Small Degrees Of Partial Melting Research Articles

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.

Lunar Crater volcanic field (Reveille and Pancake Ranges, Basin and Range Province, Nevada, USA)

The Lunar Crater volcanic field (LCVF) in central Nevada (USA) is dominated by monogenetic mafic volcanoes spanning the late Miocene to Pleistocene. There are as many as 161 volcanoes (there is some uncertainty due to erosion and burial of older centers); the volumes of individual eruptions were typically ∼0.1 km 3 and smaller. The volcanic field is underlain by a seismically slow asthenospheric domain that likely reflects compositional variability relative to surrounding material, such as relatively higher abundances of hydrous phases. Although we do not speculate about why the domain is in its current location, its presence likely explains the unusual location of the LCVF within the interior of the Basin and Range Province. Volcanism in the LCVF occurred in 4 magmatic episodes, based upon geochemistry and ages of 35 eruptive units: episode 1 between ca. 6 and 5 Ma, episode 2 from ca. 4.7 to 3 Ma, episode 3 between ca. 1.1 and 0.4 Ma, and episode 4, ca. 300 to 35 ka. Each successive episode shifted northward but partly overlapped the area of its predecessor. Compositions of the eruptive products include basalts, tephrites, basanites, and trachybasalts, with very minor volumes of trachyandesite and trachyte (episode 2 only). Geochemical and petrologic data indicate that magmas originated in asthenospheric mantle throughout the lifetime of the volcanic field, but that the products of the episodes were derived from unique source types and therefore reflect upper mantle compositional variability on spatial scales of tens of kilometers. All analyzed products of the volcanic field have characteristics consistent with small degrees of partial melting of ocean island basalt sources, with additional variability related to subduction-related enrichment processes in the mantle, including contributions from recycled ocean crust (HIMU source; high-µ, where µ = 238 U/ 204 Pb) and from hydrous fluids derived from subducted oceanic crust (enriched mantle, EM source). Geochemical evidence indicates subtle source heterogeneity at scales of hundreds of meters to kilometers within each episode-scale area of activity, and temporary ponding of magmas near the crust-mantle boundary. Episode 1 magmas may have assimilated Paleozoic carbonate rocks, but the other episodes had little if any chemical interaction with the crust. Thermodynamic modeling and the presence of amphibole support dissolved water contents to ∼5–7 wt% in some of the erupted magmas. The LCVF exhibits clustering in the form of overlapping and colocated monogenetic volcanoes that were separated by variable amounts of time to as much as several hundred thousand years, but without sustained crustal reservoirs between the episodes. The persistence of clusters through different episodes and their association with fault zones are consistent with shear-assisted mobilization of magmas ponded near the crust-mantle boundary, as crustal faults and underlying ductile deformation persist for hundreds of thousands of years or more (longer than individual episodes). Volcanoes were fed at depth by dikes that occur in en echelon sets and that preserve evidence of multiple pulses of magma. The dikes locally flared in the upper ∼10 m of the crust to form shallow conduits that fed eruptions. The most common volcanic landforms are scoria cones, agglomerate ramparts, and ‘a‘ā lava fields. Eruptive styles were dominantly Strombolian to Hawaiian; the latter produced tephra fallout blankets, along with effusive activity, although many lavas were likely clastogenic and associated with lava fountains. Eroded scoria cones reveal complex plumbing structures, including radial dikes that fed magma to bocas and lava flows on the cone flanks. Phreatomagmatic maar volcanoes compose a small percentage of the landform types. We are unable to identify any clear hydrologic or climatic drivers for the phreatomagmatic activity; this suggests that intrinsic factors such as magma flux played an important role. Eruptive styles and volumes appear to have been similar throughout the 6 m.y. history of the volcanic field and across all 4 magmatic episodes. The total volume and time-volume behavior of the LCVF cannot be precisely determined by surface observations due to erosion and burial by basin-fill sediments and subsequent eruptive products. However, previous estimates of a total volume of 100 km 3 are likely too high by a factor of ∼5, suggesting an average long-term eruptive flux of ∼3–5 km 3 /m.y.

The role of pargasitic amphibole in the formation of major geophysical discontinuities in the shallow upper mantle

Several explanations have been proposed for variation in geophysical properties and depths for the lithosphere–asthenosphere boundary (LAB) and mid-lithospheric discontinuities (MLD). Here, we investigate the proposal that the dehydration solidus of pargasitic amphibole-bearing upper mantle with very low bulk water (hundreds ppm) may be one of the main reasons for the observed geophysical anomalies. The dehydration solidus may be associated with a very small degree of partial melting in the upper mantle at temperatures and pressures in excess of 1050 °C (for geochemically more depleted) or 1100 °C (for geochemically less depleted upper mantle) and from 1 to 3 GPa (~30 to 90 km) respectively. This small amount of partial melt may be responsible for changes in geophysical properties (e.g. lower seismic velocity, higher attenuation of seismic waves, higher electrical conductivity) in association with the LAB and MLD. This simple petrologic model is tested on the abundant geophysical data of the Carpathian–Pannonian region (CPR), central Europe. The high resolution heat flow data available in the CPR allows us to estimate the depths to intersection of area specific depth-temperature curves with the dehydration solidus temperatures (1050 and 1100 °C isotherms). There is relatively small mismatch (<5 km) between the position of these intersections and the geophysically determined LAB in the central area of the CPR. These observations lend support for the proposition that the dehydration solidus may be largely responsible for depth variation of the LAB in young continental rift areas. Towards the margins of the CPR, however, where the heat flow is lower (≲70 mW/m2), the predictive capability of the dehydration solidus model deteriorates. This is because, for lower geothermal gradients, pargasitic amphibole breaks down at ~90 km (or ~3 GPa) before temperature exceeds the dehydration solidus temperatures. Consequently, at ~90 km depth we expect no changes in geophysical properties indicative of hydrous silicate melt, in areas where surface heat flow is lower (i.e. Precambrian cratonic shields, Phanerozoic continental lithospheres or, possibly older oceanic plates). Alternatively, in these areas, the intersection of the geotherm with pargasitic amphibole breakdown may cause small changes in properties which correlate with the MLD rather than the LAB, which is at deeper levels.

The Late Cretaceous diamondiferous pyroclastic kimberlites from the Fort à la Corne (FALC) field, Saskatchewan craton, Canada: Petrology, geochemistry and genesis

The Late Cretaceous (ca. 100Ma) diamondiferous Fort à la Corne (FALC) kimberlite field in the Saskatchewan (Sask) craton, Canada, is one of the largest known kimberlite fields on Earth comprising essentially pyroclastic kimberlites. Despite its discovery more than two decades ago, petrological, geochemical and petrogenetic aspects of the kimberlites in this field are largely unknown. We present here the first detailed petrological and geochemical data combined with reconnaissance Nd isotope data on drill-hole samples of five major kimberlite bodies. Petrography of the studied samples reveals that they are loosely packed, clast-supported and variably sorted, and characterised by the presence of juvenile lapilli, crystals of olivine, xenocrystal garnet (peridotitic as well as eclogitic paragenesis) and Mg-ilmenite. Interclast material is made of serpentine, phlogopite, spinel, carbonate, perovskite and rutile. The mineral compositions, whole-rock geochemistry and Nd isotopic composition (Nd: +0.62 to −0.37) are indistinguishable from those known from archetypal hypabyssal kimberlites. Appreciably lower bulk-rock CaO (mostly <5wt%) and higher La/Sm ratios (12–15; resembling those of orangeites) are a characteristic feature of these rocks. Their geochemical composition excludes any effects of significant crustal and mantle contamination/assimilation. The fractionation trends displayed suggest a primary kimberlite melt composition indistinguishable from global estimates of primary kimberlite melt, and highlight the dominance of a kimberlite magma component in the pyroclastic variants. The lack of Nb-Ta-Ti anomalies precludes any significant role of subduction-related melts/fluids in the metasomatism of the FALC kimberlite mantle source region. Their incompatible trace elements (e.g., Nb/U) have OIB-type affinities whereas the Nd isotope composition indicates a near-chondritic to slightly depleted Nd isotope composition. The Neoproterozoic (~0.6–0.7Ga) depleted mantle (TDM) Nd model ages coincide with the emplacement age (ca. 673Ma) of the Amon kimberlite sills (Baffin Island, Rae craton, Canada) and have been related to upwelling protokimberlite melts during the break-up of the Rodinia supercontinent and its separation from Laurentia (North American cratonic shield). REE inversion modelling for the FALC kimberlites as well as for the Jericho (ca. 173Ma) and Snap Lake (ca. 537Ma) kimberlites from the neighbouring Slave craton, Canada, indicate all of their source regions to have been extensively depleted (~24%) before being subjected to metasomatic enrichment (1.3–2.2%) and subsequent small-degree partial melting. These findings are similar to those previously obtained on Mesozoic kimberlites (Kaapvaal craton, southern Africa) and Mesoproterozoic kimberlites (Dharwar craton, southern India). The striking similarity in the genesis of kimberlites emplaced over broad geological time and across different supercontinents of Laurentia, Gondwanaland and Rodinia, highlights the dominant petrogenetic role of the sub-continental lithosphere. The emplacement of the FALC kimberlites can be explained both by the extensive subduction system in western North America that was established at ca. 150Ma as well as by far-field effects of the opening of the North Atlantic ocean during the Late Cretaceous.

Late Permian basalts in the Yanghe area, eastern Sichuan Province, SW China: Implications for the geodynamics of the Emeishan flood basalt province and Permian global mass extinction

We report the finding of a ∼20m thick sequence of massive pyroxene–plagioclase–phyric basalt lava flows in the Yanghe area of the northeastern Sichuan Basin, within the Yangtze craton of SW China, which were previously considered to be located outside the Emeishan flood basalt province. This basaltic sequence above the middle Permian Maokou Formation (Fm.) is overlain by the late Permian Longtan Fm. Thus, the Yanghe basalts should be stratigraphically correlated with the Emeishan flood basalts. The Yanghe basalts show typical oceanic island basalt (OIB) affinity, and geochemically resemble Emeishan basalts, especially in the case of high-Ti (HT) basalts from the eastern domain of the Emeishan flood basalt province. The rocks have low age-corrected (87Sr/86Sr)t (t=260Ma) ratios (0.704158–0.704929) and Pb isotopic ratios [206Pb/204Pb(t) (18.264–18.524), 207Pb/204Pb(t) (15.543–15.58), and 208Pb/204Pb(t) (38.147–38.519)], and positive εNd(t) values (+3.15 to +3.61), suggesting that the lavas have not undergone any significant crustal contamination. The crystallization temperature of clinopyroxene is estimated to be 1368–1420°C, suggesting anomalously thermal inputs from a mantle source and a possible plume-head origin. The fractionation of middle rare earth elements (MREE) to heavy REE (HREE) suggests that these rocks were produced by small degrees of partial melting of mantle peridotite within the garnet–spinel transition region. The stratigraphic relationships and similar geochemical signatures with the Emeishan flood basalts suggest that the Yanghe basalts are part of the Emeishan flood basalt province and can be considered as the northeastern limit of the Emeishan flood basalt province. Our finding extends the diameter of the Emeishan flood basalt province to ∼1200–1400km, covering an area of up to ∼7×105km2, two times more than previously estimated. The larger areal extent and giant eruption volume, incorporating the Sichuan Basin, lend support to the correlation between the end-Guadalupian mass extinction and the Emeishan flood basalt province.

Source and evolution of the alkaline Pilanesberg Complex, South Africa

The Pilanesberg Complex (South Africa) is one of the world's largest but least studied alkaline complexes. It consists of trachytes, phonolites, syenites and nepheline syenites (foyaites) and the preservation of the volcanic carapace makes it unique among the larger alkaline complexes. The intrusive history of the Pilanesberg Complex shows similarities to the Greenland Kangerlussuaq Intrusion, and our new whole-rock major and trace element analyses, combined with existing data, show that the complex belongs to the Sr-rich type of evolved alkaline rocks, more similar to the complexes of the Kola Peninsula than Ilímaussaq. Despite the absence of mafic lithologies, comparison with experimental studies shows that the parental magma was most likely an alkali basalt. Significant iron enrichment is caused by an early stage of fractionation involving clinopyroxene and amphibole rather than olivine and plagioclase, reflecting water-rich compositions and intermediate levels of oxygen fugacity. This fractionation trend has led to strong enrichment in Sr and Ba, but only moderate levels of Y and middle to heavy rare earth elements, and minimal Eu-anomaly. Late-stage water-rich fluids caused significant autometasomatism in most units. New U-Pb dating of titanite constrains the age of the Pilanesberg Complex as 1395 +10/−11Ma. Initial 87Sr/86Sr isotope ratios around 0.7028 (εSr1395=−1) are typical for a moderately depleted mantle source, unlike the local lithospheric mantle. The combination of an enriched trace-element signature and depleted isotopic characteristics is evidence for small degrees of partial melting. Epsilon Sr values are similar to those reported for other alkaline complexes worldwide, emplaced in crust with contrasting geological histories; this likely reflects lithospheric metasomatism shortly before magmatism and minimal crustal contamination.

Magmatism in Western Cuddapahs: The Mafic Sills and Lava Flows of Vempalle and Tadpatri Formations

Abstract: The western part of Cuddapah basin has been considered as a foreland to the fold-thrust belt represented by eastern Nallamalais and Nellore schist belt. The igneous activities, in the form of lava flows and sills, in western Cuddapahs have been a subject of controversy since tectonogenetic models ranging from a mantle plume to lithospheric extension have been suggested. The present work covers a detailed study of the mafic sills and lava flows of Vempalle and Tadpatri formations in western Cuddapahs, which earlier workers have also noted. A detailed field and laboratory work in this study brings out, for the first time, five distinct petrographic and geochemical varieties of sills. Geochemical studies also suggest that the different sill types have originated from varying degrees of partial melting of a metasomatized sub-crustal lithospheric mantle, the metasomatism of source being attributed to older Archaean convergent settings/greenstone belts that the basin unconformably overlies. The possible contribution of any small plume is only heating and doming of overlying mantle that induced incipient crustal rifting. Our geochemical studies on the lava flows of Vempalle Formation also suggest that the magma was derived from small degree of partial melting of spinel-garnet peridotite mantle in a rift setting.

Fluorine partitioning between eclogitic garnet, clinopyroxene, and melt at upper mantle conditions

In this experimental study we obtained new mineral/melt (DF=cmineral/cmelt) partitioning data for fluorine in a bimineralic hydrous eclogite under Earth's upper mantle conditions (4–6GPa, 1460–1550°C). Omphacitic clinopyroxene displays mineral/melt partition coefficients between DF=0.056±0.005 and DF=0.074±0.001. Garnet partition coefficients are consistently lower with an average partition coefficient of DF=0.016±0.003. We found that omphacitic clinopyroxene is the dominant nominally fluorine-free phase in subducted oceanic crust and hence omphacite is expected to be the major fluorine carrier during subduction of crust into the deeper mantle. Together with previously obtained partitioning data we propose that the oceanic crust can host more fluorine per mass unit than the underlying depleted oceanic mantle.If the majority of entrained fluorine is recycled into Earth's transition zone it is possible that the fluorine is either incorporated into high-pressure transition zone phases or released during high-pressure phase transformations and forming fluorine-rich small degree partial melts. Both scenarios are supported by elevated fluorine concentration in ocean island basalts, kimberlites, and lamproites.Combining the fluorine partitioning data with water partitioning data yields a plausible process to generate lamproitic magmas with a high F/H2O ratio. The enrichment of fluorine relative to H2O is triggered by multiple episodes of small degree melting that deplete the residual more in H2O than in fluorine, caused by the approximately three times smaller mineral-melt partition coefficients of H2O.

Is gold solubility subject to pressure variations in ascending arc magmas?

Magmas play a key role in the genesis of epithermal and porphyry ore deposits, notably by providing the bulk of ore metals to the hydrothermal fluid phase. It has been long shown that the formation of major deposits requires a multi-stage process, including the concentration of metals in silicate melts at depth and their transfer into the exsolved ore fluid in more superficial environments. Both aspects have been intensively studied for most of noble metals in subsurface conditions, whereas the effect of pressure on the concentration (i.e., solubility) of those metals in magmas ascending from the sublithospheric mantle to the shallow arc crust has been quite neglected. Here, we present new experimental data aiming to constrain the processes of gold (Au) dissolution in subduction-linked magmas along a range of depth. We have conducted hydrous melting experiments on two dacitic/adakitic magmas at 0.9 and 1.4GPa and ∼1000°C in an end-loaded piston cylinder apparatus, under fO2 conditions close to NNO as measured by solid Co–Pd–O sensors. Experimental charges were carried out in pure Au containers, the latter serving as the source of gold, in presence of variable amounts of H2O and, for half of the charges, with elemental sulfur (S) so as to reach sulfide saturation. Au concentrations in melt quenched to glass were determined by LA-ICPMS. When compared to previous data obtained at lower pressures and variable redox conditions, our results show that in both S-free and sulfide-saturated systems pressure has no direct, detectable effect on melt Au solubility. Nevertheless, pressure has a strong, negative effect on sulfur solubility. Since gold dissolution is closely related to the behavior of sulfur in reducing and moderately oxidizing conditions, pressure has therefore a significant but indirect effect on Au solubility. The present study confirms that Au dissolution is mainly controlled by fO2 in S-free melts and by a complex interplay of fO2 and melt S2− concentration in sulfide-saturated melts, at given temperature. In addition, we propose that the transition from sulfide (S2−) to sulfate (S6+) species in melt is shifted towards more oxidizing conditions when pressure and the degree of melt polymerization increase. If this is true, this may have important consequences during mantle melting and magma ascent. In particular, if mantle melting occurred in moderately oxidizing conditions, a small degree of partial melting would allow the primary melts to become Au-enriched but the relatively high pressure would move the sulfide-sulfate transition to more oxidizing conditions, making the primary melts saturated with sulfide phases that would sequester gold from the melt. During magma ascent, the decreasing pressure would favor the destabilization of sulfides and the release of gold to the silicate melt. However, at shallow levels, decreasing pressure, magma evolution, and varying redox conditions would be continuously competing to concentrate or fractionate gold.

Experimental Constraints on the Formation of Silicic Magmas

A rich history of experimental petrology has revealed the paths by which silicic igneous rocks follow mineral–melt equilibria during differentiation. Subdividing these rocks by ‘molar Al versus Ca + Na + K’ illustrates first-order differences in mineralogy and gives insight into formation mechanisms. Peraluminous magmas, formed by partial melting of sediments, largely owe their attributes and compositions to melting reactions in the protoliths, whereas most metaluminous felsic magmas record both continental and mantle inputs. Peralkaline rhyolites are mainly derived from either protracted crystallization or small degrees of partial melting of basalt, with only a marginal crustal contribution. Most silicic magmas hold 3–7 wt% H2Omelt, which is inversely correlated with pre-eruptive temperature (700 °C to >950 °C) but unrelated to their reduced/oxidized state.

Age, geochemistry and Sr-Nd-Pb isotopic compositions of alkali volcanic rocks from Mt. Melbourne and the western Ross Sea, Antarctica

New K/Ar ages and geochemical and isotope data (Sr, Nd, Pb) of submarine samples from the Terror Rift Region and subaerial lavas from Mt. Melbourne Volcanic Field (MMVF) in the western Ross Sea, Antarctica, are presented. The MMVF samples are classified into Groups A and B based on their temporal and spatial distribution. All samples are alkaline, ranging from basanite to trachybasalt, and exhibit the Ocean Island Basalt (OIB)-like patterns of trace element distribution, with a prominent depletion in K and Pb. New K/Ar ages and geochemical data of the studied samples show no correlations between age and their compositions and suggest that they represent products of three different magmatic episodes. The Terror Rift submarine lavas (0.46–0.57 Ma) display a distinct trend, with more primitive geochemical characteristics (higher MgO (7.2–9.8 wt%) and CaO (9.9–11.9 wt%) and stronger HIMU signature (higher 206Pb/204Pb and 143Nd/144Nd ratios, and less radiogenic 87Sr/86Sr) than those of MMVF basalts. Results from a rare earth element (REE) model suggest that the Terror Rift submarine lavas are derived from small degrees (1–2%) of partial melting of an amphibole-bearing garnet peridotite mantle source. Despite the distinctly different ages and locations of the MMVF Group A (0.16–0.33 Ma) and B (1.25–1.34 Ma) basalts, they show similar geochemical and isotopic features, indicating the sharing of common mantle sources and magma processes during magma generation. Incompatible trace element ratios (e.g., Ba/Nb = 6.4–13.2, La/YbN = 14.4–23.2, Dy/Yb = 2.2–3.0) and isotopic compositions of the MMVF Group A and B volcanics suggest derivation from higher degrees (2–5%) of partial melting of an amphibole bearing garnet peridotite source and strong influence of an EMI-type mantle source. The stronger HIMU signature of the Terror Rift submarine lavas appears to be related to smaller degrees of partial melting, suggesting predominant contribution of the HIMU component to the less partially melted rocks from the Cenozoic NVL magmatism. In contrast, the higher degree of MMVF A and B magmas can be explained by greater interaction with heterogeneous lithospheric mantle, resulting in a diluted HIMU signature compared with that of the Terror Rift submarine lavas. We assume that HIMU- and EMI-type mantle components incorporated in the Cenozoic NVL magmas originated from sub-continental lithospheric mantle metasomatized by plume or subduction-related fluids prior to the breakup of Gondwanaland.

Characterization of the sub-continental lithospheric mantle beneath the Cameroon volcanic line inferred from alkaline basalt hosted peridotite xenoliths from Barombi Mbo and Nyos Lakes

We carried out detailed petrographic, major and trace element geochemical, microstructural and FTIR analyses on eight characteristic ultramafic xenoliths from Nyos and Barombi Mbo Lakes in the continental sector of the Cameroon Volcanic Line (CVL). The studied xenoliths are spinel lherzolites showing lithologies similar to the other xenoliths reported previously along the CVL. They have protogranular and porphyroclastic textures. One of the Barombi xenolith contains amphibole, which had not been previously reported in this locality. Amphibole is common in the Nyos xenoliths suite. Peridotite xenoliths from both localities show some chemical heterogeneity, but Barombi xenoliths generally are less depleted in basaltic elements with respect to Nyos xenoliths. Trace element compositions of Nyos spinel lherzolites show a moderately depleted initial (premetasomatic) composition and variable enrichment in REE. Evidence for both modal and cryptic metasomatism is present in Nyos xenoliths. Rare earth element patterns of clinopyroxene suggest that interaction between mafic melts and the upper mantle occurred beneath the Nyos locality. Barombi Mbo xenoliths, on the other hand, record a small degree of partial melting. The Barombi Mbo xenoliths have weak, dominantly orthorhombic olivine crystal preferred orientations, whereas Nyos ones have strong axial-[010] patterns, which may have formed in response to transpression. Nominally anhydrous mantle minerals (NAMs) of the Barombi Mbo xenoliths show generally higher bulk concentrations of ‘water’ (70–127 ppm) than Nyos xenoliths (32–81 ppm). The Barombi Mbo xenoliths could originate from a juvenile segment of the lithospheric mantle, which had been originally part of the asthenosphere. It became a part of the lithosphere in response to thermal relaxation following the extension, forming a weakly deformed lower lithospheric mantle region along the CVL. The Nyos xenoliths, however, represent a shallow lithospheric mantle bearing imprints of several depletion and enrichment events probably prior or following the extension (at ∼30 Ma).

Petrogenesis of Mesoproterozoic lamproite dykes from the Garledinne (Banganapalle) cluster, south-western Cuddapah Basin, southern India

We report mineral chemistry and whole-rock major and trace-element geochemistry for a recent find of Mesoproterozoic (~1.4 Ga) lamproites from the Garledinne (Banganapalle) cluster, south-western part of the Paleo-Mesoproterozoic Cuddapah Basin, southern India. The Garledinne lamproites occur as WNW–ESE-trending dykes that have undergone varying degree of pervasive silicification and carbonate alteration. Nevertheless, their overall texture and relict mineralogy remain intact and provide important insights into the nature of their magmas. The lamproite dykes have porphyritic to weakly porphyritic textures comprising pseudomorphed olivine macrocrysts and microphenocrysts, titanian phlogopite microphenocrysts, spinel having a compositional range from chromite to rarely magnesiochromite, Sr-rich apatite and niobian rutile. The Garledinne and other Cuddapah Basin lamproites (Chelima and Zangamarajupalle) collectively lack sanidine, clinopyroxene, potassic richterite, and titanite and are thus mineralogically distinct from the nearby Mesoproterozoic lamproites (Krishna and Ramadugu) in the Eastern Dharwar Craton, southern India. The strong correlation between various major and trace elements coupled with high abundances of incompatible and compatible trace elements imply that alteration and crustal contamination have had a limited effect on the whole-rock geochemistry (apart from K2O and CaO) of the Garledinne lamproites and that olivine fractionation played an important role in their evolution. The Garledinne lamproites represent small-degree partial melts derived from a refractory (previously melt extracted) peridotitic mantle source that was subsequently metasomatised (enriched) by carbonate–rich fluids/melts within the garnet stability field. The involvement of multiple reservoirs (sub-continental lithospheric mantle and asthenosphere) has been inferred in their genesis. The emplacement of the Garledinne lamproites is linked to extensional events, across the various Indian cratons, related to the break-up of the Proterozoic supercontinent of Columbia.

Petrogenesis of the 2.1 Ga Lushan garnet-bearing quartz monzonite on the southern margin of the North China Craton and its tectonic implications

The Lushan garnet-bearing quartz monzonite was emplaced into the western Taihua Complex at 2134±17Ma on the southern margin of the North China Craton (NCC). The rocks contain SiO2 contents of 56.98–59.05wt.%, with high K2O+Na2O (7.46–9.14wt.%) and Al2O3 (15.87–17.58wt.%), and low CaO (2.38–2.77wt.%) and MgO (1.18–1.50wt.%) contents, and belong to an intermediate shoshonitic series. Magmatic garnet grains in the rocks are chemically homogeneous, with relatively high CaO (6.36–7.92wt.%), but low MnO (1.45–1.94wt.%) and MgO (2.47–3.45wt.%) contents and high Fe/Mn ratios. They contain 61.94–66.39mol.% almandine, 18.60–23.40mol.% grossular, 10.06–15.11mol.% pyrope and 1.09–4.32mol.% spessartine, similar to high-pressure garnets formed from M-type magma but different from those found in S-type granites and metapelites. Phase equilibrium diagrams in combination with trace element distribution coefficients between zircon and garnet suggest whole-rock zircon saturation temperatures varying from 891°C to 951°C and the mineral assemblage of garnet+plagioclase+amphibole or biotite in these rocks is stable at >15kbar and 890–950°C with H2O<3wt.% in the magma. This temperature range permits the breakdown of biotite and Ca-amphibole (T>800°C). All of the rocks are enriched in LILE and LREE with low Nb/La ratios (0.24–0.61), similar to those of lithospheric mantle, which has a Nb/La ratio of ∼0.5. Therefore, the garnet and whole-rock chemical compositions indicate that the Lushan quartz monzonite possibly formed by small degrees of partial melting of a phlogopite-bearing lithospheric mantle. The variable zircon ɛHf (t) values (−3.5 to +4.1) with TDMC from 2.34 Ga to 2.51Ga suggest involvement of crustal materials. The rocks have slightly negative Nb and Ta anomalies, characteristics of island arc basalts (IAB). However, their high Zr (598–926ppm) and Zr/Y ratios (17.1–21.8) suggest an affinity with ocean island basalt (OIB). Hence, the mixed IAB- and OIB-like geochemical signatures are indicative of continental intra-plate rifting, possibly related to extension and thinning of the lithosphere and upwelling of asthenosphere. Our study proposes that an intra-continental rifting event occurred at 2.2–2.0Ga on the southern margin of the NCC after the final amalgamation of NCC.

New isotopic data for Mid-Atlantic Ridge basalts from the Arkhangelsk-Sierra Leone fracture zone (central Atlantic)

This study presents Sr, Nd, and Pb isotopic data for basalts collected from the MAR axis between the Arkhangelsk transform fault and 5° N. The isotopic data for basalts from the MAR segment between the Arkhangelsk and Vernadsky faults indicate the presence of N-MORB depleted tholeiites, which are similar to some basalts from the MAR segment between the Cape Verde and Marathon faults, at the location of the 14° N geochemical anomaly. Broad positive along-axis gradients in isotopic values (and negative for 143Nd/144Nd) are observed from south to north, superimposed by isotopic variations in the 9.6° N anomalous basalts. A north-south increase in (La/Sm)n suggests a smaller degree of partial melting. The range of isotopic variations in basalts south of the Bogdanov fault is similar to that of basalts recovered at the more southerly 1.7° N anomaly. P-MORB compositions with a large proportion of the HIMU-like component are widely distributed in the vicinity of the 5.4° N rise. The isotopic ratios decrease gradually (with increasing 143Nd/144Nd) farther to the north toward the Bogdanov transform fault. A similar trend is observed for (La/Sm)n. Therefore, the basalts recovered to the south and north of Bogdanov fault have different sources of radiogenic isotope enrichment: 5.4° N and 9.6° N rises. Basalts recovered to the north of the Vernadsky transform fault have higher 143Nd/144Nd and lower 206Pb/204Pb at similar 87Sr/86Sr ratios. We conclude that the localized manifestations of enriched basalts with isotopic characteristics close to the HIMU-like plume-derived component, including the 14° N and 1.7° N geochemical anomalies, may have been formed by microplumes entrained at the base of the upper mantle by asthenospheric mantle flows arranged into a system of cylindrical convective cells, 450–500 km in diameter. Northward of the Bogdanov transform, the upper mantle is contaminated with lower continental crust material, which was entrained during tectonic erosion of the lithosphere beneath the equatorial segment of Gondwanaland during its late stage opening.

Geochemical fingerprint of the primary magma composition in the marine tephras originated from the Baegdusan and Ulleung volcanoes

The intraplate Baegdusan (Changbai) and Ulleung volcanoes located on the border of China, North Korea, and East/Japan Sea, respectively, have been explained by appeals to both hotspots and asthenospheric mantle upwelling (wet plume) caused by the stagnant Pacific plate. To understand the origin of the Baegdusan and Ulleung volcanism, we performed geochemical analyses on the tephra deposits in the East/Japan Sea basins originating from the Baegdusan and Ulleung volcanoes. The volcanic glass in the tephra from the Baegdusan and Ulleung volcanoes ranged from alkaline trachyte to peralkaline rhyolite and from phonolite to trachyte, respectively. The tephra from the two intraplate volcanoes showed highly enriched incompatible elements, such as Tb, Nb, Hf, and Ta, distinct from those of the ordinary arc volcanoes of the Japanese islands. The straddle distribution of the Th/Yb and Ta/Yb ratios of the tephra deposits from the Baegdusan volcano may originate from the alkali basaltic magma resulting from mixing between the wet plume from the stagnant Pacific plate in the transition zone and the overlying shallow asthenospheric mantle. In contrast, the deposits from the Ulleung volcano show a minor contribution of the stagnant slab to the basaltic magma, implying either partial melting of a more enriched mantle, smaller degrees of partial melting of a garnet-bearing mantle source, or a combination of both processes as the magma genesis. Our study indicated that the Baegdusan and Ulleung volcanoes have different magma sources and evolutionary histories.

A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints

The origin and petrogenesis of the Cameroon Volcanic Line (CVL), composed of volcanoes that form on both the ocean floor and the continental crust, are difficult to understand because of the diversity, heterogeneity, and nature of available data. Major and trace elements, and Sr-Nd-Pb isotope data of volcanic rocks of the CVL spanning four decades have been compiled to reinterpret their origin and petrogenesis. Volcanic rocks range from nephelinite, basanite and alkali basalts to phonolite, trachyte and rhyolite with the presence of a compositional gap between SiO2 58–64 wt.%. Similarities in geochemical characteristics, modeled results for two component mixing, and the existence of mantle xenoliths in most mafic rocks argue against significant crustal contamination. Major and trace element evidences indicate that the melting of mantle rocks to generate the CVL magma occurred dominantly in the garnet lherzolite stability field. Melting models suggest small degree (<3%) partial melting of mantle bearing (6–10%) garnet for Mt. Etinde, the Ngaoundere Plateau and the Biu Plateau, and <5% of garnet for the oceanic sector of the CVL, Mt. Cameroon, Mt. Bambouto, Mt. Manengouba and the Oku Volcanic Group. The Sr-Nd-Pb isotope systematics suggest that mixing in various proportions of Depleted MORB Mantle (DMM) with enriched mantle 1 and 2 (EM1 and EM2) could account for the complex isotopic characteristics of the CVL lavas. Low Mg number (Mg# = 100 × MgO/(MgO + FeO)) and Ni, Cr and Co contents of the CVL mafic lavas reveal their crystallization from fractionated melts. The absence of systematic variation in Nb/Ta and Zr/Hf ratios, and Sr-Nd isotope compositions between the mafic and felsic lavas indicates progressive evolution of magmas by fractional crystallization. Trace element ratios and their plots corroborate mantle heterogeneity and reveal distinct geochemical signatures for individual the CVL volcanoes.

Petrochemistry of ultrapotassic tephrites and associated cognate plutonic xenoliths with carbonatite affinities from the late Quaternary Qa’le Hasan Ali maars, central Iran

The Quaternary Qa’le Hasan Ali (QHA) maars in central Iran occur at the intersection of the north–south Nayband fault, which defines the western boundary of the Lut micro-continental block, and a system of northwest–southeast faults, subparallel to the Zagros suture zone, that formed during the Arabian–Eurasian collision. These post-collisional maars intrude Eocene volcanic rocks of the Urumieh–Dokhtar magmatic belt, which was generated by the subduction of Neotethys oceanic lithosphere below Iran. The highly potassic, Ti-phlogopite+Mg-rich (Fo89–92) olivine+diopside-augite+aegirine–augite basanite tephrites forming the tuff rims of the QHA maars contain tephrite-coated plutonic xenoliths, some of which are interpreted as co-genetic with the tephrites based on their similar mineralogy and Sr isotopic composition (87Sr/86Sr=0.70590). Cognate plutonic xenoliths have up to ∼20vol% calcite, considered to be magmatic calcite because of (1) its grain size, which is similar to feldspars and aegirine–augite pyroxenes in these rocks, (2) the occurrence of fine-grained inclusions of pyroxene and apatite within these calcite grains, and (3) the similarity of the Sr-isotopic composition of this calcite with the other minerals in these rocks. The fact that the magmatic calcite has remained intact and did not volatilize during the transport of these xenoliths to the surface in the hot tephrite magma implies a short transit time, indicating that they are samples of a shallow plutonic complex, as does the presence of anorthoclase in these plutonic xenoliths. Their high modal proportion of magmatic calcite suggests that this shallow plutonic complex has affinities with carbonatites. The magmatic calcite-bearing plutonic xenoliths have high LREE/HREE ratios and contain REE-rich allanite (with up to ∼20wt% LREE) and britholite (∼60wt% LREE) that make up ∼3 modal percent of the most calcite-rich samples. Similar to many post-collisional highly potassic rocks formed in the Alpine–Himalayan collision zone, the highly potassic basanite tephrites of the QHA maars have high 87Sr/86Sr and low C̵Nd (−1.3 to −3.4), distinct from convecting asthenosphere, and they are depleted in Nb and Ta relative to Ba and La. They formed by small degrees of partial melting of lower continental lithospheric mantle metasomatized by both H2O and CO2-rich fluids derived from subducted Neotethys oceanic crust and sediments during the collision of the Arabian and Eurasian plates.

Trace-element modelling of mare basalt parental melts: Implications for a heterogeneous lunar mantle

The heterogeneous-source model of mare basalt formation indicates that Lunar Magma Ocean (LMO) overturn produced an uneven mixture of early-formed olivine and pyroxene, and late-formed, ilmenite-rich cumulates, which subsequently partially melted to give rise to mare magmas. These heterogeneous cumulate source regions would not only have been characterised by different mineral modal abundances, but also by different trace element compositions. The aim of this work was to investigate the petrology and geochemistry of a diverse suite of Apollo mare basalts, and utilise trace-element modelling in order to understand their petrogenetic history. Chemical modelling confirms that the mare basalts were produced by relatively small degrees of partial melting (<10%) of the LMO cumulates, and that the dominant melting type (batch vs. fractional) varies among different basalt groups. Similarly, single-source mineralogy cannot be applied to all mare basalt types, confirming that the lunar mantle was heterogeneous at the time of generation of mare magmas. Plagioclase is not required in the source of most mare basalts, with the notable exception of the Apollo 14 high-Al basalts. Addition of more than 1% plagioclase to the source of other basalts produces weaker negative Eu anomalies than those observed in the samples. AFC calculations demonstrate the compositional differences between materials assimilated into the Apollo 14 high-Al and Apollo 11 high-K mare basalt partial melts, highlighting the complexities of mare basalt petrogenesis.

Geochemical diversity in first rocks examined by the Curiosity Rover in Gale Crater: Evidence for and significance of an alkali and volatile‐rich igneous source

AbstractThe first four rocks examined by the Mars Science Laboratory Alpha Particle X‐ray Spectrometer indicate that Curiosity landed in a lithologically diverse region of Mars. These rocks, collectively dubbed the Bradbury assemblage, were studied along an eastward traverse (sols 46–102). Compositions range from Na‐ and Al‐rich mugearite Jake_Matijevic to Fe‐, Mg‐, and Zn‐rich alkali‐rich basalt/hawaiite Bathurst_Inlet and span nearly the entire range in FeO* and MnO of the data sets from previous Martian missions and Martian meteorites. The Bradbury assemblage is also enriched in K and moderately volatile metals (Zn and Ge). These elements do not correlate with Cl or S, suggesting that they are associated with the rocks themselves and not with salt‐rich coatings. Three out of the four Bradbury rocks plot along a line in elemental variation diagrams, suggesting mixing between Al‐rich and Fe‐rich components. ChemCam analyses give insight to their degree of chemical heterogeneity and grain size. Variations in trace elements detected by ChemCam suggest chemical weathering (Li) and concentration in mineral phases (e.g., Rb and Sr in feldspars). We interpret the Bradbury assemblage to be broadly volcanic and/or volcaniclastic, derived either from near the Gale crater rim and transported by the Peace Vallis fan network, or from a local volcanic source within Gale Crater. High Fe and Fe/Mn in Et_Then likely reflect secondary precipitation of Fe3+ oxides as a cement or rind. The K‐rich signature of the Bradbury assemblage, if igneous in origin, may have formed by small degrees of partial melting of metasomatized mantle.