Plio-Pleistocene changes in water mass exchange and erosional inputs in the Fram Strait

We determined the isotopic composition of neodymium (Nd) and lead (Pb) of past seawater to reconstruct water mass exchange and erosional input between the Arctic Ocean and the Norwegian-Greenland Seas over the past 5 Ma. For this purpose, sediments of ODP site 911 (leg 151) located at 900 m water depth on the Yermak Plateau in the Fram Strait were used. The paleo-seawater variability of Nd and Pb isotopes was extracted from the sea water-derived metal oxide coatings on the sediment particles following the leaching method of Gutjahr et al. (2007). All radiogenic isotope data were acquired by Multi-Collector (MC) ICP-MS. The site 911 stratigraphy of Knies et al. (2009) was applied. Surface sediment Sr and Nd isotope data, as well as downcore Sr isotope data obtained on the same leaches are close to seawater and confirm the seawater origin of the Nd and Pb isotope signatures. The deep water Nd isotope time series extracted from site 911 was in general more radiogenic ("Nd = -7.5 to -10) than present day deep water ("Nd = -9.8 to -11.8) in the area of the Fram Strait (Andersson et al., 2008) and does not show a systematic trend with time. In contrast, the radiogenic isotope composition of Pb evolved from 206Pb/204Pb ratios around 18.7 to more radiogenic values around 19.2 between 2 Ma and today. The data indicate that mixing of water masses from the Arctic Ocean and the Norwegian-Greenland Seas has controlled the Nd isotope signatures of deep waters on the Yermak Plateau over the past 5 Ma. Prior to 1.7 Ma the Nd isotope signatures on the Yermak Plateau were less radiogenic than waters from the same depth in the central Arctic Ocean (Haley et al., 2008) pointing to a greater influence from the Norwegian-Greenland Seas. After 1.7 Ma the central Arctic and Yermak Plateau data have varied around similar values indicating water mass mixing overall similar to today. In contrast, the Pb isotope composition of deep waters in the Fram Strait appears to have been dominated by weathering inputs from glacially weathering old continental landmasses, such as Greenland or parts of Svalbard since 2 Ma. A similar control over the Pb isotope evolution of seawater since the onset of Northern Hemisphere Glaciation was recorded by ferromanganese crusts that grew from North Atlantic DeepWater in the western North Atlantic. References: Gutjahr, M., Frank, M., Stirling, C.H., Klemm, V., van de Flierdt, T. and Halliday, A.N. (2007): Reliable extraction of a deepwater trace metal isotope signal from Fe-Mn oxyhydroxide coatings of marine sediments.- Chemical Geology 242, 351-370 Haley B. A., M. Frank, R.F. Spielhagen and A. Eisenhauer (2008): Influence of brine formation on Arctic Ocean circulation over the past 15 million years. Nature Geoscience 1, 68–72 Andersson, P.S., Porcelli, D., Frank, M., Björk, G., Dahlqvist, R. and Gustafsson, Ö. (2008): Neodymium isotopes in seawater from the Barents Sea and Fram Strait Arctic- Atlantic gateways.- Geochim. Cosmochim. Acta 72, 2854-2867 Knies, J., J. Matthiessen, C. Vogt, J.S. Laberg, B.O. Hjelstuen, M.Smelror, E. Larsen, K. Andreassen, T. Eidvin and T.O. Vorren (2009): The Plio-Pleistocene glaciation of the Barents Sea–Svalbard region: a new model based on revised chronostratigraphy - Quaternary Science Reviews 28, 9-10, 812-82

Holocene fluctuations of neodymium isotope ratios in eastern Fram Strait sediments - An indication for deepwater variability?

EGU2012-11739 The Fram Strait as the only deep water connection of the world’s oceans to the Arctic plays a substantial role for the heat influx to the Arctic Ocean and controls freshening of the Nordic Seas through Arctic sea ice export. Large amounts of warm and saline Atlantic Water derived from the North Atlantic Drift transport most of the heat through eastern Fram Strait to the Arctic basin, resulting in year-round ice-free conditions. Arctic sea ice and cold and fresh waters exit the western part of the strait southward along the Greenland shelf. However, little is still known about the water mass transport at intermediate and bottom water depths in the Fram Strait. High-resolution Holocene sediment sequences from the Western Svalbard margin have been investigated for its neodymium isotope ratios stored in ferromanganese oxyhydroxide coatings of the sediment to derive information on the source of bottom seawater passing the site. The radiogenic isotope data are compared to a multitude of proxy indicators for the climatic and oceanographic variability in the eastern Fram Strait during the past 8,500 years. In order to obtain a calibration of the Nd isotope compositions extracted from sediments to modern bottom water mass signatures in the area, a set of core top and water samples from different water depths in the Fram Strait was additionally investigated for its present-day Nd isotope signatures. A significantly higher inflow of deepwater produced in the Nordic Seas to the core site is inferred for the earlier periods of the Holocene. Cooler surface water conditions and increased sea ice abundances during the late Holocene coincide with more radiogenic Nd isotope ratios likely resembling the neoglacial trend of the northern North Atlantic

Die Rekonstruktion der Entwicklung des Wassermassenaustauschs und der Klimaschwankungen in der Norwegisch- Grönländischen See und Nordatlantischen Ozeans während des Plio- und Pleistozäns

Global climate of the Plio-Pleistocene has been largely impacted by changes of the Atlantic Meridional Overturning Circulation (AMOC), which has most likely to a large extent been controlled by the formation of North Atlantic Deep Water (NADW) ventilating the entire Atlantic Ocean. The main areas contributing to the formation of NADW have been the Labrador Sea, the Nordic Seas, and North Atlantic Ocean. The circulation pattern in these areas underwent dramatic changes during the Plio – and Pleistocene, in particular during the onset and intensification of the Northern Hemisphere Glaciation (NHG) at 2.7 Ma and the Mid Pleistocene Transition (MPT) 1.5 – 0.6 Ma. The main goal of this study is to improve the understanding of the role of changes in ocean circulation in these key areas and their relationships to glaciations controlling and responding to climate. To investigate the past water mass mixing and erosional input between the climatically important areas of Nordic Seas and North Atlantic Ocean, the radiogenic isotope signatures of neodymium (Nd), lead (Pb) and strontium (Sr) of seawater-derived ferromanganese coatings on sediment particles, as well as of detrital material have been analyzed. Sediments obtained from six ODP (Ocean Drilling Program) Sites covering intermediate to deep waters in the Nordic Seas and North Atlantic Ocean are the basis of this study. Two Sites are located in the North Atlantic Ocean on Bjørn Drift (Site 984) in 1648 m water depth and on Rockall Plateau (Site 982) in 1135 m water depth. The sites in the Nordic Seas are located in the region influenced by Arctic Intermediate Water on Iceland Plateau (Site 907, 1800 m water depth) and of the inflowing Atlantic waters on Vøring Plateau (Site 644, 1220 m water depth), on the Svalbard margin (Site 986, 2051 m deep) and on Yermak Plateau in the Fram Strait (Site 911, 906 m water depth). The extraction of seawater Nd and Pb isotope compositions from authigenic ferromanganese coatings, as well as Nd isotope composition from the calcitic foraminiferal shells from locations in the western part of the study area (Sites 907 and 984) gave no reliable results as compared to the present seawater signatures in this area, mainly as a consequence of partial dissolution of basaltic ash particles during the extraction procedures. In contrast, for the sites (982, 644, 986, 911) in the eastern part of the Nordic Seas and in the North Atlantic Ocean reliable past bottom water signatures were extracted using different leaching methods and allow paleoreconstructions for the past 3 - 5 Ma. The Nd and Pb isotope records of ferromanganese coatings of Site 911 on the Yermak Plateau were extracted with a leach method based on studies without a preceding decarbonation step. These records show only small variations prior to 2.7 Ma pointing to a continuous Atlantic inflow under generally warmer climatic conditions. In contrast, since the intensification of the Northern Hemisphere Glaciation the Nd isotope composition has shown a high variability as a consequence of the waxing and waning of ice sheets on Svalbard, the Barents Sea and Eurasia. The Pb record at the same time reveals a trend to more radiogenic values as a result of increasing glacial weathering inputs from old continental landmasses after 2 Ma. A similar evolution is evident from the Nd and Pb isotope records of Sites 986, 644 and 982, which were extracted from seawater derived ferromanganese coatings applying the leaching method with a preceding decarbonation step. Comparison of the Nd isotope records of these three ODP sites shows periods of a warmer climate, moderate glacial conditions and an enhanced Atlantic inflow into the Nordic Seas prior to the onset of the NHG at 2.7 Ma and between 2.2 Ma and 1.5 Ma. In contrast, at the beginning of the MPT between 1.5 and 1.2 Ma the Atlantic inflow was significantly reduced, expressed as marked differences between the Nd isotope records in the Nordic Seas and North Atlantic Ocean. Sediment provenance obtained from Nd and Pb isotope compositions of the detrital material shows a dominant influence of Svalbard, Barents Sea and Eurasian shelf at the sites on the Svalbard margin and Yermak Plateau. The Vøring Plateau has been dominated by sediment input from Scandinavia to which material from the Barents Sea via icebergs and sea ice was added during glaciations. In contrast, the Rockall Plateau shows the lowest erosional influence due to its location distant from land. However, the Pb isotope evolution obtained from ferromanganese coating of sediment of Site 982 nevertheless points to a slightly increased erosional input from Greenland and Iceland after 1.2 Ma. The results of this study provide a deeper insight into the overall climate control mechanisms of the past 5 Myr and will contribute to improve the understanding of future climatic change.Das globale plio-pleistozäne Klima ist in hohem Maße durch die atlantische thermohaline Zirkulation beeinflusst worden, die wahrscheinlich hauptsächlich durch die Bildung von Nordatlantischem Tiefenwasser angetrieben wurde, das heute den gesamten Atlantischen Ozean belüftet. Hauptgebiete für die Entstehung des Nordatlantischen Tiefenwassers sind die Labrador See, die Grönländisch-Norwegische See und der Nordatlantische Ozean. Das Zirkulationsmuster in diesen Gebieten unterlag während des Plio-Pleistozäns signifikanten Änderungen, besonders während der Intensivierung der Nordhemisphärenvereisung vor 2.7 Millionen Jahren und des Mittelpleistozänen Klimawandels vor 1.5 bis 0.6 Millionen Jahren. Das Hauptziel dieser Studie ist die Verbesserung des Verständnisses der Zusammenhänge zwischen Änderungen der Zirkulation und der kontinentalen Vereisung und des Klimas. Um die Wassermassenverteilung und den Verwitterungseintrag zwischen den klimarelevanten Gebieten der Grönländisch-Norwegischen See und dem Atlantischen Ozean zu untersuchen, wurden die radiogenen Isotopenverhältnisse von Neodym (Nd), Blei (Pb) und Strontium (Sr) aus dem authigenen Mangan-Eisenoxidcoating der Partikel und die Isotopensignaturen der Sedimentpartikel selbst analysiert. Sechs Sedimentkerne des Ocean Drilling Programms (ODP) aus intermediären und tiefen Wasserschichten in der Grönländisch-Norwegischen See und dem Atlantischen Ozean waren die Basis dieser Studie. Zwei Kerne wurden im Atlantischen Ozean auf dem Bjørn Drift (Site 984) in 1648 m Wassertiefe und auf dem Rockall Plateau (Site 982) in 1135 m Wassertiefe gewonnen. Die Kerne in der Grönländisch-Norwegischen See befinden sich im Einflussbereich des Arktischen Zwischenwassers auf dem Island Plateau (Site 907, 1800 m tief) und des einströmenden Atlantikwassers auf dem Vøring Plateau (Site 644, 1220 m tief), auf dem Svalbard-Schelf (Site 986, 2051 m tief) und auf dem Yermak Plateau in der Framstraße (Site 911, 906 m tief). Die Nd- und Pb- Isotopenverhältnisse aus dem authigenen Mangan-Eisenoxidcoating, sowie die Nd-Isotopenverhältnisse aus Foraminiferenkalzit an den Kernlokationen im westlichen Teil des Gebiets (Site 907 und 984) wurden verglichen mit den heutigen Wassermassenzusammensetzungen in diesem Gebiet und ergaben keine verlässlichen Daten, hauptsächlich durch die teilweise Lösung von basaltischen Aschepartikeln. Für die Kerne (982, 644, 986 und 911) im östlichen Teil der Grönländisch-Norwegischen See und dem Nordatlantischen Ozean war es möglich, verlässliche Tiefenwassersignaturen unter Verwendung verschiedener Laugungsmethoden zu gewinnen, die die Erstellung weitergehender Paläorekonstruktionen über die vergangenen 3 bis 5 Millionen Jahre erlaubte. Die Nd- und Pb-Isotopenzeitserien der Mangan-Eisenoxidcoatings von Site 911 auf dem Yermak Plateau wurden durch Anwendung der Laugungsmethode ohne einen vorherigen Entkarbonatisierungsschritt gewonnen. Diese Daten zeigen nur geringe Schwankungen im Zeitraum vor 2.7 Millionen Jahren, was auf einen kontinuierlichen Zufluss von Atlantikwasser unter generell wärmeren Klimabedingungen hindeutet. Seit dem Beginn der Nordhemisphären-Vereisung zeigten die Neodym-Isotopendaten eine höhere Variabilität als Folge der wachsenden und schrumpfenden Eisschilde auf Svalbard, der Barents See und dem Eurasischen Schelf. Die Blei-Isotopenzeitserien zeigen einen Trend zu radiogeneren Werten als Konsequenz des zunehmenden glazialen Verwitterungseintrags von den alten kontinentalen Landmassen im Zeitraum nach 2 Millionen Jahren. Eine ähnliche Entwicklung konnte in den Nd- und Pb-Isotopenzeitserien von Site 986, 644 und 982, die nach der Laugungsmethode mit vorherigen Entkarbonatisierungsschritt extrahiert wurden, beobachtet werden. Der Vergleich der Nd- Isotopendaten dieser drei ODP Kerne zeigt Perioden von wärmerem Klima, moderaten Vereisungsbedingungen und einem verstärktem Atlantikwassereinflusses in die Grönländisch-Norwegische See vor dem Einsetzen der Nordhemisphären-Vereisung (2.7 Millionen Jahre vor heute) sowie zwischen 2.2 und 1.5 Ma. Zu Beginn des mittelpleistozänen Klimawandels zwischen 1.5 und 1.2 Millionen Jahren hingegen war der Atlantikwassereinfluss vermindert, was sich in einer signifikant unterschiedlichen Nd-Isotopie in der Grönländisch-Norwegischen See und dem Nordatlantischen Ozean ausdrückte. Die Rekonstruktion der Quellen der detritischen Sedimente selbst, die auf der Basis der Nd- und Pb- Isotopenverhältnisse des detritischen Materials erstellt wurde, zeigt einen dominanten Einfluss von Svalbard, der Barents See und des Eurasischen Schelfs auf die Kerne am Svalbard-Schelf und auf dem Yermak Plateau. Das Vøring Plateau war bestimmt durch den Sedimenteintrag von Skandinavien und teilweise von der Barents See durch den Transport durch Eisberge und Meereis während der zunehmenden Vereisung. Das Rockall Plateau hingegen zeigt einen geringeren Verwitterungseintrag aufgrund der größeren Entfernung zum Land. Dennoch deuten die Blei-Isotopenverhältnisse auf einen leichten Anstieg des Erosionseintrags von Grönland und Island in den letzten 1.2 Millionen Jahren hin. Die Ergebnisse dieser Studie öffnen einen tieferen Einblick in die übergeordneten Klimakontrollmechanismen der letzten 5 Millionen Jahre und werden das Verständnis der zukünftigen Klimaentwicklungen verbessern

Reconstructing the Plio-Pleistocene evolution of the water mass exchange and climate variability in the Nordic Seas and North Atlantic Ocean

Global climate of the Plio-Pleistocene has been largely impacted by changes of the Atlantic Meridional Overturning Circulation (AMOC), which has most likely to a large extent been controlled by the formation of North Atlantic Deep Water (NADW) ventilating the entire Atlantic Ocean. The main areas contributing to the formation of NADW have been the Labrador Sea, the Nordic Seas, and North Atlantic Ocean. The circulation pattern in these areas underwent dramatic changes during the Plio – and Pleistocene, in particular during the onset and intensification of the Northern Hemisphere Glaciation (NHG) at 2.7 Ma and the Mid Pleistocene Transition (MPT) 1.5 – 0.6 Ma. The main goal of this study is to improve the understanding of the role of changes in ocean circulation in these key areas and their relationships to glaciations controlling and responding to climate. To investigate the past water mass mixing and erosional input between the climatically important areas of Nordic Seas and North Atlantic Ocean, the radiogenic isotope signatures of neodymium (Nd), lead (Pb) and strontium (Sr) of seawater-derived ferromanganese coatings on sediment particles, as well as of detrital material have been analyzed. Sediments obtained from six ODP (Ocean Drilling Program) Sites covering intermediate to deep waters in the Nordic Seas and North Atlantic Ocean are the basis of this study. Two Sites are located in the North Atlantic Ocean on Bjørn Drift (Site 984) in 1648 m water depth and on Rockall Plateau (Site 982) in 1135 m water depth. The sites in the Nordic Seas are located in the region influenced by Arctic Intermediate Water on Iceland Plateau (Site 907, 1800 m water depth) and of the inflowing Atlantic waters on Vøring Plateau (Site 644, 1220 m water depth), on the Svalbard margin (Site 986, 2051 m deep) and on Yermak Plateau in the Fram Strait (Site 911, 906 m water depth). The extraction of seawater Nd and Pb isotope compositions from authigenic ferromanganese coatings, as well as Nd isotope composition from the calcitic foraminiferal shells from locations in the western part of the study area (Sites 907 and 984) gave no reliable results as compared to the present seawater signatures in this area, mainly as a consequence of partial dissolution of basaltic ash particles during the extraction procedures. In contrast, for the sites (982, 644, 986, 911) in the eastern part of the Nordic Seas and in the North Atlantic Ocean reliable past bottom water signatures were extracted using different leaching methods and allow paleoreconstructions for the past 3 - 5 Ma. The Nd and Pb isotope records of ferromanganese coatings of Site 911 on the Yermak Plateau were extracted with a leach method based on studies without a preceding decarbonation step. These records show only small variations prior to 2.7 Ma pointing to a continuous Atlantic inflow under generally warmer climatic conditions. In contrast, since the intensification of the Northern Hemisphere Glaciation the Nd isotope composition has shown a high variability as a consequence of the waxing and waning of ice sheets on Svalbard, the Barents Sea and Eurasia. The Pb record at the same time reveals a trend to more radiogenic values as a result of increasing glacial weathering inputs from old continental landmasses after 2 Ma. A similar evolution is evident from the Nd and Pb isotope records of Sites 986, 644 and 982, which were extracted from seawater derived ferromanganese coatings applying the leaching method with a preceding decarbonation step. Comparison of the Nd isotope records of these three ODP sites shows periods of a warmer climate, moderate glacial conditions and an enhanced Atlantic inflow into the Nordic Seas prior to the onset of the NHG at 2.7 Ma and between 2.2 Ma and 1.5 Ma. In contrast, at the beginning of the MPT between 1.5 and 1.2 Ma the Atlantic inflow was significantly reduced, expressed as marked differences between the Nd isotope records in the Nordic Seas and North Atlantic Ocean. Sediment provenance obtained from Nd and Pb isotope compositions of the detrital material shows a dominant influence of Svalbard, Barents Sea and Eurasian shelf at the sites on the Svalbard margin and Yermak Plateau. The Vøring Plateau has been dominated by sediment input from Scandinavia to which material from the Barents Sea via icebergs and sea ice was added during glaciations. In contrast, the Rockall Plateau shows the lowest erosional influence due to its location distant from land. However, the Pb isotope evolution obtained from ferromanganese coating of sediment of Site 982 nevertheless points to a slightly increased erosional input from Greenland and Iceland after 1.2 Ma. The results of this study provide a deeper insight into the overall climate control mechanisms of the past 5 Myr and will contribute to improve the understanding of future climatic change

Plio-Pleistocene evolution of water mass exchange and erosional input at the Atlantic-Arctic gateway

Water mass exchange between the Arctic Ocean and the Norwegian-Greenland Seas has played an important role for the Atlantic thermohaline circulation and Northern Hemisphere climate. We reconstruct past water mass mixing and erosional inputs from the radiogenic isotope compositions of neodymium (Nd), lead (Pb), and strontium (Sr) at Ocean Drilling Program site 911 (leg 151) from 906 m water depth on Yermak Plateau in the Fram Strait over the past 5.2 Myr. The isotopic compositions of past bottom waters were extracted from authigenic oxyhydroxide coatings of the bulk sediments. Neodymium isotope signatures obtained from surface sediments agree well with present-day deepwater εNd signature of −11.0 ± 0.2. Prior to 2.7 Ma the Nd and Pb isotope compositions of the bottom waters only show small variations indicative of a consistent influence of Atlantic waters. Since the major intensification of the Northern Hemisphere Glaciation at 2.7 Ma the seawater Nd isotope composition has varied more pronouncedly due to changes in weathering inputs related to the waxing and waning of the ice sheets on Svalbard, the Barents Sea, and the Eurasian shelf, due to changes in water mass exchange and due to the increasing supply of ice-rafted debris (IRD) originating from the Arctic Ocean. The seawater Pb isotope record also exhibits a higher short-term variability after 2.7 Ma, but there is also a trend toward more radiogenic values, which reflects a combination of changes in input sources and enhanced incongruent weathering inputs of Pb released from freshly eroded old continental rocks

The role of airborne volcanic ash for the surface ocean biogeochemical iron-cycle: a review

Iron is a key micronutrient for phytoplankton growth in the surface ocean. Yet the significance of volcanism for the marine biogeochemical iron-cycle is poorly constrained. Recent studies, however, suggest that offshore deposition of airborne ash from volcanic eruptions is a way to inject significant amounts of bio-available iron into the surface ocean. Volcanic ash may be transported up to several tens of kilometers high into the atmosphere during large-scale eruptions and fine ash may stay aloft for days to weeks, thereby reaching even the remotest and most iron-starved oceanic regions. Scientific ocean drilling demonstrates that volcanic ash layers and dispersed ash particles are frequently found in marine sediments and that therefore volcanic ash deposition and iron-injection into the oceans took place throughout much of the Earth's history. Natural evidence and the data now available from geochemical and biological experiments and satellite techniques suggest that volcanic ash is a so far underestimated source for iron in the surface ocean, possibly of similar importance as aeolian dust. Here we summarise the development of and the knowledge in this fairly young research field. The paper covers a wide range of chemical and biological issues and we make recommendations for future directions in these areas. The review paper may thus be helpful to improve our understanding of the role of volcanic ash for the marine biogeochemical iron-cycle, marine primary productivity and the ocean-atmosphere exchange of CO2 and other gases relevant for climate in the Earth's history

Trace Metals and Their Isotopes in the Tropical Atlantic Ocean - Cruise No. M81/1, February 04 – March 08, 2010, Las Palmas (Canary Islands, Spain) – Port of Spain (Trinidad & Tobago)

Summary Meteor Cruise M81/1 was dedicated to the investigation of the distribution of dissolved and particulate trace metals and their isotopic compositions (TEIs) in the full water column of the tropical Atlantic Ocean and their driving factors including main external inputs and internal cycling and ocean circulation. The research program is embedded in the international GEOTRACES program (e.g. Henderson et al., 2007), which this cruise was an official part of and thus corresponds to GEOTRACES cruise GA11. This cruise was completely dedicated to the trace metal clean and contamination-free sampling of waters and particulates for subsequent analyses of the TEIs in the home laboratories of the national and international participants. Besides a standard rosette for the less contaminant prone metals, trace metal clean sampling was realized by using a dedicated and coated trace metal clean rosette equipped with Teflon-coated GO-FLO bottles operated via a polyester coated cable from a mobile winch that was thankfully made available by the U.S. partners of the GEOTRACES program for this cruise. The particulate samples were also collected under trace metal clean conditions using established in-situ pump systems. The cruise track led the cruise southward from the Canary Islands to 11°S and then continued northwestward along the northern margin of South America until it reached Port of Spain, Trinidad & Tobago. The track crossed areas of major external inputs including exchange with the volcanic Canary Islands, the Saharan dust plume, as well as the plume of the Amazon outflow. In terms of internal cycling the equatorial high biological productivity band, as well as increased productivity associated with the Amazon Plume were covered. All major water masses contributing the Atlantic Meridional Overturning Circulation, as well as the distinct narrow equatorial surface and subsurface east-west current bands were sampled. A total of 17 deep stations were sampled for the different dissolved TEIs, which were in most cases accompanied by particulate sampling. In addition, surface waters were continuously sampled under trace metal clean conditions using a towed fish

Chemistry and Sr–Nd isotope signature of amphiboles of the magnesio-hastingsite-pargasite-kaersutite series in Cenozoic volcanic rocks: Insight into lithospheric mantle beneath the Bohemian Massif

Amphibole phenocrysts and xenocrysts from Cenozoic volcanic rocks of the Bohemian Massif (BM) belong to the magnesio-hastingsite-pargasite-kaersutite series. Their host rocks are mostly basaltic lavas, dykes and breccia pipe fills, less commonly also felsic rocks from rift zones along lithospheric block boundaries of the BM. The calculated p–T conditions suggest that almost all amphiboles crystallized in a relatively narrow temperature range (1020–1100 °C) at depths of 20–45 km (0.7–1.2 GPa) during the magma ascent. The initial 143Nd/144Nd and 87Sr/86Sr ratios of amphiboles (0.51266–0.51281 and 0.70328–0.70407, respectively) are similar to those of their whole rocks (0.51266–0.51288 and 0.70341–0.70462, respectively). This testifies to locally elevated proportions of recycled Variscan crustal material during melting of mantle peridotites rich in clinopyroxene–amphibole veins. These veins were formed by metasomatic fluids enriched in High Field Strength Elements and are isotopically similar to EM-1 mantle type.Fenokrysty a xenokrysty amfibolů kenozoických vulkanických hornin Českého masivu (ČM) náleží svým složením do magnesiohastingsit-pargasit-kaersutitové série. Jejich hostitelské horniny jsou především bazaltické lávy, žíly nebo brekciovité výplně komínů, méně často také felsické horniny z riftových zón podél hranic litosférických bloků ČM. Vypočtené p-T podmínky ukazují, že téměř všechny amfiboly krystalizovaly v relativně úzkém teplotním rozmezí (1020–1100 °C) v hloubkách 20–45 km (0,7–1,2 GPa) během výstupu magmatu. Iniciální izotopové poměry 143Nd/144Nd a 87Sr/86Sr v amfibolech jsou v rozmezí 0,51266–0,51281 a 0,70328–0,70407. To vypovídá o lokálně zvýšeném množství recyklovaného variského korového materiálu během tavení plášťového peridotitu bohatého na klinopyroxen-amfibolové žíly. Tyto žíly vznikly z metasomatických fluid obohacených o prvky s velkým iontovým potenciálem a jsou izotopově podobné obohacenému plášti typu 1 (EM-1)