G.R. Tilton

Timing of collision of the Sino-Korean and Yangtse cratons: U-Pb zircon dating of coesite-bearing eclogites

The suture zone between the Sino-Korean and Yangtse cratons in central China is marked by a series of fault-bounded belts composed of crustal protoliths. In the Dabie Mountains some of these blocks contain coesite and diamond, indicating subduction to minimum depths of ∼120 km as a result of continental collision. The blocks range from the structurally lowest ultrahigh-pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest, low greenschist facies belt. Furthermore, the faults mark large contrasts in metamorphic grade. These features form a structure that is similar to metamorphic core complexes, indicating extension as part of the exhumation process. Timing of the continental collision has previously been inferred as Late Triassic, from regional geologic relations, or Caledonian, from 40 Ar/ 39 Ar dating of the adjacent and possibly related Qinling metamorphic belt. U-Pb dating of zircon from ultra-high-pressure eclogites yields a metamorphic age of 209 ±2 Ma. This age coincides with later stages of collision and is therefore a minimum estimate of the time of collision.

SrNdPb isotopic evolution of Gran Canaria: Evidence for shallow enriched mantle beneath the Canary Islands

We report the Sr, Nd and Pb isotopic compositions (1) of 66 lava flows and dikes spanning the circa 15 Myr subaerial volcanic history of Gran Canaria and (2) of five Miocene through Cretaceous sediment samples from DSDP site 397, located 100 km south of Gran Canaria. The isotope ratios of the Gran Canaria samples vary for 87Sr/86Sr: 0.70302–0.70346, for 143Nd/144Nd: 0.51275–0.51298, and for 206Pb/204Pb: 18.76–20.01. The Miocene and the Pliocene-Recent volcanics form distinct trends on isotope correlation diagrams. The most SiO2-undersaturated volcanics from each group have the least radiogenic Sr and most radiogenic Pb, whereas evolved volcanics from each group have the most radiogenic Sr and least radiogenic Pb. In the Pliocene-Recent group, the most undersaturated basalts also have the most radiogenic Nd, and the evolved volcanics have the least radiogenic Nd. The most SiO2-saturated basalts have intermediate compositions within each age group. Although the two age groups have overlapping Sr and Nd isotope ratios, the Pliocene-Recent volcanics have less radiogenic Pb than the Miocene volcanics. At least four components are required to explain the isotope systematics of Gran Canaria by mixing. There is no evidence for crustal contamination in any of the volcanics. The most undersaturated Miocene volcanics fall within the field for the two youngest and westernmost Canary Islands in all isotope correlation diagrams and thus appear to have the most plume-like (high 238U/204Pb) HIMU-like composition. During the Pliocene-Recent epochs, the plume was located to the west of Gran Canaria. The isotopic composition of the most undersaturated Pliocene-Recent volcanics may reflect entrainment of asthenospheric material (with a depleted mantle (DM)-like composition), as plume material was transported through the upper asthenosphere to the base of the lithosphere beneath Gran Canaria. The shift in isotopic composition with increasing SiO2-saturation in the basalts and degree of differentiation for all volcanics is interpreted to reflect assimilation of enriched mantle (EM1 and EM2) (cf. [1]) in the lithosphere beneath Gran Canaria. This enriched mantle may have been derived from the continental lithospheric mantle beneath the West African Craton by thermal erosion or delamination during rifting of Pangaea. This study suggests that the enriched mantle components (EM1 and EM2) may be stored in the shallow mantle, whereas the HIMU component may have a deeper origin.

Isotopic lead ages of chondritic meteorites

Abstract The isotopic composition of lead was determined for two carbonaceous, two H, and two L chondrites. All are falls. The206Pb/204Pb ratios cover a range from 9.45 to 37.33; the207Pb/204Pb ratios range from 10.39 to 26.10. The isotopic data define a207Pb/206Pb age of 4.635 AE. Uranium and lead concentration data indicate that the isotopic lead ages for the Bruderheim chondrite are concordant within approximately 20%. This contrasts with lead data in the literature for chondrites, which consistently indicate discordant isotopic lead ages due to large excesses of radiogenic lead by factors of two or more. The isotopic lead ages for Pultusk may be concordant; those for a sample of Richardton are not. The lack of concordance for Richardton is not due to the analytical procedures, rather it is likely a result of the handling history of the chondrite prior to analysis. The L-3 chondrite, Mezo-Madaras contains such a high concentration of lead - 5.27 PPM - that accurate ratios for primordial lead can be obtained. These are 206Pb/204Pb= 9.310;207Pb/204Pb= 10.296, values which are in close agreement with a recently reported measurement on lead in troilite from the Canyon Diablo iron meteorite.

Large volume recycling of oceanic lithosphere over short time scales: geochemical constraints from the Caribbean Large Igneous Province

Oceanic flood basalts are poorly understood, short-term expressions of highly increased heat flux and mass flow within the convecting mantle. The uniqueness of the Caribbean Large Igneous Province (CLIP, 92–74 Ma) with respect to other Cretaceous oceanic plateaus is its extensive sub-aerial exposures, providing an excellent basis to investigate the temporal and compositional relationships within a starting plume head. We present major element, trace element and initial Sr–Nd–Pb isotope composition of 40 extrusive rocks from the Caribbean Plateau, including onland sections in Costa Rica, Colombia and Curacao as well as DSDP Sites in the Central Caribbean. Even though the lavas were erupted over an area of ∼3×106 km2, the majority have strikingly uniform incompatible element patterns (La/Yb=0.96±0.16, n=64 out of 79 samples, 2σ) and initial Nd–Pb isotopic compositions (e.g. 143Nd/144Ndin=0.51291±3, ϵNdi=7.3±0.6, 206Pb/204Pbin=18.86±0.12, n=54 out of 66, 2σ). Lavas with endmember compositions have only been sampled at the DSDP Sites, Gorgona Island (Colombia) and the 65–60 Ma accreted Quepos and Osa igneous complexes (Costa Rica) of the subsequent hotspot track. Despite the relatively uniform composition of most lavas, linear correlations exist between isotope ratios and between isotope and highly incompatible trace element ratios. The Sr–Nd–Pb isotope and trace element signatures of the chemically enriched lavas are compatible with derivation from recycled oceanic crust, while the depleted lavas are derived from a highly residual source. This source could represent either oceanic lithospheric mantle left after ocean crust formation or gabbros with interlayered ultramafic cumulates of the lower oceanic crust. High 3He/4He in olivines of enriched picrites at Quepos are ∼12 times higher than the atmospheric ratio suggesting that the enriched component may have once resided in the lower mantle. Evaluation of the Sm–Nd and U–Pb isotope systematics on isochron diagrams suggests that the age of separation of enriched and depleted components from the depleted MORB source mantle could have been ≤500 Ma before CLIP formation and interpreted to reflect the recycling time of the CLIP source. Mantle plume heads may provide a mechanism for transporting large volumes of possibly young recycled oceanic lithosphere residing in the lower mantle back into the shallow MORB source mantle.

ISOTOPIC COMPOSITION AND DISTRIBUTION OF LEAD, URANIUM, AND THORIUM IN A PRECAMBRIAN GRANITE

The isotopic compositions and concentrations of lead and uranium have been determined in some separated minerals and the composite of a granite from Monmouth township, Haliburton County, Ontario. The chemical and mass spectrometric methods that were used are described. The age of the zircon from the granite is 1050 million years. Much of the lead, uranium, and thorium exists in chemically unstable and presumably interstitial phases of the granite. A comparison of the observed amounts of uranium, thorium, and lead in the various minerals with those amounts that should have been present, had these three elements existed within the minerals as closed systems, shows a non-balance of these elements in every case. It appears that the granite as a whole has closely approximated a closed system since it was formed with respect to uranium and its decay products, but has been an open system with respect to thorium and its decay products. Interpretations concerning the relationship of these data to lead ores are discussed.

Isotopic lead investigations on the Allende carbonaceous chondrite

Abstract Uranium and lead concentrations and the isotopic compositions of lead were determined on samples of total rock, matrix, white inclusion, pink inclusion, white aggregate and four chondrules from the Allende carbonaceous chondrite. Observed 206Pb/204Pb ratios varied from 10.004 to 107.29; 207Pb/204Pb ratios from 10.695 to 69.07; 206Pb/204Pb ratios from 30.062 to 207.96. In a 207Pb/204Pb-206Pb/204Pb diagram a regression line fitted to all of data has a slope of 0.6240 ± 0.0015, corresponding to a single stage model age of 4.565 ± 0.004 AE. The regression line also includes the ratios for primordial lead as determined in previous investigations from Canyon Diablo troilite and the Mezo-Madaras chondrite. Although the lead in the matrix is not very radiogenic, the 207Pb/206Pb ages of four samples average 4.505 AE, a value 0.06 AE younger than that of the chondrules and inclusions. The matrix age agrees closely with a total rock Pb/Pb model age previously reported for Allende by Tatsumoto, Knight and Allegre. The matrix Pb/Pb model age is also 0.06 AE younger than the Pb/Pb isochron ages determined by previous investigators on total samples of H and L chondrites. The H and L chondrite and Allende chondrule and inclusion Pb/Pb ages are indistinguishable. The lead isotope systematics require either that the matrix is ca. 0.06 younger than the silicate inclusions and chondrules (or that radiogenic lead was inherited from a younger external source) or that the initial lead in the matrix differed from primordial lead. The lead data cannot be reconciled to a model in which the bulk material of Allende first crystallized 4.57 AE ago, followed by transfer of radiogenic lead between phases since that time. In a concordia diagram four chondrules and three inclusions plot along a chord intersecting concordia at 4.57 and 0.28 ± 0.07 AE. This indicates disturbance of the U-Pb systems relatively recently, perhaps around 0.3 AE ago. The time of disturbance is not readily understood and needs further confirmation. It correlates most closely with a possible cut-off in K-Ar and U, Th-He ages of chondrites. Although the Th/U ratios of the bulk samples and matrix are around the normal value of 3.8, much higher values are observed in some of the inclusions, the highest being 9.0.

Evolution of depleted mantle: The lead perspective☆

Abstract Isotopic data have established that, compared to estimated bulk earth abundances, the sources of oceanic basaltic lavas have been depleted in large ion lithophile elements for at least several billions of years. Various data on the Tertiary-Mesozoic Gorgona komatiite and Cretaceous Oka carbonatite show that those rocks also sample depleted mantle sources. This information is used by analogy to compare Pb isotopic data from 2.6 billion year old komatiite and carbonatite from the Suomussalmi belt of eastern Finland and Munro Township, Ontario that are with associated granitic rocks and ores that should contain marked crustal components. Within experimental error no differences are detected in the isotopic composition of initial Pb in either of the rock suites. These observations agree closely with Sr and Nd data from other laboratories showing that depleted mantle could not have originated in those areas more than a few tenths of billions of years before the rocks were emplaced. On a world-wide basis the Pb isotope data are consistent with production of depleted mantle by continuous differentiation processes acting over approximately the past 3 billion years. The data show that Pb evolution is more complex than the simpler models derived from the Rb-Sr and Sm-Nd systems. The nature of the complexity is still poorly understood.

Pb-Sr-Nd isotopic behavior of deeply subducted crustal rocks from the Dora Maira Massif, Western Alps, Italy

Abstract Pb, Nd and Sr isotope data are reported on mineral separates and whole rock samples of Mg-rich metapelites and associated rocks that have been subducted to depths of ca . 100 km in the mantle within the last 100 Ma and returned to the surface, and for which metamorphic conditions were estimated to be 700–800°C at > 28 kilobars pressure. Three rock types, fine- and coarse-grained pyrope quartzites and NaFe-rich jadeite-kyanite quartz layers, were collected from a single outcrop in the coesite-bearing portion of the Dora Maira massif. Samples analyzed include pyrope, pseudomorphs after pyrope, phengite, zircon, the new mineral ellenbergerite, and whole rocks. Isotope data from all three decay systems indicate minimal, if any, isotopic exchange with mantle rocks. Isotope ratios from minerals with low parent/daughter ratios cluster around 143 Nd/ 144 Nd = 0.51225: ϵ (Nd) = −7.6; 87 Sr/ 87 /Sr = 0.7 = 15.75–15.80 with 206 Pb/ 204 Pb = 19.4–20.7. These compare closely with ratios from Precambrian granitic rocks or their derived sediments. From the Pb isotope data and the Sm/Nd whole rock model ages we estimate a mean age of 2000 ± 500 Ma for the sediment protoliths. Pyrope mineral separates record approximate ages for Alpine metamorphism of 34–38 Ma by the Sm/Nd method; the U/Pb method applied to zircon and ellenbergerite yields possible additional metamorphic ages of 50–55 Ma. These data illustrate a possible mechanism for creating isotopically enriched mantle. Had these rocks yielded their isotopes to the mantle, “enriched mantle” could have resulted, the magnitude of the enrichment depending upon the total volume attained by the mixing process. Similarly the metasediments could also provide the high potassium abundances and appropriate Pb, Sr and Nd isotope ratios that are postulated to accompany mantle metasomatic processes.

SR-ND-PB ISOTOPE RELATIONSHIPS IN LATE ARCHEAN CARBONATITES AND ALKALINE COMPLEXES : APPLICATIONS TO THE GEOCHEMICAL EVOLUTION OF ARCHEAN MANTLE

Isotopic studies of strontium, neodymium, and lead in young carbonatites from several continents have shown that the data generally plot within the fields of ocean island basalts in isotope correlation diagrams. These and other data suggest that the magmas originate in mantle sources and generally pass through continental crust with minimal contamination. Further studies have shown that initial isotopic ratios in carbonatites and alkaline complex rocks from the Canadian Shield spanning an age range from 0.1 to 2.7 Ga appear to trace the evolution of “depleted” subcontinental mantle over the past 2.7 Ga. However, the data were based upon carbonatites between 0 and 1.9 Ga, and substituted syenites at 2.7 Ga due to lack of known carbonatites of that age. This raised the question of possible crustal contamination in the syenites, particularly for lead. Recently two carbonatite bodies with ages of 2.68 Ga have been identified at the Lac Shortt mine and Dolodau dykes in south-central Quebec. Lead, strontium, and neodymium isotope data from these carbonatites fit the evolution patterns already established from the syenites, with slightly more radiogenic neodymium and slightly less radiogenic strontium and lead. The initial ϵ(Nd) from eight carbonatite samples is + 2.8 ± 0.3; ϵ(Sr) = −0.3 ± 0.3 (87Sr86Sr = 0.70127 ± 0.00002). Lead ratios from combined carbonatites and syenites define a regression line with a slope of 0.71 ± 0.04 in a 207Pb-206Pb correlation diagram, corresponding to radiogenic lead evolved in a closed system between ca. 3.8 and 2.7 Ga; however, we ascribe this trend to mixing of mantle components rather than to crustal contamination processes. Data from young (<0.2 Ga) carbonatites from five continents closely fit a model corresponding to mixing between EM1 and HIMU mantle components in isotope correlation diagrams. Similar diagrams for the Canadian Shield 2.7 Ga samples exhibit no clear mixing trends, and 87Sr86Sr-206Pb204Pb ratios tend to cluster closely around model “bulk silicate Earth” in an isotope correlation diagram. Mantle differentiation processes appear to have changed in fundamental ways around 3 Ga ago, with most sialic continental crust produced after that time.

Applications of lead and strontium isotopic relationships to the petrogenesis of granitoid rocks, central Sierra Nevada batholith, California

Pb isotope data are given on 80 whole-rock and/or feldspar samples from 51 dated samples covering a traverse across the central Sierra Nevada batholith, largely between latitudes 36° and 37°N. In addition, Sr isotope data were measured on 30 whole-rock samples. No correlation is observed between age and isotope ratios. In agreement with previously published data, 87 Sr/ 86 Sr ratios are low (∼0.704) along the western margin and increase to high values (>0.707) at the eastern margin. 206 Pb/ 204 Pb ratios increase from ∼18.6 at the western margin to ∼19.5 in the middle of the batholith, then decrease regularly to ∼18.5 toward the eastern margin. 2O8 Pb/ 204 Pb ratios mirror the 206 Pb/ 204 Pb data, but do not exhibit the decrease in values toward the eastern margin. Pb and Sr isotopic data from the western-margin plutons plot within or close to the field of primitive island-arc magmas, indicating predominantly mantle sources. The high 206 Pb/ 204 Pb ratios in the central batholith suggest assimilation of sediments derived from Precambrian basement rocks. The decrease in 206 Pb/ 204 Pb ratios in the eastern segment of the batholith without corresponding reduction in 208 Pb/ 204 Pb ratios is attributed to assimilation of lower crustal granulitic/amphibolitic rocks by mafic magmas. Initial Pb ratios for samples with 87 Sr/ 86 Sr > 0.706 from the central and eastern segments of the batholith indicate components from recycled crustal rocks with a mean 207 Pb/ 206 Pb age of ca. 1.8 Ga. Nd and O isotopic data from the literature are consistent with the Pb-Sr model obtained in this study. Very similar isotope patterns to the present model are reported across the Salinian block.