Constraints on paleowater dissolved loads and on catchment weathering over the past 16 ka from Sr/Sr ratios and Ca/Mg/Sr chemistry of freshwater ostracode tests in sediments of Lake Constance, Central Europe

Sr isotope and Ca/Mg/Sr chemical compositions of freshwater ostracode tests separated from a sediment core represent the last 16 ka of sedimentation in Lake Constance, Central Europe. The chemical evolution of the paleowater's dissolved load of Lake Constance was estimated by correcting the ostracode data for Ca/Mg/Sr fractionation due to biogenic calcification. Since the Late Pleistocene deglaciation, the Ca/Sr molar ratios of paleowaters increased systematically from about 100 (a near marine signature) to about 200. Ca/Mg molar ratios varied in the range of 1–25. The ⁸⁷Sr/⁸⁶Sr ratios indicate Late Pleistocene paleowater compositions of 0.7086–0.7091, significantly more radiogenic than present day waters (0.7085). Sr isotopes and Ca/Mg/Sr chemical data together show that weathering of Mesozoic evaporites consistently dominated the dissolved Sr load (80–90%). Carbonate and silicate weathering were less important (1–10%). Trends of Sr dissolved loads were therefore not related to Mg which was mainly mobilized by carbonate weathering. Biotite weathering was an important source of radiogenic Sr in the paleowaters. The short-term release (duration about 600–800 years) of radiogenic Sr during glacier retreat started 15.2 ka ago and was due to enhanced biotite weathering at the glacier base. Long-term release of radiogenic Sr was due to biotite weathering in glacial soils and silicate rocks, and has gradually declined since the Late Pleistocene/Holocene transition.     

In‐situ analysis of trace elements and Sr–Pb isotopes of K‐feldspars from Tongshankou Cu–Mo deposit,SE Hubei Province,China: Insights into early potassic alteration of the porphyry mineralization system

In the Tongshankou porphyry deposit (SE Hubei Province, South China), three types of K‐feldspars are recognized: (I) the phenocryst type in the porphyry that crystalized during the magmatic stage, (II) the megacryst type and (III) the vein type in the altered porphyry and orebody that was produced by hydrothermal fluids. A detailed in‐situ analysis of trace elements and Sr–Pb isotopes was carried out on K‐feldspars in an attempt to unravel their formation processes and to trace the element sources during potassic alteration. The Type III K‐feldspars show lower Sr contents and Sr‐isotope ratios but higher Pb contents and Pb‐isotope ratios than the Type I and II K‐feldspars, possibly reflecting a contribution from the country carbonate rocks with less radiogenic Sr but more radiogenic Pb sources, and indicate that the ore‐forming fluids and materials may have been partially derived from external sources such as the host sedimentary rocks during the early potassic alteration stage.     

Major and trace element, and Sr-Nd isotope constraints on the origin of Paleogene volcanism in South China prior to the South China Sea opening

Paleogene volcanic rocks crop out in three sedimentary basins, namely, Sanshui, Heyuan and Lienping, in the attenuated continental margin of south China. Lavas from the Sanshui basin which erupted during 64-43 Ma are bimodal, consisting of intraplate tholeiitic basalt and trachyte/rhyolite associations. Similar to Cretaceous A-type granites from the nearby region, the felsic member shows peralkaline nature [Na₂O + K₂O ≈ 10–12%; (Na + K)/Al≈ 0.98−1.08], general enrichment in the incompatible trace elements and significant depletion in Ba, Sr, Eu, P and Ti. Although both types of the Sanshui lavas have rather uniform Nd isotope compositions [_Nd(T) ≈ +6 to +4]that are comparable to Late Cenozoic basalts around the South China Sea, the felsic rocks possess apparently higher initial Sr isotope ratios (I_Sr up to 0.713) and form a horizontal array to the right in the Nd vs. Sr isotope plot. Closed system differentiation of mantle-derived magmas in a ‘double diffusive’ magma chamber is considered for the bimodal volcanism, in which the trachytes and rhyolites represent A-type melts after extensive crystal fractionation in the upper portion of the chamber. Such A-type melts were later contaminated by small amounts (1–3%) of upper crustal materials during ascent. On the other hand, composition of lavas in the other two basins varies from tholeiitic basalt to andesite. Their Sr and Nd isotope ratios [I_Sr ≈ 0.705 to 0.711; _Nd(T) ≈ +1 to − 5] and generally correlative Nb-Ta depletions suggest a distinct magma chamber process involving fractional crystallization concomitant with assimilation of the country rock. We conclude that these Paleogene volcanic activities resulted from the lithospheric extension in south China that migrated southwards and eventually led to opening of the South China Sea during 30-16 Ma.     

The strontium isotopic budget of Himalayan rivers in Nepal and Bangladesh

Himalayan rivers have very unusual Sr characteristics and their budget cannot be achieved by simple mixing between silicate and carbonate even if carbonates are radiogenic. We present Sr, O, and C isotopic data from river and rain water, bedload, and bedrock samples for the western and central Nepal Himalaya and Bangladesh, including the monsoon season. Central Himalayan rivers receive Sr from several sources: carbonate and clastic Tethyan sediments, High Himalayan Crystalline (HHC) gneisses and granitoids with minor marbles, carbonates and metasediments of the Lesser Himalaya (LH), and Miocene-Recent foreland basin sediment from the Siwaliks group and the modern flood plain. In the Tethyan Himalaya rivers have dissolved [Sr] ≈ 6 μmol/l and ⁸⁷Sr/⁸⁶Sr ≈ 0.717, with a large contribution from moderately radiogenic carbonate. Rivers draining HHC gneisses are very dilute with [Sr] ≈ 0.2 μmol/l and ⁸⁷Sr/⁸⁶Sr ≈ 0.74. Lesser Himalayan streams also have low [Sr] ≈ 0.4 μmol/l and are highly radiogenic (⁸⁷Sr/⁸⁶Sr ≥ 0.78). Highly radiogenic carbonates of the LH do not contribute significantly to the Sr budget because they are sparse and have very low [Sr]. In large rivers exiting the Himalaya, Sr systematics can be modeled as a mixture between Tethyan rivers, where slightly radiogenic carbonates (mean ⁸⁷Sr/⁸⁶Sr ≈ 0.715) are the main source of Sr, and Lesser Himalaya waters, where extremely radiogenic silicates (>0.8) are the main source of Sr. HHC waters are less important because of their low [Sr]. Rivers draining the Siwaliks foreland basin sediments have [Sr] ≈ 4 μmol/l and ⁸⁷Sr/⁸⁶Sr ≈ 0.725. Weathering of silicates in the Siwaliks and the flood plain results in a probably significant radiogenic (0.72–0.74) input to the Ganges and Brahmaputra (G-B), but quantification of this flux is limited by uncertainties in the hydrologic budget. The G-B in Bangladesh show strong seasonal variability with low [Sr] and high ⁸⁷Sr/⁸⁶Sr during the monsoon. Sr in the Brahmaputra ranges from 0.9 μmol/l and 0.722 in March to 0.3 μmol/l and 0.741 in August. We estimate the seasonally weighted flux from the G-B to be 6.5 × 10⁸ mol/yr with ⁸⁷Sr/⁸⁶Sr = 0.7295.     

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 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.     

Phlogopite in the generation of olivine-melilitites from Namaqualand, South Africa and implications for element fractionation processes in the upper mantle

Major and trace element and Sr, Nd and Pb isotope analyses are presented for thirteen olivine-melilitites from Namaqualand, South Africa. Major element variations are consistent with derivation from carbonated garnet-peridotite at depths of at least 100 km and trace element abundances indicate melt fractions of 4%. Ubiquitous negative K anomalies and low, buffered K₂O concentrations are interpreted to reflect the effect of residual phlogopite during melting. It is suggested that phlogopite stability and low melt potassium saturation concentrations are enhanced by high CO₂/(CO₂ + H₂O) conditions. Residual phlogopite can also account for low measured Rb/Sr, Ba/Sr and Th/U ratios in the melilitites. REE abundances are controlled by residual garnet and hence Sm/Nd ratios are low (0.13–0.18). U/Pb ratios vary from 0.05 to 5 and are a function of Pb concentration which is in turn controlled by residual Pb-rich phase (probably sulphide). Nd and Sr isotopes are comparable with OIB from St. Helena, although two samples extend to higher ⁸⁷Sr/⁸⁶Sr ratios. Present day Pb isotopes are much more variable and partly reflect radiogenic growth since emplacement as a result of the highly variable U/Pb ratios.

Many of the trace element characteristics of the melilitites are distinct from those of within-plate potassic magmas despite both being derived from phlogopite-bearing, enriched mantle source regions. This can be attributed to the depth at which source enrichment occurred and the subsequent control exerted by phlogopite and carbonate during melting. In contrast to melilitites, potassic magmas are derived from shallower depths under low CO₂/(CO₂ + H₂O) conditions and at higher temperatures at which phlogopite melts more readily.

The incompatible element ratios of the melilitites are also similar to those both observed in HIMU ocean island basalts (OIB) and inferred for HIMU OIB source regions from isotope variations (viz, low Sm/Nd, Rb/Sr, K/Nb, Th/U and high U/Pb and Ce/Pb). It is suggested that HIMU OIB's may be derived from sources that have been subject to enrichment by a melt generated in the presence of residual phlogopite.     

The timing and isotopic character of regional hydrothermal alteration and associated epigenetic mineralization in the western sector of the Kaapvaal Craton (South Africa)

The Kaapvaal Craton of South Africa comprises an Archaean core of ≈3.5 Ga lithospheric and crustal rocks surrounded by younger accreted terrains of ≈3.0–2.7 and ≈2.1–1.9 Ga. The craton is covered by relatively undeformed 3.0–2.4 Ga supracrustal rocks, which show the effects of thermal and hydrothermal interaction. Part of this activity is manifested by a large number of epigenetic Pb–Zn (±Ag, Au, Cu, F) deposits in the cover rocks of the Kaapvaal Craton. These include small volcanic and breccia hosted deposits in mafic and felsic volcanic rocks of the 2.7 Ga Ventersdorp Supergroup and the Mississippi Valley-type (MVT) deposits in the carbonates of the Transvaal Supergroup.MVT mineralization at the Pering (and other Zn–Pb deposits) is hosted in fracture-generated N–S breccia bodies in the Paleoproterozoic carbonate succession of the western Kaapvaal Craton. The fluids carrying the metals were focused in vertical bodies within the fracture zones (FZ), the metals and the sulphur being carried together and precipitated in organic-rich sectors of the basin. Two small Pb–Zn deposits within mafic rocks of the Ventersdorp Supergroup, stratigraphically below the basin-hosted MVTs on the southwestern part of the Kaapvaal Craton have secondary chlorite which is extremely Rb-rich, associated with the mineralization. This chlorite and the associated altered basaltic host rocks give a Rb–Sr date of ≈1.98 Ga, and the associated galena Pb isotope data plot on the same array as those of other Pb–Zn deposits, the radiogenic intercept giving a date of ≈2.0 Ga. We interpret these data to indicate a craton-wide epigenetic fluid-infiltration event, which exploited the Maquassie Quartz Porphyry (MQP) as the aquifer and metal source.Sr isotopic results for the ore-zone gangue minerals show highly radiogenic ⁸⁷Sr/⁸⁶Sr ratios (>0.710) which support earlier models that the origin of radiogenic Sr isotopic composition in the calcite cements is the felsic tuffs (MQP) of the Ventersdorp Supergroup occurring at deeper levels within the basin. Relationships between δ¹⁸O and δ¹³C performed on carbonate cements within the aquifers are complex: the range in δ¹³C for some of the cements represents a mixture from two sources and with a progression from heavy carbon in the host to somewhat lighter carbon in the cements. Similarly, the lighter δ¹⁸O values have a narrow range indicative of rapid exchanges between hydrous fluid and rock.     

Iron colloids/organic matter associated transport of major and trace elements in small boreal rivers and their estuaries (NW Russia)

The chemical status of major and trace elements (TE) in various boreal small rivers and watershed has been investigated along a 1500-km transect of NW Russia. Samples were filtered in the field through a progressively decreasing pore size (5, 0.8 and 0.22 μm; 100, 10, and 1 kD) using a frontal filtration technique. All major and trace elements and organic carbon (OC) were measured in filtrates and ultrafiltrates. Most rivers exhibit high concentration of dissolved iron (0.2–4 mg/l), OC (10–30 mg/l) and significant amounts of trace elements usually considered as immobile in weathering processes (Ti, Zr, Th, Al, Ga, Y, REE, V, Pb). In (ultra)filtrates, Fe and OC are poorly correlated: iron concentration gradually decreases upon filtration from 5 μm to 1 kD whereas the major part of OC is concentrated in the <1–10 kD fraction. This reveals the presence of two pools of colloids composed of organic-rich and Fe-rich particles. According to their behavior during filtration and association with these two types of colloids, three groups of elements can be distinguished: (i) species that are not affected by ultrafiltration and are present in the form of true dissolved inorganic species (Ca, Mg, Li, Na, K, Sr, Ba, Rb, Cs, Si, B, As, Sb, Mo) or weak organic complexes (Ca, Mg, Sr, Ba), (ii) elements present in the fraction smaller than 1–10 kD prone to form inorganic or organic complexes (Mn, Co, Ni, Zn, Cu, Cd, and, for some rivers, Pb, Cr, Y, HREE, U), and (iii) elements strongly associated with colloidal iron in all ultrafiltrates (P, Al, Ga, REE, Pb, V, Cr, W, Ti, Ge, Zr, Th, U). Based on size fractionation results and taking into account the nominal pore size for membranes, an estimation of the effective surface area of Fe colloids was performed. Although the total amount of available surface sites on iron colloids (i.e., 1–10 μM) is enough to accommodate the nanomolar concentrations of dissolved trace elements, very poor correlation between TE and surface sites concentrations was observed in filtrates and ultrafiltrates. This strongly suggests a preferential transport of TE as coprecipitates with iron oxy(hydr)oxides. These colloids can be formed on redox boundaries by precipitation of Fe(III) from inflowing Fe(II)/TE-rich anoxic ground waters when they meet well-oxygenated surface waters. Dissolved organic matter stabilizes these colloids and prevents their aggregation and coagulation. Estuarine behavior of several trace elements was studied for two small iron- and organic-rich rivers. While Si, Sr, Ba, Rb, and Cs show a clear conservative behavior during mixing of freshwaters with the White sea, Al, Pb and REE are scavenged with iron during coagulation of Fe hydroxide colloids.     

云南腾冲全新世火山岩地球化学特征及其成因

通过对腾冲火山群的黑空山、打莺山和马鞍山火山岩主量元素、微量元素、Sr-Nd-Pb同位素地球化学的研究表明,腾冲全新世火山岩为高钾钙碱性系列,包括粗面玄武岩、玄武粗安岩、粗面安山岩和英安岩。该套火山岩富集大离子亲石元素和轻稀土元素,亏损Nb-Ta-Ti不相容元素,具有高的⁸⁷Sr/⁸⁶Sr比值(0.705862～0.710614),低的¹⁴³Nd/¹⁴⁴Nd比值(0.511941～0.512526)和较高的放射性成因Pb同位素组成(²⁰⁸Pb/²⁰⁴Pb=38.962～39.155;²⁰⁷Pb/²⁰⁴Pb=15.620～15.659;²⁰⁶Pb/²⁰⁴Pb=17.872～18.269)。主量、微量和同位素地球化学研究表明,腾冲全新世三座火山的岩浆源区为富集地幔,地幔富集与古洋壳俯冲有关。它们是由相同的地幔源区经历了不同的岩浆演化阶段的产物,打莺山岩浆演化程度最高,黑空山次之,马鞍山岩浆演化程度最弱。通过腾冲全新世火山岩与藏南碰撞后钾质火山岩的地球化学成分对比,Sr-Nd-Pb同位素地球化学成分存在着明显的不同,暗示两者的源区存在差异。     

Accreted fragments of the Late Cretaceous Caribbean–Colombian Plateau in Ecuador

The eastern part of the Western Cordillera of Ecuador includes fragments of an Early Cretaceous (≈123 Ma) oceanic plateau accreted around 85–80 Ma (San Juan–unit). West of this unit and in fault contact with it, another oceanic plateau sequence (Guaranda unit) is marked by the occurrence of picrites, ankaramites, basalts, dolerites and shallow level gabbros. A comparable unit is also exposed in northwestern coastal Ecuador (Pedernales unit).

Picrites have LREE-depleted patterns, high Nd_i and very low Pb isotopic ratios, suggesting that they were derived from an extremely depleted source. In contrast, the ankaramites and Mg-rich basalts are LREE-enriched and have radiogenic Pb isotopic compositions similar to the Galápagos HIMU component; their Nd_i are slightly lower than those of the picrites. Basalts, dolerites and gabbros differ from the picrites and ankaramites by flat rare earth element (REE) patterns and lower Nd; their Pb isotopic compositions are intermediate between those of the picrites and ankaramites. The ankaramites, Mg-rich basalts, and picrites differ from the lavas from the San Juan–Multitud Unit by higher Pb ratios and lower Nd_i.

The Ecuadorian and Gorgona 88–86 Ma picrites are geochemically similar. The Ecuadorian ankaramites and Mg-rich basalts share with the 92–86 Ma Mg-rich basalts of the Caribbean–Colombian Oceanic Plateau (CCOP) similar trace element and Nd and Pb isotopic chemistry. This suggests that the Pedernales and Guaranda units belong to the Late Cretaceous CCOP. The geochemical diversity of the Guaranda and Pedernales rocks illustrates the heterogeneity of the CCOP plume source and suggests a multi-stage model for the emplacement of these rocks. Stratigraphic and geological relations strongly suggest that the Guaranda unit was accreted in the late Maastrichtian (≈68–65 Ma).     

The late Cretaceous lithospheric mantle beneath the Central Andes: Evidence from phase equilibria and composition of mantle xenoliths

Temperature estimates and chemical composition of mantle xenoliths from the Cretaceous rift system of NW Argentina (26°S) constrain the rift evolution and chemical and physical properties of the lithospheric mantle at the eastern edge of the Cenozoic Andean plateau. The xenolith suite comprises mainly spinel lherzolite and subordinate pyroxenite and carbonatized lherzolite. The spinel lherzolite xenoliths equilibrated at high-T (most samples >1000 °C) and P below garnet-in. The Sm–Nd systematics of compositionally unzoned clino- and orthopyroxene indicate a Cretaceous minimum age for the high-T regime, i.e., the asthenosphere/lithosphere thermal boundary was at ca. 70 km depth in the Cretaceous rift. Major elements and Cr, Ni, Co and V contents of the xenoliths range between values of primitive and depleted mantle. Calculated densities based on the bulk composition of the xenoliths are <3280 kg/m³ for the estimated P–T conditions and indicate a buoyant, stable upper mantle lithosphere. The well-equilibrated metamorphic fabric and mineral paragenesis with the general lack of high-T hydrous phases did not preserve traces of metasomatism in the mantle xenoliths. Late Mesozoic metasomatism, however, is obvious in the gradual enrichment of Sr, U, Th and light to medium REE and changes in the radiogenic isotope composition of an originally depleted mantle. These changes are independent of the degree of depletion evidenced by major element composition. ¹⁴³Nd/¹⁴⁴Nd_i ratios of clinopyroxene from the main group of xenoliths decrease with increasing Nd content from >0.5130 (depleted samples) to ca. 0.5127 (enriched samples). ⁸⁷Sr/⁸⁶Sr_i ratios (0.7127–0.7131, depleted samples; 0.7130–0.7134, enriched samples) show no variation with variable Sr contents. Pb_i isotope ratios of the enriched samples are rather radiogenic (²⁰⁶Pb/²⁰⁴Pb_i 18.8–20.6, ²⁰⁷Pb/²⁰⁴Pb_i 15.6–15.7, ²⁰⁸Pb/²⁰⁴Pb_i 38.6–47) compared with the Pb isotope signature of the depleted samples. The large scatter and high values of ²⁰⁸Pb/²⁰⁴Pb_i ratios of many xenoliths indicates at least two Pb sources that are characterized by similar U/Pb but by different Th/Pb ratios. The dominant mantle type in the investigated system is depleted mantle according to its Sr and Nd isotopic composition with relatively radiogenic Pb isotope ratios. This mantle is different from the Pacific MORB source and old subcontinental mantle from the adjacent Brazilian Shield. Its composition probably reflects material influx into the mantle wedge during various episodes of subduction that commenced in early Paleozoic or even earlier. Old subcontinental mantle was already replaced in the Paleozoic, but some inheritance from old mantle lithosphere is represented by rare xenoliths with isotope signatures indicating a Proterozoic origin.     

High-Mg potassic rocks from Taiwan: implications for the genesis of orogenic potassic lavas

Taiwan is an active mountain belt formed by oblique collision between the Luzon arc and the Asian continent. Regardless of the ongoing collision in central and southern Taiwan, a post-collisional extension regime has developed since the Plio–Pleistocene in the northern part of this orogen, and led to generation of the Northern Taiwan Volcanic Zone. Emplaced at 0.2 Ma in the southwest of the Volcanic Zone, lavas from the Tsaolingshan volcano are highly magnesian (MgO≈15 wt.%) and potassic (K₂O≈5 wt.%; K₂O/Na₂O≈1.6–3.0). Whereas these basic rocks (SiO₂≈48 wt.%) have relatively low Al₂O₃≈12 wt.%, total Fe₂O₃≈7.5 wt.% and CaO≈7.2 wt.%, they are extremely enriched in large ion lithophile elements (LILE, e.g. Cs, Rb, Ba, Th and U). The Rb and Cs abundances, >1000 and 120 ppm, respectively, are among the highest known from terrestrial rocks. In addition, these rocks are enriched in light rare earth elements (LREE), depleted in high field strength elements (HFSE), and display a positive Pb spike in the primitive mantle-normalized variation diagram. Their REE distribution patterns mark with slight Eu negative anomalies (Eu/Eu*≈0.90–0.84), and Sr and Nd isotope ratios are uniform (⁸⁷Sr/⁸⁶Sr≈0.70540–0.70551; ¹⁴³Nd/¹⁴⁴Nd≈0.51268–0.51259). Olivine, the major phenocryst phase, shows high Fo contents (90.4±1.8; 1σ deviation), which are in agreement with the whole rock Mg-values (83–80). Spinel inclusions in olivine are characterized by high Cr/Cr+Al ratios (0.94–0.82) and have compositions similar to those from boninites that originate from highly refractory peridotites. Such petrochemical characteristics are comparable to the Group I ultrapotassic rocks defined by Foley et al. [Earth-Sci. Rev. 24 (1987) 81], such as orogenic lamproites from central Italy, Span and Tibet. We therefore suggest that the Tsaolingshan lavas resulted from a phlogopite-bearing harzburgitic source in the lithospheric mantle that underwent a recent metasomatism by the nearby Ryukyu subduction zone processes. The lavas exhibit unique incompatible trace element ratios, with Rb/Cs≈8, Ba/Rb≈1, Ce/Pb≈2, Th/U≈1 and Nb/U≈0.8, which are significantly lower than the continental crust values and those of most mantle-derived magmas. Nonmagmatic enrichment in the mantle source is therefore required. Based on published experimental data, two subduction-related metasomatic components, i.e., slab-released hydrous fluid and subducted sediment, are proposed, and the former is considered to be more pervasive for causing the extraordinary trace element ratios observed. Our observations lend support to the notion that dehydration from subducting slabs at convergent margins, as a continuing process through geologic time, can account for the fractionation of these elemental pairs between the Earth's crust and mantle.     

Trace element and Pb isotope variability during rainy events in the NW Mediterranean: constraints on anthropogenic and natural sources

Rain samples collected over the period 1995–1999 on the French Mediterranean coast have been analyzed for their trace elements and Pb isotopic compositions, both in the dissolved and particulate loads. Our results show that metal Enrichment Factors (EF) relative to Upper Erodable Crust are high for Cu, Pb, Zn and Cd and comparable to values reported about 10 years ago for North Western Mediterranean rains. Overall unradiogenic Pb isotopic compositions indicate the still visible influence of French gasoline lead on both dissolved and particulate loads. This influence is also seen on aerosols, although more variable. The examination of air mass trajectories allows us to distinguih the Pb isotopic signatures of main sources, in particular Marseille/North Italy/North Africa. Rains collected and analyzed sequentially show intra-rain variations for metal contents, metal EF and Pb isotopes, which may also be related to the main paths and altitudes of the air masses. Metal EFs and Pb isotopic data on rains from Southern France in the late 1990s are similar to recent data published on French aerosols or sediments, which are interpreted as reflecting the phasing out of leaded gasoline in the atmosphere.     

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.     

Isotopic and trace-element indications of lithospheric and asthenospheric components in Tertiary alkalic basalts, northeastern Brazil

Plate reconstructions at 30 Ma place northeastern Brazil over the Fernando de Noronha hotspot, presently 250 km offshore northeastern Brazil. Tertiary basaltic centers in northeastern Brazil may therefore be products of the Fernando de Noronha hotspot. We examined alkalic basalt from seven 30–13 Ma old centers in Rio Grande do Norte and Pernambuco states to assess this possible link. Compositions are primitive, where MgO concentrations range from 12.5 to 17 wt.%, and SiO₂ from 41 to 48 wt.%. Trace-element abundances and Sr, Nd, and Pb isotopic compositions compare well with those of ocean island basalt: =0.7038–0.7051, =0.51266–0.51280, and =18.52–19.35. There are correlations among SiO₂-undersaturation, incompatible-element abundances, relative percentages of partial melting (based on La/Yb and La/Y ratios), and the degree of isotopic ‘enrichment' inherited from mantle sources. There is also a negative correlation for La/Nb (0.6–0.86) vs. Ba/Nb (6.6–10.6), where lower La/Nb samples represent larger percentages of melting of a source comparatively enriched in radiogenic Sr. We attribute these compositional relationships to the lavas representing mixing of melts mainly from asthenosphere of largely HIMU plus DM characterization, probably the Fernando de Noronha plume, with melts from subcontinental lithosphere that was isotopically closer to EM1. Isotopic and trace-element compositions of the northeastern Brazil basalts are generally similar to those of Fernando de Noronha lavas (12–2 Ma), and some minor trace-element differences observed (e.g., more Zr, Nb, and less Th compared to northeastern Brazil basalts) are probably due to heterogeneity within the asthenospheric plume and to melt contributions from delaminated subcontinental lithosphere that may underlie Fernando de Noronha.     

Chemical and isotopic (Pb,Sr) zonation in a peraluminous granite pluton: role of fluid fractionation

The Davis Lake pluton (DLP, ~800 km²) of southwestern Nova Scotia, Canada, part of the large peraluminous South Mountain batholith of ca. 380 Ma (U/Pb zircon, Ar/Ar mica), consists of granite and subordinate topaz–muscovite leucogranite that hosts greisen tin-base metal mineralization. A new Pb–Pb isochron age for leucogranite from the most evolved part of the DLP indicates a crystallization age of 378±3.6 Ma, coincident with other radiometric ages of the DLP (Rb–Sr, Re–Os, Pb–Pb). The intrusion displays a compositional zonation defined by lead and strontium isotopic ratios, as well as some major elements (e.g., Si, F), incompatible trace elements (e.g., Li, Rb, Ta, U, Sn), and elemental ratios (e.g., K/Rb and Nb/Ta). The greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for Pb and Sr, and their chemical-isotopic trends are extensions of the trends displayed by the less evolved granites. The covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the Pb–Pb and Rb–Sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma. The granitoid magma underwent extensive fractional crystallization of feldspars, minor biotite and accessory minerals (monazite, apatite and zircon) in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation, during which time the internally derived fluorine-rich fluids modified the Rb/Sr, U/Pb and Th/Pb ratios, leading to distinct variations of ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb isotopic ratios. These data therefore document the evolution of a granitic magma through magmatic (i.e., crystal fractionation), orthomagmatic (i.e., crystal-fluid fractionation) and hydrothermal (i.e., fluid fractionation) stages that culminated in the formation of a tin-base metal deposit. The Pb isotope data also constrain the source region for the DLP as being Avalonian basement that, by inference, must underlie much of the Meguma Terrane.Editorial responsibility: T.L. Grove     

Primitive and contaminated basalts from the Southern Rocky Mountains,U.S.A.

Basalts in the Southern Rocky Mountains province have been analyzed to determine if any of them are primitive. Alkali plagioclase xenocrysts armored with calcic plagioclase seem to be the best petrographic indicator of contamination. The next best indicator of contamination is quartz xenocrysts armored with clinopyroxene. On the rocks and the region studied, K₂O apparently is the only major element with promise of separating primitive basalt from contaminated basalt inasmuch as it constitutes more than 1 % in all the obviously contaminated basalts. K₂O: lead (> 4 ppm) and thorium (> 2 ppm) contents and Rb/Sr (> 0.035) are the most indicative of the trace elements studied. Using these criteria, three basalt samples are primitive (although one contains 1.7% K₂O) and are similar in traceelement contents to Hawaiian and Eastern Honshu, Japan, primitive basalts.Contamination causes lead isotope ratios, ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb, to become less radiogenic, but it has little or no effect on ⁸⁷Sr/⁸⁶Sr. We interpret the effect on lead isotopes to be due to assimilation either of lower crustal granitic rocks, which contain 5–10 times as much lead as basalt and which have been low in U/Pb and Th/Pb since Precambrian times, or of upper crustal Precambrian or Paleozoic rocks, which have lost much of their radiogenic lead because of heating prior to assimilation. The lack of definite effects on strontium isotopes may be due to the lesser strontium contents of granitic crustal rocks relative to basaltic rocks coupled with lack of a large radiogenic enrichment in the crustal rocks.Lead isotope ratios were found to be less radiogenic in plagioclase separates from an obviously contaminated basalt than in the primitive basalts. The feldspar separate that is rich in sodic plagioclase xenocrysts was found to be similar to the whole-rock composition for ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb whereas a more dense fraction probably enriched in more calcic plagioclase phenocrysts is more similar to the primitive basalts in lead isotope ratios.The primitive basalts have: ²⁰⁶Pb/²⁰⁴Pb 18.09–18.34, ²⁰⁷Pb/²⁰⁴Pb 15.5, ²⁰⁸Pb/²⁰⁴Pb 37.6–37.9, ⁸⁷Sr/⁸⁶Sr 0.704–0.705. In the primitive basalts from the Southern Rocky Mountains the values of ²⁰⁶Pb/²⁰⁴Pb are similar to values reported by others for Hawaiian and eastern Honshu basalts and abyssal basalts, whereas ²⁰⁸Pb/²⁰⁴Pb tends to be equal to or a little less radiogenic than those from the oceanic localities. ⁸⁷Sr/⁸⁶Sr appears to be equal to or a little greater than those of the oceanic localities. These ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios are distinctly less radiogenic and ⁸⁷Sr/⁸⁶Sr values are about equal to those reported by others for volcanic islands on oceanic ridges and rises.Publication authorized by the Director, U.S. Geological Survey     

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.     

Primary magmas at Oldoinyo Lengai: The role of olivine melilitites

The paper describes olivine melilitites at Oldoinyo Lengai, Tanzania, and from tuff cones from the Tanzanian rift valley in the vicinity of Oldoinyo Lengai. Oldoinyo Lengai is the only active carbonatite volcano and is distinguished by its alkali-rich natrocarbonatites. Lengai is also unique for its extreme peralkaline silicate lavas related directly to the natrocarbonatites. Primitive olivine melilitites are, according to their Mg# and Ni, Cr contents, the only candidates in the Lengai area for primary melt compositions. Incompatible trace elements, including REE, constrain the melting process in their sub-lithospheric sources to very low degrees of partial melting in the garnet stability field. The strong peralkaline trend at Oldoinyo Lengai is already recognisable in these primary or near-primary melts. More evolved olivine melilitites, with Mg# < 60 allow the fractionation line in its major and trace element expressions to be followed. Nevertheless, a large compositional gap separates the olivine melilitites and olivine-poorer melilitites from the phonolites and nephelinites that form the bulk of the Lengai cone. These silicate lavas show a high degree of peralkalinity and are highly evolved with very low Mg, Ni and Cr. Prominent examples of the recent evolution are the combeite–wollastonite nephelinites that are unique for Lengai. In their Sr, Nd, and Pb isotope relationships the olivine melilitites define a distinct group with the most depleted Sr and Nd ratios and the most radiogenic Pb isotopes. They are closest to a supposed HIMU end member of the Lengai evolution, which is characterised by an extreme spread in isotopic ratios, explained as a mixing line between HIMU and EM1-like mantle components.