Karsten M. Haase

Petrology and geochemistry of crustally contaminated komatiitic basalts from the Vetreny Belt, southeastern Baltic Shield: Evidence for an early Proterozoic mantle plume beneath rifted Archean continental lithosphere

New isotope and trace element data are presented for komatiitic basalts and a related peridotite Vinela Dike from the large Vetreny Belt in the southeastern Baltic Shield. The MgO contents of the erupted and intruded magmas are inferred to increase from 13 to 17% towards the center of the belt, which implies the respective increase in liquidus temperatures from 1370 to 1440°C. The elevated liquidus temperatures suggest that the source of the komatiite magmas had a substantially higher potential temperature (1630°C) than the ambient mantle (1480°C) and are regarded as evidence for the existence of a mantle plume underlying the region at ~2.45 Ga. Parental magmas to the lavas and the Vinela Dike were shown to have komatiite composition and were derived from a long-term LREE-depleted mantle source with [epsilon]Nd(T) of ca. +2.6. The evolution of these magmas en route to the surface was mainly controlled by 4-15% contamination with older felsic crustal rocks, which resulted in substantial changes in incompatible trace element and isotope ratios. The obtained Sm---Nd internal isochron ages of 2449 ± 35 and 2410 ± 34 Ma for the lavas, 2430 ± 174 Ma for the Vinela Dike, and a whole-rock Pb---Pb age of 2424 ± 178 Ma for the lavas together with a U---Pb zircon age of 2437 ± 3 Ma are identical to the reported U---Pb zircon and baddeleyite ages for numerous mafic-ultramafic layered intrusions in central and northern Karelia. From their chemical and isotope similarities, it is likely that these rocks had allied parental magmas. These magmas may have been emplaced in a continental rift setting during the interaction of a mantle plume and continental crust. Impinging of a plume head beneath the continental lithosphere resulted in its thinning, stretching, and rifting but failed to open a new ocean. This extensive magmatic event was responsible for a substantial contribution of early Proterozoic juvenile material to the Archean continental crust in the Baltic Shield.

Evidence for a Neoproterozoic ocean in south-central Africa from mid-oceanic-ridge-type geochemical signatures and pressure-temperature estimates of Zambian eclogites

Precambrian eclogites, metagabbros, and gabbros occur in an similar to200-km-long by 40-km-wide zone in central Zambia. Pressure-temperature (P-T) estimates of kyanite-bearing eclogites (kyanite eclogites) throughout the zone give temperatures of 590-750 degreesC at minimum pressures of 20 kbar. Phengite-bearing eclogites equilibrated at 720-755 degreesC and 26-28 kbar and show evidence for a clockwise P-T path. These P-T conditions imply a low geothermal gradient of similar to8 degreesC/km and a subduction depth of similar to90 km. The eclogites, metagabbros, and gabbros show incompatible element patterns similar to those of recent mid-oceanic-ridge basalts, and thus are interpreted to represent former oceanic crust. The low geothermal gradient indicates a cold subducted oceanic lithosphere, implying long-lived, fast convergence and a relatively large (>1000 km) associated ocean basin. A Sm-Nd isochron defines an age of 595 +/- 10 Ma for the eclogite facies metamorphism. These results imply that a Neoproterozoic suture zone exists between the Congo and Kalahari cratons. Suturing occurred during the same orogenic cycle that formed the Zambezi belt and is related to the assembly of Gondwana.

The relationship between the age of the lithosphere and the composition of oceanic magmas: Constraints on partial melting, mantle sources and the thermal structure of the plates

On the basis of different proportions and chemical compositions of shield and post-shield magmas, three types of oceanic intraplate volcanism appear to exist. The average SiO2 contents of primitive melts of most Pacific and Atlantic intraplate lavas show a regular decrease with increasing age of the lithosphere up to 70 Ma. The average pressures of melting of most magmas lie beneath the thermal boundary layer defined by the 1300°C isotherm, in accordance with geophysical models. The average melting pressures of shield tholeiites erupting at the largest hotspots on Earth suggest that erosion of the plate is restricted to strong plumes. Increasing average ratios of (Ce/Yb)N(=chondrite-normalized) and (Tb/Yb)N with increasing age of the lithosphere imply that residual garnet has an increasing influence on the melting of most magmas. An influence of MORB material in intraplate magmas is observed in volcanoes erupting on lithosphere younger than 15 Ma. Correlations between SiO2 and the rare earth element ratios suggest that the rare earth elements are more strongly influenced by the pressure of melting than by differences in source composition. Lavas with extremely low 143Nd/144Nd (e.g. Gough-Tristan da Cunha) have high (Nd/Sm)N for a given SiO2, in accordance with a long-term enriched mantle source. After a correction for the fractionation occurring at high melting pressures (a recalculation of all averages to 50% SiO2) the (Nd/Sm)N of most lavas can be modeled by 3–15% melting of depleted mantle sources.

Mantle dynamics, element recycling, and magma genesis beneath the Kermadec Arc‐Havre Trough

New geochemical and isotopic data are presented for lavas from three sites in the Havre Trough-Lau Basin back arc and six volcanoes along the Kermadec arc. The back arc basalts range from MORB-like to arc-like in composition and contain a variable contribution from the underlying slab. The least contaminated MORB-like back arc lavas from 24°–29°S are low degree partial melts of a source with Pacific MORB isotopic characteristics. A transition occurs at 30°S between the strongly depleted northern Kermadec (and Tonga) arc lavas and the mildly depleted southern Kermadec arc lavas. This transition does not correlate with changes in the back arc extension rate or width but may reflect inhibited mantle wedge replenishment behind the shallower-dipping northern Kermadec-Tonga slab. Northern Kermadec lavas require mixing between two components: (1) depleted Havre Trough mantle and (2) fluid derived from altered MORB crust with a slight input of sediment lead. Inter-volcano differences in fluid compositions probably reflect local variations on the subducting slab rather than mineralogical variation in the mantle wedge. Southern Kermadec lavas require an additional component: (3) Pacific sediment melt. This sediment melt is only detected where the subduction rate is 650°C before passing through the sub-arc melt generation zone.

Young volcanism and related hydrothermal activity at 5°S on the slow‐spreading southern Mid‐Atlantic Ridge

The effect of volcanic activity on submarine hydrothermal systems has been well documented along fast- and intermediate-spreading centers but not from slow-spreading ridges. Indeed, volcanic eruptions are expected to be rare on slow-spreading axes. Here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated, apparently magmatically robust segment center on the slow-spreading southern Mid-Atlantic Ridge near 5°S. Three high-temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase-separated, vapor-type fluids. Exit temperatures at one of the fields reach up to 407°C, at conditions of the critical point of seawater, the highest temperatures ever recorded from the seafloor. Fluid and vent field characteristics show a large variability between the vent fields, a variation that is not expected within such a limited area. We conclude from mineralogical investigations of hydrothermal precipitates that vent-fluid compositions have evolved recently from relatively oxidizing to more reducing conditions, a shift that could also be related to renewed magmatic activity in the area. Current high exit temperatures, reducing conditions, low silica contents, and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source, probably related to a young volcanic eruption event nearby, in which basaltic magma is actively crystallizing. This is the first reported evidence for direct magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge.

Magma genesis by rifting of oceanic lithosphere above anomalous mantle: Terceira Rift, Azores

[1] The Terceira Rift formed relatively recently (∼1 Ma ago) by rifting of the old oceanic lithosphere of the Azores Plateau and is currently spreading at a rate of 2–4mm/a. Together with the Mid-Atlantic Ridge, the Terceira Rift forms a triple junction that separates the Eurasian, African, and American Plates. Four volcanic systems (Sao Miguel, Joao de Castro, Terceira, Graciosa), three of which are islands, are distinguished along the axis and are separated by deep avolcanic basins similar to other ultraslow spreading centers. The major element, trace element and Sr-Nd-Pb isotope geochemistry of submarine and subaerial lavas display large along-axis variations. Major and trace element modeling suggests melting in the garnet stability field at smaller degrees of partial melting at the easternmost volcanic system (Sao Miguel) compared to the central and western volcanoes, which appear to be characterized by slightly higher melting degrees in the spinel/garnet transition zone. The degrees of partial melting at the Terceira Rift are slightly lower than at other ultraslow mid-ocean ridge spreading axes (Southwest Indian Ridge, Gakkel Ridge) and occur at greater depths as a result of the melting anomaly beneath the Azores. The combined interaction of a high obliquity, very slow spreading rates, and a thick preexisting lithosphere along the axis probably prevents the formation and eruption of larger amounts of melt along the Terceira Rift. However, the presence of ocean islands requires a relatively stable melting anomaly over relatively long periods of time. The trace element and Sr-Nd-Pb isotopes display individual binary mixing arrays for each volcanic system and thus provide additional evidence for focused magmatism with no (or very limited) melt or source interaction between the volcanic systems. The westernmost mantle sources beneath Graciosa and the most radiogenic lavas from the neighboring Mid-Atlantic Ridge suggest a mantle flow from Graciosa toward the Mid-Atlantic Ridge and hence a flux of mantle material from one spreading axis into the other. The Terceira Rift represents a unique oceanic rift system situated within the thickened, relatively old oceanic lithosphere and thus exhibits both oceanic and continental features.

Chlorine in oceanic intraplate basalts: Constraints on mantle sources and recycling processes

Submarine volcanic glass data from different hotspot regions indicate that the Cl inventory and the Cl/K ratios of the mantle are variable. The majority of hotspot lavas have higher Cl/K ratios than depleted mid-oceanic-ridge basalts, consistent with the presence of recycled crustal components in the mantle-plume sources of hotspots. Enriched mantle sources (EM1 and EM2) have relatively low Cl/K ratios, suggesting significant devolatilization of the subducted sedimentary material. Lavas from HIMU-type hotspots (high μ, μ = 238U/204Pb) have the highest but variable Cl/K, most likely due to the presence of recycled altered oceanic lithosphere in their source. Near-ridge hotspots show correlations between Cl/ K ratios and radiogenic isotopes, supporting mixing between plume and depleted upper-mantle material. The variable Cl/K ratios in the HIMU-type magmas and the low Cl/K ratios in the EM-type magmas suggest that the quantity of Cl recycled into the mantle via subduction is not uniform.

Geochemistry of lavas from the Ahu and Tupa volcanic fields, Easter Hotspot, southeast Pacific: Implications for intraplate magma genesis near a spreading axis

Lavas from two young volcanic fields of the Easter Hotspot consist mainly of enriched tholeiites, with incompatible element and isotopic compositions similar to enriched MORB from the neighbouring spreading axis. The major element composition of these intraplate tholeiites suggests that they originate from melting at greater pressures, and REE models indicate slightly lower degrees of partial melting than beneath the ridge. This can be explained by the thickening of the lithosphere away from the spreading axis, accompanied by increasing mantle temperature as the Easter plume is approached. These processes combine to depress the melting zone to greater depth in the intraplate region. The relatively low degrees of partial melting and low volumes of melt compared, for example, to Galapagos imply a low excess temperature (∼ 100°C?) for the Easter plume. A few depleted tholeiites found on the Ahu volcanic field were generated by shallow melting of an extremely depleted (more depleted than MORB

Geochemical constraints on magma sources and mixing processes in Easter Microplate MORB (SE Pacific): a case study of plume-ridge interaction

The Easter Microplate is bounded in the east and west by two large overlapping spreading centres called the East and West Rift, respectively. The East Rift is located close to the Easter Hotspot, the site of young intraplate volcanism above a mantle plume. Easter Hotspot material creates a geochemical anomaly on the East Rift spreading axis of the microplate and mainly northward-directed material flow occurs beneath the propagating East Rift. The incompatible element-depleted mantle of the Easter Microplate consists of two different sources, one of which mixes with enriched Easter Hotspot material forming the East Rift Mid-ocean ridge basalt (MORB). The other depleted mantle source is present beneath the West Rift and mixes with an enriched endmember apparently distinct from the Easter Hotspot source. While the Easter Hotspot material leads to a bathymetric anomaly on the East Rift, no increased degree of melting is observed at the point of maximum inflow of enriched plume material, implying low excess temperatures (<50°C) of the mantle. The mixing processes between the Easter Hotspot and MORB material probably take place in the partially molten asthenosphere during the horizontal flow of the plume material. Melting above the plume and at the spreading axes in the Easter Microplate region occurs over a range of pressures but there is no sign of melts from enriched garnet pyroxenite.

Nb-depleted andesites from the Pacific-Antarctic Rise as analogs for early continental crust

Sampling of the Pacific-Antarctic Rise close to the Foundation hotspot in the South Pacific reveals that silicic lavas erupt together with basalts on a 130-km-long part of this mid-oceanic ridge, representing the most extensive occurrence of andesites on the submarine spreading system. Most of these tholeiitic andesites and dacites have negative Nb and Ta anomalies, a signature that has so far been attributed only to subduction-related magmas and the continental crust. The silicic lavas formed by fractional crystallization from basalts and assimilation of melts from hydrothermally altered amphibolite in the oceanic crust. The presence of andesites with negative Nb and Ta anomalies on the Pacific-Antarctic Rise implies that such magmas are not restricted to subduction zones but can form at a plume-influenced mid-oceanic ridge. The andesites and dacites from the Pacific-Antarctic Rise may represent analogs to some Precambrian volcanic rocks and early continental crust.