Strontium Isotope Study Research Articles

Dissolution methods for strontium isotope stratigraphy: Guidelines for the use of bulk carbonate and phosphorite rocks

Diverse carbonate dissolution methods have been applied to bulk carbonate rocks in order to target least altered components for strontium isotope stratigraphy (SIS). This is especially important for Precambrian and Cambrian studies for which no suitable skeletal material is available. One proven method for bulk limestones involves the removal of up to a third of the powdered sample using an acid pre-leach before partial dissolution of the rest of the sample using a weak acid or acid buffer solution. We applied a similar technique to dissolve a range of lithologies (limestone, dolostone and phosphorite) and compared the strontium isotopic composition of various leaches to contemporaneous seawater. Our results vindicate this approach and allow us to conclude that some dolomitic and phosphatic rock components may retain a near-primary seawater 87Sr/86Sr composition once contaminant strontium from secondary calcite and other phases has been removed. Commonly applied trace element (e.g. Mn/Sr, Mg/Ca) and isotopic (C, O) screening cut-offs were also examined in the light of these results, and proved unreliable particularly in the case of dolostones and partially dolomitized limestones. We recommend that rigid sample selection and sequential leaching procedures be applied to all SIS studies of bulk materials and propose a general protocol for strontium isotope studies on marine authigenic rocks which may also be applicable to rare earth element and other geochemical studies of marine authigenic minerals in the future.

インド、始生代ダールワール岩体、チトラドゥルガ片岩帯における変成堆積岩の炭素・硫黄・酸素・ストロンチウム同位体研究

インド、始生代ダールワール岩体、チトラドゥルガ片岩帯における変成堆積岩の炭素・硫黄・酸素・ストロンチウム同位体研究

Fracture-controlled paleohydrogeology in a basement-cored, fault-related fold: Sheep Mountain Anticline, Wyoming, United States

New geochemical and microstructural data constrain the origins and pathways of paleofluids during the growth of Sheep Mountain Anticline, Wyoming, United States. Oxygen, carbon, and strontium isotope studies were performed on prefolding and fold-related calcite veins and their sedimentary host rocks and combined to fluid inclusion microthermometry results. We show that most of the cements precipitated from Paleogene meteoric fluid. Stable isotopes and fluid inclusion homogenization temperatures further indicate that most veins were mineralized from upward moving fluids after these fluids were heated at depth (T > 110°C). This implies that fluids migrated along the basement thrust underlying the fold and/or at the base of the cover. Above the fault tip, the fluids circulated rapidly in the diffuse synfolding (and early folding) fracture network. The zone of preferential migration of the warm fluids is currently located in the backlimb of the fold, which supports some of the previously published structural interpretation of the subsurface. This study also highlights the potential of combined fracture analysis and geochemical analyses of paleofluid flows in fractures to constrain both the deformation history and the fluid flow during basement-involved shortening in Laramide-style forelands.

In search of the australopithecines

Evidence from strontium isotope ratios preserved in fossil teeth provides a glimpse into the group dynamics and ranging habits of the australopithecines that can be compared with the patterns for modern primates. See Letter p.76 How do you estimate the home ranges and land-use habits of extinct species? One method is to measure the strontium isotope content of fossil teeth, because strontium isotope ratios are a good indicator of the source of the water that the creature drank during its life. This watermark is determined by the underlying geology. A strontium-isotope study of Australopithecus africanus and Paranthropus robustus specimens from South Africa shows that the smaller individuals, inferred to have been females, ranged further than the males. This suggests that females tended to move away from their natal groups and joined others, whereas males tend to stay at home, a behaviour characteristic of humans and chimpanzees, but not of most gorillas and other primates.

Strontium and Lead Isotopic Study of the Carbonate‐hosted Xujiashan Antimony Deposit from Hubei Province, South China: Implications for its Origin

AbstractThe Xujiashan antimony deposit is hosted by marine carbonates of the Upper Sinian Doushantuo and Dengying Formations in Hubei Province, South China. Our Sr isotopic data from pre‐ and syn‐mineralization calcites that host the mineralization show that the pre‐mineralization calcite displays a narrow range of 87Sr/86Sr ratios (0.7096 to 0.7097), similar to the ratios of the Sinian seawater, and high Sr concentrations (2645 to 8174 ppm). In contrast, the syn‐mineralization calcite exhibits low Sr concentrations (785 to 2563 ppm) and high 87Sr/86Sr ratios (0.7109 to 0.7154), which is interpreted as the result of addition of radiogenic strontium during the antimony mineralization. The study of Sr isotopes suggests that their Sr component to the pre‐mineralization calcite derived directly from the host rocks (i.e. the Sinian marine carbonates), while radiogenic 87Sr for the syn‐mineralization calcite derived from the underlying Mesoproterozoic Lengjiaxi Group basement through hydrothermal fluid circulation along the major fault that hosts the mineralization.The Pb isotopic ratios of stibnite are subdivided into two groups (Group A and Group B), Group A is characterized by higher radiogenic lead, with 206Pb/204Pb = 18.874 to 19.288, 207Pb/204Pb = 15.708 to 15.805, and 208Pb/204Pb = 38.642 to 39.001. Group B shows lower lead isotope ratios (206Pb/204Pb = 17.882 to 18.171, 207Pb/204Pb = 15.555 to 15.686, and 208Pb/204Pb = 37.950 to 38.340). The single‐stage model ages of Group A are mainly negative or slightly positive values (‐258 to 3 Ma), while those of Group B range from 636 to 392 Ma, with an average of 495 ± 65 Ma. In addition, there are positive linear correlations among Pb isotopic ratios. These results suggest that the lead of Group A stibnite was mainly derived from the Sinian marine carbonates, and that of Group B stibnite from the underlying Lengjiaxi Group basement. This conclusion is consistent with the results of the Sr isotopes.These results indicate that the Xujiashan deposit is not syngenetic sedimentary and in situ reworked origin as previously considered. The metal (mainly Sb) of this deposit was not only derived from the Sinian host rocks, but also partly derived from the underlying Mesoproterozoic Lengjiaxi Group basement.

Isotopes as tracers of sources of lead and strontium in aerosols (TSP & PM 2.5) in Beijing

Even after its being phased out in gasoline in the late 90s, lead (Pb) is still present at relatively high levels in the atmosphere of Beijing, China (0.10–0.18 μg m −3). Its origin is subject to debate as several distinct sources may contribute to the observed pollution levels. This study proposes to constrain the origin(s) of Pb and strontium (Sr) in aerosols, by coupling both Pb and Sr isotope systematics. The characterisation of the main pollution sources (road traffic, smelters, metal refining plants, coal combustion, cement factories, and soil erosion) shows that they can unambiguously be discriminated by the multi-isotope approach ( 206Pb/ 204Pb and 87Sr/ 86Sr). The study of total suspended particulates (TSP) and fine particles (PM 2.5) from Beijing and its vicinity indicates that both size fractions are controlled by the same sources. Lead isotopes indicate that metal refining plants are the major source of atmospheric lead, followed by thermal power stations and other coal combustion processes. The role of this latter source is confirmed by the study of strontium isotopes. Occasionally, emissions from cement plants and/or input from soil alteration are isotopically detectable.

Tracing the interaction of acid mine drainage with coal utilization byproducts in a grouted mine: Strontium isotope study of the inactive Omega Coal Mine, West Virginia (USA)

Abstract In order to ameliorate acidic discharge, the inactive Omega Coal Mine, West Virginia was partially filled by injection of a grout consisting of 98% coal utilization byproducts (CUB), including fluidized bed combustion ash and fly ash, and 2% Portland cement. In this study, discharge chemistry and Sr isotope ratios were determined to identify and quantify the extent of interaction between mine waters and the CUB–cement grout. Eight sampling sites were monitored around the downdip perimeter of the mine. The major and trace element chemistry of the discharges was generally not sufficient to distinguish between discharges that interacted with grout and those that did not. Elements that showed the most separation include K and As, which were elevated in some waters that interacted with CUB–cement grout. In contrast, the Sr isotope ratios clearly distinguished discharges from grouted and non-grouted areas. Discharges that bypassed the grouted portions had 87 Sr/ 86 Sr ratios ranging from 0.71510 to 0.71594, while two discharges that interacted with grout had ratios in the range of 0.71401–0.71456. The Treatment Inlet, which includes both grouted and ungrouted discharges, yielded intermediate isotopic ratios. Leaching experiments on CUB–cement grout, coal and surrounding rocks are consistent with the isotopic trends observed in the discharges. Based on these results, waters that interacted with grout received 30–40% of their Sr from the CUB–cement grout material. These results suggest that the grout material is chemically eroding at a rate of approximately 0.04% per year. This novel application of the Sr isotope system illustrates its ability to sensitively track and quantify fluid interaction with coal and CUB-based grout.

Sr-isotope stratigraphy of the Upper Jurassic of central Portugal (Lusitanian Basin) based on oyster shells

Strontium isotope stratigraphy was performed on oyster shells from the Late Jurassic of the Lusitanian Basin (central Portugal). This represents the first approach to obtain numerical ages for these strata. The new chronostratigraphic data provide a more precise age determination of several units. After a basin-wide hiatus sedimentation in the Late Jurassic is proven in the Cabo Mondego and Cabacos formations to resume as early as the Middle Oxfordian. The Alcobaca formation can be placed in the latest Late Oxfordian to Late Kimmeridgian, while data from the upper part of the Abadia Formation indicate an Early to Late Kimmeridgian age. The Farta Pao formation ranges from the latest Kimmeridgian to the latest Tithonian. The largely synchronous Sobral, Arranho I, and Arranho II members are overlain by the late Early to Late Tithonian Freixial Member. The brief, local carbonate incursion of the Arranho I member marks the Kimmeridgian–Tithonian boundary. Oysters are shown once more to be suitable for strontium isotope studies. Their calcitic shells are often unaffected by diagenesis. In particular for marginal marine Jurassic and Cretaceous strata, where belemnites are usually absent, oysters may serve as a valuable tool for isotope stratigraphy.

Sr isotope diversity of hot spring and volcanic lake waters from Zao volcano, Japan

The ratio of 87Sr/ 86Sr was measured from different water samples of thermal/mineral (hot spring as well as crater lake) and meteoric origins, in order to specify the location and to verify the detailed model of a volcano-hydrothermal system beneath Zao volcano. The ratio showed a trimodal distribution for the case of thermal/mineral water: 0.7052–0.7053 (Type A, Zao hot spring), 0.7039–0.7043 (Type B, Okama crater lake and Shin-funkiko hot spring), and 0.7070–0.7073 (Type C, Gaga, Aone, and Togatta hot springs), respectively. However, in comparison, the ratio was found to be higher for meteoric waters (0.7077–0.7079). The water from the central volcanic edifice (Type B) was found to be similar to that of nearby volcanic rocks in their Sr isotopic ratio. This indicates that the Sr in water was derived from shallow volcanic rocks. The 87Sr/ 86Sr ratio for water from the Zao hot spring (Type A) was intermediate between those of the pre-Tertiary granitic and the Quaternary volcanic rocks, thus suggesting that the water had reacted with both volcanic and granitic rocks. The location of the vapor–liquid separation was determined as the boundary of the pre-Tertiary granitic and the Quaternary volcanic rocks by comparing the results of this strontium isotopic study with those of Kiyosu and Kurahashi [Kiyosu, Y., Kurahashi, M., 1984. Isotopic geochemistry of acid thermal waters and volcanic gases from Zao volcano in Japan. J. Volcanol. Geotherm. Res. 21, 313–331.].

Migration in the Nile Valley during the New Kingdom period: a preliminary strontium isotope study

The value of strontium isotope analysis in identifying immigrants at numerous archaeological sites and regional areas has been demonstrated by several researchers, usually by comparing 87Sr/ 86Sr values of human tooth enamel and/or bone with the local strontium isotope signature determined by faunal and environmental samples. This paper examines the feasibility of using 87Sr/ 86Sr ratios to investigate residential mobility in the Nile Valley region, specifically at the New Kingdom period (∼1050–1400 BC) archaeological site of Tombos (ancient Nubia). Archaeological and textual information regarding this period indicates that immigrant Egyptians and local native Nubians were likely interacting at this site during a period of Egyptian colonial occupation. The results of this study suggest that non-local individuals may be distinguished from locals using 87Sr/ 86Sr values and that colonial agents in the Tombos population were probably both local native Nubians and immigrants.

C, O, S and Sr isotope studies on the genesis of Fe-carbonate and barite mineralizations in the Attepe iron district (Adana, Southern Turkey)

The Attepe district consists of Precambrian, Lower–Middle Cambrian, Upper Cambrian–Lower Ordovician and Mesozoic formations. It contains several iron deposits and occurrences. Three types of iron-mineralizations can be distinguished in the area; (i) Sedimentary Fe-sulfide in Precambrian bituminous metapelitic rocks, and Fe-oxides in Precambrian metasandstones (SISO), (ii) vein-type Fe-carbonate and oxides composed of mainly siderite, ankerite and hematite including barite in Lower–Middle Cambrian metacarbonates of the Çaltepe Formation (HICO), (iii) karstic Fe-oxides and hydroxides essentially in the Lower–Middle Cambrian metacarbonates and the unweathered Fe-carbonates (KIO). The latter type is more widespread and located at the upper parts of the most important mineable iron deposits like Attepe deposit. Oxygen-, carbon-, sulfur- and strontium-isotope studies have been performed on siderites and barites in the vein-type ores, and on calcites in the recrystallized Çaltepe Limestones to investigate the sources and formation mechanism of primary ore-forming constituents. The δ 13C values of siderites and calcites in limestones of the Çaltepe Formation range from −10.10‰ to −8.20‰, and from −0.8‰ to 2.30‰. Both carbonate minerals show δ 18O values between 17.50–18.30‰ and 16.20–23.00‰, respectively. The δ 13C and δ 18O isotopic variations do not indicate any direct or linear relations between siderites and limestones. However, it is possible that the carbon and oxygen isotopic compositions of carbonate minerals could be changed to some extent, when limestones were subjected to hydrothermal processes or thermal alterations during metamorphism. The isotopic values of barites display 32.40–38.30‰ for δ 34S and 12.20–14.70‰ for δ 18O. The strontium isotope ratios (0.717169–0.718601) of barites and the sulfur isotope compositions of barites and pyrites suggest that there are no direct linkages of ore-forming compounds neither with a magmatic source nor with sedimentary pyrite formations in the Precambrian bituminous shales of the Attepe formation. According to the field observations and the stable isotope data, siderites and ankerites should be formed by interaction between iron-rich hydrothermal fluids and Çaltepe limestones, whereas isotope ratios of barites indicate that they were formed by mixing of sulfur-rich meteoric waters and deeply circulated hydrothermal solutions.

Kimberlite melts rich in alkali chlorides and carbonates: A potent metasomatic agent in the mantle

Kimberlite magmas, as the deepest probe into Earth's mantle (>150 km), can supply unique information about volatile components (hydrogen, carbon, chlorine, sulfur) in mantle-derived melts and fluids. All known kimberlite rocks are not suitable for studies of mantle volatiles because of their pervasive postmagmatic alteration; however, this study discusses an exceptionally fresh group I kimberlites (<0.5 wt% H2O) from the Udachnaya-East diamondiferous pipe in Siberia. Kimberlite groundmass, in addition to euhedral olivine and calcite, is extremely enriched (at least 8 wt%) in water-soluble alkali chlorides, alkali carbonates, and sulfates (ratio 5:3:1), and often shows immiscibility textures. A primary magmatic origin of alkali chlorides and alkali carbonates is confirmed by the study of strontium isotopes in the water- and dilute acid-leachates of the groundmass (Sr-87/Sr-86 = 0.7069 and 0.7050) that contrast with much more radiogenic isotope composition of the Cambrian platform sedimentary rocks and the Udachnaya-East mine-site brines. Melt inclusions in groundmass olivine, composed of halite, sylvite, alkali-Ca carbonates, phlogopite, olivine, and CO2 fluid, were used to determine the composition and evolution of the kimberlite melt prior to emplacement. Melt inclusions show immiscibility between chloride and carbonate liquids at <600oC in heating stage experiments. The chloride and carbonate enrichment in the kimberlite parental magma suggests the presence of a powerful agent for chemical modifications (metasomatism) in the mantle and crust.

Heterogeneity of initial 87Sr/86Sr ratios within a single pluton: evidence from apatite strontium isotopic study

Magmatic apatite in samples of the unmetamorphosed undeformed Cretaceous pluton of the Habu Granodiorite, southwestern Japan, was examined to determine its viability as an indicator of initial 87Sr/86Sr heterogeneity in magmatic bodies. The Habu Granodiorite is a zoned pluton in the Ryoke metamorphic belt of southwestern Japan. A published Rb–Sr whole-rock isochron age of 124.2±10.8 Ma is significantly older than Th–U–total Pb monazite ages of ca. 86 Ma. A Rb–Sr mineral isochron of 90.0±2.5 Ma for a sample of granodiorite from the margin of the pluton has an initial 87Sr/86Sr (SrI) of 0.7056±0.0001, similar to a 87Sr/86Sr of 0.70539 for coexisting euhedral apatite. Similarly, a Rb–Sr mineral isochron of 91.4±2.5 Ma for a sample of adamellite from the center of the pluton has an SrI of 0.7068±0.0001, corresponding to a 87Sr/86Sr ratio of 0.70673–0.70684 for three fractions of coexisting euhedral apatite. The uniformity of 87Sr/86Sr values of apatite in individual samples and accordance of 87Sr/86Sr values of apatite and SrI from the mineral isochron suggest that magmatic apatite can be considered as a reliable indicator of SrI in granitic rocks. The Sr isotopic compositions of euhedral apatite, coupled with those determined through the mineral isochrons and whole-rock data, indicate that SrI varies between samples from 0.7054 to 0.7074. This provides direct evidence of SrI heterogeneity within a compositionally zoned pluton. Such heterogeneity can produce spurious whole-rock age estimates in granitic plutons, even where the true age of a pluton is uniform within measurable error. Assimilation cannot account for chemical differences between granodiorite and adamellite as well as Sr-isotopic heterogeneity. It is likely that the heterogeneity of SrI is caused by magma source materials.

Rare earth element and strontium isotopic study of seamount-type limestones in Mesozoic accretionary complex of Southern Chichibu Terrane, central Japan: Implication for incorporation process of seawater REE into limestones.

Ishimaki and Tahara limestones occur as exotic blocks juxtaposed in the Mesozoic (Jurassic) accretionary complex of Southern Chichibu Terrane in eastern Aichi Prefecture, central Japan. They are supposed to be of the seamount-type limestone, since they have no terrigenous materials and are intimately associated with greenstones. REE (rare earth elements) and Sr isotopic studies for the limestones have been made in order to know their geochemical characteristics, ages and origins. Their 87Sr/86Sr ratios, when referred to the seawater 87Sr/86Sr curve and relevant geological data, suggest that Ishimaki and Tahara limestones are the late Permian and the Carboniferous to the Early Permian, respectively. Two greenstone fragments found inside the Ishimaki limestone block and one greenstone sample associated with Tahara limestone block, resemble the Hawaiian alkali basalt in their REE and Y patterns. This is supporting the idea that the limestone blocks may be parts of reef limestones on ancient volcanic seamounts. All the limestone samples, except three unusual Tahara ones, show seawater REE and Y signatures in their chondrite-normalized patterns. Their REE/Ca ratios, however, are 102-104 times as high as those ratios of modern biogenic carbonates like corals and the seawater. Accordingly, seawater REE and Y were incorporated into the limestones, when originally biogenic carbonates transformed into inorganic calcite and its secondary growths occurred in diagenesis in contact with sufficient seawater. This view is favored by the reported REE partition experiment between calcite overgrowths and seawater solution. The seawater Ce anomaly as a function of water depth in the modern ocean is a key to infer the water depth of the REE and Y incorporation. The Ce anomalies given by log(Ce/Ce∗) for about a half of Ishimaki samples and most of Tahara ones are between -0.5 and -0.2, which are compatible with the shallow water origin. Another half of Ishimaki samples, however, have log(Ce/Ce∗) values between -0.7 and -1.0, suggesting moderately deep waters (ca. 500-1000 m deep or more). This may reflect such a situation that water depths of REE incorporation into the seamount-type limestones are generally greater than the depositional water depths of original biogenic carbonates because of the fate of limestone-capped volcanic seamounts decided by the oceanic plate motion.

Strontium and neodymium isotopic study of Libyan Desert Glass: Inherited Pan‐African age signatures and new evidence for target material

Abstract— Libyan Desert Glass (LDG) is an impact‐related, natural glass of still unknown target material. We have determined Rb‐Sr and Sm‐Nd isotopic ratios from seven LDG samples and five associated sandstones from the LDG strewn field in the Great Sand Sea, western Egypt. Planar deformation features were recently detected in quartz from these sandstones. 87Sr/86Sr ratios and ∍‐Nd values for LDG range between 0.71219 and 0.71344, and between –16.6 and –17.8, respectively, and hence are distinct from the less radiogenic 87Sr/86Sr ratios of 0.70910–0.71053 and ∍‐Nd values from –6.9 to –9.6 for the local sandstones from the LDG strewn field. Previously published isotopic ratios from the Libyan BP and Oasis crater sandstones are generally incompatible with our LDG values. LDG formation undoubtedly occurred at 29 Ma, but neither the Rb‐Sr nor the Sm‐Nd isotopic system were rehomogenised during the impact event, as we can deduce from Pan‐African ages of ˜540 Ma determined from the regression lines from a total of 14 LDG samples from this work and the literature. Together with similar Sr and Nd isotopic values for LDG and granitoid rocks from northeast Africa west of the Nile, these findings point to a sandy matrix target material for the LDG derived from a Precambrian crystalline basement, ruling out the Cretaceous sandstones of the former “Nubian Group” as possible precursors for LDG.

Short-Term Storage

GEOLOGY About 770,000 years ago, a massive volcanic eruption in Long Valley, California, formed a caldera 20 kilometers across and deposited volcanic ash that formed the Bishop Tuff. One particularly provocative result from the study of strontium and other isotopes in the ash was that the magma that

Mid-Neoproterozoic (∼830–750 Ma) isotope stratigraphy of Australia and global correlation

Carbon and strontium isotope studies of numerous drillcore and outcrop sections within individual Australian Neoproterozoic basins, in a tectonic and sedimentologic framework, have allowed the construction of isotopic records for the interval 830–750 Ma. This, combined with the discovery of a restricted stratigraphic range for the acritarch Cerebrosphaera buickii in mid-Neoproterozoic strata of Australia, makes possible correlation with successions in Canada, Namibia and Spitsbergen. That part of the mid-Neoproterozoic succession in Spitsbergen which contains C. buickii was previously considered younger than the Sturtian glacial epoch, but this study has indicated that the Akademikerbreen Group is older than that. A correlation scheme is proposed along five tie lines, and is based on a correlation between Australia and Canada at ∼830 Ma, and Australia, Spitsbergen and Namibia at ∼760 Ma. The lowest ever recorded seawater 87Sr/ 86Sr ratios in the upper Shaler Supergroup of Canada (0.70561–0.70670) compare with ratios in evaporites of the ∼830 Ma Bitter Springs Formation, Amadeus Basin, Australia (0.70568–0.70720). Comparable δ 13C carb records support a correlation. At about 760 Ma, a correlation is made between the heaviest mid-Neoproterozoic δ 13C carb values of 7.2‰ (upper Burra Group, Adelaide Rift Complex) and 8.1‰ (Kanpa Formation, western Officer Basin) in Australia, 8.5‰ in Spitsbergen (Backlundtoppen Formation, Akademikerbreen Group), and 8.5‰ in Namibia (Ombombo Subgroup, Congo Craton). The stratigraphic range of C. buickii in Australia and Spitsbergen supports this correlation. A compilation of δ 13C carb and 87Sr/ 86Sr records between about 830 and 750 Ma from Australia, Canada, Namibia and Spitsbergen reveals numerous negative excursions, which are not associated with preserved glacial deposits. It is necessary, therefore, to consider additional models to the ‘Snowball Earth’ to explain the large and rapid carbon isotopic excursions.

Strontium-isotope studies of “brown water” (organic-rich groundwater) from Denmark

Groundwater from some Quaternary and upper Tertiary aquifers in western Jutland, Denmark, is heavily influenced by "brown water", i.e., groundwater with a high content of naturally occurring dissolved organic matter. Stable-isotope analyses (18O/16O and D/H) indicate that both Quaternary and upper Tertiary aquifers are dominated by meteoric water. However, strontium-isotope analyses make it possible to distinguish between water samples from Quaternary and Miocene aquifers. Relatively low 87Sr/86Sr ratios, i.e., ∼0.7083, in Sr-rich water samples indicate that the majority of dissolved Sr in groundwater from Miocene as well as Quaternary strata is probably derived from Miocene marine skeletal carbonate matter in the subsurface. This situation is probably the result of Quaternary glacial reworking of upper Tertiary material and/or hydraulic contact between Quaternary and Miocene aquifers. A positive correlation between Sr contents and non-volatile organic carbon indicates that the remarkably high contents of organic matter recorded in these aquifers almost certainly are derived from Miocene sources as well.

The Mediterranean water body in the late Messinian: interpreting the record from marginal basins on Sicily

The stratigraphic record of climatic and palaeoceanographic changes in the Mediterranean during the late Messinian is controversial. On Sicily, sedimentary facies show a steady transgression, suggesting that sub-basins were hydrodynamically linked to a larger water body. Here we test this hypothesis using strontium isotope studies from Messinian materials collected from a range of sites in the Caltanissetta Basin of central Sicily. The strata include a regionally regressive ‘First Cycle’ of early-mid-Messinian age and a younger, transgressive ‘Second Cycle’. These cycles are separated by an inter-regional unconformity which may be correlated with base-level low stand in the Mediterranean. 87 Sr/ 86 Sr ratios for all First Cycle gypsum fall within the expected global marine composition (0.70891–0.70897). All Second Cycle analyses fall within a grouping of significantly lower values (0.70868–0.70878), a surprisingly tight but discrepant grouping for data collected from shells of a brackish water fauna and from gypsum. Analyses from these different Second Cycle materials are statistically indistinguishable. These results indicate that the strontium isotopic composition of waters from different sub-basins on Sicily are indistinguishable regardless of salinity during the late Messinian. Therefore these basins must have mixed with a larger, homogenized reservoir which we infer was the ancestral Mediterranean. Thus circum-Mediterranean basins may indeed chart regional palaeoceanographic and climatic events. By the end of Messinian times the base-level of the Mediterranean was within the range of the world's oceans but the water-body probably had a distinctly different but internally homogeneous strontium isotopic composition.

Basement-hosted quartz-barite sulfide veins in the French Alps; a record of alpine tectonic fluid expulsion in the external crystalline massifs; structural, fluid inclusion, and isotope (S and Sr) evidences

Quartz-barite sulfide veins hosted by a late Variscan monzonite in the polymetamorphic basement of the Aiguilles Rouges massif, western Alps, France, have been the subject of a combined structural, microthermometric fluid inclusion, and sulfur and strontium isotope study. The mineralized veins have a maximum age of 40 Ma and were emplaced along vertical tension fractures formed, or possibly reopened, during the Mesoalpine thrust phase. Two mineralization stages have been recognized: a major stage of quartz-barite veins with minor and irregularly distributed sulfides and sulfosalts, and a minor crosscutting stage of quartz-stibnite veins. Fluid inclusion microthermometry, homogeneous 87 Sr/ 86 Sr ratios of the barite, and the texture and mineralogical composition of the polymetallic quartz-barite veins indicate that they were formed mainly due to the cooling of a single aqueous fluid with a salinity around 9 wt percent NaCl equiv. Contemporaneous pressure variations cannot be excluded. Sulfur isotope thermometry yields formation temperatures between 255 degrees and 270 degrees C, and combined with the fluid inclusion isochores they indicate pressures of 0.45 to 1.55 kbars, corresponding to a depth of 3.6 (super +2.3) (sub -1.9) km assuming lithostatic conditions. Sulfur in the fluid was dominated by H 2 S which was oxidized at the ore site due to cooling of the fluid. The vein barites are enriched in 87 Sr with respect to the host rocks and indicate that the strontium and the barium were leached from deeper lithologies. After the emplacement of the quartz-barite veins, the hydrothermal system was opened toward shallower structural levels, allowing the ingress of meteoric water. The late quartz-stibnite veins are associated with a distinct and warmer CO 2 -bearing fluid. We conclude that the H 2 O-NaCl ore-bearing fluid originated at a depth of about 15 km by dewatering of Mesozoic sedimentary rocks during the collisional-thrust Mesoalpine phase. Subhorizontal thrust planes between the basement crystalline rocks and overlying sedimentary units inhibited mixing or regeneration of the system with superficial fluids. Subvertical fluid migration was restricted to faults in the polymetamorphic basement. Late Alpine extension tectonics allowed the upward migration of deep CO 2 -bearing fluids and also favored the influx of surface fluids that had remained isolated from the H 2 O-NaCl hydrothermal system.