High Chemical Index Of Alteration Research Articles

Determination of the geochemical weathering indices and trace elements content of new volcanic ash deposits from Mt. Talang (West Sumatra) Indonesia

Since the Indonesian archipelago is part of the very active and dynamic Pacific Ring of Fires, the volcanic eruptions occur from time to time. Immediately after the eruption of Mount Talang in West Sumatra (April 12, 2005), volcanic ashes, both unleached and leached were collected. The deposits from Mt. Talang were andesitic to basaltic in composition. The volcanic ash consisted of volcanic glass, plagioclase feldspar in various proportions, orthopyroxene, clinopyroxene, olivine, amphibole, titanomagnetite. We conducted the total elemental analysis of the bulk samples of the volcanic ash. The contents of major, trace and rare elements as well as heavy metals were determined by wet chemical methods and x-ray fluorescence (XRF) analyses. Although the volcanic ash of Mt. Talang are still very new, an evaluation of the geochemical weathering indices was performed with the objective of showing the volcanic ash condition at the early stage of weathering. Eight weathering indices were evaluated. The results showed that the unleached volcanic ash has higher Ruxton Ratio (R), Weathering Index of Parker (WIP), Product of Weathering Index (PWI) and Silica Titanium Index (STI) values compared to the leached ash, while the leached ash exhibited higher Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Vogt’s Residual Index (VO), and Plagioclase Index of Alteration (PIA). These weathering indices can be used to quantify the condition of the volcanic ashes at the initial stage of weathering, to evaluate their fertility, to provide a better understanding of element mobility during weathering, and predict the source of soil nutrients as well as determine the products of primary minerals alteration.

Reorganization of ice sheet flow patterns in Arctic Canada and the mid‐Pleistocene transition

Evidence for the evolution of Laurentide Ice Sheet (LIS) basal thermal regime patterns during successive glaciations is poorly preserved in the geologic record. Here we explore a new approach to constrain the distribution of cold‐based ice across central Baffin Island in the eastern Canadian Arctic over many glacial‐interglacial cycles by combining till geochemistry and cosmogenic radionuclide (CRN) data. Parts of the landscaped with geomorphic evidence for limited glacial erosion are covered by till characterized by high chemical index of alteration (CIA) values and CRN concentrations requiring complicated burial‐exposure histories. Till from regions scoured by glacial erosion have CIA values indistinguishable from local bedrock and CRN concentrations that can be explained by simple exposure following deglaciation. CRN modeling results based on these constraints suggest that the weathered tills were deposited by 1.9 to 1.2 Ma, and by that time the fiorded Baffin Island coastline must have developed close to its modern configuration as piracy of ice flow by the most efficient fiord systems resulted in a major shift in the basal thermal regime across the northeastern LIS. The resultant concentration of ice flow in fewer outlet systems may help explain the cause of the mid‐Pleistocene transition from 41‐ to 100‐kyr glacial cycles.

Petrography and whole-rock geochemistry of the Tertiary Sylhet succession, northeastern Bengal Basin, Bangladesh: Provenance and source area weathering

Detrital modes and the geochemical compositions of sandstones and mudrocks from the Paleocene to Plio-Pleistocene Jaintia, Barail, Surma, Tipam and Dupitila Groups of the Sylhet basin in Bangladesh have been examined to identify their provenance and source area weathering conditions. Barail and Surma Group sandstones are quartz-rich, feldspar-poor, and are rich in sedimentary and metamorphic lithic fragments, indicating a recycled orogen source. Geochemically the succession is divisible into three parts. Paleocene–Late Eocene Jaintia Group mudrocks form separate trends, and have very high Chemical Index of Alteration (CIA) values, suggesting derivation from a stable cratonic source. The second part, the Late Eocene to Miocene Barail and Surma Groups, forms single trends on variation diagrams. Maximum CIA ratios decrease from the Barail to the Surma Group, suggesting rapid influx of detritus from a common felsic source undergoing active uplift, compatible with weathering and erosion proxies from the Bengal and Indus fans. Average bulk geochemical and REE characteristics of these two groups are identical, as are A–CN–K trends. These features and comparison with potential Himalayan sources support derivation of both the Barail and Surma Groups from that orogen, and hence supply of Himalayan detritus to the Sylhet basin as early as the Late Eocene, rather than from the Neogene. The third part, the non-marine Tipam (Late Miocene–Pliocene) and Dupitila (Pliocene–Pleistocene) Groups, forms slightly differing geochemical trends and has higher maximum CIA values. REE fractionation between sandstones and mudrocks is more marked, but REE patterns and parameters remain unchanged, suggesting that the overall source remained unchanged. Higher CIA and greater REE fractionation in the Tipam and Dupitila Groups may be a product of their fluvial deposition and more intense weathering on the floodplain during intensification of the South Asian monsoon from 4 Ma onwards.

Detrital mode, mineralogy and geochemistry of the Sidi Aïch Formation (Early Cretaceous) in central and southwestern Tunisia: Implications for provenance, tectonic setting and paleoenvironment

The chemical and mineralogical composition of the Sidi Aïch Formation sandstones in central and southwestern Tunisia has been investigated in order to infer the provenance and tectonic setting, as well as to appraise the influence of weathering. The sixteen studied samples are mainly composed of quartz, feldspar, kaolinite and/or illite. Sidi Aïch sandstones are mainly arkosic, potassic feldspar-rich and immature. Much of the feldspar was transformed to kaolinite. Concerning the relation between sandstone detrital composition and their depositional setting, the Sidi Aïch Formation sandstone in the major studied localities, probably accumulated in relatively proximal small basins within the continental interior. However, for the Khanguet El Ouara study site, sandstones may have been deposited in a foreland basin which received recycled sediments from an adjacent orogenic belt. The source area may have included quartzose sedimentary rocks. The dominance of quartz and enrichment in immobile elements suggest that the depositional basins were associated with a passive margin. The petrography and geochemistry reflect a stable continental margin and sediments were derived from granitic and pegmatitic sources located in the southern parts of the Gafsa basin. High values for the chemical index of alteration (CIA) indicate that recycling processes might have been important. Particularly high CIA values in the Garet Hadid locality indicate more intense chemical alteration, either due to weathering processes or tectonic control.

Geochemical record of chemical weathering and monsoon climate change since the early Miocene in the South China Sea

The chemical index of alteration (CIA) and elemental ratios that are sensitive to chemical weathering, such as Ca/Ti, Na/Ti, Al/Ti, Al/Na, Al/K, and La/Sm, were analyzed for detrital sediments at Ocean Drilling Program Site 1148 from the northern South China Sea to reveal information of chemical weathering in the source regions during the early Miocene. High CIA values of ∼80, coupled with high Al/Ti and Al/Na and low Na/Ti and Ca/Ti, are observed for the sediments at ∼23 Ma, indicating a high chemical weathering intensity in the north source region, i.e., south China. This was followed by gradual decreases in Al/Ti, Al/Na, La/Sm, and Al/K ratios, as well as the CIA values, and increases in Ca/Ti and Na/Ti ratios. These records together with other paleoclimate proxies, such as black carbon δ13C and benthic foraminifer δ18O, give reliable information on the climate changes in south China. Our results show that the climate in south China was warm and humid in the early Miocene (∼23 Ma) according to the chemical weathering records. The humidity in south China decreased from the early Miocene to Present with several fluctuations centering at approximately 15.7 Ma, 8.4 Ma, and 2.5 Ma, coincident with the global cooling since the middle Miocene. These climate changes implied that the summer east Asian monsoon has dramatically affected south China in the early Miocene, whereas the influence of the summer monsoon on this region has decreased continuously since that time, probably because of the intensification of the winter monsoon. Such an evolution for the east Asian monsoon is different from that for the Indian monsoon.

Reply to: Comments on “Trace element and isotopic evidence for Archean basement in the Lonar crater impact breccia, Deccan volcanic province” by Ramananda Chakrabarti and Asish R. Basu

The ∼1.88 km diameter Lonar impact crater formed ∼570 ka ago and is an almost circular depression hosted entirely in the Poladpur suite of the ∼65 Ma old basalts of the Deccan Traps. To understand the effects of impact cratering on basaltic targets, commonly found on the surfaces of inner Solar System planetary bodies, major and trace element concentrations as well as Nd and Sr isotopic compositions were determined on a suite of selected samples composed of: basalts, a red bole sample, which is a product of basalt alteration, impact breccia, and impact glasses, either in the form of spherules (<1 mm in diameter) or non-spherical impact glasses (>1 mm and <1 cm). These data include the first highly siderophile element (HSE) concentrations for the Lonar spherules. The chemical index of alteration (CIA) values for the basalts and impact breccia (36.4–42.7) are low while the red bole sample shows a high CIA value (55.6 in the acid-leached sample), consistent with its origin by aqueous alteration of the basalts. The Lonar spherules are classified into two main groups based on their CIA values. Most spherules show low CIA values (Group 1: 34.7–40.5) overlapping with the basalts and impact breccia, while seven spherules show significantly higher CIA values (Group 2: >43.0). The Group 1 spherules are further subdivided into Groups 1a and 1b, with Group 1a spherules showing higher Ni and mostly higher Cr compared to the Group 1b spherules. Iridium and Cr concentrations of the spherules are consistent with the admixture of 1–8 wt% of a chondritic impactor to the basaltic target rocks. The impactor contribution is most prominent in the Group 1a and Group 2 spherules, which show higher Ni/Co, Ni/Cr and Cr/Co ratios compared to the target basalts. In contrast, the Group 1b spherules show major and trace element compositions that overlap with those of the impact breccia and are characterized by high EFTh (Enrichment Factor for Th defined as the Nb-normalized concentration of Th relative to that of the average basalt) as well as fractionated La/Sm(N), and higher large ion lithophile element (LILE) concentrations compared to the basalts. The relatively more radiogenic Sr and less radiogenic Nd isotopic composition of the impact breccia and non-spherical impact glasses compared to the target basalts are consistent with melting and mixing of the Precambrian basement beneath the Deccan basalt with up to 15 wt% contribution of the basement to these samples. Variations in the moderately siderophile element (MSE) concentration ratios of the impact breccia as well as all the spherules are best explained by contributions from three components – a chondritic impactor, the basaltic target rocks at Lonar and the basement underlying the Deccan basalts. The large variations in concentrations of volatile elements like Zn and Cu and correlated variations of EFCu-EFZn, EFPb-EFZn, EFK-EFZn and EFNa-EFZn, particularly in the Group 1a spherules, are best explained by evaporation-condensation effects during impact. While most spherules, irrespective of their general major and trace element composition, show a loss in volatile elements (e.g., Zn and Cu) relative to the target basalts, some spherules, mainly of Group 1, display enrichments in these elements that are interpreted to reflect the unique preservation of volatile-rich vapour condensates resulting from geochemical fractionation in a vertical direction within the vapour cloud.

Geochemistry of shales from the late-Archean Chitradurga Greenstone Belt, Karnataka, Southern India

Archean shales have been collected from three different stratigraphic horizons namely Vanivilas, Ingaldhal and Hiriyur Formations representing the lower and upper levels of Chitradurga greenstone belt (CGB). These shales/pelites are characterized by mica-quartz-chlorite+graphite mineralogy. The high Al2O3/TiO2 values, enriched incompatible and HFSE indicate a greater abundance of detrital and residual heavy minerals. REE patterns of the three different formations are similar, absence of negative Eu anomalies, LREE enrichment and flat HREE (Fig. 1). One sample belonging to the Vanivilas Formation shows strong negative Ce anomaly indicating perhaps prevalence of localized oxygenated environment as evidenced by the occurrence of stromatolites. These patterns are also similar to the Fig Tree Groups in South Africa [1] and indicate contribution of detritus from source rock types to the basin. La/ Th, Th/Sc, Co/Th and Cr and Ni distributions in these pelites show a mixed provenance. High Chemical Index of Alteration (CIA) and depletion of Sr and CaO in the multi-element plot normalized against average Archean Upper Crust (AUC) indicate intense chemical weathering of the source terrain. The Chitradurga pelites on an average are more mafic in composition than the post-Archean sediments indicating strong similarities to that of Archean turbidite deposits.

Geochemical characteristics of water and sediment from the Dal Lake, Kashmir Himalaya: constraints on weathering and anthropogenic activity

Two hundred and forty water samples (in four seasons) and seventeen sediment samples have been analyzed to monitor the natural and anthropogenic influences on the water and sediment chemistry of the Dal Lake, Kashmir Himalaya. The scatter diagrams [(Ca+Mg)/total cations (TZ+), (Ca+Mg)/HCO3, (Ca+Mg)/(HCO3+SO4), (Na+K)/TZ+; (Ca+Mg)/(Na+K)] and the geological map of the study area suggest predominance of carbonate and silicate weathering. Lower pH and high total dissolved solids, electrical conductivity and \( {\text{NO}}^{ - }_{{\text{3}}} \) values in the Gagribal basin and in some patches of other basins reflect anthropogenic inputs in the form of sewage from surrounding population, houseboats, hotels, etc. The Dal Lake is characterized by high chemical index of alteration (CIA: 87–95), reflecting extreme weathering of the catchment area. Relative to the average carbonates, the lakebed sediments are enriched in Al, Ti, Zn, Cu and Co and depleted in Ni and Mn. Compared to the post-Archean Shale the sediments have higher Al, Zn and Cu contents and lower Ni and Co. There are distinct positive anomalies of Al, Mn, Zn and Cu and negative anomalies of Ni and Pb with respect to the upper continental crust. Geoaccumulation index (Igeo) and the US Environmental Protection Agency sediment quality standards indicate that the Gagribal basin and some patches of the Nagin basin are polluted with respect to Zn, Cu and Pb. These data suggest that the Dal Lake is characterized by differential natural and anthropogenic influences.

Petrology and geochemistry of the Neoproterozoic Nguba and Kundelungu Groups, Katangan Supergroup, southeast Congo: Implications for provenance, paleoweathering and geotectonic setting

The Nguba and Kundelungu Groups constitute the middle and upper parts of the Neoproterozoic Katangan Supergroup, respectively, and consist of conglomerates, sandstones, mudrocks and carbonates. During deposition, the Katangan basin received sediments originating from both northern and southern sources. The Nguba and Kundelungu Groups siliciclastic rocks have elemental abundances and ratios suggestive of a relatively felsic TTG source, although slightly more mafic compositions occur in the Nguba Group and the overlying “Petit Conglomérat” Formation at the base of the Kundelungu Group. Modal compositions of the Nguba Group rocks indicate a basement uplift provenance, and geochemical parameters indicate the source of both the Nguba and Kundelungu Groups had an active continental margin character. Source area weathering was moderate in the Nguba Group. Low Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration (PIA) indices and relatively uniform chemical compositions of the “Grand Conglomérat” and the “Petit Conglomérat” Formations lying respectively at the bases of the Nguba and Kundelungu Groups are compatible with deposition in a cool or frigid climate, and support their presumed petrographic based glaciogenic origin. High CIA and PIA indices in Upper Kalule rocks in the middle part of the Kundelungu Group point to the intensification of source weathering, possibly under tropical to subtropical climate under steady state conditions. Geochemical similarities between the Nguba Group and the “Petit Conglomérat” are compatible with a change from an extensional setting to compression, with derivation of the “Petit Conglomérat” by reworking of the underlying units during basin inversion. Change in provenance signatures and weathering indices in the Upper Kalule Formation may reflect reduced tectonism and resumption of supply of more weathered extrabasinal detritus, similar to that which fed the basal Roan Group. Overall the data suggest derivation mainly from pre-Katangan Proterozoic sources with continental arc characteristics. The adjacent Paleoproterozoic Ubendian Belt, particularly the Bangweulu block calcalkaline plutonic and volcanic province, is a suitable candidate as the source for the Nguba and Kundelungu Group sedimentary rocks. However, Mesoproterozoic and Archaean terrains have also contributed a minor component to the basin.

Provenance studies of very low- to low-grade metasedimentary rocks of the Puncoviscana complex, northwest Argentina

Abstract A provenance study of Neoproterozoic to Lower Cambrian rocks for the entire Puncoviscana Basin was conducted, using 119 samples from 15 different outcrops. Petrographic data (Q t 60–80, F15–35, L5–20, P/F 0.2–0.4, L v /L = 0) show a composition comparable to foreland-basin successions. Lithoclasts are of metamorphic and metasedimentary origin. Volcanic debris is detected only in the form of sanidine, and volcanic lithoclasts were probably decomposed to form pseudo-matrix. Framework clasts are sub-angular to sub-rounded, and the rocks are poorly sorted. Major element geochemistry shows a moderate to high Chemical Index of Alteration (56–77) and failed to provide coherent provenance and rock classification. Trace element geochemistry suggests a rhyodacitic composition overall. Rare earth element patterns are comparable to those of model upper continental crust (UCC), as are concentrations of Nb, Ta, Ti, Th-Sc and Eu/Eu* (0.45–0.87; 95% between 0.4 and 0.7); reworking signatures are not detected. The uniform mineralogical and geochemical composition reflects supra-crustal source(s) for the entire basin, including significant metamorphic rock debris. The Puncoviscana complex is interpreted as a peripheral Pampean foreland basin, fed mainly from an eastern fold-thrust belt, but includes relicts of pre- and syn-collisional magmatic activity as well. A source area of UCC composition to the west is represented by the Arequipa block.

Water and sediment geochemistry of major Kumaun Himalayan lakes, India

Four major Himalayan lakes in Kumaun region in India namely, Nainital, Bhimtal, Sattal and Naukuchiatal, have been studied to understand water and sediment geochemistry. The lakes are characterised by varying degrees of natural and human influences; the Nainital is intensively used and is a major tourist destination, whereas the other three lakes have attracted much attention lately. The lakes have a tectonic origin and are characterised by dolomites, limestones, shales and silicate rocks. The water chemistry is dominated by Ca, Mg and HCO3, an indication of a carbonate lithology source for the water composition. About 55–80% of Na and only 30–40% of Ca in the lakes are derived from silicate lithology. The Nainital lake is also characterised by a high chemical index of alteration, which is reflected in its high carbonate and the clay mineral content of the sediments. Model calculations show that 33 and 67% of nitrate in Sattal Lake is contributed by agriculture and cultural effects, respectively. The 87Sr/86Sr ratios in the lakes are very high (>0.72), with samples from Sattal Lake being extremely radiogenic (>0.82), similar to many Himalayan rivers. The 1/Sr–87Sr/86Sr in sediments show two major sources for Sr-silicates and carbonates. The contributions of Sr from carbonate and silicate lithology is estimated to be 25 and 60%, respectively, in Bhimtal Lake.

Precambrian superplumes and supercontinents: a record in black shales, carbon isotopes, and paleoclimates?

Prominent maxima in black shale abundance and in black shale/total shale ratio occur at 2.0–1.7 Ga, with less prominent peaks in the Late Neoproterozoic (800–600 Ma) and in the Late Archean (2.7–2.5 Ga). Peaks in chemical index of alteration (CIA) of shales at the same times suggest corresponding warm paleoclimates. The peaks in CIA and black shale abundance are correlated in time at a 94% confidence level. The black shale and CIA peaks may reflect the combined effects of mantle superplume events and supercontinent formation at 2.7 and 1.9 Ga. Mantle superplume events may have introduced large amounts of CO 2 into the atmosphere–ocean system, increasing depositional rates of carbon and increasing global warming. Increased black shale deposition may reflect some combination of: (1) increased oceanic hydrothermal fluxes (introducing nutrients); (2) anoxia on continental shelves; and (3) disrupted ocean currents. The apparent absence of carbon isotope anomalies at these times reflects an increase in the deposition and burial rate of both reduced and oxidized carbon. Peaks in black shale abundance at ∼2.1 Ga and 800–600 Ma correlate with peaks in δ 13C in marine carbonates, increases in atmospheric oxygen, and with high CIA values in shales. These are all consistent with higher rates of organic carbon burial in black shales at these times. These peaks may record the breakup of supercontinents at 2.2–2.0 Ga and again at 800–600 Ma, which resulted in increased numbers of partially closed marine basins, disruption of ocean currents, and increased hydrothermal vents at ocean ridges, all of which led to widespread anoxia.

Mineralogical and chemical composition and distribution of rare earth elements in clay-rich sediments from central Uganda.

Clay-rich sediments from the Kajjansi, Kitiko, Kitetika, and Ntawo valleys in central Uganda were analyzed for mineralogical and chemical composition, including the rare earth element (REE) contents. The valleys are filled with Quaternary to Recent alluvial and lacustrine sands, silt, and gravels, which formed from the bed rock metasediments of the Buganda-Toro System and from granitoid rocks that include rocks of the basement. The sediments are dominated by kaolinite and quartz, and minor phases include smectite, chlorite, and illite/muscovite. Whole rock chemistry shows that sediment samples rich in SiO 2 have low Al, Fe, Sc and Cr contents. The high chemical index of alteration (CIA) values (87 to 96), chemical index of weathering (CIW) values around 98 and low contents of the alkali and alkali earth elements of the clay-rich sediments suggest a relatively more intense weathering source area. Barium, Rb, Ca, and Mg were probably flushed out by water during sedimentation. The chondrite-normalized REE patterns of the clay-rich sediments show LREE enrichments and a negative Eu anomaly. The high chondritenormalized La/Yb ratios, and Gd/Yb ratios lower than 2.0, indicate that the sediments are enriched in the LREEs and are similar to typical post-Archean shales, such as Post-Archean Australian Shale (PAAS). The mineralogical composition, REE contents, and elemental ratios in these sediments suggest a provenance from mainly felsic rocks, with only minor contributions from basic sources. The basic sediments were most likely derived from metasedimentary rocks, such as muscovite-biotite schists, which are characteristic of the Buganda-Toro System rocks, whereas the felsic sediments are derivatives of granitoid rocks of the basement. The most significant geochemical finding is that despite intense weathering, which has affected most elements, the REE, Th, and Sc remain immobile.

Geochemistry of the Mesoproterozoic Lakhanda shales in southeastern Yakutia, Russia: implications for mineralogical and provenance control, and recycling

Shales of the Lakhanda Group of Late Mezoproterozoic age (1050–1000 Ma) from the southeastern Siberian craton in Russia have been analyzed for major elements and a number of trace elements, including the REE's. Shales along the Maya River formed as platform sediments in a deeper shelf facies, whereas, shales along the Belaya River formed in more active and open environments of an upper shelf carbonate ramp. The log of most elemental compositions to Al 2O 3 ratios are the same in the Maya and Belaya River samples, suggesting a similar source rock composition for rocks in the two areas. The log of SiO 2, MgO, Na 2O, K 2O, Rb, Ba, and Ni to Al 2O 3 ratios are significantly higher and the log of TiO 2 to Al 2O 3 ratios are significantly lower in shales from the Belaya River than the Maya River sections. The CIA (chemical index of alteration) is thus significantly lower in the Belaya shales than the Maya shales, suggesting less weathering of the in the Belaya shales than the Maya shales. The ICVs (Index of Compositional Variability=Fe 2O 3+K 2O+Na 2O+CaO+MgO+TiO 2/Al 2O 3) of the Lakhanda shales are less than 1, suggesting that they are compositionally mature and were likely dominated by recycling. Several samples have ICV>1, suggesting some first cycle input. The low K 2O/Al 2O 3 ratios of these shales suggest that minimal first cycle alkali feldspar was present in the initial source. Most shales of the Lakhanda plot parallel and along the A–K line in A–CN–K plots suggestive of intense chemical weathering (high CIA) and do not indicate any clear-cut evidence of K-metasomatism or direct weathering back to the original source. If K-metasomatism produced these rocks, then they could have formed from tonalites to basalts. If weathering produced these rocks then they could have been produced from varied amounts of mostly granodiorite to granite. Elemental ratios critical of provenance (La/Sc, La/Cr, La/Co, Th/Sc, Th/Cr, Th/Co, and Eu/Eu*) are not significantly different between the Maya River and Belaya River shales, and the ratios are similar to fine-fractions derived from the weathering of mostly granitoids and not basic rocks. The Eu/Eu*, Th/Sc and low K 2O/Al 2O 3 ratios of most shales suggest weathering from mostly a granodiorite source rather than a granite source, consistent with a source from old upper continental crust. Some samples at the bottom of the Belaya River section contain very low Eu/Eu* (0.35), suggesting significant input of first cycle detritus from highly differentiated granitoids similar to those from the Aldan Shield.

Major element geochemistry and paleoclimatology of the Permo-Carboniferous glacigene Dwyka Formation and postglacial mudrocks in southern Africa

A Chemical Index of Alteration (CIA) is used to quantify the degree of weathering of fine-grained materials in the glacigenic Permo-Carboniferous Dwyka Formation and succeeding stratigraphic units of southern Africa. Diamictites in the Dwyka Formation have low CIA values compatible with their glacigenic origin. Interbedded mudrocks show a wide scatter of values, suggesting that at least some of them were deposited during periods when ice had retreated from the source areas. Samples from the Prince Albert shale, which immediately overlies the Dwyka Formation, show high CIA values, suggesting a rapid transition to a much more intensive weathering regime (warmer climatic conditions?). Samples from the succeeding Whitehill shale have a low average value, so that weathering was once more inhibited, perhaps by a temporary return to a colder climate. The highest stratigraphic units sampled, the “upper shales”, have an average CIA value comparable to that of “average” shales, and suggesting “normal” weathering conditions. Major element geochemical data can supplement sedimentological and paleontological information and provide a valuable contribution to the detailed paleoclimatic interpretation of such glacigenic successions.

Samples and Principles

Major, trace element concentrations and Nd, Sr isotope ratios were measured in micro-drilled samples of a 2.37 Ga-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 1.2–1.3 Ga 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 (1.2 Ga) of the weathered samples coincides with the εNd (1.2 Ga) 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.