The geodynamic evolution of the South Armenian Block (SAB) within the Tethyan realm during the Palaeozoic to present-day is poorly constrained. Much of the SAB is covered by Cenozoic sediments so that the relationships between the SAB and the neighbouring terranes of Central Iran, the Pontides and Taurides are unclear. Here we present new geochronological, palaeomagnetic, and geochemical constraints to shed light on the Gondwanan and Cimmerian provenance of the SAB, timing of its rifting, and geodynamic evolution since the Permian. We report new 40Ar/39Ar and zircon U-Pb ages and compositional data on magmatic sills and dykes in the Late Devonian sedimentary cover, as well as metamorphic rocks that constitute part of the SAB basement. Zircon age distributions, ranging from ∼3.6 Ga to 100 Ma, firmly establish a Gondwanan origin for the SAB. Trondhjemite intrusions into the basement at ∼263 Ma are consistent with a SW-dipping active continental margin. Mafic intraplate intrusions at ∼246 Ma (OIB) and ∼234 Ma (P-MORB) in the sedimentary cover likely represent the incipient stages of breakup of the NE Gondwanan margin and opening of the Neotethys. Andesitic dykes at ∼117 Ma testify to the melting of subduction-modified lithosphere. In contrast to current interpretations, we show that the SAB should be considered separate from the Taurides, and that the Armenian ophiolite complexes formed chiefly in the Eurasian forearc. Based on the new constraints, we provide a geodynamic reconstruction of the SAB since the Permian, in which it started rifting from Gondwana alongside the Pontides, likely reached the Iranian margin in Early Jurassic times, and was subject to episodes of intraplate (∼189 Ma) and NE-dipping subduction-related (∼117 Ma) magmatism.
Hollow tubular structures in subaqueously-emplaced basaltic glass may represent trace fossils caused by microbially-mediated glass dissolution. Mineralized structures of similar morphology and spatial distribution in ancient, metamorphosed basaltic rocks have widely been interpreted as ichnofossils, possibly dating to ~3.5 Ga or greater. Doubts have been raised, however, regarding the biogenicity of the original hollow tubules and granules in basaltic glass. In particular, although elevated levels of biologically-important elements such as C, S, N and P as well as organic compounds have been detected in association with these structures, a direct detection of unambiguously biogenic organic molecules has not been accomplished. In this study, we describe the direct detection of proteins associated with tubular textures in basaltic glass using synchrotron X-ray spectromicroscopy. Protein-rich organic matter is shown to be associated with the margins of hollow and partly-mineralized tubules. Furthermore, a variety of tubule-infilling secondary minerals, including Ti-rich oxide phases, were observed filling and preserving the microtextures, demonstrating a mechanism whereby cellular materials may be preserved through geologic time.
Hydrogen solubility and hydroxyl substitution mechanism in olivine at upper-mantle conditions are not only a function of pressure, temperature, water fugacity and hydrogen fugacity, but are also influenced by silica activity. Olivine synthesized in equilibrium with magnesiowüstite displays hydroxyl stretching bands in the wavenumber range from 3640 to 3430 cm–1. In contrast, olivine in equilibrium with orthopyroxene shows absorption bands in a narrower wavenumber range from 3380 to 3285 cm–1. The two fundamentally different spectra are assigned to hydroxyl in tetrahedral and octahedral sublattices, respectively. Olivine in equilibrium with orthopyroxene is also less capable of incorporating hydroxyl, relative to olivines in equilibrium with magnesiowüstite, by about a factor of ten. A comparison of spectra obtained as part of this study with hydroxyl spectra of natural mantle olivines shows that the latter display hydroxyl stretching patterns reminiscent of equilibrium with magnesiowüstite, although undoubtedly olivine in the Earth's mantle coexists with orthopyroxene. This may be attributed to a metasomatic overprint by a low-silica fluid and/or melt that was in reaction relationship with orthopyroxene. A likely metasomatic agent is a carbonatitic melt. When carbonatitic melts decompose to oxides and CO2, they may temporarily impose a low-aSiO2 environment inherited by the olivine structure. If this suggestion proves true, Fourier transform IR spectroscopy may be used to fingerprint metasomatic episodes in the lithospheric mantle.
Volatiles in magmas play an important role in the eruption style and the geology of volcanic landforms, determine presence of fluid phase, the depths of volatile exsolution, and the rates of magma accent. In kimberlites the original proportions of the two main volatiles, H2O and CO2, are obscured by complex origin of the groundmass minerals. Volatiles affect diamond preservation and determine the character of surface features produced on diamond faces during dissolution in kimberlites. Experiments demonstrated that oxidation of diamonds in magmas with H2Oor CO2-rich fluids and in the absence of fluid produce distinctively different types of surface features. In addition, water fugacity of kimberlitic magma can be estimated using water content in phenocrystic olivine measured by FTIR spectroscopy. We apply these two independent methods to several kimberlites in order to constrain the behavior of volatiles and their effect on diamond population and the geology of kimberlites. The study uses diamond parcels, olivine concentrates, and kimberlite core from six EKATI Mine kimberlites, Northwest Territories, Canada. These kimberlites have similar emplacement ages, erosion level, and country rocks, but different geology, composition, and diamond populations. The surface features on diamonds studied under optical and scanning electron microscopes were compared to the diamond surfaces produced experimentally in the presence and absence of fluid. Concentration and occurrence of hydroxyl in kimberlitic olivine were measured using FTIR spectroscopy. We found that Leslie and Grizzly kimberlites filled with hypabyssal facies, with low grade and quality of diamonds, show very sharp dissolution forms on diamond surfaces. Such features indicate absence of a free fluid phase during the last stages of kimberlite emplacement. Panda, Beartooth, Misery, and Koala kimberlites filled with volcaniclastic kimberlite, with higher grade and quality of diamonds have diamond surfaces with well-developed trigon pits, rounded edges with striation, and hillocks. Such features suggest emplacement in an H2O-fluidrich environment. However, diamond populations of Panda and Beartooth are dominated by octahedral unresorbed stones and IR spectra of their olivines give higher concentration of water in olivines <600 ppm and the depth of fluid separation greater than 2GPa. On the contrary, Misery and Koala diamonds are mostly rounded with high degree of resorption. Their olivines contain <450 ppm of H2O and fluid separated at more shallow depths. Group 2 OH IR absorption bands are absent in FTIR spectra of olivine from kimberlites filled with hypabyssal facies and present in olivine from all volcaniclasticfilled kimberlites. This can provide a possible link to an early loss of magmatic fluid. The excellent agreement between the two independent datasets suggests that both are linked to the activity of water in the system. We further apply these results to explain the differences between the geology of these kimberlite pipes and their diamond populations.
Lavas and pyroclastic products of Nisyros volcano (Aegean arc, Greece) host a wide variety of phenocryst and cumulate assemblages that offer a unique window into the earliest stages of magma differentiation. This study presents a detailed petrographic study of lavas, enclaves and cumulates spanning the entire volcanic history of Nisyros to elucidate at which levels in the crust magmas stall and differentiate. We present a new division for the volcanic products into two suites based on field occurrence and petrographic features: a low-porphyricity andesite and a high-porphyricity (rhyo)dacite (HPRD) suite. Cumulate fragments are exclusively found in the HPRD suite and are predominantly derived from upper crustal reservoirs where they crystallised under hydrous conditions from melts that underwent prior differentiation. Rarer cumulate fragments range from (amphibole-)wehrlites to plagioclase-hornblendites and these appear to be derived from the lower crust (0.5–0.8 GPa). The suppressed stability of plagioclase and early saturation of amphibole in these cumulates are indicative of high-pressure crystallisation from primitive hydrous melts (≥ 3 wt% H2O). Clinopyroxene in these cumulates has Al2O3 contents up to 9 wt% due to the absence of crystallising plagioclase, and is subsequently consumed in a peritectic reaction to form primitive, Al-rich amphibole (Mg# > 73, 12–15 wt% Al2O3). The composition of these peritectic amphiboles is distinct from trace element-enriched interstitial amphibole in shallower cumulates. Phenocryst compositions and assemblages in both suites differ markedly from the cumulates. Phenocrysts, therefore, reflect shallow crystallisation and do not record magma differentiation in the deep arc crust.
The composition and crystallization conditions of the parental melts of avachites were elucidated
by studying melt inclusions in olivine (Fo85.8–90.7) phenocrysts. The melt inclusions captured
during the crys- tallization of primitive magmas subsequently reequilibrated with their host
minerals and became partly recrys- tallized and decrepitated. The diffusion-controlled
reequilibration of the melt inclusions with the olivine occurred at temperatures close to ~1100°e
and was associated with the crystallization of daughter phases: oli- vine, high-Ca pyroxene, and
spinel. The composition of the pyroxene and spinel in the inclusions evolved toward extremely high
Al contents, which is atypical of pyroxene in the rocks and was controlled by plagioclase
absence from the daughter phase assemblage of the inclusions. Magma decompression induced the
partial decrepitation of the melt inclusions, a process that was associated with the escape of
fluid components (ee2 and c2e) and variable amounts of the residual silicate material from the
inclusions. The initial compositions of the melt inclusions, which were reconstructed using
techniques of experimental homogenization and mod- eling, show broad ranges in the contents of
major and trace elements. Compared with the composition of the rocks, the compositions of
inclusions in the olivine Fo > 90% are higher in CaO, Al2O3, and Na2O at lower concentrations of
SiO2. Their geochemical characteristics are identical to those of low-Si ankaramite melts occurring
in many island arcs. The carbonatite metasomatism of the arc mantle, the derivation of nepheline-
normative ankaramite magmas, and the significant crustal contamination of these magmas during their
fraction- ation can be spread more widely than is currently assumed in models for island-arc
petrogenesis. The evolution of the avachite primitive magmas was controlled by the crystallization
of early olivine, high-Ca pyroxene, spinel, and, perhaps, the assimilation of crustal rocks in the
magmatic chambers at different depths (from 5 to
30 km). During two (or more) crystallization stages, olivine–pyroxene cumulates were produced,
remobilized, and transported to the surface by the differentiated hypersthene-normative magmas.
Avachites are hybrid cumu-
lative rocks, which were produced in a long-lived open magmatic system.
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