Phase relations of natural aphyric high-alumina basalts and their intrusive equivalents were determined through rock-melting experiments at 2 kb, H2O-saturated with fO2 buffered at NNO. Experimental liquids are low-MgO high-alumina basalt or basaltic andesite, and most are saturated with olivine, calcic plagioclase, and either high-calcium pyroxene or hornblende (±magnetite). Cr-spinel or magnetite appear near the liquidus of wet high-alumina basalts because H2O lowers the appearance temperature of crystalline silicates but has a lesser effect on spinel. As a consequence, experimental liquids follow calcalkaline differentiation trends. Hornblende stability is sensitive to the Na2O content of the bulk composition as well as to H2O content, with the result that hornblende can form as a near liquidus mineral in wet sodic basalts, but does not appear until liquids reach andesitic compositions in moderate Na2O basalts. Therefore, the absence of hornblende in basalts with low-to-moderate Na2O contents is not evidence that those basalts are nearly dry. Very calcic plagioclase (>An90) forms from basaltic melts with high H2O contents but cannot form from dry melts with normal are Na2O and CaO abundances. The presence of anorthite-rich plagioclase in high-alumina basalts indicates high magmatic H2O contents. In sum, moderate pressure H2O-saturated phase relations show that magmatic H2O leads to the early crystallization of spinel, produces calcic plagioclase, and reduces the total proportion of plagioclase in the crystallizing assemblage, thereby promoting the development of the calc-alkaline differentiation trend.

Experimental investigations of the role of H2O in calc-alkaline differentiation and subduction zone magmatism

Aleutian magnesian andesites: Melts from subducted Pacific ocean crust

A multicomponent melting-mixing model for the origin of volcanic arc basalt magmas implies signifi cant recycling of K, Rb, Ba, and other elements in the crust-upper mantel system. The model accounts for chemical differences in alkali, alkaline earth, and rare earth content between arc basalts and a wide range of oceanic and intraplate basalt types. In essence, the differences are due to modification of mantle peridotite by recycled continental components (derived from sediment and sea water) at convergent plate margins. In a specific model tested by mass balance calculations, residual periodotite (partly depleted in a basalt fraction) in a subducted lithospheric plate thermally equilibrates with the surrounding hotter mantle at depth beneath island arcs, and being less dense, rises as kilometer size diapires. The diapirs are modified by addition of small proportions of melt or saline supercritical fluid from the denser oceanic basalt (eclogite) and sediment. The diapire serves as a source for arc basalts...

Volcanic Arc Magmas: Implications of a Melting-Mixing Model for Element Recycling in the Crust-Upper Mantle System

Experimental evidence is used to estimate H2O contents in low-MgO high-alumina basalts (HABs) ( 8wt.% MgO) could have H2O contents reaching no more than 1–2wt.%. The more common low-MgO HABs could have existed as liquids within the crust with H2O contents of 4 wt.% or higher at temperatures<1100°C. Magmas with these high H2O contents will saturate with and exsolve aqueous fluid upon approaching the surface. They cannot erupt as liquids and must grow crystals at shallow depths, thus accounting for the abundant phenocrysts in low-MgO HABs and BAs.

Temperatures and H 2 O contents of low-MgO high-alumina basalts

Isotopic and incompatible element constraints on the genesis of island arc volcanics from Cold Bay and Amak Island, Aleutians, and implications for mantle structure

The historic eruptions of Bogoslof volcano were characterized by hornblende andesite in AD 1796, and nepheline-normative, hornblende basalt in AD 1883 and 1926-1927 Rare earth element (REE) abundances for the 1796 and 1926-1927 eruptions show enrichment of the light REE relative to the heavy REE, overall higher abundances in the andesite and a slight positive Eu anomaly in the basalt The $$^{87}Sr/^{86}Sr$$ ratios are $$070343 \pm 7$$ for the andesite and $$070296 \pm 7$$ for the 1927 basalt Lead isotopic abundances are similar for the two eruptions and the rocks are more radiogenic than mid-ocean ridge basalts Microprobe data for phenocrysts of potassio ferroan pargasite, calcic augite and plagioclase show a considerable compositional range and degree of compositional overlap Mafic inclusions in basalt and andesite contain the same phases as the host lavas and a similar compositional range, suggesting a syngenetic origin for inclusions and host One inclusion contains quartz and biotite which 

S. S. Sun

Papers

The Alkalic Rock Suite of Bogoslof Island, Eastern Aleutian Arc, Alaska