Partial Melting Processes Research Articles

Petrogenesis of mafic dykes from the western Bastar craton of Central India and their relation to outgrowth of Columbia supercontinent

We report mineral compositions and bulk rock geochemistry of mafic dykes intruded in the western part of Bastar craton, comprising of Archaean Amgaon Group and Proterozoic Dongargarh Supergroup of rocks. Field relations show two distinct trends of these dykes which are almost perpendicular to each other but having similar mineralogical and geochemical characteristics. Dykes are mostly composed of pyroxenes, plagioclase and subordinate amount of amphiboles and Fe-Ti oxides (magnetite and ilmenite). These hypersthene normative basaltic dykes show tholeiitic trend and are characterised by narrow compositional variations of MgO (6.06–7.08 wt%), FeOt (15.06–17.78 wt%), TiO2 (1.18–2.24 wt%), Al2O3 (11.96–15.54 wt%) and low Mg# [atomic Mg/(Mg + Fe2+) × 100] values in the range of 37–48. Low loss on ignition (LOI) values <2 wt% and significant trends of trace elements (Nb, La, Th, Sr) with Zr indicate insignificant effects of post magmatic processes in these dykes. Smooth correlations between major oxides and MgO, among trace element ratios (Ce/La, Th/Yb, Nb/Yb) and negative Nb-Ta anomalies without positive Zr and Hf anomalies negate the crustal contamination effects. The correlations of compatible (e.g. Cr, Ni) and incompatible (e.g. Ba, Rb) elements show involvement of both fractional crystallisation and partial melting processes in their formation. Flat heavy rare earth element (HREE) pattern with low (Tb/Yb)n values reveal their genesis from a mantle source without involvement of garnet and geochemical models suggested in the present study indicate melting from spinel lherzolite mantle source. Strong geochemical similarities of present dykes with those of earlier reported Lakhna (1.46 Ga) and Bandimal (1.42 Ga) dykes of northern Bastar craton suggest a widespread mafic magmatic event across the Bastar craton during 1.42–1.46 Ga. Present dykes therefore represent a subduction related outgrowth of Columbia supercontinent due to the accretion of continental margins.

S-type granites in the western Superior Province: a marker of Archean collision zones

Detailed field observations indicate that Neoarchean S-type granites were emplaced along and (or) proximal to some terrane (tectonic) boundary zones in the western Superior Province, southeastern Manitoba. These S-type granites are characterized by the presence of at least one diagnostic indicator mineral, such as sillimanite, cordierite, muscovite, garnet, and tourmaline. They are medium- to high-K calc-alkaline, moderately to strongly peraluminous (ANKC &gt;1.1), and contain &gt;1% CIPW normative corundum. Compared with more voluminous, older I-type granitoids in tonalite–trondhjemite–granodiorite suites in the region, the S-type granites occur as relatively small intrusions and have high (SiO2 &gt;72 wt.%) contents with a small silica range (SiO2 = 72.2–81.2 wt.%), but a large range of Zr/Hf (17.1–43.8) and Nb/Ta (0.3–16.0) ratios. These geochemical characteristics suggest that the S-type granites were derived from partial melting of heterogeneous sedimentary rocks deposited as synorogenic flysch that underwent burial and crustal thickening during terrane collision. Although the S-type granites display geochemical variations in individual and between different plutons, their low Sr (&lt;400 ppm) and Yb (&lt;2 ppm) contents and low Sr/Y (&lt;40) and La/Yb (&lt;20) ratios are consistent with a partial melting process that left a granulite-facies residue consisting of plagioclase, pyroxene, and ± garnet. The S-type granites display low zircon saturation temperatures (mostly &lt;800 °C) and low emplacement pressures (&lt;300 MPa), similar to strongly peraluminous leucogranites formed in the Himalayas. Therefore, we propose that the Neoarchean S-type granites in the western Superior Province, whose source rocks were deposited between colliding tectonic blocks between 2720 and 2680 Ma, may serve as a geological marker of some Archean terrane boundary zones.

Preface: Metamorphism and Orogenic Belts—Response from Micro- to Macro-Scale

The geological toolbox for the analysis of orogenic processes has seen substantial additions over the recent decades. Major advances have been made in the ability to simulate geological conditions and processes by computer modeling, we have a much improved knowledge of geochemical processes (trace elements and isotopes in particular), and, last but not least, the range and versatility of micro-analytical methods and instrumentation available to us has been expanded dramatically. The latter aspect has had a particular impact on metamorphic petrology as the conventional tools to determine pressure-temperature-time paths via rock and mineral analysis combined with isotope geochronology have generally gained in precision and accuracy through a much improved resolution. Geochronology has moved from whole-grain analysis to spot analyses on zoned single grains while analyzing trace elements and isotopes at the same time. It is therefore possible to recognize complex multi-stage orogenic events in single samples or single grains. Diffusion profiles in minerals provide us with time-scales of metamorphic and magmatic processes. Major and trace elements as well as isotope profiles of mineral grains give us insight into changing ambient conditions during mineral growth. This special issue focuses on pressure-temperature conditions and geochronology of metamorphic rocks, isotope studies of magmatic rocks, partial melting and metasomatic processes, all with a view at tectonic implications and orogeny. Much of the evidence has been gained from small-scale observation and analysis. Diligent field work and detailed microscopic examination remain indispensable, though, as demonstrated clearly by the “expansion” of the P-T space as applicable to crustal metamorphic processes towards extreme pressures and temperatures. As a consequence, much interest has thus focused on UHP and UHT rocks and the orogenic processes that generate them. There is petrological evidence for subduction to, and subsequent exhumation from, depths in excess of 250 km. While this impacts substantially on the large-scale geodynamic models of orogenic processes, the evidence is gathered primarily from small-scale petrological observation. The same applies in principle to UHT granulites. To generate temperatures in the 900 to 1 100 °C range within the sillimanite stability field in orogenic heat budget models remains a serious challenge, however. The large overlap of solid-state metamorphism, partial anatexis and magmatic crystallization in P-T space suggests close interrelationships, and the role and contribution of the diverse magmatic activities in forming orogenic belts, from the onset of subduction to late-orogenic delamination or slab-break-off is crucial for our understanding of geodynamics, with focus on geochemistry and geochronology. Aspects of metasomatism in the framework of orogeny are given some special consideration in this issue. The topics include the formation of sagvandite, pyrope quartzite (or whiteschist), jadeitite and rodingite. One of the key questions addressed is the source and the nature of the fluids that triggered the different types of metasomatic events. Finally, it is shown how small-scale methods contribute to a better understanding of the formation of deep-seated mantle xenoliths.

Effects of Extrusion Processing on Microstructure of 7075Al Alloy in the Semi-solid State

A recrystallization and partial melting (RAP) process was introduced to prepare the semi-solid 7075 aluminum alloy used for thixoforming. In order to obtain an ideal semi-solid microstructure, a series of extrusion experiments were conducted to comparatively investigate the optimum extrusion process parameters. Commercial 7075Al alloy samples were frstly extruded with varying extrusion ratios below the recrystallization temperature followed by homogenization, then these samples were reheated to the semi-solid state and held in the range of 5 to 50 minutes. The experimental results show that varying process cause the difference in the deformation degree and microstructure for as-extruded samples, resulting in various semi-solid microstructure. It is verifed that the formation of equiaxed grains in semi-solid microstructure depends on recrystallization behavior of extruded samples during partial melting. Both relative high extrusion temperature and low extrusion ratio lead to high volume fraction of recrystallized area, thus entirely equiaxed solid grains in semi-solid 7075Al alloy samples can be obtained finally. In addition, Ostwald ripening was determined as the dominate coarsening mechanism of solid grains in semi-solid state for this 7075 Al alloy during the RAP route. The infuence of pre-deformation on recrystallization behavior of this 7075 Al alloy was discussed in detail.

Reconstruction the Process of Partial Melting of the Retrograde Eclogite from the North Qaidam, Western China: Constraints from Titanite U-Pb Dating and Mineral Chemistry

Retrograde eclogite and garnet amphibolite of the Luliangshan unit of the Shenglikou area, North Qaidam, were studied with emphasis on rutile and titanite. A special focus is on the formation of rutile and its corona of titanite (Ttn1) in retrograde eclogite and on coarse-grained titanite (Ttn2) from the garnet amphibolite. Using zirconium (Zr)-in-rutile and Zr-in-titanite thermometers, the temperatures estimated for the formation of an early generation of rutile are 823–884 °C at 2.5–2.8 GPa, while 812–894 °C at 1.3–1.5 GPa are derived for the formation of coronitic Ttn1 in the retrograde eclogite. Therefore, isothermal decompression must have occurred during exhumation, which also has triggered the partial melting of the retrograde eclogite. Ttn2 of the garnet amphibolite has high REE contents and high Th/U ratios, indicating that it is newly grown from a Ti, Ca, and LREE enriched anatectic melt derived from the partial melting of retrograde eclogite. LA-ICP MS U-Pb dating yields a lower intercept age of 423±4 Ma for Ttn2, which is consistent with the granulite-facies metamorphic age of the retrograde eclogite. Moreover, a temperature of 781–823 °C at 1.0–1.2 GPa is obtained for Ttn2, which fits the P-T conditions of the HP granulite-facies metamorphic stage (P=1.07–1.24 GPa and T=774–814 °C), and documents that the crystallization of the melt occurred at the granulite-facies stage at 423 Ma. The high amount of REE of the garnet amphibolite is a consequence of the formation of Ttn2 from the melt. The contents and ratios of Zr and Hf in rutile and Ttn2 differ from those in the garnet amphibolite, and the whole rock Zr/Hf ratios of retrograde eclogite and garnet amphibolite are both higher than the respective ratios in rutile and Ttn2, suggesting that rutile and titanite cannot be the major carriers of Zr and Hf accounting for the high whole rock Zr/Hf ratios.

Potassic/ultrapotassic intrusions at the southwestern margin of the Yangtze Craton, southwestern China: Petrogenesis and implications for the metal and fluid source of non-arc porphyry Cu–(Mo–Au) deposits

Magmatic rocks can reveal partial melting processes in the lower crust and mantle. Therefore, the petrogenesis of both ore-barren and ore-bearing magmatic rocks can provide key insights into the deep geological processes that control porphyry Cu–(MoAu) deposits (PCDs). Here, we investigate the petrogenesis of potassic/ultrapotassic igneous rocks in the southwestern Yangtze Craton (SWYC) to constrain their magmatic sources and assess their role in the formation of PCDs. These igneous rocks can be divided into three suites, ore-barren granite porphyry, lamprophyre, and ore-bearing quartz monzonite. UPb dating of zircons indicates that these potassic/ultrapotassic intrusions crystalized simultaneously between 35.4 and 33.6 Ma. The lamprophyres have low SiO2 contents and high MgO and K2O contents, exhibiting an ultrapotassic affinity. They are depleted in high field strength elements (HFSEs; Nb, Ta, Zr, Hf, and Ti) and enriched in large ion lithophile elements (LILEs; Rb, K, Sr, and Ba). The enriched Sr, Nd, and Hf isotopic compositions of the ore-barren lamprophyres suggest that they were derived from low-degree partial melting of lithospheric mantle that had been metasomatized by subduction-related fluids during the Proterozoic. The ore-barren granite porphyries have high SiO2 and K2O contents, with high-K calc-alkaline to shoshonitic characteristics. These rocks have low MgO, Cr, Ni, and Co contents. All samples exhibit high Sr/Y ratios, and highly fractionated rare earth elements (REEs) patterns with weak Eu anomalies. Combined with their low total REE contents (75–81 ppm) and εHf(t) values, which are similar to Neoproterozoic granulite xenoliths, we conclude that the ore-barren porphyries are adakites derived from thickened Neoproterozoic lower crust. The ore-bearing intrusions show variable SiO2 and high Na2O + K2O contents, plotting in the monzonite to quartz monzonite fields in a SiO2 vs Na2O + K2O diagram. These samples have much higher total REE contents (346–671 ppm) than the ore-barren intrusions. The homogeneous SrNd isotopic compositions and linear trends on Harker diagrams suggest that the magma that formed the ore-bearing rocks underwent fractional crystallization of mainly amphibole and accessory minerals. Water contents are estimated to have been 9–12 wt% at 0.8 GPa on the basis of the retention of plagioclase and low zircon saturation temperatures, which suggests that the primitive magmas of the ore-bearing intrusions were derived from water-fluxed melting rather than dehydration melting. The additional water, required to promote water-fluxed melting, was probably derived from underplated poassic/ultrapotassic mafic magmas. The high total REE contents, Sr/Y and (La/Yb)N ratios indicate that the primitive magmas of ore-bearing intrusions were probably derived from partial melting of potassic/ultrapotassic lower crust, formed by the underplating of mantle-derived potassic/ultrapotassic magma. These results, combined with previous research, show that mantle-derived potassic/ultrapotassic magmas can provide metals and fluids required for the formation of PCDs.

Geochemistry and petrogenesis of the Tsirub nephelinite intrusions, southern Namibia

Field studies and geochemical analysis of a nephelinite dyke and two plug-like bodies - Tsirub (south) and Tsirub (north), occurring in southern Namibia are reported. Bulk rock geochemical analyses show that the nephelinites have limited but significant compositional variation (MgO = 8.7–12.1 wt%; Mg# = 0.61–0.71; Zr = 415–525 ppm; Nb = 152–267 ppm; Ni = 105–305 ppm; Zr/Nb = 1.8–3.1; La/Smn = 3.8–6.1). In detail, systematic minor variations in incompatible trace element abundances and ratios and Sr and Nd isotope compositions (87Sr/86Sr = 0.70335–0.70441; 143Nd/144Nd = 0.51278–0.51272) are best accounted for by small differences in degree of partial melting and some source heterogeneity. Quantitative modelling of partial melting processes indicates that compositional variations can be accounted for by low but variable degrees of partial melting (2–6%) of a peridotitic mantle. Negative K and to a lesser extent Ti, anomalies on primitive mantle normalised diagrams require melting to have occurred in the presence of minor amphibole, constraining the depth of melting to less than ~38 kbar, i.e. within the subcontinental lithospheric mantle. Heat required to raise the ambient geotherm (to ~55 mWm−2) to allow low degrees of melting of a volatile-rich (H2O–CO2 undersaturated) peridotitic lithospheric mantle is attributed to the passage of the African plate over the Vema mantle plume at 40–50 Ma. The close spatial proximity of the Tsirub nephelinite to the Dicker Willem carbonatite, Schwarzeberg nephelinite and alkaline intrusive rocks associated with the Klinghardt phonolites is taken to indicate a similar emplacement age of ~30–50 Ma and a related cause of magmatism.

Refining the timing and mechanism of the Triassic partial melting in the Sulu UHP orogen, China: Zircon and garnet evidence from a felsic vein and its host granitic gneiss

Partial melting of high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in subduction zones has great influence on both the chemical and physical properties of deeply subducted continental crust. Felsic veins in HP-UHP metamorphic rocks are products of partial melting and thus would be a key target to trace partial melting in HP-UHP terranes. This contribution is an integrated study of petrology, zircon and titanite U-Pb ages, as well as zircon Hf-O and major minerals O isotope compositions for a felsic vein and its host granitic gneiss in the Sulu UHP terrane to decipher the timing and mechanism of partial melting. Zircon U-Pb dating for the inherited magmatic zircon cores of the granitic gneiss constrains the protolith age at ca. 750 Ma. The inherited zircons had an average δ18O value of 0.03 ± 0.15‰ and yielded a weighted mean εHf(t) value of −1.16 ± 0.54 and two-stage Hf model ages of 1707 ± 65 Ma. These results suggest that the protolith was derived from reworking of Paleoproterozoic crust in a Neoproterozoic rift. In the two samples, broad garnet core/mantle domains are of metamorphic origin and narrow garnet rims of peritectic origin. There are large differences in δ18O values between the protolith magmatic zircon (δ18O = 0.03 ± 0.15‰) and metamorphic garnet (−4.14 ± 0.21‰ for the felsic vein and −4.13 ± 0.11‰ for the host granitic gneiss), suggesting that the negative δ18O fingerprints might result from hydrothermal alteration or multi-episodes of rift magmatism after the formation of the protolith. Zircon from the felsic vein yielded a U-Pb age of 212 ± 3 Ma. Titanites from the two studied samples show undistinguishable U-Pb ages of 227 ± 14 Ma and 212 ± 6 Ma within the analytical uncertainties. These ages register the timings of the partial melting. The approximately consistent δ18O values of the metamorphic zircon (−4.78 ± 0.15‰) and metamorphic garnet indicate that the melt was sourced from the surrounded gneiss in a closed system, which might be triggered by breakdown of phengite. According to a compilation of partial melting in the Sulu UHP terrane, there exists a clear spatio-temporal variation. The ca. 237-224 Ma partial melting in the northern UHP zone might be due to its relatively high temperature metamorphism; the most extensive partial melting occurred at ca. 220-215 Ma in both the southern and northern UHP zones, which might be triggered by the detachment of the preceding subducted slab; and partial melting of the northern UHP zone continued to ca. 210 Ma due to heat supply from the underneath mantle. Such a variation might be dictated by the tectonic evolution of the different UHP slices. Accordingly, the whole partial melting process of UHP rocks in the Sulu UHP terrane might have lasted for about 27 Myr with two major episodes at ca. 230–224 and 220-215 Ma.

The Microstructural Evolution of a Cold Rolled AlSi9Mg Aluminum Alloy after Semi-Solid Isothermal Treatment Process

The key factor in semi-solid metal forming process is the preparation of semi-solid billet with fine and spherical microstructure. The recrystallization and partial melting (RAP) process consists of cold deformation and semi-solid isothermal treatment (SSIT) is a promising method, because that many alloys are always supplied in the deformed state. In this work, a cold rolled AlSi9Mg aluminum alloy was used directly to fabricate semi-solid billets. During the RAP process, samples cut from the cold rolled AlSi9Mg aluminum alloy were treated with different isothermal holding temperatures and times, and the effects of isothermal treatment parameters on the microstructures of AlSi9Mg alloy semi-solid billets were investigated. The results show that, with increasing the isothemal holding temperature and time, the average grain size of solid grains is gradually increased, the spheroidization degrees of solid grains are sharply increased and then follows by a slight improvement. The high-quality semi-solid billets can be prepared with SSIT process at 570 °C for 5~10 min.

Multi-stage evolution of forearc mantle peridotites of Kağızman Ophiolite (Ağrı-eastern Anatolia)

The Kağızman Ophiolite, constituting the easternmost part of Tethys ophiolites in Anatolia, is located on the İzmir-Ankara-Erzincan ophiolitic belt. Ophiolitic rocks include a range of mantle peridotites, gabbros, pillow basalts and isolated sheeted dykes. The studied mantle peridotites are investigated with regard to geochemical and petrographical characteristics. MgO content of peridotites varies from 41.24 to 45.63 wt % while that of Al2O3 is in the range of 0.20–0.32 wt % indicating a high partial melting degree. Highly depleted mantle peridotites represent residues of primitive mantle formed by 17–23% melting. Whole rock trace element patterns point to variable partial melting conditions. Petrographical and geochemical results reveal that mantle peridotites of Kağızman Ophiolite are the residues of two-stage partial melting process. The first melting (consuming) stage occurred in a mid-ocean ridge setting which was later metasomatized in a subduction zone by interaction between peridotites and fluids derived from the subducted crust. According to all data, these rocks are hypothesized as forearc peridotites affected by high degree partial melting.

Influence of laser surface engineering of AISI P20-improved mold steel on wear and corrosion behaviors

The wear and corrosion characteristics of a mold steel (AISI P20-improved, KP4M) were investigated before and after the laser surface engineering. The laser surface engineering of AISI P20-improved steel was conducted at different laser energy densities by a high-power diode laser. The microstructure of the base metal changed from tempered martensite to lath-type martensite. In addition, the surface hardness was enhanced from 307 HV to 545, 657, and 610 HV when the laser energy density was 220, 315, and 420 J/mm2, respectively. An electron probe micro-analyzer revealed that microstructural homogenization was introduced by the laser surface engineering at 220 and 315 J/mm2, whereas an elemental segregation of chromium and manganese was observed with the laser surface engineering at 420 J/mm2 due to partial melting and solidification process. The laser surface-engineered samples showed an enhancement in the wear resistance in both the fretting and block-on-ring tests as compared to that of the base metal, and the highest wear resistance was detected for the laser surface-engineered at 315 J/mm2 owing to the largest increment in the hardness. In terms of corrosion resistance, the laser surface-engineered AISI P20-improved steel at 315 J/mm2 showed a marginal improvement in the corrosion resistance as compared to that of the base metal because of the microstructural homogenization. Contrastingly, a marginal deterioration of the corrosion resistance was examined for the laser surface-engineered AISI P20-improved steel at 420 J/mm2 because of the elemental segregation.

Potassium isotope fractionation during magmatic differentiation of basalt to rhyolite

High-temperature equilibrium and kinetic stable isotope fractionation during partial melting, fractional crystallization, and other igneous differentiation processes has been observed in many isotope systems, but due to the relative nascence of high-precision analytical capabilities for K, it is still unclear whether igneous processes induce systematic and resolvable K isotope fractionation. In this study, we look to the natural laboratory of Hekla volcano in Iceland to investigate the behavior of K isotopes during magmatic differentiation of basalt to rhyolite. Using a novel MC-ICP-MS method, we analyzed 24 geochemically diverse samples from Hekla, including 7 basalts, 8 basaltic andesites, 3 andesites, 4 dacites, and 2 rhyolites, along with 2 additional samples from Burfell, Iceland, for comparison (1 basalt and 1 trachyte). We observed extremely limited variation of 41K/39K ratios throughout our suite of samples, which is not resolvable within the best current analytical uncertainty. The average value of all samples is δ41KNIST SRM3141a = −0.46 ± 0.07‰ (2sd). This value agrees with the Bulk Silicate Earth value previously defined by average global oceanic basalts in literature. The lack of variation throughout this suite of samples from a single volcano system indicates that K does not fractionate during magmatic differentiation (of basalt to rhyolite) through processes such as partial melting and fractional crystallization. This conclusion is important to the estimation of the Bulk Silicate Earth K isotope composition, to placing a more robust estimate on the composition bulk continental crust, and to fostering a better understanding of the behavior of K isotopes during differentiation of the terrestrial planets.

Minerals of the Pb-As-Sb-S и Cu-Pb-As-Sb-S systems in the ores of berezitovoe gold-polymetallic deposit (Upper Amur region, Russia)

Composition and genesis of arsenium-antimony sulfosalts of lead from the gold-bearing ores of the Berezitovoe deposit located in the eastern part of Mongol-Okhotsk orogenic belt (Upper Amur Area) were considered. Lead sulfosalts (Cu and Pb) are presented by tsugaruite, dufrenoysite, boulangerite, menehinite, bismuthic menehinite (with Bi up to 11.5 mass%), as well as minerals of jordanite-geocronite and bournonite-seligmannite series. The studies have revealed basic features of the lead sulfosalts relationship with ore and silicate minerals in different mineral association of vain ores and regularities in their composition variations. It was found that the composite arsenium-antimony sulfosalts form quasicontinuous series of solid compounds strongly differing from each other by the rate of semimetals as well as semimetals-Pb ratio in their composition.&#x0D; The suggestion has been made that the main typomorphic features of composition of the arsenium-antimony sulfosalts of lead from ores of the Berezitovoe deposit were determined by the specific processes of partial melting of sulfides under the high temperature metamorphism of primary polymetallic ores.

Potassium isotope systematics of oceanic basalts

Abstract High-temperature isotope fractionation during partial melting and other igneous differentiation processes has been observed in many non-traditional isotope systems. The potassium (K) isotope system has not been extensively investigated historically due to the lack of high-precision analysis methods; however, the recent development of the Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) now allows for high-precision potassium isotope analysis. In this study, we utilized this new method to analyze 51 geologically, geographically, and geochemically diverse oceanic basalt samples including 32 mid-ocean ridge basalts (MORB), 3 back-arc basin basalts (BABB), and 16 oceanic island basalts (OIB). We observed a limited variation of 41K/39K ratios across our spread of samples. This variation in mantle-derived rocks is restricted compared to the large K isotopic fractionation observed in low-temperature systems. The averages of MORBs, BABBs, and OIBs are −0.44 ± 0.17‰ (2sd), −0.44 ± 0.08‰, and −0.41 ± 0.16‰, respectively, and there is no geographical variation (e.g., Indian vs. Pacific MORBs) in terms of K isotopes. Among all samples, there are two outliers, in which we have observed evidence of secondary mineral formation (i.e., palagonite) due to interaction with seawater. These two outliers have a K isotopic composition significantly heavier than other unaltered samples, close to the K isotopic composition of seawater. The grand average of all pristine samples is −0.43 ± 0.17‰ (2sd) which agrees well with the Bulk Silicate Earth (BSE) value previously defined. This new study indicates the homogeneity of K isotopes in the mantle and suggests that, since K will not resolvably fractionate during partial melting, any observable fractionation of K isotopes in primitive basalts is likely due to low-temperature, post-eruptive alteration processes. This conclusion is critical for understanding the initial bulk composition of the Earth and it is essential for any interplanetary comparison.

The Upper Critical Zone in the Swartklip Sector, north-western Bushveld Complex, on the farm Wilgerspruit 2JQ: II. Origin by intrusion of ultramafic sills with concomitant partial melting of host norite-anorthosite cumulates

Abstract In the Bushveld Complex, the ultramafic (orthopyroxenite/harzburgite with chromitite) layers that host most of the PGE and chromite mineralization in the Upper Critical Zone display well-documented discordant basal contacts with their anorthositic and noritic host rocks. Whilst not so well documented, there is evidence that the upper contacts of these units are also discordant. We review the nature of the contacts between the ultramafic units and adjacent plagioclase-rich lithologies. These include contact phenomena like pegmatoidal lithologies and thin magmatic reaction chromite stringers. We conclude that most, if not all, ultramafic layers were intruded as sills into pre-existing norite/anorthosite cumulates. The sequence of norites and anorthosites that hosts the ultramafic layers was built up by a prior series of multiple tholeiitic (A-type) magma intrusions. The spectrum of lithologies from melanorite through to (mottled) anorthosite represents differing degrees of partial melting in response to these successive magma influxes. Density and competence contrasts between layers of plagioclase-rich rocks in turn provided pathways for sill propagation of subsequent ultramafic (U-type) magmas. The ultramafic magmas further modified the host norites and anorthosites by processes of partial melting and metasomatism. The ultramafic units themselves accumulated as composite sills in response to multiple magma injections. In the western Bushveld Complex, particularly including the Swartklip Sector in the north-western part of the complex, the Merensky Reef is represented by various facies that occur at different levels in the host stratigraphy. This phenomenon has been referred to by the term “regional potholing”, and has been attributed to the erosion of footwall cumulates by new influxes of magma. We suggest that a series of step-and-stair-type transitions of intruding sills to successive stratigraphic levels might more appropriately explain the various facies of the Merensky Reef.

Petrogenesis and Tectonic Implications of the Paiku Leucogranites, Northern Himalaya

The Himalayan leucogranites provide insights into the partial melting behavior of relatively deeper crustal rocks and tectono-magmatic history of the Himalayan Orogen. The Paiku leucogranites of northern Himalaya can be subdivided into two-mica leucogranite (TML), garnet-bearing leucogranite (GL), cordierite-bearing leucogranite (CL), and tourmaline-bearing leucogranite (TL). All of them are high-K, peraluminous, calc-alkalic to alkali-calcic rocks. They are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), and show pronounced negative anomalies of Sr, Ba, K and Ti, but positive anomalies of Nb and Rb. LA-ICP-MS U-Pb zircon dating of one TML, one GL, and two CL samples yielded variable 206Pb/238U ages ranging from 23.6 to 16.1 Ma, indicating the Paiku leucogranites underwent a low degree of partial melting process. Combining with previous studies, we suggest the Paiku leucogranites were derived from partial melting of metasedimentary rocks of the Higher Himalayan Sequence (HHS). The GL and TL mainly resulted from the muscovite-dehydration melting, whereas the TML and CL were mainly derived from the biotite-dehydration melting. Finally, it is concluded that the Paiku leucogranites were probably formed during the subduction of the Indian crust.

Minerals of the Pb–As–Sb–S and Cu–Pb–As–Sb–S Systems in Ores of the Berezitovoe Gold-Polymetallic Deposit, Upper Amur Region, Russia

Composition and genesis of arsenium-antimony sulfosalts of lead from the gold-bearing ores of the Berezitovoe deposit located in the eastern part of Mongol-Okhotsk orogenic belt (Upper Amur Area) were considered. Lead sulfosalts (Cu and Pb) are presented by tsugaruite, dufrenoysite, boulangerite, menehinite, bismuthic menehinite (with Bi up to 11.5 mass%), as well as minerals of jordanite-geocronite and bournonite-seligmannite series. The studies have revealed basic features of the lead sulfosalts relationship with ore and silicate minerals in different mineral association of vain ores and regularities in their composition variations. It was found that the composite arsenium-antimony sulfosalts form quasicontinuous series of solid compounds strongly differing from each other by the rate of semimetals as well as semimetals-Pb ratio in their composition. The suggestion has been made that the main typomorphic features of composition of the arsenium-antimony sulfosalts of lead from ores of the Berezitovoe deposit were determined by the specific processes of partial melting of sulfides under the high temperature metamorphism of primary polymetallic ores.

Ferrous oxide-rich asteroid achondrites

Ferrous oxide (FeO)-rich asteroid achondrites can be defined as asteroid-derived samples that experienced incipient partial melting processes in the early Solar System (>4.5 Ga) leading to melt-residues and cumulate and melt rocks that have high FeO in silicate grains (molar Mg/[Mg + Fe] < 80), implying relatively oxidative conditions (fO2 of IW +1 to +3). These achondrites include olivine-dominated brachinite and brachinite-like achondrite meteorites, ungrouped meteorites including Lewis Cliff 88763, Northwest Africa (NWA) 6693 and NWA 6704, Tafassasset, NWA 011/1296, and the oligoclase-rich meteorites Graves Nunataks (GRA) 06128 and GRA 06129. Ferrous oxide-rich asteroidal achondrites differ from other partially-melted achondrites, including ureilites and acapulcoite-lodranites in that the latter have higher molar Mg/ (Mg + Fe) in silicate grains, and lower fO2 (IW 0 to −2). New mineral chemical, whole-rock major- and trace-element and highly siderophile element (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, Au) abundance data, and O and Os isotope data are presented for FeO-rich achondrite meteorites Allan Hills 84025 (brachinite), Miller Range (MIL) 090206 and MIL 090405 (brachinite-like achondrites), and NWA 6693 (ungrouped). These results, combined with available data for FeO-rich asteroidal achondrites, reveal that these rocks include nearly-pure residues after partial melting, to samples formed by melt-rock reaction and as cumulates, requiring variable to extensive Fe-Ni-S partial melting, and between 1 and 20% silicate partial melting. The FeO-rich asteroidal achondrites originate from at least four distinct parent bodies, based on O-Cr-Ti isotope systematics, and occur for both carbonaceous and non-carbonaceous chondrite precursor sources. The initial water and volatile contents of FeO-rich asteroid achondrites were similar to carbonaceous chondrite groups, implying both carbonaceous and non-carbonaceous precursor materials generated water-rich partially-melted asteroidal bodies. The existence and recognition of FeO-rich asteroid achondrites explains the otherwise enigmatic nature of some iron meteorite groups (e.g., IVA, IVB) that require segregation from an oxidized asteroid parent body. The internal structure of some asteroid parent bodies was likely to be complex, reflecting early differentiation processes of nascent core formation, Fe-Ni-S melt pooling, variable silicate partial melting, igneous differentiation and the important role of melt-rock reaction, melt refertilization and late-stage C- (reduced bodies) or S-rich (oxidized bodies) fluid and vapor reactions.

Siderophile element characteristics of acapulcoite–lodranites and winonaites: Implications for the early differentiation processes of their parent bodies

AbstractWe have studied magnetic fractions of five acapulcoites, three lodranites, and two winonaites to investigate chemical compositions of their precursor materials and metallic partial melting processes occurring on their parent bodies. One winonaite metal is similar in composition to low Au, low Ni IAB iron subgroup, indicating genetic relationship between them. Magnetic fractions of chondrule‐bearing acapulcoite and winonaite have intermediate chemical compositions of metals between H chondrites and EL chondrites. This fact indicates that the precursor materials of acapulcoite–lodranites and winonaites were similar to H and/or EL chondrites in chemical compositions. Magnetic fractions in acapulcoite–lodranites have a large variety of chemical compositions. Most of them show enrichments of W, Re, Ir, Pt, Mo, and Rh, and one of them shows clear depletion in Re and Ir relative to those of chondrule‐bearing acapulcoite. Chemical compositional variations among acapulcoite–lodranite metals cannot be explained by a single Fe‐Ni‐S partial melting event, but a two‐step partial melting model can explain it.

Mineralogical Evidence for Partial Melting and Melt-Rock Interaction Processes in the Mantle Peridotites of Edessa Ophiolite (North Greece)

The Edessa ophiolite complex of northern Greece consists of remnants of oceanic lithosphere emplaced during the Upper Jurassic-Lower Cretaceous onto the Palaeozoic-Mesozoic continental margin of Eurasia. This study presents new data on mineral compositions of mantle peridotites from this ophiolite, especially serpentinised harzburgite and minor lherzolite. Lherzolite formed by low to moderate degrees of partial melting and subsequent melt-rock reaction in an oceanic spreading setting. On the other hand, refractory harzburgite formed by high degrees of partial melting in a supra-subduction zone (SSZ) setting. These SSZ mantle peridotites contain Cr-rich spinel residual after partial melting of more fertile (abyssal) lherzolite with Al-rich spinel. Chromite with Cr# &gt; 60 in harzburgite resulted from chemical modification of residual Cr-spinel and, along with the presence of euhedral chromite, is indicative of late melt-peridotite interaction in the mantle wedge. Mineral compositions suggest that the Edessa oceanic mantle evolved from a typical mid-ocean ridge (MOR) oceanic basin to the mantle wedge of a SSZ. This scenario explains the higher degrees of partial melting recorded in harzburgite, as well as the overprint of primary mineralogical characteristics in the Edessa peridotites.