Abstrakt
Water and melts are crucial in controlling the chemistry, architecture, and dynamics of the Earth's crust. However, how water and melts influence chemical and textural changes of pervaded rocks during transport and how they chemically react with surrounding minerals remain cryptic.
We present experimental data illustrating how water-undersaturated and potassium-rich melt in partially molten felsic granulite, or melt lost from such granulite affects the chemistry and texture of an initially, nearly anhydrous eclogite at high-pressure and high-temperature (1.2 GPa, 950 degrees C) for 20 h. The first experiment simulates interaction of eclogite with granulite having a small proportion of melt (melt = 2 vol%, solids = 98 vol%, with H2O = 0.4 wt% in the system granulite+melt, called in this paper granulite).
The second experiment simulates interaction of eclogite with melt (melt = 90 vol%, solids = 10 vol%, H2O = 4 wt% in the system granulite+melt, called in this paper granulite partial melt). Our results show that during granulite-eclogite interaction: i) the melt in the granulite crystallises as a consequence of water diffusion into the eclogite; ii) this water causes partial replacement of garnet by plagioclase-orthopyroxene-magnetite-(+/- amphibole-) symplectite, whose volumetric proportion correlates with water content in the granulite; and iii) a small proportion of melt from the granulite (<= 2 vol%) infiltrates into the eclogite, as documented by mineral overgrowths in granulite and by glass pools with increased potassium content in symplectites and along grain boundaries in the eclogite.
Water and melt transfer appears to be effective in causing textural changes, but appears to be ineffective in changing the bulk chemistry of the eclogite, even if local enrichment in potassium and water occurs. Such chemically stealthy diffusion of water and migration of limited amount of melt is able to cause major textural changes accompanied by localised chemical changes at grain-scale that would not be otherwise observed at the scale of the bulk rock composition.