Garnet stability during crustal melting: implications for chemical mohometry and secular change in arc magmatism and continent formation

Understanding how new felsic crust is formed and subsequently evolves through time is critical to identifying the geodynamic regimes that have dominated various parts of Earth history, and have important implications for feedbacks between the lithosphere and biosphere, such as controlling the influx of continental detritus into the oceans. In recent years, several trace element-based geochemical proxies have been proposed to allow determination of paleo-crustal thicknesses, which have been calibrated primarily using data collected from modern-day arcs. The application of these proxies through deep time has revealed surprising results, including the suggestion that the mid-Proterozoic continents were atypically thin compared to those in the Archean and the Phanerozoic; however, a range of factors may influence commonly cited trace element ratios (e.g. Sr/Y) rather than just crustal depth, leading to additional and unexpected magnitudes of uncertainty. Here we perform geochemical modelling to deduce the effect of variable bulk-rock composition and geothermal gradient on the trace element signature of felsic melts generated in arc systems. Using a range of protoliths representative of deep arc crust, the results show that considerable care must be taken when analysing simple trace element ratios of granitoid melts and making direct interpretations of the pressure of crystallisation. In particular, changes in geothermal gradients and differences in arc basalt composition impart strong controls on the relative stability of garnet and plagioclase during metamorphism and partial melting, and wide ranges of Sr/Y and La/Yb may be produced in derivative felsic melts produced at the same crustal depth. The interpretation of mid-Proterozoic continental arcs being atypically thin may instead be an artefact of underestimation of the active geothermal gradient at the time of magma formation, which acts to reduce Sr/Y and La/Yb ratios, even in normal thickness (∼35–40 km) crust. Furthermore, we argue that the potentially garnet-free residua during the formation of mid-Proterozoic felsic magmas points to crust formation without lower crustal foundering, and thus, that this commonly invoked paradigm for formation of the continental crust may only be applicable to certain periods of Earth history

Constraints on the thermal evolution of metamorphic core complexes from the timing of high-pressure metamorphism on Naxos, Greece

Metamorphic core complexes are classically interpreted to have formed during crustal extension, although many also occur in compressional environments. New U–(Th)–Pb allanite and xenotime geochronologic data from the structurally highest Zas Unit (Cycladic Blueschist Unit) of the Naxos metamorphic core complex, Greece, integrated with pressure–temperature–time (P–T–t) histories, are incorporated into a thermal model to test the role of crustal thickening and extension in forming metamorphic core complexes. Metamorphism on Naxos is diachronous, with peak metamorphic conditions propagating down structural section over a ~30–35 m.y. period, from ca. 50 Ma to 15 Ma. At the highest structural level, the Zas Unit records blueschist-facies metamorphism (~14.5–19 kbar, 470–570 °C) at ca. 50 Ma, during northeast-directed subduction of the Adriatic continental margin. The Zas Unit was subsequently extruded toward the SW and thrust over more proximal continental margin and basement rocks (Koronos and Core units). This contractional episode resulted in crustal thickening and Barrovian metamorphism from ca. 40 Ma and reached peak kyanite-sillimanite–grade conditions of ~10–5 kbar and 600–730 °C at 20–15 Ma. Model P–T–t paths, assuming conductive relaxation of isotherms following overthrusting, are consistent with the clockwise P–T–t evolution. In contrast, extension results in exhumation and cooling of the crust, which is inconsistent with key components of the thermal evolution. Barrovian metamorphism on Naxos is therefore interpreted to have resulted from crustal thickening over a ~30–35 m.y. time period prior to extension, normal faulting, and rapid exhumation after a thermal climax at ca. 15 Ma

Iron Behaving Badly: Inappropriate Iron Chelation as a Major Contributor to the Aetiology of Vascular and Other Progressive Inflammatory and Degenerative Diseases

The production of peroxide and superoxide is an inevitable consequence of aerobic metabolism, and while these particular "reactive oxygen species" (ROSs) can exhibit a number of biological effects, they are not of themselves excessively reactive and thus they are not especially damaging at physiological concentrations. However, their reactions with poorly liganded iron species can lead to the catalytic production of the very reactive and dangerous hydroxyl radical, which is exceptionally damaging, and a major cause of chronic inflammation. We review the considerable and wide-ranging evidence for the involvement of this combination of (su)peroxide and poorly liganded iron in a large number of physiological and indeed pathological processes and inflammatory disorders, especially those involving the progressive degradation of cellular and organismal performance. These diseases share a great many similarities and thus might be considered to have a common cause (i.e. iron-catalysed free radical and especially hydroxyl radical generation). The studies reviewed include those focused on a series of cardiovascular, metabolic and neurological diseases, where iron can be found at the sites of plaques and lesions, as well as studies showing the significance of iron to aging and longevity. The effective chelation of iron by natural or synthetic ligands is thus of major physiological (and potentially therapeutic) importance. As systems properties, we need to recognise that physiological observables have multiple molecular causes, and studying them in isolation leads to inconsistent patterns of apparent causality when it is the simultaneous combination of multiple factors that is responsible. This explains, for instance, the decidedly mixed effects of antioxidants that have been observed, etc...Comment: 159 pages, including 9 Figs and 2184 reference