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In the absence of an externally applied stress, the segregation of small amounts of granitic or tonalitic melts from their
residual mafic crystals is possible only if the melt forms an interconnected network phase. Accordingly, this research focuses
on melt connectivity at low melt fraction (<4 wt% or 5 vol.%). Connectivity of granitic and tonalitic melts in amphibole-rich
rock was assessed by performing two types of piston-cylinder experiments at 1 GPa and 800 °C. The first involved annealing
samples that consisted of either alternating layers or homogeneous mixtures of calcic amphibole and metaluminous obsidian
powder. The second type of experiment involved creating diffusion couples. Here, an upper cylinder of amphibole-saturated
granitic or tonalitic melt was placed against a lower cylinder consisting of an amphibole-rich rock containing zero or a small
melt (granitic or tonalitic) fraction. The upper part of the diffusion couple was doped with β emitter (151Sm or 14C) and functioned as an infinite melt reservoir. The lower part of the diffusion couple was considered to be the host rock.
The experiments approached textural equilibrium which allowed us to characterize the wetting behaviour of the calcic amphibole
by the hydrous silicic melt (granitic or tonalitic). These particular experiments also provided information concerning diffusive
transport, because the β emitter could diffuse through the connected melt (liquid) in the amphibole-rich rock. The dihedral
angle measurements show that melt connectivity was achieved. This conclusion is based on the fact that the dihedral angles,
θ, consistently yielded median apparent values of 53°<θ<58° for an amphibole-rich rock/granitic melt system, and 46°<θ<48°
for an amphibole-rich rock/tonalitic melt system. However, the frequency distribution of θ angles is found to be relatively
broad. The results of the diffusion-couple experiments, assessed using the β radiographic technique, complement the dihedral
(wetting) angle measurements by showing that melt connectivity is achieved at a melt fraction less than 4wt% (5 vol.%).
Received: 15 April 1997 / Accepted: 23 September 1998 相似文献
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Lead isotope signatures of epithermal and porphyry-type ore deposits from the Romanian Carpathian Mountains 总被引:3,自引:0,他引:3
Lead isotope analyses have been performed on the two major Miocene mining districts of Romania, Baia Mare and Apuseni Mountains. These two districts have different non-overlapping 206Pb/204Pb isotopic signatures ranging from 18.752 to 18.876 and 18.497 to 18.740. In the Baia Mare district, epithermal deposits are overall homogeneous in their lead isotopic compositions and have values similar to the average of the calc-alkaline volcanic rocks. These results suggest a magmatic signature for the Pb (and possibly other metals) in the hydrothermal fluids. However, magmas in this district show isotopic evidence of crustal assimilation. In the southern Apuseni Mountains, the lead isotope compositions of sulfide minerals in porphyry copper deposits are clustered, confirming that Pb, and probably other metals, were derived principally from associated porphyry stocks. On the other hand, lead isotope data on sulfides in epithermal ore deposits are much more scattered, indicating a notable contribution of Pb from local country rocks. In the Apuseni Mountains, 'fertile' volcanics are few and appear to come from a more primitive mantle-derived source. Most of the analysed volcanic rocks seem 'barren'. Differences in lead isotopic compositions between the Baia Mare district and the Apuseni Mountains are due to a different basement, and probably to variations in crustal assimilation superimposed on variations in the mantle source composition. In the Apuseni Mountains, Pb may be partly inherited from the previous Mesozoic magmatic-hydrothermal stage. From a geodynamic point of view, it seems that the nature and the source of volcanic rocks and their position related to the collision area of the Carpathian arc are not the only factors controlling the 'fertility' of a volcanic district. 相似文献
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