Fractional crystallization and mantle-melting controls on calc-alkaline differentiation trends

Abstract

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.

Chemical, isotopic, and temporal variations during crustal differentiation : insights from the Dariv Igneous Complex, Western Mongolia

“…Currently, the primary evidence of alkalibasalt fractionation arises from phlogopite-bearing ultramafic xenoliths in alkali-basalts and lamprophyres, which indicate that the fractionation sequence involves the crystallization of olivine + clinopyroxene + phlogopite ± apatite (Buhlmann et al 2000;Dawson and Smith 1992;Downes et al 2004;Giannetti 1982;Giannetti and Luhr 1990;Righter and Rosas-Elguera 2001). Experimental phase-equilibria of hydrous alkali-rich magmas corroborate the limited evidence from high-K ultramafic xenoliths (Barton and Hamilton 1978;Barton and Hamilton 1979;Edgar et al 1980;Elkins-Tanton and Grove 2003;Esperança and Holloway 1987;Melzer and Foley 2000;Righter and Carmichael 1996), yet, until now, an exposed, continuous plutonic sequence documenting such a fractionation sequence has not been described.…”

“…have found olivine + clinopyroxene + phlogopite ± spinel at multiple saturation (Edgar et al 1980;Elkins-Tanton and Grove 2003;Esperança and Holloway 1987;Melzer and Foley 2000;Righter and Carmichael 1996). Only two studies obtained conditions conducive to orthopyroxene saturation and both required CO 2 to stabilize it (Edgar et al 1980;Sato 1997).…”

“…Field ) and experimental studies (Müntener et al 2001;Grove et al 2003;) have corroborated the importance of fractional crystallization in producing this stratified crust, however many questions remain concerning the compositional and isotopic variations, as well as, timescales associated with fractional crystallization in the arc crust.…”

“…; Grove et al 2003;Sisson and Grove 1993) and in the field (e.g., Jagoutz et al 2011;), yet the fractionation of primitive alkali-rich basalts remains poorly defined. In part, the reason for the paucity of studies on alkaline basalt differentiation in arcs is the lack of field exposure and the limited, fragmentary evidence from cumulate xenoliths.…”

Geochemistry, zircon U–Pb ages and Hf isotopes of the Paleozoic volcanic rocks in the northwestern Chinese Altai: Petrogenesis and tectonic implications

“…Mixing end members: Depleted mantle M1 (e Nd (t): +9.86, Nd: 3.53 ppm), Habahe sediments S1 (e Nd (t): À5.29, Nd: 20 ppm), Habahe sediments S2 (e Nd (t): À3.45, Nd: 50 ppm), Global subducting sediment Gloss (e Nd (t): À5.2, Nd: 27 ppm, Plank and Langmuir, 1998). it is impossible for the Mg-rich dacite to be derived from mantle peridotite metasomatized by slab-derived adakitic melts (Rapp et al, 1999;Defant and Kepezhinskas, 2001;Kelemen et al, 2003). Recent experiments have demonstrated that partial melting of mantle peridotite can produce primitive high-Mg andesitic melts with high Mg # ($70) and e Nd (t) values similar to the depleted mantle (Grove et al, 2003;Wood and Turner, 2009). The Mg # (51-53) and e Nd (t) values (À4.6 to À3.2) of the Mg-rich dacite are remarkably lower than those of the primitive melts of hydrated peridotite, therefore excluding an origin from partial melting of the hydrated mantle peridotite.…”