Primary Magma Compositions Research Articles

Fast, furious, and gassy: Etna's explosive eruption from the mantle

The 3930 BP Fall Stratified (FS) eruption at Mt. Etna is a rare example of a highly explosive eruption of primitive (picritic) magma directly from the mantle. The eruption produced ash plumes up to an estimated 20 km height, leading to a volcanic explosivity index (VEI) 4 (subplinian). Given its volatile-rich and primitive nature, the FS magma may have ascended rapidly from great depths to avoid fractionation and mixing within the extensive plumbing system beneath Etna. To determine the pressures from which the FS magma derived, we perform rehomogenization experiments on melt inclusions hosted in Fo90–91 olivines to resorb shrinkage bubbles and determine the initial H2O and CO2 in the melt. With measured CO2 concentrations of up to 9600 ppm, volatile solubility models yield magma storage pressures of 630–800 MPa. These correspond to depths of 24–30 km, which are comparable to the seismologically estimated Moho. Therefore, the magma's high CO2 concentration must come from carbon in the mantle (likely from subducted carbonates), as opposed to assimilation of shallow (<10 km) crustal carbonates.Diffusion modeling of H2O and forsterite zonation profiles in clear, euhedral, and crystallographically oriented olivines indicates rapid ascent of magma directly from its source region to the surface. Forsterite profiles exhibit a narrow rim of growth zoning but no detectable diffusional zoning, reflecting maximum ascent times of 1–5 days. Eighteen measured H2O profiles result in remarkably uniform decompression rates of 0.47 MPa/s (95% confidence interval of 0.16–1.28 MPa/s), which is among the fastest measured for basaltic-intermediate magmas. These decompression rates indicate that the final stage of magma ascent over the region in which H2O degasses (between the surface and ∼ 15 km) occurred extremely fast at ∼ 17.5 m/s. This eruption may provide a link between primary magma composition and eruption intensity: we propose that the unusually explosive nature of this picritic eruption was driven by high H2O and CO2 concentrations, which led to continuously rapid ascent without stalling, all the way from the Moho.

Melt inclusion heterogeneity in nakhlite and chassignite meteorites and evidence for complicated, multigenerational magmas

AbstractNakhlite and chassignite meteorites are cumulate rocks thought to originate from the same location on Mars. Petrogenetic relationships between nakhlites and chassignites are not fully constrained, and the two cumulus phases in nakhlites—olivine and clinopyroxene—possibly formed either together from one magma or separately from different magmas. Primary magma compositions can potentially be determined from studies of melt inclusions (MIs) trapped within early‐formed mineral phases. MIs frequently undergo post‐entrapment effects, and when such processes occur, there can be significant changes to their compositions. Here, we report major, minor, and trace element abundances for MIs in cumulus phases in nakhlites and chassignites. The melt compositions that they record are variable (MgO = 2.50–13.5 wt%, K2O = 0.03–3.03 wt%, La/Yb = 2.46%–16.4%) and are likely affected by diffusive reequilibration with changing magma composition outside of their host phases. Evidence for diffusive reequilibration suggests that nakhlite and chassignite magmas were generated in an open system, and cumulus phases may have undergone magma storage and mixing. Such processes may be akin to those that occur in terrestrial intrusive magmatic systems by open‐system magma recharge. MIs within the nakhlite and chassignite suite therefore provide insights into magmatic processes during magma storage and transit on Mars.

Origin of the Paleoproterozoic basaltic dikes from the central and eastern Dharwar Craton and sills and volcanics from the adjoining Cuddapah Basin, southern India

Reverse fractionation modeling considering energy-constrained assimilation-fractional crystallization is performed to estimate primary magma compositions, degree of crustal contamination, pressure–temperature of equilibrium with mantle, and potential temperatures for the origin of the Paleoproterozoic (~ 2.37–1.88 Ga) basaltic dikes in central and eastern Dharwar Craton and sills and volcanics in the adjoining Cuddapah Basin, southern India. Mineral thermobarometry indicates that the dikes crystallized at upper crustal conditions (~ 1–6 kbar/ ~ 1120–1210 °C). Hence, the reverse fractionation calculations are performed at low pressures by adding olivine + plagioclase + clinopyroxene, olivine + plagioclase and only olivine in equilibrium with melt, and simultaneously subtracting an upper crustal partial melt in small steps until the melt is multiply saturated with lherzolite at a high pressure. The results indicate that the basalts are 5–30% contaminated, and their enriched light rare earth element (REE) patterns can be attributed to upper crustal assimilation. The upper crust was pre-heated to 665–808 °C during dike emplacement. The primary magmas of all basalts were last equilibrated with spinel lherzolite at 10–16.5 kbar/1291–1366 °C, and they resemble pooled polybaric incremental melts generated along a ~ 1450 °C adiabat. The estimated mantle potential temperatures (1293–1515 °C) are similar to Paleoproterozoic ambient mantle temperatures. All basalts and their primary magmas show lower chondrite-normalized DyN/YbN ratios than the plume-derived mid-Proterozoic Mackenzie dikes of Canadian Shield, and the primary magmas show flat REE patterns indicating spinel lherzolite melting. The low estimated potential temperatures, low DyN/YbN ratios, and a spinel-bearing mantle source are at odds with an origin of the basalts from mantle plumes.

A peridotite source for strongly alkalic ultrabasic HIMU lavas of the Oslo Rift, Norway

High alkali (Na2O + K2O) low SiO2 lavas generally have highly fractionated trace element compositions and their petrogenesis is debated. Both pyroxene-rich (pyroxenite) and olivine-rich (peridotite) mantle sources have been proposed for such lavas. HIMU (high 238U/204Pb) lavas usually have such compositions and are often interpreted as being derived from garnet-pyroxenite (eclogite) residue of subducted recycled oceanic crust, possibly including a carbonate component. In some cases, an origin from a garnet peridotite source has also been considered.We present new Mg, Pb, Nd, and Sr isotopic data and elemental data for alkalic ultrabasic lavas (melilitites, nephelinites) from the Oslo Rift, Norway. The magmatic province of the Permo-Carboniferous Oslo Rift is interpreted as part of the much larger Skagerrak-Centered Large Igneous Province (SCLIP) which may be related to a mantle plume source originating from the African Large Low Shear Velocity Province (LLSVP) at the core-mantle boundary. The range of δ26Mg is narrow (−0.32 to −0.28) and is similar to typical mantle values, and therefore is inconsistent with a carbonated source as has been suggested for other similar lavas. The 206Pb/204Pb values of all the lavas are high, which identifies them as HIMU lavas. The εNd of +1 to +2 are substantially lower than for typical HIMU lavas, and in the εNd-εSr diagram they plot on the LoNd array between HIMU and EMI. The lavas are more like archetypal kimberlites (formerly, Group I kimberlites), as these also have εNd slightly higher than the chondritic value and Pb isotopes consistent with a HIMU source. The REE patterns of the lavas are also almost as fractionated as in Group I kimberlites.We used major and trace elements to infer the primary magma composition of these lavas. This primary magma composition is compared with experimental melts of both carbonated peridotite and carbonated eclogite source compositions and shows that their major element compositions are only consistent with either a very high degree of melting of eclogite or a very low degree melting of garnet peridotite. The strongly fractionated REE patterns of the lavas can only be made by low-degree melting of either eclogite or garnet peridotite, which therefore effectively rules out eclogite as a major component of the mantle source of these lavas. Further trace element modeling points to a mantle source composition with >97% garnet peridotite that was lightly carbonated (<0.5 wt% CO2) and melted to a very low degree (∼0.4%) to produce these lavas. The modeled source trace element pattern was constrained by the Nd, Sr, and Pb isotopic compositions of the lavas. The measured high Ti contents of these lavas (3.8–6 wt%) are quantitatively shown to be the result of enrichment by both partial melting and fractional crystallization, using experimentally determined DTi for melting of this type of source. We also show that a low degree of melting of a slightly carbonated source is sufficient to result in the likely very high CO2 content (10–15 wt%) of the primary magma.The rather uniform Pb isotope composition of HIMU magmas suggests that the HIMU source is rather uniform in composition, but the Pb isotope composition requires this source to form late in Earth's history. The similarity of these lavas and kimberlites suggests a very deep source, possibly in the LLSVP that gave origin to the SCLIP. This means that LLSVPs do not represent some primordial layer around the core that escaped homogenization of the mantle by the Moon-forming giant impact. Instead, this layer must have formed late in Earth's history. The constraint that this source contains <3% of garnet-pyroxenite (eclogite) residue of recycled basaltic oceanic crust shows that this layer is primarily not a subducted slab graveyard. The process by which the LLSVPs formed, however, remains obscure.

Off-rift Axis Channelized Melt and Lithospheric Metasomatism along Mid-ocean Ridges—A Case Study from Iceland on the Limits of Melt Channelling

AbstractMagmatism in Iceland is classically explained by the interaction of the mid-Atlantic ridge with the Iceland plume. The growth of Iceland through time is the result of volcanic activity at the rift axis. However, Holocene volcanism (0–11.5 ka) is not restricted to the rift zone (RZ) but also occurs off-axis, specifically in the western Snæfellsnes Volcanic Belt (SVB) and in the Southern Flank Zone (SFZ). The rift and off-axis postglacial volcanic zones are separated by a gap of ~60–80 km. While the volcanic activity of the SFZ seems correlated with the actual location of the Iceland plume, the plume relation to the SVB is uncertain. To address the origin and relationships between rift and off-rift magmas, we present new data from two transects perpendicular to the Reykjanes rift. The lavas in the SVB are characterized by transitional to alkaline compositions, with elevated incompatible trace element content. In contrast, the RZ volcanic rocks have tholeiitic compositions with trace element signatures slightly more enriched than Mid-Ocean Ridge Basalt (MORB). Rift and off-rift Iceland lavas are all characterized by positive Ba and Nb anomalies, particularly in alkaline rocks. Tholeiitic and alkaline lavas show distinct differentiation sequences, with the main difference being the delayed crystallization of plagioclase in the fractionating assemblage of alkaline magmas. We apply these sequences to calculate primary magma compositions, which are then used to constrain melting conditions. Geochemical modelling indicates that Iceland rift and off-rift magmas can be produced from a peridotitic mantle source if lithospheric processes are involved. We demonstrate that recycled crust in the form of pyroxenite is not required to generate Snæfellsnes alkaline lavas. The low solidus temperature and high productivity of pyroxenite favour early and more extensive melting producing primary magmas that are not sufficiently enriched in incompatible trace elements to explain the compositional variation of Snæfellsnes magmas. An alternative mechanism to involve pyroxenite in the source of Snæfellsnes lavas relates to the hybridization of recycled oceanic crust with peridotite, but such reacted pyroxenite requires specific compositions to reproduce the Ba and Nb anomalies. As an alternative, we suggest that Snæfellsnes alkaline lavas are the result of channelized low-degree melts produced on the periphery of the melting column at distances exceeding 65 km from the ridge axis. These melts accumulate and percolate into the lithosphere producing amphibole ± phlogopite-bearing hydrous cumulates. Incongruent melting of these cumulates via renewed magmatic activity and melt-peridotite reaction can reproduce the alkaline compositions observed in the SVB, including the Ba and Nb anomalies. Numerical simulations of melt extraction below mid-ocean ridges suggest that low-degree melts produced as far as ~65 km from the central ridge axis rise vertically to the base of the lithosphere and are then focussed towards the ridge axis in decompaction channels. We propose that these melts interact with hydrous cumulates previously formed during the development of decompaction channels at the lithosphere–asthenosphere boundary and acquire specific Ba and Nb anomalies. The mixing of these distal enriched melts with more depleted melts extracted from the central part of the melting regime explains the composition of RZ lavas. Alkaline lavas observed in the SFZ show numerous analogies with the Snæfellsnes magmas, suggesting that similar lithospheric processes control their chemistry. The high thermal regime in Iceland and thick lithosphere explains the difference between Icelandic tholeiite and typical MORB. Our results highlight the importance of mantle dynamics below mid-ocean ridges and lithospheric interaction to produce off-axis magmatism with enriched alkaline signatures.

Carbonatite, aillikite and olivine melilitite from Zandkopsdrift, Namaqualand, South Africa: Constraints on the origin of an unusual lamprophyre-dominated carbonatite complex and the nature of its mantle source

We present a chemical and stable and radiogenic isotope data set for carbonatite, aillikite (ultramafic lamprophyre) and olivine melilitite from the Zandkopsdrift carbonatite complex in Namaqualand, South Africa. This carbonatite complex is associated with the ≈500-km-long, NE-SW oriented, age-progressive (40–80 Ma) Namaqualand-Bushmanland-Warmbad ultramafic alkaline magmatic lineation that consists mainly of olivine melilitite and kimberlite. Zandkopsdrift has a radiometric age of 55 ± 4 Ma and is unusual among southern African carbonatites because the dominant rock type exposed is aillikite (a variety of carbonated ultramafic lamprophyre), with carbonatite occurring mainly as a network of subvertical dykes in the southwest of the complex and olivine melilitite occurring as meter-sized blocks of float derived from the subsurface. Calcite and ferroan dolomite carbonatites are present but Mg-rich dolomitic carbonatite varieties are lacking. All rock types appear to have experienced at least moderate differentiation and all show elevated δ18OSMOW values for carbonate minerals, whereas phlogopite and clinopyroxene from these rock types fall near mantle values (with ranges of +13.3 to +21.8‰ and + 4.9 to +5.4‰ for carbonates and silicates, respectively), indicative of interaction with hydrous fluids at low-temperature. Sr-Nd-Pb isotope compositions indicate moderate to strong HIMU signatures (e.g., 206Pb/204Pbi = 19.4 to 21.0) with limited SrNd isotope variation and subvertical NdHf isotope arrays extending well below the ocean island basalt field and terrestrial array. Uncertainty regarding the differentiation history of the three rock types complicates estimation of primary magma compositions but strong chemical similarities between the carbonatites and aillikites, plus their close physical intermingling, strongly suggests that they are related by fractional crystallization to a common parental magma, whereas their relationship to the melilitites is less straightforward. The aillikites and melilitites have compositions of the REE and other elements similar to near-primary versions of these rock types from other alkaline igneous provinces globally, which have been explained as the products of low degrees of partial melting at 4–7 GPa (for aillikites) and slightly higher degrees of melting at 3–3.5 GPa (for melilitites). Lower pressures of generation and distinct trace element characteristics of the Zandkopsdrift melilitites could be explained by these originating as melts of a garnet wehrlite halo around the magmatic conduit formed by metasomatic reaction between carbonated melts and lithospheric peridotite. Most Zandkopsdrift carbonatites have high Lu/Hf ratios which, combined with their relatively low 176Hf/177Hf values, suggests that their mantle sources could not have formed more than 30–50 Myr prior to emplacement of the complex. This implies that a mantle plume had a role in their formation, either as a direct source of melts or as a source of heat with which to melt pre-existing metasomatic heterogeneities.

Olivine in picrites from continental flood basalt provinces classified using machine learning

Abstract Picrites, dominantly composed of highly forsteritic olivine, can serve as important constraints on primary magma composition and eruption dynamic processes in global continental flood basalt (CFB) provinces. Picrites are commonly divided into high-Ti and low-Ti groups based on whole-rock TiO2 content or Ti/Y ratio. Here, we use an artificial neural network (ANN) to classify the individual olivine in picrites from global CFB provinces according to whether their parental magma is high-Ti or low-Ti to better understand the primary origin and magmatic processes. After training the ANN on 1000 olivine major element compositions data points, the network was able to differentiate chemical patterns for high-Ti and low-Ti olivine and classify olivine into correct types with an accuracy of &amp;gt;95%. Moreover, we find that two types of olivine mix in some single samples from Etendeka, Emeishan, and Karoo CFB provinces. Combining the results with chemical markers of source lithology, we suggest that the two types of olivine originate from two different sources and their olivine populations mixed during the ascent. This mixing then makes the spatial and temporal variation of picrites types in some CFB provinces unclear.

Reconstruction of primary alkaline magma composition from mineral archives: Decipher mantle metasomatism by carbonated sediment

Alkaline magmas play a key in understanding crust-mantle interaction and deep carbon recycling. Here, petrography and in situ mineral analyses of evolved alkaline rocks from the North China Craton are reported to decipher mantle metasomatism by recycled carbonated sediments via reconstruction of primary magma composition. The alkaline rocks are nepheline syenites for which new, consistent zircon and titanite UPb ages of ~235 Ma are presented. The rocks are highly evolved (SiO2 = 58.1–60.0 wt%) with negative anomalies for Sr, Ba, Ti, P and Eu, and relative depletion of MREE, indicating fractional crystallization of feldspar, apatite, titanite and amphibole. Core-rim variation of 87Sr/86Sr (0.7052–0.7092) of late-crystallized low Mg# clinopyroxene and feldspar are attributed to crustal assimilation.The origin of primary alkaline magma is recorded in early high Mg# (70–84) clinopyroxenes with weak Eu anomalies (Eu/Eu* = 0.94–1.29), suggesting that they crystallized from a little-differentiated mantle-derived magma (Mg# > 60). Their variable 87Sr/86Sr ratios (0.7031–0.7060) correlate positively with Sr/Y and (La/Yb)N and negatively with Ti/Eu ratios, but are not correlated with Mg# or Eu/Eu*. Moreover, the REE patterns of melt equilibrated with high 87Sr/86Sr clinopyroxene is similar to coeval alkaline rocks from adjacent areas. These features indicate that the primary alkaline magma was derived from the mixing of depleted mantle and enriched mantle metasomatized by carbonatite melts from subducted crustal materials. Low zircon εHf(t) values (−1.4 to −2.8) imply that the subducted crustal material was carbonated sediments. Calcite and CO2 inclusions occur in early-crystallized olivine, clinopyroxene and zircon, and amphibole and fluorapatite both reveal a volatile-rich (H2O, F and CO2) alkaline magma derived from mantle enriched by carbonate metasomatism. The mineral archives of alkaline rocks not only record mantle metasomatism by recycled carbonated sediments, but also fractional crystallization and assimilation during magma evolution.

Origin of the primitive, strongly SiO2-undersaturated alkalic rocks from the Deccan Traps by low-degree mantle melting and high-pressure fractional crystallization

Strongly SiO2-undersaturated alkalic rocks (Mg# > 50, SiO2 ≤ 45 wt%, Na2O + K2O ≥ 3 wt%) occur in three early-stage (Sarnu-Dandali, Mundwara, Bhuj) and one late-stage (Murud-Janjira) rift-associated volcanic complexes in the Cretaceous-Paleogene Deccan Traps flood basalt province of India. Thermobarometry based on clinopyroxene-liquid equilibrium suggests that they mostly crystallized beneath the Moho at ~ 15 kbar/1270 °C to ~ 11–12 kbar/1115–1156 °C pressures and temperatures. Primary magma compositions in equilibrium with lherzolite were estimated through reverse fractionation calculations by incrementally adding equilibrium phases to the rocks in olivine:clinopyroxene:spinel:phlogopite = 12:68:20:15 proportions at low temperatures followed by olivine:clinopyroxene:spinel = 12:68:20 proportions at higher temperatures. A comparison of the primary magmas with experimentally generated melts shows that their compositions are consistent with an origin from garnet lherzolite sources with < 1 wt% H2O and CO2. Hornblendite, pyroxenite (except for some Bhuj rocks) and carbonated eclogite are unlikely sources for the Deccan alkalic rocks. The Sarnu-Dandali and Bhuj alkalic rocks and the Murud-Janjira lamprophyres probably originated by < 5% melting of ~ 1.3 times Ti-enriched lherzolitic sources compared to primitive mantle. The primary magmas of the Murud-Janjira basanites calculated through reverse assimilation-fractional crystallization by assimilating lower crustal and mantle xenoliths found in younger lamprophyre dikes of the same area indicate that contamination by the Indian lithosphere was unlikely during their ascent. The basanites evolved by mixing with phonotephritic melts, and they probably originated from a Ti-poor (0.7 times) lherzolite source. The temperature of the melts at the base of the lithosphere was ~ 1325 °C beneath Sarnu-Dandali and ~ 1285 °C beneath Bhuj and Murud-Janjira.

Temporally variable crustal contributions to primitive mantle-derived Columbia River Basalt Group magmas

The Steens Formation is one of the earliest and most primitive (>7 wt. MgO) eruptive products of the Columbia River Basalt Group (CRBG) and the CRBG-Yellowstone-Snake River large igneous province. New major-, trace-, and highly siderophile-element abundance and 87Sr/86Sr, 143Nd/144Nd and 187Os/188Os data are reported for the lower and upper Steens Formation to examine likely mantle sources and the nature of magmatic differentiation and crustal contamination acting on lavas. Examined Steens Formation basalts are relatively mafic (7–9 wt% MgO), incompatible trace element enriched, and have weaker Nb and Ta anomalies compared to other CRBG lavas. The most primitive basalts have isotopic compositions at the time of crystallization consistent with originating from a mantle source that was relatively depleted (87Sr/86Sr = ~0.7033; εNdi = ~ + 6.5; γOsi = ~ + 1). Primary magma compositions for the Steens Formation do not provide compelling evidence for a subducted slab component, instead suggesting derivation from primitive mantle sources more similar to those of other Mesozoic continental flood basalts (CFB; e.g., Deccan, North Atlantic Igneous Province). Onset of sulfide saturation in the CRBG occured at lower MgO (<7 wt%) than in other CFB (~ 8 wt%) leading to the high Os contents in the Steens Formation. Collectively, the Steens Formation exhibits decreasing Os contents, εNdi values and increasing 187Os/188Os with decreasing MgO. These geochemical signatures are consistent with increasing crustal contamination to parent melts with time, a feature that is also shared for the CRBG as a whole. Calculations based on Os and Nd isotopes of likely mantle and crust components to different formations of the CRBG indicates a progressive increase in the quantities of crustal contamination from ~1 to 2% from Steens Formation magmatism to more than 6% during Grande Ronde and Wanapum eruptions. These results would indicate increasing crustal contamination and enhanced potential cryptic degassing of CO2 in the later, more voluminous stages of CRBG magmatism, after ~16.5 Ma. Unless mantle-derived melts can produce sufficient greenhouse gas emission, there is likely an offset between the inception of the mid-Miocene Climatic Optimum at 17 Ma and maximum CO2 release, indicating that CRBG eruption was a contributing factor to climate change at that time, but was not the trigger for it.

Crystallization and melt extraction of a garnet-bearing charnockite from South China: Constraints from petrography, geochemistry, mineral thermometry, and rhyolite-MELTS modeling

AbstractSince granitic rocks in high-grade terranes commonly undergo amphibolite-granulite facies meta-morphic overprint, recovering magmatic records from the metamorphic modification remains a major challenge. Here, we report an early Paleozoic, garnet-bearing Yunlu charnockite that outcropped in the Yunkai terrane of the Cathaysia block from South China and underwent amphibole-grade metamorphic overprint in the late Devonian. Field observation, micro-texture, and mineral geochemistry combined with diffusion modeling constrain that the metamorphic overprint with an extremely short duration of ~0.2–0.5 Ma only influences a narrow rim of &amp;lt;100 μm for most minerals. The magmatic information can be retrieved by combining rhyolite-MELTS modeling with mineral thermobarometry using mineral core compositions to quantitatively estimate magmatic pressure, temperature, and melt H2O contents. Rhyolite-MELTS modeling results are evaluated by comparison with experimentally determined phase relations for a peraluminous granite with ~69.83 wt% SiO2 at a pressure of ~500 MPa. The comparison suggests that the modeling reproduces phase relationships of feldspars and quartz within 20–60 °C when the melt H2O contents are below 7.0 wt%, but fails to account properly for all the phases when the melt H2O contents are higher than 7.0 wt%. The modeling results using reconstructed primary magma composition of the Yunlu charnockite combined with the orthopyroxene-garnet-plagioclase-quartz thermobarometry and fluid inclusion analyses suggest that the magma was emplaced at a pressure of ~600 MPa, a temperature of &amp;gt;900 °C, and an initial H2O content of ~4.0 wt% with rare CO2 components. The orthopyroxene-garnet, biotite-garnet, and biotite-orthopyroxene thermometers yield a consistent temperature range of 770–820 ± 60 °C, which is significantly higher than the H2O-saturated solidus temperature of ~630 °C estimated from experimental results and two-feldspar thermometry. These results indicate that the early crystallized minerals (e.g., garnet, orthopyroxene, and some euhedral biotite) of the Yunlu charnockite equilibrate at higher temperatures with crystallinities of ~30–45%, rather than the H2O-saturated solidus conditions. We thus propose a hypothesis of melt extraction at 780–820 °C in a deep-seated, slowly cooling, partially crystalline magma reservoir. The melt extraction physically segregates the early crystallized minerals from residual interstitial melts, which inhibits element diffusion equilibration between these minerals and interstitial melts. Granite thermometry commonly yields a large range of temperature estimations, which may be related to melt extraction events. Our study shows that melt extraction recorded in granites can be identified by combining micro-texture, mineral thermometry and rhyolite-MELTS modeling, which further provides quantitative insights into the fractionation process of silicic magmas.

Controls of mantle source and condition of melt extraction on generation of the picritic lavas from the Emeishan large igneous province, SW China

Like many continental flood basalt (CFB) provinces in the world, the source mantle compositions and melting conditions for generation of the Emeishan CFB are highly debated. We have carried out an integrated Sr-Nd-O-Mg isotopic study of the Emeishan picritic lavas to evaluate possible effects of mantle heterogeneity on the magma composition. Moreover, we have used PRIMELT3 (Herzberg and Asimow, 2015) to calculate the primary magma compositions and melting conditions for these picritic lavas. Picrites from Maoniuping, Tanglanghe and Wuguijing exhibit depleted Sr-Nd isotopic compositions (87Sr/86Sri = 0.7038–0.7050; εNd = 0.27–3.89), normal mantle-like δ26Mg (−0.27 to −0.2‰) and δ18O (5.0–5.4‰), which can be explained by partial melting of plume materials with negligible contribution of sedimentary carbonates. The Wulongba picrites are characterized by depleted Sr-Nd isotopic compositions (87Sr/86Sri = 0.7041–0.7045; εNd = 2.57–2.85), slightly lower δ26Mg (−0.36 to −0.23‰) and higher δ18O (5.6–5.7‰), which were attributed to involvement of minor sedimentary carbonates (magnesite) in their mantle source. The calculated mantle potential temperatures for the Emeishan picrites range from 1516 to 1596 °C. The initial melting pressures range from 4 to 5 GPa. The final melting pressures for the Wulongba picrites (2–3 GPa) are lower than those for the Tanglanghe, Maoniuping and Wuguijing picrites (>3 GPa). This suggests that the Wulongba picrites were generated in shallower garnet-spinel transition zone of the upper mantle. Picrites from other three localities are entirely generated in garnet stability field of the upper mantle.

Experiments on the Saturation of Fluorite in Magmatic Systems: Implications for Maximum F Concentration and Fluorine-Cation Bonding in Silicate Melt

The effects of melt composition, temperature and pressure on the solubility of fluorite (CaF2), i.e., fluorine concentration in silicate melts in equilibrium with fluorite, are summarized in this paper. The authors present a statistic study based on experimental data in literature and propose a predictive model to estimate F concentration in melt at the saturation of fluorite (C F in melt Fl-sat ). The modeling indicates that the compositional effect of melt cations on the variation in C F in melt Fl-sat can be expressed quantitatively as one parameter FSI (fluorite saturation index): FSI=(3AlNM+Fe2++6Mg+Ca+1.5Na-K)/(Si+Ti+AlNF+Fe3+), in which all cations are in mole, and AlNF and AlNM are Al as network-forming and network-modifying cations, respectively. The dependence of C F in melt Fl-sat on FSI is regressed as: C F in melt Fl-sat =1.130−2.014·exp (1 000/T)+2.747·exp (P/T)+0.111·C melt H2O +17.641·FSI, in which T is temperature in Kelvin, P is pressure in MPa, C melt H2O is melt H2O content in wt.%, and C F in melt Fl-sat is in wt.% (normalized to anhydrous basis). The reference dataset used to establish the expression for conditions within 540–1 010 °C, 50–500 MPa, 0–7 wt.% melt H2O content, 0.4 to 1.7 for A/CNK, 0.3 wt.%–7.0 wt.% for C F in melt Fl-sat . The discrepancy of C F in melt Fl-sat between calculated and measured values is less than ±0.62 wt.% with a confidence interval of 95%. The expression of FSI and its effect on C F in melt Fl-sat indicate that fluorine incorporation in silicate melts is largely controlled by bonding with network-modifying cations, favorably with Mg, AlNM, Na, Ca and Fe2+ in a decreasing order. The proposed model for predicting C F in melt Fl-sat is also supported by our new experiments saturated with magmatic fluorite performed at 100–200 MPa and 800–900 °C. The modeling of magma fractional crystallization emphasizes that the saturation of fluorite is dependent on both the compositions of primary magmas and their initial F contents.

Geochemistry of continental alkali basalts in the Sabzevar region, northern Iran: implications for the role of pyroxenite in magma genesis

A geochemical study was undertaken on basaltic lava flows intercalated with Oligocene to Miocene strata in the Sabzevar region, northern Iran, to examine their petrogenesis in a regional tectonic framework. The lavas are either aphyric or have phenocrysts and micro-phenocrysts of olivine and, to a lesser extent, clinopyroxene. Geochemically, the lavas are silica-undersaturated alkali basalts characterized by relatively high Mg# (~ 57–66) and Na2O/K2O (~ 2.0–6.7). They have distinctive trace element patterns characterized by strong rare-earth element fractionation, negative Nb–Ta and Zr-Hf anomalies and a positive Sr anomaly. Significant contamination by crustal materials either in the magma source or during ascent is ruled out on the basis of trace element compositions and Sr–Nd isotopic compositions (87Sr/86Sr = 0.7037–0.7048 and 143Nd/144Nd = 0.5128–0.5130), both of which differ markedly from continental crustal rocks. Phenocryst assemblage, analysis of multiple saturation points in lherzolite systems, and covariations of La/Yb with MgO of the studied lavas are generally consistent with an origin involving high-pressure fractionation of peridotite-derived melts. Primary magma compositions calculated by reversed fractionation of clinopyroxene and olivine for the relatively primitive samples (> 9 wt.% MgO), however, do not plot on the lherzolite multiple saturation points. Also, high-pressure fractionation predicts increasing trends of silica undersaturation and alkalinitiy with differentiation, and such trends are not indicated by the geochemical data. We suggest that the mixed trends shown by the data might be related to melt generation from both peridotite and silica-deficient pyroxenite sources, superimposed by variable degrees of high-pressure fractionation. The role of pyroxenite in magma genesis is indicated not only by the positive Sr anomaly shown by the trace element patterns, but also first-row transition element systematics of the studied lavas. The silica-deficient pyroxenites contributing to melt generation might have been transformed from mafic–ultramafic cumulates in subducted, lower oceanic crust, or might have formed in the lower crust or mantle lithosphere under continents during earlier magmatic episodes.

Petrogenesis of the ~740 Korab Kansi mafic-ultramafic intrusion, South Eastern Desert of Egypt: Evidence of Ti-rich ferropicritic magmatism

The Neoproterozoic Korab Kansi mafic-ultramafic intrusion is one of the largest (100 km2) intrusions in the Southern Eastern Desert of Egypt. The intrusion consists of Fe-Ti-bearing dunite layers, amphibole peridotites, pyroxenites, troctolites, olivine gabbros, gabbronorites, pyroxene gabbros and pyroxene-hornblende gabbros, and also hosts significant Fe-Ti deposits, mainly as titanomagnetite-ilmenite. These lithologies show rhythmic layers and intrusive contacts against the surrounding granites and ophiolitic-island arc assemblages. The wide ranges of olivine forsterite contents (Fo67.9-85.7), clinopyroxene Mg# (0.57–0.95), amphibole Mg# (0.47–0.88), and plagioclase compositions (An85.8-40.9) indicate the role of fractional crystallization in the evolution from ultramafic to mafic rock types. Clinopyroxene (Cpx) has high REE contents (2–30 times chondrite) with depleted LREE relative to HREE, like those crystallized from ferropicritic melts generated in an island-arc setting. Melts in equilibrium with Cpx also resemble ferropicrites crystallized from olivine-rich mantle melts. Cpx chemistry and its host rock compositions have affinities to tholeiitic and calc-alkaline magma types. Compositions of mafic-ultramafic rocks are depleted in HFSE (e.g. Nb, Ta, Zr, Th and U) relative to LILE (e.g. Li, Rb, Ba, Pb and Sr) due to the addition of subduction-related hydrous fluids (rich in LILE) to the mantle source, suggesting an island-arc setting. Fine-grained olivine gabbros may represent quenched melts approximating the primary magma compositions because they are typically similar in assemblage and chemistry as well as in whole-rock chemistry to ferropicrites. We suggest that the Korab Kansi intrusion crystallized at temperatures ranging from ~700 to 1100 °C from ferropicritic magma derived from melting of metasomatized mantle at <5 Kbar. These hydrous ferropicritic melts were generated in the deep mantle and evolved by fractional crystallization under high ƒO2 at relatively shallow depth. Fractionation formed calc-alkaline magmas during the maturation of an island arc system, reflecting the role of subduction-related fluids. The interaction of metasomatized lithosphere with upwelling asthenospheric melts produced the Fe and Ti-rich ferropicritic parental melts that are responsible for precipitating large quantities of Fe-Ti oxide layers in the Korab Kansi mafic-ultramafic intrusion. The other factors controlling these economic Fe-Ti deposits beside parental melts are high oxygen fugacity, water content and increasing degrees of mantle partial melting. The generation of Ti-rich melts and formation of Fe-Ti deposits in few layered intrusions in Egypt possibly reflect the Neoproterozoic mantle heterogeneity in the Nubian Shield. We suggest that Cryogenian-Tonian mafic intrusions in SE Egypt can be subdivided into Alaskan-type intrusions that are enriched in PGEs whereas Korab Kansi-type layered intrusions are enriched in Fe-Ti-V deposits.

A mid-Cretaceous alkaline volcano in the Davis Strait

The AT2-1 well in the Davis Strait between Canada and Greenland penetrated an approximately 1.2 km thick sequence of alkaline volcanic rocks with some intercalated sediments at depths between 3690 to 4850 m. These volcanic rocks can be mapped on 2D seismic data and constitute a cone-shaped 5 km × 10 km wide and &gt;1.2 km high structural high named the Atammik Volcano. This sequence comprises two distinct parts, an upper part of phono-tephrite to basaltic trachy-andesite and a lower part of tephriphonolite and phonolite. Rock textures and structures testify to a volcanic origin, with the uppermost units showing textural evidence of being subaerially extruded. Zircon crystals found in a sample of phonolite from 4453 m were dated by in situ laser ablation ICP–MS technique to yield ages between 98 and 93 Ma, indicating a maximum age of the formation of the phonolitic volcano of 93 Ma (Turonian). Further, detrital zircons from the clastic material have been dated yielding Archean ages. The gamma ray log indicates three internal cycles within the phonolites, each cycle displaying a stratigraphically upwards decrease in potassium content, suggesting the existence of a longer-lived system undergoing repeated magmatic differentiation and eruption events. The upper volcanic sequence is less evolved and less alkaline than the lower, suggesting a change in primary magma compositions towards progressively higher degrees of melting of the underlying mantle. This fits into a scheme of progressively higher degrees of melting with time, which in a regional context probably corresponds to a rifting event.

Zircon U‐Pb Ages and Geochemistry of Permo‐Carboniferous Mafic Intrusions in the Xilinhot Area, Inner Mongolia: Constraints on the Northward Subduction of the Paleo‐Asian Ocean

The Central Asian Orogenic Belt (CAOB) resulted from accretion during the Paleozoic subduction of the Paleo‐Asian Ocean. The Xilinhot area in Inner Mongolia is located in the northern subduction zone of the central‐eastern CAOB and outcropped a large number of late Paleozoic mafic intrusions. The characteristics of magma source and tectonic setting of the mafic intrusions and their response to the closure process of the Paleo‐Asian Ocean are still controversial. This study presents LA‐ICPMS zircon U‐Pb ages and geochemical features of mafic intrusions in the Xilinhot area to constrain the northward subduction of the Paleo‐Asian Ocean. The mafic intrusions consist of gabbro, hornblende gabbro, and diabase. Their intrusion times can be divided into three stages of 326–321 Ma, 276 Ma and 254 Ma by zircon U‐Pb ages. The first two stages of the 326–276 Ma intrusions mostly originated from subduction‐modified continental lithospheric mantle sources that underwent a variable degree partial melting (5–30%), recording the subduction of oceanic crust. The third stage of the 254 Ma mafic rocks also show arc‐related features. The primary magma compositions calculated by PRIMELT2 modeling on three samples of ∼326 Ma and two samples of ∼254 Ma show that these mafic samples are characterized by a variable range in SiO2 (47.51–51.47 wt%), Al2O3 (11.46–15.55 wt%), ΣFeO (8.27–9.61 wt%), MgO (13.01–15.18 wt%) and CaO (9.13–11.67 wt%), consisting with the features between enriched mantle and lower continental crust. The source mantle melting of mafic intrusions occurred under temperatures of 1302–1351°C and pressures of 0.92–1.30 GPa. The magmatic processes occurred near the crust‐mantle boundary at about 33–45 km underground. Combined with previous studies, it is concluded that Carboniferous to early Permian (∼326–275 Ma) northward subduction of the Paleo‐Asian oceanic crust led to the formation of the mafic magmatism in the Baolidao arc zone. The whole region had entered the collision environment at ∼254 Ma, but with subduction‐related environments locally. The final collision between the North China craton and the South Mongolian microcontinent may have lasted until ca. 230 Ma.

The geochemical nature of mantle sources for two types of Cretaceous basaltic rocks from Luxi and Jiaodong in east-central China

The composition of mantle-derived basalts reflects the geochemical nature of their source regions in the mantle. Olivine is generally the first mineral to crystallize from basaltic melts, with the composition of this mineral providing important constraints on primary magma compositions. This study presents new whole-rock major- and trace-element, Sr-Nd-Hf isotopic, olivine compositional, and mineral O isotopic data for Cretaceous basalts within the Luxi and Jiaodong areas of east-central China. These data provide insights into the processes that caused the compositional differences between the two types of basaltic rocks. The first type was erupted during the Early Cretaceous with high SiO2 contents and Mg# values [100 Mg/(Mg + Fe2+), molar ratio], and low (Fe2O3)T and TiO2 contents. These basalts have island-arc basalts (IAB)-like trace-element patterns and enriched Sr-Nd-Hf isotopic compositions with high (87Sr/86Sr)i ratios and negative εNd(t) and εHf(t) values. They also contain phenocrysts of olivine and clinopyroxene that have consistently high δ18O values. The most primitive olivine phenocrysts within these basalts have Ni and Mn contents and Fe/Mn ratios similar to those of olivine from mid-ocean-ridge basalts (MORB). The second type generally has basanite compositions and was erupted during the Late Cretaceous, with low SiO2 contents and Mg# values, and high (Fe2O3)T and TiO2 contents. These basanites have ocean-island basalt (OIB)-like trace-element patterns and depleted Sr-Nd-Hf isotopic compositions with low (87Sr/86Sr)i ratios and positive εNd(t) and εHf(t) values. Olivine and orthopyroxene phenocrysts within these basalts have low δ18O values whereas the clinopyroxene phenocrysts have variable δ18O values. The most primitive olivine phenocrysts contain more Ni but less Mn and have higher Fe/Mn ratios than olivine formed in MORB settings. The contrasting geochemistry of these two types of basalts is indicative of derivation from mantle source regions with different compositions, reflecting different types of subduction-related crust-mantle interaction. The Early Cretaceous basalts were derived from an arc-like, isotopically enriched mantle domain that was formed at subarc depths by the metasomatic reaction of mantle wedge peridotite with felsic melts derived from the subducted continental crust during a Triassic continental collision event. In contrast, the Late Cretaceous basanites were derived from an OIB-like isotopically depleted mantle domain generated at postarc depths by the metasomatic reaction of mantle wedge peridotite with felsic melts derived from subducting oceanic crust of the Paleo-Pacific Plate.

Coupled Assimilation and Fractional Crystallization (AFC) and Mantle Plume Source(s) Contribution in the Generation of Paleoproterozoic Mafic Dykes of the Eastern Dharwar Craton, Southern India

Abstract The eastern Dharwar craton (EDC) of the southern Indian Shield hosts five geochronologically distinct Paleoproterozoic mafic dyke swarms emplaced at 2.37, 2.21, 2.18, 2.08 and 1.89 Ga. Trace element geochemical data available for these dykes display the ‘arc signals’ viz., negative Nb-Ta anomalies and elevated Zr/Nb, Th/Yb and Th/Ta values, which are conventionally interpreted to represent involvement of subduction in their genesis. It is shown that these ‘arc signals’ resulted from coupled assimilation and fractional crystallization (AFC) processes that modified these mantle-derived melts. Since, mafic dykes under study are highly evolved, an attempt has been made to estimate (using PRIMELTS2.xls software) the composition of the primary magma from the most primitive sample available from the 2.21 and 2.37 Ga swarms. The mantle potential temperature derived from the estimated primary magma compositions revealed anomalously hot mantle source regions compared to the known ambient upper mantle temperatures during Paleoproterozoic, thus implying the possible involvement of thermal plumes in their genesis.

Nishinoshima volcano in the Ogasawara Arc: New continent from the ocean?

AbstractNishinoshima, a submarine volcano in the Ogasawara Arc, approximately 1 000 km south of Tokyo, Japan, suddenly erupted in November 2013, after 40 years of dormancy. Olivine‐bearing phenocryst‐poor andesites found in older submarine lavas from the flanks of the volcano have been used to develop a model for the genesis of andesitic lavas from Nishinoshima. In this model, primary andesite magmas originate directly from the mantle as a result of shallow and hydrous melting of plagioclase peridotites. Thus, it only operates beneath Nishinoshima and submarine volcanoes in the Ogasawara Arc and other oceanic arcs, where the crust is thin. The primary magma compositions have changed from basalt, produced at considerable depth, to andesite, produced beneath the existing thinner crust at this location in the arc. This reflects the thermal and mechanical evolution of the mantle wedge and the overlying lithosphere. It is suggested that continental crust‐like andesitic magma builds up beneath submarine volcanoes on thin arc lithosphere today, and has built up beneath such volcanoes in the past. Andesites produced by this shallow and hydrous melting of the mantle could accumulate through collisions of plates to generate continental crust.