Large peridotite massifs are scattered along the 1500km 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 fO sub(2) (SiC, nitrides, carbides, native elements) and/or ultrahigh pressures (UHP) (diamond, TiO sub(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 plus or minus cpx; reconstructions suggest that these are the breakdown products of majoritic garnets, with estimated minimum pressures to>13GPa. 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 (375Ma) 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-fO sub(2) phases. A proposed P-T-t path involves the original formation of chromitites in mantle-wedge harzburgites, subduction of these harzburgites at c. 375Ma, residence in the upper Transition Zone for >200 Myr, and rapid exhumation at c. 170-150Ma or 130-120Ma. 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.
Coarse-grained xenoliths of hibonite + grossite + Mg-Al-V spinel from Cretaceous pyroclastic rocks on Mt. Carmel, N. Israel, and from Sierra de Comechingones, Argentina, include spherules, rods and ...dense branching structures of native vanadium and VAl alloys. Microstructures suggest that vanadium melts became immiscible with the host Ca-Al-Mg-Si-O melt, and nucleated as droplets on the surfaces of the oxide phases, principally hibonite. Many extended outward as rods or branching structures as the host oxide crystal grew. The stability of V0 implies oxygen fugacities ≥9 log units below the Iron-Wustite buffer, suggesting a hydrogen-dominated atmosphere. This is supported by wt%-levels of hydrogen in gasses released by crushing, by Raman spectroscopy, and by the presence of VH2 among the vanadium balls. The oxide assemblage formed at 1400–1200 °C; the solution of hydrogen in the metal could lower the melting point of vanadium to these temperatures. These assemblages probably resulted from reaction between differentiated mafic melts and mantle-derived CH4 + H2 fluids near the crust-mantle boundary, and they record the most reducing magmatic conditions yet documented on Earth.
•Hibonite-grossite-vanadium xenoliths in basalt record Earth's most reducing conditions.•Vanadium (V0) melts were immiscible with Ca-Al-Mg-Si oxide melts at T > 1200 °C.•Melting T of V0 was lowered by solution of H2; VH2 phase coexists with V0.•V0 “dendrites” contain ca 3% void spaces, developed by expulsion of H2 on cooling.•Raman spectra suggest that some H2 diffused into host hibonite.
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•Ultra-low fO2 reflects fluxing of basaltic differentiates by mantle-derived CH4.•Fractionation involved progressive reduction + multiple stages of melt immiscibility.•Upper-mantle ...CH4-H2 fluids accompany magmatism in many tectonic settings worldwide.
Oxygen fugacity (ƒO2) controls the speciation of COH fluids in Earth’s mantle; a major question is whether the sublithospheric mantle is metal-saturated, maintaining ƒO2 near the Iron-Wüstite (IW) buffer reaction. If so, then COH fluids from this source will be dominated by CH4 + H2, rather than the more oxidized CO2-H2O fluids commonly considered in petrological studies. A key to this question is found in rare but widespread examples of natural mineral assemblages that require unusually low ƒO2. We summarize an investigation of super-reduced mineral assemblages in corundum xenocrysts from Late Cretaceous alkali-basalt volcanoes on Mt Carmel, northern Israel and related Plio-Pleistocene alluvial deposits. P-T estimates indicate that the corundum xenocrysts crystallized in the uppermost mantle. The well-documented geological controls on the origin of these deposits, and radiometric dating of the super-reduced phases, ensure the “naturalness” of the controversial assemblages and make these mineral parageneses a benchmark for evaluation of related occurrences worldwide.
The tuffs contain a “basalt-megacryst” mineral suite (zircon, sapphire, ilmenite, spinel). The megacryst chemistry and the geochronology of the zircons indicate that the megacrysts crystallized from broadly syenitic melts that differentiated at subcrustal levels (P ca 1 GPa) within a thick gabbroic underplate built up from Permian through Pliocene time and perhaps into the Pleistocene. Reaction of mantle-derived CH4-H2 fluids with these syenitic melts led to the separation of immiscible Fe0 and Fe-Ti oxide melts near fO2 = IW. Trace-element distributions suggest the syenitic melts then separated into immiscible Si-Al-Na-K-rich and FeO-rich oxide melts; the latter were enriched in HFSE, REE, P and Zr as in other natural and synthetic examples of melt-melt immiscibility.
In a model magma chamber the FeO-rich melts would sink, leaving the Si-Al-Na-K melts in an upper zone, both still fluxed by CH4-H2 fluids. At fO2 of ΔIW-6 to -7 the removal of immiscible Fe-Ti-Si-C silicide melts from the FeO-rich melt would leave a desilicated Ca-Al-Si oxide melt that crystallized high-Ti corundum hibonite cumulates with inclusions requiring fO2 from ΔIW + 2 to ΔIW-9, while the less-reduced conjugate silicate melts in the upper levels crystallized low-Ti corundum. Aggregates of skeletal, strongly Ti-zoned corundum crystals. reflect rapid crystallization from very reduced melt-fluid mixtures, probably in fluid-escape channels. Explosive eruptions sampled individual magma chambers at different depths and with different initial compositions, fluid mixtures and fluid dynamics to produce Mt Carmel’s mineralogical diversity.
A review of similar occurrences worldwide suggests that the Mt Carmel assemblages reflect a fundamental process – the rise of CH4-H2 fluids into the upper mantle -- that accompanies mantle-derived magmatism in many tectonic settings. The interaction of these fluids with lithospheric mantle rocks and melts can lead to extreme fractionation via the separation of immiscible Fe-Ti-Si-C melts and residual desilicated melts. The oxidation of CH4-H2 fluids in the lithospheric mantle may be the ultimate source of metasomatic fluids dominated by CO2 + H2O, and of many diamonds. More attention should be paid to the role of methane and other reduced fluids in mantle petrology, and their relevance to metasomatic processes and global carbon cycles.
Xenocrystic zircons from Cretaceous pyroclastic vents on Mt. Carmel, N. Israel, document two major periods of earlier mafic magmatism: Permo-Triassic (285–220 Ma) and Jurassic (200–160 Ma). Related ...alluvial deposits also contain these zircon populations. However, most alluvial zircons are Cretaceous (118–80 Ma) or younger, derived from Miocene to Pliocene volcanic episodes. The Permo-Triassic-Jurassic zircons are typically large and glassy; they have irregular shapes and a wide variety of internal zoning patterns. They appear to have grown in the interstitial spaces of coarse-grained rocks; many show evidence of recrystallization, including brecciation and rehealing by chemically similar zircon. Grains with relict igneous zoning have mantle-like δ18O (5.5 ± 1.0‰), but brecciation leads to lower values (mean 4.8‰, down to 3.1‰). Hf-isotope compositions lie midway between the Chondritic Uniform Reservoir (CHUR) and Depleted Mantle (DM) reservoirs; Hf model ages suggest that the source region separated from DM in Neoproterozoic time (1500–1000 Ma). Most Cretaceous zircons have 176Hf/177Hf similar to those of the older zircons, suggesting recrystallization and/or Pb loss from older zircons in the Cretaceous thermal event. The Permo-Jurassic zircons show trace-element characteristics similar to those crystallized from plume-related magmas (Iceland, Hawaii). Calculated melts in equilibrium with them are characterized by strong depletion in LREE and P, large positive Ce anomalies, variable Ti anomalies, and high and variable Nb, Ta, Th and U, consistent with the fractionation of monazite, zircon, apatite and Ti-bearing phases. We suggest that these coarse-grained zircons crystallized from late differentiates of mafic magmas, ponded near the crust-mantle boundary (ca 30 km depth), and were reworked repeatedly by successively younger igneous/metasomatic fluids.
The zircon data support a published model that locates a fossil Neoproterozoic plume head beneath much of the Arabia-Levant region, which has been intermittently melted to generate the volcanic rocks of the region. The Cretaceous magmas carry mantle xenoliths derived from depths up to 90 km, providing a minimum depth for the possible plume head. Post-Cretaceous magmatism, as recorded in detrital zircons, shows distinct peaks at 30 Ma, 13 Ma, 11.4 ± 0.1 Ma (a major peak; n = 15), 9–10 Ma and 4 Ma, representing the Lower and Cover Basalts in the area. Some of these younger magmas tapped the same mantle source as the Permian-Jurassic magmatism, but many young zircons have Hf-isotope compositions extending up to DM values, suggesting derivation of magmas from deeper, more juvenile sources.
•Zircon xenocrysts in Cretaceous pyroclastics on Mt. Carmel date gabbroic differentiates.•Two major magmatic pulses – Permo-Triassic, Jurassic – underplated the lithosphere.•LuHf and trace-element data have OIB signatures, and O-isotopes are mantle-like.•The LuHf data of zircons are consistent with derivation of magmas from a stalled Proterozoic plume head.
Piston-cylinder experiments with natural rocks and mineral separates were carried out at 750–900 °C and 2.9 GPa, conditions relevant to hot subduction zones, to study the mechanisms of metasomatic ...alteration of mantle-wedge rocks such as dunite and lherzolite, and the transfer of trace elements released from a carbonate-bearing amphibolite during its eclogitization. Element transfer from the slab to the mantle lithologies occurred in porous-, focused- and diffusive-flow regimes that remove melt and carbon, and partially water, from the metabasite layer. Porous flow is recorded by dissolution of clinopyroxene and growth of orthopyroxene ± garnet ± magnesite ± chlorite along grain boundaries in the peridotite layers, but is invisible in the metabasite layers. Porous flow of the same fluids/melts produces harzburgite mineralogy in both dunite and lherzolite. The transformation of lherzolite to harzburgite reflects breakdown of clinopyroxene in the lherzolite and diffusion of the liberated calcium into the metabasite layer, i.e. against the direction of major fluid/melt flow. Focused flow develops along the side walls of the capsules, producing a melt-free omphacite ± phengite ± quartz paragenesis in the metabasite, and melt segregations, separated from the host peridotite layers by newly-formed omphacite ± garnet ± phlogopite + orthopyroxene + magnesite. Diffusive flow leads to the formation of orthopyroxene ± magnesite ± garnet reaction zones at the metabasite-peridotite interface and some melt-peridotite interfaces.
Melt segregations in the peridotite layers at 850–900 °C are rich in LREE and LILE, strongly depleted in Y and HREE, and have higher Sr/Y and La/Yb ratios than island arc andesites, dacites and rhyolites. These features, and negative anomalies in Nb–Ta and low Nb/Ta, resemble those of high-silica adakites and TTGs, but K2O is high compared to TTGs. Metasomatism in the dunite layer changes the REE patterns of dunite, recording chromatographic fractionation during porous melt flow. During metabasite-lherzolite interaction, the metabasite layer becomes mildly enriched in LREE; the lherzolite layer, in contrast, is generally depleted in LREE relative to the initial composition. This also indicates element transfer against the direction of fluid flow. Trace-element profiling reveals the development of Eu anomalies in the peridotite layers and the diffusion of many trace elements out of both layers toward the contact zone. The documented processes may be applicable to both Phanerozoic and Precambrian subduction zones.
•Three fluid/melt flow regimes may operate simultaneously in a mantle wedge.•Slab-derived acid melts/fluids can generate harzburgite by depletion of lherzolite.•Many elements migrate out of metabasite and peridotite layers toward the reaction zone.•Fluid/melt flow transports carbon from metabasite to peridotite layers.
Cataract surgery in diabetics is more technically challenging due to a number of factors including poor intraoperative pupil dilation and a higher risk of vision threatening complications. This study ...evaluates the safety and efficacy of an intracameral combination of 2 mydriatics and 1 anesthetic (ICMA, Mydrane) for cataract surgery in patients with well-controlled type-2 diabetes.
Post-hoc subgroup analysis of a phase 3 randomized study, comparing ICMA to a conventional topical regimen. Data were collected from 68 centers in Europe and Algeria. Only well-controlled type-2 diabetics, free of pre-proliferative retinopathy, were included. The results for non-diabetics are also reported. The primary efficacy variable was successful capsulorhexis without additional mydriatic treatment. Postoperative safety included adverse events, endothelial cell density and vision.
Among 591 randomized patients, 57 (9.6%) had controlled type 2 diabetes 24 (42.1%) in the ICMA Group and 33 (57.9%) in the Topical Group; intention-to-treat (ITT) set. Among diabetics, capsulorhexis was successfully performed without additional mydriatics in 24 (96.0%; modified-ITT set) patients in the ICMA Group and 26 (89.7%) in the Topical Group. These proportions were similar in non-diabetics. No diabetic patient 1 (0.5%) non-diabetics in the ICMA Group had a significant decrease in pupil size (≥3 mm) intraoperatively compared to 4 (16.0%; modified-ITT set) diabetics 16 (7.3%) non-diabetics in the Topical group. Ocular AE among diabetics occurred in 2 (8.0%; Safety set) patients in the ICMA Group and 5 (16.7%) in the Topical Group. Endothelial cell density at 1 month postoperatively was similar between groups in diabetics (P = 0.627) and non-diabetics (P = 0.368).
ICMA is effective and can be safely used in patients with well-controlled diabetes, with potential advantages compared to a topical regimen including reduced systemic risk, better corneal integrity and reduced risk of ocular complications.
The trial was registered at (reference # NCT02101359) on April 2, 2014.
Totally implantable portal systems are widely used for long-term central venous access. A new venous portal system inserted via the brachial veins (P.A.S. Port system, Pharmacia Deltec Inc, U.S.A.) ...was studied in five centres. From January 1988 through May 1989 61 systems were implanted. Fifty-two patients had malignant diseases. Nine cases had non-malignant disorders. The portals were implanted subcutaneously in the fore-arm and catheterization was done percutaneously (46) or by cutdown-technique (15) under local anesthesia. Catheter tip position was controlled by fluoroscopy or x-ray. The basilic vein (49) and the cephalic vein (12) were used. The total follow-up time for all systems was 323 months. Forty-five systems were still in use at the end of the observation period, six were explanted electively at the end of infusion therapy and six systems were still functioning at the time of the patient's death (at a maximum of 14 months after implantation). Temporary armphlebitis was noticed on the first postoperative week in five patients. Two P.A.S. Port systems were explanted due to infection and one because of skin rupture at the wound. One intact system was removed as it was thought to be leaking because of needle displacement. The P.A.S. Port system is easy and safe to implant with a high success rate and a low complication rate. It is well accepted by patients and nurses. The device should be advantageous in patients unsuitable for standard venous portal systems and offers an excellent alternative system for venous access.
Abstract
Aggregates of corundum crystals with skeletal to hopper morphology occur in pyroclastic rocks erupted from Cretaceous basaltic volcanoes on Mt Carmel, N. Israel. The rapid growth of the ...crystals trapped volumes of the parental Al
2
O
3
-supersaturated melt; phenocrysts of tistarite (Ti
2
O
3
) in the trapped melts indicate crystallization at oxygen fugacities 6–7 log units below the Iron-Wüstite buffer (
f
O
2
= ΔIW − 6 to − 7), induced by fluxes of mantle-derived CH
4
-H
2
fluids. Cathodoluminescence images reveal growth zoning within the individual crystals of the aggregates, related to the substitution of Ti
3+
in the corundum structure. Ti contents are < 0.3 wt% initially, then increase first linearly, then exponentially, toward adjacent melt pockets to reach values > 2 wt%. Numerical modelling indicates that the first skeletal crystals grew in an open system, from a moving magma. The subsequent linear increase in Ti reflects growth in a partially closed system, with decreasing porosity; the exponential increase in Ti close to melt pockets reflects closed-system growth, leading to dramatic increases in incompatible-element concentrations in the residual melts. We suggest that the corundum aggregates grew in melt/fluid conduits; diffusion modelling implies timescales of days to years before crystallization was terminated by explosive eruption. These processes probably operate in explosive volcanic systems in several tectonic settings.
Zircon megacrysts from the Mud Tank carbonatite, Australia, are being used in many laboratories as a reference material for LA‐ICP‐MS U‐Pb dating and trace element measurement, and LA‐MC‐ICP‐MS ...determination of Hf isotopes. We summarise a database of > 10000 analyses of Mud Tank zircon (MTZ), collected from 2000 to 2018 during its use as a secondary reference material for simultaneous U‐Pb and trace element analysis, and for Hf‐isotope analysis. Trace element mass fractions are highest in dark red‐brown stones and lowest in colourless and gem‐quality ones. Individual unzoned grains can be chemically homogeneous, while significant variations in trace element mass fraction are associated with oscillatory zoning. Chondrite‐normalised trace element patterns are essentially parallel over large mass fraction ranges. A Concordia age of 731.0 ± 0.2 Ma (2s, n = 2272) is taken as the age of crystallisation. Some grains show lower concordant to mildly discordant ages, probably reflecting minor Pb loss associated with cooling and the Alice Springs Orogeny (450–300 Ma). Our weighted mean 176Hf/177Hf is 0.282523 ± 10 (2s, n = 9350); the uncertainties on this ratio reflect some heterogeneity, mainly between grains. A few analyses suggest that colourless grains have generally lower 176Hf/177Hf. MTZ is a useful secondary reference material for U‐Pb and Hf‐isotope analysis, but individual grains need to be carefully selected using CL imaging and tested for homogeneity, and ideally should be standardised by solution analysis.
Key Points
Summary of > 10000 analyses of Mud Tank Zircon (MTZ) for U‐Pb, TE and Hf isotopes.
Concordia age of 731.0 ± 0.2 Ma (2s, n = 2272) taken as age of crystallisation.
Mean 176Hf/177Hf is 0.282523 ± 10 (2s, n = 9350).
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