Lead isotopic compositions of South Sandwich Island volcanic rocks and their bearing on magmagenesis in intra-oceanic island arcs

Pb isotope ratios have been measured in 12 volcanic rocks from the South Sandwich Islands. The ranges are ${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 18.51–18.66;}{}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 15.55–15.64;}{}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 38.42–38.64}$. In ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{-}{}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{-}{}^{206}\text{Pb}{}^{204}\text{Pb}$ correlation diagrams, the South Sandwich data plot distinctly above the fields for ocean ridge basalts, and yield trends showing apparent mixing with a sedimentary end member similar to South Atlantic pelagic sediments as reported by Chow and Patterson (1962) and this study. Armstrong and Cooper (1971) have likewise shown that volcanics from the Lesser Antilles show mixing trends with North Atlantic sediments in Pb isotope correlation diagrams. The North Atlantic sediments have distinctly higher ${}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios compared to the South Atlantic sediments. The parallel relationships between sediments and volcanic island arc rocks of the North and South Atlantic provide strong evidence for a component of Pb from subducted sediments in the lavas of the west Atlantic basin. In contrast to these data, lavas from the Mariana Arc in the western Pacific show little or no component of Pb from pelagic sediments. The reason for the different behaviors in the two settings is speculative.     

Sr and Pb isotopes,U and Th chemistry of the alkaline Monteregian and White Mountain igneous provinces,eastern North America

The Monteregian Hills and younger White Mountain alkaline intrusions were emplaced into the Cambro-Ordovician sediments of the St. Lawrence Lowlands and the folded and thrusted Lower Paleozoic sequence of the Appalachian orogen. Age relations indicate that there is a fine-scale structure to the igneous activity, with slightly undersaturated to critically saturated rocks emplaced between 141 and 128 Ma and strongly undersaturated rocks emplaced between 121 and 117 Ma.Sr and Pb isotopic data for the mantle-derived alkali picrite, alkali olivine basalt and basanite magmas, indicate derivation from a depleted mantle similar to that which produces present-day oceanic island basalts. For the most isotopically primitive samples, decay-corrected ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.7030–0.7037}$, ${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 19.05–19.72}$, ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 15.56–15.65}$, and ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 38.64–39.26}$. On Pb-Sr isotope correlation diagrams the data define trends similar to those for MOR basalts, implying mantle heterogeneity which requires the presence of a component enriched in radiogenic Pb relative to Sr. The interaction of these isotopically primitive magmas with the crust can be defined in terms of a three component system: depleted mantle-Grenville age crust-Lower Paleozoic age crust. The granitic magmas were apparently derived from the Lower Paleozoic crust of the Appalachian orogen.For the mantle-derived magmas, Th/U ratios vary from 2.5 (estimated ratio for MORB source) to 5.1, with the mean value near that of the bulk earth. The variations in Th/U suggest mantle heterogeneity on a local scale, and the high Th/U of some samples suggests that the mantle was enriched in incompatible elements shortly before melting. The magmas derived by partial melting of the crust have Th/U of 3.3 to 8.7, and the higher ratios are associated with rocks crystallized from magmas that originated by melting of Lower Paleozoic sediments.The Sr and Pb isotopic data support the conclusion of Bellet al. (1982) that the subcontinental mantle under eastern Canada underwent a Precambrian depletion event. This depleted mantle apparently extends under the White Mountain province and is isotopically similar to the mantle which gives rise to oceanic island basalts. In contrast, Pb isotopic ratios for the New England Seamount chain (TARAS and HART, 1983), which apparently represents the oceanic extension of this magmatic activity, are significantly more radiogenic. It is possible that a mantle plume provided the heat energy, and perhaps metasomatic fluids, to trigger melting in the subcontinental mantle, whereas in the case of the oceanic extension the plume directly contributed to the observed magmatism, as reflected in the more radiogenic Pb ratios.     

Initial Pb of the Amîtsoq gneiss,West Greenland,and implications for the age of the Earth

Pb isotopic abundances and U-Th-Pb concentrations are reported for feldspar megacrysts from the 3.59 AE old Amîtsoq gneisses, Godthaab District, West Greenland. The distinctive Pb in the feldspars is the most primitive terrestrial Pb so far observed. It is observed in feldspars which are from different geographic localities and which exhibit varying degrees of deformation and recrystallization. This appear to be either the initial Pb in the Amîtsoq gneiss or the initial Pb only slightly modified by subsequent metamorphism in a low ${}^{238}\text{U}{}^{204}\text{Pb}$ environment. ${}^{238}\text{U}{}^{204}\text{Pb}$ in the feldspars is low and the corrections for in situ produced Pb are only 0.4% for ${}^{207}\text{Pb}{}^{206}\text{Pb}$ and 0.6% for ${}^{204}\text{Pb}{}^{206}\text{Pb}$. The mean corrected isotopic abundances are ${}^{204}\text{Pb}{}^{206}\text{Pb}\text{= 0.08720}$, ${}^{207}\text{Pb}{}^{206}\text{Pb}\text{= 1.1513}$, and ${}^{208}\text{Pb}{}^{206}\text{Pb}\text{= 2.7350}$. The feldspars contain a very small amount of easily leachable radiogenic Pb which is correlated with U and which indicates the formation of U-rich phases at about 2.7 AE. The matrix surrounding the feldspar megacrysts contains Pb which is much evolved relative to the megacrysts and this matrix does not appear to have behaved as a simple closed system. Element redistribution and open system behavior at about 2.7 AE is also suggested by Pb in feldspar from a dike cutting across the gneiss. Assuming that the Amîtsoq gneiss feldspar Pb corrected for in situ U decay was the initial Pb in the gneiss at 3.59 AE (Baadsgaard, 1973), a single-stage “age of the earth” is determined as 4.47 ± 0.05 AE and μ is 8.5. This is indistinguishable from the single-stage age for modern rocks and is distinctly younger than the 4.55 AE age of some meteorites. If the feldspar Pb was modified by metamorphism at 2.7 AE the model age of the earth is calculated as 4.53 AE which is similar to the 4.55 AE age of some meteorites. Two-stage models using the nominal 3.59 AE initial Pb indicate that if the earth is ~4.55 AE old then μ values were low in the early Earth's history and differentiation occurred within a few hundred million years after the planet formed.     

UThPb systematics of the eucrite “Juvinas”: Precise age determination and evidence for exotic lead

The U-Th-Pb isotope systematics of the eucrite “Juvinas” have been studied in whole rock fragments as well as in plagioclases and pyroxenes. The results show that this monomict breccia crystallized with a very high $\text{U}\text{Pb}$ initial ratio at T = 4.539 ± 0.004 AE ago. There is evidence for a less radiogenic Pb component (${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 13.0}$; ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 13.5}$; ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 32.71}$) interpreted as “exotic lead” induced by a meteoritical impact at the surface of the Juvinas parent body, 1.92 ± 0.06 AE ago.     

A detailed Pb isotopic study of crustal contamination/assimilation: The Edgecumbe volcanic field,SE Alaska

To better understand the process of crustal contamination/assimilation, 23 Pb isotopic compositions and 12 concentrations have been measured on lavas and basement rocks from the Edgecumbe volcanic field, SE Alaska. Measured isotopic ratios have the following ranges: ${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 18.477–19.161}$; ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 15.562–15.679}$; ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 38.17–38.85}$. While the data form well-constrained linear arrays on Pb-Pb diagrams, no simple correlation exists with major element composition. Basaltic lavas (≤ 51 wt% SiO₂) are characterized by two isotopic groups. The olivine basalt (≤ 48% SiO₂) is more radiogenic than the plagioclase basalt (48–51%) which also shows more heterogeneity. In the silica range 52–55%, Pb isotopic ratios increase significantly but remain fairly constant in the range 55–70% SiO₂. Lead concentrations vary from 1 ppm in the basalts to 7 ppm in the rhyodacites. Analyzed basement rocks are more radiogenic than any of the lavas $\text{(}{}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 19.20}$; ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 15.65}$; ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 38.86}$. The Pb isotopic data are qualitatively consistent with the contamination process described by Myerset al. (1984). However, because of fundamental differences in the mixing relations between the Sr system studied earlier and the Pb system, the new Pb data have revealed details of the process not apparent from the Sr data alone. In particular, it has been shown that the parent magma was more primitive than originally assumed, and that two contamination events are recorded in the lavas. The first event, involving a mafic parent and different crustal contaminants, produced the intermediate and siliceous hybrids in cupolas located above the main basaltic chamber. The types of country rock intruded as well as the degree of partial fusion achieved in individual cupolas controlled the range of hybrid compositions produced while the eruption sequence was determined by the order in which the cupolas were tapped. The second contamination event produced the plagioclase basalt, the most voluminous basaltic unit, by mixing the mafic parent with the olivine basalt, an independent, primary magma. Our results suggest crustal contamination models that assume bulk assimilation of crustal end members may be too simplistic.     

Propagation of experimental errors in lead isotope ratio measurements using the double spike method

Computer simulation of random errors in the measurements for the double spike technique applied to Pb isotope analyses, indicates that normalized error multiplication factors for the calculated values of isotope ratios lie in the range 2 to 3. The optimum isotopic composition of the tracer is about ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 2.0}$ and the optimum ratio of amount of tracer to amount of unknown is about P = 1.0. Determination of errors in the fractionation factor alone does not adequately describe the ultimate errors in the calculated isotope ratios except under special circumstances. Uncertainty in the actual composition of the tracer is closely proportional to uncertainty in the unknown composition. The purity of the spike is unimportant at least as long as the separated ²⁰⁷Pb and ²⁰⁴Pb isotopes are greater than 90% pure.     

Pb isotope geochemistry of a massif-type anorthositic-charnockitic body: The Hidra Massif (Rogaland,S.W. Norway).

The Hidra Massif (Rogaland complex, S.W. Norway) is a massif-type anorthositic-charnockitic body. It consists of undeformed anorthosites and leuconorites, grading into fine-grained jotunites at the contact with the granulite facies gneisses of the metamorphic envelope. A stockwork of charnockitic dykes cross-cuts the massif. The Pb isotopic compositions of the anorthosites and leuconorites are comparable or slightly less radiogenic than those of the jotunites (${}^{206}\text{Pb}{}^{204}\text{Pb}$ from 18.079 to 19.307,(${}^{207}\text{Pb}{}^{204}\text{Pb}$ from 15.568 to 15.657 and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ from 37.617 to 38.493). These values are compatible with an upper mantle origin for the parental magma of jotunitic composition and for the plagioclasic cumulates, but show the incorporation of lower crustal material (U-depleted and thus less radiogenic). The charnockitic dykes have significantly less radiogenic Pb isotopic compositions (${}^{206}\text{Pb}{}^{204}\text{Pb}$ from 17.472 to 19.171, ${}^{207}\text{Pb}{}^{204}\text{Pb}$ from 15.489 to 15.620and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ from 36.991 to 40.922) which can be explained by a larger proportion of lower crustal contamination material. The contaminant could be the granulite facies gneisses of the metamorphic envelope. This interpretation is compatible with the K-Rb relationships of these rocks and with the O and Sr isotopic geochemistry.The proportion of contaminating lead in the charnockitic dykes can be estimated at 55 ± 15% considering the border facies jotunite as the uncontaminated parental magma and the least radiogenic gneiss of the metamorphic envelope as the contaminant.     

Rb-Sr and U-Th-Pb systematics of alkaline rocks: the alkaline rocks from Italy

The isotopic composition of Pb and Sr and the abundances of Rb, Sr, U, Th, and Pb were determined for whole rock samples from all major volcanic centres of the Cenozoic alkaline volcanism of Central and South Italy, together with some samples from the contemporaneous anatectic Tuscan volcanism. The Sr and Pb isotopic compositions of the alkaline rocks show a negative correlation combined with a regional trend: the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios decrease from 0.711 in the north-west to 0.704 in the south-east, while the ${}^{206}\text{Pb}{}^{204}\text{Pb}$ ratios increase from 18.7 to 20.0. Variations in both isotopic compositions are generally small throughout erupted rock sequences for any volcanic centre.The Pb and Sr isotopic abundance variations are interpreted on the basis of two alternative models, which correspond to two groups of geological processes: variations can result (i) from a time dependent development in subsystems with different $\text{Rb}\text{Sr}$ or $\text{U(Th)}\text{Pb}$ ratios or, (ii) from mixing of Sr or Pb with different isotopic compositions. Combining both Pb and Sr isotope abundance measurements it is shown that the source of each volcanic centre is formed by various degrees of mixing between two components. One component and the most southern Tuscan anatectic rocks most likely have a common source, whereas the other component of the mixing process is suggested to be a liquid fraction derived from a small degree of partial fusion of a hydrous mantle. Thus at least a two-stage evolution of the Italian alkaline rocks is indicated: first a mixing process leading to the formation of the parental material followed by differentiation processes leading to the formation of the erupted rock sequences.The geodynamic model which explains the data best is that of a lateral inhomogeneous mantle. The lateral inhomogeneities in the mantle would be the result of mixing between originally mantle and crustal derived material. The mixing process itself would not have any primary connection with the Quarternary volcanic activity.     

The lead isotope geochemistry and geochronology of late-kinematic intrusives from the Abitibi greenstone belt,and the implications for late Archaean crustal evolution

Pb isotope abundances are reported for six late-kinematic granitoid intrusives from the Quebec sector of the Abitibi greenstone belt. Leaching experiments on K-feldspar separates reveal the presence of radiogenic Pb, attributed to in situ decay of U and Th. Pb-Pb mineral isochrons were constructed with the K-feldspar data plus results obtained on the total-rock, sphene, apatite and other mineral phases; five localities show no evidence of post-emplacement disturbance and yield ages ranging from 2616 ± 19 to 2718 ± 12 Ma. These ages, which are corroborated by U-Pb dating of small populations of sphene, imply that the orogenic events in the Abitibi belt were terminated 2700–2710 Ma ago, and followed by a period of granitization which lasted for 80 to 100 Ma.The initial Pb isotope composition of the magmas shows that their source regions were isotopically heterogeneous; the time integrated ${}^{238}\text{U}{}^{204}\text{Pb}$ values for the source regions vary from 7.62 to 7.92 and the K-feldspar data indicate that similar heterogeneities were present at the scale of a single intrusion. The range of isotopic composition spans the compositional domain of the mantle, defined by sulfides associated with komatiites and some galenas, and that of the continental crust, defined by sulfides associated with Abitibi iron-formations. Consequently, the granitoid magmas are interpreted as partial melts of a continental crust comprising juvenile, mantle-derived rocks and non-negligible amounts of earlier formed sialic material. The Pb isotope data for the latter are consistent with the presence in the area of 3.0 to 3.4 Ga old sialic crust. The episode of crustal anatexis occurred as a consequence to the orogenic events which resulted in burial of altered supracrustal rocks rich in water and heat-producing elements.     

Unscrambling the lead model ages

A linear relation is derived for the secular evolution of ${}^{206}\text{Pb}{}^{204}\text{Pb}$, ${}^{207}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios, that permits tests to be made for open system evolution on each system independently. Application of the method to conformable ore bodies of various geological age indicates that the available data do not demand an open system evolution for the last 3.8 b.y. ${}^{238}\text{U}{}^{204}\text{Pb}$ and ${}^{232}\text{Th}{}^{204}\text{Pb}$ of 9.66 ± 0.15 and 37.65 ± 1.14 respectively fit best the data for this time interval.A single stage evolution since 4.5 b.y. is unlikely, however, and the major events of continent formation postdate the Earth accretion by at least 400 m.y. The larger scatter of ${}^{207}\text{Pb}{}^{204}\text{Pb}$ data about the evolution line relative to the other isotopic ratios is also interpreted as resulting from a series of continental differentiation events taking place at 3.85 ± 0.15 b.y.     

Single-grain 207Pb206Pb and U/Pb age determinations with a 10-μm spatial resolution using the ion microprobe mass analyzer (IMMA)

Analytical techniques have been developed for using a secondary ion mass spectrometer, the ion microprobe mass analyzer (IMMA), to determine, in situ, ${}^{207}\text{Pb}{}^{206}\text{Pb}$ and U/Pb ages on approximately 10-μm areas of individual mineral phases containing relatively abundant radiogenic Pb isotopes. Standard samples of known age and U, Th and Pb contents, together with the Andersen-Hinthorne local thermal equilibrium (LTE) model for predicting ionization parameters are used to establish a semi-empirical relationship for correcting observed U/Pb intensities to atom ratios. Measurements of isotope standards show that mass fractionation corrections are not required and that the accuracy and precision of analysis are generally limited by Poisson counting statistics.Many U-rich accessory minerals yield spectra which consist only of Pb at masses 204, 206, 207 and 208; thus the measurement of ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages is accomplished by simply measuring the intensities of these peaks and the background. An excellent correspondence of the ages determined using the IMMA with those from more conventional techniques is demonstrated for terrestrial, lunar and meteoritic phases. For example, the following ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages were obtained from polished thin sections of crystalline lunar samples: 10047, 3.75 ± 0.05 (2σ) Ga; 14310, 3.96 ± 0.03 Ga; and 15555, 3.36 ± 0.06 Ga. From small U-rich phases in CaAl-rich inclusions in the Allende carbonaceous chondrite, seven ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages were obtained, averaging 4.60 ± 0.09 (2σ) Ga.A method of correcting low-resolution Pb-isotope data for molecular ion interferences in zircon and baddeleyite is presented and shown to be practical through the analysis of terrestrial and lunar samples.     

Mineral isotopic age relationships in the polymetamorphic Amîtsoq gneisses,Godthaab district,West Greenland

U-Th-Pb, Pb-Pb, Rb-Sr and K-Ar radiometric relationships in the minerals from six selected Amîtsoq gneiss samples reveal a complicated history of variable mineral response to polymetamorphism.K-Ar dates on biotite range from 2170 to 3220 m.y. (excess argon present), on hornblende from 2340 to 2510, and on a single muscovite at 1670 m.y.Rb-Sr whole rock results give an apparent isochron of at least 4065 m.y., but this result is likely fortuitous from a small sample selection since Pb-Pb whole rock analyses give ~ 3600 m.y. and the zircons in these rocks yield a concordia-discordia intersection at 3600 m.y. Rb-Sr mineral analyses generally give a confusing and variable pattern of isotopic relationships; but hornblende, K-feldspar, apatite, allanite and sphene appear to have last responded to metamorphism at 2200–2600 m.y. Rb-Sr in biotite, epidote and, in part, plagioclase have been affected by an event at ~ 1550 m.y.U-Th-Pb data from sphene, apatite and allanite give almost concordant dates at 2500–2600 m.y. $\text{soul}{}^{207}\text{Pb}{}^{204}\text{Pb}$ vs $\text{soul}{}^{206}\text{Pb}{}^{204}\text{Pb}$ plots yield two separate lines for apatite (slope age 2435 m.y.) and for sphene + allanite (slope age 2530 m.y.), indicating apatite to have a different (less-radiogenic) ‘initial’ Pb than that for sphene and allanite. A similar pattern is found for the $\text{soul}{}^{208}\text{Pb}{}^{204}\text{Pb}$ vs $\text{soul}{}^{207}\text{Pb}{}^{204}\text{Pb}$ plot for sphene and apatite. The Pb-isotopic composition of the feldspars is very homogeneous and the least-radiogenic of all components, pointing towards a homogeneous parent material for the now lithologically diverse Amîtsoq gneisses. Using a₀ = 9.307, b₀ = 10.294, C₀ = 29.476, t₀ = 4.56 b.y., ω = 6.9 and $\text{soul}{}^{232}\text{Th}{}^{204}\text{Pb = 27.1}$; the feldspars give a model Pb age of 3500–3600 m.y. by either U-derived or Th-derived Pb. The segregation of the present Amîtsoq gneisses from the homogeneous parent material was apparently accompanied by a U and Th loss with preservation or enrichment of Pb at ~ 3600 m.y. ago. No consistent treatment of the present U-Th-Pb data will produce viable data indicating an age > 3600 m.y. for the parent materials of the Amîtsoq gneiss.Petrographie observations generally concur with radiometric results and permit the postulation of the reaction: Hbl + K-feld→ biotite + epidote + sodic plag, to account for some of the effects of the latest metamorphism.The total internal radiometric evidence indicates three major metamorphic events affected the Amîtsoq gneisses close to 3600, 2500 and 1550 m.y.     

Sr and Nd isotope geochronology,geologic history,and origin of the Adirondack Anorthosite

We have analyzed samples from the Adirondack Marcy massif for Rb-Sr and Sm-Nd isotopes in an attempt to determine directly the primary crystallization age of a Proterozoic massif-type anorthosite rock suite. The oldest age obtained (1288 ± 36Ma) is from a 4 point Sm-Nd isochron defined by igneous-textured whole-rock and mineral separate data from a local layered sequence gradational from oxiderich pyroxenite to leuconorite. This age is older than Silver's (1969) 1113 Ma zircon age of associated charnockites, but is within the window of permissible anorthosite ages based on previous geochronology and field relationships. As such, 1288 Ma may represent the time of crystallization of the massif. For the most part, however, both Sm-Nd and Rb-Sr isotopic systems did not survive granulite facies metamorphism. Internal isochrons based on whole rocks and minerals yield ages between 995 and 919 Ma. These isotopic data suggest that the granulite fades metamorphism experienced by the massif was a prograde event that occurred a minimum of 100 Ma and as much as 350 Ma after crystallization of the massif. The relatively large range in Rb abundance, and in calculated initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ (0.7039–0.7050) and ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios among anorthosite suite rocks, particularly those at or near the contacts of the Marcy massif is explicable by variable contamination with “crustal” materials and/or fluids, derived from surrounding acidic metaplutonic rocks, paragneisses, and marbles. Despite uncertainies caused by crustal contamination and metamorphic resetting of primary ages, Marcy samples have epsilon Nd values between +0.44 and +5.08, implying a source for the massif with long-term depletion in light rare earth elements. A probable source material would be depleted mantle.     

Chemical and isotopic evidence for mixing between depleted and enriched mantle,northwestern U.S.A.

Combined elemental and Sr, Nd, Pb and O isotopic data for late Cenozoic olivine tholeiite lavas from the northwestern Great Basin indicate derivation from at least two chemically and isotopically distinct mantle source regions with no significant modification by interaction with continental crust. The lack of crustal involvement is a direct reflection of the extensional tectonic environment which favors rapid ascent of magmas, minimal residence time in crustal magma chambers and scattered fissure eruptions.The observed chemical and isotopic variations in the tholeiite suite are attributed to mixing between depleted oceanic type mantle (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{~ 0.7030}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{~ 0.51305}$) and old, chemically heterogeneous, isotopically enriched subcontinental mantle (${}^{87}\text{Sr}{}^{86}\text{Sr}\text{~ 0.7078}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{~ 0.51233}$). Model incompatible element concentrations suggest strong similarities between the depleted mantle and the mantles beneath normal oceanic ridge segments and back-arc basins and between the enriched mantle and the mantle beneath enriched oceanic ridge segments such as the Azores. Superimposed upon the characteristics derived from the two component mixing model may be the effects of a third mantle source which is identifiable only by its apparent radiogenic ${}^{206}\text{Pb}{}^{204}\text{Pb}$ ratios. If present, this third source may reflect a component derived from the downgoing slab of an ancient subduction zone.     

Isotopic and incompatible element constraints on the genesis of island arc volcanics from Cold Bay and Amak Island,Aleutians, and implications for mantle structure

Cold Bay and Amak Island, two Quaternary volcanic centers in the eastern Aleutians, are orthogonal relative to the trench and separated by ~50 km. Sr, Nd and Pb isotopic compositions of the calc-alkaline andesite magmas show no sign of contamination from continental crust (average ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{= 0.70323}$, ${}^{143}\text{Nd}{}^{144}\text{Nd}\text{= 0.51301}$, ${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 18.82}$, ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 15.571}$). These samples plot within the mantle arrays for Sr-Nd and for Pb and are similar to arcs such as the Marianas and New Britain (Sr-Nd) and Marianas and Tonga (Pb). Incompatible element ratios for the Aleutian andesites (K/Rb ~ 332, K/Cs ~ 10,600, K/Sr ~ 22.4, K/Ba ~ 18.3, Ba/La ~ 60) are within the range reported for arc basalts, despite the difference in degree of fractionation.Average K content, K/Rb, K/Ba and K/Sr are approximately the same for basalts from arcs and from oceanic islands (OIB); K/Cs is a factor of 4 lower and Ba/La almost 3 times higher in arcs. Abundance ratio correlations indicate that arcs are enriched in Cs and depleted in La relative to OIB, with other incompatible element abundances very similar. Histograms of Sr and Nd isotopic compositions for MORB, OIB, and intraoceanic arcs show remarkably similar peaks and distribution patterns for intraoceanic arcs and OIB.A “plum pudding” model for the upper mantle best accommodates a) geochemical coherence of OIB and IAV, b) the existence of mantle plumes at some oceanic islands, and c) the presence of a MORB-type source at back arc spreading centers. In this model, OIB plums are imbedded in a MORB matrix; small degrees of melting generate OIB-type magmas while larger degrees of melting dilute the OIB magma with MORB matrix melts.OIB plums are merely less robust lower mantle plumes (i.e., blobs) which are distributed throughout the upper mantle by convection. The existence of at least two types of OIB, as indicated by Sr, Nd, and Pb isotopes, suggests that nuggets of recycled oceanic lithosphère may coexist with lower-mantle plums and that both may be tapped in arcs and intraplate environments.     

Strontium isotope ratios in volcanic rocks from the northwestern part of the Hellenic arc

${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios have been determined in fifteen volcanic rocks from the northwestern part of the Hellenic arc. They range from 0.7041 to 0.7134. There is no apparent correlation of strontium isotope values with any major chemical component or with Rb/Sr ratios. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios appear to increase in a general way with increasing depth to the Benioff zone. The strontium isotope ratios are higher than from most island arcs; this is believed to be due to contamination.     

Isotope and trace element geochemistry of sediments from the Barbados Ridge-Demerara Plain region,Atlantic Ocean

Twenty-four piston core sediment samples and 13 sediments and 3 basalts from DSDP Leg 78 Site 543 were analyzed for Sr, Nd and Pb isotopic compositions. The results show sediment with highly radiogenic Pb${}^{206}\text{Pb}{}^{204}\text{Pb}$ up to 19.8) and rather radiogenic Sr and unradiogenic Nd has been deposited in the region since the Cretaceous. The source of this sediment is probably the Archean Guiana Highland, which is drained by the Orinoco River. Pb and Sr isotopic compositions and sediment thickness decrease and${}^{143}\text{Nd}{}^{144}\text{Nd}$ increases northward due to a decrease in turbiditic component. This decrease is partly due to the damming action of basement ridges. Rare earth concentrations in the sediments are somewhat low, due to the abundance of detrital and biogenic components in the sediment and rapid sedimentation rates. Both positive and negative Ce anomalies occur in the surface sediments, but only positive Ce anomalies occur in the Site 543 sediments. It is unlikely that sediment subducted to the source region of Lesser Antilles arc magmas could be the cause of negative Ce anomalies in those magmas.Isotopic compositions of Site 543 basalts show some effect of contamination by seawater-basalt reaction products and sediments. Beyond this, however, they are typical of “normal” depleted MORB.     

Recognition of extraneous argon components through incremental-release 40Ar39Ar analysis of biotite and hornblende across the Grenvillian metamorphic gradient in southwestern Labrador

${}^{40}\text{Ar}{}^{39}\text{Ar}$ incremental-release ages have been determined for 15 hornblende and 20 biotite concentrates separated from rocks collected across the garnet and kyanite zones of Grenvillian metamorphism in southwestern Labrador. Most hornblende spectra from the kyanite zone are slightly discordant, with low-temperature increments yielding ages older than the ca 1000 Ma date suggested for culmination of Grenvillian metamorphism in the area. However, all the hornblende concentrates record well-defined plateau ages. These range from 968 to 905 Ma across the kyanite zone and date times of diachronous post-metamorphic cooling. The discordant spectra are interpreted to result from low-temperature liberation of excess ⁴⁰Ar components from grain margins. Two hornblende concentrates from the garnet zone display very discordant spectra (total-gas ages of 2100 and 3017 Ma) in which incremental dates systematically decrease during analysis. This pattern of discordance suggests that excess argon components are inhomogeneously distributed throughout these hornblende grains.Most biotites from the garnet and kyanite zones record total-gas or plateau ages in excess of 1000 Ma (2066-857 Ma), reflecting the widespread presence of excess argon components. Because most of the ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age spectra are internally concordant, the ratios of excess ⁴⁰Ar relative to radiogenic ⁴⁰Ar must have been uniform in the various gas fractions liberated from each sample. This is also reflected in the inability of isotope correlation diagrams to differentiate between excess, radiogenic, and atmospheric argon components. The biotite total-gas or plateau dates show marked local variation. This is interpreted to indicate that the biotite grains were in contact with a post-metamorphic intergranular vapor phase that was characterized by large and variable ${}^{40}\text{Ar}{}^{36}\text{Ar}$ ratios. Such ratios most likely resulted from widespread diffusion of the argon liberated from adjacent Archean basement gneisses during the Grenvillian metamorphic overprint.     

Sr and Nd isotopic evidence for petrogenesis of mid-tertiary felsic volcanism in the mineral district of Zacatecas,Zac. (Sierra Madre Occidental), Mexico

Twelve samples of mid-Tertiary felsic volcanic rocks from Zacatecas and San Luis Potosí (both belonging to the Sierra Madre Occidental) and one sample of Lower Tertiary porphyritic andesite from Zacatecas are analyzed for ${}^{87}\text{Sr}{}^{86}\text{Sr}$, K, Rb, and Sr. Eight selected samples are also analyzed for ${}^{143}\text{Nd}{}^{144}\text{Nd}$. A linear regression of the present-day ${}^{87}\text{Sr}{}^{86}\text{Sr}\text{and}{}^{87}\text{Rb}{}^{86}\text{Sr}$ of the felsic volcanic rocks in Zacatecas gives an approximate date of 30 ± 8 Ma. The initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios are high and widely distributed (from 0.705 to 0.712 or higher) whereas the initial ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios are somewhat low and show a narrow range (0.5125–0.5127). The available isotopic and trace-element data are best explained in terms of a binary mixing model in which the magmas derived from a slightly depleted-mantle fractionate and mix with varying proportions of the overlying middle/upper continental crust and undergo further shallow-level fractional crystallization before eruption. This model is also compatible with the trace-element and Sr isotopic data published from other areas of the Sierra Madre Occidental for which a purely mantle origin has been proposed.     

U-Th-Pb isotope systematics related to igneous rocks and ore Pb,Mount Isa,Queensland

This study is a search for a genetic relationship between Pb sulphide ore and igneous rocks in the region of Mount Isa, Queensland. The approach involves derivation of Pb isotope initial ratios by the whole-rock isochron method, and comparison of the initial ratios (Pb²⁰⁶/Pb²⁰⁴, Pb²⁰⁷/Pb²⁰⁴ and Pb²⁰⁸/Pb²⁰⁴) with the isotopic composition of the ore Pb. Data are reported for four igneous units; Kalkadoon granodiorite, Kalkadoon adamellite, Sybella granite and Eastern Creek volcanics. The results display considerable scatter for each of the units, and reveal the effects of recent surficial loss of U. The positioning of isochrons is aided by previous Rb-Sr geochronological data wherever possible. Comparison of initial ratios and ore Pb suggests that none of the igneous rock units is co-genetic with the ore deposit. Both phases of the Sybella Granite are more radiogenic and are apparently younger than the ore Pb. The Kalkadoon Granite is possibly related to the ore through some post-emplacement process of extraction and transport of Pb (e.g. by erosion or by anatectic magma generation) to the present site of the orebodies.