Beschreibung: The western Indian Ocean has been warming faster than any other tropical ocean during the 20th century, and is the largest contributor to the global mean sea surface temperature (SST) rise. However, the temporal pattern of Indian Ocean warming is poorly constrained and depends on the historical SST product. As all SST products are derived from the International Comprehensive Ocean-Atmosphere dataset (ICOADS), it is challenging to evaluate which product is superior. Here, we present a new, independent SST reconstruction from a set of Porites coral geochemical records from the western Indian Ocean. Our coral reconstruction shows that the World War II bias in the historical sea surface temperature record is the main reason for the differences between the SST products, and affects western Indian Ocean and global mean temperature trends. The 20th century Indian Ocean warming pattern portrayed by the corals is consistent with the SST product from the Hadley Centre (HadSST3), and suggests that the latter should be used in climate studies that include Indian Ocean SSTs. Our data shows that multi-core coral temperature reconstructions help to evaluate the SST products. Proxy records can provide estimates of 20th century SST that are truly independent from the ICOADS data base.

Beschreibung: Highlights • First comprehensive data set of the seamounts from the Walvis Ridge. • The seamounts are 20–40 Myr younger than the age progressive Walvis Ridge basement. • The composition of the seamounts extends from the St. Helena HIMU to EMORB. • The seamounts are derived from a distinct source compared to the Walvis Ridge. • The temporal change from EM I to HIMU could reflect the compositional heterogeneities of the LLSVP. Abstract Volcanic activity at many oceanic volcanoes, ridges and plateaus often reawakens after hiatuses of up to several million years. Compared to the earlier magmatic phases, this late-stage (rejuvenated/post-erosional) volcanism is commonly characterized by a distinct geochemical composition. Late-stage volcanism raises two hitherto unanswered questions: Why does volcanism restart after an extended hiatus and what is the origin of this volcanism? Here we present the first 40Ar/39Ar age and comprehensive trace element and Sr–Nd–Pb–Hf isotopic data from seamounts located on and adjacent to the Walvis Ridge in the South Atlantic ocean basin. The Walvis Ridge is the oldest submarine part of the Tristan-Gough hotspot track and is famous as the original type locality for the enriched mantle one (EM I) end member. Consistent with the bathymetric data, the age data indicates that most of these seamounts are 20–40 Myr younger than the underlying or nearby Walvis Ridge basement. The trace element and isotope data reveal a distinct compositional range from the EM I-type basement. The composition of the seamounts extend from the St. Helena HIMU (high time-integrated 238U/204Pb mantle with radiogenic Pb isotope ratios) end member to an enriched (E) Mid-Ocean-Ridge Basalt (MORB) type composition, reflecting a two-component mixing trend on all isotope diagrams. The EMORB end member could have been generated through mixing of Walvis Ridge EM I with normal (N) MORB source mantle, reflecting interaction of Tristan-Gough (EM I-type) plume melts with the upper mantle. The long volcanic quiescence and the HIMU-like geochemical signature of the seamounts are unusual for classical hotspot related late-stage volcanism, indicating that these seamounts are not related to the Tristan-Gough hotspot volcanism. Two volcanic arrays in southwestern Africa (Gibeon-Dicker Willem and Western Cape province) display similar ages to the late-stage Walvis seamounts and also have HIMU-like compositions, suggesting a larger-scale event at ∼77–49 Ma. We propose that the EM I-like mantle plumes rise from the edges of the African Large Low Shear Velocity Province (LLSVP; Tristan-Gough, Discovery and Shona hotspot), whereas the HIMU-dominated intraplate lavas (St. Helena, Gibeon-Dicker Willem and Western Cape province) and the late-stage Walvis seamounts tap material from internal portions of the African LLSVP, suggesting possible lateral and/or vertical chemical zonation of the African LLSVP.

Beschreibung: The 541 ± 4 Ma-old magnesian, weakly peraluminous, calc-alkalic Donkerhoek Onanis granite is part of the ca. 6000 km2 large Donkerhoek batholith in the Southern Zone of the Damara orogen of Namibia. Linear major and trace element variations and decreasing MgO, FeO, Al2O3, CaO, K2O, Na2O, Ba and Sr concentrations with increasing SiO2 indicate that this part of the batholith represent a coherent mass and underwent fractional crystallization processes. The Donkerhoek Onanis granites are isotopically evolved (initial εNd: −4.7 to −12.3, initial 87Sr/86Sr: 0.7099–0.7157) with moderately radiogenic Pb isotope ratios (206Pb/204Pb: 17.26–18.22; 207Pb/204Pb: 15.59–15.67; 208Pb/204Pb: 37.60–38.06). Beside heterogeneities imparted by the sources, an evaluation of LREE fractionation and Nd isotope data suggests that AFC processes also modified some samples. Based on the chemical and isotope data, the Donkerhoek Onanis granites cannot be derived by partial melting of Al- and Fe-rich metasedimentary rocks of the Kuiseb formation in which they intruded. Instead, melting of meta-igneous crustal sources with Proterozoic crustal residence ages is more likely. Three igneous to meta-igneous rock suites from the area (Matchless amphibolites, Proterozoic mafic to felsic gneisses from the southern Kalahari craton basement, syn-tectonic Salem granodiorites to granites) are potential sources. An evaluation of chemical and isotope data suggests that remelting of early syn-orogenic Salem-type granites is the most likely process which would also explain the existence of ca. 563 ± 4 Ma-old zircon in the Donkerhoek Onanis granites. Comparison of the Donkerhoek Onanis granites with experimentally derived melt compositions from an intermediate igneous parent indicates temperatures between 800 and 850 °C. It is suggested that the Pan-African igneous activity in this part of the Damara Belt was a moderate-temperature intra-crustal event. Although there are some compositional similarities with juvenile granites generated in subduction zones, unradiogenic Pb isotope ratios and moderately radiogenic Sr and unradiogenic Nd isotopes suggest that reprocessed crustal rocks are more likely sources. Previously obtained high δ18O values of the Donkerhoek Onanis granites ranging from 11.8 to 13.6‰, covering the range of δ18O values obtained on Salem-type granites from the area (12.5–13.3‰) confirm this view. In contrast to igneous processes along active continental margins that produce juvenile batholiths with calc-alkaline affinities, this igneous event was not a major crust-forming episode and the Donkerhoek Onanis granites represent reprocessed crustal material.

Beschreibung: Highlights • Melt inclusions from southern Payenia have highly variable element enrichment • Magmas formed by mixing of asthenospheric high Nb/U and lithospheric low Nb/U melts • Low Nb/U type inclusions are similar in composition to alkaline lamprophyres • Low Nb/U melts were formed by fractionation of high Nb/U melts in the SCLM • The percolative fractional crystallization involved cpx, rutile and apatite Abstract We present major and trace element compositions of melt inclusions from three alkali basalts from the Río Colorado volcanic field in the Payenia backarc province, Argentina. Modeling of diffusion profiles around the inclusions showed that most inclusions equilibrated 〈14 days after formation, indicating a short crustal residence time for the magmas and nearly direct ascent through the crust. Despite overlapping host rock isotopic compositions, the inclusions show a large variation in their degree of enrichment, and display trends that we interpret as mixing between asthenospheric OIB-type low K2O-high Nb/U melts and enriched high K2O-low Nb/U lithospheric mantle melts similar in composition to alkaline lamprophyres. The low Nb/U magmas are excessively enriched in the elements Cs, Rb, Ba, Th, U, K, Pb and Cl relative to Nb, Ta and REEs. The enriched low Nb/U components are interpreted to have formed by percolative fractional crystallization of asthenospheric high Nb/U melts in the lithospheric mantle involving crystallization of clinopyroxene, apatite and rutile. The residual fluid-rich melts either mixed directly with new batches of high Nb/U melts or metasomatized and veined the lithospheric mantle which later re-melted during continued volcanism. The major element compositions of the high K2O-low Nb/U components are distinct for the whole rocks and melt inclusions, and most enriched inclusions have lower SiO2 and higher TiO2 contents indicating derivation by melting of amphibole-bearing veins. In contrast, most wr low Nb/U basalts have higher SiO2 and lower TiO2 and were most likely formed by melting of pyroxenitic veins or peridotitic metasomatized lithospheric mantle.

Beschreibung: Highlights • First data on the composition of deep crust and primitive rocks of high-Ti magmatic series of Manihiki Plateau • High potential mantle temperature for Manihiki sources (〉1460oC) suggests a lower mantle plume origin • EM1 signature in high-Ti Manihiki basalts could originate from recycled lower continental crust or re-fertilized SCLM • The presence of refractory mantle in the Manihiki plume explains 30% lower crustal thickness compared to OJP • Manihiki and OJP could have been formed from a geochemically zoned plume or from two spatially separated mantle plumes Abstract Geochemical studies revealed two major (high- and low-Ti) magmatic series composing the Manihiki Plateau in the Western Pacific. Here we report new geochemical data (major and trace element and Sr-Nd-Pb isotope compositions) of the Manihiki rocks. The rocks belong to the previously rarely sampled high-Ti Manihiki series and represent a section of deep crust of the plateau. The rocks were collected by remotely operated vehicle ROV Kiel 6000 during R/V SONNE SO225 expedition from a tectonic block at a stretched and faulted boundary between the Northern and Western Manihiki sub-plateaus. Additional data is presented on samples obtained by dredging during the same cruise. Judging from the age of stratigraphically higher lavas, most samples must be ≥125 Ma old. They comprise fully crystalline microdolerites, aphyric and Ol-Px-Pl-phyric basalts and breccias metamorphosed under greenschist to amphibolite facies with peak metamorphic temperatures of 636–677 °C and pressures of 2.0–2.7 kbar. A single sample of hornblende gabbro was also recovered and likely represents a late stage intrusion. Despite strong metamorphism, the samples from the ROV profile reveal only minor to moderate chemical alteration and their initial compositions are well preserved. The rocks are relatively primitive with MgO up to 13 wt%, range from enriched to depleted in LREE (LaN/SmN = 0.7–1.1), exhibit variable but mostly depleted Nb contents (Nb/Nb* = 0.8–1.3) and display only a narrow range in isotope compositions with strong EM1 characteristics (εNd (t) = 1.8–3.6, 206Pb/204Pb (t) = 17.9–18.1, 207Pb/204Pb (t) = 15.49–15.53, 208Pb/204Pb (t) = 38.08–38.42). The parental magmas are interpreted to originate from a thermochemical plume with a potential mantle temperature 〉1460 °C. The trace element and isotope EM1 signature of the high-Ti rocks reflects the presence of recycled lower continental crust material or re-fertilized subcontinental lithospheric mantle in the plume source. A highly refractory mantle was the primary source of the low-Ti basalts and could also contribute to the origin of high-Ti basalts. On average a more depleted mantle source for the Manihiki rocks can explain ~30% lower crustal thickness of this plateau compared to Ontong Java Plateau, which was mainly formed by melting of similarly hot but more fertile mantle. The presently available data suggest that the sources of Ontong Java and Manihiki Plateaus were compositionally different and could represent two large domains of a single plume or two contemporaneous but separate plumes.

Beschreibung: It was proposed to utilize siderite FeCO3 in mid to late Archaean Superior type banded as a proxy to constrain the CO2 partial pressure of Archaean atmospheres. Implicit in this proposition is that siderite was a primary carbonate mineral that crystallized directly from Fe2+ enriched Archaean seawater, in equilibrium with atmospheric CO2. To our knowledge that proposition has not been demonstrated to be valid. We test with water-gas exchange experiments under controlled CO2 partial pressures if siderite can be stabilized as a primary mineral in Fe2+ bearing seawater. Reduced seawater proxies enriched in Fe2+ and Mn2+ are equilibrated with reduced N2-CH4-CO2-H2 gas phases with variable CO2. The solid phases stabilized in Fe2+ enriched water compositions are amorphous ferrous iron hydroxy carbonates. Crystalline siderite FeCO3 is not found to be a stable phase. The phases precipitating from Mn2+ enriched water include crystalline rhodochrosite MnCO3 and possibly amorphous Mn-enriched phases. Based on these results we advise against using siderite in banded iron formations as a CO2 sensor for the Archaean atmosphere.

Beschreibung: The strong dependence of vanadium partitioning between olivine and silicate melt (DVOl-M) on redox conditions (fO2) can be used as sensitive oxybarometer in magmatic systems. Here we extend the experimental database on DVOl-M, obtained so far at high temperatures (mainly above 1250 °C), to lower temperatures that are typical for island-arc basalts. Crystallization experiments were performed using a composition from Mutnovsky volcano (Kamchatka), and the investigated temperature, pressure, and oxygen fugacity ranges were 1025–1150 °C, 0.1 and 0.3 GPa, and ΔQFM of –0.5 to +3.2, respectively. The water content in melts ranged from 0.6 to ∼6.5 wt% H2O. The data demonstrate a strong negative correlation between DVOl-M and oxygen fugacity, similar to the behavior observed previously at higher temperatures and in MgO-rich compositions. The correlation between DVOl-M and ΔQFM in the range from –0.5 to +3.2 is described for melts with MgO 〈 12 wt% and Na2O 〈 4 wt% at temperatures ≤1250 °C by the empirical equation: ΔQFM = −3.07−0.29+0.26 logDVOl-M – 3.34−0.49+0.40 with the standard error (SE) as a function of logDVOl-M: 2SE(ΔQFM) = –0.275logDVOl-M + 0.4. We suggest that this equation can be used as an oxybarometer, which is particularly well applicable to the hydrous island-arc magmas at relatively low temperature. Application of the equation to the composition of melt inclusions and their host olivine phenocrysts from basalts of Mutnovsky volcano, containing vanadium concentrations in the range of 250–370 and 4–6 ppm, respectively, reveals an oxygen fugacity in the range ΔQFM +1.9 to +2.3. The estimates are in a good agreement with olivine-spinel oxybarometry for Mutnovsky basalts and may be typical for moderately evolved island-arc magmas.

Beschreibung: We evaluate the potential of ophiolites as archives of paleoseawater and hydrothermal fluid compositions by analysing the chemical and isotopic composition of abiogenic carbonates, precipitated from fluids within the oceanic crust of the 91 Ma Troodos Ophiolite, Cyprus. Calculated variations in fluid Mg/Ca, Sr/Ca and Sr-87/ Sr-86 with temperature within the upper sections of the ophiolite are similar to those from drilled oceanic crust, and yield literature values for late Cretaceous seawater Mg/Ca, Sr/Ca and Sr-87/ Sr-86. This indicates that carbonates from ophiolites could be used to estimate the composition of ancient seawater at times before the age of the oldest preserved in-situ oceanic crust. Whereas most carbonates recovered from in-situ oceanic crust were precipitated at temperatures 〈 60 degrees C, abiogenic carbonates from the Troodos Ophiolite formed over a temperature range of 7 degrees C to 218 degrees C. These provide unique insights into the chemical and mineralogical processes that transform seawater into a high temperature hydrothermal fluid within the oceanic crust. We use 'hydrothermal variation diagrams' of Mg/Ca, Sr/Ca, Sr-87/ Sr-86 and delta(44)/Ca-40 versus calculated temperature (delta O-18) to trace this fluid evolution within the Troodos oceanic crust. We find that successive fluid-crust-interaction, the precipitation of Mg- and Ca-bearing minerals and the early formation of anhydrite (〉 44 degrees C) gradually transform Cretaceous seawater into a Troodos hydrothermal fluid. Comparison of the Troodos data with a global dataset of abiogenic carbonates from in-situ oceanic crust shows that the chemical pathways of low-temperature fluid evolution are similar for all Cretaceous sites. These different sites represent varied geotectonic settings (midocean ridge vs. suprasubduction zone), with different basement composition (basalt, basaltic andesite/boninite) and situated in different ocean basins (Atlantic, Pacific, Mediterranean [Tethys]). The similarity in the carbonate record indicates that these differences do not significantly influence seafloor weathering and hydrothermal alteration at low temperatures. However, abiogenic carbonates from younger oceanic crust differ from the Cretaceous trends and follow different fluid evolution pathways. This indicates, that temporal variations in the composition of seawater may control the nature and the extent of seafloor weathering and hydrothermal alteration at low temperatures. A thermodynamic model of fluid-crust interaction, in which modern and Cretaceous seawater are heated to 200 degrees C while an average Troodos basaltic andesite is successively added under otherwise identical conditions predicts that fluid evolution and alteration of the oceanic crust were different in the Cretaceous than they are today, and that initial seawater chemistry affects the nature and the extent of seafloor alteration up to moderate fluid temperatures. For example, twice the amount of carbonate formed during alteration of the oceanic crust in the Cretaceous compared to modern times, indicating that the flux of CO2 from the hydrosphere-atmosphere system into the oceanic crust was greater in the Cretaceous than it is nowadays, and that it probably varied throughout geologic time.

Beschreibung: Tertiary rift-related intraplate basanites from the Batain basin of northeastern Oman have low SiO2 (〈 45.6 wt.%), high MgO (〉 9.73 wt.%) and moderate to high Cr and Ni contents (Cr 〉 261 ppm, Ni 〉 181 ppm), representing near primary magmas that have undergone fractionation of mainly olivine and magnetite. Rare earth element systematics and p-T estimates suggest that the alkaline rocks are generated by different degrees of partial melting (4–13%) of a spinel-peridotite lithospheric mantle containing residual amphibole. The alkaline rocks show restricted variations of 87Sr/86Sr and 143Nd/144Nd ranging from 0.70340 to 0.70405 and 0.51275 to 0.51284, respectively. Variations in Pb isotopes (206Pb/204Pb: 18.59–18.82, 207Pb/204Pb: 15.54–15.56, 208Pb/204Pb: 38.65–38.98) of the alkaline rocks fall in the range of most OIB. Trace element constraints together with Sr–Nd–Pb isotope composition indicate that assimilation through crustal material did not affect the lavas. Instead, trace element variations can be explained by melting of a lithospheric mantle source that was metasomatized by an OIB-type magma that was accumulated at the base of the lithosphere sometimes in the past. Although only an area of less than 1000 km2 was sampled, magmatic activity lasted for about 5.5 Ma with a virtually continuous activity from 40.7 ± 0.7 to 35.3 ± 0.6 Ma. During this period magma composition was nearly constant, i.e. the degree of melting and the nature of the tapped source did not change significantly over time.

Beschreibung: Highlights • Widespread Holocene tephra KHG, from Kamchatka, is found as a cryptotephra in the NGRIP ice-core. • This is the first identification of tephra from the Kamchatka Peninsula in Greenland ice. • NGRIP KHG has an age of 7872 ± 50 a BP and improves age models for Kamchatka. • Existing 14C age estimates for the KHG eruption are too young. Abstract Contiguous sampling of Holocene ice from the NGRIP core, Greenland, has revealed a new rhyolitic cryptotephra that is geochemically identical to the KHG tephra, a widespread marker deposit originating from the Khangar volcano, Kamchatka. This is the first identification of tephra from the Kamchatka Peninsula in Greenland ice and the first finding of the KHG tephra outside Kamchatka. The NGRIP KHG has an age of 7872 ± 50 a BP 1950, and this date will help improve age models for Kamchatka, where existing age estimates of KHG are too young, thus highlighting the importance of locating long-range, low-concentration cryptotephra deposits in well-dated ice cores. In Greenland KHG is located close to the termination of the 8.2 ka BP cooling event that is also a climate feature in palaeo-records of Kamchatka. This tie-point therefore provides a unique opportunity to synchronise records of environmental change in distal locations.