Description: Plastic deformation of diamond has long been associated with the generation of color, specifically brown and pink. Extensive previous optical and spectroscopic characterization of natural pink Type I (nitrogen containing) diamonds has revealed two clear groupings, with distinct geographical origins. Group 1 pinks, which have low concentrations of nitrogen and are relatively highly aggregated (IaA ≤ B), have only been found in the Argyle lamproite pipe (Australia) and Santa Elena alluvial deposits (Venezuela). Group 2 pinks, which have much higher nitrogen concentrations and exhibit low levels of aggregation, have been found in deposits from southern Africa, Canada, and Russia. Pink color is intimately associated with deformation lamellae on the {111} crystal planes, and understanding their formation and structure has been a priority with respect to defining the source of this gemologically valuable color center. In group 2 pinks, these {111} lamellae have been characterized as deformation microtwins by both transmission electron microscopy and X-ray diffraction. Subsequently the {111} lamellae in group 1 pinks have been assumed to also be deformation microtwins. In this paper we report electron backscatter diffraction (EBSD) studies of three brown and six pink naturally deformed diamonds with varying nitrogen concentrations and aggregation states. The results show that there are no deformation microtwins in the group 1 pink or brown diamonds. The study also highlights the usefulness of orientation contrast imaging as a simple and rapid method for determining the presence of microtwins. Our results suggest that the color in the group 1 pink diamonds is not directly related to the presence of deformation twins. However, we propose that twins may have been present but subsequently removed by de-twinning, a process that utilizes the same Shockley partial dislocations involved in the original twinning event. Therefore, it may be the process of twinning (and de-twinning) that creates the defect responsible for pink color, as opposed to the actual structure of microtwins themselves. In addition, a large laboratory data set of pink diamond analyses reveals the occurrence of group 1 pink diamonds in the Namibian marine (secondary) deposits. This would appear to suggest an additional source of group 1 pink diamonds in southern Africa, but the antiquity of these diamonds means that a common source on the former Pangaea supercontinent cannot be ruled out.

Description: The Paleoproterozoic Mojave and Yavapai crustal provinces in southwestern Laurentia contain evolved and juvenile crust, respectively, but the nature of the province boundary remains uncertain. 1.78–1.35 Ga crystalline basement rocks of the Mojave Province preserve an evolved isotopic signature reflecting an Archean crustal component in several isotopic systems (Nd, Pb, Hf). However, no Archean rocks have been found, and hence the origin and tectonic significance of this Archean component are also unclear. This paper analyzes the U-Pb age and Hf isotopic composition of zircons from both the oldest granodiorite plutons (1.84–1.71 Ga) and the oldest metasedimentary rocks (1.75 Ga Vishnu Schist) across a 180-km-long cross-strike transect in Grand Canyon. This transect crosses the Crystal shear zone, which has been proposed as the location of a suture separating the provinces. Our results show that the characteristically bimodal population of detrital zircons in the Vishnu Schist (2.5 Ga and 1.8 Ga modes) yields mixed Hf(t) values, primarily between +5 to –5, that are uniform across the transect. Another new finding is that the 1.84 Ga Elves Chasm pluton, on which the Vishnu Schist was deposited, yields juvenile Hf(t) values of +5 to +12 and was not the dominant source for the ca. 1.85 Ga peak in the 1.75 Ga Vishnu Schist. Instead, the Vishnu Schist was derived from an Archean craton mixed with intermediate to evolved 1.85 Ga crust. Metasediments show no evidence in support of the proposed suture. Paradoxically, plutons east and west of the Crystal shear zone do support models for a crustal suture. Plutons east of the Crystal shear zone dated at 1.74–1.71 Ga yield juvenile Hf(t) values of +5 to +12 that are characteristic of the Yavapai Province. Plutons west of the Crystal shear zone show juvenile to evolved Paleoproterozoic grains ( Hf(t) of –5 to +10) as well as xenocrystic Archean and 1.85 Ga grains ( Hf(t) of –12 to +10). These data support the proposition that the Crystal shear zone marks a sharp boundary between the Mojave and Yavapai crustal provinces. However, the overlapping Vishnu Schist suggests a more complicated crustal architecture. The depositional setting of the Vishnu Schist remains unclear; however, we interpret the ultimate geometry of the transect to reflect an ~200-km-wide middle-crustal duplex system in which the 1.75 Ga Vishnu Schist was deposited across both Mojave and Yavapai crust. This system was subsequently imbricated in an accretionary complex. The ultimate architecture is of a distributed boundary with slivers of plutons that carry the isotopic signature of their respective provinces imbricated within metasediments.

Description: 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.

Description: 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.

Description: 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.

Description: 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.

Description: 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.