Basalt Research Articles

Situational Analysis of Groundwater Resources in Kenyan Drylands, Case study of Turkana County

Recent climate changes have increased the incidences of severe droughts and floods, which have increased the vulnerability of pastoralists in the Kenyan drylands. Thus, there is a need to carry out a situational analysis of groundwater as it is the main source of water which provides baseline information useful for planning Analysis of results indicates that Turkana County is dominated by shallow wells and boreholes with the shallowest wells located along the seasonal rivers and Lake Turkana. The borehole depth was found to range between 20 to 200m. Most of the boreholes located near Lake Turkana were shallower compared to those far away from the lake with a depth ranging between 0 to 52m. Most of the boreholes had a low yield that ranged between 1.2 to 4.6m3/hr be attributed to the basement rocks which are prevalent in the county and are classified as poor aquifers. The water points were mainly dense in urban areas, which was mainly the central region where the water demand was high and this was attributed to the high population. The groundwater levels occurred mostly at 13m indicating that Turkana County has mostly shallow aquifers that predominantly occurred along the river valleys and at the edge of the volcanic deposits. The groundwater quality is mainly saline as most of the boreholes had high TDS, EC and chloride levels. The presence of high fluoride levels indicates the presence of high volcanic rocks that have high fluoride ions that are largely prevalent in the study region. The Sulphate, Nitrite and Nitrate Levels in all the water samples analyzed were below the EU, WHO and KEBs standards indicating the low anthropogenic activities carried out in the drylands as most farmers are pastoralists. The most prevalent cation was sodium in some of the boreholes indicating why the groundwater was saline.

A New Approach of Adjustment Factor 2023 (NAAF23) for Modified Slope Mass Rating (M-SMR)

The Modified Slope Mass Rating (M-SMR) system is a SMR-based geomechanical classification system utilized for rock slope characterization in the Crocker Formation. The M-SMR rating is derived from the sum of the basic Rock Mass Rating (RMRb) and an adjustment factor. However, it has been observed that the parallelism correction parameter, F1, within both the M-SMR and SMR systems, can sometimes be overestimated, especially for toppling failures when the discontinuity dip direction (αj) is less than the slope dip direction (αs). This study was conducted on six rock-cut slopes to not only evaluate the production of a convincing F1 value but also to introduce a simplified New Approach of Adjustment Factor 2023 (NAAF23) diagram for the M-SMR. This adjustment factor (F) includes four correction parameters (F1, F2, F3, and F4), similar to those used in SMR, but modifies the calculation approach for F1. The calculation now involves subtracting the higher value from the lower value among the discontinuity dip, slope dip, or intersection line orientations. The symbols A, B, C, and D represent the subtracted values, with A and B used when the discontinuity dip direction is higher than the slope dip direction and vice versa, and C and D used when the intersection line is higher than the slope dip direction and vice versa. For plane failures, A or B becomes the value, while for wedge failures, C and D are used. For toppling failures, the formula is 180 − A or B if A or B is less than 180, and A or B − 180 if A or B is greater than 180, eliminating the need for absolute symbols. A comparison F1 calculation using SMR is also conducted. The results show that F1 values become more convincing when using NAAF23.

The magma evolutional constrains on the genesis of proximal Zn skarn mineralization: A case study from the Yaojialing deposit in Tongling district, eastern China

The major factors especially the roles of the magma evolution controlling the Zn/Cu mineralization in skarn deposits are still controversial. Yaojialing is a large-sized skarn Zn polymetallic deposit (1.74 Mt Zn at 3.6 %, 30.4 t Au at 4.2 g/t and 24.8 t Cu at 0.83 %) located in the Tongling district of the Middle–Lower Yangtze belt (MLYB), both the proximal and distal areas of the deposit exhibit high Zn/Cu mineralization. The intrusions in Yaojialing primarily consist of quartz monzonite porphyry (QMP) and granodiorite porphyry (GDP), and the QMP is related to skarn mineralization. Both the QMP and GDP display relatively high (87Sr/86Sr)i values (0.707907 to 0.709390), low εNd(t) values (−9.05 to −8.17) and negative εHf(t) values (−8.51 to −11.72), suggesting that they originated from a mixed source of enriched mantle and lower crust. Both the QMP and GDP contain type I and type II amphiboles, while type III amphibole exists only in QMP. Type I amphibole is acicular crystals shape, type II amphibole is euhedral in shape and relatively large in size (0.6–2.0 mm), while type III amphibole crystallized around the margin of type II amphibole. The type I amphibole from QMP and GDP show similar calculated temperatures (948–1010 ℃), pressures (2.9–6.5 kbar, corresponding depths at 11.0 to 24.4 km), fO2 (ΔFMQ = 0.2–1.9) and H2O content (4.2–6.8 wt%). Type II amphibole crystallized at 815–941 ℃, 1.2–2.9 kbar (corresponding to depth of 4.6–11.1 km), ΔFMQ = 0.7–2.1, and 4.4–5.7 wt% H2O. Type III amphibole have a lower temperature (679–795 ℃), pressure (<0.5 kbar) and water content (2.5–4.3 wt%) but higher fO2 value (ΔFMQ = 3.1–3.8) compared to type I and type II amphiboles. Reverse zoning of plagioclase and higher Mg# (74 to 89) of type III amphibole in QMP are resulted from injection of mafic magma at shallow depths, which provide sufficient metal and favorable conditions for the formation of QMP parental magma. Modeling of magma H2O solubility indicates that QMP begins to exsolve fluid at depth of 6.2–11.1 km. Initial low oxygen fugacity and ore-forming element differential release during fluid exsolution process resulted in the high Zn/Cu mineralization at Yaojialing.

Widespread dolomite recrystallization and porosity modification of upper Permian Zechstein carbonates, Symra discovery, Utsira High, Norwegian North Sea

The Utsira High is a prominent intrabasinal basement structure in which the eroded remnants of a widespread, Late Permian, Zechstein carbonate shelf are preserved locally. The western margin of the central Utsira High is mainly characterized by weathered basement rocks with a thin Mesozoic cover. However, the recently discovered Symra Field forms an isolated sedimentary inlier basin of the area, where deeply eroded Zechstein shelf carbonates are preserved within a half-graben. The Zechstein carbonates consist of mainly ZS2 marginal marine carbonate facies and are similar to those described elsewhere from the Zechstein Basin. They are unconformably overlain by Paleogene chalks. Based on detailed facies analysis of two recently drilled cores combined with detailed petrographic and stable isotopic analysis and supplemented by age dating of selected carbonate phases using U-Pb geochronology, we show that the Zechstein carbonates have been subjected to several phases of near-surface diagenetic alterations. Volumetrically the most dominant diagenetic product comprises nonplanar dolomites interpreted to have formed by recrystallization of a precursor reflux-type dolomite phase. The recrystallized dolomites retain enhanced reservoir quality in comparison to stratigraphic equivalent ZS2 reflux-type dolomites found elsewhere on the Utsira High. Based on U-Pb-derived age constraints, the recrystallization took place during a long-lived Late Triassic exposure event of the Utsira High, and a near-surface origin for the recrystallized dolomites is proposed. The porosity enhancement occurred contemporaneously with the dolomite recrystallization process and was facies controlled. Reservoir modifications associated with the exposure led to the dissolution of CaSO4 cement, and metastable dolomite phases, in addition to the enlargement of existing pores which had created zones of weakness prone to further dissolution. Overall, the Zechstein carbonates preserved on the Utsira High illustrate the complexity of the diagenetic alterations resulting from the uplift of a carbonate shelf, and its importance for reservoir quality.

The Derim Derim Dolerite, greater McArthur Basin, Australia: Using subsurface data to characterise a mesoproterozoic magma plumbing system

The application of high-resolution seismic reflection data has spurred major advances in knowledge of the emplacement of sub-volcanic mafic magma plumbing systems in sedimentary basins, highlighting the importance of interconnected sheet intrusions in facilitating lateral magma transport, the links between host rock mechanical properties and emplacement processes, and providing insights into how intrusive activity in basins impacts their resource potential. However, most studies have focused on Mesozoic-Cenozoic mafic magma plumbing systems situated along offshore rifted margins characterised by extensive subsurface datasets. The extensive Mesoproterozoic (c. 1300 Ma) Derim Derim Dolerite, which intrude the greater McArthur Basin in northern Australia, provide a unique opportunity to study the emplacement of a Proterozoic magma plumbing system due to its penetration by numerous hydrocarbon and mineral drillholes, in addition to seismic reflection coverage. Understanding the emplacement of this system is important because of its interactions with prospective unconventional shale reservoirs in the Velkerri and Kyalla Formations, which represent one of the world's oldest known petroleum systems. This paper focuses on characterising the intrusion emplacement and magma plumbing system using an array of subsurface data, to constrain the distribution, morphology, and emplacement mechanisms of the Derim Derim Dolerite.The morphology of the large, strata-concordant intrusions encountered at shallow present-day depths (<2 km) in the greater McArthur Basin is indicative of greater original emplacement depths of >3–4 km, suggesting a significant extent of uplift and erosion. Density-derived porosity values for the Velkerri and Kyalla Formations are anomalously low for their present-day depths, corroborating a greater palaeo-depth at time of emplacement. The extent of alteration of the host rock surrounding the Derim Derim Dolerite is highly variable, with a small number of occurrences of graphitisation of the organic matter adjacent to intrusions. However, this appears to be highly localised and the detrimental impact of the Derim Derim Dolerite on potential reservoir and/or source rocks appears generally minimal.

Geochemical and isotopic features of the Early Cretaceous volcanism of the Torey Volcanic Field (Eastern Transbaikalia, Russia) as a record of the transition from pyroxenite to eclogite mantle source

The Mesozoic magmatic activation in Central and Northeast Asia resulted in the formation of a large volume of volcanic rocks with diverse compositions. The most dramatic compositional change occurred at the end of the Early Cretaceous, when mainly alkaline basaltic lavas began to erupt after subalkaline differentiated lavas. The nature of crustal and mantle processes that led to this change in volcanism remains unclear. The Torey Volcanic Field (TVF) of Eastern Transbaikalia (Russia) demonstrates a similar compositional change. Therefore, the TVF is crucial to studying the cause of the difference in geochemical and isotopic signatures of the Mesozoic volcanism in Central and Northeast Asia.TVF belongs to the northeastern end of the Eastern Mongolia Volcanic Area (EMVA). Like other volcanic fields of the EMVA, TVF formed in two stages: early (∼121–129 Ma) and late (∼101–119 Ma). The TVF is composed of subalkaline and alkaline basaltic trachyandesites – trachyandesites. All TVF rocks are characterized by negative Ti and Sr anomalies and positive Ba and Pb anomalies. Compared to the late TVF rocks, the early TVF rocks have distinct negative Ta and Nb anomalies and are highly enriched in light rare earth elements relative to heavy rare earth elements. TVF rocks have the following isotopic characteristics: 87Sr/86Sr(t) = 0.70477–0.70540, εNd(t) = − 0.9 – +2.4, 206Pb/204Pb(t) = 17.9–18.4, and 207Pb/204Pb(t) = 15.5–15.6. However, older rocks mainly have higher εNd(t) values and 206Pb/204Pb(t) and 207Pb/204Pb(t) ratios.Geochemical and isotopic data of samples suggest that the TVF formed by melting in the continental metasomatized lithospheric mantle (CMLM). Phlogopite-amphibole-rutile-bearing pyroxenite veins played a major role in the formation of older rocks. Eclogite, represented by the recycled oceanic crust or the buried lower continental crust, was dominant in the source of younger rocks. The common source for both groups of the TVF rocks was metasomatized hydrous peridotite.Lithospheric extension and subsequent asthenospheric upwelling led to melting in the CMLM and the formation of the TVF. At the early stage of the volcanism, melting occurred at relatively low temperatures where amphibole, phlogopite, and rutile were stable. Due to ongoing lithosphere extension, the melting of eclogite occurred at a higher temperature and/or lower pressure at the late stage of the volcanism.

Delineation of deep-seated crustal structures from magnetic data in the southeastern part of the Niger Delta basin, Nigeria

Regional magnetic data in the southeastern segment of the Nigerian Niger Delta were evaluated with the aim of mapping deep-seated tectonic elements. Enhanced filtering operations and 3D forward modelling were applied on the magnetic data. These geologic features triggered the formation of rollover anticlines and faults that serve as structural traps in the study area. The filtered residual magnetic data revealed geologic structures characterized with NE - SW, N - S, and E - W orientations. The 3-D models detected the faulted crustal blocks, gradient zones, and intra-basement compositional magnetic variations. Furthermore, some prominent horst and graben structures as well as related normal faults characterized with distinct magnetic signatures were observed. Faults of base magnetic (of various compositions) were observed to be the fabricating mechanisms of the magnetic anomalies. Collectively, these structures influenced the patterns of magnetic anomalies with direct effects on the hydrocarbon trapping systems, as well as the pathways and accumulation zones for hydrothermal minerals. On the whole, the interpreted results revealed that the basement surface is rippling. Additionally, the depth result showed sedimentary thicknesses that ranged from 4–10 km. Again, the estimated crustal thickness varied from 14 to 19 km. This study has displayed the capabilities of the magnetic method in mapping the depth and configuration of basement rocks, which are crucial in controlling the formation of structural traps. Identifying these basement structures early helps in understanding the overall geological framework and potential hydrocarbon systems.

Karlleuite Ca2MnO4 – a first mineral with the Ruddlesden-Popper type structure from Bellerberg volcano, Germany

AbstractKarlleuite, ideally Ca2MnO4, is a newly approved accessory mineral found in the xenolith sample within the basaltic lava from the Caspar quarry, Bellerberg volcano, Eifel, Germany. It usually occurs as thin tabular/plate crystals, which range from 40 to 80 μm in diameter, and is associated with other members of the perovskite supergroup such as srebrodolskite, brownmillerite, sharyginite, perovskite, and lakargiite distributed within rock-forming minerals represented by reinhardbraunsite, fluorellestadite, fluorapatite, larnite, gehlenite, and several hydrated Ca aluminosilicates. Karlleuite crystals are brown with sub-metallic lustre, a light brown streak, and a good cleavage along (001). It is non-fluorescent, brittle and has an uneven fracture, a Mohs hardness of 3.5 and calculated density Dx = 3.79 g/cm3. The empirical formula of the holotype karlleuite calculated based on O = 4 atoms per formula is (Ca1.97Ce3+0.06)2.03(Mn4 + 0.39Ti0.36Fe3+0.19Al0.09)1.03O4, which shows that it is a multicomponent phase characterised by various substituents at the octahedral site. Karlleuite is tetragonal I4/mmm (no. 139), with a = 3.7683(2) Å, c = 11.9893(8) Å, V = 170.254(17) Å3, and Z = 2. The calculated strongest lines in the X-ray powder diffraction pattern are [d in Å (I) hkl]: 5.995 (43), 2.742 (100), 2.665 (91), 2.023 (25), 1.998 (28), 1.884 (61), 1.553 (38), 1.371 (24). The new mineral is the first natural phase which exhibits a first order of Ruddlesden-Popper type structure, which indicates a modular nature and consists of Ca(Mn, Ti, Fe, Al)O3 perovskite layers, packed between CaO rock-salt layers arranged along the c-axis. Raman spectroscopy supports the interpretation of the chemical and structural data. Mineral association, structural data, as well as the study of the synthetic Ca-Mn-O system suggest that karlleuite could form under high-temperature conditions, above 1000˚C.

SIMS U-Pb dating of baddeleyite-zircon pairs in mafic rocks to determine the ages of magmatic mineralization and hydrothermal remobilization in a Ni-Cu-(PGE) deposit

Ni-Cu-(PGE) sulfide deposits associated with mafic–ultramafic rocks are the most important sources of Ni and platinum-group elements (PGE) in the world. These deposits also contain significant amounts of Cu. Such deposits are generally considered to have formed by multiple stages of primary magmatic concentration and secondary hydrothermal remobilization. However, constraining the ages of Ni-Cu-(PGE) sulfide deposits has proven challenging, particularly those that have experienced post-magmatic hydrothermal overprinting and remobilization. The lack of robust geochronology data has hindered understanding of the geological controls on this important type of mineralization. The Chibaisong Ni-Cu-(PGE) sulfide deposit in the North China Craton is characterized by disseminated and net-textured ores formed by magmatic processes and overprinted by hydrothermal Cu-rich vein stockwork. The Cu-rich vein stockwork is fracture-controlled, and contains a mineral assemblage of Ni-Cu sulfides (chalcopyrite, pyrrhotite, pentlandite), amphibole and quartz. Chalcopyrite in the hydrothermal stockwork is relatively depleted in Ni but enriched in Ag and Cd compared with disseminated magmatic chalcopyrite, typical of hydrothermal superimposed ores as documented elsewhere. We identified baddeleyite-zircon pairs associated with Ni-Cu sulfides in hydrothermal superimposed ores of the Chibaisong deposit, in which the baddeleyite is unequivocally of primary igneous origin whereas the zircon represents the replacement product of baddeleyite during hydrothermal alteration. In situ SIMS U-Pb dating of baddeleyite and zircon yielded weighted mean 207Pb/206Pb ages of 2181 ± 21 Ma and 1892 ± 33 Ma, respectively, interpreted as the timing of magmatic mineralization and subsequent hydrothermal remobilization of the Ni-Cu-(PGE) deposit. Our results explain the large discrepancies in sulfide Re-Os ages previously obtained from this deposit. This study demonstrates that on the basis of detailed BSE imaging, in situ SIMS U-Pb dating of baddeleyite-zircon pairs can provide robust age constraints for both magmatic mineralization and hydrothermal remobilization of Ni-Cu-(PGE) sulfide deposits hosted in mafic–ultramafic intrusions, although further studies are needed to test whether the formation of such deposits commonly entails multiple stages of hydrothermal remobilization. Since baddeleyite or baddeleyite-zircon pairs are common accessory minerals in Ni-Cu-(PGE)-bearing mafic–ultramafic rocks, the dating approach employed in this study may be applicable to similar deposits elsewhere.

Hydrothermal alteration and elemental mass balance for the Surda Copper Deposit, Singhbhum Shear Zone, Eastern India: implications for both copper and magnetite–apatite mineralization

This paper explores the geological relationships, mineralogical and geochemical characteristics, and quantitative elemental mass balance constraints of the hydrothermal alteration zones within the uraniferous Surda copper deposit in the Singhbhum Shear Zone in eastern India, which is affiliated with an iron oxide–copper–gold system. Five distinct alteration zones are identified: a proximal potassic (biotitization) zone containing magnetite–apatite; a central chloritization zone containing copper, uranium, gold and magnetite; distal silicification (quartz) and albitization zones; and a transitional chlorite–sericitization zone. Evidence suggests these zones originated from mafic precursors linked to a subduction zone. Magnetite mineralization shows both magmatic ( δ 18 O + 2.0 to +4.8‰) and hydrothermal ( δ 18 O + 0.4 to +0.6‰) signatures. Hydrothermal magnetite–apatite formed at temperatures of 440 to 550°C, with continued crystallization at lower temperatures (280 to 360°C) concurrent with copper sulfide and uranium precipitation from an Fe-rich, isotopically heavy fluid ( δ 34 S + 4.55 to +9.66‰). Major elements (Ca, Na, Mg) and some minor and rare earth elements decrease from weakly altered mafic rocks to late alteration zones, reflecting the breakdown of Ca-bearing hornblende, biotite and plagioclase. Potassium decreases from early to late alteration zones due to mineral transformations. Mass-balanced geochemical data align with hydrothermal alteration mineral assemblages, indicating processes such as mineral dissolution, element absorption and sulfide behaviour. Overall, the study highlights the complex hydrothermal alteration processes shaping the Surda copper deposit and provides insights into ore formation mechanisms.

Analysis of Mineral Types, Density, and Porosity in the Lam Teuba Formation Using Infrared Spectroscopy Method

The Lam Teuba Formation is one of the local fault areas of the Seulimuem segment located in the Grand Mosque sub-district of Aceh Besar regency. The Lam Teuba Formation has several types of rocks such as basaltic andesite lava, pyrolastic flows, pyroclastic falls, and sedimentary rocks, namely claystone, limestone sandstone, agglomerate, limestone. Rocks have physical characteristics, namely density and porosity which have different values in each rock. This difference can be caused by the minerals contained in the rock. The method used to calculate the density and porosity values used the Archimedes method, and the identification of mineral types using the infrared spectroscopy method. The results of data processing obtained the highest original density value at location 1 sample 1 with a value of 3.42 and the lowest value at location 5 sample 2. The highest dry density value was in location 1 of sample 1 with a value of 3.33 and the lowest value was in location 5 of sample 2. The highest saturation density value was in location 1 of sample 1 with a value of 3.43, the lowest value was in location 5 of sample 2 with a value of 2.07. The average porosity value was the highest with a value of 5.41 in the poor quality category in sample 4, while the lowest value was 4.62 in the very bad category in sample 1. The types of minerals found in the Lam Teuba formation are illite, montmorillonite, kaolinite minerals with a mixture of smectite which is a clay mineral group and siderite minerals which are carbonate mineral groups.

A subduction-dismembered Neoproterozoic large igneous province in the Qinling Orogenic Belt, China: Implications for 850 Ma–initiated mantle plume activity in the greater Yangtze Block

Large igneous provinces (LIP) provide invaluable clues for recognizing the growth, breakup, and cycles of supercontinents and constraining the activity of ancient mantle plumes (or superplumes). The widespread intraplate magmatism is considered as a possible fingerprint of ancient plume activities and LIPs. Here, we present the results of an integrated study on protoliths of eclogites and amphibolite-facies bimodal volcanic rocks from North Qinling Ultrahigh-pressure Metamorphic (UHPM) Terrane, Qinling Orogen, and identify a dismembered Neoproterozoic LIP. These rocks share similar protolith ages of ca. 850–800 Ma and experienced high- to ultrahigh-pressure metamorphism at ca. 500–480 Ma. The mafic rocks are mainly tholeiitic and display enrichment of LILEs and HFSEs, similar to the E-MORB/OIB characters. They also have typical features of continental flood basalts (CFBs) with low CaO/Al2O3 (0.68–0.87), Lu/Yb (0.14–0.15), Zr/Nb (8–13), high Nb/La (0.87–1.24, mean 1.07), La/Nb (0.80–1.15), La/Ta (11–20), Nb/Yb (3.0–6.2) ratios, and positive εNd(t) (+4.3 to + 5.4). Their occurrence, geochemical features, age data, and high mantle potential temperature (1500–1550 °C) suggest that they are remnants of Neoproterozoic CFBs with mantle plume origin. Based on the lateral distribution length (ca. 300 km) of meta-basaltic rocks and the maximum subduction depths (ca. 100–200 km) recorded by UHP eclogites, the identified ca. 850 Ma-initiated CFBs in the North Qinling terrane should represent an LIP, covering more than 100,000 km2. This Neoproterozoic LIP, together with the coeval volcanic sequences in the South Qinling terrane and the northern Yangtze, would have occupied a maximum landmass of 300,000 km2, which lends support for the onset of a mantle plume at ca. 850 Ma within the Rodinia supercontinent. We conclude that the Qinling Block is one of the fragments of the Rodinia supercontinent with a passive margin covered by CFBs, which was subducted to mantle depths in the Early Paleozoic.

Molybdenum isotope evidence for recycled oceanic crust in the mantle sources of continental intraplate mafic lavas, Emeishan Large Igneous Province

The chemical and lithological heterogeneities of Earth's mantle in oceanic settings have long been attributed to the presence of recycled materials such as subducted crust, oceanic or subcontinental lithosphere mantle and sediment, as evidenced by Mid-Ocean Ridge Basalts (MORB) and Ocean Island Basalts (OIB). However, the extent to which recycled materials contribute to mantle heterogeneity in continental settings remains uncertain. Here we present systematic Sr-Nd-Pb-Mo isotope data for the Pingchuan picritic porphyries and associated mafic rocks (basalts and gabbros) in the central Emeishan Large Igneous Province, southwestern China, to resolve this issue. In contrast to the gabbro samples that show continental crust contamination in variations of δ98/95Mo (relative to NIST SRM 3134, −0.35 ± 0.03 to 0.03 ± 0.02 ‰), the Pingchuan picritic porphyries and basalts show a large range of δ98/95Mo (−0.32 ± 0.01 to −0.15 ± 0.06 ‰) and broadly negative trend between δ98/95Mo and Ce/Mo ratios, which cannot be explained by post-magmatic alteration, crustal contamination, fractional crystallization, olivine accumulation, metasomatism or fluid-activity in the mantle source. They were most likely derived from incongruent melting of a heterogeneous mantle source, i.e., a deep mantle with chondritic-like δ98/95Mo (−0.15 ± 0.01 ‰) containing recycled oceanic crust (+ sediment) with light δ98/95Mo (< −0.32 ‰). The basalts display low δ98/95Mo (−0.32 ± 0.01 to −0.24 ± 0.04 ‰), high 87Sr/86Sri (0.7046 to 0.7059), TiO2, Sm/Yb, Dy/Yb and Ce/Mo, pointing to characteristics of melts produced by MORB-type eclogite (+ sediment) at relatively low degrees of partial melting. With increasing degrees of partial melting, melts produced by peridotite (87Sr/86Sri = 0.7035 to 0.7043, chondritic-like δ98/95Mo = −0.18 ± 0.03 to −0.15 ± 0.06 ‰) come to dominate, resulting in low TiO2, Sm/Yb, Dy/Yb and Ce/Mo of the Pingchuan picritic porphyries. From a global perspective, continental basalts also plot on the negative trend defined by the OIB datasets (δ98/95Mo vs. Ce/Mo). As such, Mo isotope may provide a robust tracer of recycled oceanic crust in shaping mantle heterogeneities that occur in both oceanic and continental settings.

Mercury isotope evidence for the importance of recycled fluids in collisional ore systems.

The sources of fluids and metals in porphyry systems of continental-collision settings are poorly constrained. Mercury isotopes display unique mass-independent fractionation (expressed as Δ199Hg) and may provide important constraints on metal and volatile sources given that Hg is a highly volatile metal. Here, we report Hg isotope data on ore-forming porphyries, barren magmatic rocks, and mantle-derived mafic magmas from southern Tibet. The fertile porphyries and coeval mafic magmas display mainly positive Δ199Hg values (up to +0.25 per mil), while Δ199Hg values in barren magmatic rocks and mafic magmas are largely negative (-0.54 to 0.00 per mil). The positive Δ199Hg values observed here are consistent with seawater and marine sediments, suggesting that the ultimate source of fluids involved in the genesis of post-subduction porphyry copper deposits was the mantle lithosphere metasomatized by previous oceanic plate subduction. Our Hg isotope data provide an alternative view to current metallogenetic models on collisional porphyry systems that focus on melting of the lower continental crust.

Submarine cores record magma evolution toward a catastrophic eruption at Kikai Caldera

Magma evolution toward a caldera-forming eruption remains uncertain in many cases owing to the lack of successive volcanic records before catastrophic eruptions. Here we take an approach to this issue by analyzing a submarine core sampled near Kikai Caldera, southern Japan, which has recorded two caldera-forming eruptions at 95 and 7.3 ka and small eruptions between them. Discovery of mafic glass fragments in the submarine deposits of the 95-ka eruption, which had not been recognized in subaerial outcrops, implies the involvement of mafic magma in felsic magma-driven caldera-forming eruption. Inter-caldera volcanic activity resumed with binary mafic and felsic magma extrusions but then shifted to eruptions predominated by felsic magmas. In the final stage preceding the 7.3-ka caldera-forming eruption, the most felsic composition did not appear in glass fragments. We suggest that this period was the phase of felsic melt accumulation to grow a magma reservoir toward the next catastrophic eruption.

High‐Resolution Ice‐Core Analyses Identify the Eldgjá Eruption and a Cluster of Icelandic and Trans‐Continental Tephras Between 936 and 943 CE

AbstractThe Eldgjá eruption is the largest basalt lava flood of the Common Era. It has been linked to a major ice‐core sulfur (S) spike in 939–940 CE and Northern Hemisphere summer cooling in 940 CE. Despite its magnitude and potential climate impacts, uncertainties remain concerning the eruption timeline, atmospheric dispersal of emitted volatiles, and coincident volcanism in Iceland and elsewhere. Here, we present a comprehensive analysis of Greenland ice‐cores from 936 to 943 CE, revealing a complex volatile record and cryptotephra with numerous geochemical populations. Transitional alkali basalt tephra matching Eldgjá are found in 939–940 CE, while tholeiitic basalt shards present in 936/937 CE and 940/941 CE are compatible with contemporaneous Icelandic eruptions from Grímsvötn and Bárðarbunga‐Veiðivötn systems (including V‐Sv tephra). We also find four silicic tephra populations, one of which we link to the Jala Pumice of Ceboruco (Mexico) at 941 ± 1 CE. Triple S isotopes, Δ33S, spanning 936–940 CE are indicative of upper tropospheric/lower stratospheric transport of aerosol sourced from the Icelandic fissure eruptions. However, anomalous Δ33S (down to −0.4‰) in 940–941 CE evidence stratospheric aerosol transport consistent with summer surface cooling revealed by tree‐ring reconstructions. Tephra associated with the anomalous Δ33S have a variety of compositions, complicating the attribution of climate cooling to Eldgjá alone. Nevertheless, our study confirms a major S emission from Eldgjá in 939–940 CE and implicates Eldgjá and a cluster of eruptions as triggers of summer cooling, severe winters, and privations in ∼940 CE.

Geochemistry of Quaternary sediments in the northwestern South China Sea: Sediment provenance and mid-Pleistocene transition

The Quaternary sediments in the northwestern South China Sea (NW-SCS) provide valuable information about provenance, climate and sea level changes. However, the correlation between the geochemical records in marine sediments and these influencing factors remains less understood in the NW-SCS. Two wells penetrated Quaternary sediments on the shelf and shelf margin of the NW-SCS and provide an excellent dataset. In this study, the major, trace, and rare earth elements in the sediments were analyzed to reveal the Quaternary provenance changes that correlate with the climatic Middle Pleistocene transition (MPT). These results indicate that the core sediments were mainly derived from felsic source rocks and the degree of chemical weathering in the source areas is relatively low (CIA averaged at 58). The Quaternary provenance of the NW-SCS did not undergo significant changes, recording mixed sedimentation from the Red River, Hainan Island, and central Vietnamese sources. The felsic source rocks and negative Eu anomaly indicated the shelf area received sediments primarily from the Red River and Hainan Island. However, a positive Eu anomaly on the shelf margin suggests additional contributions from mafic rocks sourced from central Vietnam. During the MPT (1.3–1.4 to 0.8–0.9 Ma), the dominant periodicity of climate cycles changed from a 40 kyr obliquity cycle to a 100 kyr eccentricity cycle; the cooling climate led to a slight weaking in chemical weathering and a minor decrease in sediment supply from terrestrial sources. After the MPT, the mixed sourced sediments in the shelf and shelf margin areas of the NW-SCS likely suffered from multiple hydrodynamic forces under the influence of the Pleistocene high-frequency and high-amplitude sea level fluctuations. These climate induced changes led to significant fluctuations in element content that provide new insights into Quaternary sources and climatic events in the NW-SCS.

Reliability of the palaeomagnetic signal recorded in a lava flow erupted on 4 December 2021 in La Palma (Canary Islands, Spain)

SUMMARY A basaltic lava flow erupted from the Tajogaite volcano on 4 December 2021, in La Palma (Canary Islands, Spain) was sampled to find out to what extent reliable and correct information on both intensity and direction of the Earth's magnetic field can be obtained from the palaeomagnetic signal recorded in a lava flow which erupted under known conditions. Samples were taken every few centimetres across a flow up to a total of 27 oriented cores. Palaeomagnetic experiments showed a strong viscous overprint in many samples. Nevertheless, the mean palaeomagnetic direction obtained agrees well with the actual value from IGRF-13. Rock magnetic experiments were performed to obtain additional information about the quality and reliability of the results and the reasons for unsuccessful determinations. Analysis of mostly irreversible thermomagnetic curves showed that the carriers of remanence were magnetite and titanomagnetite of low and/or intermediate Curie-temperature. Hysteresis parameter ratios showed a pronounced variability across the flow. Analyses of frequency dependent susceptibility, IRM acquisition coercivity spectra and FORCs showed a noticeably presence of very low coercivity grains (multidomain and superparamagnetic-single domain boundary). Multimethod palaeointensity experiments were performed with the Thellier-Coe, multispecimen and Tsunakawa-Shaw methods. Only three of 25 cores from the flow yielded successful Thellier-Coe determinations, in agreement with the expected field value of 38.7 μT (IGRF-13). However, palaeointensities of 60 per cent of the specimens agree with the expected value performing an informal analysis without considering criteria thresholds. Four of six Tsunakawa-Shaw determinations performed on samples from the flow yielded correct results, but three multispecimen determinations providing apparently successful determinations largely underestimate the expected field intensity. Combination of three Thellier-Coe and four Tsunakawa-Shaw successful determinations yields a multimethod palaeointensity result B = (36.9 ± 2.0) μT in good agreement with the expected field intensity.

Olivine Time-Capsules Constrain the Pre-Eruptive History of Holocene Basalts, Mount Meager Volcanic Complex, British Columbia, Canada

Abstract The Canadian segment of the Cascade Volcanic Arc (i.e. the Garibaldi Volcanic Belt) comprises more than 100 eruptive centres, spanning the entire Quaternary period (Pleistocene to Holocene in age), and with deposits ranging in composition from alkaline basalt to rhyolite. At least one of the volcanoes is currently active; Mount Meager / Q̓welq̓welústen erupted explosively 2360 years BP and has ongoing fumarolic activity. Long-term forecasting of eruption frequency and style depends on reconstruction of the history and timescales of magmatic processes preceding previous volcanic eruptions. Utilising diffusion chronometry, we investigate the Mount Meager Volcanic Complex focusing on Holocene olivine-phyric basalts (Lillooet Glacier basalts) exposed by the retreat of the Lillooet Glacier. We identify two distinct olivine populations in samples of quenched, glassy basalt lavas that record different magmatic processes and histories. Glomerocrysts of Fo83 olivine phenocrysts, entrained and transported by a hot mafic input, form Population 1. These exhibit resorption and normally zoned outermost rim compositions of Fo76–78; a third of them also show interior reverse compositional zoning. A second population of skeletal microphenocrysts have the same composition as the phenocryst rims (i.e. Fo76–78) and are in equilibrium with the adjacent matrix glass. We estimate the pre-eruptive temperature-fO2 conditions in a shallow reservoir (100 MPa; ~3 km) for a melt with H2O content of 0.5 to 1 wt % as ~1097°C to 1106°C (± 30°C), and NNO + 0.5 (±1.1), respectively. Using these input parameters, we report Fe-Mg diffusion chronometry results for 234 normally zoned profiles from 81 olivine phenocrysts. Diffusion modelling of compositional profiles in oriented crystals indicates pre-eruptive magmatic residence times of 1 to 3 months. These remarkably short residence times in shallow reservoirs prior to eruption suggest very short periods of unrest may precede future eruptions.

Evolution of a confined gravelly river to a braided-meandering river in the Late Cretaceous Khasi group, southern Shillong plateau, NE India

This paper intends to unravel the depositional condition of the Basal Conglomerate/Jadukata Formation and the Lower Mahadek Formation in southern Shillong Plateau, NE India deposited during the Late Cretaceous period. The studied succession is entirely siliciclastics in composition and was built up on the Precambrian basement rocks. State-of-art facies analysis and palaeocurrent evidence helped us to reconstruct the palaeogeography and palaeoenvironment. The facies tracts reveal that frequent hyperconcentrated mass flow deposits dominated the proximal part of the river channels. Gradually, the river evolved to a sandy braided and finally mud-dominated, meandering system. The river channel changed its pattern through onlapping and progressive elimination of the accommodation space created by the tectonic subsidence/upliftment. Major shifts in palaeocurrent direction at different stages of the river evolution confirm the role of tectonics on the river channel evolution. QFL detrital modes of the sandstones reflecting lithic wacke composition and the recycled orogeny corroborate the interpretation. Rapid lateral and vertical facies transition suggests a pull-apart type basin due to ongoing rifting. Geochemical analyses also show signatures of the overlapping tectonic conditions supporting the pull-apart basin model.