Beschreibung: Chromite from Los Congos and Los Guanacos in the Eastern Pampean Ranges of Córdoba (Argentinian Central Andes) shows homogenous and exsolution textures. The composition of the exsolved phases in chromite approaches the end-members of spinel (MgAl 2 O 4 ; Spl) and magnetite ( $${\mathrm{Fe}}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{O}}_{4}$$ ; Mag) that define the corners of the spinel prism at relatively constant Cr 3+ /R 3+ ratio (where R 3+ is Cr+Al+Fe 3+ ). The exsolution of these phases from the original chromite is estimated to have accounted at ≥600 °C on the basis of the major element compositions of chromite with homogenous and exsolution textures that are in equilibrium with forsterite-rich olivine (Fo 95 ). The relatively large size of the exsolved phases in chromite (up to ca. 200 μm) provided, for the first time, the ability to conduct in situ analysis with laser ablation-inductively coupled plasma-mass spectrometry for a suite of minor and trace elements to constrain their crystal-crystal partition coefficient between the spinel-rich and magnetite-rich phases $$({D}_{\mathrm{i}}^{\mathrm{Spl}/\mathrm{Mag}})$$ . Minor and trace elements listed in increasing order of compatibility with the spinel-rich phase are Ti, Sc, Ni, V, Ge, Mn, Cu, Sn, Co, Ga, and Zn. $${D}_{\mathrm{i}}^{\mathrm{Spl}/\mathrm{Mag}}$$ values span more than an order of magnitude, from $${D}_{\mathrm{Ti}}^{\mathrm{Spl}/\mathrm{Mag}}=0.30\pm 0.06$$ to $${D}_{\mathrm{Zn}}^{\mathrm{Spl}/\mathrm{Mag}}=5.48\pm 0.63$$ . Our results are in remarkable agreement with data available for exsolutions of spinel-rich and magnetite-rich phases in other chromite from nature, despite their different Cr 3+ /R 3+ ratio. The estimated crystal-crystal partitioning coefficients reflect the effect that crystal-chemistry of the exsolved phases from chromite imposes on all investigated elements, excepting Cu and Sc (and only slightly for Mn). The observed preferential partitioning of Ti and Sc into the magnetite-rich phase is consistent with high-temperature chromite/melt experiments and suggests a significant dependence on Fe 3+ substitution in the spinel structure. A compositional effect of major elements on Ga, Co, and Zn is observed in the exsolved phases from chromite but not in the experiments; this might be due to crystal-chemistry differences along the $${\mathrm{MgFe}}_{-1}{-\mathrm{Al}}_{2}{\mathrm{Fe}}_{-2}^{3+}$$ exchange vector, which is poorly covered experimentally. This inference is supported by the strong covariance of Ga, Co, and Zn observed only in chromite from layered intrusions where this exchange vector is important. A systematic increase of Zn and Co coupled with a net decrease in Ga during hydrous metamorphism of chromitite bodies cannot be explained exclusively by compositional changes of major elements in the chromite (which are enriched in the magnetite component). The most likely explanation is that the contents of minor and trace elements in chromite from metamorphosed chromitites are controlled by interactions with metamorphic fluids involved in the formation of chlorite.

Beschreibung: The minimum oxygen fugacity ( f O 2 ) 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 ( f O 2 = 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 f O 2 (IW 〈 –10). One abundant phase is tistarite (Ti 2 O 3 ), 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 CH 4 + H 2 fluids from the deep mantle, specifically related to deep-seated volcanic plumbing systems at plate boundaries.

Beschreibung: Cells engage numerous signaling pathways in response to oxidative stress that together repair macromolecular damage or direct the cell toward apoptosis. As a result of DNA damage, mitochondrial DNA or nuclear DNA has been shown to enter the cytoplasm where it binds to “DNA sensors,” which in turn initiate signaling cascades. Here we report data that support a novel signaling pathway in response to oxidative stress mediated by specific guanine-rich sequences that can fold into G-quadruplex DNA (G4DNA). In response to oxidative stress, we demonstrate that sequences capable of forming G4DNA appear at increasing levels in the cytoplasm and participate in assembly of stress granules. Identified proteins that bind to endogenous G4DNA in the cytoplasm are known to modulate mRNA translation and participate in stress granule formation. Consistent with these findings, stress granule formation is known to regulate mRNA translation during oxidative stress. We propose a signaling pathway whereby cells can rapidly respond to DNA damage caused by oxidative stress. Guanine-rich sequences that are excised from damaged genomic DNA are proposed to enter the cytoplasm where they can regulate translation through stress granule formation. This newly proposed role for G4DNA provides an additional molecular explanation for why such sequences are prevalent in the human genome.

Beschreibung: High-Mg andesites (HMAs) and adakitic rocks are purported to occur exclusively in subduction zones in the modern Earth. In the North China Craton, early Cretaceous HMAs and adakitic dacites were erupted in a continental setting, apparently unrelated to subduction given their location distal (〉1000 km) to the trench at that time. Here we report petrological, mineralogical and geochemical data for these rocks with the aim of constraining their petrogenesis and elucidating the role of water in intraplate magmatism and cratonic destruction. The HMAs can be subdivided into olivine (Ol-)HMAs and clinopyroxene (Cpx-)HMAs. The former have high MgO (〉9·8 wt %) and Mg# (〉71), with rare high-Fo (up to 91) olivine phenocrysts, corresponding to (near-)primary magmas that equilibrated with mantle peridotite. The latter have moderate MgO (7·8–8·8 wt %) and Mg# (mostly 〈70) and low-Fo (mostly 〈 83) olivine phenocrysts. The Cpx-HMAs are interpreted as magmas differentiated from the Ol-HMAs by olivine-dominated fractionation at lower-crust levels. P–T–X H2O estimations show that the primary HMAs are melts of shallow (1·1–1·2 GPa), hot (~1250°C) and wet (H 2 O 〉 3 wt %) lithospheric mantle. The coexisting adakitic dacites are hydrous (H 2 O ≥ 5 wt %) magmas with high SiO 2 (〉63 wt %), Sr/Y ratios (≥39) and Yb SN (source-normalized), low (Sm/Yb) SN , and negligible Eu anomalies. They also have unradiogenic whole-rock Nd [ Nd ( t ) = –19 to –9] and zircon Hf [ Hf ( t ) = –23 to –21] isotopic compositions consistent with derivation by melting of ancient lower crust at depths 〈 40 km. Melting may have been induced by heating and addition of H 2 O from underplated HMAs. Mixing between Cpx-HMAs and low-Mg adakitic dacites in magma chambers produced high-Mg adakitic rocks. The petrogenetic model presented here explains the occurrence of intraplate HMAs and adakitic magmas elsewhere in the North China Craton. The P–T–X H2O conditions inferred for HMA generation imply that the subcontinental lithospheric mantle beneath the craton was hot and hydrous in the early Cretaceous, which may have triggered the destruction of the cratonic root. The occurrence of young HMAs and adakitic rocks in an intraplate extensional environment also casts doubts on the common use of a similar igneous rock association as an indicator of subduction processes in Archean time.

Beschreibung: Aims To examine the relationship between peripheral arterial disease (PAD) and atrial fibrillation (AF) in a population-based cohort study of older adults. Methods and results We examined the relationship between PAD and AF in 5143 participants (85% white, 43% male) in the Cardiovascular Health Study (CHS), a longitudinal, observational study of adults aged 65 years and older. Peripheral arterial disease was defined by abnormal ankle-brachial index (ABI) values (〈1.0 or 〉1.4). Incident AF events were ascertained by self-reported history, study electrocardiograms, and hospitalization discharge records. Cox regression was used to compute hazard ratios (HRs) and 95% confidence intervals (CIs) for the association between PAD and AF. Over a median follow-up of 11.7 years, a total of 1521 participants developed AF. The incidence rate (per 1000 person-years) of AF was higher in those with PAD (incidence rate = 32.9, 95% CI = 29.5, 36.7) than those without PAD (incidence rate = 23.3, 95% CI = 22.0, 24.6). In a multivariate Cox regression analysis, PAD was associated with an increased risk for AF (HR = 1.52, 95% CI = 1.34, 1.72). Each 0.1 decrease in the ABI was associated with a 6% increase in the risk for AF (HR = 1.06, 95% CI = 1.02, 1.10). The associations of high (〉1.4) and low (〈1.0) ABI values with AF were examined separately and were in the same direction as the main result for PAD (ABI 〈 1.0: HR = 1.24, 95% CI = 1.08, 1.42; ABI 〉 1.4: HR = 1.33, 95% CI = 0.95, 1.86). Conclusion The presence of PAD should alert practitioners to the increased risk of AF. Elderly patients with PAD possibly will benefit from routine electrocardiographic screening to identify AF events.

Beschreibung: Large peridotite massifs are scattered along the 1500 km length of the Yarlung–Zangbo Suture Zone (southern Tibet, China), the major suture between Asia and Greater India. Diamonds occur in the peridotites and chromitites of several massifs, together with an extensive suite of trace phases that indicate extremely low f O 2 (SiC, nitrides, carbides, native elements) and/or ultrahigh pressures (UHP) (diamond, TiO 2 II, coesite, possible stishovite). New physical and isotopic (C, N) studies of the diamonds indicate that they are natural, crystallized in a disequilibrium, high- T environment, and spent only a short time at mantle temperatures before exhumation and cooling. These constraints are difficult to reconcile with previous models for the history of the diamond-bearing rocks. Possible evidence for metamorphism in or near the upper part of the Transition Zone includes the following: (1) chromite (in disseminated, nodular and massive chromitites) containing exsolved pyroxenes and coesite, suggesting inversion from a high- P polymorph of chromite; (2) microstructural studies suggesting that the chromitites recrystallized from fine-grained, highly deformed mixtures of wadsleyite and an octahedral polymorph of chromite; (3) a new cubic Mg-silicate, with the space group of ringwoodite but an inverse-spinel structure (all Si in octahedral coordination); (4) harzburgites with coarsely vermicular symplectites of opx + Cr–Al spinel ± cpx; reconstructions suggest that these are the breakdown products of majoritic garnets, with estimated minimum pressures to 〉 13 GPa. Evidence for a shallow pre-metamorphic origin for the chromitites and peridotites includes the following: (1) trace-element data showing that the chromitites are typical of suprasubduction-zone (SSZ) chromitites formed by magma mixing or mingling, consistent with Hf-isotope data from magmatic (375 Ma) zircons in the chromitites; (2) the composition of the new cubic Mg-silicate, which suggests a low- P origin as antigorite, subsequently dehydrated; (3) the peridotites themselves, which carry the trace element signature of metasomatism in an SSZ environment, a signature that must have been imposed before the incorporation of the UHP and low- f O 2 phases. A proposed P – T – t path involves the original formation of chromitites in mantle-wedge harzburgites, subduction of these harzburgites at c . 375 Ma, residence in the upper Transition Zone for 〉200 Myr, and rapid exhumation at c . 170–150 Ma or 130–120 Ma. Os-isotope data suggest that the subducted mantle consisted of previously depleted subcontinental lithosphere, dragged down by a subducting oceanic slab. Thermomechanical modeling shows that roll-back of a (much later) subducting slab would produce a high-velocity channelized upwelling that could exhume the buoyant harzburgites (and their chromitites) from the Transition Zone in 〈 10 Myr. This rapid upwelling, which may explain some characteristics of the diamonds, appears to have brought some massifs to the surface in forearc or back-arc basins, where they provided a basement for oceanic crust. This model can reconcile many apparently contradictory petrological and geological datasets. It also defines an important, previously unrecognized geodynamic process that may have operated along other large suture zones such as the Urals.

Beschreibung: The genesis of primitive arc magmas has had a major impact on continent formation through time, but the rarity of exposures of deep arc sections limits our understanding of the details of melt migration and differentiation. Abundant pyroxenites are exposed within a 600 m thick section of arc-related mantle harzburgites and dunites in the Herbeira massif of the Cabo Ortegal Complex, Spain. We report a combination of field and petrographic observations with in situ and whole-rock geochemical studies of these pyroxenites. After constraining the effects of secondary processes (serpentinization, fluid or melt percolation and amphibolitization), we determine that the low Al content of pyroxenes, high abundance of compatible elements and the absence of plagioclase reflect melt–peridotite interaction and crystal segregation from primitive hydrous melts at relatively low pressure (〈1·2 GPa). Olivine clinopyroxenites and olivine websterites preserving dunite lenses (type 1 and 3 pyroxenites) represent the products of partial replacement of peridotites at decreasing melt/rock ratio following the intrusion of picritic melts. Massive websterites (type 2) may represent the final products of this reaction at higher melt/rock ratios. They crystallized from more Si-rich (boninitic) melts, potentially generated through differentiation of the initially picritic melts or intruded as dykes and veins. Rare opx-rich websterites (type 4) were produced by interaction of these melts with dunites. Chromatographic re-equilibration accompanied late-magmatic crystallization of amphibole from migrating or trapped residual melts. This percolative fractional crystallization produced a range of rare earth element (REE) patterns from spoon-shaped in type 1 pyroxenites to strongly light REE (LREE)-enriched in type 2 and 3 pyroxenites. Particularly high CaO/Al 2 O 3 ratios (2·2–11·3) and the selective enrichment of large ion lithophile elements (LILE) over high field strength elements (HFSE) in Cabo Ortegal pyroxenites suggest the generation of Ca-rich picritic–boninitic parental melts via low-degree, second-stage melting of a refractory lherzolite at 〈2 GPa, following percolation of slab-derived fluids and/or carbonatite melts. Pyroxenites and their host peridotites record high-temperature deformation followed by the development of sheath folds and mylonites. Peak metamorphism was then reached under eclogite-facies conditions (1·6–1·8 GPa and 780–800°C) as recorded by undeformed garnet coronas around spinel. We suggest that this episode corresponds to the delamination of an arc root owing to gravitational instabilities arising from the presence of abundant pyroxenites within mantle harzburgites. Retrograde metamorphism and hydration under amphibolite-facies conditions were recorded by abundant post-kinematic amphibole, which corresponds to the exhumation of the arc root after its intrusion into a subduction zone. The Cabo Ortegal Complex thus preserves a unique section of delaminated arc root, providing evidence for the significant role of melt–peridotite interaction during the differentiation of primitive arc magmas at depth.