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Determining high precision, in situ, oxygen isotope ratios with a SHRIMP II Analyses of MPI-DING silicate-glass reference materials and zircon from contrasting granites [查看] R.B.Ickert、J.Hiess、I.S.Williams、P.Holden、T.R.Irel、P.Lanc、N.Schram、J.J.Foster、S.W.Clement

The development of new techniques and instrumentation on the ANU SHRIMP II ion microprobe has made it possible to measure the oxygen isotope ratios of insulating and conducting phases (e.g. silicates, carbonates, phosphates and oxides) on a 25 μm scale with better than 0.4‰ precision and accuracy at 95% confidence.Instrumentation changes include the installation of a multiple collector, charge neutralization using an oblique-incidence low-energy electron gun, and the addition of Helmholtz coils to counter mass dispersion by the Earth's magnetic field. A redesign of sample mounts and mount holders has effectively eliminated differences in variable isotope fractionation across the mount surface during analysis. Techniques have been developed to minimize the effect of electron-induced secondary ionization of oxygen. During a 6-minute analysis involving 100–140 s of data collection, δ18O values can be measured on one 25 μm spot with an internal precision of better than 0.2‰ (2 standard errors). Analyses of MPI-DING silicate-glass reference material demonstrate that the external reproducibility of single spots can be better than 0.4‰ at 95% confidence, and that for matrix-matched samples and reference material, accuracy is commensurate with precision. MPI-DING glasses are acceptable ion microprobe reference materials for oxygen isotope measurements of glasses, although KL2-G is possibly heterogeneous. Zircon reference materials TEMORA 2 and FC1 appear to be acceptable as preliminary oxygen isotope reference materials. SHRIMP II analyses of FC1 indicate that it has a δ18O value of 5.4‰ (VSMOW). Analyses of zircon oxygen isotopic compositions from a gabbro, a tonalite and a granodiorite from southeastern Australia are presented. Zircon from the gabbro has a δ18O value of 5.6‰, the tonalite has an I-type affinity and slightly heterogeneous δ18O values around 6.6‰, and the granodiorite has an S-type affinity and a range of igneous, melt precipitated zircon δ18O values between 8.2 and 10.2‰. These results suggest that the gabbro is mantle-derived and slightly contaminated with crustal material, and that the I-type granodiorite has evolved in a similar manner from a mantle-derived source. The δ18O values of the zircon from the S-type granodiorite are not only higher than from the I-type, but also more heterogeneous, consistent with partial melting of a poorly-mixed, metasedimentary source.

Cambrian ensialic rift-related magmatism in the Ossa-Morena Zone(Évora–Aracena metamorphic belt, SW Iberian Massif) Sm-Nd isotopes and SHRIMP zircon U-Th-Pb geochronology [查看] M.Chichorro、M.F.Pereira、M.Díaz-Azpiroz、I.S.Williams、C.Fernández、C.Pin、J.B.Silva

The Late Ediacaran (c. 560–550 Ma) Série Negra sediments of the Évora–Aracena metamorphic belt, Ossa-Morena Zone, SW Iberian Massif, preserve a record of the erosion of an Avalonian–Cadomian magmatic arc and subsequent related turbiditic sedimentation. Detrital zircon from the Série Negra is characterized by predominantly Ediacaran and Cryogenian ages, with few Paleoproterozoic and Archean cores, and a marked lack of Grenvillian ages. These features, when combined with the metasediments' enrichment in LREE (La/Yb=14), negative Eu-anomalies, low 147Sm/144Nd values (0.121) and negative εNd550=−5.5, indicate that the protolith Série Negra sediments were derived from a continental magmatic arc. A period of Late Cadomian (ca. 560–540 Ma) tectonism was followed by an extended episode of widespread bimodal magmatism related to Cambrian (ca. 540–500 Ma) rifting. This tectonic inversion is expressed in the geological record by a regional Early Cambrian unconformity.SHRIMP zircon U–Th–Pb ages from four felsic orthogneisses from the Évora Massif record Cambrian (527±10 Ma, 522±5 Ma, 517±6 Ma and 505±5 Ma) crystallization ages for their igneous protoliths. This confirms the existence of widespread Lower Paleozoic igneous activity in the Ossa-Morena Zone: (i) a Lower Cambrian (ca. 535–515 Ma) igneous–felsic dominated–sedimentary complex (with calc-alkaline signature and associated carbonate and siliciclastic deposition), and (ii) a Middle Cambrian–?Ordovician (ca. 515–490 Ma) igneous–bimodal–sedimentary complex (with calc-alkaline and tholeiitic signatures and associated dominant siliciclastic deposition, but also carbonate sediments).The Cambrian felsic magmatism was characterized by negative Eu-anomalies, (La/Lu)N=0.8–11, 147Sm/144Nd=0.1289–0.1447 and εNd500 ranging from −1.5 to −0.8. A tendency towards peraluminous compositions suggests late fractionation, low degrees of partial melting, or the mixing of crustal and mantle-derived material in the magma source region. Some felsic rocks possibly represent the last residual melts of hightemperature,zircon-undersaturated mafic magmas later affected by crustal contamination, while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids.The transition from early felsic dominated to later more mafic magmatism suggests the gradual opening of the system to tholeiitic N–E-MORB products (ThN/TaNb1.0). The as yet undated (Cambrian–?Ordovician) E-MORB amphibolites have 147Sm/144Nd=0.1478–0.1797 and εNd500 values ranging from +6.4 to +7.3, while the N-MORB amphibolites have 147Sm/144Nd=0.1818–0.1979 and εNd500 values of +5.8 and +7.0,reaching a maximum of +9.1. In contrast, other amphibolites have a negative Ta-anomaly (1.35bThN/TaNb2.41) reminiscent of lavas from “orogenic” settings or alternatively, typical of crustally-contaminated within-plate magmas. These “VAB-like” amphibolites have 147Sm/144Nd values ranging from 0.1639 to 0.1946 and εNd500 values of +3.5 to +5.2, suggesting derivation by crustal assimilation processes. The subalkaline igneous precursors of the amphibolites were most likely generated in a rift setting by asthenospheric upwelling.

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