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The phase relations of primitive magnesian andesites and basaltic andesites from the Mt. Shasta region, N California have been determined over a range of pressure and temperature conditions and H 2 O contents. The experimental results are used to explore the influence of H 2 O and pressure on fractional crystallization and mantle melting behavior in subduction zone environments. At 200-MPa H 2 O-saturated conditions the experimentally determined liquid line of descent reproduces the compositional variation found in the Mt. Shasta region lavas. This calc-alkaline differentiation trend begins at the lowest values of FeO*/MgO and the highest SiO 2 contents found in any arc magma system and exhibits only a modest increase in FeO*/MgO with increasing SiO 2 . We propose a two-stage process for the origin of these lavas. (1) Extensive hydrous mantle melting produces H 2 O-rich (>4.5-6 wt% H 2 O) melts that are in equilibrium with a refractory harzburgite (olivine + orthopyroxene) residue. Trace elements and H 2 O are contributed from a slab-derived fluid and/or melt. (2) This mantle melt ascends into the overlying crust and undergoes fractional crystallization. Crustallevel differentiation occurs under near-H 2 O saturated conditions producing the distinctive high SiO 2 and low FeO*/MgO characteristics of these calc-alkaline andesite and dacite lavas. In a subset of Mt. Shasta region lavas, magnesian pargasitic amphibole provides evidence of high pre-eruptive H 2 O contents (>10 wt% H 2 O) and lower crustal crystallization pressures (800 MPa). Igneous rocks that possess major and trace element characteristics similar to those of the Mt. Shasta region lavas are found at Adak, Aleutians, Setouchi Belt, Japan, the Mexican Volcanic Belt, Cook Island, Andes and in Archean trondhjemite-tonalite-granodiorite suites (TTG suites). We propose that these magmas also form by hydrous mantle melting.

DefinitionsD VSMOW is the difference between the measured D/H ratio of a sample and the D/H ratio of Vienna Standard Mean Ocean Water (VSMOW) expressed in per mil (‰):

aragonite precipitation by marine organisms is affected by seawater chemistry, B/Ca may 36 also prove useful in reconstructing seawater chemistry. A simplified boron purification 37 protocol based on amberlite resin and the organic buffer TRIS is also described. 38 39

[1] We reared primary polyps (new recruits) of the common Atlantic golf ball coral Favia fragum for 8 days at 25°C in seawater with aragonite saturation states ranging from ambient (W = 3.71) to strongly undersaturated (W = 0.22). Aragonite was accreted by all corals, even those reared in strongly undersaturated seawater. However, significant delays, in both the initiation of calcification and subsequent growth of the primary corallite, occurred in corals reared in treatment tanks relative to those grown at ambient conditions. In addition, we observed progressive changes in the size, shape, orientation, and composition of the aragonite crystals used to build the skeleton. With increasing acidification, densely packed bundles of fine aragonite needles gave way to a disordered aggregate of highly faceted rhombs. The Sr/Ca ratios of the crystals, measured by SIMS ion microprobe, increased by 13%, and Mg/Ca ratios decreased by 45%. By comparing these variations in elemental ratios with results from Rayleigh fractionation calculations, we show that the observed changes in crystal morphology and composition are consistent with a >80% decrease in the amount of aragonite precipitated by the corals from each ''batch'' of calcifying fluid. This suggests that the saturation state of fluid within the isolated calcifying compartment, while maintained by the coral at levels well above that of the external seawater, decreased systematically and significantly as the saturation state of the external seawater decreased. The inability of the corals in acidified treatments to achieve the levels of calcifying fluid supersaturation that drive rapid crystal growth could reflect a limit in the amount of energy available for the proton pumping required for calcification. If so, then the future impact of ocean acidification on tropical coral ecosystems may depend on the ability of individuals or species to overcome this limitation and achieve the levels of calcifying fluid supersaturation required to ensure rapid growth.

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