Nd and Sr isotopes in the Aleutians: multicomponent parenthood of island-arc magmas

Abstract

Young volcanic rocks from different sections of the Aleutian Islands-Alaska Peninsula Arc have been measured for ^(87)Sr/^(86)Sr, ^(143)Nd/^(144)Nd and some trace elements. We found the ^(143)Nd/^(144)Nd to be highly restricted in range (ɛ_(Nd)=6 to 7) and low as compared to midocean ridge basalts (MORB). This indicates that the source of the Aleutian Arc magmas is different from MORB and remarkably isotopically homogeneous with respect to Nd. The range reported here for arc rocks is substantially smaller than found by other workers. However, the Sr isotope ratios vary considerably (ɛ_(Sr)=−24 to −14). Those samples from small volcanic centers north of the main arc (second arc) are characterized by low ɛ_(Sr). Our data in combination with previous studies suggest that there are slight geochemical differences between discrete sections of the arc. The general uniformity of Nd isotope ratios are thought to be the surface expression of an efficient mixing or homogenization process beneath the arc plate, but which still causes a wide dispersion in Sr isotopic composition. To relate the arc rocks to the broader tectonic setting and to identify possible sources of arc magmas, measurements were done on volcanic and sedimentary rocks from the North Pacific/Bering Sea area. Alkali basalts from the back-arc islands St. George, Nunivak and St. Lawrence and alkali-rich tholeiites from the fore-arc have ɛ_(Nd)=+4 to +9 and are correlated on the ɛ_(Sr)-ɛ_(Nd)diagram parallel to the mantle array but shifted to lower ɛ_(Sr). These samples are thought to be isotopically representative of the mantle transported to that region. A tholeiitic basalt from the Kamchatka Basin ocean floor (back-arc), however, yielded typical MORB values (ɛ_(Nd)=10, ɛ_(Sr)=−24). Composite sediment samples were made from DSDP cores in the Aleutian Abyssal Plain, Gulf of Alaska and the Alka Basin which represent mixtures of continentally and arc-derived materials. These composites have intermediate Nd isotopic (ɛ_(Nd)= −2 and +2) and high Sr isotopic values (ɛ_(Sr)=+9 and +37). These data show that possible source materials of the Aleutian Arc volcanics are isotopically different from and much more heterogeneous than the arc rocks themselves. On the basis of this study and of literature data, we developed a set of alternative models for volcanic arc magma generation, based on the restricted range in ɛ_(Nd) and the wider range in ɛ_(Sr) for arc rocks. Different isotopic and trace element characteristics found in different arcs or arc sections are explained by varying mixing proportions or concentrations in source materials. The basic observations require rather strict mixing ratios to obtain constant ɛ_(Nd). The preferred model is one where the melting of subducted oceanic crust is controlled by the amount of trapped sediment with the melting restricted to the upper part of the altered basaltic layer. Homogenization within the upper part of the oceanic crust is brought about by hydrothermal circulation attending dewatering of the slab during subduction and possibly some oxygen exchange of the magmas on ascent.

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