Δ18 OVSMOW Research Articles

The absolute δ18 O value for SLAP with respect to VSMOW reveals a much lower value than previously established.

SLAP is one of the two calibration materials for the isotopic water scale. By consensus the established δ18 O value is -55.5‰, although several expert laboratories measure significantly more negative δ18 OSLAP values. The real δ18 OSLAP value as such does not influence the isotopic water scale; however, knowledge of the size of isotopic scale contraction in stable isotope measurements is vital for second-order isotopes. This study describes the quantification of δ18 OSLAP with respect to δ18 OVSMOW . SLAP-like water was quantitatively mixed with highly 18 O-enriched water to mimic VSMOW. The 18 O concentration was determined using an electron ionization quadrupole mass spectrometer. The isotopic composition of the SLAP-like and VSMOW-like waters was measured using an optical spectrometer, alongside original VSMOW and SLAP. This study resulted in a much more negative δ18 O value for SLAP than expected. The averaged outcome of seven independent experiments is δ18 OSLAP = -56.33 ±0.03‰. There is a large discrepancy between the actual isotopic measurements of even the most carefully operating isotope laboratories and the true δ18 O value. Although this finding as such does not influence the use of the VSMOW-SLAP scale, it raises the intriguing question of what we actually measure with our instruments and why even a fully corrected measurement can be so far off. Our result has consequences for issues like the transfer of δ18 O from and to the VPDB scale, various fractionation factors, and Δ17 O. The absolute 18 O abundance for SLAP was calculated as (1887.98 ± 0.43) × 10-6 based on the absolute 18 O abundance of VSMOW and the presented δ18 OSLAP in this paper.

Progressive carbonation and Ca-metasomatism of serpentinized ultramafic rocks: insights from natural occurrences and hydrothermal experiments

Hydration, carbonation, and related metasomatism of mantle peridotite play a significant role in the global geochemical cycle. In this study, we combined an analysis of carbonated serpentinite with hydrothermal experiments on carbonation and Ca-metasomatism for samples from the Manlay ophiolite, southern Mongolia to investigate that carbonation mechanism of the serpentinite body after serpentinization. Samples show that the serpentinite was either transected by calcite and dolomite veins or was completely replaced by carbonates (calcite with minor dolomite) and quartz, in which the original mesh texture of serpentinite was preserved. Carbonation occurred after low-temperature serpentinization (lizardite/chrysotile), suggesting that carbonation occurred at temperatures lower than 300 ˚C. Calcite in the serpentinite showed δ13 CVPDB values ranging from -8.83 to -5.11 ‰ and δ18 OVSMOW from + 20.1 to + 24.4 ‰, suggesting that CO2 in the fluids could be derived from the degradation of organic material or methanotrophic processes rather than the origin of seafloor limestone. Three batch-type experiments, i.e., single step experiments (1) Olivine + NaHCO3,aq + CaCl2,aq and (2) Chrysotile + NaHCO3,aq + wollastonite (Ca source), and two steps experiment (3) Olivine carbonation and Ca-metasomatism, were conducted at 275 °C and 5.7 MPa to constrain the mechanism of calcite replacement of serpentinite. We found that calcite precipitated from the solution directly in the first two experiments, but replacement of serpentinite by calcite was not observed. In contrast, the third experiment caused the initial carbonation to form magnesite and then changed to calcite by later alteration. The natural occurrences and experiments revealed the possibility that the carbonation of olivine followed by Ca-rich fluid infiltration produced calcite in the carbonated serpentinite. Such Ca-metasomatism of Mg carbonates could easily occur in the ultramafic bodies and significantly affect the global carbon cycle.

The origin of carbonate cements in the Hildasay reservoir, Cambo Field, Faroe-Shetland Basin; clumped isotopic analysis and implications for reservoir performance

The early Eocene paralic sandstones of the Hildasay Member of the Flett Formation form the major oil-bearing reservoir in the Cambo Field, located in the Faroe-Shetland Basin. The sandstones locally contain calcite-cemented intervals that vary in thickness from decimetre to over 1 m. A calcite-cemented interval from well 204/10a-5 has been analysed petrographically and using clumped isotopes to determine its mode of formation and potential lateral extent. Petrographic analysis shows the cemented interval to consist of ferroan calcite, with a consistent dull red cathodoluminescence, suggesting a single phase of precipitation. The centre of the cemented interval comprises a finer grained unit with detrital kerogen and early sphaerosiderite, while the rest comprises homogeneous porosity-occluding ferroan calcite. The early sphaerosiderite in the centre is replaced by ferroan calcite with a high δ13 carbon isotopic signature (δ13 CVPDB = 7.53‰) suggesting that it formed during anaerobic methanogenesis or fermentation. Samples from the intermediate zone have a lower δ13 carbon isotopic composition (δ13 CVPDB = 0.72 to −3.68‰). In comparison the outer margin of the cemented unit has an even lower δ13 carbon isotopic signature (δ13 CVPDB = −15.5 to -15.9‰) more typical of a strong aerobic oxidation source. The oxygen isotopic signature of the cements is similar (δ18 OVPDB = −10.9 to −12.2‰). Analysis of the clumped isotopes suggest that the ferroan calcite formed at ∼40–50 °C, from pore waters that were predominantly meteoric in origin (δ18 OVSMOW = −4.8 to −5.7‰). Burial history modelling would suggest that the cements formed at a depth of ∼500–1000 m. Meteoric water was likely to have been introduced during the formation of the mid-Lutetian unconformity ∼45Mya, approximately 10 Mya after the sandstones were deposited. The model proposed is that ferroan calcite precipitation was initiated in the fine lag deposits that contained the kerogen and sphaerosiderite, and then grew outwards. If the model is correct, the cemented units should be restricted to this facies and consequently of limited lateral extent. Consequently, it is likely that the cemented intervals will have a limited impact upon the reservoir performance and are unlikely to act as major barriers to fluid flow.

Micromorphology and Textural Variations in the Ane Ghat Waterfall Tufa Deposits from Upland Deccan Traps and their Genesis

ABSTRACT Calc tufa associated with waterfalls and rapids from Ane Ghat in the western Deccan Traps have been studied on the basis of their field occurrence, morphology and microtextures. The calc tufas are semi-consolidated and show porous, spongy morphology and occasionally ‘soda straw’ structure. Blue green algae, moss and diatoms are the predominant biotic components of the calc tufa. The growth of such calcifying biota has played a constructive role in building calc tufa deposits at Ane Ghat. Mineralogically, the tufas are made of calcite and contain variable carbonate percentage (53 to 78%). Petrographically, they contain fragments of basalt, devitrified glass, plagioclase, quartz, agate, zeolites, etc. set in a clay-rich and clay-poor micrite. Irregular pore spaces and voids are partially filled with spary calcite lined by thin layers of clay. SEM images of pristine calc tuffa surfaces reveal micro-morphological and textural features that dominantly reflect precipitation. Images of insoluble residue of phases like glass, plagioclase and zeolites indicate some dissolution and corrosive features indicating post depositional diagenetic processes. δ18O V-SMOW values range from 27.03 to 28.92‰ and δ13C V-PDB values of -3.58 to -6.02‰ are recorded for the Ane Ghat samples. The calc tufas were precipitated from waters at paleo-temperatures of 16.3 to 27.1°C. The calc tufa from Ane Ghat represents biological and physico-chemical calcification processes in response to variability in calcium carbonate saturation and water chemistry in temperate to semi-arid climate.

Jurassic ore-forming systems during the Tethyan orogeny: constraints from the Shamlugh deposit, Alaverdi district, Armenia, Lesser Caucasus

The Shamlugh polymetallic deposit located in the Alaverdi mining district, Armenia, is hosted by Middle Jurassic magmatic rocks of the Somkheto–Karabakh island arc, Lesser Caucasus. Lithogeochemistry and radiogenic isotope geochemistry indicate a subduction setting with an evolution from tholeitiic to transitional Middle Jurassic magmatism to calc-alkaline to high-K calc-alkaline Late Jurassic magmatism. A progressively more juvenile mantellic input in the magmatic source is noted. Ore consists of stratabound massive sulfide ore bodies, ore-bearing breccia, stockwork-style mineralization, and disseminated ore hosted by a pyroclastic host rock unit below a subvolcanic rhyolitic sill. Towards depth, the stratabound ore is connected to subvertical high-grade sulfide veins hosted by basaltic andesitic, andesitic, and dacitic tuff and lava breccia. Ore minerals include principally pyrite and chalcopyrite with minor sphalerite and hematite. Hydrothermal alteration consists of quartz, chlorite, carbonate, and sericite in the footwall pyroclastic rock, tuff, and lava breccia, which are also crosscut by late-stage carbonate and barite veins. The rhyolitic sill has been altered by silica and sericite and contains rare sulfides. Sulfur isotope values for sulfides range from + 0.7 to + 4.6‰ and suggest an essentially magmatic source for sulfur. Strontium isotopic compositions of hydrothermal carbonates and sulfates (0.70507–0.70660), sulfur isotopes of sulfates (+ 16.8 to + 17.3), and carbon and oxygen isotopic compositions of late-stage carbonates (δ18O VSMOW + 15.9 to + 17.6‰; δ13C VPDB − 1.8 to − 0.8‰) not only are permissive with seawater incursion in the ore-forming hydrothermal system but can also be attributed to fluid–rock interaction with hidden metamorphic basement rocks and Jurassic magmatic rocks. The lead isotopic composition of sulfides (206Pb/204Pb 18.505 to 18.586, 207Pb/204Pb 15.611 to 15.636, 208Pb/204Pb 38.516 to 38.640) implies leaching of the Middle Jurassic host rocks as the main source of Pb. LA-ICP-MS U–Pb zircon dating of the hanging wall rhyolitic sill affected by hydrothermal alteration yielded an age of 155.0 ± 1.0 Ma and provides a maximum age for mineralization. Ore formation at Shamlugh is younger than the immediate Middle Jurassic host rocks and is related to the Late Jurassic calc-alkaline and high-K calc-alkaline magmatic evolution. Late Jurassic magmatism includes a tonalite dated by LA-ICP-MS U–Pb zircon geochronology at 152.9 ± 0.7 Ma that hosts the 146-Ma-old Teghout porphyry deposit. In the absence of other age constraints, the genetic relationship of the polymetallic Shamlugh deposit with the Teghout porphyry deposit remains open to question. Nevertheless, hydrothermal minerals and alteration paragenesis and the lithologically and structurally controlled ore body geometry at Shamlugh are consistent with a low to intermediate sulfidation epithermal model.

Fluid inclusion and O–H–C isotopic constraints on the origin and evolution of ore-forming fluids of the Cenozoic volcanic-hosted Kuh-Pang copper deposit, Central Iran

The Kuh-Pang copper deposit (2.8 Mt @ 1.65% Cu, 0.52 g/t Au, 34 g/t Ag) is a volcanic-hosted epithermal deposit in the central part of the Late Eocene-Oligocene Urmia-Dokhtar Magmatic Arc of Iran. Three stages of fluid evolution are identified at the Kuh-Pang deposit: (i) early pre-ore stage: with homogenization temperature of 205–372 °C (298 ± 45 °C), salinity of 11.3 ± 2.9 wt% NaCl equiv., fluid trapped at ∼200 bars, (ii) main-ore stage: with homogenization temperature of 175–310 °C (253 ± 43 °C), salinity of 2.1–12.5 wt% NaCl equiv. at ∼150 bars, and (iii) late post-ore stage: with homogenization temperatures of 148 to 231 °C (199 ± 24 °C), and salinity of 1.1–9.8 wt% NaCl equiv. These data record an evolution of mineral precipitation from deeper (>2 km) to shallower environments (<500 m). Fluids trapped in the early (quartz I) and main-ore stages (quartz II) yielded δ18Ofluid values of +5.7 to +5.9‰ and +3.2 to +3.8‰, and δDfluid values of −77 to −41‰ and −84 to −60‰, respectively. These data indicate a major magmatic fluid source, with slightly increasing mixing of meteoric waters. Calcite in the late post-ore stage has δ13CV–PDB values of −5.8 to −5.6‰ and δ18OV–SMOW values of 8.2–8.6‰, with calculated δ18Ofluid and δ13Cfluid values of −1.0 to −1.4‰ and −5.4 to −5.2‰ respectively, that are also consistent with a predominantly magmatic carbon source and a significant fluid mixing by meteoric waters.

N and O isotope (δ15 Nα , δ15 Nβ , δ18 O, δ17 O) analyses of dissolved NO3- and NO2- by the Cd-azide reduction method and N2 O laser spectrometry.

The nitrogen and oxygen (δ15 N, δ18 O, δ17 O) isotopic compositions of NO3- and NO2- are important tracers of nutrient dynamics in soil, rain, groundwater and oceans. The Cd-azide method was used to convert NO3- or NO2- to N2 O for N and triple-O isotopic analyses by N2 O laser spectrometry. A protocol for laser-based headspace isotope analyses was compared with isotope ratio mass spectrometry. Lasers provide the ability to directly measure 17 O anomalies which can help discern atmospheric N sources. δ15 N, δ18 O and δ17 O values were measured on N/O stable isotopic reference materials (IAEA, USGS) by conversion to N2 O using the Cd-azide method and headspace N2 O laser spectrometry. A 15 N tracer test assessed the position-specific routing of N to the α or β positions in the N2 O molecule. A data processing algorithm was used to correct for isotopic dependencies on N2 O concentration, cavity pressure and water content. NO3- /NO2- nitrogen is routed to the 15 Nα position of N2 O in the azide reaction; hence the δ15 Nα value should be used for N2 O laser spectrometry results. With corrections for cavity pressure, N2 O concentration and water content, the δ15 NαAIR , δ18 OVSMOW and δ17 OVSMOW values (‰) of international reference materials were +4.8 ± 0.1, +25.9 ± 0.3, +12.7 ± 0.2 (IAEA NO3 ), -1.7 ± 0.1, -26.8 ± 0.8, -14.4 ± 1.1 (USGS34) and +2.6 ± 0.1, +57.6 ± 1.2, +51.2 ± 2.0 (USGS35), in agreement with their values and with the isotope ratio mass spectrometry results. The 17 O excess for USGS35 was +21.2 ± 9‰, in good agreement with previous results. The Cd-azide method yielded excellent results for routine determination of δ15 N, δ18 O and δ17 O values (and the 17 O excess) of nitrate or nitrite by laser spectrometry. Disadvantages are the toxicity of Cd-azide chemicals and the lack of automated sampling devices for N2 O laser spectrometers. The 15 N-enriched tracer test revealed potential for position-specific experimentation of aqueous nutrient dynamics at high 15 N enrichments by laser spectrometry, but exposed the need for memory corrections and improved spectral deconvolution of 17 O.

The Early Jurassic Chang'anbu porphyry Cu–Mo deposit in Northeastern China: Constraints from zircon U–Pb geochronology and H–O–S–Pb stable isotopes

Central Jilin is tectonically subordinate to the Lesser Xing'an Range–Zhangguangcai Range polymetallic metallogenic belt, an important region for Cu–Mo prospecting in NE China. Dozens of large‐scale molybdenum deposits, including Fu'anbu, Chang'anbu, Jide, and Dashihe, have been recently discovered in Central Jilin, whereas porphyry Cu or Cu–Mo deposits have not been found to date. One such example of an intracontinental porphyry Cu–Mo deposit is the Chang'anbu Cu–Mo deposit in Shulan, Jilin Province, hosted in early Yanshanian rocks. Here in this contribution, we described detailed geology based on our field observation and conducted a comparative study on the metallogenic epoch and the ore‐forming sources of the Chang'anbu Cu–Mo deposit by using zircon U–Pb dating and H–O–S–Pb stable isotopes. We propose that the Chang'anbu deposit is rare in the Lesser Xing'an Range–Zhangguangcail Range metallogenic belt and differs from other porphyry deposits that consist solely of Mo, indicating a unique mechanism of metallogenesis. Zircon U–Pb ages indicate emplacement of a granite pluton which is the main metallogenetic rock body during the Early Jurassic (182.10 ± 1.20 Ma). The pluton is spatially and temporally associated with Cu–Mo mineralization and led to large‐scale porphyry Cu–Mo mineralization during the early Yanshanian. Sulphur, Pb, H, and O isotope data suggest that magma generated by subduction of the Paleo‐Pacific oceanic crust was the main ore‐forming source of this deposit (206Pb/204Pb = 18.046–18.734; Pb207/Pb204 = 15.502–15.655; δ34S = 0.3–2‰; δDV–SMOW = −102.2–93.4‰; δ18OV–SMOW = 9.1–11.6‰). The Chang'anbu porphyry Cu–Mo deposit is representative of large‐scale polymetallic metallogenic events in Central Jilin that resulted from magmatism related to crust and mantle melting during the early Yanshanian.

Scientific study of 13C/12C carbon and 18O/16O oxygen stable isotopes biological fractionation in grapes in the Black Sea, Don Basin and the Western Caspian regions

The report presents the results of a study of carbon and oxygen stable isotopes in carbohydrates and intracellular water of red and white grapes of 2016 wine-growing season in the Crimean peninsula areas, South-west coast of the Greater Caucasus, the Don basin and the Western Caspian region. The mass concentration of reducing sugars in the studied grape samples has been from 17.5 to 25.0 g/100 ml, titrated acids concentration (based on tartaric acid) – from 6.0 to 9.1 g/l, the buffer capacity 34.1–63.2 mg-Eq/l. Red and white wine made from respective grapes contained from 0.5 to 3.6 g/l of residual sugar; from 11.1 to 14.5% ethanol by volume; buffer capacity was 35.2–52.6 mg-Eq/l. It has been found that the δ13 CVPDB values for carbohydrates of red and white grape varieties as a result of biological fractionation of carbon isotopes in the agro-climatic conditions of plant growth for the studied geographical areas are ranging from − 26.74 to − 20.74‰ (the Crimean peninsula); from − 27.31 to − 21.58‰ (South West Coast of the Greater Caucasus), from − 27.33 to − 24.73‰ (Don Basin) and from − 26.64 to − 23.17‰ (West Caspian). The δ13 CVPDB values for ethanol of the red and white dry wines range from − 28.52 to − 24.26‰ (the Crimean peninsula); from − 29.23 to − 24.52‰ (South West Coast of the Greater Caucasus); from − 28.97 to − 26.22‰ (Don Basin); from − 29.14 to − 25.22‰ (Western Caspian). Compared with the surface water and groundwater (averages from δ18 OVSMOW − 13.90 to − 6.38‰) and with precipitation (averages from δ18 OVSMOW − 10.30 to − 9.04‰) the δ18 OVSMOW values in intracellular water of grapes are the following: for the Crimean peninsula grapes, from 0.40 to 4.97‰; the South West Coast of the Greater Caucasus, from -2.11 to 6.29‰; the Don Basin, from − 2.21 to 6.26‰; the Western Caspian, from − 0.24 to 1.44‰. It has been noted that in conditions of water shortage caused by low rainfall or lack of rainfall and irrigation, grapes are prone to the accumulation of “heavy” 13 C carbon isotope due to changes in isotopic exchange with the environment through a partial reduction in the intensity of transpiration and photosynthesis.

Grapes from the geographical areas of the Black Sea: Agroclimatic growing conditions and evaluation of stable isotopes compositions in scientific study

The report considers the agroclimatic conditions in the Black Sea districts of cultivation and processing of grapes - the Black Sea Lowland, the Crimean Peninsula and the South-west coastal areas of the Greater Caucasus. The IRMS/SIRA techniques - Flash combustion (FC-IRMS/SIRA) & Isotopic equilibration (EQ-IRMS/SIRA) - were first applied for the evaluation of carbon and oxygen isotopes ratios in the components of grapes from the Crimean Peninsula. The 13 C/12 C ratios were studied by the FC-IRMS/SIRA in carbohydrates and organic acids in authentic samples of 8 grape varieties from the 2015 harvest. The EQ-IRMS/SIRA was applied to measure the 18 O/16 O ratios in intracellular water of grapes. The measured δ13 CVPDB value ranges from − 25.01 to − 21.01‰ (for carbohydrates), and from − 25.09 to − 21.30‰ (for organic acids). To evaluate the extent of biological isotope fractionation the 18 O/16 O ratios were measured in ground water and water of atmospheric precipitates from the Crimean Peninsula. Compared to ground (δ18 OVSMOW from − 10.85 to − 8.14‰) and atmospheric (average δ18 OVSMOW − 2.85‰) waters, the intracellular water of Crimean grape varieties is found to be enriched with 18 O isotope. The δ18 OVSMOW value of the grape intracellular water varies from 2.34 to 5.29‰ according to agroclimatic conditions of the season in 2015.

Precambrian banded iron formations in the North China Craton: Silicon and oxygen isotopes and genetic implications

Banded iron formations (BIFs) are Precambrian chemical marine sedimentary formations that record major transitions of chemical composition, and oxidation–reduction state of oceans at the time of their deposition. In this paper, we report silicon and oxygen isotope compositions of a variety of BIFs from the North China Craton (NCC) in order to deduce the mechanism of their formation. Quartz in the various types of BIFs from the NCC are generally depleted in 30Si, where δ30SiNBS-28 values range from −2.0‰ to −0.3‰ (average, −0.8‰), similar to δ30SiNBS-28 values measured from modern submarine black chimneys and sinters. The δ18OV-SMOW values of quartz in the BIFs are relatively high (8.1‰–21.5‰; average, 13.1‰), similar to those of siliceous rock formed by hydrothermal activities. The δ30SiNBS-28 values of quartz in magnetite bands are commonly lower than those of quartz in adjacent siliceous bands within the same sample, whereas δ18OV-SMOW values are higher in the magnetite bands. A negative correlation is observed between δ30SiNBS-28 and δ18OV-SMOW values of quartz from siliceous and magnetite bands in BIF from Fuping, Hebei Province. The isotopic compositions of silicon and oxygen of quartz in BIFs provide insights for the formation mechanisms of silicon–iron cyclothems in BIFs. After the silicon- and iron-rich hydrothermal solution was injected onto the seabed, the abrupt temperature drop caused oversaturation of silicic acid, resulting in rapid precipitation of SiO2 and deposition of siliceous layers. Ferric hydroxide was precipitated later than SiO2 because of low free-oxygen concentration in the ocean bottom. Progressive mixing of hydrothermal solution with seawater caused a continuous drop in temperature and an increase in Eh values, resulting in gradual oxidation of hydrothermal Fe2+ and deposition of iron-rich layers. In summary, each silicon–iron cyclothem marks a large-scale submarine hydrothermal exhalation. The periodic nature of these exhalations resulted in the formation of regular silicon–iron cyclothems. The widespread distribution of BIFs indicates that volcanism and submarine hydrothermal exhalation were extensive; the low δ30SiNBS-28 and high δ18O V-SMOW values of the BIFs indicate that the temperature of seawater was relatively high at the time of BIF formation, and that concentrations of Fe2+ and H4SiO4 in seawater were saturated.

Onshore to offshore trends in carbonate sequence development, diagenesis and reservoir quality across a land-attached shelf in SE Asia

Although isolated Miocene buildups in SE Asia commonly form prolific hydrocarbon reservoirs, their equivalents on clastic-dominated land-attached shelves remain poorly known and underexplored. Here, onshore to offshore trends in carbonate development and reservoir quality are assessed across the NW Borneo shelf through study of surface outcrops and subsurface wells. A multidisciplinary programme of fieldwork, petrography and geochemical analyses allowed evaluation of spatio-temporal variations in deposition, diagenesis and pore system development, together with an assessment of controlling influences. In addition to field logging and sample collection >200 samples were studied via transmitted light, cathodoluminescent and scanning electron microscopy together with stable isotopic characterisation (O, C and Sr).Carbonates developed as localised low-, and higher-relief buildups, as well as more continuous sheet-like deposits in near-coast to shelf margin positions. Molluscs, corals, larger benthic foraminifera and coralline algae are common constituents. Most samples show evidence for marine micritisation, and just in shelf margin positions isopachous cements. However, burial diagenesis predominates in the form of compaction, neomorphism, fracturing, late leaching and dolomitisation. Near-coastal carbonates commonly contain siliciclastics, as do some shelf margin deposits that interdigitate with, or are covered in siliciclastics. Some early, probable meteoric leaching affected inner shelf deposits prior to pervasive neomorphic to blocky/poikilotopic calcite cement formation. On the basis of δ18O V-PDB values of −4.5 to −7.9‰ equivalent to δ18O V-SMOW values of 0 to −4‰ at 25–40 °C and δ13C V-PDB values of −0.6 to +1.6‰ cementation probably reflects alteration from terrestrial groundwaters in meteoric aquifers derived from the humid landmass of Borneo. Despite this cementation, moderate energy inner- to mid-shelf grainstones from the core of mounded carbonates still retain, or have enhanced porosity (<8%) over their lower energy counterparts (<4% porosity). Retention of primary porosity and/or late burial dissolution (often associated with saddle dolomite formation) enhancing predominantly primary and minor secondary porosity is key to reservoir quality development in outer-shelf deposits. Best porosity (<20–35%) is in high energy grainstones and rudstones from outer-shelf to shelf-margin positions that experienced minimal clastic influx, most commonly from backstepping to aggradational carbonate sequences. Although stable isotopes for shelf margin calcite cements are consistent with precipitation from marine-derived fluids (δ18O V-PDB values of −3.6 to −5.4‰), those for the late dolomites are suggestive of fluids of meteoric origin (δ18O V-PDB values of −5.2 to −7.4‰ equivalent to values of −0.3 to −6.3‰ V-SMOW at 40–60 °C). Critical factors for reservoir quality development in carbonates from siliciclastic-dominated shelves in the equatorial tropics are: (1) development and preservation of primary porosity, (2) cementation associated with meteoric aquifers draining large humid equatorial landmasses, and (3) burial leaching and fluid pathways.

Evidence from Dwyka tillite cobbles of Archaean basement beneath the Kalahari sands of southern Africa

We have found a new source of information about what lies beneath the Kalahari sands. The Kheis and Rehoboth Provinces of southern Africa were thought to be underlain by either an ~1800Ma orogenic belt, or a northern branch of the ~1200Ma Namaqua–Natal Province. Glacial diamictites of the Permocarboniferous Dwyka Group exposed at Rietfontein west of the Kalahari sands carry cobbles plucked from the bedrock by the ice sheet which covered the Gondwana supercontinent about 300Ma ago. Despite altered mineralogy, the cobbles are not severely altered geochemically. Their normative mineral compositions give classifications as trondhjemites and granites, supported by rare earth element diagrams. Microbeam U–Pb zircon dating of the granitic cobbles shows that they contain no evidence of crustal growth or orogeny at either 1800 or 1200Ma. Rather they testify to the presence of 2500 to 2900Ma Archaean trondhjemitic and granitic crust beneath the Kalahari, with a lesser ~2050Ma granite component. The pebble assemblages from the diamictites we sampled lack the diagnostic banded iron formation (BIF), stromatolitic limestone and other supracrustal pebbles which characterise diamictites derived from the Kaapvaal Craton, thus we envisage shorter transport distances and derivation from the region now beneath the Kalahari sands. Three of the Archaean granite cobbles have unusual less-than-mantle zircon oxygen isotope values around +3 (δ18O VSMOW), which may reflect interaction of their source with high-temperature, originally meteoric water before melting to produce the granites. The mafic cobbles described in a companion paper are much younger and are related to intrusions of the 1.1Ga Umkondo Large Igneous Province, probably located on the Kalahari Line or Rehoboth Province.Five trondhjemitic granites from the westernmost outcrops of the Kaapvaal Craton were dated, the oldest being 3061±9Ma and four others between 2882±7Ma and 2854±7Ma, reflecting the cratonisation of the Kimberley Terrane. Four of the Archaean Dwyka cobbles we dated are younger than the 2.7Ga Kaapvaal cover sequence and are thus too young to be derived from the craton.All the Dwyka cobbles described here are most likely derived from either the Rehoboth Province or the Kalahari Line with origins from the Kheis Province, Kaapvaal Craton, or further afield considered unlikely. We envisage the Rehoboth Province to consist of an Archaean core supplemented by Palaeoproterozoic granitoids, which was joined to the Kaapvaal Craton at an early stage of crustal development and played an important role during later tectonic events. This has important implications not only for the tectonic framework and assembly of Southern Africa, but also for exploration for diamonds and other ore deposits.