Behaviour of high field strength elements in subduction zones: constraints from Kamchatka–Aleutian arc lavas
نویسندگان
چکیده
Models explaining the characteristic depletion of High Field Strength Elements (HFSE) relative to elements of similar compatibility in subduction zone magmas invoke either (1) the presence of HFSE-rich minerals in the subduction regime or (2) a selectively lower mobility of HFSE during subduction metasomatism of the mantle. In order to investigate the properties of HFSE in subduction regimes closer, we performed high precision measurements of Nb/Ta, Zr/Hf, and Lu/Hf ratios together with Hf/Hf analyses on arc rocks from Kamchatka and the western Aleutians. The volcanic rocks of the Kamchatka region comprise compositional end members for both fluid and slab melt controlled mantle regimes, thus enabling systematic studies on the HFSE mobility at different conditions in the subarc mantle. Hf–Nd isotope and systematic Zr/Hf and Lu/Hf covariations illustrate that Zr–Hf and Lu are immobile in fluid-dominated regimes. Hf–Nd isotope compositions furthermore indicate the presence of ‘‘Indian’’ type depleted mantle beneath Kamchatka, as previously shown for the Mariana and Izu–Bonin arcs. In addition to a depleted mantle component, Hf–Nd isotope compositions enable identification of a more enriched mantle wedge component in the back-arc (Sredinny Ridge) that most likely consists of mantle lithosphere. The ratios of Nb/Ta and Zr/Hf are decoupled in rocks from fluid-dominated sources, indicating that Nb and Ta can be enriched in the mantle by subduction fluids to a small extent. In contrast to the fluid-dominated regime in Central Kamchatka, the budget of HFSE and Lu in rocks from the Northern Kamchatka Depression and in adakitic rocks from the western Aleutians is significantly affected by slab melts that originate from subducted oceanic crust. Compositions of the rocks with the highest slab melt components in their source (Sr/ Y>30) provide no evidence that either Nb/Ta or Zr/Hf ratios are fractionated at a globally significant scale during melting of subducted oceanic crust. Subduction processes are therefore an unlikely candidate to explain the terrestrial Nb–Ta paradox (i.e., the subchondritic Nb/Ta ratios in all accessible terrestrial silicate reservoirs). D 2004 Elsevier B.V. All rights reserved.
منابع مشابه
High-Mg# andesitic lavas of the Shisheisky Complex, Northern Kamchatka: implications for primitive calc-alkaline magmatism
Primitive arc magmatism and mantle wedge processes are investigated through a petrologic and geochemical study of high-Mg# (Mg/Mg ? Fe [ 0.65) basalts, basaltic andesites and andesites from the Kurile-Kamchatka subduction system. Primitive andesitic samples are from the Shisheisky Complex, a field of Quaternary-age, monogenetic cones located in the Aleutian–Kamchatka junction, north of Shiveluc...
متن کاملLarge-scale deformation related to the collision of the Aleutian Arc with Kamchatka
The far western Aleutian Island Arc is actively colliding with Kamchatka. Westward motion of the Aleutian Arc is brought about by the tangential relative motion of the Pacific plate transferred to major, right-lateral shear zones north and south of the arc. Early geologic mapping of Cape Kamchatka (a promontory of Kamchatka along strike with the Aleutian Arc) revealed many similarities to the g...
متن کاملTrace element transport rates in subduction zones: evidence from Th, Sr and Pb isotope data for Tonga-Kermadec arc lavas
Trace element and Th, Sr and Pb isotope data for young lavas from the Tonga-Kermadec arc in the southwest Pacific suggest that geochemical variations in the lavas along the arc are linked to differences in the material being subducted beneath the arc. Lavas from the southern (Kermadec) segment of the arc have relatively radiogenic Pb isotope compositions, which reflects a contribution from subd...
متن کاملAdakites without slab melting: High pressure differentiation of island arc magma, Mindanao, the Philippines
New geochemical data for Pleistocene magmatic rocks from the Surigao peninsula, eastern Mindanao, the Philippines, demonstrate typical adakitic traits, including elevation of Sr /Y and depletion of the heavy rare earth elements. Sr / Sr and Nd / Nd ratios of the adakites do not support melting of the subducted Philippine Sea Plate but resemble Pliocene arc lavas generated in the same subduction...
متن کاملDiapirs as the source of the sediment signature in arc lavas
Island arc lavas, erupted above subduction zones, commonly contain a geochemical component derived from partial melting of subducted sediment. It is debated whether this sediment melt signature, with enriched trace element concentrations and isotope ratios, forms at relatively low or high temperatures. Here we compile and analyse the geochemistry of metamorphosed sedimentary rocks that have bee...
متن کامل