Hot fluids or rock in eclogite metamorphism?

Abstract

Arising from: A. Camacho, J. K. W. Lee, B. J. Hensen & J. Braun Nature 435, 1191–1196 (2005); Camacho et al. reply.

The mechanisms by which mafic rocks become converted to denser eclogite in the lower crust and mantle are fundamental to our understanding of subduction, mountain building and the long-term geochemical evolution of Earth. Based on larger-than-expected gradients in argon isotopes, Camacho et al.¹ propose a new explanation — co-seismic injection of hot (700 °C) aqueous fluids into much colder (400 °C) crust — for the localized nature of eclogite metamorphism during Caledonian crustal thickening, as recorded in the rocks of Holsnøy in the Bergen arcs, western Norway. We have studied these unusual rocks^2,3,4, which were thoroughly dehydrated under granulite facies conditions during a Neoproterozoic event (about 945 million years (945 Myr) ago); we also concluded that fracture-hosted fluids were essential as catalysts and components in the conversion to eclogite about 425 Myr ago⁵. However, we are sceptical of the assertion by Camacho et al. that eclogite temperatures were reached only in the vicinity of fluid-filled fractures. Determining whether these rocks were strong enough to fracture at depths of 50 km because they were cold or because they were very dry is crucial to understanding the mechanics of the lower crust in mountain belts, including, for example, the causes of seismicity in the Indian plate beneath the modern Himalayas⁶.

Main

The interpretation by Camacho et al. of their argon-isotope data is inconsistent with reasonable magnitudes of heat and fluid advection, as well as with structural field relations in these rocks, for two reasons. First, immense quantities of fluid — equal at a minimum to the mass of eclogitized rock — would have been required to heat the rocks to 300 °C above the ambient temperature. Moreover, the source of any such superheated fluids is not obvious for the Holsnøy complex. There is no evidence of magmatism at the time of the eclogite metamorphism and, because the rocks resided somewhere within the overthickened Caledonian crust, any fluids derived from a subducting slab would have had to travel long distances (20 km, assuming a geothermal gradient of 15 °C km⁻¹) without losing their heat in order to trigger eclogite-grade reactions in cold rocks.

Second, Camacho et al. suggest that the observed occurrence of eclogite in 10–100-m-wide shear zones could be explained by multiple seismically triggered episodes of hot fluid injection into the same sites. However, consideration of structural relationships make this scenario implausible. Although most of the eclogites occur as shear zones, cross-cutting relationships show that these shear zones were secondary results of, rather than the primary conduits for, fluid infiltration. Brittle tensile and shear fractures, as well as pseudotachylyte (glassy rock generated by frictional melting during seismic slip), are seen exclusively in the almost anhydrous granulite facies rocks, whereas only eclogitized areas show evidence of pervasive ductile strains that overprint the granulite-facies fabric⁵.

These ubiquitous structural relationships indicate that the dry granulite was much stronger than the slightly rehydrated (phengite- and zoisite-bearing) eclogites, which accommodated very large ductile shear strains following their metamorphic conversion. Although it is surprising to see evidence of frictional sliding behaviour at such depths and temperatures, the virtual absence of hydrous phases in the gabbroic to anorthositic granulites can account for their unusual rheological properties at high temperatures^7,8.

Once formed, the eclogites on Holsnøy were apparently too weak to fracture, seismically or otherwise, and they responded to far-field stresses instead by ductile deformation — making repeated, rapid infiltration of large volumes of fluid into the same sites unlikely. The resultant shear zones may have been loci of slow, diffuse fluid flow, but under such conditions the fluids would have steadily lost their heat to the surrounding rocks if the ambient temperature had been only 400 °C. Hence, the scenario proposed by Camacho et al.¹ cannot readily account for the broadest belts of eclogite, such as the Hundskjeften shear zone (shown in Fig. 1 of ref. 1), which is as much as 500 m wide. We suspect that the profound decrease in strength upon conversion to eclogite caused the metamorphic process to be self-limiting and that it resulted in the observed ‘patchiness’ of the eclogitized areas⁵.

We find that the model proposed by Camacho et al. is at odds not only with plausible geophysical constraints but also with some of the principal characteristics of this remarkable rock complex, which provides a rare glimpse of how different geological processes may be when rocks are exceptionally dry.