High Chemical Index Of Alteration Research Articles

Geochemical anatomy of a spheroidally weathered diabase

Major, trace element concentrations and Nd, Sr isotope ratios were measured in micro-drilled samples of a 2.37Ga-old, hand-specimen sized spheroidally weathered diabase from southern India. A sample of the un-weathered diabase dike was also analyzed. X-ray micro-CT imaging of the weathered sample shows three dominant mineral phases which are plagioclase, pyroxene, and a Fe-bearing phase (possibly hematite and/or ilmenite). This imaging documents the pervasive nature of two generations of ribbon-like, cross-cutting fractures. The older fracture is sealed while the more recent fracture is open without any in-filling. The values of the Chemical Index of Alteration (CIA) of the samples show a wide range but are less than 50. Despite being a relatively less weathered rock, we observe that concentrations of major, minor and trace elements vary significantly with the percentage relative standard deviation (%RSD) for the elements ranging from 10.2–41.8. The CIA of the samples do not show any trend with the position of the sample in the hand-specimen. Barring Ca and Li, whose concentrations decrease from the core to the rim of the sample, there is no significant spatial trend in the concentrations of the elements.Concentrations of Na, Al, and Sr increase with increasing CIA values while concentrations of Mg, Fe, and Sc decrease with increasing CIA. The strong positive correlations of Na and Al, as well as Na and Sr indicates preferential weathering of plagioclase in the diabase. Na/Ca increases while Mg/Al, Mg/Na, Mg/Ca, Fe/Al and Sc/Sr decrease with increasing CIA values and the un-weathered rock plots in the middle of these trends. Such variations are explained in terms of differential weathering of plagioclase (in samples with lower CIA than the un-weathered rock, W1-type) and pyroxene (in samples with higher CIA than the un-weathered rock, W2-type) which have varying resistance to weathering. At the hand-specimen scale, the variability in the weathering indices like CIA are controlled by differential weathering of minerals and might not accurately reflect the intensity of weathering. Chondrite-normalized La/Sm and Gd/Lu co-vary with CIA values indicating mobility of the REEs during spheroidal weathering even at the hand-specimen scale. The Eu anomaly also increases with increasing CIA values which is explained by differential weathering of pyroxene and plagioclase. We observe large percentage deviations of the Nb-normalized concentrations of elements from the un-weathered rock in specific samples but no spatial trend is observed. Overall, the variations in element concentrations can be explained by varying fluid mobility of the elements, selective weathering of the minerals in the diabase, and ambient environmental conditions.Considerable Nd and Sr isotopic variability is observed at the hand-specimen scale and is explained in terms of weathering-related fractionation of parent/daughter ratios. This elemental fractionation must have happened long time ago to allow for radiogenic decay of the long-lived isotopes of 87Rb and 147Sm. The spread (%RSD) in the initial Sr and Nd isotope compositions of the weathered samples reach a minimum value around 1.2–1.3Ga which we interpret as the timing of the peak weathering event which led to fractionation of the parent/daughter ratios. For Nd isotopes, the average εNd (1.2Ga) of the weathered samples coincides with the εNd (1.2Ga) of the un-weathered rock. The timing of the weathering event coincides with the timing of the breakup of the Columbia supercontinent and follows wide-spread alkaline volcanism in the Indian subcontinent. This is the first such attempt to determine the timing of a weathering event in rocks using long-lived radioactive isotopes.

Provenance of >2.8 Ga Keonjhar Quartzite, Singhbhum Craton, Eastern India: Implications for the Nature of Mesoarchean Upper Crust and Geodynamics

AbstractWe determined the provenance, tectonic setting, and weathering characteristics of weakly metamorphosed Mesoarchean siliciclastic rocks from the Singhbhum craton, eastern India, on the basis of their petrographic and geochemical compositions. The 1.5-km-thick siliciclastic succession, formally introduced here as the Keonjhar Quartzite, unconformably overlies the Singhbhum Granite (3.1–3.4 Ga) and has been assigned a Mesoarchean depositional age on the basis of a youngest detrital zircon of 3.01 Ga and intrusive granitoids with ages of not less than 2.8 Ga. Their highly matured and recycled nature is reflected in several geochemical proxies. The highly variable chemical index of alteration (CIA) values and the pattern in the A-CN-K plot indicate variable degrees of weathering, which in turn suggest non-steady-state weathering conditions. The high CIA values also suggest a stable tectonic setting that caused the erosion rates to decrease, which in turn caused a rise in the flux of weathered materials...

Oxygen isotope perspective on crustal evolution on early Earth: A record of Precambrian shales with emphasis on Paleoproterozoic glaciations and Great Oxygenation Event

We present stable isotope and chemical data for 206 Precambrian bulk shale and tillite samples that were collected mostly from drillholes on all continents and span the age range from 0.5 to 3.5 Ga with a dense coverage for 2.5–2.2 Ga time interval when Earth experienced four Snowball Earth glaciations and the irreversible rise in atmospheric O2. We observe significant, downward shift of several ‰ and a smaller range of δ18O values (7 to 9‰) in shales that are associated with the Paleoproterozoic and, potentially, Neoproterozoic glaciations. The Paleoproterozoic samples consist of more than 50% mica minerals and have equal or higher chemical index of alteration than overlying and underlying formations and thus underwent equal or greater degrees of chemical weathering. Their pervasively low δ18O and δD (down to −85‰) values provide strong evidence of alteration and diagenesis in contact with ultra-low δ18O glacial meltwaters in lacustrine, deltaic or periglacial lake (sikussak-type) environments associated with the Paleoproterozoic glaciations. The δDsilicate values for the rest of Precambrian shales range from −75 to −50‰ and are comparable to those for Phanerozoic and Archean shales. Likewise, these samples have similar ranges in δ13Corg values (−23 to −33‰ PDB) and Corg content (0.0 to 10 wt%) to Phanerozoic shales. Precambrian shales have a large range of δ18O values comparable to that of the Phanerozoic shales in each age group and formation, suggesting similar variability in the provenance and intensity of chemical weathering, except for the earliest 3.3–3.5 Ga Archean shales, which have consistently lower δ18O values. Moreover, Paleoproterozoic shales that bracket in age the Great Oxidation Event (GOE) overlap in δ18O values. Absence of a step-wise increase in δ18O and δD values suggests that despite the first-order change in the composition of the atmosphere, weathering cycle was not dramatically affected by the GOE at ∼2.4–2.3 Ga. Shales do not show comparable δ18O rise in the early Phanerozoic as is observed in the coeval δ18O trends for cherts and carbonates. There is however a sharp increase in the average δ18O value from the Early Archean to the Late Archean followed by a progressively decelerating increase into the Phanerozoic. This decelerating increase with time likely reflects declining contribution of mantle-extracted, normal-δ18O crust and lends support to crustal maturation and increasing 18O sequestration into the crust and recycling of high-δ18O (and 87Sr/86Sr) sedimentary rocks. This secular increase in the δ18O composition of the continental crust could have also had a mild effect on seawater δ18O composition.

Insights into chemical weathering of the upper continental crust from the geochemistry of ancient glacial diamictites

Glacial diamictites, with ages ranging from ∼2900 to 0.01Ma, record the changing composition of the upper continental crust through time (Gaschnig et al., 2014). Li concentrations and isotopic compositions, combined with Pb isotopic compositions, chemical index of alteration (CIA) values and relative Sr concentrations are used here to assess the degree of chemical weathering recorded in these deposits and the origin of this signature. The δ7Li values of most of the diamictites (ranging from −3.9 to +3.5) are lower than those of mantle-derived basalts (+3.7±2, 2σ), and the low δ7Li values are generally accompanied by high CIA and low Sr/Sr∗ values (or Sr depletion factor, Sr/Sr∗=Sr/(Ce∗Nd)0.5), reflecting a weathering signature that may have derived from pre-depositional, syn-depositional, and/or post-depositional weathering processes. Profiles through three glacial diamictites with relatively high CIA (a fresh road cut of the Neoproterozoic Nantuo Formation (CIA=62–69), and drill cores through the Paleoproterozoic Timeball Hill (CIA=66–75) and Duitschland Formations (CIA=84–91)) do not show evidence of significant post-depositional weathering. High Th/U, reflecting loss of uranium during oxidative weathering, is seen in all Paleozoic and Neoproterozoic diamictites and a few Paleoproterozoic deposits. Pb isotopic systematics suggest that this signature was largely inherited from preexisting crust, although a subset of samples (the Neoproterozoic Konnarock, Paleozoic Dwyka, and several of the Paleoproterozoic Duitschland samples) appears to have experienced post-depositional U loss. Modern glaciomarine sediments record little weathering (CIA=47, Sr/Sr∗=0.7, δ7Li=+1.8), consistent with the cold temperatures accompanying glacial periods, and suggesting that limited syn-depositional weathering has occurred. Thus, the chemical weathering signature observed in ancient glacial diamictites appears to be largely inherited from the upper continental crust (UCC) over which the glaciers traversed. The strength of this weathering signature, based on the CIA, is greatest in the Mesoarchean and some of the Paleoproterozoic diamictites and is weaker in the Neoproterozoic and Phanerozoic glacial diamictites. Combining these data with data for Archean shales and other types of post-Paleoproterozoic sedimentary rocks (i.e., shales, mudstones, etc.), it appears that post-Paleoproterozoic upper continental crust experienced less intense chemical weathering, on average, than Archean and Paleoproterozoic upper continental crust.

Occurrence of red clay horizon in soil profiles of the Yellow River Delta: Implications for accumulation of heavy metals

The source-area weathering and pedogenesis processes in the alluvial soil profiles might affect depth distribution of heavy metals. Red clay horizon (RCH) with a thickness of 5–50cm in a 1m soil profile has been found ubiquitously in the Yellow River Delta (YRD). The occurrence of this RCH was supposed to be related with the frequent shifting of the Yellow River tail channel in the Yellow River Delta (YRD). The geochemical features of the RCH were distinct from its upper or lower yellow silt horizon (YSH). The average median grain size of the RCH (10.5μm) was almost three times lower than that of the YSH (29.9μm). Meanwhile, the RCH was characterized of higher chemical index of alteration (CIA), magnetic susceptibility (χlf) and frequency-dependent magnetic susceptibility (χfd) values than the YSH, which implied a stronger source-area weathering and pedogenesis intensity of the RCH. Besides the distinctive characteristics of the RCH, it also accumulated significantly (p<0.05) higher mean contents of Cu, Zn, Pb, Cr, Ni and Co, and maximum content of Cd in the RCH than that in the YSH. The principal component analysis (PCA) suggested that distribution of the heavy metals in the YRD soil profiles was significantly related to the content of aluminosilicates, oxides, clay fraction, χlf and χfd, however, such a correlation was not found except for Pb in the YSH. In addition, result of BCR sequential extraction indicated that a higher percentage of Fe–Mn oxides associated fraction was in the RCH than in the YSH for the heavy metals of Pb and Co. Cadmium was observed at higher percentage of exchangeable fraction in the RCH than in the YSH, implying a higher environmental risk of the Cd in the RCH.

Sediment generation in humid Mediterranean setting: Grain-size and source-rock control on sediment geochemistry and mineralogy (Sila Massif, Calabria)

Grain-size control on sediment composition is investigated in modern proximal sediment from the Sila Massif, where basic to felsic intrusive rocks are exposed in a Mediterranean humid–temperate upland climate. Samples were taken from small creeks and weathering profiles from three areas reflecting different bed rock compositions. Samples were separated into eleven grain size fractions from very coarse sand to clay and analyzed by (i) X-ray fluorescence for chemical composition and (ii) X-ray diffraction and Mineral Liberation Analysis for mineralogical composition. The chemical composition vs. grain size relations were modeled by compositional linear regression. Mineralogical composition of selected samples is used to substantiate the interpretations based on geochemistry. Resultsreveal a high degree of chemical weathering with chemical index of alteration (CIA) up to 92. High CIA values are restricted to the fine-grained fractions, while sand-sized sediment average at low to moderate CIA values (~60). Although strongly weathered, the three sample suites reflecting basic to felsic plutonic bed rock can be effectively discriminated across all grain-size classes using trace elements such as V, Rb, and Sr. Linear trend modeling and mineralogical data reflect similar patterns for all sample suites implying similar processes independent of source rock composition. This includes overall decrease of quartz and K-feldspar over the full grain-size range from very coarse sand to clay, which is contrasted by overall increase of sheet silicates from coarse to fine. Among the latter, increase of clay minerals strongly outpaces the increase of micas in silt to clay fractions. A more complex behavior is shown by plagioclase, which is most abundant in intermediate grain-size fractions for all sample suites. This is likely caused by initial hydrolysis along cleavage and twinning planes and subsequent breakage of plagioclase crystals into smaller fragments. Towards finer grain size, intense hydrolysis has destroyed most feldspars.

Preservation of glacial and interglacial phases in Tethys Himalaya: evidence from geochemistry and petrography of Permo-Carboniferous sandstones from the Spiti region, Himachal Pradesh, India

Through a multidisciplinary approach, including petrology and geochemistry, the sedimentary provenance and paleo-weathering of the Permo-Carboniferous Spiti sandstones of Tethys Himalaya are investigated. The Spiti region consists of texturally immature to mature sandstones composed of unsorted to sorted and subangular to subrounded clastic grains dominated by variable amounts of quartz and feldspar accompanied by lithic fragments (mostly metasedimentary, sedimentary, and plutonic grains). They are characterized by moderate to high SiO2 contents, moderate K2O/Na2O ratios, but relatively low mafic contents. Uniform rare earth element (REE) patterns similar to upper continental crustal (UCC) with light REE (LREE) enrichment (LaN/SmN = 3.91), flat heavy REE (HREE; GdN/YbN = 1.21–2.5), and negative Eu anomalies with variable amounts of ΣREE and Eu anomalies (0.4–0.8) suggest that hydraulic sorting played a significant role. The striking similarities of the multi-elemental spider diagrams of the Spiti sandstones and the Himalayan granitoids indicate that sediments are sourced from the Proterozoic and Cambro-Ordovician orogenic belts of the Himalayan region. The nature of the feldspar observed in thin sections from most altered to euhedral pristine minerals corresponding to Carboniferous to lower Permian sandstones strongly indicates a change in climate from most favorable conditions for rapid feldspar alteration (humid) to conditions where negligible alteration is possible (arid and glacial). It is found that the chemical index of alteration (CIA) values of these sandstones accorded with inferences based on modified chemical index of alteration (CIX) and αCa values, and sedimentologic and paleontological evidence, discriminating well between warm-humid (indicated by high CIA values) and arid-glacial (representing low CIA) conditions in the Spiti basin. Thus, these results document a complete record of glacial and interglacial phases in the Permocarboniferous Spiti sandstones, and the interpretations are consistent with other such studies on the Phanerozoic glaciation events on Gondwana supercontinent.

Petrography and geochemistry of the Kamlial Formation, southwestern Kohat plateau, Pakistan: implications for paleoclimate of the Western Himalayas

The Middle to Late Miocene Kamlial Formation that largely consists of sandstone and interbedded clay/mudstone sequences is exposed in the southwestern part of the Kohat plateau, which constitutes the westernmost extension of the Himalayan Foreland Basin. Whereas the sandstone is gray to brownish gray, fine- to medium-grained, and mostly thick-bedded, the interbedded clay/mudstone sequence is brownish gray to maroon red and occurs as continuous beds as well as lenses. Some mudstone/clay beds are bioturbated and seem to be pedogenic surfaces. Results of geochemical investigation of fresh (unaltered) representative sandstone and mudstone samples from three different sections of the formation in the southwestern Kohat plateau are presented and discussed. The average chemical index of alteration (CIA) values of both the sandstone (70-86) and mudstone (71-85) suggest moderate to slightly intense weathering in the source area. However, the high CIA values may also be due to the presence of abundant sedimentary rock fragments, which occur in the studied sandstone, rather than a result of severe weathering. Furthermore, the possibility of intensive chemical weathering in the Himalayas orogenic belt is highly unlikely, as it requires tectonic quiescence for a long period, higher temperature, and humidity. Accordingly, the range of the index of chemical variability values (0.6-2.1) of mudstone and low contents of Rb and Cs in both the mudstone and sandstone indicate somewhat moderate weathering. Furthermore, the Th/U and Rb/Sr ratios of the Kamlial Formation are lower than the corresponding average values for the upper continental crust and post-Archean average Australian shale, which shows that these sediments are first-cycled in origin. However, the Zr/Sc ratio indicates minor contributions from recycled sedimentary sources. The values of authigenic U and the U/Th, V/Cr, Cu/Zn, and Ni/Co ratios all suggest that the Kamlial sediments were deposited under oxidizing conditions.

Petrographic and geochemical study of the Maastrichtian Ajali Sandstone, North Central Nigeria

The Upper Cretaceous Ajali Sandstone is an extensive stratigraphic unit of the Anambra Basin in southern Nigeria. It consists of friable, white cross-bedded sandstones exhibiting a fining upward sequence. Structures such as bioturbation, planar and herringbone cross beds indicate variability in depositional environment ranging from fluviatile to deltaic. Petrographic and geochemical compositions (major and trace elements) of these sandstones have been investigated to determine their provenance, tectonic setting and weathering conditions. Fifteen sandstone samples examined are rich in quartz but poor in feldspar and lithic fragments. Texturally, the framework grains of the sandstones are fine- to medium-grained, sub-angular to sub-rounded, moderately sorted to poorly sorted, positively skewed and leptokurtic. The sandstones are texturally immature as depicted by their sub-angular edges of grains, but mineralogically mature in terms of high percentage of quartz. The high chemical index of alteration (CIA) values (71.0–99.2%) for these sandstones suggests that they were derived from highly weathered rocks in the source area. The composition of the major oxides in the sandstones revealed that SiO 2 (49.1–99.7%), Al 2 O 3 (0.2–30.3%), Fe 2 O 3 (0.4–1.8%) and TiO 2 (0.06–3.2%) were the most abundant elements in all sandstone samples indicating a high detrital quartz and clay mineral content. The Al 2 O 3 /TiO 2 ratios (1.47 to 12.48), Ti/Zr (6.48–18.63) and Zr/Cr (2.24–22.36) suggest that the sandstones were derived from variable basement complex rocks, including some contribution from mafic or ultramafic components. Inferences from the plots of K 2 O/Na 2 O versus SiO 2 indicated a passive margin tectonic setting for the sandstones. The high loading of CaO and Na 2 O in more than 50% of the samples is indicative of terrigenous input, mainly in the form of carbonates and silicates. These results are generally consistent with a derivation of the sandstones from adjacent igneous and metamorphic basement complexes (Cameroon–Adamawa highlands and Oban Massif) while the extensive and blanket-like geometry of the Ajali Sandstone is indicative of an excellent reservoir for groundwater in the Anambra Basin.

Bulk and clay mineral composition indicate origin of terra rossa soils in Western Herzegovina

The B horizons of terra rossa soils developed on three different carbonate lithologies having variable insoluble residue contents were studied in Western Herzegovina. Comparison of their composition and properties illustrates to what extent mineral (especially clay mineral assemblage) and particle size composition of those horizons and the insoluble residue of the underlying carbonate rocks can be used as indicators of the polygenetic nature of terra rossa in this region. Terra rossa B horizons have characteristic red colours, neutral to slightly acid pH, high base saturation with calcium as the predominant cation and high CIA (Chemical Index of Alteration). The CIA values obtained are generally in accordance with mineral composition and particle size distribution of the analysed B horizons. The predominant clay mineral phases in B horizons and related insoluble residues match. Kaolinite is the predominant clay mineral phase in the B horizons overlying carbonate rocks containing low amounts of insoluble residue, while smectite predominates in calcarenites areas with a high insoluble residue content. However, the presence of plagioclase, gibbsite, chlorite-vermiculite mixed layer mineral and vermiculite in B horizons overlying carbonate rocks containing low amounts of insoluble residue support a polygenetic origin for the terra rossa. In contrast, terra rossa formed on calcarenites containing high amounts of insoluble residue might have formed almost exclusively from the parent carbonate rock although some influence of external materials (e.g. gibbsite) cannot be excluded. This investigation shows that in Western Herzegovina, an area with no important aeolian input, the content and mineral composition of carbonate rock insoluble residue plays a major role in terra rossa composition. We can tentatively conclude that the lower the insoluble residue content of the parent materials, the greater is the expectation of a more polygenetic origin for the terra rossa.

Provenance of quaternary and modern alluvial deposits of the Amazonian floodplain (Brazil) inferred from major and trace elements and Pb–Nd–Sr isotopes

Chemical and Pb, Nd and Sr isotope data from floodplain sedimentary deposits in the Solimões River, northwest Brazil, constrain the variability of the deposits through the Quaternary and help reconstruct the paleogeography of Amazonia since the uplift of the Andes. Compared to the modern alluvial deposits in the active channel, the Quaternary terrace deposits in the Solimões River have a higher chemical index of alteration (CIA), higher SiO2/Al2O3 ratios, higher concentrations of immobile elements, higher radiogenic Sr isotopic compositions, more negative εNd(t=0) values, and older Nd TDM ages. These geochemical characteristics indicate that the Quaternary sediment sources were composed by more felsic and/or recycled old continental crust derived rock materials than the younger source areas, which contributed major arc volcanic materials to the flood plain deposits. This shift in the composition of the sediment source areas indicates that a significant reorganization of the architecture of the Solimões River has occurred since the uplift of the Andes.

Mineralogy and Geochemistry of the Numidian Formation Central-Northern Sicily): intra-formation variability and provenance evaluation

Mudrocks and sandstones of Numidian Formation in Sicily have been analysed for mineralogical, petrographic, major element and selected trace element compositions to constrain the intraformational variability and the sediment provenance. The mudrocks are characterized by a kaolinite-rich signature while the sandstones have a quartz-arenitic composition. The high Chemical Index of Alteration and Plagioclase Index of Alteration strongly suggest that recycling processes have been responsible for the present compositions. The mudrocks geochemical data suggested a compositional variability function of the paleogeographic depositional environments. In particular, the Numidian Formation of the more external Serra Del Bosco Unit is affected by the terrigenous contribution of epicontinental-type sediments. With regard to provenance, the geochemical data indicate a derivation from post-Archean sedimentary siliciclastic rocks. The contribution of African cratonic sediments was confirmed by the trace elements ratios and by the end-member modelling processed on mudrocks compositional data. In particular the suitability of the Nubian sediments as one of the sources of sediment supply is also attested.

Geochemistry of the Paleocene-Eocene and Miocene-Pliocene clayey materials of the eastern part of the Wouri River (Douala sub-basin, Cameroon): Influence of parent rocks

Major and trace element concentrations of clay deposits of the Missole II and Bomkoul respectively from the Paleocene-Eocene N’Kapa Formation and the Miocene-Pliocene-Matanda-Wouri Formation in the eastern part of the Wouri River in the Douala sub-basin of Cameroon have been investigated to identify the parent rocks. To carry out this study, X-ray diffraction, inductively coupled plasma–atomic emission spectrometry (ICP/AES) and inductively coupled plasma–mass spectrometry (ICP/MS) were performed to determine respectively the mineralogical and chemical data of Missole II and Bomkoul clayey materials. Clay sediments are essentially kaolinitic and illitic, and kaolinitic and smectitic respectively in both sites. They are generally siliceous, aluminous with small iron and bases (MgO, CaO, Na2O, and K2O) contents. Samples of Missole II profiles are more siliceous than those from the Bomkoul grey and dark grey clayey materials. Clayey materials have high Chemical Index of Alteration (CIA=80–99.34) which suggests that they are strongly weathered under humid tropical climate before and after their deposition in the coastal plain. The value of Eu/Eu* (0.48–0.61), La/Sc (2.15–20.50), Th/Sc (0.74–2.25), Th/Co (1.08–8.33), and Cr/Th (5.24–13.55) ratios support essentially a silicic or felsic parent rocks. Total REE concentrations reflect the variations in their grain-size fractions. Chondrite-normalised REE patterns with LREE enrichment, flat HREE, and negative Eu anomaly are attributed essentially to silicic or felsic parent rocks like those from weathered materials developed from the gneisses around the coastal plain in the littoral part of Cameroon (Noa Tang et al., 2012), main characteristic of Paleocene-Eocene and Miocene-Pliocene clay sediments of Missole II and Bomkoul areas.

Lawsonite- and glaucophane-bearing blueschists from NW Qiangtang, northern Tibet, China: mineralogy, geochemistry, geochronology, and tectonic implications

The occurrence of high-pressure (HP) blueschists within the central Qiangtang terrane of northern Tibet has a significant bearing on plate-suturing processes. In order to contribute to the ongoing debate on whether the central Qiangtang metamorphic belt represents an in situ suture within the Qiangtang terrane, we examined lawsonite- and glaucophane-bearing blueschists from the northwest Qiangtang area (84° 10′–85° 30′ E, 34°10′–34° 45′ N). All studied rocks are metapelites, metasandstones, or metabasalts, characterized by lawsonite + glaucophane + phengite, lawsonite + glaucophane + epidote + albite + quartz, or glaucophane + phengite + quartz assemblages. The meta-mafic rocks contain very high TiO2 and low Al2O3 contents. They are typified by abundant ferromagnesian trace elements, and an absence of Eu anomalies and Nb–Ta deletions; all the above features indicate that these mafic rocks represent oceanic island basalt (OIB) protoliths. Most of the metasediments contain high SiO2, moderate Al2O3 + K2O, and low TiO2 + Na2O. They display high CIA (chemical index of alteration) values (74% ± 5%) and distinctly negative Eu anomalies (Eu/Eu* = 0.64 ± 0.05). This, along with their high field strength elemental characteristics, indicates that they were deposited in a passive continental margin environment, intercalated with OIB-type basalts. We estimate the peak metamorphic conditions for these blueschists as T = 330–415°C and P = 9–11.5 kbar. This HP event occurred at ca. 242 Ma, indicated by a well-defined 40Ar/39Ar plateau age for glaucophane. Retrograde metamorphism occurred at T = 280–370°C, P = 6.5–9.5 kbar, t = ca. 207 Ma (40Ar/39Ar dating of phengite). Therefore, a cold subduction (geotherm ∼8°C/km) attended the passive continental margin during the Triassic when the eastern Qiangtang collided with the western Qiangtang. The northwest Qiangtang HP metamorphic belt is an extension of the central Qiangtang metamorphic belt that defines the suture between eastern and western Qiangtang, and indicates an anticlockwise, diachronous closure of the Shuanghu Palaeo-Tethys.

Geochemical evidence for volcanic activity prior to and enhanced terrestrial weathering during the Paleocene Eocene Thermal Maximum

The Re–Os and major element geochemistry of an expanded gray shale sequence from the Central Basin in Spitsbergen, Svalbard Archipelago (Norway) over the Paleocene Eocene Thermal Maximum (PETM) is presented. The data suggest that a significant volcanic episode, as indicated primarily by a large drop in 187Os/188Os, occurred just prior to or coincident with the onset of the PETM. The distinct geochemistry of the inferred ash component is consistent with the geochemistry of contemporaneous ashes previously sampled in Denmark, and persists in the sedimentary record for ∼8ka. Based on the substantial size of the 187Os/188Os decrease (Δ187Os/188Os ∼−0.23), the volcanic event must have involved widespread ash deposition in the local catchment. The terrestrially deposited ash weathered rapidly, shifting the 187Os/188Os of the basin water column to <0.5. Temporally, the volcanic event coincides with (or predates by a few thousand years) the onset of the PETM, suggesting northwest European shelf volcanism as the trigger for the PETM. As the planet warmed, both dissolved and detrital indicators of weathering indicate a <10ka pulse of more weathered material to the basin, shifting authigenic 187Os/188Os to more radiogenic values and depositing clastic materials with higher Chemical Index of Alteration and lower Na/Ti. Persistently anoxic sedimentary pore fluids, interpreted to reflect bottom water anoxia, initiated close to the onset of the PETM. Anoxic, mainly euxinic, conditions persisted for ∼50ka, ending abruptly ∼55.77Ma. Anoxia appears related to enhanced high-latitude hydrological cycling and consequent basinal stratification during the PETM.

Geochemical evolution of the Tertiary succession of the NW shelf, Bengal basin, Bangladesh: Implications for provenance, paleoweathering and Himalayan erosion

Whole rock major, trace and rare earth element (REE) compositions of Paleogene to Neogene sedimentary rocks of the NW shelf succession (Province 1) of Bangladesh contain a record of interaction of the India and Asian plates, Himalayan tectonism, and climatic development. Analyses of 66 sandstones and mudrocks from the Tertiary succession of Bangladesh were made to examine provenance, source weathering, and the influence of paleoclimate and tectonism. The sediments display linear geochemical trends due to quartz dilution, and varying quartz–clay ratios produced by hydrodynamic sorting. Chondrite-normalized REE patterns for both sandstones and mudrocks from different groups are similar to upper continental crust, with moderate to high LREE enrichment (lithotypes within formations average LaN/YbN 5.31–11.41) and marked negative Eu anomalies (Eu/Eu* 0.51–0.69). Based on geochemical criteria the succession can be divided into three parts (Jaintia; Barail–Surma; and Dupi Tila). Very high silica contents in Jaintia Tura sandstones and high Chemical Index of Alteration (CIA) indices in Kopili mudrocks (Fe-shales) suggest derivation from a deeply weathered and stable cratonic source (India). The Tura sandstones are interpreted as first-cycle quartz arenites, produced while the Indian plate drifted across equatorial regions during the Paleocene–Eocene Thermal maximum (PETM). The Barail–Surma and Dupi Tila sediments were derived from a felsic orogen (the Himalaya). The Barail–Surma sediments were mainly derived from the Trans Himalayan Batholith and associated granitoids, with significant contribution from the Lesser Himalaya. Mafic input is also evident, probably from intraoceanic arc material within the Himalaya. Barail mudrocks have uniformly high CIA values (92–95), suggesting intense steady-state weathering of their proto-Himalayan source, and warm and humid climate. In contrast, CIA values of Surma mudrocks range from 66 to 93 (average 84), suggesting non-steady state weathering related to active uplift in the Himalaya. The Dupi Tila sediments were derived from a more felsic Lesser Himalaya source, with significant contribution from the Trans-Himalaya and very little or no ophiolitic or arc material. Dupi Tila mudrocks have CIA ratios of 62–99 (average 72), also indicating non-steady state weathering in the rising Himalayan source. Geochemical compositions of the NW shelf sediments are comparable to coeval successions in the Surma basin (Province 2) of Bangladesh and the Siwaliks (India), indicating similar source. Evolution of the Indian monsoon and associated high precipitation caused intense chemical weathering of the Surma and Dupi Tila source, despite rapid uplift. The Surma Group thus bears the signature of evolution of the Asian monsoon in the Bengal basin at 21Ma, simultaneous with the development of the East Asian monsoon. This supports proposals that both monsoon systems developed at the same time.

Climatic control on the composition of Carboniferous–Permian Gondwana sediments, Khalaspir basin, Bangladesh

Late Paleozoic or Permo-Carboniferous Gondwana successions world-wide are marked by widespread and lengthy glaciation (~67Ma), and multiple transitions from Icehouse to Greenhouse state. Mineralogical and compositional changes in sandstones and mudstones of the Gondwana succession in drill hole GDH-45 from the Khalaspir basin of Bangladesh are interpreted in a climatic framework, using the proxies of the Mineralogical Index of Alteration (MIA) and the Chemical Index of Alteration (CIA). Low MIA (<75) and CIA (<70) values in the lowermost unit (Unit 1) of the Khalaspir Gondwana sequence indicates minimal chemical weathering of the source and imply cold and dry glacial climatic conditions. Values increase progressively in the overlying Unit 2, suggesting warming and deglaciation. Very high MIA (80–100) and CIA (90–100) values in Unit 3 indicate intense chemical weathering associated with warm and humid climatic conditions in a post-glacial setting. These changes are recorded in both sandstones and mudstones, illustrating the value of acquiring data for a variety of size grades. The stratigraphic changes are also well recorded by progressively decreasing K2O/Al2O3 and increasing Ga/Rb ratios, the combination of which may comprise a new geochemical proxy of climate and weathering. Upward increase in the modal Q/(F+L) and Qp/(F+L) ratios are also consistent with change from cool to warm and humid climate in the source region. The change in climatic conditions within the Khalaspir sequence is also well correlated with the Gondwana successions of other continental blocks, and records the climatic fluctuations and extent of climatic impact in Gondwanaland during the Permo-Carboniferous.

Weathering intensity in the Mesoproterozoic and modern large-river systems: A comparative study in the Belt-Purcell Supergroup, Canada and USA

The Mesoproterozoic Belt-Purcell Supergroup (BPS) preserves a thickness of 17km of dominantly siliciclastic rocks deposited between 1470 and 1400Ma. The total range of Chemical Index of Alteration (CIA) values, corrected for diagenetic K-addition, are 62–88 for argillites and 55–80 for sandstones, with averages of 72±6 and 68±7, respectively. More intense CIA values, in conjunction with low absolute contents of Sr, Ca and Na, and high Rb/Sr ratios (average 4), reflect an intensely weathered provenance in a hot-wet climate with hot-arid intervals resulting in evaporitic sediments. Covariations of CIA-Eu/Eu* and Sr-Eu/Eu* are consistent with a large catchment area including extensive provenance terranes of weathered recycled sedimentary rocks for the most extreme CIA and Eu/Eu* values, with smaller less intensely weathered juvenile terranes represented by lower CIA values. Accordingly, variations of CIA within the BPS stratigraphic sequence may record some combination of shifting catchment terranes and weathering intensity. Stratigraphic trends in CIA within the Appekunny and Grinnell Formations of 80±6 to 66±4 to 79±5 record the variation in this combination with time.Siliciclastic rocks record a first order trend of CIA values from CIA ∼80–100 in the Mesoarchean, through ∼80–90 in the Neoarchean, and ∼70–85 in the Proterozoic, to ∼72 for global Phanerozoic shales. These values reflect progressive drawdown of greenhouse gases that promote silicate weathering by their sequestration into carbonates and black shales, as preserved in the geologic record. Second order secular peaks in CIA values correlate in time with mantle plumes that emit greenhouse gases, which enhance silicate weathering. Some of the more intense CIA values in the BPS may also stem from release of volcanic gases during magmatism that accompanied rifting of Laurentia during breakup of the Supercontinent Columbia at ∼1.4Ga. Overall, CIA values are within the range of modern humid-temperate and humid-tropical climatic catchment areas drained by large river systems such as the Orinoco, Nile and Amazon rivers.Proterozoic rivers have been viewed as mostly braided systems due to the lack of influence of rooted vegetation, which resulted in fast channel lateral migration, high run-off rates, and low bank stability. Many large-scale Proterozoic siliciclastic basins have been preserved, formed by river systems up to pan-continental scale. However, their significance as archives of continental weathering intensity remains under-explored. This study suggests that BPS CIA values reflect more aggressive chemical weathering, since Proterozoic rivers had less sediment residence times due to a lack of vegetation cover, and therefore faster transport time than their modern counterparts. To achieve high CIA values in shorter periods of time without vegetation cover, more intense chemical weathering conditions must have been present.

Environmental changes recorded by major elements in Loulan Stupa Section during Early-Middle Holocene

Holocene palaeoweathering conditions and environmental changes of Loulan (楼兰) Stupa Section were reconstructed by major elements analysis results and optical stimulated luminescence dating results. Major element results show source rock change little, with the composition equivalent to granodiorite. The low R value and high CIA (chemical index of alteration) value indicating the section at primary weathering phases. The muddy silt and silty clay have low R value and high CIA value and total iron content. In contrast, the sandy clay silt and sandy silt have high R value and low CIA and total iron value. Deglaciation time climate was relatively humid. In comparison to previous and posterior hiatus, the Middle-Holocene climate was relatively wet but still displayed high-frequency and high-amplitude fluctuations.

Palaeoclimatic Control on the Composition of Palaeozoic Shales from Southern Ghana, West Africa

The Late Ordovician to Early Cretaceous Sekondian Group of southern Ghana is made up of seven major sedimentary formations. One important feature of the Sekondian Group is the abundance of fairly fresh plagioclase in the lowest two formations, i.e., the Ajua Shale and Elmina Sandstone, and at the uppermost formation (i.e., Essikado Sandstone); but a virtual absence throughout the rest of the Group. The basal part of the Sekondian Group has been interpreted as of glacial origin whereas the upper formations are thought to have formed in a warmer stage. The abundant plagioclase in the lowest two formations could, therefore, be due to less pervasive chemical weathering rather than erosion of a distinct source. In this paper, geochemical data for fine-grained sedimentary rocks of the Ajua Shale and the Takoradi Shale (that overlie the Elmina Sandstone) formations are used to investigate the influence of palaeoweathering on the composition of the Sekondian Group.The concentrations of the transition trace metals Cr, Ni, V, and Cu, and Cr/V-, Zr/Y-, and Ba/Co-ratios are similar in the samples from both formations and suggest that they were largely derived from the granitoids that intrude the Birimian supracrastal rocks. However, the Takoradi Shale samples are enriched in Rb, Cs, Th, U, and Nb relative to the Ajua Shale samples, and this may be related to differences in the degree of weathering in the source rocks. The Takoradi Shale samples are characterized by high chemical index of alteration (CIA) values and Rb/Sr ratios (CIA > 75; Rb/Sr > 1) indicating high degrees of weathering of their source rocks. In contrast, the Ajua Shale samples have low CIA values and Rb/Sr ratios (CIA < 62; Rb/Sr < 0.5) that indicate low degrees of weathering of their source. The tectonic setting at the time of deposition of both formations was passive margin suggesting that the change in paleoclimatic conditions rather than erosion rates played a more dominant role in the observed compositional changes in these shales.