Low-degree Melting Research Articles

Melt evolution beneath a rifted craton edge: 40Ar/39Ar geochronology and Sr–Nd–Hf–Pb isotope systematics of primitive alkaline basalts and lamprophyres from the SW Baltic Shield

A new high-precision 40Ar/39Ar anorthoclase feldspar age of 176.7±0.5Ma (2-sigma) reveals that small-volume alkaline basaltic magmatism occurred at the rifted SW margin of the Baltic Shield in Scania (southern Sweden), at a time of global plate reorganization associated with the inception of Pangea supercontinent break-up. Our combined elemental and Sr–Nd–Hf–Pb isotope dataset for representative basanite and nephelinite samples (>8wt.% MgO) from 16 subvolcanic necks of the 30 by 40km large Jurassic volcanic field suggests magma derivation from a moderately depleted mantle source (87Sr/86Sri=0.7034–0.7048; εNdi=+4.4 to +5.2; εHfi=+4.7 to +8.1; 206Pb/204Pbi=18.8–19.5). The mafic alkaline melts segregated from mixed peridotite–pyroxenite mantle with a potential temperature of ∼1400°C at 2.7–4.2GPa (∼90–120km depths), which places ultimate melt generation within the convecting upper mantle, provided that the lithosphere–asthenosphere boundary beneath the southern Baltic Shield margin was at ⩽100km depth during Mesozoic–Cenozoic rifting. Isotopic shifts and incompatible element enrichment relative to Depleted Mantle reflect involvement of at least 20% recycled oceanic lithosphere component (i.e., pyroxenite) with some minor continent-derived sediment during partial melting of well-stirred convecting upper mantle peridotite. Although pargasitic amphibole-rich metasomatized lithospheric mantle is excluded as the main source of the Jurassic magmas from Scania, hydrous ultramafic veins (i.e., hornblendite) may have caused subtle modifications to the compositions of passing sublithospheric melts. For example, modeling suggests that the more radiogenic Hf (εHfi=+6.3 to +8.1) and Pb (206Pb/204Pbi=18.9–19.5) isotopic compositions of the more sodic and H2O-rich nephelinites, compared with relatively homogenous basanites (εHfi=+4.7 to +6.1; 206Pb/204Pbi=18.8–18.9), originate from minor interactions between rising asthenospheric melts and amphibole-rich metasomatic components. The metasomatic components were likely introduced to the lithospheric mantle beneath the southern Baltic Shield margin during extensive Permo-Carboniferous magmatic activity, a scenario that is supported by the geochemical and isotope compositions of ca. 286Ma lamprophyres from Scania (87Sr/86Sri=0.7040–0.7054; εNdi=+2.0 to +3.1; εHfi=+6.1 to +9.0; 206Pb/204Pbi=17.8–18.2).Strong variations in lithosphere thickness and thermal structure across the southern Baltic Shield margin may have caused transient small-scale mantle convection. This resulted in relatively fast and focused upwellings and lateral flow beneath the thinned lithosphere, where mafic alkaline magmas formed by low degrees of decompression melting of sublithospheric mantle. Such a geodynamic scenario would allow for enriched recycled components with low melting points to be preferentially sampled from the more depleted and refractory convecting upper mantle when channeled along a destabilizing craton edge. Similar to the ‘lid effect’ in oceanic island volcanic provinces, lithospheric architecture may exert strong control on the mantle melting regime, and thus offer a simple explanation for the geochemical resemblance of continental and oceanic intraplate mafic alkaline magmas of high Na/K affinity.

Timing of Partial Melting and Cooling across the Greater Himalayan Crystalline Complex (Nyalam, Central Himalaya): In-sequence Thrusting and its Implications

The timing of crustal melting and cooling has been investigated across the migmatites of the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, central Himalaya. Monazite U–Pb ages vary from 32 to 14 Ma and are linked to metamorphic conditions on the basis of monazite internal zoning, mineral inclusions, and changes in heavy rare earth element and Y composition. Metamorphic temperatures were estimated by Zr-in-rutile thermometry and cooling rates were further constrained by rutile U–Pb ages. The results reveal two distinct blocks within the GHC of the Nyalam region. The upper GHC experienced higher peak metamorphic temperatures (730–750°C) and a higher degree of melting (15–25%). Partial melting was dominated by muscovite dehydration melting, which lasted from ∼32 to 25 Ma, possibly until ∼20 Ma. The lower GHC experienced lower peak metamorphic temperatures (640–675°C) and a lower degree of melting (0–10%) mainly via H2O-saturated melting from 19 to 16 Ma. At different times, both upper and lower blocks experienced initial slow cooling (rates 35 ± 8 and 10 ± 5°C Ma–1, respectively) followed by rapid cooling (100 ± 20°C Ma–1). The documented diachronous metamorphism implies the presence of the ‘High Himalayan Thrust’ that was active at ∼25–16 Ma within the GHC of the central Himalaya. Different degrees and durations of partial melting in the investigated section suggest that a channel flow process dominated the exhumation of the upper GHC migmatites at 25–16 Ma, whereas a critical taper process dominated the exhumation of the relatively lower-grade lower GHC rocks and cooled upper GHC migmatites at 16–10 Ma. We suggest that propagating thrusts along large tectonic boundaries together with low-viscosity lateral crustal flow could contribute to exhumation of high-grade metamorphic rocks in the Himalaya and other similar collisional orogens.

The redox geodynamics linking basalts and their mantle sources through space and time

The Earth's mantle redox state regulates the fate and transfer of metals by magmatism, buffers the igneous inputs of volcanic gases in the atmosphere and controls the depth of mantle melting. It therefore strongly affects ore forming processes, biogeochemical cycles and deep geodynamic processes. This paper reviews the current knowledge on the redox state of the upper mantle and of magmas produced by mantle melting. The geochemical processes likely to control and modify it through space and time are discussed.We analyze the link between the redox state of magma and that of their mantle source and we conclude that melts produced in the mantle may well all equilibrate in a narrow range of oxidation state, where the speciation of sulfur in basalts shifts from sulfide to sulfate, that is, FMQ+1±1 (1 log unit below and above the oxygen fugacity buffered by the assemblage fayalite–magnetite–quartz). Subsequently, degassing and partial crystallization of melts can affect their redox states, producing most of the range of redox states observed on magmas reaching Earth's surface. The asthenosphere sourcing basaltic magmas may therefore be more oxidized than the FMQ−1 value generally assumed.We also discuss redox transfers from the mantle to the atmosphere via volcanic degassing and the backward fluxes via subduction processes of the hydrothermalized oceanic lithosphere. Arc-magmas are oxidized (up to FMQ+4) but it is unclear when this feature is acquired since strongly oxidized primary arc-basalts have yet to be found. The oxidizing event may be the assimilation of slab-derived SO3-rich fluids by primary basalt generated by decompression melting in the mantle-wedge.Overall, subduction must result in a transfer of oxygen from the Earth's surface down to the mantle. This must imply that subduction and its initiation can hardly be the trigger of the great oxidation event at the end of the Archaean. In contrast, the cooling of the Earth's interior through time must have impacted on the redox state of basalts, by decreasing the depth of mantle melting. According to the long-established vertical stratification of the Earth's mantle, ancient primary magmas are therefore likely to have been more reduced (i.e. <FMQ−3) than present-day ones. However, geochemical observations on ancient basalts suggest a constant oxidation state since the early Archaean.We conclude that large uncertainties in the calibration of mineralogical oxygen barometer probably explains why we have difficulties in identifying (i) ancient primary basalts being more reduced than recent ones and (ii) primary basalts from subduction zones being as oxidized as arc-lavas reaching the surface. Finally, the degree of mantle melting is certainly a key issue for the interpretation of the mantle oxidation state. Extremely oxidized melts, enriched in C–H–S volatile species, produced by very low degrees of mantle melting may be indicative of an Earth's mantle more oxidized than usually considered.

Melting of the primitive martian mantle at 0.5–2.2 GPa and the origin of basalts and alkaline rocks on Mars

We have performed piston–cylinder experiments on a primitive martian mantle composition between 0.5 and 2.2 GPa and 1160 to 1550 °C. The composition of melts and residual minerals constrain the possible melting processes on Mars at 50 to 200 km depth under nominally anhydrous conditions. Silicate melts produced by low degrees of melting (<10 wt.%) were analyzed in layers of vitreous carbon spheres or in micro-cracks inside the graphite capsule. The total range of melt fractions investigated extends from 5 to 50 wt.%, and the liquids produced display variable SiO2 (43.7–59.0 wt.%), MgO (5.3–18.6 wt.%) and Na2O + K2O (1.0–6.5 wt.%) contents. We provide a new equation to estimate the solidus temperature of the martian mantle: T(°C)=1033+168.1P(GPa)−14.22P2(GPa), which places the solidus 50 °C below that of fertile terrestrial peridotites. Low- and high-degree melts are compared to martian alkaline rocks and basalts, respectively. We suggest that the parental melt of Adirondack-class basalts was produced by ∼25 wt.% melting of the primitive martian mantle at 1.5 GPa (∼135 km) and ∼1400 °C. Despite its brecciated nature, NWA 7034/7533 might be composed of material that initially crystallized from a primary melt produced by ∼10–30 wt.% melting at the same pressure. Other igneous rocks from Mars require mantle reservoirs with different CaO/Al2O3 and FeO/MgO ratios or the action of fractional crystallization. Alkaline rocks can be derived from mantle sources with alkali contents (∼0.5 wt.%) similar to the primitive mantle.

New 40Ar/39Ar, unspiked K/Ar and geochemical constraints on the Pleistocene magmatism of the Samtskhe-Javakheti highlands (Republic of Georgia)

The Samtskhe-Javakheti volcanic plateau (Republic of Georgia) is the northernmost and youngest expression of the magmatism following the Arabia-Eurasia collision. Here, we present whole rock elemental and twenty-one new unspiked K/Ar and 40Ar/39Ar ages for the volcanic sequence well exposed east of the plateau. Based on our new radio-isotopic ages, we have identified three magmatic episodes. The oldest one (2.84–1.08 Ma), corresponding to the “Javakheti plateau s.s.”, is mainly constituted of medium-K alkaline mafic lavas (basalt, basaltic trachyandesite) and of intermediate composition domes (dacite). The more recent volcanic activity has constructed an impressive N–S trending volcanic ridge (Samsari s.s.) composed of evolved rocks (medium-K dacite to rhyolite). Within this ridge, two main periods of activity could be depicted: Middle Pleistocene (439–189 ka) and Late Pleistocene (90–13 ka). The youngest activity is restricted to the northern edge of this prominent magmatic structure and linked to the Tavkvetili volcano activity. According to these young ages, this area can be considered as a potential zone of volcanic hazards. The oldest volcanic activity shaping the Javakheti plateau is distributed between major strike slip faults in pull-apart position. The emplacement of the volcanism is controlled by a localized upper crustal extension. This is particularly outlined by the N–S linear array of domes that constitutes the Samsari ridge. This volcanic structure emplaced indeed on top of two major N–S faults that have probably played a key role to control the Middle to Late Pleistocene volcanism. The new 40Ar/39Ar ages date between 2.32 and 1.54 Ma the fauna assemblage of the Tsalka paleontological site. Rocks from the Samtskhe-Javakheti volcanic plateau derived from a low degree of melting of a metasomatized lithospheric mantle source (spinel facies). Except the obsidians from the Chickiani dome, they all derived from this source and evolved following a crystallization sequence involving mainly clinopyroxene, garnet, and/or amphibole. A crustal contamination component modified the composition of the youngest products (Samsari ridge rocks). According to the geochemical signature of these rocks, it seems that the magmatism does not fit with models involving asthenospheric upwelling in this region.

Geochemical Stratigraphy of the Keweenawan Midcontinent Rift Volcanic Rocks with Regional Implications for the Genesis of Associated Ni, Cu, Co, and Platinum Group Element Sulfide Mineralization

The development of the North American Midcontinent rift can be understood in the context of a tectonic and magmatic event in which the far-field effects of continental drift influenced magmatism produced by a mantle plume. We report and interpret new data for samples collected from stratigraphically controlled sequences of basaltic rocks in the Osler Volcanic Group and the Mamainse Point Volcanic Group in Ontario, Canada. We confirm that the earliest phase of rift development involved primitive melts that produced basalts in the Lower Formation of the Osler Volcanic Group with Fo74–80 olivine (1,490–2,280 ppm Ni). The earliest basalts are primitive in both the Mamainse Point Volcanic Group and Osler Volcanic Group sections, with elevated MgO, Cr, and Ni; these rocks also have high TiO2 and high Gd/Yb, which are features that indicate low degrees of melting of a garnet-bearing lherzolite mantle. The Central Formation A and B basalts of the Osler Volcanic Group that overlie the Lower Formation lavas record an upward increase and then decrease in the degree of contamination of basaltic rocks with normal Ni and platinum group element (PGE) abundance levels that require no sulfide saturation event to accompany contamination. The Upper Formation of the Osler Volcanic Group is characterized by a limited number of PGE-depleted flows; although these basalts are highly fractionated and typically develop textures with stellate to laminated plagioclase feldspar, they are usually not significantly contaminated. The Lower and Upper formations of the Mamainse Point Volcanic Group show a diversity in geochemistry consistent with contributions from different mantle sources, but contamination is restricted to a small number of flows with normal Ni and PGE abundance levels. There is no indication of a regional metal depletion signature on the scale of the Nadezhdinsky Formation at Noril’sk, but there is evidence that local S saturation events were triggered in the unexposed conduits feeding some of the upper sequence flows in response to fractionation of the magma. Massive to heavy disseminated Ni-Cu-PGE sulfide mineralization in the Midcontinent rift is hosted by small primitive intrusions that were formed early in the evolution of the rift and are typically associated with transtensional structures in the cratonic blocks adjacent to the main rift. Examples include the Eagle deposit in Michigan, the Tamarack mineralization in Minnesota, and the Current Lake Complex in Ontario. The American examples are associated with splays of the Great Lakes tectonic zone and the Canadian example is associated with the Quetico fault zone. The intrusions share the geochemical fingerprints of the more primitive lowermost volcanic rocks of the Midcontinent rift and they are broadly contemporaneous with the development of the lower stratigraphy of the Osler Volcanic Group and Mamainse Point Volcanic Group. Although the low-grade but large-tonnage Cu-Ni-PGE deposits of the Duluth were formed significantly later than the small primitive intrusions, their host rocks share the same lithophile trace element signature as these early intrusions. The parental magmas that produced all of these intrusions were derived from deeper mantle depths, where garnet was a stable phase, than the magmas that produced the Central Formation basalts and some of the Upper Formation basalts investigated in this study.

Whole-rock geochemistry and Sr–Nd–Pb isotope systematics of the Late Carboniferous volcanic rocks of the Awulale metallogenic belt in the western Tianshan Mountains (NW China): Petrogenesis and geodynamical implications

The Awulale metallogenic belt (AMB) of the western Tianshan (NW China) includes Late Carboniferous (ca. 320Ma) ore-bearing volcanic rocks of the Dahalajunshan Formation. The petrogenesis and tectonic setting of these volcanic rocks are important for the understanding of the tectonic evolution and metallogeny of the western Tianshan. This paper presents new major and trace elements and Sr–Nd–Pb isotope data from the Dahalajunshan volcanic rocks, which are mainly calc-alkaline basaltic trachy-andesite and trachy-andesite with subordinate basalt, trachy-basalt and rhyolite. The variations of major and trace elements in the mafic and intermediate volcanic rocks indicate the fractionation of pyroxene and magnetite or hornblende, magnetite, apatite and plagioclase, respectively, during their petrogenesis. The Dahalajunshan volcanic rocks have similar primitive mantle-normalized diagrams and chondrite-normalized rare-earth element (REE) patterns suggesting their similar mantle source(s). They are characterized by enrichment in large ion lithophile elements (LILEs) and light REEs (LREEs), depletion in heavy REEs (LaN/YbN≈2.80 to 9.59) and high field strength elements (HFSEs) and εNd(t) ranging from +1.2 to +6.0 at 86Sr/87Sr(t)=0.7047–0.7063 and 206Pb/204Pbi=17.49–18.19. Both the geochemical and isotopic data indicate that the volcanic rocks were probably derived by low-degree melting of sub-arc lithospheric mantle modified by fluids in a continental arc setting. Our obtained results, in conjunction with previous published data, allow us to suggest that the southward subduction of Junggar oceanic crust continued until the Late Carboniferous and was followed by a tectonic shift from continental arc to post-collisional extension environment.

Middle Paleozoic mafic magmatism and ocean plate stratigraphy of the South Tianshan, Kyrgyzstan

The tectonic history of the Kyrgyz South Tianshan in the western Central Asian Orogenic Belt (CAOB) remains controversial, first of all, due to the limited amount of geochemical and isotope data. Our paper presents the first results of a detailed geochemical study (major and trace elements, Sr, Nd and Pb isotopes) of Middle Paleozoic mafic volcanic and subvolcanic rocks of the Ferghana and Atbashi–Kokshaal accretionary belts of the South Tianshan orogen in Kyrgyzstan, which formed during the evolution of the Turkestan Ocean. A special focus is given to the relation between magmatic rocks and sedimentary units of marine origin, chert, siliceous shale/mudstone, volcanogenic–carbonate clastics, seamount carbonates, and turbidites, which we consider as elements of Ocean Plate Stratigraphy (OPS). The age range of marine sediments is Late Silurian to Early Carboniferous, but the age of the most volcanic rocks associated with fossil-bearing OPS sediments is Devonian. The magmatic rocks have geochemical affinity to oceanic island basalts (OIB-type) and, to a lesser extent, mid-oceanic ridge (MORB-type) basalts associated with hemipelagic siliceous mudstone and pelagic chert. The rocks with OIB-type affinity are associated with chert, siliceous shale and carbonaceous clastics and carbonates. They are enriched in TiO2, LREE (La/Smn=1.9), and Nb (Zr/Nbav.=10), have differentiated HREE (Gd/Ybn=2.0), medium to low εNd (~5.7) and are characterized by clear Nb positive anomalies in normalized multi-element plots (Nb/Thpm=1.3, Nb/Lapm=1.1). The OIBs formed by relatively low degrees of melting (<5%) of mantle sources in the garnet stability field and erupted in an oceanic island setting. The MORB-type samples associated with siliceous mudstone and chert are less enriched in incompatible elements, possess flat REE and multi-element pattern, and show higher εNd values (~9.1); they were probably produced by high-degree melting of spinel lherzolite and/or harzburgite and erupted in a mid-oceanic ridge setting. The geological, lithological and geochronological data suggest that the OPS units with dominantly OIB-type basalts formed at one or several seamount chains of the Turkestan Ocean, which were accreted to the Kazakhstan continent, and thus contribute to our understanding of the Paleozoic tectonic evolution of the western CAOB during the Serpukhovian–Bashkirian.

Anatexis at the roof of an oceanic magma chamber at IODP Site 1256 (equatorial Pacific): an experimental study

Replenished axial melt lenses at fast-spreading mid-oceanic ridges may move upward and intrude into the overlying hydrothermally altered sheeted dikes, resulting in high-grade contact metamorphism with the potential to trigger anatexis in the roof rocks. Assumed products of this process are anatectic melts of felsic composition and granoblastic, two-pyroxene hornfels, representing the residue after partial melting. Integrated Ocean Drilling Program Expeditions 309, 312, and 335 at Site 1256 (eastern equatorial Pacific) sampled such a fossilized oceanic magma chamber. In this study, we simulated magma chamber roof rock anatectic processes by performing partial melting experiments using six different protoliths from the Site 1256 sheeted dike complex, spanning a lithological range from poorly to strongly altered basalts to partially or fully recrystallized granoblastic hornfels. Results show that extensively altered starting material lacking primary magmatic minerals cannot reproduce the chemistry of natural felsic rocks recovered in ridge environments, especially elements sensitive to hydrothermal alteration (e.g., K, Cl). Natural geochemical trends are reproduced through partial melting of moderately altered basalts from the lower sheeted dikes. Two-pyroxene hornfels, the assumed residue, were reproduced only at low melting degrees (<20 vol%). The overall amphibole absence in the experiments confirms the natural observation that amphibole is not produced during peak metamorphism. Comparing experimental products with the natural equivalents reveals that water activity (aH2O) was significantly reduced during anatectic processes, mainly based on lower melt aluminum oxide and lower plagioclase anorthite content at lower aH2O. High silica melt at the expected temperature (1000–1050 °C; peak thermal overprint of two-pyroxene hornfels) could only be reproduced in the experimental series performed at aH2O = 0.1.

Early stages of core segregation recorded by Fe isotopes in an asteroidal mantle

Ureilite meteorites are achondrites that are debris of the mantle of a now disrupted differentiated asteroid rich in carbon. They provide a unique opportunity to study the differentiation processes of such a body. We analyzed the iron isotopic compositions of 30 samples from the Ureilite Parent Body (UPB) including 29 unbrecciated ureilites and one ureilitic trachyandesite (ALM-A) which is at present the sole large crustal sample of the UPB. The δ56Fe of the whole rocks fall within a restricted range, from 0.01 to 0.11‰, with an average of +0.056±0.008‰, which is significantly higher than that of chondrites. We show that this difference can be ascribed to the segregation of S-rich metallic melts at low degrees of melting at a temperature close to the Fe–FeS eutectic, and certainly before the onset of the melting of the silicates (<1100°C), in agreement with the marked S depletions, and the siderophile element abundances of the ureilites. These results point to an efficient segregation of S-rich metallic melts during the differentiation of small terrestrial bodies.

Petrogenesis of OIB-like basaltic volcanic rocks in a continental collision zone: Late Cenozoic magmatism of Eastern Iran

Hundreds of compositionally diverse volcanoes have erupted on the Turkish–Iranian Plateau in the last 15Myr, attributed to one or more of the processes of Middle Miocene Tethyan slab break-off, localised lithospheric delamination and small-scale convection. Magmatism post-dates the initial Late Eocene or Early Oligocene Arabia–Eurasia collision by >10Myr. By contrast, in the east of the plateau in Eastern Iran there has been intermittent magmatism from the Late Oligocene to the Quaternary. Magma types include alkali basalt flows and scoria cones and adakite-like intermediate–felsic lavas and domes. New elemental and Nd–Sr–Pb–Hf isotope data from Quaternary mafic alkaline flows near Quchan in the Meshkan area in the NE of the country are combined with existing data from Miocene–Quaternary volcanic centres in the Binalud Range and the Nayband/Neh Fault zones. These mafic, incompatible element-enriched rocks, with positive Nb–Ta anomalies, are OIB-like and are argued to have formed by low-degree melting of a heterogeneous mantle source which contained DMM- and EMII-like components. At least some of the melting took place in the garnet stability field. Significant crustal contamination during magma ascent is recognised in the Nayband/Neh samples. Penecontemporaneous adakite-like rocks are argued to be high-pressure fractionates of basaltic melts which may have a separate source to the OIB-like basalts. Late Cenozoic mantle melting was aided by (1) localised extension and (2) enhanced small-scale asthenospheric convection related to Makran subduction, low mantle viscosity owing to the prior Tethyan subduction history of the region, and possibly edge-driven convection on the margin of the thickened Zagros lithosphere.

Extreme radium deficit in the 1957 AD Mugogo lava (Virunga volcanic field, Africa): its bearing on olivine-melilitite genesis

We report here U-series and trace element data for an olivine-melilitite lava flow, erupted in 1957 in the Virunga volcanic field and known as the Mugogo eruption. Petrological and geochemical data show that the Mugogo magma represents a primary mantle melt, derived from a low degree of melting of a metasomatised mantle source. It is highly enriched in very incompatible trace elements (e.g. more than 300-fold compared to primitive mantle for Th), with a distinctly lower enrichment in Rb, Ba and Sr. The high Th/U and low (230Th/232Th) ratios (4.4 and 0.750, respectively, with a (230Th/238U) ratio of 1.09) are close to the values found for the Nyamuragira basanites. But the most striking feature is the very low (226Ra/230Th) ratio of 0.48, the lowest ever measured in a mafic volcanic rock. The most probable origin of this Ra deficit is the presence of phlogopite (having a high Ra partition coefficient) in the lithospheric mantle, either as a residual phase during low-degree isobaric melting (in batch-melting or a diffusion-controlled melting models), or in a phlogopite-bearing upper mantle through which the melt migrates and equilibrates (in an equilibrium porous flow model commonly used to describe adiabatic melting). The disequilibrium pattern (230Th) > (238U) > (210Pb) ≫ (226Ra), reversed compared to the pattern observed in the Oldoinyo Lengai natrocarbonatite, suggests that a carbonatite melt might have been involved to explain the Ra deficit. We thus discuss the possibility of an early separation of a Ra-enriched carbonate melt either from the olivine-melilitite melt or from a carbonated mantle source, followed by the production of a low-degree melt of olivine-melilitite composition, but the lack of available experimental, petrological and geochemical evidences makes this process somewhat speculative at present. Further studies of recent (<5 ky) olivine-melilitites are needed to check whether the large Ra deficit is a general characteristic of this type of magma that can be used to constrain models of its formation.

Re‐Os isotope and platinum group elements of a FOcal ZOne mantle source, Louisville Seamounts Chain, Pacific ocean

AbstractThe Louisville Seamount Chain (LSC) is, besides the Hawaiian‐Emperor Chain, one of the longest‐lived hotspot traces. We report here the first Re‐Os isotope and platinum group element (PGE) data for Canopus, Rigil, and Burton Guyots along the chain, which were drilled during IODP Expedition 330. The LSC basalts possess (187Os/188Os)i = 0.1245–0.1314 that are remarkably homogeneous and do not vary with age. A Re‐Os isochron age of 64.9 ± 3.2 Ma was obtained for Burton seamount (the youngest of the three seamounts drilled), consistent with 40Ar‐39Ar data. Isochron‐derived initial 187Os/188Os ratio of 0.1272 ± 0.0008, together with data for olivines (0.1271–0.1275), are within the estimated primitive mantle values. This (187Os/188Os)i range is similar to those of Rarotonga (0.124–0.139) and Samoan shield (0.1276–0.1313) basalts and lower than those of Cook‐Austral (0.136–0.155) and Hawaiian shield (0.1283–0.1578) basalts, suggesting little or no recycled component in the LSC mantle source. The PGE data of LSC basalts are distinct from those of oceanic lower crust. Variation in PGE patterns can be largely explained by different low degrees of melting under sulfide‐saturated conditions of the same relatively fertile mantle source, consistent with their primitive mantle‐like Os and primordial Ne isotope signatures. The PGE patterns and the low 187Os/188Os composition of LSC basalts contrast with those of Ontong Java Plateau (OJP) tholeiites. We conclude that the Re‐Os isotope and PGE composition of LSC basalts reflect a relatively pure deep‐sourced common mantle sampled by some ocean island basalts but is not discernible in the composition of OJP tholeiites.

Peridotite-basalt association at MAR between 19°42′ and 19°59′ N: Evaluation of petrogenetic conditions and material balance during hydrothermal transformation of the oceanic crust

Petrogenetic conditions are evaluated for a poorly studied segment of the MAR axial zone. Mantle peridotites from MAR at 19°42′–19°59′ N are determined to show low to moderately low degrees of melting. The geochemistry of the peridotites is close to that of typical abyssal peridotites hosting active hydrothermal fields in the Atlantic (such as the Ashadze-Semenov-Logachev), except that plagioclase lherzolite occurs among the peridotites. Some of our samples collected in the area represent magnesian metabasites which could have been produced by metasomatic transformation (rodingitization) of harzburgite at contact with gabbroids. Cr and Ni concentrations in serpentine were proved to be indicative of the precursor mineral of the serpentine. The distribution of the isotopic composition data points of the MAR abyssal peridotites in the 87Sr/86Sr-143Nd/144Nd diagram suggests that outcrops of mantle peridotites and related plutonic rocks in the crest zone of the ridge expose blocks of deep-sitting rocks that were not simultaneously brought to the surface. The MAR axial segment between 19°42′ and 19°59′ N can be viewed as prospective for searches for hydrothermal ore mineralization. Compositional parameters of basalt in our rock collection correspond to those of the most depleted MORB varieties in the Atlantic.

Origin and geodynamic relationships of the Late Miocene to Quaternary alkaline basalt volcanism in the Pannonian basin, eastern–central Europe

Alkaline basaltic volcanism has been taking place in the Carpathian–Pannonian region since 11 Ma and the last eruptions occurred only at 100–500 ka. It resulted in scattered low-magma volume volcanic fields located mostly at the margins of the Pannonian basin. Many of the basalts have compositions close to those of the primitive magmas and therefore can be used to constrain the conditions of the magma generation. Low-degree (2–3 %) melting could occur in the convective asthenosphere within the garnet–spinel transition zone. Melting started at about 100 km depth and continued usually up to the base of the lithosphere. Thus, the final melting pressure could indicate the ambient lithosphere–asthenosphere boundary. The asthenospheric mantle source regions of the basalts were heterogeneous, presumably in small scale, and included either some water or pyroxenite/eclogite lithology in addition to the fertile to slightly depleted peridotite. Based on the prevailing estimated mantle potential temperature (1,300–1,400 °C) along with the number of further observations, we exclude the existence of mantle plume or plume fingers beneath this region. Instead, we propose that plate tectonic processes controlled the magma generation. The Pannonian basin acted as a thin spot after the 20–12 Ma syn-rift phase and provided suction in the sublithospheric mantle, generating asthenospheric flow from below the adjoining thick lithospheric domains. A near-vertical upwelling along the steep lithosphere–asthenosphere boundary beneath the western and northern margins of the Pannonian basin could result in decompressional melting producing low-volume melts. The youngest basalt volcanic field (Persani) in the region is inferred to have been formed due to the dragging effect of the descending lithospheric slab beneath the Vrancea zone that could result in narrow rupture at the base of the lithosphere. Continuation of the basaltic volcanism cannot be excluded as inferred from the still fusible condition of the asthenospheric mantle. This is reinforced by the detected low-velocity seismic anomalies in the upper mantle beneath the volcanic fields.

Geochemistry of the Osnitsk-Mikashevichy volcanoplutonic complex of the Ukrainian shield

This paper addresses the geochemistry of intrusive (Osinitsk complex) and volcanic (Klesov Group) rocks of the Osnitsk-Mikashevichy volcanoplutonic belt (OMVPB) of the Ukrainian shield, which is an active continental margin existing approximately 1980–2000 Ma ago. The Osnitsk complex comprises a wide range of rocks, from ultrabasics to granitoids, and the Klesov Group is dominated by extrusive rocks of basaltic and rhyolitic compositions metamorphosed under epidote-amphibolite facies conditions. The Sr-Nd-Hf isotopic systematics (ɛSr1990 from −4 to +10, ɛNd1990 from −0.6 to +2.3, and ɛHf1990 from 0.1 to 1.4) indicates a juvenile source for the OMVPB rocks. Geochemical data suggest independent origin of the rocks of gabbroid (SiO2 60 wt %) series. The gabbroids are subdivided into pre- and post-granite groups on the basis of the higher contents of incompatible trace elements and lower contents of compatible elements in the post-granite rocks. The geochemical characteristics of the two groups of basic rocks indicate their formation in a convergent tectonic setting. The origin of the granitoid melts is attributed to the low-degree (eutectoid) melting of basic rocks at relatively low temperatures.

Hot mantle geotherms stabilize calcic carbonatite magmas up to the surface

The eruption of calciocarbonatites at Earth’s surface is at odds with them being equilibrated with the mantle at depth because high-pressure experimental studies predict that significant magnesium contents should be expected. Here we report on new high-pressure experiments that demonstrate extreme calcium enrichment of carbonatites en route to the surface. We have monitored the decompression of partially molten carbonated peridotite using a multianvil apparatus coupled to synchrotron radiation. The experimental charge was molten at high pressure and high temperature, before being decompressed along a path that avoided the so-called “carbonate ledge” (a boundary that prevents carbonatitic melts from reaching the surface). Reaction with clinopyroxene yields calcium enrichment and magnesium depletion. The resulting Ca/(Ca + Mg) of the quenched melt reaches 0.95, which compares well with the composition of erupted calcic carbonatites [Ca/(Ca + Mg) ∼0.96–0.99] and of calcic melts trapped in mantle xenoliths from ocean islands [Ca/(Ca + Mg) ∼0.84–0.97]. Our results demonstrate that it is possible to bring carbonatites very close to the surface, without breakdown, and therefore without catastrophic CO2 release. Such occurrence appears to be favored by hot geotherms, meaning that higher temperatures tend to stabilize carbonatitic melts at shallow mantle pressure. Carbonatitic magmas are usually associated with low temperatures, because of the assumed low melting degree or low eruption temperature of the only active carbonatite volcano (i.e., Oldoinyo Lengai, Tanzania). Here we show that emplacement of carbonatites at or near the surface necessitates a hot environment.

Origin of Permian basalts and clastic rocks in Napo, Southwest China: Implications for the erosion and eruption of the Emeishan large igneous province

Abstract The Linghao Formation is composed of interbedded clastic and basaltic rocks in the Napo region of Youjiang Basin, Southwest China. The basalts are geochemically divided into low-Ti and high-Ti groups. Relative to the high-Ti group, the low-Ti group exhibits lower Ti/Y ratios, higher ratios of Th/Nb and Th/La, more significant negative Nb–Ta anomalies and lower e Nd (t) values (− 7.26 to + 0.29). The high-Ti group is characterized by primitive mantle normalized OIB-like incompatible element patterns. These geochemical signatures are also comparable to those of the Emeishan high-Ti and low-Ti basalts in Southwest China, respectively, and support a view that both of the low-Ti and high-Ti basalts at Napo are part of the Emeishan large igneous province (ELIP). The geochemical features imply that the high-Ti basalts could have been generated by low degree of melting of the garnet peridotite, whereas the low-Ti basalts may be derived from an EMII-like source. All clastic rocks exhibit no Nb–Ta anomalies on primitive mantle normalized elemental diagrams. Detrital zircons from clastic rocks yield U–Pb ages of ~ 260 Ma and have a geochemical affinity to within-plate-type magmas, implying a sedimentary source dominated by the ELIP. Most clastic rocks from the upper part of the Linghao Formation show higher TiO 2 contents and lower ratios of Al 2 O 3 /TiO 2 , Th/Sc, and Zr/Sc than those from the lower part. The former were related to the Emeishan high-Ti basaltic rocks, whereas the latter may have mixed felsic source compositions. In terms of chemostatigraphic correlation, we propose that the eruption of the ELIP may be more complex than previously thought and a prolonged and punctuated history of ELIP formation may exist in the Youjiang Basin.

Oceanic island basalts in accretionary complexes of SW Japan: Tectonic and petrogenetic implications

The paper presents the results of a detailed geochemical study (major and trace elements, Sr, Nd and Pb isotopes) of early Carboniferous to Miocene volcanic rocks of the Akiyoshi, Mino–Tamba, Chichibu and Shimanto accretionary complexes of Japan in combination with an analysis of their geological relations with sedimentary units and their petrogenesis (melting modeling). Most of the samples are oceanic island basalts (OIBs) associated with Carboniferous–Cretaceous sediments of Ocean Plate Stratigraphy (OPS): carbonate “cap” limestones, hemipelagic siliceous mudstone and pelagic radiolarian chert. The basalts are enriched in TiO2, LREE (La/Smn=1.9), and Nb (Zr/Nbav.=10), have differentiated HREE (Gd/Ybn=2.0), medium to low εNd (7.8–2.2) and are characterized by clear Nb positive anomalies in multi-element plots (Nb/Thpm=1.3, Nb/Lapm=1.1). The Carboniferous–Cretaceous OIBs originated at relatively low degrees of melting (<5%) from mantle sources in the spinel and garnet stability fields and erupted in an oceanic island setting. The Shimanto AC also includes Miocene andesites having lower TiO2 (0.7wt.%) and other HFSE (Hf, Y, Zr, P2O5 and Nb), strongly negative Nb anomalies in multi-element diagrams and higher εNd values (average 6.8–7.4). They may be derived from high degree melting of depleted harzburgite and probably erupted in an intra-oceanic arc setting. The studied OIBs suggest an about 250Ma period of intra-plate magmatism in the Paleo-Pacific Ocean probably related to mantle plume activity.

10Be, 18O and radiogenic isotopic constraints on the origin of adakitic signatures: a case study from Solander and Little Solander Islands, New Zealand

Subduction-related Quaternary volcanic rocks from Solander and Little Solander Islands, south of mainland New Zealand, are porphyritic trachyandesites and andesites (58.20–62.19 wt% SiO2) with phenocrysts of amphibole, plagioclase and biotite. The Solander and Little Solander rocks are incompatible element enriched (e.g. Sr ~931–2,270 ppm, Ba ~619–798 ppm, Th ~8.7–21.4 ppm and La ~24.3–97.2 ppm) with MORB-like Sr and Nd isotopic signatures. Isotopically similar quench-textured enclaves reflect mixing with intermediate (basaltic-andesite) magmas. The Solander rocks have geochemical affinities with adakites (e.g. high Sr/Y and low Y), whose origin is often attributed to partial melting of subducted oceanic crust. Solander sits on isotopically distinct continental crust, thus excluding partial melting of the lower crust in the genesis of the magmas. Furthermore, the incompatible element enrichments of the Solander rocks are inconsistent with partial melting of newly underplated mafic lower crust; reproduction of their major element compositions would require unrealistically high degrees of partial melting. A similar argument precludes partial melting of the subducting oceanic crust and the inability to match the observed trace element patterns in the presence of residual garnet or plagioclase. Alternatively, an enriched end member of depleted MORB mantle source is inferred from Sr, Nd and Pb isotopic compositions, trace element enrichments and eHf ≫ 0 CHUR in detrital zircons, sourced from the volcanics. 10Be and Sr, Nd and Pb isotopic systematics are inconsistent with significant sediment involvement in the source region. The trace element enrichments and MORB-like Sr and Nd isotopic characteristics of the Solander rocks require a strong fractionation mechanism to impart the high incompatible element concentrations and subduction-related (e.g. high LILE/HFSE) geochemical signatures of the Solander magmas. Trace element modelling shows that this can be achieved by very low degrees of melting of a peridotitic source enriched by the addition of a slab-derived melt. Subsequent open-system fractionation, involving a key role for mafic magma recharge, resulted in the evolved andesitic adakites.