ALBERT

Description: High-Mg, low-Ti volcanic rocks from the Manihiki Plateau in the Western Pacific share many geochemical characteristics with subduction-related boninites such as high-Ca boninites from the Troodos ophiolite on Cyprus, which are believed to originate by hydrous re-melting of previously depleted mantle. In this paper we compare the Manihiki rocks and Troodos boninites using a new dataset on the major and trace element composition of whole rocks and glasses from these locations, and new high-precision, electron microprobe analyses of olivine and Cr-spinel in these rocks. Our results show that both low-Ti Manihiki rocks and Troodos boninites could originate by re-melting of a previously depleted lherzolite mantle source (20–25% of total melting with 8–10% melting during the first stage), as indicated by strong depletion of magmas in more to less incompatible elements (Sm/Yb 〈 0.8, Zr/Y 〈 2, Ti/V 〈 12) and high-Cr-spinel compositions (Cr# 〉 0.5). In comparison with Troodos boninites, the low-Ti Manihiki magmas had distinctively lower H2O contents (〈 0.2 vs. 〉 2 wt% in boninites), ~ 100 °C higher liquidus temperatures at a given olivine Fo-number, lower fO2 (ΔQFM 〈 + 0.2 vs. ΔQFM 〉 + 0.2) and originated from deeper and hotter mantle (1.4–1.7 GPa, ~ 1440 °C vs. 0.8–1.0 GPa, ~ 1300 °C for Troodos boninites). The data provide new evidence that re-melting of residual upper mantle is not only restricted to subduction zones, where it occurs under hydrous conditions, but can also take place due to interaction of previously depleted upper mantle with mantle plumes from the deep and hotter Earth interior.

Description: During cruise SO201-1b of the joint Russian–German expedition on the R/V Sonne in 2009, mantle peridotites affected by varying secondary alteration were dredged on the eastern slope of the north- western segment of the Stalemate transverse ridge adjacent to the eponymous fracture zone. The collection discussed in this paper included four samples of silicified serpentinites after dunites and 11 lherzolite samples serpentinized to a varying degree. The abundance of amorphous silica and quartz, very high SiO2 content (up to 88.7 wt %), and unusually low MgO (up to 1.4 wt %) in the serpentinized dunites strongly distinguish these rocks from the known products of hydrothermal alteration and low-temperature (seafloor) weathering of peridotites in the oceanic crust. In order to determine the conditions and processes resulting in the silicifica- tion of peridotites at the Stalemate Fracture Zone, thermodynamic modeling accounting for the kinetics of mineral dissolution implemented in the GEOCHEQ program package was used in this study. The results of modeling allowed us to suppose that the geochemical and mineralogical effects observed in the silicified ser- pentinized dunites of the Stalemate Fracture Zone are consequences of low-temperature deserpentinization of oceanic materials under subaerial conditions.

Description: New geochemical data from the Cocos Plate constrain the composition of the input into the Central American subduction zone and demonstrate the extent of influence of the Galapagos Hotspot on the Cocos Plate. Samples include sediments and basalts from Ocean Drilling Program (ODP) Site 1256 outboard of Nicaragua, gabbroic sills from ODP Sites 1039 and 1040, tholeiitic glasses from the Fisher Ridge off northwest Costa Rica, and basalts from the Galapagos Hotspot Track outboard of Central Costa Rica. Site 1256 basalts range from normal to enriched MORB in incompatible elements and have Pb and Nd isotopic compositions within the East Pacific Rise MORB field. The sediments have similar Pb-206/Pb-204 and only slightly more radiogenic Pb-207/Pb-204 and Pb-208/Pb-204 isotope ratios than the basalts. Altered samples from the subducting Galapagos Hotspot Track have similar Nd and Pb isotopic compositions to fresh Galapagos samples but have significantly higher Sr isotopic composition, indicating that the subduction input will have a distinct geochemical signature from Galapagos-type mantle material that may be present in the wedge beneath Costa Rica. Gabbroic sills from Sites 1039 and 1040 in East Pacific Rise (EPR) crust show evidence for influence of the Galapagos Hotspot similar to 100 km beyond the morphological hotspot track

Description: This paper reports the results of a mineralogical study of 14 mantle peridotite samples dredged in 2009 from the eastern slope of the northwestern segment of the Stalemate Ridge in the northwestern Pacific during cruise SO201-KALMAR Leg 1b of the R/V Sonne. The sample collection included four serpentinized and silicified dunites and ten variably serpentinized lherzolites. The compositions of primary minerals (clinopyroxene, orthopyroxene, and spinel) change systematically from the lherzolites to dunites. Spinel from the lherzolites shows higher Mg# and lower Cr# values (0.65-0.68 and 0.26-0.33, respectively) compared with spinel from the dunites (Mg# = 0.56-0.64 and Cr# = 0.38-0.43). Clinopyroxene from the lherzolites is less magnesian (Mg# = 91.7-92.4) than clinopyroxene from dunite sample DR37-3 (Mg# = 93.7). Based on the obtained data, it was concluded that the lherzolites of the Stalemate Fracture Zone were derived by 10-12% near-fractional melting of a DMM-type depleted mantle reservoir beneath the Kula-Pacific spreading center. The dunites were produced by interaction of residual lherzolites with sodium- and titaniumrich melt and are probably fragments of a network of dunite channels in the shallow mantle. The moderately depleted composition of minerals clearly distinguishes the lherzolites from the strongly depleted peridotites of the East Pacific Rise and indicates the existence of slow-spreading mid-ocean ridges in the Pacific Ocean during the Cretaceous-Paleogene.

Description: The paper presents data on the petrology and geochemistry of plutonic rocks dredged from the Stalemate Fracture Zone, Northwest Pacific Ocean, during Cruise SO201-1 of the R/V “Sonne”. We proposed also the reconstruction of their formation conditions and interpretation of their tectonic evolution. The genesis of gabbroids found among plutonic rocks composing the Cretaceous–Paleogene basement of the northwestern part of the Pacific Ocean was related to magmatism at the ancient spreading center and provides record of the evolution of the parental magmatic melts of N-MORB. Along with related peridotites, basalts, and dolerites, these rocks can be attributed to the disintegrated the Cretaceous–Paleogene oceanic lithosphere of the Pacific Ocean. The shallow mantle beneath the ancient oceanic crust of this area is made up of depleted magmatic spinel lherzolite, harzburgite, and dunite. The fact that gabbro-diorite and diorite that are not genetically related to the rocks of the Cretaceous–Paleogene basement of the Northwest Pacific occur at the eastern termination of the Stalemate Fracture Zone possibly reflects the complicated structure of the tectonic collage of rocks of different age that were produced in different geodynamic environments and were later tectonically brought together near the frontal portion of the Aleutian island arc. Judging by the isotopic–geochemical characteristics of these rocks, they cannot be classed with the family of oceanic plagiogranites. Deformations of the oceanic basement can be discerned throughout the whole Stalemate Fracture Zone as brecciation and large-amplitude vertical displacements within the oceanic lithosphere.

Description: The paper presents data on plutonic and metamorphic rocks dredged during Cruise 249 of the German R/V Sonne to the Stalemate Ridge, Northwest Pacific Ocean and the Shirshov Rise, western Bering Sea. Dredges in the northwestern sector of the Stalemate Ridge and central portion of the Shirshov Rise show that the plutonic and metamorphic rocks obtained here are amazingly similar. Our petrologic and geochemical data led us to view the rocks as members of a mafic–ultramafic assemblage typical of cumulate portions of ophiolite complexes and backarc spreading centers. The plutonic complexes of the Shirshov Rise and Stalemate Ridge show similarities not only in the petrography and mineralogy of their protoliths but also in the character of their metamorphic transformations. Plutonic rocks from both areas display mineralogical evidence of metamorphism within a broad temperature range: from the high-temperature amphibolite facies to the greenschist facies. Relations between the index mineral assemblages indicate that the metamorphic history of plutonic complexes in the Stalemate Ridge and Shirshov Rise proceeded along a retrograde path. Hornblende schists accompanying the plutonic rocks of the Stalemate Ridge and Shirshov Rise are petrographically close to foliated amphibolites in subophiolitic metamorphic aureoles. Within the framework of geodynamic interpretations of our results, it is realistic to suggest that the examined plutonic complexes were exhumed from subduction zones of various age.

Description: The Osborn Plateau is a large intraplate rise in the eastern part of the Indian Ocean, which has been poorly studied by the geological and geophysical methods. In cruise SO258/1 on the R/V Sonne, new data were collected using Parasound seismic profiling and a multibeam echo-sounder survey. Faults in the sedimentary cover, which extend to the bottom surface, indicate high neotectonic activity in the Osborn Plateau area. It may continue up to the present, as well as in the adjacent segment of the Ninetyeast Ridge, where strong earthquakes have been recorded. Two reflectors in the upper part of the sedimentary cover mark the global lowering of the World Ocean level at the Miocene/Pliocene and Pliocene/Pleistocene boundaries. The reflector in the sediments at the Lower/Upper Pliocene boundary is associated with a change in the regional hydrodynamic regime that occurred at that time in the eastern Indian Ocean. The rocks dredged on Osborn Plateau are identical to some volcanic rocks of the Ninetyeast Ridge, confirming their assumed genetic link, but they are more similar to the basalts of the Kerguelen Plateau. Extremely altered vitroclastic tuffs appear to have been formed as a result of explosive volcanic eruptions of alkali basalts or foidites under subaeral or relatively shallow water conditions and represent the most recent eruptions in the region.

Description: The first systematic rock sampling of volcanoes along the Galápagos hotspot tracks (the aseismic Cocos, Carnegie, Malpelo and Coiba ridges and adjacent seamounts) in the area between the Galápagos Islands and Central and South America was carried out on R/V Sonne cruise 144-3. Guyot-shaped seamounts, paleo-beach or intertidal wave-cut platform deposits, the structure and texture of volcanic rocks, and low sulfur contents of fresh glasses dredged at these volcanoes imply that ocean islands existed continuously above the Galápagos hotspot for at least the past 17 million years. These new data significantly extend the time period over which the unique endemic Galápagos fauna could have evolved, providing a complete solution to the long-standing enigma of the evolution of Galápagos land and marine iguanas

Description: The Quepos, Nicoya and Herradura oceanic igneous terranes in Costa Rica are conspicuous features of a Mid to Late Cretaceous regional magmatic event that encompasses similar terranes in Central America, Colombia, Ecuador and the Caribbean. The Quepos terrane (66 Ma), which consists of ol-cpx phyric, tholeiitic pillow lavas overlain by highly vesicular hyaloclastites, breccias and conglomerates, is interpreted as an uplifted seamount/ocean island complex. The Nicoya (∼90 Ma) and Herradura terranes consist of fault-bounded sequences of sediments, tholeiitic volcanics (pillow lavas and massive sheet flows) and plutonic rocks. The volcanic rocks were emplaced at relatively high eruption rates in moderate to deep water, possibly forming part of an oceanic plateau. Major and trace element data from Nicoya/Herradura tholeiites indicate higher melting temperatures than inferred for normal mid-ocean-ridge basalts (MORB) and/or a different source composition. Sr–Nd–Pb isotopic ratios from all three terranes are distinct from MORB but resemble those from the Galápagos hotspot. The volcanological, petrological and geochemical data from Costa Rican volcanic terranes, combined with published age data, paleomagnetic results and plate tectonic reconstructions of this region, provide strong evidence for a Mid Cretaceous (∼90Ma) age for the Galápagos hotspot, making it one of the oldest known, active hotspots on Earth. Our results also support an origin of the Caribbean Plate through melting of the head of the Galápagos starting plume.

Description: The Osborn Plateau is a large intraplate elevation in the eastern Indian Ocean, which has been poorly studied by geological and geophysical methods. On cruise SO258/1 aboard the R/V Sonne, new data were collected by Parasound seismic processing, multibeam survey, and, for the first time, dredging. Faults in the sedimentary cover, which extend to the seafloor surface, indicate high neotectonic activity in the area of the Osborn Plateau. It may have continued up to the present, as well as in the adjacent segment of the Ninetyeast Ridge, where strong earthquakes are recorded. Two reflectors A and C in the upper part of the sedimentary cover mark global regressive changes in the World Ocean level at the Pliocene–Pleistocene and Miocene–Pliocene and boundaries. Reflector B in sediments at the Lower–Upper Pliocene boundary is associated with a change in the regional hydrodynamic regime at the time in the eastern Indian Ocean. Reflector B at the Lower–Upper Pliocene boundary is associated with a change in the local hydrodynamic regime in the region of the plateau and was caused by uncompensated sedimentation. As a result of dredging, strongly altered vitroclastic tuffs were obtained, consisting of palagonized ash particles and lapilli. Analysis of geochemical data on the composition of palagonite made it possible to reconstruct the main geochemical features of the primary composition of glass in tuffs, in particular, high enrichment in incompatible elements (Nb, Zr, Ti, La, etc.). The results of studying the dredged rocks suggest that tuffs dredged on the Osborn Plateau were formed as a result of explosive volcanic eruptions of alkaline basalts under subaerial or relatively shallow conditions and represent the latest eruption products in the region.