Geochemistry Of Rocks Research Articles

Deciphering a cryptic unconformity in the Surat Basin using high frequency detrital zircon U-Pb geochronology: Insights into basin dynamics on the north-east margin of Gondwana

The Surat Basin is one of a series of shallow intracratonic basins that comprise the Great Australian Superbasin and developed in response to the initiation of Gondwanan breakup. Within the Surat Basin, the Walloon Coal Measures (WCM) are a significant target for Coal Seam Gas (CSG) production and the identification of the boundary with the overlying Springbok Sandstone (SST) is critical to the stratigraphic framework of the basin, and identification of groundwater flow units it delineates. Identification of the contact on a local (tenement) and regional (basin) scale between the SST and WCM is challenged in CSG producing areas by (i) overall lithological similarity of the units (macro- and microscopically, whole rock geochemistry, wireline signature), (ii) lateral heterogeneity of lithologies on a regional scale due to lateral facies and depositional environment variation in the uppermost WCM and lowermost SST and (iii) lack of consistent regional stratigraphic markers like erosion surfaces or correlated tuff beds. Sediment provenance analysis via detrital zircon (DZ) U-Pb geochronology allows constraint of spatial and temporal changes in sediment provenance by sampling across stratigraphic units along a lateral transect. The resulting large-scale stratigraphic correlations facilitate identification of basin dynamics within a tectonically quiescent basin. Here we present the results of DZ distributions from 26 samples that were sampled from four wells along a 300 km transect from the eastern Eromanga Basin to easternmost extent of the Surat Basin. Our data show a brief pulse of basement derived material at the base of the SST which interrupts the predominant volcaniclastic sediment provenance in the SST and WCM. This brief change in zircon provenance is interpreted as a precursor to higher energy depositional environments in the SST and can be utilised as a regional marker across the Surat and Eromanga basins, indicating a change in source provenance and basin dynamics associated with the SST – WCM boundary.

The genesis of granite mass in the volcanic area of Litang, Chuanxi province: constraints from rock geochemistry and U–Pb zircon dating

The genesis of granite mass in the volcanic area of Litang, Chuanxi province: constraints from rock geochemistry and U–Pb zircon dating

Geochemistry, Age, and Geodynamic Setting of the Volcanic Rocks of the Indigirka Section of the Uyandina-Yasachnaya Volcanic Belt (Northeast Asia)

The Uyandina–Yasachnaya volcano-plutonic belt (UYVB) is one of the largest structures of this type in Northeast Asia. It strikes in the north-west direction for 900 km from the upper reaches of the Kolyma River to the upper reaches of the Selenyakh River. The belt is characterized by the strong facies variability and zoning, which is expressed in a change from mafic volcanic rocks (Ilin’-Tass zone) in the northeast to felsic volcanic rocks (Darpir zone) in the southwest. Poor knowledge of the UYVB results in an ambiguous interpretation of its geodynamic nature and volcanic evolution. The paper presents new detailed geochemical, isotopic, and geochronological data on the volcanic rocks of the Indigirka section of the Darpir zone of the UYVB. The U-Th-Pb SIMS zircon dating showed that volcanic rocks of the section previously attributed to the Oxfordian–Tithonian stages have the younger age within 150 ± 2–152 ± 2 Ma, which corresponds to the Tithonian. Andesites from the section base show Ta–Nb depletion, indicator element ratios Th/Nbpm, La/Nbpm, and La/Smpm 1, and extremely low εNd = (–8), which may indicate a crustal contamination of mantle sources. Felsic volcanic rocks overlying the andesites have the postcollisional signatures. They are also characterized by the wide variations of εNd from –2.4 to –6.5 and model ages. The systematic position of the rocks in the section as well as upward increase of alkalinity and silica content from basaltic andesites to rhyolites in the upper parts of the section, which is typical of island-arc buildups, suggest that suprasubduction melts were generated beneath the collisional crust. Source of these melts could be a mantle domain that was enriched in fluid during previous subduction event.

Origin of Upper Cretaceous marine ironstones of Ayat Formation (Turgay depression, Northern Kazakhstan)

This work studies the mineralogical and geochemical features of ooidal ironstone formation of the Upper Cretaceous Ayat Formation located in the Turgay depression (Kazakhstan). The origin of ooidal ironstone deposits has been the subject of many discussions for a long time. These results present diagenetic conditions of the marine ironstone precipitation in the Ayat Formation. Various authigenic minerals, such as siderite, glauconite, goethite, pyrite, hydroxylapatite, wurtzite, and barite indicate fluctuations in the different geochemical conditions during the diagenesis of marine sediments of the Ayat Formation. The precipitation of in situ pyrite and wurtzite was controlled by an oxygen-deficient environment at the interface between water and sediment, which was accompanied by bacterial sulphate reduction under seabed conditions and, in consequence, by the concentration of sulfides. The authigenic assemblage of siderite, wurtzite, and barite indicates an integrated process of their input to site the sedimentation. The stable isotopic composition of siderite supports the microbial carbonate origin by decomposing the organic matter. However, geochemical features (the pattern Co/Zn vs (Co + Ni + Cu)) evidence the hydrogenous precipitation of iron. It involves the interaction of two central in situ processes to form Ayat ooidal ironstones. One of them is the iron precipitation from the bottom water, and the second is the microbial generation of hydrocarbons. Marine Ayat and channel Lisakovsk ironstones have similar features of bulk rock geochemistry. It reflects the exact environments of the ooidal iron ore formation, while the wall rocks have distinctive facies conditions.

The Potential of Sukadana Basalt Province as a New Geothermal Resources in The Back Arc of Sumatra

Geothermal resources are currently obtained from areas within volcanic arcs, such as the Pertamina Ulu Belu and Supreme Energy Rajabasa Geothermal Fields. However, this understanding may change in the future, as the Quaternary Sukadana Basalt Province (SBP), located in the back arc, is believed to have potential as a future geothermal energy resource. This research aims to explore the various factors that contribute to the high heat flow in the SBP region and generate a new perspective on geothermal energy particularly in the Lampung province. The methods used integrate previous research findings, such as heat flow data, regional tectonics, and geological structures, with new petrography-whole rock geochemistry. The whole rock geochemistry was determined using X-Ray Fluorescence (XRF), Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES), and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The SBP was formed by the Paleogene northwest-southeast striking fault and influenced by the Quaternary northeast-southwest striking fault, which may serve as conduits for hydrothermal fluid in addition to their vesicular structures. Geochemical analysis suggests the presence of both mantle plume and subduction-related processes. The magmatism linked to subduction-plume tectonic mechanisms and the thinning of the crust due to pull-apart motion caused by the movement of two large faults (Sumatra Fault Zone and Bangka Shear) can increase regional heat flow to 100±10 mW/m2. As a result, the SBP has significant potential as a source of geothermal energy for electricity generation in the future.

Syndepositional Uptake of Uranium, Molybdenum and Vanadium into Modern Bahamian Carbonate Sediments during Early Diagenesis

Syndepositional diagenesis is a complicating factor when interpreting geochemical proxies in carbonate sedimentary environments. Previous studies have suggested that carbonate deposits may preserve the geochemical and isotopic signatures of seawater that can be used for paleo-redox reconstructions. However, more work is necessary to understand how these trace metals are preserved. The present study examines shallow marine carbonate sediments from the Bahamas to better understand diagenetic effects on trace metal uptake and sequestration. Analysis of diagenetic effects and trace metal uptake follows a multi-method approach, combining sequential extraction, stable isotope analyses, and rare earth elemental analysis. Stable isotopes track bacterial sulfate reduction, denitrification, and organic matter source and provide insight into thresholds and processes for the authigenic trace metal uptake. Importantly, exchangeable phases exhibit authigenic accumulation of molybdenum, uranium and vanadium, and intensified bacterial sulfate reduction is evidenced by most depleted sulfur isotope signatures. In addition, rare earth element values are very indicative proxies that suggest altered primary seawater trace element in carbonates (no cerium or lanthanum anomaly, moderate heavy rare earth element enrichment, decreased y/ho ratios and positive correlations between aluminum, manganese, and iron). Taken together, these results allow the development of a framework to better understand how to apply sedimentary geochemistry of carbonate rocks to paleo-environments as this study shows significant authigenic accumulation of redox-sensitive trace metals by exchangeable phases.

Geochemistry, geochronology, and tectonic setting of the Cretaceous volcanic rocks in east Mongolia

Late Mesozoic volcanic rocks in East Mongolia exhibit high-K calc-alkaline and shoshonitic series and display features of bimodal-like volcanic with a mafic and felsic members. The mafic member is composed mainly of trachybasalt and basaltic trachyandesite, with a minor amount of trachyandesite. The mafic rocks have elevated incompatible trace element concentrations and significantly negative Nb-Ta and Ti anomalies, features of typical subduction-related magmas, distinguished from OIBs. The mafic magmas have undergone fractional crystallization dominated by pyroxene and/or olivine and crustal contamination. The mafic magma was derived from low-degree partial melting of an enriched lithospheric mantle source that might have been metasomatized by subduction-derived fluids. The felsic member consists of trachydacite and rhyolite. The geochemical data indicate they produced from partial melting of a crustal source that is dominated by juvenile mafic rocks. Our new K-Ar dating and previous age data demonstrated that the late Mesozoic volcanism in East Mongolia took place during Late Jurassic-Early Cretaceous at between 156-99 Ma. The model of the arc‒back-arc extension possibly induced by slab roll-back of the westward (paleo) Pacific-subduction can explain the geodynamic setting and the eastward young trend of the late Mesozoic volcanism in East Mongolia and its adjacent NE China.

An intraoceanic juvenile arc of shoshonite and adakitic andesite in the Nemuro Belt, the Lesser Kuril Arc, across the K/Pg boundary

In situ eruptions and accreted blocks of Mesozoic basalts exhumed in present-day forearc settings, around the Pacific Rim record paleovolcanic events in intraplate oceanic settings since the closure of the Panthalassa Ocean. Basaltic and andesitic volcanic and intrusive rocks occur in the present-day forearc area of the Late Cretaceous Nemuro Belt in northeastern Japan. Most basalts in this area are K-rich (i.e. shoshonites), similar to rocks erupted in oceanic back arc or petit-spot settings. In this study, we investigated the occurrence, geochemistry, and age of these shoshonitic rocks (ca. 70 Ma), along with associated adakitic andesites (ca. 62 Ma). The petrogenesis of these igneous rocks and petrography of the associated Nemuro sandstones were used to constrain the nature of the Nemuro paleo-arc during the Late Cretaceous to early Paleogene. The shoshonitic, adakitic, and basaltic rocks in the Nemuro sandstones formed in an intraoceanic juvenile arc setting. Previous paleomagnetic studies have suggested that the Nemuro and Lesser Kuril paleo-arcs were approximately ENE–WSW-trending, located in the central paleo-Pacific, and extensions of the Olyutorsky (eastern Kamchatka) and Terpenia (eastern Sakhalin) paleo-arcs near the mid-ocean ridges of the Kula/Izanagi and Pacific plates in the central Pacific. Our findings are consistent with these previous results, although the shoshonites in the lower part of the Nemuro Group indicate that the Nemuro paleo-arc drifted rapidly northward or northwestward after ca. 65 Ma.

Evidence, or not, for the late Tonian break-up of Rodinia? The Dalradian Supergroup, Scotland

The Tonian–Cambrian Dalradian Supergroup in Scotland is a siliciclastic–carbonate succession that can be up to 10 km thick. The consensus view is that its lower part, the mid- to late Tonian Grampian and Appin groups, formed in rift basins: the deep marine turbidites of the Grampian Group infilled rift depocentres, whereas the shallow marine strata of the Appin Group mark basin-bounding palaeohighs. This scenario is used as a key line of evidence to infer the onset of the break-up of Rodinia between Laurentia and Baltica. However, deformation during the mid-Ordovician Caledonian Orogeny obscured the original depositional frameworks. Reconstructing these frameworks (and hypothesized rift basins) has relied on the trace and major element log-ratio geochemistry of minor carbonate rocks to assign the units to either the Grampian or Appin group – that is, to rift depocentres or basin-bounding palaeohighs, respectively. We report new carbon and oxygen isotope and geochemical data and use these to create a revised stratigraphic framework for the Grampian and Appin groups. Our findings show that the previous geochemical-based correlations are unreliable and that there is no evidence for palaeohighs or rift basins. Instead, the Grampian–Appin groups are a deeper marine flysch to a shallower marine molasse succession formed in response to the mid-Tonian Knoydartian Orogeny. From a Scottish perspective, evidence for the break-up of Rodinia is recorded higher in the Dalradian succession during the deposition of the early Cryogenian Argyll Group. Supplementary material: Geochemical analysis of samples is available at https://doi.org/10.6084/m9.figshare.c.6317830 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Manganese carbonate-bearing mudstone of the Witwatersrand-Mozaan succession in southern Africa as evidence for bacterial manganese respiration and availability of free molecular oxygen in Mesoarchaean oceans

AbstractThe Mesoarchaean (2.96 to 2.91 Ga) Witwatersrand-Mozaan succession of southern Africa contains multiple units that show evidence for the presence of free molecular oxygen in oceanic water columns approximately 500 million years prior to the Great Oxidation Event. The lithostratgraphically correlatable Thalu and Brixton formations of the Mozaan and West Rand groups, respectively, now yield further evidence for an oxygen-containing water column. The two formations contain multiple beds of manganese carbonate-bearing mudstone. This study documents these beds and their stratigraphy, mineralogy, petrography, whole rock geochemistry and stable carbon and oxygen isotopes from deep-level drill cores.The manganese carbonate-bearing beds occur towards the base of upward-coarsening units, indicating deposition during higher sea levels, followed by regressions. The mudstones show sharp contacts and compaction around manganiferous carbonate concretions, suggesting early diagenetic growth of the latter. The concretions are composed of either rhodochrosite cores and kutnahorite rims, or kutnahorite cores and ankerite rims, illustrating a decrease in manganese concentration from core to rim. Relative to the surrounding mudstone, the carbonate concretions are markedly enriched in manganese. The rare earth element and yttrium contents in the concretions, normalised to shale, show heavy over light rare earth element enrichment and positive europium and yttrium anomalies, indicating precipitation from mixed marine-hydrothermal water. The carbon and oxygen isotopes of concretionary carbonates are depleted in 13C (-12.2 to -21.30/00) and 18O (-14.9 to -23.90/00) relative to Pee Dee Belemnite, respectively.It is concluded that the main mode of deposition for iron and manganese was by precipitation likely facilitated by iron- and manganese-oxidising bacteria. The iron and manganese were sourced as dissolved Fe2+ and Mn2+ from a distal, high-temperature hydrothermal plume. The stratigraphic position of the studied units suggests that deposition occurred on the middle to outer shelf where iron oxidation and deposition proceeded far enough so that the environment could transition to manganese oxidation and deposition. These precipitates then reacted with organic carbon to form rhodochrosite that nucleated very locally to grow concretions. As the available manganese was all reduced and incorporated into the carbonates, Fe3+-oxyhydroxides were also reduced by any excess organic carbon and incorporated into the concretion rims during later stages of growth.The main implication of the proposed model for concretion formation is that free molecular oxygen was available in the water column of the shelf for manganese-oxidising microaerophyllic chemolithoautotrophs to function. The oxygen concentration was in excess of approximately 5 μM. This concentration, along with the depositional setting of the studied units, falls within the ranges and parameters of previous studies indicating conditions in “oxygen oases” of Archaean oceans.

Geochemistry and geochronology of basic igneous rocks in Bairin Right banner, southeastern inner Mongolia, China: Implications for the final closure of the Paleo—Asian Ocean along the Xar Moron suture zone

This paper reports the first discovery of basic igneous rocks in Bairin Right Banner in southeastern Inner Mongolia, China. To understand the genesis and tectonic evolution of these rocks, we have carried out geochronology and geochemistry research on them. The results show that LA-ICP-MS zircon U-Pb concordant mean ages of massive basalt, diabase, and pillow basalt are 272.0 ± 2.8 Ma, 261.5 ± 3.2 Ma, and 256.5 ± 2.5 Ma, respectively, suggesting their formation during the Late Permian. The massive basalt, diabase, and pillow basalt are tholeiitic with low Mg# values in the ranges of 46.2–66.8, 54.4–71.0, and 53.5–58.2, respectively. They have similar chondrite-normalized rare earth element (REE) patterns. The intensity of REE differentiation is similar to that of typical enriched mid-ocean ridge basalts (E-MORB). The pillow basalts are relatively enriched in Ba and U, while slightly depleted in Rb and Th, and show geochemical affinity to E-MORB. The massive basalts and diabases have the same primitive mantle-normalized patterns and are relatively enriched in Rb, Ba, and U, while depleted in Nb and Ta, similar to the typical island arc magmas. Our new findings indicate that the intra-oceanic subduction of the Paleo-Asian Oceanic slab was still ongoing during the late Permian in the Bairin Right Banner area and, combined with previous studies, it might have been closed during the Late Permian to Early Triassic along the Xar Moron suture zone.

Geochronology and geochemistry of igneous rocks of the Dassa region, Central-Benin: evidence of an Ediacaran emplacement of alkali-calcic and alkaline plutonic and volcanic magmas

Geochronology and geochemistry of igneous rocks of the Dassa region, Central-Benin: evidence of an Ediacaran emplacement of alkali-calcic and alkaline plutonic and volcanic magmas

Whole Rock Geochemistry and Geodynamic Evolution of Paleoproterozoic Gneisses, Ako’ozam-Njabilobe Area, Southwestern Cameroon

The Ako’ozam–Njabilobe area, which is part of the Nyong group, is characterized mainly by highly deformed gneisses that host amphibolites, and metadolerites as enclaves. The gneisses within the Ako’ozam-Njabilobe area were investigated to constrain their geotectonic environment as well as understand their geodynamic evolution within the Nyong group. Three litho-types of gneisses outcrop within the Ako’ozam-Njabilobe area, consisting of horblende-biotite, quartzofeldspathic, and garnet gneisses. They are strongly peraluminous, with low Yb<SUB>N</SUB>, Rb/Sr, K₂O/Na₂O, and enriched in large ion lithophile elements (LILE). The gneisses within the Ako’ozam-Njabilobe area have a trachy-andesite parentage. The negative anomalies in high field strength elements (HFSE) and enrichments in LILE and light rare earth elements (LREE) suggest a subduction setting for this magmatism. The geochemical signatures including, low K₂O/Na₂O ratio, silica content (67.18 to 73.65 wt%), Yb and Y contents less than 1 and 10 ppm respectively, and low heavy REE values of the studied gneisses are akin to chemical signatures of archean crustal rocks. The Nb/Th < 1 and Th/Yb >1 of the studied gneisses indicate that the magma that sourced their protolith is mantle-derived with some degree of crustal contamination. The similarity in whole rock data of the three lithotypes of gneisses within the Ako’ozam-Njabilobe area suggests a possible mechanical mixing during emplacement. The chemistry of the gneisses indicates a possible Archean inheritance during the emplacement of their protoliths, as such could provide insights into the petrogenetic processes controlling crustal growth and aid in understanding the Archean-Proterozoic transition within the Nyong group.

The System KCl–CaCO3–MgCO3 at 3 GPa

Inclusions in mantle minerals and xenoliths from kimberlites worldwide derived from depths exceeding 100 km vary in composition from alkali-rich saline to carbonatitic. Despite the wide distribution of these melts and their geochemical importance as metasomatic agents that altered the mineralogy and geochemistry of mantle rocks, the P-T range of stability of these melts remains largely undefined. Here we report new experimental data on phase relations in the system KCl–CaCO3–MgCO3 at 3 GPa obtained using a multianvil press. We found that the KCl–CaCO3 and KCl–MgCO3 binaries have the eutectic type of T-X diagrams. The KCl-calcite eutectic is situated at K2# 56 and 1000 °C, while the KCl-magnesite eutectic is located at K2# 79 and 1100 °C, where K2# = 2KCl/(2KCl + CaCO3 + MgCO3) × 100 mol%. Just below solidus, the KCl–CaCO3–MgCO3 system is divided into two partial ternaries: KCl + magnesite + dolomite and KCl + calcite–dolomite solid solutions. Both ternaries start to melt near 1000 °C. The minimum on the liquidus/solidus surface corresponds to the KCl + Ca0.73Mg0.27CO3 dolomite eutectic situated at K2#/Ca# 39/73, where Ca# = 100∙Ca/(Ca + Mg) × 100 mol%. At bulk Ca# ≤ 68, the melting is controlled by a ternary peritectic: KCl + dolomite = magnesite + liquid with K2#/Ca# 40/68. Based on our present and previous data, the KCl + dolomite melting reaction, expected to control solidus of KCl-bearing carbonated eclogite, passes through 1000 °C at 3 GPa and 1200 °C at 6 GPa and crossovers a 43-mW/m2 geotherm at a depth of 120 km and 37-mW/m2 geotherm at a depth of 190 km.

Study on Epithermal Gold Mineralization System at Shwebontha Prospect, Monywa Copper-Gold Ore Field, Central Myanmar

The Shwebontha prospect area is situated in the Central Volcanic Belt, central Myanmar, where the well-known Sagaing Fault serves as its eastern boundary. This study aims to document key the mineralogy, host rock geochemistry and ore mineralizing fluids. The mineralization, hosted by Upper Oligocene to Middle Miocene rhyolites, displays a strong lithological control. Mineralization is characterized by gold-bearing silicified massive ore and chalcedonic quartz veins in which sulfides are clustered and disseminated not only in quartz gangue but also in rhyolite host rocks. The significant ore minerals in the mineralized veins include pyrite, sphalerite, galena, chalcopyrite, and gold. Common hydrothermal alterations such as silicic, argillic and propylitic alteration types are recognized. According to the fluid inclusion data and interpretation, ore mineralizing fluids in the research area are characterized by formation temperatures of 260-280 °C and salinity of 0.35-2.41 % wt. NaCl eq. respectively. Mixing of hydrothermal fluids was generally considered to be an effective mechanism for ore transport and deposition.

Petrogenesis of the Limerick Igneous Suite: insights into the causes of post-eruptive alteration and the magmatic sources underlying the Iapetus Suture in SW Ireland

The Limerick Igneous Suite (LIS) in County Limerick, SW Ireland, contains two distinct basaltic igneous units: the Knockroe and Knockseefin, which are expressed as hypabyssal intrusions, porphyritic dykes, diatremes, lava flows, agglomerates and tuffs. These units make up two distinct evolutionary trends: the Knockroe igneous units which range from alkaline basalts to trachyandesites, and the Knockseefin igneous units which range from alkaline basalts to basanites. U‒Pb dating of apatite establishes a primary crystallization age of c. 350 Ma for the Knockroe units. Strontium and Nd isotopes from the least-altered Knockroe bulk rock samples range over 0.70301–0.70454 and 0.512457–0.512493, respectively. The Sr isotopes for the least-altered Knockseefin samples are similar, ranging over 0.70325–0.70386, but the Nd values are slightly more radiogenic, spanning 0.512431–0.512437. Altered samples are buffered against changes in Nd, but some show excursions towards radiogenic Sr, suggesting contamination from Carboniferous seawater and/or introduction of Rb. Rb–Sr calculations conducted on altered Knockroe samples returned ages within uncertainty of the U‒Pb dates. The bulk rock isotope values, normalized trace element plots as well as Zr/Sm, Ce/Pb and Nb/U values demonstrate the LIS is comparable to ocean island basalts and Ce/Y and Zr/Nb values suggest the units likely originated from low degrees of partial melting likely caused by extension related to the amalgamation of Laurussia and Gondwana. Supplementary material: ESM 1, whole rock geochemistry data; ESM 2, apatite chemistry and geochronology data; and ESM 3, supplementary figures are available at https://doi.org/10.6084/m9.figshare.c.6297395

Clastic rock geochemistry of the Sirohi Basin, Aravalli Craton, NW India: Implications for paleoweathering, provenance and geodynamics

• Petrographic and geochemical data of clastic rock formations in the Sirohi Basin. • Two wacke horizons with distinct textural and chemical characteristics. • Geological evolution of the Sirohi Basin and relation with geodynamics of the Indian Shield. Sirohi Basin of Trans-Aravalli Region, Rajasthan preserves a thick clastic sequence composed of phyllite and metasandstone. Based on their petrochemical properties, metasandstone is classified as K-feldspar rich wacke (Wacke I) and plagioclase rich wacke (Wacke II). Wacke I is characterised by higher K 2 O, Fe 2 O 3 and lower CaO, Na 2 O, and Na 2 O/K 2 O ratios than Wacke II. Chemical Index of Alteration (CIA) and the A-CN-K systematic prescribe least chemical weathering for the sediments of Wacke II, moderate for Wacke I and moderately intense for phyllite of the Sirohi Basin. Chemical traits perceived from UCC spidergrams, discrimination function diagrams, etc. in conjunction with immobile element ratios (e.g., Al 2 O 3 /TiO 2 , Th/Sc, La/Sc, La/Co, Th/Co, Eu/Eu*, La/Sm N and Gd/Yb N ) establish dual-source terrains for Sirohi clastics, one with preponderance of felsic componets and other mafic plus felsic ingrediatns. The petrochemical signatures of Sirohi clastics, though suggest procurement of detritus form diffrent sources but consanguineous evolution in the Sirohi Basin, in response to extensional stresses. :

The eruption interface between carbonatitic dykes and diatremes – The Gross Brukkaros volcanic field Namibia

Carbonatites exist as intrusive and extrusive rocks, with the former dominating the rock record. The geochemical link between intrusive and extrusive equivalents and the processes during eruption of carbonatite melt are essentially unknown. This contribution aims at providing new insights into the transition from intrusive to extrusive carbonatites with special emphasis on the trace element budget. The Gross Brukkaros natural laboratory in central Namibia was chosen to study the interface between fine-grained dolomite‑carbonatite dykes and associated diatremes. Whole rock geochemistry combined with micro-scale petrography via BSE imaging and microXRF mapping provide evidence that the carbonatite dykes contain significant amounts of silica and aluminium, which is inherited from crustal xenoliths by resorption and leaching. At the transition from carbonatite dyke to diatreme, the CO2 component (likely together with most of the other volatiles e.g. Cl, H2O, F, S) of the carbonatitic brine-melt vaporised and was released into the atmosphere. During this process the Si, Ca, Mg and Fe transported together with a high amount of trace elements became precipitated from a resulting primarily magmatic fluid as a mixture of cryptocrystalline quartz (quartz I) and aegirine-augite (plus minor magnetite) with frequently occurring microcrystalline quartz I grains. This mineral assemblage forms the matrix in the diatreme breccia and precipitated by rapid temperature drop in the course of decompression from the hydrothermal fluid. In the post-eruption stage the influx of meteoric waters (into the system), which mixed with the remaining magmatic fluid, caused precipitation of a second quartz generation (quartz II; euhedral grains), which is depleted in trace elements. All measured trace elements show significantly higher contents in quartz I compared to quartz II (with exception of Li). The application of the TitaniQ geothermometer indicates that Ti incorporation in quartz is kinetically controlled for quartz I. During quartz II formation the system thermodynamically equilibrates and ends up with realistic precipitation temperatures of 290–350 °C. Therefore, this study shed light on the trace element behaviour at the interface between feeder dykes and diatreme roots of a strongly contaminated carbonatite melt.

Understanding regional groundwater flow using water-rock interaction processes and anthropic signatures in a fractured volcanic aquifer

This research evaluated the hydrochemistry of a fractured volcanic aquifer at Paraná 3 Basin, in the western region of the state of Paraná, southern Brazil. Geochemical rock data from the literature were used to describe the distinctive characteristics of the principal geological units in the area, Pitanga and Paranapanema Formations, both belonging to the Serra Geral Group. Data from x-ray diffraction were obtained to deepen the study of hydrogeochemical processes. Previous studies did not consider the differences in the lithogeochemistry of the group, in the terms contemplated in this research. In addition, the research also approaches the evaluation of the flow depth contribution in the groundwater chemistry and the analysis of hydrogeochemistry processes. For this analysis, a set of chemical data was utilized on 35 groundwater samples from water supply boreholes at Paraná 3 Basin. From shallow to deep flow, there was an increase in pH, electrical conductivity, total dissolved solids, total alkalinity and a decrease in CO2. These parameters exert the main physical-chemical groundwater controls, which are identified by changes in the major ions along the flow lines, as well as by variations in trace elements. The water-rock interaction becomes increasingly evident in the intermediate and deep flows, standing the higher alkalinity, pH, electrical conductivity, total dissolved solids (TDS) and most of the trace elements at Pitanga Formation. Pitanga Formation presents higher medians of alkalinity (95.5 mg.L-1), electrical conductivity (225 μS.cm-1) and TDS (140.0 mg.L-1) than Paranapanema Formation (57.0 mg.L-1; 130 μS.cm-1 and 90.0 mg.L-1, respectively). Furthermore, the pH distribution shows greater values starting at the 60% percentile in Pitanga Formation (8.24), compared to Paranapanema (8.10). Despite such differences, it was not possible to distinguish the geological units with the hydrochemical characterization, probably because both units are mainly represented by rocks of basic composition, with a similar mineralogical basis. The shallow hydrogeochemical processes are characterized by the weathering of Fe–Mg silicates and plagioclase, changing to a predominance of ion exchange in deeper flows. Calcite precipitation is a secondary process, associated with calcium consumption towards deeper flows. Anthropic influences are mainly observed with more elevated concentrations of NO3-, PO43-, Cl- and K+, occurring in the entire basin, and they are strongly evidenced by the correlation between Cl- and NO3- in most samples.

Laser Induced Breakdown Spectroscopy (LIBS) for whole rock geochemistry

To meet the ever-increasing global demand for metals, new mineral deposits need to be discovered. However, as the exploration frontier progressively moves towards deeper buried prospective areas, sampling is becoming costlier and deposit discovery rates are in decline. New technologies and analytical procedures able to deliver fast, reliable, and cheaper samples, analysis, and data, compared to current procedures are therefore, crucial in decreasing the risk of exploration targeting and can represent a step change in mineral exploration. In this study we demonstrate how to emulate laboratory whole-rock geochemical data for major elements (Al, Ca, Fe, K, Mg, Na, Si, and Ti) by averaging LIBS spot analyses performed over single, 1-mm spaced transects, that imitate downhole trajectories, along ~1-meter length drill core intervals for chemically, texturally, and mineralogically diverse rocks. Data was collected using a benchtop prototype LIBS instrument that could be reconfigured to fit within a ≥ 75 mm diameter drill hole. The prototype comprises a customised 8 ns pulsed 532 nm Nd:YAG laser with maximum pulse energy of 40 mJ at 5 Hz; two high-resolution spectrometers that together cover a spectral range from 190 to 830 nm, hence allowing all elements to be analysed; a motorised stage; and optics. Estimated LIBS geochemistry shows strong correlations with laboratory whole rock geochemistry for the selected major elements, particularly for Si, Al, and Na, and to a lesser extent K, although the LIBS estimation can breakdown for elements found in low concentrations (e.g., 0.06 wt%). Factors including the variability in mineralogy and element deportment, or roughness of the analysed surface can result in under- or over-estimation in LIBS whole rock geochemistry. In addition, for more efficient data collection processes, investigations of the optimal number of LIBs spot analysis showed that optimal ~30 to ~560 spot analyses per meter are required to emulate whole rock geochemistry (e.g., 1 % error and 95 % confidence). The strongest influence on the optimal number of LIBS analysis required for estimation of whole rock geochemistry is the range of element concentration and to a lesser extent the number of minerals that host an element as well as grain size. The results of this study show promise in development of strategies for rapid whole rock geochemical analysis down a drill hole.