Slab melting in the Aleutians: implications of an ion probe study of clinopyroxene in primitive adakite and basalt

Abstract

An ion probe study of trace elements in Mg-rich clinopyroxene phenocrysts in primitive Aleutian lavas provides constraints on the genesis of Aleutian adakites, and possible insights into the source of common Aleutian magmas. Clinopyroxene (cpx) phenocrysts in the primitive adakites have high Sr and Nd=Yb compared to cpx in Aleutian basalts. In the adakites, Sr and Nd=Yb are highest for high Mg# cpx, and these concentrations decrease toward lower Mg# compositions. These trends are the opposite of those seen in basalt cpx which generally show increasing incompatible trace element contents with decreasing Mg#, and are unlike antithetic compatible-incompatible trace element trends produced by chemical or kinetic effects of crystal growth. Petrographic observations and major and trace element zonation in cpx phenocrysts indicate that primitive Aleutian adakites are in part the product of mixing between primitive and relatively evolved magmas. The adakite trace element signature (high Sr, Nd=Yb) is clearly associated with the primitive mixing end-member. This observation supports the idea that adakites are derived by equilibration of slab melts with mantle olivine, and appears to rule out an origin by melting in the lower crust. Adakites are relatively rare in the Aleutians, but arc-wide correlations between Sr and La=Yb indicates that an adakite-type slab melt component may be present in the magmatic source throughout the arc.

Late Triassic high-Mg andesite/dacite suites from northern Hohxil, North Tibet: Geochronology, geochemical characteristics, petrogenetic processes and tectonic implications

“…5c). The andesite samples exhibit high wt.%), Mg # (64-72), and Cr (272-531 ppm) and Ni (66.7-636 ppm) contents, which are similar to typical Cenozoic high-Mg andesites in arc settings, e.g., "adakitic" and "Piip" high-Mg andesites in the Aleutian islands (Alaska) and the Kamchatka Strait to the west of the Aleutian islands (e.g., Yogodzinski et al, 1994Yogodzinski et al, , 1995Yogodzinski et al, , 2001Yogodzinski and Kelemen, 1998), bajaites in Baja California (Mexico) (Calmus et al, 2003;Saunders et al, 1987), sanukitoids in the Setouchi Volcanic Belt (SW Japan) (Shimoda et al, 1998;Tatsumi, 2001;Tatsumi and Ishizaka, 1981) and boninites of the Bonin Islands (Japan) (e.g., Taylor et al, 1994;Umino, 1986) (Fig. 6a-e).…”

“…The data for bajaites in the Baja area, California, Mexico are from Calmus et al (2003) and Saunders et al (1987). The data for Adakitic high-Mg andesites of Adak Island (Aleutian islands) and the Kamchatka Strait to the west of Aleutian islands are from Yogodzinski et al (1995Yogodzinski et al ( , 1998Yogodzinski et al ( , 2001, and references therein; the data for Piip-type high-Mg andesites of the Piip volcano (Aleutian islands) are from Yogodzinski et al (1994). HMA-high Mg andesites.…”

“…5 for other data sources. Xu et al, 2008;Yogodzinski et al, 1994Yogodzinski et al, , 1995Yogodzinski and Kelemen, 1998).…”

“…3-4). The compositional disequilibrium characteristics of the augites suggest that the Hohxil high-Mg andesites were likely derived by interaction between melts and mantle (e.g., Rapp et al, 1999;Yogodzinski and Kelemen, 1998). However, the origin of the melts requires further clarification, given that similar melts have been attributed to partial melting of delaminated or subducted continental crust (e.g., Gao et al, 2004;Wang et al, 2006bWang et al, , 2008cXu et al, 2008), subducted basaltic oceanic crust, or sediments (e.g., Kay et al, 1993;Kelemen et al, 2003;Shimoda et al, 1998;Tatsumi, 2001;Tatsumi and Hanyu, 2003;Yogodzinski and Kelemen, 1998;Yogodzinski et al, 1994Yogodzinski et al, , 1995.…”

Thermal structure due to solid-state flow in the mantle wedge beneath arcs

“…(1) Partial melting of subducted metabasalt (or metagabbro) at eclogite facies conditions is inferred based on the similarity of trace element patterns in rare, primitive andesites with partial melts of eclogite [e.g., Defant and Kepezhinskas, 2001;Grove et al, 2001;Tatsumi et al, 2001;Yogodzinski et al, 2001;Rapp et al, 1999;Yogodzinski and Kelemen, 1998;Yogodzinski et al, 1995;Yogodzinski et al, 1994;;Defant and Drummond, 1990;Drummond and Defant, 1990;Kay, 1978]. Such lavas are primarily observed where subduction rates are slow, and/or the subducting oceanic crust is young (< 20 million years old), and/or the subducting plate terminates along strike allowing heating from the side as well as from the top [Yogodzinski et al, 2001;Defant and Drummond, 1990].…”