Trace Elements and Li Isotope Compositions Across the Kamchatka Arc: Constraints on Slab‐Derived Fluid Sources
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Abstract Chalcophile elements and lithium (Li) isotopes were measured on lavas from a 220‐km transect across the Kamchatka arc in order to investigate the fluid variations below arc volcanoes and to trace the geochemical behaviour of Li in convergent plate margins. From the Eastern Volcanic Front (EVF), through the Central Kamchatka Depression (CKD), into the Sredinny Range (SR) volcanic zones, chalcophile element ratios (e.g., As/Ce and Sb/Ce) show clear across‐arc variations, decreasing (e.g., As/Ce: 0.20 to 0.03 and Sb/Ce: 0.013 to 0.002) with increasing depth above the slab (110 to 400 km). This clearly indicates a gradually decreasing influx of slab‐derived fluids added to the mantle wedge as the slab subducts below Kamchatka. In addition, the anomalously high U/Th, La/Sm, and B/Nb ratios in the CKD lavas suggest lawsonite breakdown reaction dominates the fluid release in this area. However, Li/Y (0.07 to 1.78) and δ 7 Li (+1.8 to +5.4‰, with an exception of +8.6‰ in CKD) show limited variations and values similar to the MORB mantle. A dehydration model suggests that slab‐derived fluids, which are characterized by high Li concentration and high δ 7 Li, do not control the Li budget in Kamchatka arc lavas. Therefore, the isotopic heavy Li from slab‐derived fluids likely equilibrates in the sub‐arc mantle, which acts as a buffer for Li systematics. In addition, based on the Li isotopic signatures of Klyuchevskoy volcano, our study demonstrates insignificant Li isotopic fractionation during mantle melting and subsequent differentiation.Keywords:
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Abstract Previous studies of the Ecuadorian arc (1°N–2°S) have revealed across‐arc geochemical trends that are consistent with a decrease in mantle melting and slab dehydration away from the trench. The aim of this work is to evaluate how these processes vary along the arc in response to small‐scale changes in the age of the subducted plate, subduction angle, and continental crustal basement. We use an extensive database of 1437 samples containing 71 new analyses, of major and trace elements as well as Sr‐Nd‐Pb isotopes from Ecuadorian and South Colombian volcanic centers. Large geochemical variations are found to occur along the Ecuadorian arc, in particular along the front arc, which encompasses 99% and 71% of the total variations in 206 Pb/ 204 Pb and 87 Sr/ 86 Sr ratios of Quaternary Ecuadorian volcanics, respectively. The front arc volcanoes also show two major latitudinal trends: (1) the southward increase of 207 Pb/ 204 Pb and decrease of 143 Nd/ 144 Nd reflect more extensive crustal contamination of magma in the southern part (up to 14%); and (2) the increase of 206 Pb/ 204 Pb and decrease of Ba/Th away from ∼0.5°S result from the changing nature of metasomatism in the subarc mantle wedge with the aqueous fluid/siliceous slab melt ratio decreasing away from 0.5°S. Subduction of a younger and warmer oceanic crust in the Northern part of the arc might promote slab melting. Conversely, the subduction of a colder oceanic crust south of the Grijalva Fracture Zone and higher crustal assimilation lead to the reduction of slab contribution in southern part of the arc.
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The mechanisms underpinning the formation of a focused volcanic arc above subduction zones are debated. Suggestions include controls by: (i) where the subducting plate releases water, lowering the solidus in the overlying mantle wedge; (ii) the location where the mantle wedge melts to the highest degree; and (iii) a limit on melt formation and migration imposed by the cool shallow corner of the wedge. Here, we evaluate these three proposed mechanisms using a set of kinematically-driven 2D thermo-mechanical mantle-wedge models in which subduction velocity, slab dip and age, overriding-plate thickness and the depth of decoupling between the two plates are systematically varied. All mechanisms predict, on the basis of model geometry, that the arc-trench distance, D, decreases strongly with increasing dip, consistent with the negative D-dip correlations found in global subduction data. Model trends of sub-arc slab depth, H, with dip are positive if H is wedge-temperature controlled and overriding-plate thickness does not exceed the decoupling depth by more than 50 km, and negative if H is slab-temperature controlled. Observed global H-dip trends are overall positive. With increasing overriding plate thickness, the position of maximum melting shifts to smaller H and D, while the position of the trenchward limit of the melt zone, controlled by the wedge's cold corner, shifts to larger H and D, similar to the trend in the data for oceanic subduction zones. Thus, the limit imposed by the wedge corner on melting and melt migration seems to exert the first-order control on arc position.
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K-Ar dating was carried out on the Tertiary volcanic rocks from Okushiri Island in Japan Sea. We obtained K-Ar whole rock ages of 4.4±0.2, 20.4±0.5, 19.7±0.5 and 34.4±0.8 Ma for a basalt in the Kamuiyama Formation, an andesite in the Tsurikake Formation, an andesite in the Aonaegawa Formation and a basalt in the Matsue basalt Formation, respectively. The volcanic activity during Tertiary time in Okushiri Island are divided into main three stages, Oligocene (34-29 Ma), Early Miocene (23-20 Ma) and Pliocene (-4 Ma). The chemical compositions of representative 32 samples from the three stages were determined by XRF techniques, The Oligocene Matsue basalts (lower) are characterized by high contents of MgO, Ni and Cr whereas the upper Matsue basalts have high contents of HFS elements such as Ti, P, Zr and Nb, suggesting that the latter are different in chemical compositions from the typical island arc basalts. The Early Miocene volcanics composed mainly of calc-alkaline andesite and dacite, whose incompatible element contents are similar to those of the Quaternary volcanics from the back-arc side of the NE Japan arc. The Pliocene volcanics composed mainly of rock suites ranging from andesite to dacite of calc-alkaline series with subordinate amounts of basalt which have incompatible element contents comparable to those of the Pliocene alkali basalt from Kyuroku-shima Island, northeast Japan Sea. The NE Japan arc and the surrounding areas during the Oligocene to Early Miocene are characterized by occurrence of andesite and dacite of calc-alkaline series and tholeiitic and alkali basalts with high contents of HFS elements, and the calc-alkaline rocks were generally formed before basaltic volcanism took place.
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The Izu‐Bonin arc is an intraoceanic island arc where the Pacific plate subducts beneath the Philippine Sea plate. Along‐arc variation in thickness of the arc crust has been observed by previous active seismic surveys beneath the Izu‐Bonin arc. We have imaged three‐dimensional (3‐D) seismic velocity structures in the northern Izu‐Bonin arc using arrival time data of the earthquakes during 80 days of observations by ocean bottom seismographs and three permanent island seismic stations. Our 3‐D velocity model indicates heterogeneous structure in the mantle wedge along the arc. Low‐velocity anomalies related to upwelling flow in the mantle wedge are not uniform beneath the volcanic front. Low‐velocity anomalies extending down to the subducting slab beneath the volcanic front coincide with thicker parts of the arc crust north of Aoga‐shima and south of Sumisu‐jima. This coincidence suggests that heterogeneous mantle upwelling flow fundamentally controls the growth of the arc crust. Along topographic highs in the forearc, low‐velocity anomalies in the crust and uppermost mantle coincide with positive magnetic anomalies suggesting the presence of a remnant arc, providing further evidence of this view of arc crust formation.
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