Nd Isotopic Compositions Of Rocks Research Articles

The origin and geochemical evolution of the Woodlark Rift of Papua New Guinea

Geochemical, isotopic, and geochronologic data for exhumed rocks in the Woodlark Rift of Papua New Guinea (PNG) allow a tectonic link to be established with the Late Cretaceous Whitsunday Volcanic Province (WVP) of northeastern Australia. Most of the metamorphic rocks in the Woodlark Rift have Nd isotopic compositions (εNd=+1.7 to +6.2) similar to the Nd isotopic compositions of rocks in the WVP (εNd=+1.3 to +6.6; Ewart et al., 1992), and contain inherited zircons with 90 to 100Ma U–Pb ages that overlap the timing of magmatism in the WVP. None of the metamorphic rocks in the Woodlark Rift have the highly evolved Hf and Nd isotopic compositions expected of ancient continental crust. Magmas were erupted in the WVP during the middle Cretaceous as eastern Gondwana was rifted apart. The protoliths of felsic and intermediate metamorphic rocks in the Woodlark Rift are interpreted to be related to the magmatic products produced during this Cretaceous rifting event. Some mafic metamorphic rocks exposed in the western Woodlark Rift (eclogites and amphibolites) are not related to the WVP and instead could have originated as basaltic lavas crystallized from mantle melts at (U)HP depths in the Late Cenozoic, or as fragments of Mesozoic aged oceanic lithosphere.Isotopic and elemental comparisons between basement gneisses and Quaternary felsic volcanic rocks demonstrate that felsic lavas in the D'Entrecasteaux Islands did not form solely from partial melting of metamorphic rocks during exhumation. Instead, the isotopic compositions and geochemistry of Quaternary felsic volcanic rocks indicate a significant contribution from the partial melting of the mantle in this region. When combined with geophysical data for the western Woodlark Rift, this suggests that future seafloor spreading will commence south of Fergusson Island, and west of the present-day active seafloor spreading rift tip.

Zircon effect alone insufficient to generate seawater Nd‐Hf isotope relationships

Many studies have suggested that continental weathering inputs have controlled the dissolved oceanic budget of hafnium (Hf). However, whether the offset of seawater Nd‐Hf isotope compositions from the terrestrial array can be fully generated by incongruent weathering of continental rocks (the zircon effect) is still not well constrained. In recent years, an increasing amount of combined U‐Pb ages and Hf‐isotopic compositions of riverine detrital zircons have been published. Here a new model of the Nd‐Hf isotopic compositions of the weathered zircon‐free part of the upper continental crust is presented, which is based on published Hf isotope compositions and formation ages of modern riverine detrital zircons combined with Nd isotopic compositions of rocks from the upper continental crust. Our model results indicate that the Nd‐Hf isotopic composition of the weathered zircon‐free part of the upper continental crust is not consistent with the seawater isotopic compositions. This suggests that the elevated seawater Hf isotope compositions for given Nd isotope compositions cannot be fully explained by incongruent zircon weathering of the continents, which is also supported by a recent study demonstrating incongruent weathering of other minerals than zircon.

Formation of shoshonites from calcalkaline basalt magmas: geochemical and experimental constraints from the type locality

Independence volcano, Montana is a major center of the Absaroka volcanic field, from which absarokite, shoshonite, and banakite were originally defined. One magmatic trend at Independence volcano, from high-alumina tholeiitic basalt through shoshonite to high-K dacite, may be modeled by fractional crystallization of observed phenocryst phases (plagioclase, hypersthene, augite, and magnetite). Trace-element and Sr and Nd isotopic compositions of rocks are consistent with this model.