Partial Melting Processes Research Articles

Chemical-Abrasion U-Pb zircon geochronology reveals 150 Myr of partial melting events in the Archean crust of the São Francisco Craton

Field observations and CA-LA-ICP-MS U–Pb zircon ages and Hf isotope compositions obtained from migmatitic orthogneisses and granitoids from the Belo Horizonte Complex, southern São Francisco Craton, indicate a major period of partial melting and production of felsic rocks in the Neoarchean. Our observations show that the complex is an important site for studying partial melting processes of Archean crystalline crust. Much of the complex exposes fine-grained stromatic migmatites that are intruded by multiple leucogranitic veins and sheeted dikes. Both migmatites and leucogranite sheets are crosscut by several phases of granitoid batholiths and small granitic bodies; both of which are closely associated with the host banded gneisses. Chemical abrasion followed by detailed cathodoluminescence imaging revealed a wide variety of zircon textures that are consistent with a long-lived period of partial melting and crustal remobilization. Results of U-Pb and Hf isotopes disclose the complex as part of a much wider crustal segment, encompassing the entire southern part of the São Francisco Craton. Compilation of available U-Pb ages suggests that this crustal segment was consolidated sometime between 3000 Ma and 2900 Ma and that it experienced three main episodes of partial melting before stabilization at 2600 Ma. The partial melting episodes took place between 2750 Ma and 2600 Ma as a result of tectonic accretion and peeling off the lithospheric mantle and lower crust. This process is likely responsible for the emplacement of voluminous potassic granitoids across the entire São Francisco Craton. We believe that the partial melting of Meso-Archean crystalline crust and production of potassic granitoids are linked to a fundamental shift in the tectonics of the craton, which was also responsible for the widespread intrusion of large syenitic bodies in the northern part of the craton, and the construction of layered mafic–ultramafic intrusions to the south of the BHC.

Lower crustal attenuation in northeastern Tibetan Plateau from ML amplitude

This study investigated the crustal attenuation structures of Sg and Lg waves of the northeastern Tibetan Plateau. We collected ML amplitude data recorded at 168 permanent stations between 1985 and 2016 and 11 temporary broadband stations between 2014 and 2016. Detailed Q0 variation maps of Sg and Lg waves were obtained by applying ML amplitude tomography. The average Q0 values of the Sg and Lg wave were 440 and 220, respectively. Relatively high attenuation anomalies of both waves appeared in the central and eastern regions of the Bayan Har Block and the east edge of the Qiangtang Block, which may be related to partial melting, high geotemperature, and strong tectonic processes. High attenuation anomalies were also found in the Qilian Orogenic Belt and Hetao Graben, which may be related to their active tectonic behavior and densely distributed faults. The relatively low attenuation anomalies of both waves were revealed in the Alax and Ordos blocks, Qaidam, Tarim, Qinghai Lake, and Gonghe basins, which can be explained by the tectonically stable properties and ancient composition of geological elements. These results indicate that the path between the highly attenuated lower crust of the Bayan Har Block and the Qilian Orogenic Belt is obstructed by three adjacent low attenuated areas (i.e., the Qilian, Qinghai Lake, and Gonghe basins); thus, it appears unlikely that a crustal flow channel from the interior of the Tibetan Plateau to the Qilian Orogenic Belt will form.

Conundrum of strongly coupled supercurrent flow in both under- and overdoped Bi-2212 round wires

Understanding what makes Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_x$ (Bi-2212) the only high critical current density ($J_c$), high temperature superconductor (HTS) capable of being made as a round wire (RW) is important intellectually because high $J_c$ RW Bi-2212 breaks the paradigm that forces biaxially textured REBCO and uniaxially textured (Bi,Pb)$_2$Sr$_2$Ca$_2$Cu$_3$O$_x$ (Bi-2223) into tape geometries that reproduce the strong anisotropy of the native crystal structure and force expensive fabrication routes to ensure the best possible texture with minimum density of high angle grain boundaries. The biaxial growth texture of Bi-2212 developed during a partial melt heat treatment should favor high $J_c$, even though its $\sim$15$^{\circ}$ full width at half maximum (FWHM) grain-to-grain misorientation is well beyond the commonly accepted strong-coupling range. Its ability to be strongly overdoped should be valuable too, since underdoped cuprate grain boundaries are widely believed to be weakly linked. Accordingly, we here study property changes after oxygen underdoping the optimized, overdoped wire. While $J_c$ and vortex pinning diminish significantly in underdoped wires, we were not able to develop the prominent weak-link signature (a hysteretic $J_c$(H) characteristic) evident in even the very best Bi-2223 tapes with a $\sim$ 15$^{\circ}$ FWHM uniaxial texture. We attribute the high $J_c$ and lack of weak link signature in our Bi-2212 round wires to the high-aspect ratio, large-grain, basal-plane-faced grain morphology produced by partial-melt processing of Bi-2212 which enables $c$-axis Brick-Wall current flow when $ab$-plane transport is blocked. We conclude that the presently optimized biaxial texture of Bi-2212 intrinsically constitutes a strongly coupled current path, regardless of its oxygen doping state.

Mantle melting models of the Kızıldağ ophiolite in SE Turkey: Two types of partial melting processes in the oceanic upper mantle of southern Neo-Tethys

Many ophiolites in southern Neo-Tethys contain complete lithologic sequences, with the dikes and lavas commonly having highly variable geochemical compositions. In this study, we examine the sheeted dikes and layered and isotropic gabbros of the Kızıldağ ophiolite (southern Turkey) and suggest that the compositional variations of the rocks are likely related to melting models of the mantle. The sheeted dikes and gabbros of the ophiolite have 206Pb/204Pb(90 Ma) of 17.765–19.361, 207Pb/204Pb(90 Ma) of 15.570–15.665 and 208Pb/204Pb(90 Ma) of 37.812–39.269, straddling the fields of Indian Ocean MORB and modern marine sediments. The sheeted dikes have low rare earth element (REE) concentrations and LaN/YbN ratios (e.g., Yb < 2.5 ppm and LaN/YbN of 0.33 to 0.62 on average), comparable with those of forearc basalt (FAB), except for a few samples whose trace element concentrations and patterns are similar to those of boninite. All the studied samples have positive anomalies of large ion lithophile elements (LILE) (e.g., Rb, Ba, Th, U and Sr) on a N-MORB-normalized trace element diagram and show considerable scatter on the plot of Th/Yb vs. Nb/Yb, which is attributed to involvement of slab-derived melts/fluids in a protoarc-forearc setting. Thermodynamic modelling results suggest that the parental magmas of the sheeted dikes were combined melts derived by both polybaric continuous melting (<10%) and flux-melting (<25%) of the mantle wedge during subduction initiation. In contrast, clinopyroxene and plagioclase in the layered and isotropic gabbros have compositions similar to those of boninite. The gabbro compositions can be simulated by batch crystallization of the boninitic melts, and the relatively high trace element concentrations of some isotropic gabbros likely reflect a high fraction of interstitial liquid in the crystal mush. Thus, we propose that the compositional variations of the sheeted dikes reflect two types of mantle melting, and that the gabbros were produced by subsequent crystallization within a crystal mush having variable fractions of interstitial liquid. Not only can such complex magmatic processes explain the observed compositional variations of dikes and lavas of the Kızıldağ ophiolite, but may be appliable to other ophiolites in southern Neo-Tethys, such as the Lycian, Pozantı-Karsantı and Troodos massifs.

Mantle sources and magma evolution in Europe's largest rare earth element belt (Gardar Province, SW Greenland): New insights from sulfur isotopes

Alkaline igneous complexes are often rich in rare earth elements (REE) and other metals essential for modern technologies. Although a variety of magmatic and hydrothermal processes explain the occurrence of individual deposits, one common feature identified in almost all studies, is a REE-enriched parental melt sourced from the lithospheric mantle. Fundamental questions remain about the origin and importance of the mantle source in the genesis of REE-rich magmas. In particular, it is often unclear whether localized enrichments within an alkaline province reflect heterogeneity in the mantle source lithology (caused by prior subduction or plume activity) or variations in the degree of partial melting and differentiation of a largely homogeneous source. Sulfur isotopes offer a means of testing these hypotheses because they are unaffected by high temperature partial melting processes and can fingerprint different mantle sources. Although one must be careful to rule out subsequent isotope fractionation during magma ascent, degassing and crustal interactions.Here, we present new S concentration and isotope (δ34S) measurements, as well as a compilation of major and trace element data, for a suite of alkaline magmatic units and crustal lithologies from the Mesoproterozoic Gardar Province. Samples span all phases of Gardar magmatism (1330–1140 Ma) and include regional dykes, rift lavas and the alkaline complexes Motzfeldt and Ilímaussaq, which represent two of Europe's largest REE deposits. We show that the vast majority of our 115 samples have S contents >100 ppm and δ34S of −1 to 5‰. Only 8 samples (with low S contents, <100 ppm) show evidence for crustal interactions, implying that the vast majority of Gardar melts preserve the S isotopic signature of their magma source. Importantly, samples from across the Gardar Province have δ34S above the canonical mantle range (≤−1.4‰) and therefore require recycled surface S in their mantle source. Elevated δ34S values are explained by a period of Andean-style subduction and mantle metasomatism which took place ∼500 Ma before rift onset and are also supported by trace elements signatures (e.g. Ba/La) which match modern subduction zones.Comparing the various generations of Gardar magmas, we find that δ34S values, large ion lithophile elements (K, Ba, P) and selective incompatible elements (Nb and light REE) are particularly enriched in the Late Gardar dykes, alkaline complexes and clusters of silica-undersaturated dykes spatially associated with the alkaline complexes. These data indicate that subduction-related metasomatism of the Gardar mantle was spatially heterogeneous, and that alkaline complexes are sourced from localized mantle domains highly enriched in 34S, REE, alkalis and volatiles (particularly, F). Since alkalis and volatiles play an essential role in driving extreme differentiation of alkaline melts and fluids, we suggest the co-location of these species plus incompatible metals at high concentrations in the lithospheric mantle is a critical first-step in the genesis of a world-class alkaline REE deposit. S isotopes are powerful tools for identifying enriched mantle domains and the sources of mineralized alkaline igneous bodies.

Subgrain Boundaries as Sites of Melt Generation and Pathways for Melt Flow: Implications for Rheology From Combined Chemical and Quantitative Orientation Analysis

AbstractWe report a detailed petrographic, compositional, structural, and crystallographic investigation into the partial melting process. Migmatitic samples that record biotite‐dehydration melting in the Wet Mountains of central Colorado preserve textural evidence for partial melting locations and an incipient former melt network. Detailed analysis of partial melting locations was conducted using a combined energy dispersive X‐ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) approach. Cuspate‐shaped films of former melt, now plagioclase, with low apparent dihedral angles and adjacent to rounded and corroded reactant minerals are interpreted as former locations of partial melting. We identified former melt films infiltrating and melting along quartz grains using optical microscopy and EDS and determined crystallographic misorientation across these transects using EBSD. Forty‐two percent of these transects are associated with subgrain boundaries, and the remaining transects are between adjacent quartz grains with various misorientations. Although previous work has emphasized grain boundaries and triple junctions as sites for melting, the presence of over 40% of the former melt films developed along subgrain boundaries suggests that these are favorable and potentially preferred locations for partial melting. Consequently, melting along subgrain boundaries must be integral to the evolution of partial melt networks and will increase melt network interconnectedness and generate a finer grain size, enhancing permeability and leading to focused melt flow. Given that these factors exert a fundamental control on crustal rheology, the role of melting and migration along subgrain boundaries should be investigated within diverse crustal and mantle lithologies and incorporated into models of partial melting.

Early Palaeoproterozoic granulite-facies metamorphism and partial melting of eclogite-facies rocks in the Salma association, eastern Fennoscandian Shield, Russia

The Salma-type Archaean eclogites exposed along the northwestern boundary of the Belomorian Eclogite Province in the eastern Fennoscandian Shield formed as a result of the Mesoarchaean–Neoarchaean subduction and collision. The common protoliths of the Salma-type subduction-related eclogites were oceanic layered gabbro and volcanic-sedimentary assemblage. The eclogite-facies pillow lavas and associated alumina-siliceous sediments that fill interpillow space and intercalate with lava flows are the main objects of our work. The kyanite-garnet–phengite–quartz rocks formed after alumina-siliceous sediments contain fluid inclusions trapped in large relic quartz grains. The fluid inclusions yielded an isochore that corresponds to PT-conditions of a beginning of the Salma oceanic rock subduction from the seafloor level that generally confirms the sedimentary provenance of these rocks. The alumina-siliceous sediments underwent the eclogite-facies metamorphism at pressure no lower than 21 kbar and temperatures of 650–750 °C and transformed into kyanite-garnet–phengite–quartz rocks. During exhumation under granulite-facies conditions at temperatures up to 900 °C and pressure down to 9 kbar, eclogite facies metasediments underwent partial melting accompanied by disequilibrium breakdown of phengite + quartz association with formation complex polymineralic pseudomorphs consisting of feldspars, biotite, muscovite, kyanite, corundum, and dumortierite. U-Pb dating of Th-rich igneous zircon from melted metasedimentary and mafic rocks using the LA-ICP-MS and TIMS methods yielded the time of granulite facies event accompanied by partial melting processes at ~2.45 Ga. After this, zircon underwent fluid-induced alteration, causing partial dissolution followed by precipitation of new Th–poor zircon and zircon rims around ancient grains at ~1.9 Ga ago

Changes in the composition of anatectic melt and its complementary residue by forward-modelling using THERMOCALC

This paper presents results from forward modelling of graywacke and shale average bulk compositions assuming closed-system and open-system partial melting processes. For each process, several compositions of anatectic melt and corresponding solid residue are calculated, and the results compared in binary, ternary and normative diagrams to evaluate how their compositions evolve during an isobaric heating path. Melt compositions are peraluminous, with A/CNK increasing from 1.05 to 1.45, as P and T decrease. The mg# ranges from 0.22 to 0.39, increasing with consumption of biotite and other FeMg phases by melting reactions as T rises. On FeO + MgO, CaO + Na2O, K2O ternary, K2O vs. Na2O binary and chemical normative diagrams, compositions display trends with an increase in the K2O content of melt as T rises, and an increase in FeO + MgO content after the complete consumption of biotite. An increase in orthoclase component along the heating path, followed by an increase in the proportion of quartz component after K-feldspar/plagioclase breakdown is illustrated on mesonorm diagrams. Total melt volume produced by graywacke bulk compositions is in the range 33–53 vol% for closed-system and 21–33 vol% for open-system melting. For the shale bulk composition volumes are in the range 50–70 vol% during closed-system and 30–45 vol% during open-system melting. The rate of melt production varies with the type of melting reaction and is between 0.19 and 4.73 vol% °C−1 for water-present melting and 0.01–6.82 vol% °C−1 for hydrate-breakdown melting. Retrograde metamorphic assemblages can be formed even with the chemical depletion caused by melt extraction, but an external source of water and other components would make the consumption of the high-T assemblages more efficient. The correlation between data from this study and that from melt inclusions and natural leucosomes indicates that the melt compositions calculated via forward modelling are a useful guide in understanding how leucosome compositions and their mineral assemblages evolve.

Geochemistry of Eocene felsic volcanic rocks from the Mesa Virgen‐Calerilla, Zacatecas, Mexico: Implications for the magma source and tectonic setting

The Mesa Virgen Calerilla (MVC) is located in the state of Zacatecas, Mexico. The most intense volcanism, which occurred during the Eocene, formed extensive ignimbrite deposits exposed in some parts as lava spills of rhyolitic (felsic) composition. This felsic volcanism may represent much of the MVC. This study describes whole‐rock geochemistry and mineralogy data from felsic volcanic rocks in the MVC to address their petrogenesis and tectonic setting. The MVC covers a compositional spectrum ranging from trachyte, dacite, to high‐silica rhyolite. The petrography and mineral assemblages indicate that the felsic rocks are composed of K‐feldspar (sanidine), quartz, plagioclase, and biotite. The felsic volcanic rocks have a composition of 64.08–78.17 wt% (SiO2)adj, 0.14–0.69 wt% (TiO2)adj, and 0.11–0.62 wt% (MgO)adj with 12–54 Mg number [Mg# = 100 × (Mg2+/[Mg2+ + Fe2+])]. These felsic volcanic rocks showed enrichment in light rare earth elements (LREE; [La/Sm]N = 3.80–7.19), and are depleted in heavy rare earth elements [HREE; (Tb/Yb)N ratios 0.35–1.84], along with negative Ba, Nb, Sr, P, Eu, and Ti anomalies. The geochemical characteristics and petrogenetic modelling indicate that felsic volcanic rocks are derived from partial melting process of an upper‐middle continental crust. The tectono‐magmatic model and multidimensional tectonic discrimination diagram indicate that an extensional‐related setting prevails for the MVC.

Middle Triassic high-K calc-alkaline effusive and pyroclastic rocks from the Zagorje-Mid-Transdanubian Zone (Mt. Kuna Gora; NW Croatia): mineralogy, petrology, geochemistry and tectonomagmatic affinity

This study uses mineralogical, petrological, geochemical, and Sr and Nd isotope data along with K-Ar ages to infer the petrogenesis and geodynamic evolution of Middle Triassic high-K calc-alkaline lavas and their associated pyroclastics of Mt. Kuna Gora in NW Croatia. Their analogue mineralogy and bulk-rock geochemistry testify to the coeval origin of both rock types. Sanidine and plagioclase accompanied by inor augite and Ti-bearing magnetite are the major phases found in a matrix of devitrified volcanic glass and plagioclase microlites. Hydrothermal anddiagenetic processes in the pyroclastics originated the formation of chlorite and white mica, and mixed-layer clay minerals, respectively. Petrography reveals the following crystallization order: spinel→clinopyroxene→plagioclase→alkali-feldspar±Fe-Ti oxides. Geochemical and isotopic data suggests that the studied rocks had a complex origin that included the contamination of subduction-generated magmas by lithospheric mantle melts. This presumes an interplay between fertile arc mantle, subducted continental crust, and depleted or ocean island basalts-like mantle. A low degree of crustal contamination stands as a last step in the formation of such “hybrid” magmas. The subducted Paleotethyan oceanic lithosphere went through processes of partial melting at depths of ~45-49km and pressures of ≤1.6GPa and fractionation that produced melts which gave rise to the studied rocks. In the model we are proposing herein such formed partial melts are related to the demise of the northward subduction of the Paleotethys oceanic lithosphere during the Early to Middle Triassic epoch, which is consistent with an active, ensialic mature volcanic arc developing along Laurussian southern active margins.

Competing effects of spreading rate, crystal fractionation and source variability on Fe isotope systematics in mid-ocean ridge lavas

Two-thirds of the Earth is covered by mid-ocean ridge basalts, which form along a network of divergent plate margins. Basalts along these margins display a chemical diversity, which is consequent to a complex interplay of partial mantle melting in the upper mantle and magmatic differentiation processes in lower crustal levels. Igneous differentiation (crystal fractionation, partial melting) and source heterogeneity, in general, are key drivers creating variable chemistry in mid-ocean ridge basalts. This variability is reflected in iron isotope systematics (expressed as δ57Fe), showing a total range of 0.2 ‰ from δ57Fe = + 0.05 to + 0.25 ‰. Respective contributions of source heterogeneity and magma differentiation leading to this diversity, however, remain elusive. This study investigates the iron isotope systematics in basalts from the ultraslow spreading Gakkel Ridge in the Arctic Ocean and compares them to existing data from the fast spreading East Pacific Rise ridge. Results indicate that Gakkel lavas are driven to heavier iron isotope compositions through partial melting processes, whereas effects of igneous differentiation are minor. This is in stark contrast to fast spreading ridges showing reversed effects of near negligible partial melting effects followed by large isotope fractionation along the liquid line of descent. Gakkel lavas further reveal mantle heterogeneity that is superimposed on the igneous differentiation effects, showing that upper mantle Fe isotope heterogeneity can be transmitted into erupting basalts in the absence of homogenisation processes in sub-oceanic magma chambers.

Zircon U–Pb ages and petrogenesis of the middle Eocene Aliabad Daman pluton, Northeast Iran: implications for magmatic activity along the Doruneh fault zone

The Aliabad Daman pluton is exposed along the Doruneh fault zone of the southern edge of the Sabzevar zone, northeast Iran. This intrusive body consists of alkali feldspar granite, syenogranite, and monzogranite. U–Pb age dating results demonstrate that the magmas crystallized 45–40 Ma ago in the middle Eocene (Lutetian to Bartonian). Whole-rock samples show SiO2 ranging between 75.4 and 77.0 wt.%, variable fractionation between light and heavy rare earth elements (REE) (LaN/YbN = 11–22) with negative Eu anomalies Eu/Eu* (0.1–0.4), depletion of heavy REE, enrichment of large ion lithophile elements (LILE), and plot in the metaluminous to peraluminous, and high-K calc-alkaline fields. The major and trace element characteristics of the Aliabad Daman pluton bear the imprints of the partial melting process. The presence of inherited zircon and the combination of the new results with previous studies suggest that the magma of the Aliabad Daman pluton was derived from a continental crustal source and an enriched mantle source. Tectonic activities such as the shortening and the thickening of the continental crust, delamination of the lower crust and the lithospheric mantle, heating by upwelling asthenosphere, and tectonic activities along the Doruneh fault zone on the southern edge of the Sabzevar zone as well as the extensional collapse of the crust played important roles in the generation and emplacement of the Aliabad Daman pluton.

Petrological and Geochemical Studies on the Si-Undersaturated Rocks of the Mount Cameroon: Genesis of the Camptonite and Nephelinite at the Cameroon Hot Line

The Cameroon hot line is dominated by magmatic rocks. The variations of magma and chemistry are generally due to the difference of physical conditions and chemistry in the magma source region during the ascent of magma. The Mt Etinde and the Mt Cameroon, both edifices belong to the Cameroon Hot line, have a particularity some rare rocks such as camptonite and nephelinite. The relationship between the silica undersaturated rocks in the both edifices is characterized by the lateral variation appear through the petrography of the different rocks. The concerned geochemical data allow to compare the Mount Etinde nephelinite and Mount Cameroon camptonite where the differentiation process reflects geochemical affinities from a basaltic magma source on the Cameroon hot line. The compatible elements between the Mount Etinde nephelinite and the Mount Cameroon camptonite and basalt correlate with the difference in modal compositions of mineral phases. The lateral variation of major and trace element contents in the Mount Cameroon camptonite and Mount Etinde nephelinite seem to be related to the difference in the fractional crystallization processes of mineral phases, the difference in the partial melting processes and the metasomatism source rich in volatile. The silica-undersaturated character of the camptonite and nephelinite could be attributed to assimilation of carbonate rocks within depth-level magma chambers. Trace element AFC modelling revealed that the parental magmas of both edifice volcanic rocks were mostly affected by fractional crystallisation coupled with metasomatism process in Ca rich source.

Geochemical Variation of Miocene Basalts within Shikoku Basin: Magma Source Compositions and Geodynamic Implications

Shikoku Basin is unique as being located within a trench-ridge-trench triple junction. Here, we report mineral compositions, major, trace-element, and Sr-Nd-Pb isotopic compositions of bulk-rocks from Sites C0012 (&gt;18.9 Ma) and 1173 (13–15 Ma) of the Shikoku Basin. Samples from Sites C0012 and 1173 are tholeiitic in composition and display relative depletion in light rare earth elements (REEs) and enrichment in heavy REEs, generally similar to normal mid-ocean ridge basalts (N-MORB). Specifically, Site C0012 samples display more pronounced positive anomalies in Rb, Ba, K, Pb and Sr, and negative anomalies in Th, U, Nb, and Ta, as well as negative Nb relative to La and Th. Site 1173 basalts have relatively uniform Sr-Nd-Pb isotopic compositions, close to the end member of depleted mantle, while Site C0012 samples show slightly enriched Sr-Nd-Pb isotopic signature, indicating a possible involvement of enriched mantle 1 (EM1) and EM2 sources, which could be attributed to the metasomatism of the fluids released from the dehydrated subduction slab, but with the little involvement of subducted slab-derived sedimentary component. Additionally, the Shikoku Basin record the formation of the back-arc basin was a mantle conversion process from an island arc to a typical MORB. The formation of the Shikoku Basin is different from that of the adjacent Japan Sea and Parece Vela Basin, mainly in terms of the metasomatized subduction-related components, the nature of mantle source, and partial melting processes.

Investigation on the welding-induced multiple failures in boiler water wall tube

In this study, by performing visual inspection, chemical composition analysis, mechanical performance testing, micro-structure and micro-morphology analyses, a failure investigation is presented on the surfacing weld induced bursting of 20G water wall tube in a thermal power plant. Besides, to identify the environmental medium and deposits, the Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD) have been conducted. The tube has undergone the localized wall thinning by fly-ash erosion burst after being repaired for 4000 h. As revealed from the macroscopic examination on the tube, significant corrosive products have been produced on the water-side of the surfacing layer position of the tube. According to the metallographic analysis of the rupture region, the massive widmanstatten overheated structure has been found in the heat-affected zone (HAZ). Furthermore, in the failure parent metal, extensive intergranular cracks, severe decarburization and mechanical performance degradation (∼20 HV10) have been identified. The micro-morphology analysis has described the partial-melting process, spherical particles and the dendrite on the fracture surface. As revealed from the EDS and XRD results, the accumulation of Cu on the inner surface of the rupture region of the tube was demonstrated. The comprehensive study has suggested that the excessive welding heat-input leads to the generation of local over-heating and over-burning. Lastly, the electrochemical corrosion and the hydrogen corrosion cracking are generated in the inner wall of the welding region.

Generation of andesite through partial melting of basaltic metasomatites in the mantle wedge: Insight from quantitative study of Andean andesites

Continental crust in average exhibits a similar composition in both major and trace elements to andesites along active continental margins. For this reason, andesitic magmatism above oceanic subduction zones is considered to have played a key role in the generation of continental crust along convergent plate boundaries. With respect to the origin of andesites themselves, however, there is still a hot debate on how they have acquired their geochemcial compositions. The debate is mainly centralized on the relative contributions of crustal contamination, magma differentiation and source mixing, which reaches an impasse in the past decades. The essential reason for this kind of debates is that these three mechanisms only can account for some of the geochemical observations for andesites, leading to insufficient discrimination among them. Nevertheless, the geochemical features of andesites are primarily controled from early to late by the composition of their source rocks in addition to partial melting and magma differentiation processes. If source mixing and partial melting processes in the early stage of andesite magmatism can account for the first-order geochemical features of andesites, there is no need to invoke the late processes of magma differentiation and crustal contamination for andesite petrogenesis. This is illustrated by quantitative forward modeling of the geochemical data for Quaternary andesites from the Andean arc in South America based on an integrated interpretation of these data. The modeling has run with four steps from early to late: (1) dehydration of the subducting oceanic crust at forearc depths; (2) partial melting of the subducting terrigenous sediment and altered oceanic basalt at subarc depths to produce hydrous felsic melts; (3) the generation of basaltic metasomatites (e.g., Si-excess pyroxenite) in the mantle wedge through reaction of the mantle wedge peridotite with large amounts of the hydrous felsic melts; (4) the production of andesitic melts by partial melting of the basaltic metasomatites. The results not only testify the hypothesis that the trace element and radiogenic isotope compositions of andesites can be directly produced by the source mixing and mantle melting but also demonstrate that partial melting of the basaltic metasomatites can reproduce the lithochemical composition of andesites. The compositional variations of Andean andesites within a single volcanic zone and among different volcanic zones can be explained by incorporating different amounts of heterogeneous hydrous felsic melts into their mantle sources, followed by different degree of partial melting under different pressures and temperatures. Therefore, the source mixing and partial melting processes at subarc depths can account for the first-order geochemical features of Andean andesites. In this regard, it may be not necessary for andesite petrogenesis to invoke the significant contributions from the processes of magma differentiation and crustal contamination.

The Genesis of Gemstone based on Pillow Lava Primary Structure: A Case Study from Kumbang Formation, Limbasari

Gemstone/Jasper is an ornamental rock included in the quartz (SiO2) microcrystalline chalcedony variety. Jasper is translucent, has a microcrystal structure, and consists of several mixes of other minerals resulting in appealing color and patterns. In general, Jasper forms as a result of hydrothermal activity controlled by geological structure (secondary structure). On the other hand, Jasper is formed in Limbasari, controlled by a primary structure on pillow lava. The methods used are field data collection, petrography, and XRF (X-Ray Fluorescence). From the data collected in the field and results from petrography analysis and genesis geochemistry. We can infer that the Jasper found in the research site can be called Peperite. The jasper there is a result of unconsolidated tuff deposits still in the form of wet deposits rolled up by the movement of pillow lava and sealed in the gaps of the pillow lava primary structure, starting a partial melting process, and then cooled swiftly, resulting in glassy texture.

Fluid-fluxed melting of orogenic crust in the South Qinling Belt, central China: Implications from migmatites of the Foping dome

• Migmatites were derived from fluid-fluxed partial melting processes in the South Qinling Belt. • Melting reaction is plagioclase + quartz + biotite + fluid → amphibole + melt. • Leucosomes without obvious Eu anomalies can be formed by the partial melting processes. • Leucosomes with highly positive Eu anomalies represent melts that experienced significant crystallization of felsic minerals. • Migmatites record decompressional melting processes of the middle crust in the South Qinling Belt. Early Mesozoic partial melting event was recorded in voluminous migmatites within the Foping dome of the South Qinling Belt, central China. Petrogenesis of migmatites has not been well constrained. In this paper, we present an integrated study of petrography and geochemistry for migmatites and leucosomes exposed in the Longcaoping area of the Foping dome to decipher the partial melting processes. Mineral assemblage, geochemical characteristics and P-T estimation suggest migmatites were mainly formed under the conditions of 2.6–5.0 kbar and 695-779℃ through fluid-fluxed melting of plagioclase + quartz + biotite + fluid → amphibole + melt. Leucosomes can be roughly divided into two groups based on their petrological and geochemical data. Geochemical modelling suggests one group of leucosomes without obvious Eu-anomalies could be derived from moderate degrees of melting of the protolith leaving amphibole-plagioclase dominated residue. Another group with highly positive Eu-anomalies represents melts that experienced significant crystallization of felsic minerals. We propose migmatites and leucosomes were formed by decompression melting of protoliths in middle crust resulting from the delamination of thickened crust in the South Qingling Belt after assembly with the Yangtze Block. Field relationships suggest the significant melt migration from external sources may be an important emplacement mechanism for Late Triassic granitic plutons in the South Qinling Belt.

New Data on the Genesis and Evolution of the Primitive Magmas of Mount Cameroon: Contribution of Melt Inclusions

Mount Cameroon is a Plio-Quaternary volcanic massif, without a centralcrater, made up of more than 140 pyroclastic cones. It is one of theactive volcanoes of the Cameroon Volcanic Line. Mount Cameroon meltinclusions are found in microdroplets trapped in the early minerals (olivines)from the pyroclastic products. The analysis of these melt inclusions allowedus to find primitive liquids compared to lavas. Major elements study ofthe magmatic inclusions, trapped in the most magnesian olivines (Mg#84-86) of Mount Cameroon revealed “primitive” liquids of basanite and alkalibasalt type with variable composition compared to the much more uniformbasalts of the magmatic series of Mount Cameroon. The study of thesetrapped liquids shows that: (1) the original primitive lavas did not undergothe process of evolution by FC, but rather underwent fundamentally (orexclusively) the process of partial melting; (2) the emitted lavas, evolvedessentially by FC; (3) the variations in the trace element contents of theprimitive liquids directly reflect a variation in the rate of partial melting ofa homogeneous mantelic source. The very high La/Yb ratios of the MountCameroon melt inclusions (&gt; 20) characterize a garnet lherzolite source.Spectra of the melt inclusions show a negative anomaly or depletion in K,Rb and Ba as those of HIMU. The “primitive” liquids and lavas of MountCameroon represent a co-genetic sequence formed by varying degrees ofpartial melting of a source considered as homogeneous.

Heterogeneous sub-ridge mantle of the Neo-Tethys: Constraints from Re-Os isotope and HSE compositions of the Xigaze ophiolites

Luqu harzburgites from the Xigaze ophiolites represent ultra-refractory mantle domains in the Yarlung-Tsangbo Suture Zone, which is accepted as relics of the Neo-Tethys. These ultra-depleted harzburgites provide an excellent opportunity to constrain the ophiolite generation from the perspectives of Re-Os isotopes and highly siderophile elements. The depleted HSE patterns of several samples suggest that these harzburgites had experienced higher extents of partial melting than the normal abyssal peridotites. These conclusions were also evidenced by the petrological observations, such as the low modal contents of clinopyroxenes and the rare occurrence of residual sulfides. Re-depleted model ages (TRD) of two Luqu harzburgites with ultra-low absolute Re abundances yield an estimate of ca. 2.0 Ga for an ancient melting event, which implies that the Luqu peridotites have ancient domains. Even though the compositions of IPGE (Os, Ir, and Ru) in all Luqu samples are quite homogeneous and comparable to those of the primitive upper mantle (PUM), the variations of PPGE (Pt and Pd) and Re indicate that the Luqu harzburgites had experienced late-stage melt-rock interaction. These infiltrating melts are constrained to be silica-rich and sulfur-saturated and might be different from the melt accounted for the metasomatic clinopyroxene. The sulfide-rich melt agents are speculated to be derived from recycling ancient components, which is supported by several samples with elevated 187Os/188Os compositions and enriched Re abundances. Considering the dynamic environments and evolution history of the Neo-Tethys, these ultra-depleted mantle domains represented by Luqu harzburgites are readily formed by multi-stage processes of partial melting and melt-rock interactions in the mid-ocean ridges. Also, fragments of ancient subducted oceanic slabs within the mantle probably provide the sources of the recycling melts pervasive in the Xigaze mantle.