Sources and Fluids in the Mantle Wedge below Kamchatka, Evidence from Across-arc Geochemical Variation

Major and trace element and Sr–Nd–Pb isotopic variations in mafic volcanic rocks hve been studied in a 220 km transect across the Kamchatka arc from the Eastern Volcanic Front, over the Central Kamchatka Depression to the Sredinny Ridge in the back-arc. Thirteen volcanoes and lava fields, from 110 to 400 km above the subducted slab, were sampled. This allows us to characterize spatial variations and the relative amount and composition of the slab fluid involved in magma genesis. Typical Kamchatka arc basalts, normalized for fractionation to 6% MgO, display a strong increase in large ion lithophile, light rare earth and high field strength elements from the arc front to the back-arc. Ba/Zr and Ce/Pb ratios, however, are nearly constant across the arc, which suggests a similar fluid input for Ba and Pb. La/Yb and Nb/Zr increase from the arc front to the back-arc. Rocks from the Central Kamchatka Depression range in 87Sr/86Sr from 0·70334 to 0·70366, but have almost constant Nd isotopic compositions (143Nd/144Nd 0·51307–0·51312). This correlates with the highest U/Th ratios in these rocks. Pb-isotopic ratios are mid-ocean ridge basalt (MORB)-like but decrease slightly from the volcanic front to the back-arc. The initial mantle source ranged from N-MORB-like in the volcanic front and Central Kamchatka Depression to more enriched in the back-arc. This enriched component is similar to an ocean-island basalt (OIB) source. Variations in (CaO)6·0–(Na2O)6·0 show that degree of melting decreases from the arc front to the Central Kamchatka Depression and remains constant from there to the Sredinny Ridge. Calculated fluid compositions have a similar trace element pattern across the arc, although minor differences are implied. A model is presented that quantifies the various mantle components (variably depleted N-MORB-mantle and enriched OIB-mantle) and the fluid compositions added to this mantle wedge. The amount of fluid added ranges from 0·7 to 2·1%. The degree of melting changes from ∼20% at the arc front to <10% below the back-arc region. The rocks from volcanoes of the northern part of the Central Kamchatka Depression—to the north of the transect considered in this study—are significantly different in their trace element compositions compared with the other rocks of the transect and their source appears to have been enriched by a component derived from melting of the edge of the ruptured slab

Late Pleistocene to Holocene activity at Bakening volcano and surrounding monogenetic centers (Kamchatka): volcanic geology and geochemical evolution

The different roles of variable mantle sources and intra-crustal differentiation processes at Bakening volcano (Kamchatka) and contemporaneous basaltic monogenetic centers are studied using major and trace elements and isotopic data. Three suites of volcanic activity are recognized: (1) plateau basalts of Lower Pleistocene age; (2) andesites and dacites of the Bakening volcano, the New Bakening volcano dacitic centers nearby; and (3) contemporaneous basaltic cinder cones erupted along subduction zone—parallel N–S faults. Age-data show that the last eruptions in the Bakening area occurred only 600–1200 years ago, suggesting the volcano is potentially active. Major element variations and petrographic observations provides evidence for a fractionation assemblage of olivine, clinopyroxene, ±plagioclase, ±magnetite (?) within the basaltic suite. The fractionation in the andesites and dacites is dominated by amphibole, clinopyroxene, orthopyroxene and plagioclase plus minor amounts of magnetite and apatite. The youngest cpx-opx-andesites of Bakening main volcano deviate from that trend. Their source was probably formed by mixing of basaltic magmas into the silicic magma chamber of the Bakening volcano. Overall trace element patterns as well as the Sr–Nd–Pb isotopic compositions are quite similar in all rocks despite large differences in their chemical composition (from basalt to rhyodacite). In detail however, the andesite–dacites of the central Bakening volcano show a stronger enrichment in the more incompatible elements and depletion in HREE compared to the monogenetic basaltic centers. This results in a crossing of the REE-pattern for the two suites. The decrease in the HREEs can be explained by amphibole fractionation. A slab component is less likely because it would result in fractionation of the HREE from each other, which is not observed. The higher relative amounts of LILE in the dacitic and the large scatter in the basaltic rocks must be the result of a variable source enrichment by slab-derived fluids overprinting a variable depleted mantle wedge. The plateau basalts are less depleted in HFSE and show a more fractionated HREE pattern. These lavas could either result from a slab component or the addition of an OIB-type enriched mantle in their source