A complex magmatic system beneath the middle and northern Okinawa Trough: evidence from pyroxene characteristics

The Okinawa Trough is an initial marginal back‐arc basin. In this paper, we present a comparative study of pyroxenes in white and black pumice from the Okinawa Trough to explore the magmatic evolution of the region and the characteristics of the magmatic system. The results show two species of pyroxenes in the white pumice (clinopyroxene and orthopyroxene) but only one species in the black pumice (orthopyroxene). Many isomorphic replacements among the cations occurred during clinopyroxene crystallization. Clinopyroxene‐melt thermobarometry yielded a temperature of 847–988°C and a pressure of 2.73–7.42 kbar, which vary considerably and correspond to a maximum depth of approximately 24 km. All of the orthopyroxenes in both the white and black pumice crystallized at similar pressures ranging from 0.67 to 4.59 kbar, which correspond to a maximum depth of approximately 15 km. The clinopyroxenes crystallized mainly in the lower crust, whereas the orthopyroxenes formed at the boundary depth between the upper crust and the lower crust. The similarities in the pyroxene chemical compositions and the whole‐rock characteristics between the two types of pumice indicate that both types of pumice are linked to the same magmatic system at depth. The pyroxenes crystallized from genetically related melts within a magmatic system that had undergone various degrees of differentiation. The physicochemical characteristics of the pyroxenes show that the magma chamber module changed during the crystallization process from clinopyroxenes to orthopyroxenes. This change most likely resulted from local characteristics of the near‐surface structure. These observations support the model of a ‘two‐layered magma chamber’ in the northern Okinawa Trough. Copyright © 2016 John Wiley & Sons, Ltd.

Morphometric analysis of Early Permian fusulinids from east‐central Iran: a new approach to defining fusulinid species reliably

Fusulinid fauna from the late Sakmarian–early Artinskian Khan Formation in the Kalmard area, east‐central Iran, was studied using morphometric techniques to test the reliability of previous taxonomic descriptions. Fusulinids in the Kalmard area have been previously grouped into three genera (Eoparafusulina, Perigondwania, and Neodutkevitchia) and ‘20 species’. Results of ordination analyses (PCA and RDA) reveal that the genera are readily distinguished, whereas only a few species previously defined within the genera Eoparafusulina and Perigondwania are recognizable. This suggests that the taxonomic descriptions and measurements for different structural elements for some of these fusulinid species are not sufficient to justify erecting separate taxonomic names. Since fusulinids are really important fauna for biostratigraphic and palaeobiogeographic interpretation in the late Palaeozic, defining species based on quantitative analysis of their morphological structures should be more accurate and improve the quality of regional correlations. Copyright © 2016 John Wiley & Sons, Ltd.

Tectonic settings of continental crust formation: Insights from Pb isotopes in feldspar inclusions in zircon

Most crustal rocks derive from preexisting crust, and so the composition of newly generated (juvenile) continental crust, and hence the tectonic settings of its formation, have remained difficult to determine, especially for the first billion years of Earth’s evolution. Modern primitive mantle–derived magmas have distinct U/Pb ratios, depending on whether they are generated in intraplate (mean U/Pb = 0.37) or in subduction settings (mean U/Pb = 0.10). The U/Pb ratio can therefore be used as a proxy for the tectonic settings in which juvenile continental crust is generated. This paper presents a new way to see back to the U/Pb ratios of juvenile continental crust that formed hundreds to thousands of millions of years ago, based on ion probe analysis of Pb isotopes in alkali feldspar and plagioclase inclusions within well-dated zircons. Pb isotope data are used to calculate the time-integrated U/Pb ratios (i.e., 238U/204Pb = µ) for the period between the Hf model age and the U-Pb crystallization age of the zircons. These time-integrated ratios reflect the composition of the juvenile continental crust at the time it was extracted from the mantle, and so they can be used as a proxy for the tectonic setting of formation of that crust. Two test samples with Proterozoic Hf model ages and Paleozoic crystallization ages have feldspar inclusions with measured Pb isotope ratios that overlap within analytical error for each sample. Sample Z7.3.1 from Antarctica has Pb isotope ratios (mean 206Pb/204Pb = 16.88 ± 0.08, 1) that indicate it was derived from source rocks with low U/Pb ratios (~0.11), similar to those found in subduction-related settings. Sample Temora 2 from Australia has more radiogenic Pb isotope ratios (mean 206Pb/204Pb = 19.11 ± 0.23, 1) indicative of a source with higher U/Pb ratios (~0.36), similar to magmas generated in intraplate settings. Analysis of detrital populations with a range of Hf model ages (e.g., Hadean to Phanerozoic), and for which zircons and their inclusions represent the only archive of their parent magmas, should ultimately open new avenues to our understanding of the formation and the evolution of the continental crust through time.

First terrestrial occurrence of tistarite (Ti2O3): Ultra-low oxygen fugacity in the upper mantle beneath Mount Carmel, Israel

The minimum oxygen fugacity (fO2) of Earth’s upper mantle probably is controlled by metal saturation, as defined by the iron-wüstite (IW) buffer reaction (FeO -> Fe + O). However, the widespread occurrence of moissanite (SiC) in kimberlites, and a suite of super-reduced minerals (SiC, alloys, native elements) in peridotites in Tibet and the Polar Urals (Russia), suggest that more reducing conditions (fO2 = 6–8 log units below IW) must occur locally in the mantle. We describe pockets of melt trapped in aggregates of corundum crystals ejected from Cretaceous volcanoes in northern Israel which contain high-temperature mineral assemblages requiring extremely low fO2 (IW < –10). One abundant phase is tistarite (Ti2O3), previously known as a single grain in the Allende carbonaceous chondrite (Mexico) and believed to have formed during the early evolution of the solar nebula. It is associated with other reduced phases usually found in meteorites. The development of super-reducing conditions in Earth’s upper mantle may reflect the introduction of CH4 + H2 fluids from the deep mantle, specifically related to deep-seated volcanic plumbing systems at plate boundaries.

Recycling in debris-filled volcanic vents

Eruptions through debris-filled vents produce deposits containing magmatic juvenile lithic and recycled clasts. Recycled clasts are exposed to multiple transportation and fragmentation events. We used experiments with multiple subsurface explosions to track clasts and highlight dominant recycling processes in eruptions through analog debris-filled vents. Recycled clasts include those that fall back into and reside in the vent for extended time periods and those that return to the vent through crater growth or collapse. Clasts are recycled by any combination of lofting and fallback of material in the crater by explosion jets, mixing and churning of material at depth in the debris fill, and redistribution of extra-crater deposits by explosion-induced excavation or slumping. We compare experimental processes with natural deposits that preserve recycling signatures from discrete explosions through debris-filled vents such as maar-diatremes, Strombolian vents, and hydrothermal craters. Clasts may not preserve textures diagnostic of their complete recycling histories, but can be used to infer if that history occurred in part in the vent debris or in the eruptive jet. Experiment results and natural deposits suggest that for volcanic craters that undergo multiple explosions, clasts likely undergo some form of recycling before final deposition outside the craters. The underestimation of recycled clast contributions to deposits can lead to inaccurate estimates of thermal budgets and eruption processes.

Control of lithospheric inheritance on neotectonic activity in northwestern Canada?

Lithospheric inheritance is thought to affect the location and reactivation of tectonic structures through successive cycles of supercontinent formation and dispersal; however, its relation to neotectonic activity remains unclear. In northwestern Canada, abundant seismicity throughout the northern Canadian Cordillera (NCC) is geographically confined by several crustal-scale boundaries, yet its southern extent terminates abruptly along the inferred westward extension of a Late Cretaceous rifted margin boundary called the Liard transfer zone (LTZ). We use seismic data to show that the uppermost mantle beneath the Cordillera exhibits a sharp north-south contrast in fabric across the LTZ. South of the LTZ, fast axes of seismic wave propagation align closely with the lithospheric mantle fabric orientation of the adjacent Canadian shield. North of the LTZ, fast axes are reoriented subparallel to the motion of the Pacific plate and follow the strike of the large dextral strike-slip Tintina and Denali faults. We attribute changes in anisotropic delay times across the Tintina and Denali faults to localized shear within the lithosphere; this implies that the crust and lithospheric mantle remained mechanically coupled during shearing. We propose that the contrast in uppermost mantle structure across the LTZ reflects a change in the nature and origin of the lithospheric mantle from inherited rifted margin structures, which affects the stability of the lithosphere and limits the extent of seismic activity within the NCC. These results indicate that neotectonic activity in modern Cordilleras is controlled in part by inherited upper mantle structures.