Partial Melting Origin Research Articles

Experimental partial melting of the Allende (CV) and Murchison (CM) chondrites and the origin of asteroidal basalts

The compositions of experimental partial melts from the Allende (CV) and anhydrous Murchison (CM) chondrites depend strongly on ambient oxygen fugacity ( fo 2). At low fo 2 (IW-1) partial melts are tholeiitic and resemble the compositions of eucrites, whereas at high fo 2( IW+2) melts are silica undersaturated and resemble the compositions of angrites. In particular, the major element composition of our (1170°C, IW-1) melt of Murchison is strikingly similar to natural eucrites. Allende partial melts, while broadly eucritic, do not match most eucrites in detail. Conversely, IW+2 Allende partial melts are more like angrite compositions than are partial melts of Murchison. Chromium contents of eucrites and of our spinel-saturated Murchison and Allende experiments are extremely similar, suggesting that eucrites are also spinel saturated. This observation favors a partial melting origin for eucrites such as Sioux County, as opposed to an origin by fractional crystallization. However, most of the main group eucrites have slightly lower Mg#'s and slightly higher CaO/A1 2O 3 ratios than Sioux County, suggesting that, relative to Sioux County, the majority of the main group eucrites either have experienced small amounts of fractional crystallization or were produced by slightly smaller amounts of partial melting. If the eucrites were produced by simple partial melting, the MnO content of the eucrite parent body must be rather high compared to either Allende or Murchison. In contrast to comparisons between our IW—1 experiments and eucrites, none of our IW+2 experiments have exactly reproduced a natural angrite composition. There are also minor but significant discrepancies between our partial melting experiments and experiments on angrite compositions. While these differences may, in part, be due to our poor knowledge of the angrite parent body, they may be explicable if at least some of the angrites are not primitive partial melts but, rather, impact melts.

Miocene calc-alkaline heritage in the pliocene postcollisional volcanism of monte arci (Sardinia, Italy)

At Monte Arci alkaline (hawaiites to trachytes), subalkaline with a marked calc-alkaline character (basalts to dacites) and rhyolitic lavas were erupted almost simultaneously in Late Pliocene time. Major- and trace-element chemistry, microprobe mineralogy and isotopic data suggest a partial melting origin for both rhyolites and subalkaline rocks. Different sources are however inferred for two rock series: homogeneous, calc-alkaline in nature for subalkaline rocks; unhomogeneous, richer in 87Sr, for rhyolitic ones. Crystal fractionation differentiation from subcrustal alkali-basalts should have been the main process in the genesis of alkaline rocks. Large-scale contaminations with rhyolitic and/or alkaline rocks are evident in many of these lavas. Such a complicated magmatic association characterizes an area where volcanism related to post-collisional tensional movements in Pliocene time superimposes to Middle Miocene calc-alkaline basic volcanism related to previous subduction processes. The Pliocene volcanic history of Monte Arci emphasizes the influence of the paleogeodynamic environment on the nature of magmas erupted in post-continental collision areas, that are frequently difficult to arrange in the usual schemas connecting magma composition with tectonic setting.

Recent volcanism in Masaya-Granada Area, Nicaragua

Masaya-Granada area is located in the middle part of the Central American volcanic zone. A basaltic shield volcano with a caldera, an acidic pyroclastic flow plateau with a caldera, cinder cones, maars, a lava dome and a composite andesitic volcano were formed by recent volcanic activities. Magmas of basic and intermediate ejecta are supposed to be formed by partial melting of the upper mantle material. Most of basalts and andesites was derived from common parental magma after crystallization differentiation history, but some basalts, which have extremely high MgO content and low K2O content might be derived from primary magma of different type. There is no evidence to deny the possibility of differentiation product of acidic rock from basic magma, but compositional gap on variation diagram suggest the possibility of partial melting origin. Strike-slip fault systems might have been formed in association with plate movement, and fluidal basaltic magma was erupted also along these fault zones.

Geochemistry of the volcanics of central Mexico

The Tertiary and Recent volcanics of Mexico occur in two provinces. The Cordillera Province is made up of about 1700 m of ignimbrite sheets overlain and intercalated in the upper part by olivine basalt and basaltic andesite. The Rio Lerma Province extends transversely across Mexico and in the Valley of Mexico the lavas consist mainly of andesite and dacite, 68 % of those analysed having 62 | 4.7 % SiO2. A total of 108 chemical analyses were made for the major elements, 90 of these including determinations of Cr, Ni, Cu, Zn, Rb, Sr, Ba, Th and Pb for two areas, the Valley of Mexico in the Rio Lerma Province and the Guadalajara region which lies at the intersection of the two provinces. Computer constructions of normative components in the basalt tetrahedron and other projections support an origin of partial melting of tholeiitic to pyrolitic material for the production of andesite. The Guadalajara lavas have consistently higher K/SiO2 and K/Rb ratios and lower Mg/SiO2 ratio than the Valley of Mexico rocks suggesting generation at greater depth.