reliable U-series coral ages and errors.

Rhenium-Osmium Geochronology of Arsenopyrite in Meguma Group Gold Deposits, Meguma Terrane, Nova Scotia, Canada: Evidence for Multiple Gold-Mineralizing Events

Rhenium-osmium geochronology using arsenopyrite was undertaken for three gold deposits in the Meguma terrane, Nova Scotia, Canada, in order to better constrain their age of formation and to assess the utility of arsenopyrite for dating similar deposits globally. Analyses of arsenopyrite from bedding-concordant veins from The Ovens locality, southwest Meguma terrane, yield a precise Re-Os isochron age of 409 ± 5 Ma. Arsenopyrite analyses from a bedding-discordant vein at this locality indicate an identical Re-Os age of 407 ± 4 Ma. Saddle-reef veins from the Dufferin deposit, northeast Meguma terrane, contain arsenopyrite with a precise Re-Os isochron age of 380 ± 3 Ma. The lack of common Os in some arsenopyrite samples from both The Ovens and Dufferin permit calculation of single mineral model ages for each deposit, which are identical to those determined using the isochron method. Initial Os compositions for the two vein types at The Ovens suggest a predominately crustal source of Os in the mineralizing fluids, whereas a less radiogenic initial Os composition for arsenopyrite from Dufferin does not as clearly define a crustal metal source. At a third locality, the Touquoy deposit, the Re-Os systematics of arsenopyrite associated with disseminated gold mineralization do not define a precise formation age, possibly as a result of mixing of Re and Os derived from the mineralizing fluid and the shale host rock. The Re-Os ages of arsenopyrite indicate that there were at least two distinct periods of gold deposition in the Meguma terrane coinciding with widespread tectonothermal events: regional deformation and metamorphism associated with Acadian orogenesis, and widespread generation of meta- and peraluminous granites and high-grade metamorphism within the basement rocks under the Meguma terrane. The ca. 407 Ma age for The Ovens arsenopyrite provides the best estimate for the timing of regional Acadian deformation in the Meguma terrane and is slightly older than previous estimates based solely on 40Ar/39Ar dating.

Ancient thermal anomaly of an active fault system: Zircon fission‐track evidence from Nojima GSJ 750 m borehole samples

To understand better heat generation and transfer along faults, fission track (FT) thermochronologic analysis was performed on zircon separates from Cretaceous granitic rocks around the Nojima fault, activated during the 1995 Kobe earthquake. Samples were collected from the Geological Survey of Japan (GSJ) 750 m borehole and nearby outcrop. Zircon FTs showed reduced mean lengths and ages for samples less than ∼10 m away from the fault at depth. This is interpreted as representing an ancient thermal anomaly by heat transfer or dispersion via fluids at the deep crustal interior because the degree of FT annealing is correlated to that of deformation/alteration of borehole rocks. From extrapolation of data in the mean FT age‐length plot as well as from model age calculation, it is suggested that last cooling took place at ∼30–40 Ma.

Re–Os systematics and platinum-group element distribution in metalliferous sediments from the Troodos ophiolite

Umbers from 8 of 11 localities studied yield 187Os/ 188Os ratios that range from 0.51 to 0.57. We interpret this range as reflecting the Os isotopic composition of mid-Cretaceous seawater. 187Os/ 188Os measured in samples from the remaining three localities fall above and below this range and are believed to result from post-depositional redistribution of Os. Extremely large 187Os/ 188Os ratios occur in hydrothermal sulfides (>200) and in their oxidation products, ochres (3.6–13.2). Measured Re/Os ratios in these samples require open system behavior. Model age calculations indicate that ochre formation occurred over a 1–5 million year interval after sulfide precipitation. In most samples >99% Re loss occurred during sulfide oxidation. Comparison of element ratios in Cyprus umbers to those of modern seawater suggest the following sequence of scavenging efficiency by hydrothermal Fe-oxides: Ir>Pt>Os>Re. Cyprus sulfides exhibit the opposite pattern of element enrichment. These patterns of aqueous platinum-group element (PGE) fractionation are potentially relevant to study of recycled Os in mantle-derived rocks because they give rise to the following phenomena: (1) ephemeral Re enrichment within the basal sediments of the oceanic crust, (2) PGE concentrations in metalliferous sediments (Os up to 1 ppb and Pt up to 22 ppb) that are comparable to those in the mantle, and (3) Pt/Re ratios in Cyprus umbers that exceed those required to explain the coupled 186Os– 187Os variations in Hawaiian basalts.

Nd isotopic evolution of the upper mantle during the Precambrian: models, data and the uncertainty of both

This paper presents a dual approach to resolve the terrestrial evolution of the Sm–Nd system: a critically reviewed compilation of Precambrian Nd initial isotope ratios and, independently, a forward modelling effort of the Nd isotopic development of the upper mantle. The transport balance model used is identical to a previously published one which allows to reconcile calculated and observed parameters of the Th–U–Pb system. In order to express compositional and age heterogeneity, the continental crust is divided into four domains: upper and lower as well as younger and older. The mantle is represented by an upper and a lower reservoir. Model versions successful for Th–U–Pb and Sm–Nd produce a very narrow range of ϵ Nd development curves for the upper mantle. All these scenarios show very little recycling of continental crust into the upper mantle before 2–1.6 Ga ago. The data as well as the model show a steady increase of the ϵ Nd[ T] value of the upper mantle from ca 3 Ga on to a present day value of +10. In the Early Archean, a quasi constant ϵ Nd[ T] Nd value is seen. The model predicts that the upper mantle was not significantly different from bulk silicate Earth (BSE) in this early period. The data, however are generally one ϵ Nd unit above the accepted Chondritic Uniform Reservoir (CHUR) evolution. Lunar rocks also show this offset and, in particular, appear to indicate its existence since >4.5 Ga ago. This implies that the initial BSE and bulk silicate Moon are apparently slightly different from the conventional initial CHUR. Possible explanations for this apparent offset are some uncertainty of the CHUR model, analytical artefacts and interlaboratory bias. Published deviations from the accepted CHUR values and the effects caused by different fractionation correction procedures are all in the same direction, and make an apparent offset of ca +1 ϵ Nd unit of terrestrial and lunar data relative to CHUR plausible. Applying an apparent BSE evolution with an initial at one ϵ Nd unit above the CHUR model brings terrestrial modelling of the upper mantle and data into good alignment. As the model describes internally fully homogeneous reservoirs, a scatter of real data around the model development curves is expected as a result of imperfect internal mixing. A third order polynomial fit to ϵ Nd evolution curves generated by the model for the depleted upper mantle is given as a new reference for Nd model age calculations, via the relation: ϵ Nd[ T] sample=0.164 T 3−0.566 T 2−2.79 T+10.4.

The Deep Cratonic Mantle Roots Beneath the Canadian Craton: Mantle Xenolith Evidence from Somerset Island Kimberlites

The nature and origin of the mantle roots beneath the Canadian craton are currently poorly constrained. The recently discovered Cretaceous Nikos kimberlite (Pell, 1993) on Somerset Island, in the Canadian Arctic, consists of three individual pipes and hosts an unusually well preserved suite of large (10-20 cm) mantle xenoliths dominated by garnet-peridotite (lherzolite, harzburgite, dunite) showing coarse and porphyroclastic textures, and minor occurrences of garnet-pyroxenite. The bulk compositions and mineral data for fifty-four Nikos xenoliths indicate their highly refractory nature with high olivine abundances (avg. 80 wt.%) and olivine forsterite contents (avg. Fo = 92.3). These compositions are characteristic of peridotite xenolith suites derived from the mantle underlying Archaean cratons such as the Kaapvaal and Siberian cratons (avg. olivine Fo = 92.5; Boyd et al., 1997), but are clearly distinct from the trend defined by oceanic peridotites (avg. olivine Fo = 91.0; Boyd, 1989) and spinel-peridotites from the Canadian Cordillera (Shi et al., in press) that represent the mantle beneath accreted post-Archaean continental terranes along the western margin of the Canadian shield. The Nikos xenoliths yield pressures and temperalures of last equilibration between 20 to 55 kb and 650 to 1300~ and a number of the peridotite nodules appear to have equilibrated in the diamond stability field. The pressure and temperature data define a conductive palaeogeotherm corresponding to a surface heat flow of 44 mW/m 2. Peridotite xenoliths from the central Slave province of the Canadian craton require a lower surface heat flow (-40 mW/m2; Boyd and Canil, 1997) indicating a cooler geothermal regime than that beneath the Canadian Arctic. Many of the kimberlite-hosted peridotites from the Kaapvaal craton in South Africa and parts of the Siberian craton are characterized by high orthopyroxene contents (avg. Kaapvaal 32 wt.%, Siberia 20 wt.%; Boyd et al., 1997). The Nikos peridotites show modal mineral assemblages with moderate to low orthopyroxene contents (avg. 12 wt.%), indicating that the orthopyroxene-rich mineralogy characteristic of the Kaapvaal and Siberian cratons are not present in the subcratonic mantle beneath Somerset Island, and suggesting that the Nikos xenoliths formed by extensive partial melting of primitive mantle compositions. The isotope results obtained to date suggest that the cratonic mantle beneath Somerset Island is heterogeneous and that the pyroxenites, lherzolites, and harzburgites may represent isotopically distinct mantle reservoirs. The Pb isotope data for the Nikos peridotites show that the harzburgites have more radiogenic Pb isotopic compositions than the lherzolites. One pyroxenite sample, in particular, gives a very unradiogenic Pb isotope composition (2~176 = 17.015, 2~176 = 15.349, 2~176 = 37.285) indicative of involvement of an old, most probably Archaean mantle component. Nd and Sr isotope results for the Nikos xenoliths (143Nd/144Nd = 0.51257-0.51285; S7Sr/S6Sr = 0.70412-0.70969) are similar to data previously reported for gamet-peridotites world wide, with the lherzolites plotting at the enriched end of the data array. Nd model age calculations for the Nikos xenoliths indicate ages between 420 and 800 Ma. A Lu-Hf isochron using garnet and clinopyroxene separates from one pyroxenite sample yields an age of 680 Ma, similar to the Nd model ages. The Hf and Nd isotope results may reflect formation ages, but more likely represent mixing between an old Archaean mantle reservoir and a younger metasomatic event possibly associated with kimberlite generation.

A Re—Os isotope study of ultramafic xenoliths from the Matsoku kimberlite

Ultramafic xenoliths erupted within the Matsoku kimberlite pipe display a range of petrographic features that have been interpreted as the products of melt injection or infiltration and metasomatic processes. We have analysed Re and Os concentrations and the Os isotope composition of these samples with the aim of investigating how such processes affect the Re—Os system. In particular, we have focused on the relationships between: (1) veins, dykes and sheets, that are thought to record melt injection or infiltration processes; (2) the coarse peridotite wallrocks to the veins, dykes and sheets, which usually do not show modal metasomatism but have been affected by low-volume melt infiltration; and (3) common coarse peridoties which show little, if any, evidence of modal metasomatism or melt infiltration. Peridotite samples (i.e., groups 2 and 3 above) all display low 187Os/ 188Os and thus mimic the signature that is now well established for the sub-continental mantle lithosphere. We demonstrate a relationship between Os isotope composition and modal mineralogy that suggests a link between Re depletion and extraction of a basaltic component to establish the refractory compositions of lithospheric mantle samples. Model age calculations suggest a mean age of melt extraction older than 2200 Ma. The development of ilmenite-rutile-phlogopite-sulphide metasomatic assemblages does not appear to affect strongly Re/Os. By contrast, pyroxenite dykes have substantially higher 187Os/ 188Os which is broadly supported by elevated Re/Os. High Re/Os is consistent with the pyroxenites having formed by precipitation from melts intruded into the lithospheric mantle. Such melts, as is observed for the dykes, would also be expected to carry a low Sm/Nd signature which with time would develop negative ε Nd . Wallrock peridotites and other rocks affected only by low-volume melt migration also show low Sm/Nd but with accompanying low Re/Os, thereby demonstrating the potential for the generation of the low 143Nd/ 144Nd, low 187Os/ 188Os signature which appears to be characteristic of sub-continental lithospheric mantle.

Re-Mo-Cu-Os sulphide from the Ekojoki Ni-Cu deposit, SW Finland

This short communication reports the discovery and composition of the first Os-bearing rhenium sulphide. It occurs as a single euhedral crystal in the early Proterozoic Ekojoki Ni-Cu deposit in Finland, being enclosed by interstitial magmatic Fe-Ni sulphides. Electron microprobe analyses are most consistent with a general formula of Cu(Re,Os,Mo)5S9. The Re-Os model age calculations indicate that approximately 2/3 osmium is "common" Os that was accommodated by the sulphide at the time of crystallisation, 1/3 being produced via post-crystallisation decay of187,Re to18 7Os. We propose that this sulphide crystallised from an immiscible sulphide liquid that was unusually rich in rhenium due to extensive assimilation of black schist sulphides by the magma.

Erosion sources determined by inversion of major and trace element ratios and strontium isotopic ratios in river water: The Congo Basin case

Dissolved and suspended load river material represents the integrated products of the erosion of drainage basins. To enlarge the study of erosion processes we have determined 87Sr/ 86Sr ratios and the Cl, Na, Mg, Ca and Sr contents for the main tributaries of the Congo River Basin, both for water and suspended sediment. We have also analyzed 30 streams draining monolithological terranes. A systematic study of precipitation has permitted the estimation of a good rain correction factor. Sr isotopic ratios have shown that the seawater input correction based on riverine Cl content is not valid in the Congo Basin because a large part of the Sr, Ca and Mg come from a terrestrial source. The conventional atmospheric input correction by reference to the marine ratios underestimates the real atmospheric input because of the crustal elements carried by rainwaters. Different erosion source parameters have been obtained for carbonates, evaporites and silicates. An inversion scheme has been developed to compute the multimixing equations and allows the quantification of the input of each main reservoir (atmosphere, carbonates, evaporites and silicates) for each tributary and each element. For Ca and Mg, rainfall and carbonate dissolution are the main inputs. For Sr, the input is mainly controlled by rains and silicate weathering. By using Sr isotopic systematics we have calculated the Sr isotopic composition of the silicate weathered crust for each of the main tributaries of the Congo Basin. We obtain uniform values for the main tributaries ranging between 87Sr/ 86Sr= 0.7195 ± 0.001 and 0.7251 ± 0.005. These results allow the calculation of strontium model ages T Sr, which differ from neodymium model ages T Nd. Using T Nd, we have calculated the 87Rb/ 86Sr of the silicate weathered crust. We obtain homogeneous values close to 0.75, which is in agreement with estimates for the average silicate crust. The discrepancy between T Sr and T Nd may be linked to the vegetation impact which fractionates Rb and Sr.

Remarks on depleted mantle evolution models used for Nd model age calculation

This paper reviews published models on Nd isotopic evolution of the upper mantle, and focusses in particular on the problems of models currently in use for Nd model age (T Nd ) calculation and should help to prevent misleading T Nd interpretation. Presently, T Nd calculations are generally based on depleted mantle (DM) evolutionary models (T DM ), implying that continental crust is generated from suboceanic, convecting mantle. However, the properties of the secular evolution curve of the DM, are rather controversial. Depending on the models used, differences between calculated T DM ages for a given sample may be as much as a few hundred millions years

ESR isochron dating for teeth: A brief demonstration in solving the external dose calculation problem

The radiation sensitive ESR signal at g = 2.0018 in fossil enamel can be used to date teeth absolutely, providing a suitable U uptake model can be selected, and the external dose rate, D ext can be accurately calculated. Apart from the uptake model, the most obvious uncertainty lies in D ext. With the isochron method, D ext can be derived easily during the ESR analysis. By plotting the AD vs. the total internal dose rate for teeth where multiple subsamples have been analyzed, a regression line, the isochron, can be determined for each model age calculation. Each isochron slope represents the sample age, while its y-intercept gives the total external dose, A ext. In the preliminary test, the isochron method was used to date two teeth from Bau de l'Aubesier, Provençe. Both isochron calculations produced a good estimate for the total external dose, A ext. Although the external dose rate, D ext, so derived is now uptake-model dependent, this method should revolutionize ESR dating.

Intracrustal REE fractionation and implications for SmNd model age calculations in late-stage granitic rocks: An example from central Brazil

The light rare-earth elements (LREE) Sm and Nd are generally regarded as behaving coherently during intracrustal processes such as weathering, metamorphism, partial melting and fractional crystallization. SmNd model ages calculated with respect to a depleted mantle reservoir ( tDM) are often interpreted by assuming that Sm and Nd are not substantially fractionated within the crust. In consequence, SmNd model ages are commonly considered as crust formation ages which see through crustal reworking episodes and indicate the age of extraction of continental material from the mantle. However, major-element and REE data on late Proterozoic K-rich granitic rocks from southwestern Goiás (central Brazil) reveal major fractionation of Sm and Nd with differentiation of the granitic suites. Late-stage differentiates have high Sm/Nd ratios and yield Sm-Nd model ages ( tDM) which do not reflect differences in source rock ages. Microprobe studies on accessory minerals from two granitic plutons (Israelândia and Iporá granites) show that a significant proportion of their total REE is carried by allanite, which is relatively depleted in Sm compared to the other four LREE analyzed (La, Ce, Pr and Nd). Crystallization of a strongly LREE-enriched phase such as allanite results in the progressive depletion of the residual magma in LREE and a consequent increase of Sm/Nd ratios and tDM- values with progressive differentiation. Progressive allanite segregation can produce a large range of Sm-Nd model ages, unrelated to the age of crust formation.

An unusual mantle source region for nephelinites from the Oslo Rift, Norway

We have analyzed the trace-element and isotopic composition of a series of lavas and their constituent pyroxenes and calcite from near the town of Skien in the Permo-Carboniferous Oslo Rift. The series decreases in alkalinity upsection: the lowermost lavas are nephelinites, followed by basanites and finally alkali basalts. This sequence is accompanied by decreasing light rare earth element (LREE), Th, Nb and U concentrations. Nd isotopic change is strongly correlated with the petrologic and chemical evolution. The nephelinites are characterized by ϵ Nd (300 Ma) of 1.0 to 1.6, the basanites 1.9 to 2.2 and the alkali basalts 2.4 to 3.9. ϵ sr (300 Ma) ranges from −10 to −15 and shows little change from one group to the next. Calcite segregations in the basaltic rocks show δ 13C = −2.82‰ , δ 18O = 7.7 to 9.6‰ , ϵ sr = −6.5 to −9.9 and ϵ Nd = 0 to +2, values which are compatible with formation from magmatic CO 2 exsolved from a mantle-derived silicate melt. The Sr isotope values are unradiogenic relative to Nd for the nephelinites, causing these samples to fall to the lefthand side of most mantle-derived rocks on a ϵ Nd vs. ϵ Sr diagram. They are similar to rocks from “high μ” (HIMU) sources and the recently proposed “LoNd array” ( Hart et al., 1986). The alkali basalts, with their higher ϵ Nd , lie close to the mantle array for 300 Ma ago and resemble tholeiitic magmatism in the Oslo Rift. We interpret these trends as representing evolution in which the earliest magmas were derived from a source with HIMU-LoNd characteristics, and successive magmas gradually acquired the more dominant mantle isotopic composition. Model age calculations permit either 1) that the nephelinite source was originally depleted then subsequently re-enriched in LREE or 2) that the nephelinite source was primitive with respect to Nd but had undergone Rb depletion. The nearby Fen carbonatite complex is about 250 Ma older than Skien and its ϵ Nd is slightly more positive. A direct relationship between Fen and Skien would favor the first hypothesis.

Nd and Sr isotope systematics of clastic metasediments from Isua, West Greenland: Identification of pre‐3.8 Ga Differentiated Crustal Components

Recent field work on the Isua metasediments has established that there are two lithological sequences (A and B), which are divided into a number of distinct formations. Each clastic metasedimentary unit has a relatively unique trace element signature. The depleted mantle Sm‐Nd model ages of samples from all major units range from 3.59 to 3.98 Ga, whereas Rb‐Sr model ages cover a very wide range (1.9–4.6 Ga). The samples from the sequence A felsic gneiss formation (A6) yield a relatively good Rb‐Sr isochron age of T = 3.71±0.07 Ga. The sequence B samples do not yield a Rb‐Sr isochron; however, some of the samples plot close to the A6 Rb‐Sr isochron, and sequences A and B appear to have essentially the same deposition age. The Sm‐Nd data for both sequences yield initial εNd values in the range −1 to +3 for a deposition age TSTRAT = 3.64 Ga (if TSTRAT = 3.81 Ga, the range is 0–4). This range appears to reflect real variability of initial εNd values corresponding to the time of deposition. Data trends in εNd; versus 1/Nd diagrams suggest at least three distinct components in each of sequences A and B. Model age calculations suggest that some of these components must be derived from continental crustal sources that predate the time of deposition of these sediments by as much as ≈0.4 b.y. Other components appear to be derived from volcanic sources that are roughly coeval with sediment deposition. The very high initial εNd values implied for early depleted mantle sources (+2 to +4), suggest a very large pre‐3.8 Ga continental crust. Alternatively, these high values could reflect an early terrestrial mantle with highly fractionated cumulate layers that formed in a ≈4.5 Ga magma ocean.

UPb and RbSr geochronology and geological evolution of the Harts Range ruby mine area of the Arunta Inlier, central Australia

Supracrustal and meta-igneous rock units from the ruby mine area of the Harts Range, eastern Arunta Inlier, central Australia, have been dated by the zircon UPb and RbSr total-rock (TR) and mica methods. A well-defined zircon discordia for a weakly deformed specimen of the Bruna granite gneiss yields an age of emplacement of 1748 −4 +5 Ma, thereby constraining the minimum age of the Irindina supracrustal assemblage. Metapelitic gneiss within the supracrustals and a meta-igneous ultramafic boudin from the associated Harts Range meta-igneous complex yield highly discordant zircon data, revealing a strong early Palaeozoic overprint. RbSr TR data from anorthositic gneisses associated with the ultramafic boudin are highly disturbed, also apparently during the lower Palaeozoic. However, RbSr model age calculations and the zircon UPb data suggest a maximum age of about 2000 Ma for the supracrustal and meta-igneous rocks, and argue for new Proterozoic crust formation. Zircon UPb data from a deformed pegmatite, emplaced in the meta-igneous complex, yield an emplacement age of 520 −4 +5 Ma, further pointing to Lower Palaeozoic magmatism and deformation. Correlations of U content and calculated 206 Pb 238 U age for the ultramafic boudin zircons suggest that new growth of low-U zircons occurred during retrogression associated with this event. The Sr-isotope systematics of the anorthositic gneisses can also be interpreted in terms of introduction of Palaeozoic Sr. Our data suggest lower Palaeozoic (possibly Delamerian) tectonothermal activity to be more important in the evolution of the Harts Range area than previously recognised. On the other hand, RbSr mica ages for deformed and undeformed pegmatites, and TR isochrons for the latter, show that pervasive tectonothernal activity had ceased by about 315 Ma and that regional cooling occurred between about 345 and 325 Ma. Local shear-zone biotite resetting may have persisted to about 300 Ma, consistent with the previously recognised Alice Springs Orogeny. Possible dilational Pb loss in the Bruna zircons occurred at about 103 Ma.

Time and duration of lunar highlands crust formation

All radiometric systems indicate that crust-mantle differentiation on the moon is dominated by events which occurred very early in lunar history. However, due to remaining uncertainties in model parameters and assumptions in the calculation of model ages, it is not yet possible to resolve the precise times of occurrence of these events nor the duration of the formation of the highlands crust. The strongest time constraints are offered by direct radiometric ages of samples formed during this earliest period. Two possible candidates for this material, norites 78236 and 73255,27,45, were examined utilizing the Sm-Nd radiometric system. Sm-Nd systematics of 78236 show post-crystallization disturbance but indicate that this norite crystallized in the lunar crust about 4.34 AE ago. Data for 73255,27,45 define an isochron and yield a crystallization age of 4.23 ± 0.05 AE. The initial Nd isotopic composition of both norites is within uncertainty of a “chondritic” reference reservoir at the time of their respective crystallizations. The implications for lunar crustal formation persisting over a time span of close to 350 m.y. are discussed.

An evaluation of the zircon method of isotopic dating in the Southern Arabian Craton

A zircon study has been made on eleven samples of igneous rocks from the Saudi Arabian Craton. Ages of sized and magnetic fractions of zircon concentrates show variable degrees of discordance which seem to result from a very young disturbance that produces linear arrays in the Concordia plot. Model age calculations based on a statistically and geologically reasonable lower intercept produce very consistent internal relationships. The Pan African Orogeny, considered to be responsible for loss of radiogenic argon and strontium from minerals of many rocks, does not appear to have affected the zircon data, even though uplift had exposed the rocks of the Arabian Shield at that time. Tonalite, granodiorite, and crosscutting leucoadamellite bodies in the southern part of the An Nimas Bathylith yield ages in the time range 820−760 Ma. A narrow time range of 660 to 665 million years was indicated for ages of widely separated and compositionally different intrusive bodies all to the east of the An Nimas Bathylith. This work suggests that the younger end of the age spectrum established from regional K-Ar and Rb-Sr measurements may be underestimated, and that magmatic activity could be more episodic than previously assumed.