Partial Melting Processes Research Articles

Lu-Hf изотопный состав циркона из сиенитов Сахарйокского щелочного массива, Кольский полуостров

This study examines zircon from the alkaline and nepheline syenites of the Sakharjok massif magmatic (2.645 ± 7 Ma), hydrothermal (1.832 ± 7 Ma) and metamorphic (1.784 ± 22 Ma) stages. Zircon from the alkaline syenites shows lower ɛHf and ɛNd values compared to that from ocean island basalts (OIB) suggesting their mantle-derived prololiths, are close to the CHUR in terms of isotopic composition. It is assumed that the formation of the Neoarchean alkaline Keivy province resulted from the plume-lithospheric interaction triggered by the uplift of a Neoarchean plume that stipulated the processes of metasomatic alterations and subsequent partial melting of the subcratonic lithospheric mantle and crustal rocks. The initial melts of the Sakharjok massif could probably form during the reactivation of the plume activity that occurred 20—30 million years after the emplacement of alkaline granites of the province by selective melting of the metasomatized lithospheric mantle rocks.

Microstructural Evolution of ZL104 Aluminum Alloy Semi-Solid Billet Fabricated by RAP Process

In this study, ZL104 aluminum alloy supplied in cold rolled state was introduced in recrystallization and partial melting (RAP) process to fabricate semi-solid billets. During the RAP process, samples cut from cold rolled ZL104 aluminum plate were heated to different semi-solid temperatures, and the effects of isothermal treatment parameters on the microstructures of semi-solid billets were investigated. Results showed that, with the increase of isothermal holding temperature and time, both the average grain size and the shape factor were increased. Namely, the shape of solid grain was more and more spherical, but the size of solid grain was larger and larger, which may be not suitable for semi-solid forming. The size of liquid droplets was increased while the number of liquid droplets was decreased with increasing the isothermal holding temperature and time. Microstructural coarsening of solid grain were attributed to coalescence and Ostwald ripening mechanisms, however, the latter one played a more and more important role with the increase of isothermal holding time and temperature. Additionally, The optimal isothermal holding temperature and time are 570 °C and 5 min, respectively, and the coarsening rate constant is 1357.2 μm3/s at 570 °C.

Petrogenesis of Mesozoic granitoids and genesis of Au–polymetallic mineralization in the Sonchon district, DPR Korea

The Au–polymetallic deposit in the Sonchon district is located in the western parts of North Pyongan Province, DPR Korea. Here, we present a detailed study of the petrogenesis, magma source of the Mesozoic granitoids, and the sources of ore–forming materials and fluids. Mesozoic granitoids consist of diorite, granodiorite and biotite granite. These rocks are alkali–enriched (K2O + Na2O = 5.02–7.75 wt%) and classified as high–K calc–alkaline. The diorite is metaluminous, while the granodiorite and biotite granite are weakly peraluminous and peraluminous, respectively. The samples show high CaO/Na2O ratios of 1.66–2.16 for the diorite and 0.74–1.35 for the granodiorite, indicating a derivation from clay–poor psammite–derived melt source, and the biotite granite has low CaO/Na2O ratios of 0.14–0.38, indicating a derivation from clay–rich pelite–derived melt source. The biotite granite belongs to fractionated M–, I–, and S–type granites (FG), whereas the granodiorite and the diorite belong to unfractionated M–, I–, and S–type granites (OGT). These rocks are typically arc-related. The similar REE distribution patterns of these rocks show that they are relatively uniform with partial melting processes playing an important role in their generation. The REE distribution patterns of ore–bearing quartz are similar to those of granodiorite rather than diorite, biotite granite and wall rocks, suggesting that they were mainly inherited from the granodiorite. The barren quartz has more negative Eu anomalies than the ore–bearing quartz. The histogram of sulfur isotopic composition shows that the δ34S values are mainly distributed around +5 to +10‰ and indicate the sulfur within sulfide was derived from a magmatic source. The absence of sulfates in ore bodies indicates that the low fO2 and high pH of ore–forming fluid system. Fluid inclusions in quartz have only aqueous–gas phases without CO2–rich phase and daughter minerals, indicating that ore–forming fluids was homogenous magmatic and had low salinity. According to the values of X (Na+)/X (K+) ratios, X (Na+)/X (Ca2+ + Mg2+) ratios, and X (F−)/X (Cl−) ratios, the ore–forming fluids could be interpreted as a mixture of magmatic and meteoric water. Microthermometric analyses indicate that these inclusions have medium temperature (223.5–286.3 °C) and low salinity (1.39–4.01 wt% NaCl) and are NaCl + H2O or NaCl + KF + H2O systems. All these data in this study suggest that the Sonchon Au–polymetallic deposit is mesothermal and related to the granodiorite.

Timing and nature of the partial melting processes during the exhumation of the garnet–bearing biotite gneiss in the southern Altyn Tagh HP/UHP belt, Western China

Partial melting of the garnet (Grt)–bearing biotite gneiss from the Jianggalesayi area in the southern Altyn Tagh high pressure (HP) – ultra high pressure (UHP) metamorphic belt has been identified and reported here. Felsic veins were distributed along the foliation plane and deformed together with the Grt–bearing biotite gneiss, which dominated the main outcrop and preserved diagnostic features of anataxis. Highly cuspate and irregular quartz (Qz)-melt and plagioclase (Pl)-melt surrounded or stretched into biotite and/or quartz grains with obvious eroded boundaries further provided the microstructural evidences for partial melting of the gneiss. CL images showed that zircons from the Grt–bearing biotite gneiss were characterized by multizone structures of core – Rim I – Rim II – Rim III. The different zircon zones from the gneiss had integrated recordings of the protolith ages 836–1613 Ma; the peak metamorphic age of 503 ± 5 Ma; the retrograded age of 456 ± 4 Ma; and the anatectic age of 418 ± 3 Ma. Zircons from felsic veins also exhibited core – rim structures on CL images. The felsic veins yielded similar protolith ages (1099–1072 Ma) and anatectic age (417 ± 2 Ma) as the host gneiss. The zircons from the host gneiss and felsic veins had plotted identical crustal evolution curves in the εHf(t) vs. U-Pb age diagram with identical TDM ages. The U-Pb ages and Lu-Hf isotope both suggested that the felsic veins were internally derived by the partial melting of the host gneiss. Moreover, the high content of Al2O3 and Rb in felsic veins indicated that the decomposition of biotite accounted for the partial melting of the gneiss. The temperatures calculated by Ti–in–zircon thermometer for the anatectic zircons from the felsic veins (582–694 °C) were identical to the conditions of the amphibolite facies stage of the Grt–bearing biotite gneiss (T = 619–646 °C), which indicated that the crystallization of the melt had taken place during the amphibolite facies stage of the exhumation. The anatectic ages (418 ± 3 Ma and 417 ± 2 Ma) of the gneiss and felsic veins were determined to be within the range of the fourth episode of the early Paleozoic magmatism (420–385 Ma), which had formed in a post-collisional extensional setting. Therefore, the combination of the magmatic heat events and stress release may have played an important role in the partial melting of the gneiss.

High-Temperature Metamorphism, Anataxis and Tectonic Evolution of a Mafic Granulite from the Eastern Himalayan Orogen

The Himalayan Orogen, resulting from the Tertiary collision of Indian and Asian continents, is a natural laboratory for studying metamorphism, partial melting and granite formation of collisional orogens. However, metamorphic and anatectic conditions and timescales of meta-mafic rocks in the Greater Himalayan Sequences (GHS) in the east-central Himalaya remain controversial. In this paper, we conduct a study of petrology and geochronology of mafic granulite from the Eastern Himalayan Syntaxis (EHS). The mafic granulite with abundant leucosome bands occurs as layers within felsic granulites and is well deformed. The granulite consists of garnet, plagioclase, amphibole and quartz with minor clinopyroxene, orthopyroxene, biotite, rutile, titanite and ilmenite. The garnet has growth compositional zoning and contains abundant mineral inclusions in its core. Peak metamorphic mineral assemblage of the granulite is garnet, amphibole, plagioclase, quartz, clinopyroxene and rutile, recording a high-pressure (HP) and high-temperature (HT) peak-metamorphism under conditions of 14–15.5 kbar and 780–790 °C in the presence of melt. The reconstructed clockwise P-T path is characterized by an early heating and burial prograde metamorphism, and late isothermal and cooling decompression retrogression. The granulite witnessed a long lasting HT metamorphism, partial melting and melt crystallization process which began at ca. 39 Ma and lasted to ca. 11 Ma. The present study shows that various high-grade rocks of the GHS in the EHS core experienced similar metamorphic conditions and P-T-t paths, indicating that they occurred as a coherent slab during the subduction and exhumation of Indian lithosphere. The significant melts generated during the prograde metamorphism of the GHS rocks not only contributed to the formation of the Himalayan leucogranite, but also resulted in the rheological weakening and ductile flow of the thickened lower crust of the Himalayan Orogen.

Influence of the Roughness on the Mechanical Properties of Ti-6Al-4V Products Prepared by Direct Laser Deposition Technology

This article presents the technology of direct laser deposition. This technology is one of the most perspective new technologies. It allows realizing heterophase process during the manufacturing, in which the process of partial melting of the used powder is realized. The product is formed from a metal powder, which is supplied by a compressed gas-powder jet directly into the laser action zone. Ti-based alloys are interesting for many industrial areas, for example, engine systems, aircraft and shipbuilding, because of their corrosion resistance, low density and high strength. The influence of roughness on the mechanical properties of Ti-based products prepared by direct laser deposition was studied. It was found that the surface roughness distorts the values of the mechanical characteristics.

Evolution of the Palaeozoic basement of the North Patagonian Andes in the San Martín de los Andes area (Neuquén, Argentina): petrology, age and correlations

In San Martín de los Andes area (Argentinian Patagonia) the Palaeozoic basement consists of metamorphic and plutonic rocks. The metamorphic rocks include strongly deformed schists, gneisses and migmatites. Their geochemical and petrographic characteristics suggest that the protholith could have been a sequence of pelites and greywackes. Detrital zircon analysis (U-Pb Q-ICP-LA-MS) yielded a maximum depositional age of 501±14 Ma (Series 3 Cambrian) for this sedimentary protolith. Metasedimentary rocks are affected by a regional foliation defined by the minerals of the metamorphic peak. This is a S2 foliation, since relics of a former foliation are present in some samples. This regional foliation is locally affected by open folds that develop an incipient crenulation cleavage (S3). The high-grade metamorphism includes partial melting processes, where the incipient segregates intrude parallel to the regional foliation and also cut it in presence of abundant melt. Zircons from anatectic granites formed during this partial melting process yielded a U-Pb Concordia age of 434.1±4.5 Ma (Llandovery-Wenlock, Silurian). The age of maximum sedimentation and the anatectic age constrain the metamorphic evolution of the basement into the lower Palaeozoic (between upper Cambrian and lower Silurian). The igneous rocks of the basement are granodiorites, tonalities, and some gabbros that cut the metamorphic basement and contain xenoliths and roof pendants from the country rocks. These plutonic rocks are affected by low-grade metamorphism, with the development of discrete, centimetric to hectometric, brittle-ductile shear zones. The age of these igneous rocks has been constrained through U-Pb zircons analysis, carried out by several authors between ca. 370 and 400 Ma (Devonian). The maximum sedimentation age for the protolith and its metamorphic evolution seems to be related to an early Palaeozoic orogenic event, probably the Patagonian Famatinian orogeny. In contrast, the Devonian igneous rocks of San Martín de los Andes could represent a Devonian magmatic arc, related to subduction processes developed at the beginning of the Gondwanan orogenic cycle, which culminates with the Gondwanan orogeny.

Magnesium isotopic composition of continental arc andesites and the implications: A case study from the El Laco volcanic complex, Chile

Continental crust can dramatically modify the geochemical and isotopic compositions (e.g., Sr-Nd, Pb) of mantle-derived lavas, and has important implications in understanding magmatic processes in continental arcs which involve subducted materials. In this paper, we report the Mg isotopic compositions of continental arc andesites from El Laco in northern Chile, and evaluate the contribution of the subducted slab to the formation of continental arc lavas. The andesites in the El Laco volcanic complex (ELVC) display relatively high (87Sr/86Sr)i ratios and negative age-corrected εNd(t) (t = 1.6 Ma) values. The δ26Mg values of the ELVC andesites range from −0.26 ± 0.05‰ to −0.15 ± 0.04‰ (average δ26Mg = −0.18 ± 0.05‰), slightly heavier than that of the primitive mantle. The δ26Mg values of clinopyroxene, orthopyroxene and magnetite grains separated from andesites are −0.27 ± 0.03‰ to −0.20 ± 0.04‰, −0.24 ± 0.06‰ to −0.18 ± 0.03‰ and +0.09 ± 0.06‰ to +0.33 ± 0.06‰, respectively. Our results suggest that Mg isotope fractionation occurred during the fractional crystallization of El Laco andesite, and imply complex Mg isotopic fractionation at high-temperature conditions than previously inferred. Combing the Mg isotopes with the geochemical and Sr-Nd isotopic data, we confirm that neither the deep process of partial melting nor the shallow process of crust contamination contributed to the heavier Mg isotopes of the andesites. The heavier Mg isotopic features of these lavas were mainly inherited from aqueous fluids derived from the subducted slab, which reacted with the primitive mantle wedge to produce a metasomatic mantle source for the continental arc lavas.

Geochemistry of Mesoarchean felsic to ultramafic volcanic rocks of the Lac Guyer area, La Grande Subprovince (Canada): Evidence for plume-related magmatism in a rift setting

The Lac Guyer area of the La Grande Subprovince (Superior Province) hosts Mesoarchean east-trending volcano-sedimentary sequences subdivided into 3 stratigraphic units (Mintisch and Rouget Formations and Guyer Group), which were emplaced between 2847 and 2806 Ma on a gneissic-tonalitic basement. These approximately 35 to 175 km-long sequences comprise felsic to ultramafic volcanic rocks that have been metamorphosed to the amphibolite and granulite facies and record mainly two Archean episodes of ductile deformation.The Mesoarchean tholeiitic basalt, komatiitic basalt, and komatiite appear to be genetically related based on petrologic modeling, their close spatial association, and their major and trace element signatures. They are characterized by the following: depletion to enrichment in LREE relative to HREE; Zr/Ypm, Ti/Vpm, and La/Smpm ratios higher than or close to 1; Ti/Zrpm and Nb/Thpm ratios lower than or close to 1; common negative anomalies in Nb-Ta ± Ti; and local positive to negative anomalies in Hf-Zr. Komatiitic basalt and komatiite are mostly Al-undepleted and rarely Al-depleted. According to current komatiite melting models, this suggests that these rocks formed by a high degree of partial melting of a garnet-bearing lherzolite source that consumed any residual garnet and progressively depleted the primitive mantle source in incompatible elements. Basalt and komatiitic basalt compositions were likely produced by assimilation and fractional crystallization (AFC) processes from komatiitic magmas (∼21–23 wt% MgO), as suggested by AFC modeling and their geochemical characteristics, whereas komatiite compositions were controlled by partial melting processes and crustal contamination.The Mesoarchean calc-alkaline rhyolite, dacite, and andesite are enriched in LREE relative to HREE, enriched in Th relative to LREE, and have negative anomalies in Nb-Ta-Ti, negative to positive anomalies in Hf-Zr, as well as high Zr/Y and Ti/V ratios and low Ti/Zr ratios. Such geochemical signatures could be explained either by concurrent assimilation and fractional crystallization of the basaltic magmas in the lower crust or by magma mixing between basalt and lower crustal anatectic magmas.The spatial and temporal distribution of Mesoarchean volcanic rocks within the Lac Guyer area and their major and trace element characteristics suggest that they were emplaced into a continental rift associated with a mantle plume. The geochemical similarities between these rocks and the rift-related volcanic rocks of the younger Yasinski Group (2751–2732 Ma), and the presence of other Mesoarchean volcanic sequences with rift affinity indicate that the La Grande Subprovince was affected by two main plume-related rifting events during the Archean (ca. 2.88–2.81 and 2.75–2.71 Ga), each produced by a single or multiple magmatic plumes.

Early Cretaceous diorites in the Kenbale Cu mineralization occurrence, Tibet, China, and its geological significance

The Kenbale Cu mineralization occurrence related to the diorite is a newly discovered Cu mineralization event in the Bangong-Nujiang metallogenic belt, Tibet, China. The Cu mineralization is hosted in the contact between the monzogranite or biotite quartz diorite and the fine-grained diorite, which is the mineralization related intrusion. In order to constrain the Kenbale mineralization age, petrogenesis and tectonic setting, we conducted LA-ICP-MS zircon U-Pb dating and Hf isotopic analyses of the biotite quartz diorite and fine-grained diorite and also the whole-rock geochemical study of the biotite quartz diorite. Zircon UPb dating show that weighted mean 206Pb/238U ages of the biotite quartz diorite and fine-grained diorite are 123.5 ± 1.9 Ma (MSWD = 2.3, n = 16) and 118.9 ± 1.3 Ma (MSWD = 2.5, n = 18), respectively. The biotite quartz diorite is a high-K calc-alkaline I-type magma rock and was controlled mainly by partial melting process during the magma formation and evolution. This intrusion is characterized by positive eHf(t) values (2.6 to 5.8) and old Hf crustal model ages (813 to 1016 Ma), indicating that the magma was sourced from partial melting of the Mesoproterozoic to Neoproterozoic juvenile crust of the northern Lhasa Terrane. Compared with the biotite quartz diorite, the mineralization associated fine-grained diorite has much higher zircon eHf(t) values (8.2 to 11.4) and younger Hf crustal model ages (450 to 650 Ma). These characteristics are similar with those of the coeval magmatic rocks induced by slab break-off of the southward subducted Bangong-Nujiang Ocean. The geochronology and geochemical results show that the Kenbale Cu mineralization was controlled by the slab break-off of the southward subducted Bangong-Nujiang Ocean.

Isotopic Disequilibrium in Migmatitic Hornfels of the Gennargentu Igneous Complex (Sardinia, Italy) Records the Formation of Low 87Sr/86Sr Melts from a Mica-Rich Source

Isotopic disequilibrium is increasingly recognized as a common feature of magmatic systems, but the details of the mechanism(s) underpinning the development of isotopic disequilibrium during partial melting processes are not fully understood. Partial melting of mica-rich lithologies may be predicted to generate melts enriched in radiogenic Sr compared to the bulk protolith compositions due to the typically high Rb/Sr ratio coupled with low melting temperature of mica in crustal rocks. Here we report a puzzling case study where the Sr-isotope composition of the melt fraction (leucosome) of partially molten metapelites (migmatites) is instead less radiogenic than the restitic component (melanosome). The examined migmatites fringe (∼50 m wide zone) a low-pressure (≤200 MPa), high-temperature (∼1050 °C) quartz-dioritic intrusion, which was emplaced in the Gennargentu Igneous Complex (Sardinia, Italy) at 306 ± 26 Ma (bulk-rock Rb/Sr dating). The migmatites derive from anatexis of the muscovite-rich metapelitic wall-rocks. They include a quartzo-feldspathic leucosome and a melanosome containing cordierite, K-feldspar, plagioclase, biotite, Fe-Ti oxide minerals and both corundum and hercynite. The leucosome has a less radiogenic Sr and more radiogenic Nd isotope composition than the melanosome (87Sr/86Sr (306 Ma) = 0.71068 and 0.71536; eNd(306 Ma) = -6.4 and -9.2, respectively), with bulk migmatite samples having intermediate compositions. The significantly lower content of mica in the migmatites compared to the protolith indicates that muscovite and, to a lesser extent, biotite largely contributed to melt formation. However, the leucosome volume (∼50%) estimated through mass balance calculations is considerably higher than the amount of melt (≤10 vol.%) generally produced by mica-dehydration melting in the crust, suggesting that partial melting was enhanced by an external hydrous fluid. The O-isotope composition of the migmatites is lower than the typical metapelite values (>10‰) but overlaps with the δ18O range of the quartz-diorites (8.8-9.9‰), suggesting that such a hydrous fluid was released from the quartz-dioritic intrusion. We put forward a model whereby the anatexis temperature conditions (T < 800 °C) favored the preservation of isotopic disequilibrium of micas and plagioclase in the protolith. In this context, the leucosome formed by the preferential melting of less radiogenic plagioclase rather than more radiogenic micas. The melt was then efficiently separated from the melanosome containing restitic biotite and peritectic K-feldspar and magnetite derived from mica breakdown. During the anatectic process the quartz-diorite provided not only the heat budget, but also the fluid amount responsible for (i) the hydration of the pelitic country rock, (ii) the increase of melt fraction, and (iii) the higher mobility of anatectic magma.

Formation time of the big mantle wedge beneath eastern China and a new lithospheric thinning mechanism of the North China craton—Geodynamic effects of deep recycled carbon

High-resolution P wave tomography shows that the subducting Pacific slab is stagnant in the mantle transition zone and forms a big mantle wedge beneath eastern China. The Mg isotopic investigation of large numbers of mantle-derived volcanic rocks from eastern China has revealed that carbonates carried by the subducted slab have been recycled into the upper mantle and formed carbonated peridotite overlying the mantle transition zone, which becomes the sources of various basalts. These basalts display light Mg isotopic compositions ( δ 26Mg = –0.60‰ to –0.30‰) and relatively low 87Sr/86Sr ratios (0.70314–0.70564) with ages ranging from 106 Ma to Quaternary, suggesting that their mantle source had been hybridized by recycled magnesite with minor dolomite and their initial melting occurred at 300−360 km in depth. Therefore, the carbonate metasomatism of their mantle source should have occurred at the depth larger than 360 km, which means that the subducted slab should be stagnant in the mantle transition zone forming the big mantle wedge before 106 Ma. This timing supports the rollback model of subducting slab to form the big mantle wedge. Based on high P-T experiment results, when carbonated silicate melts produced by partial melting of carbonated peridotite was raising and reached the bottom (180–120 km in depth) of cratonic lithosphere in North China, the carbonated silicate melts should have 25–18 wt% CO2 contents, with lower SiO2 and Al2O3 contents, and higher CaO/Al2O3 values, similar to those of nephelinites and basanites, and have higher e Nd values (2 to 6). The carbonatited silicate melts migrated upward and metasomatized the overlying lithospheric mantle, resulting in carbonated peridotite in the bottom of continental lithosphere beneath eastern China. As the craton lithospheric geotherm intersects the solidus of carbonated peridotite at 130 km in depth, the carbonated peridotite in the bottom of cratonic lithosphere should be partially melted, thus its physical characters are similar to the asthenosphere and it could be easily replaced by convective mantle. The newly formed carbonated silicate melts will migrate upward and metasomatize the overlying lithospheric mantle. Similarly, such metasomatism and partial melting processes repeat, and as a result the cratonic lithosphere in North China would be thinning and the carbonated silicate partial melts will be transformed to high-SiO2 alkali basalts with lower e Nd values (to −2). As the lithospheric thinning goes on, initial melting depth of carbonated peridotite must decrease from 130 km to close 70 km, because the craton geotherm changed to approach oceanic lithosphere geotherm along with lithospheric thinning of the North China craton. Consequently, the interaction between carbonated silicate melt and cratonic lithosphere is a possible mechanism for lithosphere thinning of the North China craton during the late Cretaceous and Cenozoic. Based on the age statistics of low δ 26Mg basalts in eastern China, the lithospheric thinning processes caused by carbonated metasomatism and partial melting in eastern China are limited in a timespan from 106 to 25 Ma, but increased quickly after 25 Ma. Therefore, there are two peak times for the lithospheric thinning of the North China craton: the first peak in 135−115 Ma simultaneously with the cratonic destruction, and the second peak caused by interaction between carbonated silicate melt and lithosphere mainly after 25 Ma. The later decreased the lithospheric thickness to about 70 km in the eastern part of North China craton.

Geochemistry of silicate melt inclusions in middle and southern Okinawa Trough rocks: Implications for petrogenesis and variable subducted sediment component injection

Silicate melt inclusions (SMIs) in olivine, clinopyroxene, and orthopyroxene in basalt and trachyandesite of the middle Okinawa Trough (MOT) and in basaltic andesites of the southern Okinawa Trough (SOT) were investigated in detail to understand the evolution of magma and geochemical characteristics of the basaltic magma source. Both the SMIs and host bulk rocks are more heavily enriched with Pb and large ion lithophile elements than with high field strength elements and rare earth elements. The major element compositions of SMIs in the MOT trachyandesite show a scattered variation trend of medium‐K to shoshonitic series and suggest that a K‐rich magma source may exist in the Okinawa Trough. Two groups with distinct element distribution patterns observed in SMIs of the MOT trachyandesite may represent magmas with different evolutionary histories. Compared with fractional crystallization and partial melting processes, magma mixing may serve as a reasonable process for the production of trachyandesite in the MOT. Fractional crystallization has played an important role in basaltic magma evolution in the MOT and SOT. Simulations conducted using MELTS show that the fractional crystallization evolution trends observed correspond well to the presence of a magma chamber at 300 MPa with an H2O content level of 0.2 wt% in the MOT and to the presence of a magma chamber at 300 MPa with an H2O content level of 0.4 wt% in the SOT. Ratios of incompatible trace elements (e.g., Th/Ce, Th/Sm, Ba/Th, and Th/Nb); positive Pb anomalies; negative Nb, Ta, and Ti anomalies; and the existence of organic hydrocarbons (CnHm) in the SMIs can be attributed to the involvement of subducted sediment from the subducting slab. Modelling results suggest that more primitive magma represented by SMIs from the MOT has formed through the melting of less than 6% of a metasomatized depleted mantle with contributions of 1.2% sediment melt and 0.8% sediment fluid. In contrast, more primitive magma from the SOT formed through the melting of 4–10% of the depleted mantle with contributions of 3% sediment melt and 1% sediment fluid. The different features of the SMIs together with different tectonic settings of the MOT and SOT have formed in response to the westward subduction of the Philippine Sea Plate.

Geochemistry and Petrogenesis of Intracontinental Basaltic Volcanism on the Northwest Arabian Plate, Gaziantep Basin, Southeast Anatolia, Turkey

AbstractVolcanism along the northwest boundary of the Arabian Plate found in the Gaziantep Basin, southeast Turkey, is of Miocene age and is of alkaline and calc‐alkaline basic composition. The rare earth element data for both compositional series indicates spinel–peridotite source areas. The rare earth and trace elements of the alkaline lavas originate from a highly primitive and slightly contaminated asthenospheric mantle; those of the calc‐alkaline lavas originate from a highly heterogeneous, asthenospheric, and lithospheric mantle source. Partial melting and magmatic differentiation processes played a role in the formation of the petrological features of these volcanics. These rocks form two groups on the basis of their 87Sr/86Sr and 143Nd/144Nd isotopic compositions in addition to their classifications based on their chemical compositions (alkaline and calc‐alkaline). These isotopic differences indicate a dissimilar parental magma. Therefore, high Nd isotope samples imply a previously formed and highly primitive mantle whereas low Nd isotope samples may indicate comparable partial melting of an enriched heterogeneous shallow mantle. Other isotopic changes that do not conform to the chemical features of these lavas are partly related to the various tectonic events of the region, such as the Dead Sea Fault System and the Bitlis Suture Zone.

Variation in sub-arc mantle oxygen fugacity during partial melting recorded in refractory peridotite xenoliths from the West Bismarck Arc

It is debatable whether oxygen fugacity (fO2), the usual measure of the oxidation state of a system, can vary during partial melting in the Earth's mantle or not. Notably, samples of mantle peridotite recovered from lavas and pyroclastic deposits in island arcs are mostly more oxidized than those from other tectonic settings. However, the petrological history of sub-arc mantle peridotites, in particular the respective extents to which partial melting and post-melting metasomatism have controlled their fO2 record, are elusive. It has remained unclear whether the oxidized peridotites have preserved the oxidation state of a melt-depleted, residual mantle wedge or not.Here we report Mössbauer spectroscopy and EPMA measurements of Fe valence states in spinel (Fe3+/∑Fespinel where ∑Fe refers to Fe3++Fe2+) in a suite of markedly unaltered, sub-arc mantle-derived, harzburgite and dunite xenoliths from the active Ritter volcano (West Bismarck Arc, Papua New Guinea). These rocks, together with similar suites from the Kamchatka and Izu-Bonin arcs, have recently been interpreted to be residues of partial melting in the mantle wedge. The Fe3+/∑Fespinel decreases from 0.27 ± 0.04 to 0.11 ± 0.01 with increasing degrees of melt depletion in the West Bismarck sample suite, as monitored by decreasing Al2O3 (from 0.72 to 0.29 wt%) and modal percentage of orthopyroxene (from ~28 to ~7 wt%) in bulk rocks. Importantly, Fe3+/∑Fespinel in the most melt-depleted, orthopyroxene-poor residual samples are significantly lower (down to 0.11 ± 0.01) than those in melt-percolated harzburgite (0.29 ± 0.04) and dunite melt channel-cumulates (0.20 ± 0.01) found at the same sampling sites. The calculated fO2 in West Bismarck residual samples decreases from +1.7 ± 0.2 to −0.5 ± 0.2 log units relative to the synthetic fayalite-magnetite-quartz redox buffer (∆logfO2(FMQ)) with Al2O3 and orthopyroxene contents. The upper-end ∆logfO2 for the least melt-depleted, orthopyroxene-rich residual samples are consistent with those for sub-arc mantle harzburgite xenoliths from the Kamchatka and Izu-Bonin arcs recording similar melting degrees, but also those for more fertile lherzolite and harzburgite rocks from the northeastern Japan Arc. In turn, the most melt-depleted, orthopyroxene-poor residual samples have ∆logfO2 similar to the upper bound recorded in abyssal peridotites.Taking literature data into consideration, the fO2 spectrum recorded by the West Bismarck sub-arc mantle peridotite suite is modelled here by a two-stage partial melting process. The first-stage oxidation state is near-buffered from lherzolite to orthopyroxene-rich harzburgite by fluxed-melting with volatile-rich, slab-derived components at ca. FMQ + 0.5 to FMQ + 1.5 during the generation of high-partial melting degree, picrite-boninite-andesite oxidized liquids. The second-stage oxidation state is un-buffered during re-melting of residual harzburgite accompanying generation of low- to moderate-degree partial melts such as low-Ca boninite; these magmas preserve more variable fO2 extending to lower values (FMQ and below) owing to the progressive removal of Fe3+ from their sources with increasing melting degree. The second-stage melting event likely occurs during adiabatic decompression of residual spinel harzburgite to the uppermost mantle wedge.The data in this study support the general hypothesis that Fe3+/∑Fespinel and fO2 of residual peridotite (and of the melts formed at equilibrium) can vary during partial melting in the Earth's mantle. These results further provide direct source evidence for the controls of various subduction zone melting processes on the oxidation state of different types of primary arc melts. Melt depletion of mantle wedge sources can result in a progressive decrease in the fO2 of liquids subsequently extracted from these sources, but only in the absence of oxidized, Si- and volatile-rich components. These components are presumably derived from the subducted slab and effectively buffer fO2 during fluxed-melting. The observed fO2 variability in sub-arc mantle peridotites worldwide likely reflects the combination of fluxed- and adiabatic decompression melting in the mantle wedge.

The microstructural evolution and hardness of the equiatomic CoCrCuFeNi high-entropy alloy in the semi-solid state

The semi-solid slurries of the CoCrCuFeNi high entropy alloy (HEA) were fabricated through the recrystallization and partial melting (RAP) process by cold-rolling and partial remelting. Based on Kim's theory, the solidus-liquidus temperature range and the fraction of the liquid phase as a function of temperature were examined from the Differential Scanning Calorimetry (DSC) curve. The effect of isothermal temperature and soaking time on the microstructural evolution of the CoCrCuFeNi HEA in a semi-solid state were analyzed. The results showed that the globular structure was obtained without the new phase formed for the semi-solid slurries, containing two face-centered-cubic (FCC) solid-solution phases. The average grain size of the globular grain and the globular degree increased with increasing the isothermal temperature and soaking time. The study of the coarsening kinetics indicated that the coarsening process could be described by the modified liquid film migration model. The sluggish diffusion effect of the current HEA leaded to the low coarsening rate, compared with other conventional alloys. However, the existence of the liquid phase in the semi-solid state resulted in a higher grain growth rate for the CoCrCuFeNi HEA with a lower activation energy of 206.9 kJ/mol, compared with the diffusion couple and the grain growth process for the other HEAs at low temperatures. To evaluate the mechanical properties, Vickers hardness was measured to represent the strength of the CoCrCuFeNi HEA. The CoCrCuFeNi semi-solid slurries showed a Hall-Petch type relationship of the average grain size dependence on Vickers hardness.

Pleistocene alkaline rocks of Martin Vaz volcano, South Atlantic: low-degree partial melts of a CO2-metasomatized mantle plume

ABSTRACTSeveral alkaline massifs on inland southeastern Brazil extend offshore, roughly parallel to ~20° S, through a seamount chain of the Vitoria-Trindade ridge. This paper presents the first extensive work on the Martin Vaz volcano through whole-rock and Sr and Nd isotopic composition of volcanic and subvolcanic lithotypes from the Martin Vaz Island, located at the easternmost of this volcanic chain. These alkaline rocks were generated during the Plio-Pleistocene (~0.47 My, 40Ar/39Ar dating in whole-rock) and represent the crystallization of sodic magmas of nephelinitic composition that evolved through fractional crystallization towards phonolites. Calculations from P–TLiquidus using PELE software show temperatures of 1045°C and 818°C, viscosity of 2.47 log Poise and 5.02 log Poise, and densities of 2.57 g/cm3 and 2.26 g/cm3 for nephelinite and phonolite, respectively. Like in Trindade Island, the nephelinitic volcanism in Martin Vaz may represent a Strombolian and/or Hawaii-type eruption due to low viscosity magma according to its physical properties whereas phonolitic intrusions present higher viscosity characteristics forming lava domes. The 87Sr/86Sr (~ 0.703800) and 143Nd/144Nd (~ 0.512750) ratios of lavas from the seamounts and Martin Vaz do not vary significantly, pointing to partial melting process from a homogeneous mantle source showing isotope signature close to HIMU. Beside the restrict variation on these isotopic ratios, a conspicuous enrichment in incompatible trace elements, mainly LREE, indicates that metasomatism is a recent process and not a long-term source characteristic. Non-modal partial melting models (fractional melting and batch melting) suggest that the source of the Martin Vaz magmatism is consistent with the garnet-lherzolite mantle stability field (>90 km depth; Tb/Yb >0.7), generated about 3.0 GPa by very small degree of partial melting of an enriched wet mantle source (F = 0.03–0.04) with 2.5 wt. % of CO2.

Origin of Lherzolitic Peridotites in Ab-Bid Ultramafic Complex (Hormozgan Province); Products of Mantle Metasomatism or Partial Melting Processes?

Lherzolite is one of the main units in the Ab-Bid ultramafic complex from Esfandagheh-HadjiAbad coloured melange (South of Iran). The complex contains harzburgite, dunite, lherzolite and pyroxenite dykes and the lherzolites mainly occur in the margins. In the field, lherzolites occur as weakly foliated coarse-grained peridotites with shiny pyroxene grains and cut by neumerous pyroxenitic veins. Textural features such as elongation and undoluse extinction of minerals and porphyroclastic grains indicate that the lherzolites were part of the upper mantle and experienced high P-T deformational events. Mineral chemistry data such as Cr# values in spinels (10/33-14/04) and fo contents of olivines (90/49-93/51) from the Ab-Bid lherzolites suggest that these rocks belong to the mantle. Evidences such as CaO (1/18-3/23) and MgO (39/53-43/65) contents of whole rock compositions, Cr# (10/33-14/04) and Mg# (74/20-78) values of spinels, besides textural features and REE normalized patterns show that they have past a complex petrological history. At the first stage, they have partially melted (< 10%) in an abyssal environment, then, they refertilized by ascending melts and enriched in LREE. Tectonomagmatic discrimination diagrams indicate that Ab-Bid lherzolites belong to the abyssal peridotites and their petrogenetic evolutions are similar to those from MOR type peridotites. Our data document the dependence of Esfandagheh-HadjiAbad coloured melange to the Neotethyan oceanic lithosphere in the south of Iran.

電気石を用いた含ホウ素H2O流体活動と部分溶融過程制約の試み―ネパールヒマラヤ東部・ダンクッタ地域の例

電気石を用いた含ホウ素H2O流体活動と部分溶融過程制約の試み―ネパールヒマラヤ東部・ダンクッタ地域の例

Petrogenesis of the microcrystalline-dioritic enclaves from Jiuling granitoids in the eastern segment of Jiangnan Orogen and constraints on magma source materials

ABSTRACT Numerous dark enclaves with different shapes are found in Jiuling Neoproterozoic granitoids. Precise LA-ICP-MS U-Pb dating was conducted on zircons extracted from two microcrystalline enclave samples, yielding crystallization ages of 822.6±5.8 Ma and 822.2±6.2 Ma, respectively. The consistent ages within analytical errors with the host granitoids suggested that they were the products of the same magmatism. The microcrystalline-dioritic enclaves commonly show plastic forms and contain similar plagioclase megacrysts to the host rocks, and both of the enclaves and host granitoids showed a complex composition and structural imbalance in plagioclases. Furthermore, the apatites with a euhedral acicular shape occurred widely in the microcrystalline-dioritic enclaves. All of these petrographic features above imply magma mixing is involved in their diagenesis. The enclaves and host granitoids show a marked zircon trace element difference and Hf isotopic signatures without correlation in zircon trace element pairs but form their own system between enclaves and host granitoids. Additionally, most of the zircons show extremely high eHf (t) with eHf (t) =3.54–11.94 from the southern samples, and eHf (t) =1.0–9.09 from the central region. Some zircons with the higher eHf (t) are similar to the zircons from the juvenile island arc in the eastern segment of Jiangnan Orogen. Integrated geological and Hf isotopic characteristics suggest microcrystalline-dioritic enclaves were derived from the partial melting process of the Mesoproterozoic crust which enriched juvenile island arc materials and mixed with the granitic magma that remelted from the Mesoproterozoic continental crust which relatively enriched ancient sediments and mixed with the host granitoid in diagenesis.