Metamorphic fluids in the deep crust: evidence from the Adirondacks

Abstract

Unusual mineral assemblages of tremolitic hornblende + diopside+enstatite+quartz have been examined to estimate the temperature (T) and water fugacity ($f_{{\text{H}}_2\text{O}}$) at the time of the granulite fades metamorphism in the Adirondacks. The pyroxenes are closely represented by the system CaO–MgO–SiO₂ allowing application of experimental data¹ with only small corrections for reduced values of activities. Tremolite equilibria² buffered log $f_{{\text{H}}_2\text{O}}$= 3.0±0.1, equivalent to 0.9 kbar of water pressure ($P_{{\text{H}}_{\text{2}}\text{O}}$) at 7 kbar, 710 °C. Nearby marbles contain wollastonite+calcite+quartz and buffered $P_{\text{C}{\text{O}}_{\text{2}}}$ = 1.1 kbar, consistent with $P_{{\text{H}}_{\text{2}}\text{O}}$⩽5.9 kbar. Inferred local gradients in fluid compositions suggest that deep crustal fluid conditions are complex and heterogeneous indicating that fluid movements, whether of mantle³ or deep crustal origin, are channelized rather than pervasive. Thus some granulite facies rocks were wholly or partly closed systems with respect to externally derived fluids permitting localized buffering of fluid compositions. Recognition of restricted fluid-mixing limits theories that call for massive amounts of pervasive fluid flow to facilitate melting, cause metasomatism, or to stabilize granulite facies mineralogy.