Search

Abstract—Yellow volborthite Cu2.95(V1.91P0.09)Σ2O7(OH)1.90·2H2O, and turquoise atacamite, Cu2[Cl0.98(OH)0.02]Σ1.00(OH)3have been found incrusting fractures and the surfaces of lava blocks on the Northeast slope of the Alaid volcano (Atlasov island, Kuril Islands, Russia). The Raman spectrum of volborthite contains the following bands (with assignment) in the range 900–70 cm–1: 885 (ν1VO4), 809 (ν3VO4), 748 (ν3VO4, libration mode of water, deformation mode of OH), 507 (ν4VO4, ν1CuO6), 471 (ν4VO4, ν1CuO6), 441 (ν4VO4, ν2CuO6), 345 (ν2VO4), 257 (ν5CuO6, ν2VO4) and 241 (ν2VO4) cm–1. Volborthite is likely a supergene mineral formed as a result of primary alteration of fumarole minerals. Euchlorine and shcherbinaite could be the Cu and V sources for volborthite, respectively. Conversely, the volborthite–atacamite paragenesis could have formed in the near-surface, relatively low-temperature fumarole zone—the so-called hot hypergenesis zone, due to the interaction of primary exhalative minerals with meteoric water under the influence of volcanic gas.

Abstract: This paper presents materials on the chemical and mineralogical composition of Fe-Mn mineralization in island arcs (Kurile, Nampo, Mariana, New Britain, New Hebrides, and Kermadec) in the western part of the Pacific Ocean. The mineralization was proved to be of hydrothermal and/or hydrogenic genesis. The former is produced by hydrothermal Fe and Mn oxi-hydroxides that cement volcanic-terrigenous material in sediments. Some Fe oxi-hydroxides can be derived via the halmyrolysis of volcaniclastic material. Crusts of this stage are characterized by fairly low concentrations of trace and rare elements, and their REE composition is inherited from the volcanic-terrigenous material. The minerals of the Mn oxi-hydroxides are todorokite and “Ca-birnessite.” The Mn/Fe ratio increases away from the discharge sites of the hydrothermal solutions. The hydrogenic Fe-Mn crusts are characterized by high concentrations of trace and minor elements of both the Mn group (Co, Ni, Tl, and Mo) and the Fe group (REE, Y, and Th). The hydrogenic crusts consist of Fe-vernadite and Mn-feroxyhyte. Some of the hydrothermal crusts originally had a hydrothermal genesis. The first data were obtained on crust B30-72-10 from the Macauley Seamount in the Kermadec island arc, which contained anomalously high concentrations of Co (2587 ppm) and other Mn-related trace elements in the absence of hydrogeneous Fe oxi-hydroxides.

Abstract: A possible mechanism of the formation of chains of intraplate seamounts and islands alternative to the “hot spot” hypothesis is considered. It is related to the appearance of additional stresses in the lithospheric plate when it ascends over a mantle inhomogeneity and descends from it. The magnitude of these stresses (∼600 bar) is sufficient for formation of deep fracture zones. In the paper, schemes of formation of volcanic chains are described. The formation of the “faults-volcanoes-volcanic chains” sequence may follow two different ways. The first is controlled by the fracture zones formed along the direction of the plate movement. In this case, feeding channels are localized along the boundary of the rise. If the chain is gradually formed simultaneously with the plate motion, the age of the volcanoes along the chain may change in a more or less regular way. The second type is formed by fractures orthogonal to the movement direction. They may be formed when the plate ascends over a mantle inhomogeneity and/or descends from it. In this case, adjacent volcanoes may have similar ages. A combination of these two variants may also be encountered. The mechanism posed allowed us to explain selected facts referring to the volcanic chains of French Polynesia and may be applied to other regions of the Pacific Ocean.

A detailed heat flow study of some areas in the Middle America Trench is attempted. Forty six measurements were obtained in the region between the Tres Marias Islands and the Tehuantepec Ridge. The stations were concentrated in three detailed survey areas and 4 profiles. The obtained data show a steep decrease in the heat flow values towards the southern portion of the trench. The detailed survey area, located in the northern end of the trench (Area 20–1) has the highest heat flow average (122 mW m-2), however a characteristical pattern was observed: most data within the Rivera Plate have higher than average heat flow due to the young age of this plate and contrast with the low values associated with the continental lithosphere of the North America Plate. Areas 20–3 and 20–4 have lower averages (50 and 27 mW m-2 respectively) and they coincide with portions of the Guadalupe Plate, proposed by Klitgord and Mammerickx (1982) and assumed to be older than the Cocos Plate, though magnetic lineations have not been directly correlated with age in those areas.