Melting experiments on hydrous low-K tholeiite: Implications for the genesis of tonalitic crust in the Izu–Bonin – Mariana arc

A series of water-deficient partial melting experiments on a low-K tholeiite were carried out under lower crustal P–T–H₂O conditions (900–1200 °C, 0.7–1.5 GPa, 2 and 5 wt% H₂O added) using a piston-cylinder apparatus. With increasing temperature at 1.0 GPa, supersolidus mineral assemblages vary from amphibolitic to pyroxenitic. Garnet crystallizes in the higher pressure runs (> 1.2 GPa). Melt compositions show low-K calc-alkalic trends, and are classified as metaluminous or peraluminous tonalite. These features are similar to the felsic rocks in the Izu–Bonin – Mariana (IBM) arc, for example Tanzawa plutonic rocks. The anatectic origin of Tanzawa tonalites is consistent with geochemical modeling, which demonstrates that the rare earth element (REE) characteristics of Tanzawa plutonic rocks (which represent the middle crust of the IBM arc) can be generated by partial melting of amphibolite in the lower crust (∼ 50% melting at 1050 °C and below 1.2 GPa). Estimated densities of pyroxenitic restites (∼ 3.9 g/cm³) after extraction of andesitic melts are higher than that of mantle peridotite beneath the island arc (3.3 g/cm³). The high density of the restite could cause delamination of the IBM arc lower crust. Rhyolitic magmas in the IBM arc (e.g. Niijima) could be formed by low degrees of partial melting of the amphibolitic crust at a temperature just above the solidus (10% melting at or below 900 °C).     

Vertical profiles and storage fluxes of CO2, heat and water in a tropical rainforest at Pasoh, Peninsular Malaysia

Ambient CO₂ concentration, air temperature and relative humidity were measured intermittently for a 3-year period from the floor to the canopy top of a tropical rainforest in Pasoh, Peninsular Malaysia. Mean diurnal CO₂ storage flux ( S _c; μmol m⁻² s⁻¹) and sensible and latent heat storage fluxes ( Q _a and Q _w; W m⁻²) ranged from −12.7 to 3.2 μmol m⁻² s⁻¹, −15 to 27 W m⁻² and −10 to 20 W m⁻², respectively. Small differences in diurnal changes were observed in S _c and Q _a between the driest and wettest periods. Compared with the ranges of mean diurnal CO₂ eddy flux (−14.7 to 4.9 μmol m⁻² s⁻¹), sensible eddy flux (−12 to 169 W m⁻²) and latent eddy flux (0 to 250 W m⁻²), the contribution of CO₂ storage flux was especially large. Comparison with summertime data from a temperate Japanese cypress forest suggested a higher contribution of S _c in the tropical rainforest, probably mainly due to the difference in nighttime friction velocity at the sites. On the other hand, differences in Q _a and Q _w were smaller than the difference in S _c, probably because of the smaller nighttime sinks/sources of heat and water vapour.     

地壳表层砷的循环与污染地下水模式

为了更好地监测和预测地下水砷污染,揭示由自然原因引起的地下水砷污染规律和机制,通过多年来对内蒙古河套平原地下水砷污染的调查研究,在充分了解和掌握砷的化学与地球化学性质以及世界各国地下水砷污染状况与分布特征的基础上,总结了砷在自然环境中的分布和污染地下水的机理。文章阐述了4种地下水砷污染类型的形成机理,并提出了地壳表层砷的循环与污染地下水模式,通过该模式在基本掌握某一地区地质环境背景的情况下,可以初步预测其可能发生地下水砷污染的地段。地下水砷污染是砷在地壳表层循环过程中发生的一种环境水文地球化学现象,污染区的分布与特定的地质、地理背景和环境条件有关。在还原解吸污染类型中,封闭的环境、硫元素和有机碳的存在是地层中砷溶出的主要控制因素。今后应加强对封闭的冲积或湖积平原、河流入湖或入海三角洲等地区地下水砷污染的调查与监测。