ALBERT

Description: Highlights • High-Ti lavas have the same composition as Walvis Ridge and Gough Subtrack. • Low-Ti lavas are derived from a distinct source compare to the high-Ti lavas. • High-Ti and low-Ti basalts reflect the spatial zonation of the plume head. • Tristan-type composition has not been discovered in the plume head stage. • Sr-Nd-Pb-Hf isotopes from Etendeka flood basalts. Abstract The origin and distribution of geochemically distinct source components in continental flood volcanism (generally associated with the initial phase of a mantle plume head) are poorly understood. Here we present new geochemical (major and trace element and Sr-Nd-Pb-Hf isotope) data from the Etendeka flood basalts and associated dikes from northern and central Namibia that are believed to have been produced during the initial stage of the Tristan-Gough hotspot. Following earlier studies, the Etendeka lava flows and dikes are divided into high-Ti and low-Ti groups. The trace element and isotopic composition of the high-Ti tholeiitic basalts, exclusively outcropping in northern Etendeka (northwestern Namibia), are similar to the Gough-type enriched mantle I (EMI) composition found on the Walvis Ridge (the Atlantic type locality for the EMI end member). The low-Ti tholeiitic basalts, primarily outcropping in Southern Etendeka (central western Namibia), have higher 143Nd/144Nd and 207Pb/204Pb but lower 208Pb/204Pb ratios than the Gough composition. Combining our data with newly published 3He/4He data and estimates of the magma source’s potential temperature from 1520-1680◦C, we conclude that the source of the low-Ti basalts was also intrinsic to the Tristan-Gough plume, consistent with a spatially-zoned plume head. The low-Ti basalts were derived from a distinct EMI-type source component that has thus far only been detected in the initial Tristan-Gough plume head (∼132 Ma), but not the later submarine hotspot track.

Description: Highlights • New trace element and Sr-Nd-Pb isotope data for SVZ crustal and trench sediment samples. • New whole-rock Hf–O isotope data for crustal and trench sediment samples. • Nd model ages confirm a Mesoproterozoic crustal residence age for the Guarguaráz complex. • NSVZ trench sediments reflect the source composition of eroded mafic material. Abstract This paper provides new trace element and Sr-Nd-Hf-Pb-O isotope data on Neoproterozoic to Phanerozoic mid to upper crustal lithologies of the Andean basement in central Chile and western Argentina (33°-35°S; 69°-72°W). It also provides additional data on trench sediments being subducted offshore the northern segment of the Southern Volcanic Zone in Chile. Neoproterozoic metamorphic and igneous rocks from the Guarguaráz complex (Argentina; 33.6°S, 69.5°W), when back corrected to 350 Ma, display a narrow range in 87Sr/86Sri (0.713–0.718), 143Nd/144Ndi (0.5118–0.5121), εNdi (−8.1 to −1.1), εHfi (−11.4 to +1.2) and δ18O (9–13‰). Nd model ages (TDM = 1.08–1.65 Ga) for the Guarguaráz complex points to a Mesoproterozoic crustal residence age for these rocks. Metasedimentary rocks from the Carboniferous accretionary prism in central Chile (~34°S) overlap with these ranges, but differ by having lower initial 87Sr/86Sri (0.7052–0.7093) and higher δ18O (14–17‰). The Guarguaráz metamorphic and igneous rocks, when back corrected to 350 Ma, have similar Pb isotope ratios than the Chilean Carboniferous metasedimentary rocks (206Pb/204Pbi = 17.58–18.52 vs. 18.33–18.46; 207Pb/204Pbi = 15.50–15.64 vs. ~15.64; 208Pb/204Pbi = 37.70–38.36 vs. 37.98–38.18). Two Guarguaráz samples are shifted towards less radiogenic Pb isotope ratios, similar to samples representative of the Cuyania basement. This suggests that Chilenia hosts at least two geochemical components: (1) a component with unradiogenic Pb isotopes, similar to the Proterozoic Cuyania basement, and (2) a component with more radiogenic Pb isotopes, similar to Chilean Phanerozoic metasedimentary and igneous rocks. The ranges in Pb isotope ratios for the Chilean Mesozoic (206Pb/204Pbi = 18.44–19.86; 207Pb/204Pbi = 15.59–15.69; 208Pb/204Pbi = 38.30–40.30) and Miocene (206Pb/204Pbi = 18.43–18.57; 207Pb/204Pbi = 15.58–15.60; 208Pb/204Pbi = 38.33–38.46) igneous rocks are similar to those of the accretionary prism. The Mesozoic and Miocene intrusive rocks are characterized by low 87Sr/86Sri (0.704–0.708 and ~0.704, respectively) and high εNdi (−6.2 to +4.0 and + 3.9 to +5.9, respectively) and εHfi (+7.0 to +12.7 and + 8.5 to +10.8, respectively). They can be divided into two groups. Group (1), consisting exclusively of Mesozoic samples, has negative εNdi, 87Sr/86Sri 〉 0.706, elevated e.g., Ba/Th, Nb/Yb, Zr/Y and lower Nb/La, reflecting derivation from enriched (most likely overriding crust or mantle) material. Group (2), consisting of Mesozoic and Miocene rocks, has positive εNdi, εHfi, and lower initial 87Sr/86Sri than group (1) reflecting depleted mantle melts addition to the crust. Finally, Sr-Nd-O isotopic compositions of the trench sediments at latitude 33°-33.3°S are almost identical to those at latitude 35°-40°S, indicating a relative homogeneous material input along the SVZ, although there are subtle differences in REE and Pb isotopic compositions. Based on Nd–Hf isotopes, trench sediments offshore Chile (εNd〉 +1; εHf 〉 +2) and offshore Peru (εNd 〈 −2; εHf 〈 +1) have distinct compositions, reflecting the differences in input material. The positive εNd and εHf values suggest derivation from eroded depleted mantle-derived mafic material.

Description: Seawater Mg:Ca and Sr:Ca ratios are biogeochemical parameters reflecting the Earth-ocean-atmosphere dynamic exchange of elements. The ratios' dependence on the environment and organisms' biology facilitates their application in marine sciences. Here, we present a measured single-laboratory dataset, combined with previous data, to test the assumption of limited seawater Mg:Ca and Sr:Ca variability across marine environments globally. High variability was found in open-ocean upwelling and polar regions, shelves/neritic and river-influenced areas, where seawater Mg:Ca and Sr:Ca ratios range from ∼4.40 to 6.40 mmol:mol and ∼6.95 to 9.80 mmol:mol, respectively. Open-ocean seawater Mg:Ca is semiconservative (∼4.90 to 5.30 mol:mol), while Sr:Ca is more variable and nonconservative (∼7.70 to 8.80 mmol:mol); both ratios are nonconservative in coastal seas. Further, the Ca, Mg, and Sr elemental fluxes are connected to large total alkalinity deviations from International Association for the Physical Sciences of the Oceans (IAPSO) standard values. Because there is significant modern seawater Mg:Ca and Sr:Ca ratios variability across marine environments we cannot absolutely assume that fossil archives using taxa-specific proxies reflect true global seawater chemistry but rather taxa- and process-specific ecosystem variations, reflecting regional conditions. This variability could reconcile secular seawater Mg:Ca and Sr:Ca ratio reconstructions using different taxa and techniques by assuming an error of 1 to 1.50 mol:mol, and 1 to 1.90 mmol:mol, respectively. The modern ratios' variability is similar to the reconstructed rise over 20 Ma (Neogene Period), nurturing the question of seminonconservative behavior of Ca, Mg, and Sr over modern Earth geological history with an overlooked environmental effect.

Description: Tephra layers produced by volcanic eruptions are widely used for correlation and dating of various deposits and landforms, for synchronization of disparate paleoenvironmental archives, and for reconstruction of magma origin. Here we present our original database TephraKam, which includes chemical compositions of volcanic glass in tephra and welded tuffs from the Kamchatka volcanic arc. The database contains 7049 major element analyses obtained by electron microprobe and 738 trace element analyses obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on 487 samples collected in proximity of their volcanic sources in all volcanic zones in Kamchatka. The samples characterize about 300 explosive eruptions, which occurred in Kamchatka from the Pliocene until historic times. Precise or estimated ages for all samples are based on published 39Ar/40Ar dates of rocks and 14C dates of host sediments, statistical age modelling and geologic relationships with dated units. All data in TephraKam is supported by information about source volcanoes and analytical details. Using the data, we present an overview of geochemical variations of Kamchatka volcanic glasses and discuss application of this data for precise identification of tephra layers, their source volcanoes, temporal and spatial geochemical variations of pyroclastic rocks in Kamchatka.

Description: Highlights • The vent fluids discharged from the Lutao hydrothermal field experienced low-degree subcritical phase separation. • The temperature and chemical compositions of the vent fluids were modulated by tides. • The time delay between tides and the response of hydrothermal system was about 3 h. • The typhoon “Fung-wong” cooled the reaction zone and decreased the degree of phase separation. • The hydrothermal system began to recover after the typhoon passed by. Abstract The Lutao hydrothermal field is an intertidal arc-volcanic system located offshore southeast Taiwan, hosting a Zhudanqu (ZDQ) vent and a Huwaichi (HWC) spring with strongly contrasting fluid chemistry. Low Mg, moderately enriched Cl, and H+ with respect to seawater indicate that the ZDQ endmember was derived from the brine phase that was formed during low-degree subcritical phase separation. In contrast, the endmember for the HWC vent fluids is related to the vapor phase. Temperature and pressure of the phase separation were estimated as ~150 °C and ~7 bar, respectively. The water/rock ratio was roughly calculated as about 2. The Lutao hydrothermal system was slightly affected by semi-diurnal tides, by some combination of tidal loading and tidal currents. The time delay between tides and the response of the hydrothermal system was about 3 h. While freshwater was almost absent in the HWC vent fluids at normal conditions, the typhoon “Fung-wong” on Sep 21st, 2014, led to intrusions of freshwater into the vent fluids with a percentage of ~16%. Both the ZDQ and the HWC endmember compositions showed some changes after the typhoon event, suggesting a cooling of the reaction zone. After the typhoon passed by, the hydrothermal system began to recover, evidenced by increasing percentages of the HWC endmember and decreasing freshwater contributions. The flux of the HWC endmember was estimated as 460–560 L h−1 based on these observations. This study, for the first time, reports a shallow-depth tidal-influenced hydrothermal system that was temporarily cooled by a tropical storm.

Description: Highlights • Lithospheric mantle stabilization under Jiao-Liao-Ji Belt at 2.8Ga (SmNd isochron) • Liaohe mafic meta-igneous rocks formed in active continental margin subduction zone • Emplacement of Liaohe mafic igneous rocks at ~2.2 Ga (LuHf isochron) • Amphibolite retrograde metamorphism from exhumation at 1824 ± 19 Ma (PbPb isochron) • Cooling of terrane to ~500 °C at 1671 ± 58 Ma (RbSr isochron) Abstract The assembly and long-term evolution of the Eastern Block of the North China Craton are poorly constrained. Here we use bulk rock geochronological and geochemical data from mafic meta-igneous rocks (hornblendites, amphibolites and a metagabbro) of the Liaohe Group to reconstruct the Neoarchean to Paleoproterozoic history of the Jiao-Liao-Ji Belt, located between the Longgang and Nangrim blocks that together form the Eastern Block of the North China Craton. The mafic/ultramafic meta-igneous rocks have intrusive or tectonic contacts with the Liaoji granitic rocks (~2.2–2.0 Ga), which form the basement of the Jiao-Liao-Ji Belt. The major and trace element data indicate that the protoliths had calc-alkaline composition and formed along an active continental margin subduction zone. The mafic rocks form a whole-rock 176Lu/177Hf isochron with an age of 2.25 ± 0.31 Ga, overlapping with UPb zircon ages for mafic and granitic rocks from the Jiao-Liao-Ji Belt and consistent with being the emplacement age of the mafic protoliths along the active continental margin. In contrast, the whole-rock 147Sm/144Nd isochron age of 2.83 ± 0.18 Ga is likely to reflect the average age of the lithospheric mantle source from which the mafic/ultramafic protoliths were extracted. Together with geological evidence, we propose that the southwestern portion of the Longgang Block was an active continental margin since at least the early Paleoproteorozic. Literature age data from metamorphic zircons show that peak granulite metamorphism took place at ~1.96–1.88 Ga, resulting from the collisional event that fused the Longgang and Nangrim blocks into the Eastern Block of the North China Craton. Our bulk-rock 207Pb/206Pb age of 1824 ± 19 Ma and our 87Rb/86Sr age of 1671 ± 58 Ma reflect retrograde (cooling) stages during the exhumation of the Jiao-Liao-Ji Belt after the orogenesis.

Description: The Labrador Sea is important for the modern global thermohaline circulation system through the formation of intermediate Labrador Sea Water (LSW) that has been hypothesized to stabilize the modern mode of North Atlantic deep-water circulation. The rate of LSW formation is controlled by the amount of winter heat loss to the atmosphere, the expanse of freshwater in the convection region and the inflow of saline waters from the Atlantic. The Labrador Sea, today, receives freshwater through the East and West Greenland currents (EGC, WGC) and the Labrador Current (LC). Several studies have suggested the WGC to be the main supplier of freshwater to the Labrador Sea, but the role of the southward flowing LC in Labrador Sea convection is still debated. At the same time, many paleoceanographic reconstructions from the Labrador Shelf focussed on late deglacial to early Holocene meltwater run-off from the Laurentide Ice Sheet (LIS), whereas little information exists about LC variability since the final melting of the LIS about 7000 years ago. In order to enable better assessment of the role of the LC in deep-water formation and its importance for Holocene climate variability in Atlantic Canada, this study presents high-resolution middle to late Holocene records of sea surface and bottom water temperatures, freshening, and sea ice cover on the Labrador Shelf during the last 6000 years. Our records reveal that the LC underwent three major oceanographic phases from the mid- to late Holocene. From 6.2 to 5.6 ka, the LC experienced a cold episode that was followed by warmer conditions between 5.6 and 2.1 ka, possibly associated with the late Holocene thermal maximum. While surface waters on the Labrador Shelf cooled gradually after 3 ka in response to the neoglaciation, Labrador Shelf subsurface or bottom waters show a shift to warmer temperatures after 2.1 ka. Although such an inverse stratification by cooling of surface and warming of subsurface waters on the Labrador Shelf would suggest a diminished convection during the last 2 millennia compared to the mid-Holocene, it remains difficult to assess whether hydrographic conditions in the LC have had a significant impact on Labrador Sea deep-water formation.

Description: Microbial metabolisms in sediments play a pivotal role in marine element cycling. In hydrothermal sediments chemosynthetic microorganisms likely prevail, while in non-hydrothermally impacted sediment regimes microorganisms associated with organic matter decomposition are primarily recognized. To test how these microorganisms are distributed along the hitherto neglected transition zone influenced to different degrees by hydrothermal input we sampled four sediment sites: these were (i) near an active vent, (ii) the outer rim, and (iii) the inactive area of the Kairei hydrothermal field as well as (iv) sediments roughly 200 km south-east of the Kairei field. Chemistry and microbial community compositions were different at all sampling sites. Against expectations, the sediments near the active vent did not host typical chemosynthetic microorganisms and chemistry did not indicate current, extensive hydrothermal venting. Data from the outer rim area of the active Kairei field suggested microbially mediated saponite production and diffuse hydrothermal flow from below accompanied by increased metal concentrations. A steep redox gradient in the inactive Kairei field points towards significant redox driven processes resulting in dissolution of hydrothermal precipitates and intense metal mobilization. Local microorganisms were primarily Chloroflexi, Bacillales, Thermoplasmata, and Thaumarchaeota.

Description: Despite their large number and in contrast to seamount chains, small solitary seamounts/seamount groups have rarely been sampled and therefore their origins remain enigmatic. Here we present 40Ar/39Ar, trace element and Nd‐Pb‐Hf isotope data from the solitary Demenitskoy Seamount, the isolated Tolkien seamount group and the Krylov Seamount and Ridge in the Canary Basin. Their chemical compositions range from intraplate ocean‐island‐basalt (Demenitskoy) to normal‐ and enriched‐mid‐ocean‐ridge‐basalt types (Tolkien and Krylov system, respectively). Lavas from all three seamount groups, however, show geochemical evidence for involvement of enriched (Canary/Cape Verde) plume material. Seismic tomography shows that large areas around these mantle plumes are affected by dispersal of presumably low‐velocity material, which also could have fed nearby isolated seamounts with diffusely‐upwelling plume material. This process may be extremely widespread but has been poorly studied to date.