Diagenetic mass transport in the southern San Joaquin basin, California, U.S.A.: Implications from the strontium isotopic composition of modern pore fluids

Three types of chemically and isotopically distinct pore fluids from the southern San Joaquin basin previously recognized by J.B. Fisher and J.R. Boles also have distinctive ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations. Meteoric fluids have stable isotopic compositions which lie on or near the meteoric water line and low chlorinities. Sr concentrations are between 0.01 and 2.6 mg l⁻¹, and ⁸⁷Sr/⁸⁶Sr ratios range from 0.7061 to 0.7078. Diagenetically modified connate marine fluids have δD-and δ¹⁸O-values more positive than −35‰ and 0‰, respectively, and have chlorinities generally comparable to seawater. Sr concentration are much higher than the meteoric group (16–198 mg l⁻¹), although the ⁸⁷Sr/⁸⁶Sr ratios (0.7070–0.7081) are not distinctive. Mixed meteoric-modified connate fluids have δD, δ¹⁸O and chlorinity intermediate between the meteoric and modified connate groups. Sr concentrations are also intermediate, between 16 and 22 mg l⁻¹, but ⁸⁷Sr/⁸⁶Sr ratios (0.7080–0.7087) are generally more radiogenic than either the meteoric or modified connate groups.

All of the fluids have ⁸⁷Sr/⁸⁶Sr ratios comparable to or lower than Tertiary seawater. Alteration of detrital plagioclase is the probable origin of the low isotopic ratios. Mass-balance calculations based on the Sr data suggest that essentially no transport of Sr occurred during diagenesis of sandstones containing modified connate pore fluids, while large amounts of Sr have been transported out of meteoric reservoirs by fluid flow. The chemically anomalous mixed meteoric-modified connate fluids contain the most radiogenic strontium in the basin. These fluids are spatially associated with major faults, and may represent clay mineral dehydration waters which have been transported upward from greater depth.

These results suggest that the three types of fluids identified by Fisher and Boles represent three distinct mass transport regimes: a largely stagnant deep-basin system containing modified connate pore fluids; an actively recharging meteoric system along the basin flanks; and a third system restricted to the southern basin which may be characterized by largescale cross-formational fluid flow, rather than dilution by meteoric waters.     

Upper Cambrian carbonate sequences of the Argentine Precordillera and the Steptoean C-Isotope positive excursion (SPICE)

Carbon and Sr-isotope profiles in Upper Cambrian platformal carbonate Formations in the Precordillera, western Argentina (Zonda, La Flecha and La Silla Formations), were constructed for three representative sections: (a) Quebrada de la Flecha, Eastern Precordillera, (b) Cerro La Silla, Central Precordillera and (c) Quebrada de La Angostura, northern part of the Central Precordillera.

At Quebrada de La Angostura, upper part of the La Flecha Formation, δ¹³C_carb varies continuously up-section from − 2.0 to + 5.6‰ (PDB) and records the SPICE anomaly (+ 5‰) reported for the first time in South America. The peak of this excursion is characterized by intercalated 2 m thick beds of black shale with marl and limestone that record the onset of a sea-level change.

The Steptoean Zonda Formation dolomites at the Quebrada de la Flecha exhibit a total δ¹³C range from − 2.7 to + 0.6‰ with discrete positive anomaly about 200 m from the transition to the overlying Sunwaptan La Flecha Formation. Pronounced C-isotope anomaly (− 5.6‰) is observed in the La Flecha Formation at about 300 m below the transition to the La Silla Formation.

At the Cerro La Silla section, the Zonda Formation exhibit δ¹³C values of  − 1‰, increasing slightly at the transition to the La Flecha Formation (− 1 to 0‰). The transition of the La Flecha to the La Silla Formations is characterized by alternation of black shales and dolomitic limestone with a discrete positive C-isotope excursion, probably corresponding to the SPICE.

At the Quebrada de La Flecha, ⁸⁷Sr/⁸⁶Sr for the Zonda Formation varies from 0.70924 to 0.70955 and for the La Flecha Formation from 0.70908 to 0.70942. At Cerro La Silla this ratio varies from 0.70914 to 0.70923 for the La Flecha Formation, and from 0.70898 to 0.70980 for the La Silla Formation. At the Quebrada de La Angostura, ratios for the La Flecha carbonates range from 0.70918 to 0.70993. The overall variation of ⁸⁷Sr/⁸⁶Sr is consistent with globally reported Upper Cambrian seawater values at ca. 500 Ma.

The unambiguous record of SPICE in the La Flecha Formation at the Quebrada de La Angostura supports a Steptoean age for its deposition and allows precise local, regional, and global stratigraphic correlation. The pronounced negative C-isotope excursion recorded in the La Flecha Formation carbonates at the Quebrada de La Flecha is likely equivalent to that registered in Sunwaptan carbonates of North America and Australia, and might be tied to a global event, as a valuable tool in stratigraphic correlation (SNICE, acronym for Sunwaptan negative isotope carbon excursion).     

Isotopic equilibrium/disequilibrium in granites, metasedimentary rocks and migmatites (Damara orogen, Namibia)—a consequence of polymetamorphism and melting

The overwhelming part of the continental crust in the high-grade part of the Damara orogen of Namibia consists of S-type granites, metasedimentary rocks and migmatites. At Oetmoed (central Damara orogen) two different S-type granites occur. Their negative ε_Nd values (− 3.3 to − 5.9), moderately high initial ⁸⁷Sr/⁸⁶Sr ratios (0.714–0.731), moderately high ²⁰⁶Pb/²⁰⁴Pb (18.21–18.70) and ²⁰⁸Pb/²⁰⁴Pb (37.74–37.89) isotope ratios suggest that they originated by melting of mainly mid-Proterozoic metasedimentary material. Metasedimentary country rocks have initial ε_Nd of − 4.2 to − 5.6, initial ⁸⁷Sr/⁸⁶Sr of 0.718–0.725, ²⁰⁶Pb/²⁰⁴Pb ratios of 18.32–18.69 and ²⁰⁸Pb/²⁰⁴Pb ratios of 37.91–38.45 compatible with their variation in Rb/Sr, U/Pb and Th/Pb ratios. Some migmatites and residual metasedimentary xenoliths tend to have more variable ε_Nd values (initial ε_Nd: − 4.2 to − 7.1), initial Sr isotope ratios (⁸⁷Sr/⁸⁶Sr: 0.708–0.735) and less radiogenic ²⁰⁶Pb/²⁰⁴Pb (18.22–18.53) and ²⁰⁸Pb/²⁰⁴Pb (37.78–38.10) isotope compositions than the metasedimentary rocks. On a Rb–Sr isochron plot the metasedimentary rocks and various migmatites plot on a straight line that corresponds to an age of c. 550 Ma which is interpreted to indicate major fractionation of the Rb–Sr system at that time. However, initial ⁸⁷Sr/⁸⁶Sr ratios of the melanosomes of the stromatic migmatites (calculated for their U–Pb monazite and Sm–Nd garnet ages of c. 510 Ma) are more radiogenic (⁸⁷Sr/⁸⁶Sr: 0.725) than those obtained on their corresponding leucosomes (⁸⁷Sr/⁸⁶Sr: 0.718) implying disequilibrium conditions during migmatization that have not lead to complete homogenization of the Rb–Sr system. However, the leucosomes have similar Nd isotope characteristics than the inferred residues (melanosomes) indicating the robustness of the Sm–Nd isotope system during high-grade metamorphism and melting. On a Rb–Sr isochron plot residual metasedimentary xenoliths show residual slopes of c. 66 Ma (calculated for an U–Pb monazite age of 470 Ma) again indicating major fractionation of Rb/Sr at c. 540 Ma. However, at 540 Ma, these xenoliths have unradiogenic Sr isotope compositions of c. 0.7052, indicating depleted metasedimentary sources at depth. Based on the distinct Pb isotope composition of the metasedimentary rocks and S-type granites, metasedimentary rocks similar to the country rocks are unlikely sources for the S-type granites. Moreover, a combination of Sr, Nd, Pb and O isotopes favours a three-component mixing model (metasedimentary rocks, altered volcanogenic material, meta-igneous crust) that may explain the isotopic variabilty of the granites. The mid-crustal origin of the different types of granite emphasises the importance of recycling and reprocessing of pre-existing differentiated material and precludes a direct mantle contribution during the petrogenesis of the orogenic granites in the central Damara orogen of Namibia.     

Neo-Proterozoic rift-related syenites (Northern Damara Belt, Namibia): Geochemical and Nd–Sr–Pb–O isotope constraints for mantle sources and petrogenesis

Major element, trace element and Nd–Sr–Pb–O isotope data for a suite of Neo-Proterozic, pre-orogenic, rift-related syenites from the Northern Damara orogen (Namibia) constrain their sources and petrogenesis. New U–Pb ages obtained on euhdreal titanite of inferred magmatic origin constrain the age of intrusion of the Lofdal and Oas syenites to ca. 750 Ma compatible with previous high-precision zircon analyses from the Oas complex. Major rock types from Lofdal and Oas are mildly sodic nepheline-normative and quartz-normative syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Primitive samples from Lofdal and Oas show depletion of Rb, K and Th relative to Ba and Nb together with variable negative anomalies of P and Ti on a primitive mantle-normalized diagram. Evolved samples from Oas develop significant negative Ba, Sr, P and Ti anomalies and positive U and Th anomalies mainly as a function of crystal fractionation processes. The lack of a pronounced negative Nb anomaly in samples from Lofdal suggests that involvement of a crustal component is negligible. For the nepheline-normative samples from Lofdal, the unradiogenic Sr and radiogenic Nd isotope composition and low δ¹⁸O values suggest derivation of these samples from a moderately depleted lithospheric upper mantle with crustal-like U/Pb ratios (⁸⁷Sr/⁸⁶Sr: 0.7031–0.7035, ε Nd: ca. + 1, δ¹⁸O: 7‰, ²⁰⁶Pb/²⁰⁴Pb: ca.18.00, ²⁰⁷Pb/²⁰⁴Pb: 15.58–15.60). Primitive samples of the Oas quartz-normative syenites have identical isotope characteristics (⁸⁷Sr/⁸⁶Sr: 0.7034, ε Nd: ca. + 1, δ¹⁸O: 6.5‰, ²⁰⁶Pb/²⁰⁴Pb: ca.18.00, ²⁰⁷Pb/²⁰⁴Pb: 15.59) whereas more differentiated samples have higher ⁸⁷Sr/⁸⁶Sr ratios (0.709–0.714), slightly higher δ¹⁸O values (7.0–7.1‰), less radiogenic ε Nd values (− 1.1 to − 1.4) and more radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios up to 18.27. These features together with model calculations using Sr–Nd–Pb isotopes suggest modification of a primary syenite magma by combined AFC processes involving ancient continental crust. In this case, high Nb abundances of the parental syenite liquid prevent the development of significant negative Nb anomalies that may be expected due to interaction with continental crust.     

Isotopic constraints on fluid/rock ratios in carbonate rocks: Barite-sulfide mineralization in the Schwaz Dolomite, Tyrol (Eastern Alps, Austria)

The significance of isotopic data on constraining the physical conditions of fluid-rock interaction and mineralization processes in carbonate rocks is discussed, based on the example of barite-tetrahedrite mineralization in Lower Devonian platform carbonates of the Western Greywacke Zone (Tyrol, Austria). Available strontium, oxygen, carbon and sulfur isotopic data are complemented with oxygen isotopic data for barite. Barites are homogeneous in δ¹⁸O_V-SMOW and δ³⁴S_CDT with values of + 15.4 and + 23.5‰, respectively. Their ⁸⁷Sr/⁸⁶Sr ratios vary between 0.7128 and 0.7113 for the first generation and between 0.7117 and 0.7123 for younger remobilization. The dolomitic host rock shows a significant variation in Sr, O and C isotopic composition between non-mineralized and mineralized zones: ⁸⁷Sr/⁸⁶Sr ratios vary between 0.7076 and 0.7133, δ¹⁸O_V-SMOW-values between +28.11 and +20.65‰, and δ¹³C_PDB-values between −1.15 and + 3.06‰. Fluid/rock volume ratios on the order of 1.3–3.2 are calculated for open-system behaviour by modelling Sr, O and C isotopic shifting capacities. The isotope data combined with other geological evidence support the following genetic model: Subsequent to synsedimentary sulfide mineralization during an Early Devonian rifting stage, collision tectonics in Carboniferous time led to the expulsion of Ba- and Sr-rich orogenic brines, which evolved from metamorphic fluids consisting essentially of H₂O and some CH₄, into an external sedimentary fold-and-thrust belt. The brines remobilized the synsedimentary sulfides, mixed with meteoric waters in the platform carbonates, reacted with evaporitic horizons and finally caused the recrystallization of dolomite and the precipitation of Sr-rich barite in structurally weak zones at 70–130°C. During the later Alpine orogeny supergene oxidation products were formed, and sulfates, sulfides and carbonates were further remobilized into late faults and fractures.     

An evaporitic facies in Neoproterozoic post-glacial carbonates: The Gifberg Group, South Africa

A geological study of the hitherto poorly described Neoproterozoic Gifberg Group, with emphasis on lithogeochemistry and O, C and Sr isotopic composition of the carbonate-dominated Widouw Formation (Vredendal Outlier, westernmost South Africa) revealed that the entire group is an equivalent of the relatively well constrained Port Nolloth Group in the external, paraautochthonous part of the Pan-African Gariep Belt further north. Thus, the Vredendal Outlier can be regarded as the southern extension of the Port Nolloth Zone. Two diamictite units are recognised in the Vredendal Outlier, which can be correlated respectively with the c. 750 Ma Kaigas Formation diamictite and the 583 Ma, syn-Gaskiers Numees Formation diamictite in the Gariep Belt proper. The dominating carbonate unit in the studied area is post-glacial with respect to the older of the two diamictite units. The combined textural, structural and geochemical evidence suggests that parts of the variably dolomitised limestone succession represent former evaporite beds. Sedimentation in a restricted, very shallow and proximal basin led to a wide range in C isotope ratios (δ¹³C_PDB from − 4.2 to + 4.8‰), very high Sr concentrations (derived from original anhydrite) and initial ⁸⁷Sr/⁸⁶Sr ratios that are significantly higher (0.70785) than those of coeval seawater. As C and Sr isotopes are commonly used for chemostratigraphic correlation, and high Sr concentrations in Neoproterozic carbonates are often interpreted as evidence of former aragonite, the findings of this study should be used as warning against uncritical use of geochemical and isotopic parameters for describing ancient seawater composition. Thus C and Sr isotope ratios alone in Neoproterozoic carbonates may be less powerful proxies of ancient seawater composition, and high Sr contents are not necessarily indicative of an “aragonite sea”, as previously inferred.     

The mineral isotope composition of two Precambrian carbonatite complexes from the Kola Alkaline Province – Alteration versus primary magmatic signatures

We investigated the isotope composition (O, C, Sr, Nd, Pb) in mineral separates of the two Precambrian carbonatite complexes Tiksheozero (1.98 Ga) and Siilinjärvi (2.61 Ga) from the Karelian–Kola region in order to obtain information on Precambrian mantle heterogeneity. All isotope systems yield a large range of variations. The combination of cathodoluminescence imaging with stable and radiogenic isotopes on the same samples and mineral separates indicates various processes that caused shifts in isotope systems. Primary isotope signatures are preserved in most calcites (O, C, Sr, Pb), apatites (O, Sr, Nd), amphiboles (O), magnetites (O), and whole rocks (Sr, Nd).

The primary igneous C and O isotope composition is different for both complexes (Tiksheozero: δ¹³C = − 5.0‰, δ¹⁸O = 6.9‰; Siilinjärvi: δ¹³C = − 3.7‰, δ¹⁸O = 7.4‰) but very uniform and requires homogenization of both carbon and oxygen in the carbonatite melt. The lowest Sr isotope ratios of our carbonates and apatites from the Archaean Siilinjärvi (0.70137) and the Palaeoproterozoic Tiksheozero (0.70228) complexes are in the range of bulk silicate earth (BSE). Positive ε_Nd values of the two carbonatites point to very early Archaean enrichment of Sm/Nd in the Fennoscandian mantle. No HIMU components could be detected in the two complexes, whereas Tiksheozero carbonatites give the first indication of Palaeoproterozoic U depletion for Fennoscandia.

Sub-solidus exchange processes with water during emplacement and cooling of carbonatites caused an increase in the oxygen isotope composition of some carbonates and probably also an increase of their ⁸⁷Sr/⁸⁶Sr ratio. A larger increase of initial Sr isotope ratios was found in carbonatized silicic rocks compared to carbonatite bodies. The Svecofennian metamorphic overprint (1.9–1.7 Ga) caused reset of Rb/Sr (mainly mica) and Pb/Pb (mainly apatite) isochron systems.     

Lower crustal melting and the role of open-system processes in the genesis of syn-orogenic quartz diorite–granite–leucogranite associations: constraints from Sr–Nd–O isotopes from the Bandombaai Complex, Namibia

The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous hornblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U–Pb zircon age of 540±3 Ma.

Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (_{Nd (540 Ma)}=−6.3 to −19.8; δ¹⁸O=9.0–11.6‰) but rather low and uniform initial Sr isotope compositions (⁸⁷Sr/⁸⁶Sr_initial=0.70794–0.70982). Two leucogranites and one aplite have higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.70828–0.71559), but similar initial _Nd (−11.9 to −15.8) and oxygen isotope values (10.5–12.9‰). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K₂O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (<0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material.     

Pre-Cenozoic intra-plate magmatism along the Central Andes (17–34°S): Composition of the mantle at an active margin

Sr–Nd–Pb isotope ratios of alkaline mafic intra-plate magmatism constrain the isotopic compositions of the lithospheric mantle along what is now the eastern foreland or back arc of the Cenozoic Central Andes (17–34°S). Most small-volume basanite volcanic rocks and alkaline intrusive rocks of Cretaceous (and rare Miocene) age were derived from a depleted lithospheric mantle source with rather uniform initial ¹⁴³Nd/¹⁴⁴Nd ( 0.5127–0.5128) and ⁸⁷Sr/⁸⁶Sr ( 0.7032–0.7040). The initial ²⁰⁶Pb/²⁰⁴Pb ratios are variable (18.5–19.7) at uniform ²⁰⁷Pb/²⁰⁴Pb ratios (15.60 ± 0.05). A variety of the Cretaceous depleted mantle source of the magmatic rocks shows elevated Sr isotope ratios up to 0.707 at constant high Nd isotope ratios. The variable Sr and Pb isotope ratios are probably due to radiogenic growth in a metasomatized lithospheric mantle, which represents the former sub-arc mantle beneath the early Palaeozoic active continental margin. Sr–Nd–Pb isotope signatures of a second mantle type reflected in the composition of Cretaceous (one late Palaeozoic age) intra-plate magmatic rocks (¹⁴³Nd/¹⁴⁴Nd  0.5123, ⁸⁷Sr/⁸⁶Sr  0.704, ²⁰⁶Pb/²⁰⁴Pb  17.5–18.5, and ²⁰⁷Pb/²⁰⁴Pb  15.45–15.50) are similar to the isotopic composition of old sub-continental lithospheric mantle of the Brazilian Shield.

Published Nd and Sr isotopic compositions of Mesozoic to Cenozoic arc-related magmatic rocks (18–40°S) represent the composition of the convective sub-arc mantle in the Central Andes and are similar to those of the Cretaceous (and rare Miocene) intra-plate magmatic rocks. The dominant convective and lithospheric mantle type beneath this old continental margin is depleted mantle, which is compositionally different from average MORB-type depleted mantle. The old sub-continental lithospheric mantle did not contribute to Mesozoic to Cenozoic arc magmatism.     

Sr–Nd–Pb isotopic compositions of the Kovdor phoscorite–carbonatite complex, Kola Peninsula, NW Russia

The Sr, Nd and Pb isotopic compositions for the Kovdor phoscorite–carbonatite complex (PCC), Kola Peninsula, NW Russia, have been determined to characterize the mantle sources involved and to evaluate the relative contributions of a plume and subcontinental lithospheric mantle in the formation of the complex. The Kovdor PCC is a part of the Kovdor ultramafic–alkaline–carbonatite massif, and consists of six intrusions. The initial isotopic ratios of the analyzed samples, calculated at 380 Ma, display limited variations: ε_Nd, + 2.0 to + 4.7; ⁸⁷Sr/⁸⁶Sr, 0.70319 to 0.70361 (ε_Sr, − 12.2 to − 6.2); ²⁰⁶Pb/²⁰⁴Pb, 18.38 to 18.74; ²⁰⁷Pb/²⁰⁴Pb, 15.45 to 15.50; ²⁰⁸Pb/²⁰⁴Pb, 37.98 to 39.28. The Nd and Sr isotope data of the Kovdor PCC generally fit the patterns of the other phoscorites and carbonatites from the Kola Alkaline Province (KAP), but some data are slightly shifted from the mixing line defined as the Kola Carbonatite Line, having more radiogenic ⁸⁷Sr/⁸⁶Sr ratios. However, the less radiogenic Nd isotopic compositions and negative Δ7/4 values of Pb isotopes of the analyzed samples exclude crustal contamination, but imply the involvement of a metasomatized lithospheric mantle source. Isotopic variations indicate mixing of at least three distinct mantle components: FOZO-like primitive plume component, EMI-like enriched component and DMM-like depleted component. The isotopic nature of the EMI- and DMM-like mantle component observed in the Kovdor samples is considered to be inherited from metasomatized subcontinental lithospheric mantle. This supports the previous models invoking plume–lithosphere interaction to explain the origin of the Devonian alkaline carbonatite magmatism in the KAP.     

Oxygen, carbon, and strontium isotope geochemistry of diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav Massif, Kazakhstan

Diamond-bearing carbonate rocks from Kumdy-Kol, Kokchetav massif, Kazakhstan, were strongly altered by fluids flowing through fractures and infiltrating along grain boundaries during exhumation. Alteration includes retrogradation of high-grade silicate assemblages by hydrous minerals, replacement of diamond by graphite and of dolomite by calcite. Diamond-bearing carbonate rocks are among the most intensely altered isotopically with δ¹⁸O_VSMOW values as low as +9‰, δ¹³C_VPDB=−9‰, and ⁸⁷Sr/⁸⁶Sr as high as 0.8050. Evidence of isotopic equilibration between coexisting dolomite and high-Mg calcite during ultrahigh-pressure metamorphism (UHPM) is preserved only rarely in samples isolated from infiltrating fluids by distance from fractures. Isotopic heterogeneity and isotopic disequilibrium are widespread on a hand-specimen scale. Because of this lack of homogeneity, bulk analyses cannot provide definitive measurements of ¹³C/¹²C fractionation between coexisting diamond and carbonate. Our study adequately documents alteration on a scale commensurate with observed vein structures. But, testing the hypothesis of metamorphic origin of microdiamonds has not fully succeeded because our analytical spatial resolution, limited to 0.5 mm, is not small enough to measure individual dolomite inclusions or individual diamond crystals.     

The combined use of Sr/Sr and carbon and water isotopes to study the hydrochemical interaction between groundwater and lakewater in mantled karst

The hydrochemical interaction between groundwater and lakewater influences the composition of water that percolates downward from the surficial aquifer system through the underlying intermediate confining unit and recharges the Upper Floridan aquifer along highlands in Florida. The ⁸⁷Sr/⁸⁶Sr ratio along with the stable isotopes, D, ¹⁸O, and ¹³C were used as tracers to study the interaction between groundwater, lakewater, and aquifer minerals near Lake Barco, a seepage lake in the mantled karst terrane of northern Florida. Upgradient from the lake, the ⁸⁷Sr/⁸⁶Sr ratio of groundwater decreases with depth (mean values of 0.71004, 0.70890, and 0.70852 for water from the surficial aquifer system, intermediate confining unit, and Upper Floridan aquifer, respectively), resulting from the interaction of dilute oxygenated recharge water with aquifer minerals that are less radiogenic with depth. The concentrations of Sr²⁺ generally increase with depth, and higher concentrations of Sr²⁺ in water from the Upper Floridan aquifer (20–35 μg/L), relative to water from the surficial aquifer system and the intermediate confining unit, result from the dissolution of Sr-bearing calcite and dolomite in the Eocene limestone. Dissolution of calcite [δ¹³C = −1.6 permil (%o)] is also indicated by an enriched δ¹³C_DIC(-8.8 to -11.4% o) in water from the Upper Floridan aquifer, relative to the overlying hydrogeologic units (δ¹³C_DIC < -16%o).

Groundwater downgradient from Lake Barco was enriched in ¹⁸O and D relative to groundwater upgradient from the lake, indicating mixing of lakewater leakage and groundwater. Downgradient from the lake, the ⁸⁷Sr/⁸⁶Sr ratio of groundwater and aquifer material become less radiogenic and the Sr²⁺ concentrations generally increase with depth. However, Sr²⁺ concentrations are substantially less than in upgradient groundwaters at similar depths. The lower Sr²⁺ concentrations result from the influence of anoxic lakewater leakage on the mobility of Sr²⁺ from clays. Based on results from mass-balance modeling, it is probable that cation exchange plays the dominant role in controlling the ⁸⁷Sr/⁸⁶Sr ratio of groundwater, both upgradient and downgradient from Lake Barco. Even though groundwater from the three distinct hydrogeologic units displays considerable variability in Sr concentration and isotopic composition, the dominant processes associated with the mixing of lakewater leakage with groundwater, as well as the effects of mineral-water interaction, can be ascertained by integrating the use of stable and radiogenic isotopic measurements of groundwater, lakewater, and aquifer minerals.     

Distribution of diagenetic alterations in fluvial and paralic deposits within sequence stratigraphic framework: Evidence from the Petrohan Terrigenous Group and the Svidol Formation, Lower Triassic, NW Bulgaria

Elucidation of diagenetic alterations in the Petrohan Terrigenous Group (fluvial; highstand systems tract HST) sandstones and Svidol Formation (tide-dominated deltaic and tidal flat, transgressive systems tract TST and highstand systems tract HST, respectively) sandstones and calcarenite, Lower Triassic, NW Bulgaria was constrained within a sequence stratigraphic framework. Eogenetic alterations in the fluvial HST sandstones include (i) formation of grain-coating infiltrated clays as a result of percolation of mud-rich surface waters into underlying coarse-grained and permeable channel-fills and crevasse splay sandstones; (ii) formation of pseudomatrix by mechanical compaction of mud intraclasts that were incorporated into the coarse-grained channel sandstones during their lateral avulsion; and (iii) cementation by calcite (δ¹⁸O_VPDB = − 6.5‰ to − 3‰; δ¹³C_VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ¹⁸O_VPDB = − 6.1‰ to − 0.3‰; δ¹³C_VPDB = − 7.2‰ to − 5.8‰) in the crevasse splay and floodplain sediments. Mesogenetic alterations that are encountered in the fluvial HST sandstones include (i) illitization of grain-coating clays, mud intraclasts, and mica, possibly because of simultaneous albitization of feldspars; (ii) cementation by calcite (δ¹⁸O_VPDB = − 14.5‰ to − 8.4‰; δ¹³C_VPDB = − 7.7‰ to + 0.6‰) and dolomite (δ¹⁸O_VPDB = − 15.8‰ to − 5‰; δ¹³C_VPDB = − 7.9‰ to + 1.5‰); and (iii) limited amounts of quartz overgrowths in the channel sandstones owing to occurrence of thick grain-coating clays.

Conversely, the tide-dominated deltaic TST sandstones and the tidal flat HST calcarenite were pervasively cemented by calcite (δ¹⁸O_VPDB = − 6.6‰ to − 3.1‰; δ¹³C_VPDB = − 5.1‰ to + 0.6‰) and siderite (δ¹⁸O_VPDB = − 7.2‰ to − 5.7‰; δ¹³C_VPDB = + 0.3‰ to + 0.9‰) particularly below marine and maximum flooding surfaces, due to the presence of abundant bioclasts and prolonged residence time of the sediments under certain geochemical conditions along these surfaces. The remaining open pores were cemented during mesodiagenesis by calcite (δ¹⁸O_VPDB = − 6.6‰ to − 3.1‰ and δ¹³C_VPDB = − 5.1‰ to + 0.6‰) and dolomite (δ¹⁸O_VPDB = − 6.6‰ to − 3.1‰ and δ¹³C_VPDB = − 5.1‰ to + 0.6‰).

This study shows that constructing a conceptual model for the distribution of diagenetic alterations is possible by integration of diagenesis with sequence stratigraphy. The model shows that tide-dominated deltaic TST sandstones and tidal flat HST calcarenite were pervasively cemented by carbonates during near-surface eodiagenesis, owing to the presence of abundant bioclasts. Conversely, fluvial LST sandstones remained poorly cemented during near-surface eodiagenesis due to the lack of bioclasts, but were cemented by mesogenetic calcite, dolomite and quartz overgrowths instead.     

塔里木盆地中央隆起区上寒武统—下奥陶统白云岩地球化学特征及白云石化流体演化规律*

以详细的岩石学研究为基础,综合利用碳、氧、锶同位素等地球化学资料,深入分析了塔里木盆地中央隆起区上寒武统—下奥陶统白云石化流体演化规律以及白云岩成因机制。结果表明,上寒武统白云岩主要由泥晶—粉晶白云岩、微生物白云岩和(残余)颗粒白云岩等原始结构保留较好的白云岩构成,其C、Sr同位素与同期海水相近,O同位素值偏正,属于同生/准同生期与轻微蒸发海水有关的白云石化的产物;下奥陶统白云岩以细晶自形—半自形白云石为主,原始结构保留差,其C、Sr同位素与同期海水近似,但O同位素值略微偏负,主要为浅埋藏期白云石化的产物。部分早期白云岩在中—深埋藏过程中受埋藏重结晶和构造—热液白云石化的影响,形成细晶—粗晶他形白云岩和缝洞鞍形白云石充填物,该阶段白云石化流体主要来自于地层内封存的海源流体、深部热液以及蒸发岩层间热卤水,多期多源流体的共同作用导致该类白云岩具有较宽的Sr同位素组成和明显负偏的O同位素值。总体上,研究区白云岩具有早期形成(近地表到浅埋藏期大规模交代)、中期加强(中—深埋藏期部分重结晶)、晚期改造(热液局部调整)的整体演化趋势。     

Isotopic characteristics and petrogenesis of the lamproites and kimberlites of central west Greenland

Kimberlites which intruded the Sisimiut (formerly Holsteinsborg) region of central west Greenland during the Early Palaeozoic have initial ⁸⁷Sr/⁸⁶Sr between 0.7028 and 0.7033 and ε_Nd between + 1.3 and + 3.9. Mid-Proterozoic potassic lamproites from the same region have initial ⁸⁷Sr/⁸⁶Sr between 0.7045 and 0.7060, ε_Nd between −13 and −10 and unradiogenic initial Pb isotopic compositions. The isotopic data favour an asthenospheric mantle source for the kimberlite magmas, in common with “basaltic” kimberlites from other localities, whereas the lamproite magma sources evolved in isolation from the convecting mantle for > 1000 Ma, probably within the subcontinental lithospheric mantle of the Greenland craton, prior to emplacement of the lamproites.     

The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km² in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A₁ (1800–1780 Ma), main anorthositic A₂ (1760 Ma), early gabbroic G₃ (between 1760 and 1758 Ma), late gabbroic G₄ (1758 Ma), and a suite of dykes D₅ (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A₁ rocks have lower Sr (⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70233–0.70288) and higher Nd (εNd₍₁₇₆₀₎ = 1.6–0.9) isotopic composition. The most widespread A₂ anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70362, εNd₍₁₇₆₀₎ varies from 0.2 to − 0.7. The G₃ gabbro–norite has slightly lower εNd₍₁₇₆₀₎ varying from − 0.7 to − 0.9. Finally, G₄ gabbroic rocks show relatively high initial ⁸⁷Sr/⁸⁶Sr (0.70334–0.70336) and the lowest Nd isotopic composition (εNd₍₁₇₆₀₎ varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd₍₁₇₆₀₎ values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia.     

Temporal Variation in Sr and 87Sr/86Sr of Yangtze River: An Example from Datong Hydrological Station

Temporal variation of dissolved ⁸⁷Sr/⁸⁶Sr in the Yangtze River is poorly understood compared to other Tibetan rivers. In this study, dissolved Sr and ⁸⁷Sr/⁸⁶Sr were measured from a temporal series of water samples collected biweekly at Datong Hydrological Station over a period of one year. Our results show that Sr concentration in the Yangtze River ranges from 1.74 to 2.92 μmol/L with ⁸⁷Sr/⁸⁶Sr of 0.710125 to 0.710965. The Sr concentration and ⁸⁷Sr/⁸⁶Sr shows a distinct seasonal variation, with a general increase in ⁸⁷Sr/⁸⁶Sr ratios from summer to winter and some fluctuations during July and December, then followed by a gradually decrease till the next rainy season. The seasonal variation results from the variation of contributions from different sub-basin due to the spatially and seasonally variable rainfall across the basin. During the flood season, more contribution from upper reach (low ⁸⁷Sr/⁸⁶Sr values) due to the strong rainfall decreases the ⁸⁷Sr/⁸⁶Sr ratio at lower reach. While the severe drought which happened in the middle-lower reaches (high ⁸⁷Sr/⁸⁶Sr values) from January to May explains the decrease in the later part of the data by the decrease of the contribution from middle-lower reaches. The discharge weighted annual ⁸⁷Sr/⁸⁶Sr and annual Sr flux of the Yangtze River based on the time series data are 0.710628 and 1.9×10⁹ mol/a, respectively. It was also indicated that dissolved ⁸⁷Sr/⁸⁶Sr in the Yangtze River is well correlated to the extreme climate events and might contribute to our explanation for reconstructing past climatic changes by using ⁸⁷Sr/⁸⁶Sr ratios of the sedimentary record in the delta .     

Late Triassic granitoids of the eastern margin of the Tibetan Plateau: Geochronology, petrogenesis and implications for tectonic evolution

Late Triassic granitoids in the Songpan-Garzê Fold Belt (SGFB), on the eastern margin of the Tibetan Plateau, formed at 230 to 220 Ma and can be divided into two groups. Group 1 are high-K calc-alkaline rocks with adakitic affinities (K-adakites), with Sr > 400 ppm, Y < 11 ppm, strongly fractionated REE patterns ((La/Yb)_N = 32–105) and high K₂O/Na₂O (≈ 1). Group 2 are ordinary high-K calc-alkaline I-types with lower Sr (< 400 ppm), higher Y (> 18 ppm) and weakly fractionated REE patterns ((La/Yb)_N < 20). Rocks of both groups have similar negative Eu anomalies (Eu/Eu = 0.50 to 0.94) and initial ⁸⁷Sr/⁸⁶Sr (0.70528 to 0.71086), but group 1 rocks have higher ε_Nd(t) (− 1.01 to − 4.84) than group 2 (− 3.11 to − 6.71). Calculated initial Pb isotope ratios for both groups are: ²⁰⁶Pb/²⁰⁴Pb = 18.343 to 18.627, ²⁰⁷Pb/²⁰⁴Pb = 15.610 to 15.705 and ²⁰⁸Pb/²⁰⁴Pb = 38.269 to 3759. Group 1 magmas were derived through partial melting of thickened and then delaminated TTG-type, eclogitic lower crust, with some contribution from juvenile enriched mantle melts. Group 2 magmas were generated by partial melting of shallower lower crustal rocks. The inferred magma sources of both groups suggest that the basement of the SGFB was similar to the exposed Kangding Complex, and that the SGFB was formed in a similar manner to the South China basement. Here, passive margin crust was greatly thickened and then delaminated, all within a very short time interval ( 20 Myr). Such post-collisional crustal thickening could be the tectonic setting for the generation of many adakitic magmas, especially where there is no spatial and temporal association with subduction.     

Crustal contamination of mafic magmas: evidence from a petrological, geochemical and Sr–Nd–Os–O isotopic study of the Proterozoic Isortoq dike swarm, South Greenland

The mid-Proterozoic Isortoq dike swarm in the Gardar Province, South Greenland, comprises a variety of alkaline rocks ranging from gabbroic to syenitic in composition. Major magmatic mineral phases are olivine, clinopyroxene, Fe–Ti oxides, amphibole, plagioclase and alkali feldspar. Quartz occurs in some samples as a late magmatic phase. Liquidus temperatures of olivine-bearing samples range between 1120 and 1145 °C and solidus temperatures are 850–930 °C. Calculated silica activities are highly variable between 0.53 and unity. Oxygen fugacities vary from −3 to +1 log units relative to the fayalite–magnetite–quartz buffer.

The rocks have MgO contents <6 wt.% with Mg# between 53 and 17. Primitive mantle-normalized trace element patterns show a relative enrichment of LIL elements with Ba peaks and Nb troughs. Clinopyroxenes show a general enrichment in REE relative to chondritic values with variable slightly positive to prominent negative Eu anomalies. Two of the dikes were dated with Sm–Nd three-point isochrons at 1190±44 and 1187±87 Ma, respectively. Initial ⁸⁷Sr/⁸⁶Sr ratios of mafic mineral separates range from 0.70289 to 0.70432 and initial _Nd values vary from +0.3 to −10.7. Whole-rock initial ¹⁸⁷Os/¹⁸⁸Os ratios are highly variable including very radiogenic values of up to 7.967. δ¹⁸O_v-smow values of separated clinopyroxene and amphibole range from +5.2‰ to +6.2‰ and fall within the range of typical mantle-derived rocks, although mixing with a lower crustal component is permitted by the data. Using energy-constrained assimilation-fractional crystallization (EC-AFC) modeling equations, the Sr–Nd isotope data of the more radiogenic samples can successfully be modeled by addition of up to 10% lower crustal granulite-facies Archean gneisses as contaminants. The Os isotopic data also suggest the involvement of old radiogenic crust. In accordance with seismic data, we conclude that a wedge of Archean crust extends from West Greenland further to the south below the present erosion level.     

Comparison of petrogenetic signatures between mantle-derived alkali silicate intrusives with and without associated carbonatite, Namibia

We compare the petrogenetic and chemical signatures of two alkali silicate suites from the Cretaceous Damaraland igneous province (Namibia), one with and one without associated carbonatite, in order to explore their differences in terms of magma source and evolution. The Etaneno complex occurs in close spatial proximity to the Kalkfeld bimodal carbonatite–alkali silicate complex, and is dominated by nepheline (ne)-monzosyenites and ne-bearing alkali feldspar syenites. The Etaneno samples have isotopic compositions of ⁸⁷Sr/⁸⁶Sr(i)=0.70462–0.70508 and Nd=−0.5 to −1.5, with the highest ⁸⁷Sr/⁸⁶Sr(i) and lowest Nd values observed in evolved samples. The magma differentiated via olivine, feldspar, clinopyroxene, and nepheline (ne) fractionation in a F-rich system, which fractionated Zr from Hf, and Y from Ho. Partly glassy, recrystallized inclusions in some samples are less evolved than their host rocks and contain a cumulate component (nepheline, plagioclase). The Kalkfeld ne-foidites (ijolites) and ne-syenites have ⁸⁷Sr/⁸⁶Sr(i)=0.70285–0.70592 and Nd=0.5 to 1.1. The isotope ratios show no consistent variation with rock composition, and they are in the same range as the associated carbonatites. The Kalkfeld silicate magma fractionated nepheline and alkali-feldspar in a CO₂-dominated, F- and Ca-poor system. As a result, the rocks display some major and trace element trends distinctly different from those of the Etaneno samples.

We suggest that the Etaneno and the Kalkfeld magmas represent different melt fractions of a heterogeneous mantle source, resulting in different compositions especially with respect to CO₂ contents of the primitive, parental magmas. In this scenario, the carbonated alkali silicate Kalkfeld parental melt contained a critical CO₂ concentration and underwent liquid separation of carbonate and silicate melt fractions at crustal depths. The resulting silicate melt fraction experienced a very different mode of differentiation than the carbonate-poor Etaneno parental magma. Thus, the Kalkfeld rocks are depleted in Ca and other divalent cations, as well as F, rare-earth elements (REE), Ba, and P relative to the Etaneno syenites. We interpret these differences to reflect the partitioning of these elements into the carbonate melt fraction during immiscible separation.