Date of Award

1-1-2013

Document Type

Open Access Thesis

Department

Earth and Ocean Sciences

Sub-Department

Geology

First Advisor

Gene M Yogodzinski

Abstract

Whole-rock compositions of seafloor lavas are used to constrain the processes and source characteristics contributing to the formation of geochemically distinctive, primitive magma types located in the Aleutian back-arc. Samples were dredged primarily from small volcanic cones on the seafloor, located between the emergent volcanoes and in the back-arc up to 60 km from the volcanic front. Compositions vary from basalt to dacite, with 48-70 % SiO2 and 4-13 % MgO. Nearly 30% of the samples are primitive, with Mg/Mg+Fe >0.60. Most primitive samples are basalts, which fall into two groups, based on minor and trace element abundances. Low/med-K basalts are similar to primitive basalts throughout the arc with % K2O and 2O5. Most of these contain Sr, Rb, 4-7 ppm La, and La/Yb=3-5. Medium/high-K basalts have higher K2O and P2O5 as well as higher abundances of most incompatible trace elements except Pb (basalts). Enrichments are somewhat stronger in the large ion lithophile elements compared to less strongly incompatible elements, resulting in more fractionated trace element patterns in med/high-K basalts (La/Yb=5-10 and Sr/Y=21-39), but without an affect from residual garnet (normalized Dy/Yb=1-2). Strontium and Pb isotopes are generally less radiogenic in med/high-K basalts (87Sr/86Sr=0.7028-0.7031, 207Pb/204Pb=15.49-15.55) compared to low/med-K basalts (87Sr/86Sr=0.7030-0.7035, 207Pb/204Pb=15.53-15.59). The pattern is one of generally less radiogenic Pb and Sr in samples that are more strongly enriched in incompatible elements relative to Pb (K/Pb, Ce/Pb, Hf/Pb). Medium/high-K basalts with the least radiogenic Sr and Pb have incompatible trace element ratios that approach those of ocean ridge basalts (Ce/Pb=12, Zr/Sm=28, La/Ta=17). The broad isotopic pattern of all Aleutian lavas, and the unradiogenic Sr in med/high-K basalts appear inconsistent with the involvement of an enriched mantle component in the Aleutian back-arc. Paired isotope-incompatible element systematics suggest a stronger influence from depleted mantle in the source that produced the med/high-K basalts. Mixing relationships based on 207Pb/204Pb and Ce/Pb indicate a reduced role for subducted sediment, and an increased role for depleted mantle in the source. Higher abundances of K2O and other incompatible elements in med/high-K basalts appear to require a separate explanation. One possibility is that med/high-K basalts are also produced by significantly lower degrees of partial melting in the mantle, compared to low/med-K basalts.

Rights

© 2013, Max Thomas Siegrist

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