Melting the hydrous, subarc mantle: the origin of primitive andesites

Abstract

compositions in equilibrium with an olivine + orthopyroxene ± spinel residual assemblage (harzburgite). This model quantitatively predicts the following influences of H 2 O on mantle lherzolite melting: (1) As melting pressure increases, melt compositions become more olivine-normative, (2) as melting extent increases, melt compositions become depleted in the normative plagioclase component, and (3) as melt H 2 O content increases, melts become more quartz-normative. Natural high-Mg# [molar Mg/ (Mg + Fe 2+ )], high-MgO basaltic andesite and andesite lavas-or primitive andesites (PAs)-contain high SiO 2 contents at mantle-equilibrated Mg#s. Their compositional characteristics cannot be readily explained by melting of mantle lherzolite under anhydrous conditions. This study shows that experimental melts of a FM peridotite plus the addition of alkalis reproduce the compositions of natural PAs in SiO 2 , Al 2 O 3 , TiO 2 , Cr 2 O 3 , MgO, and Na 2 O at 1.0-1.2 GPa and H 2 O contents of 0-7 wt%. Our results also suggest that PAs form under a maximum range of extents of melting from F = 0.2-0.3. The CaO contents of the melts produced are 1-5 wt% higher than the natural samples. This is not a result of a depleted source composition or of extremely high extents of melt but is potentially caused by a very low CaO content contribution from deeper in the mantle wedge.

Sediment melting during subduction initiation: Geochronological and geochemical evidence from the Darutso high‐Mg andesites within ophiolite melange, central Tibet

“…In addition, high values of MgO, Mg # , and compatible elements (e.g., Cr) in DVRs indicate they are not the evolved products of a more mafic magma. Therefore, their SiO 2 contents (59.03–63.62 wt %) are higher than those of typical melts (<55 wt %) from a peridotite source irrespective of the volume of H 2 O [Mitchell and Grove, , and reference therein]. Although melting of pyroxenite‐bearing lithospheric mantle, formed by the interaction of mantle peridotite and a Si‐oversaturated melt derived from sediments, could potentially generate such high SiO 2 and enriched isotopic compositions [ Sobolev et al ., ; Straub et al ., ], the Fe/Mn (<50) and Ni/MgO (<11) ratios of the DVRs are much lower than those of such pyroxenite‐derived HMAs (>80 and >30, respectively) [e.g., Huang et al ., ; Liu et al ., b].…”

On edge melting under the Colorado Plateau margin

“…Estimates of melting conditions were also obtained using the thermobarometer of Mitchell and Grove [], which is calibrated for melts in equilibrium with harzburgite at P up to 2.5 GPa. Assuming the same source parameters as above, there is good agreement with pressures obtained using the Lee et al .…”

“…High‐pressure experiments bolstered by geologic observations have provided a broad framework for understanding conditions of melt generation [e.g., Takahashi and Kushiro , ; Klein and Langmuir , ; Plank and Langmuir , ]. Continuing experimental refinements of melt compositions over diverse melting conditions (compiled by the Library of Experimental Phase Relations (LEPR) [ Hirschmann et al ., ]) has led to a greater understanding of the influence of water and CO 2 , and to a recent proliferation of thermobarometers for determining melting conditions [e.g., Dasgupta et al ., ; Medard and Grove , ; Putirka , ; Lee et al ., ; Till et al , ; Mitchell and Grove , ]. The role of mantle heterogeneities, while the subject of ongoing debate, may be revealed by chemical proxies for contributions from mafic lithologies [e.g., Sobolev et al ., ; Qin and Humayun , ; Le Roux et al ., ; Yang and Zhou , ].…”

Hydrous orthopyroxene-rich pyroxenite source of the Xinkailing high magnesium andesites, Western Liaoning: Implications for the subduction-modified lithospheric mantle and the destruction mechanism of the North China Craton

“…Previous experimental research has demonstrated that HMAs can be formed by the orthopyroxene-dominant melting of peridotite at low pressures (e.g., Grove et al, 2003;Hirose, 1997;Mitchell and Grove, 2015;Tatsumi, 1982;Wood and Turner, 2009). This melting process increases the amount of olivine left in the residue.…”

“…Experiments have demonstrated that HMAs must equilibrate with mantle peridotite at low pressures (e.g., Grove et al, 2003;Hirose, 1997;Mitchell and Grove, 2015;Tatsumi, 1982;Wood and Turner, 2009). This therefore implies that the HMA-basalt association is created by the multi-stage partial melting of a single source: in this scenario, HMA magmas are separated from the mantle source at lower pressures than the basaltic magmas (Falloon and Green, 1988;Mashima, 2009aMashima, , 2009b Gao et al, 2008;Li et al, 2002;Yang and Li, 2008;Huang et al, 2007;Wang et al, 2006;Zhang et al, 2003) are similar to those of the overlying HMAs, but their Nd isotopic compositions (ε Nd(t) =~−2; Gao et al, 2008) are more depleted than those of the HMAs (ε Nd(t) = −5 to −14; Li et al, 2002;Yang and Li, 2008;Huang et al, 2007;Wang et al, 2006;Zhang et al, 2003).…”