Ethan F. Baxter

Natural constraints on metamorphic reaction rates

Abstract Quantitative constraints on the rates at which metamorphic reactions proceed in nature are now available from several sources. Most common are predictions made on the basis of laboratory kinetic data. However, the applicability of such laboratory-based predictions has long been questioned and many observations in the field now suggest much slower rates. Here, published quantitative field-based constraints on high temperature (>400 °C) reaction rates are assembled from a variety of sources. Reaction rates attending regional metamorphism are four to seven orders of magnitude slower than most laboratory-based predictions. A general rate law for regional metamorphism has been derived which best describes these field-based data: log10(Rnet)≅0.0029T−9.6±1 where Rnet is the net reaction rate (g/cm2/a) and T is temperature (°C). At the same time, natural reaction rates attending contact metamorphism differ from laboratory-based predictions by less than two orders of magnitude, and are in close agreement at higher temperatures. Thus, while existing laboratory-based kinetic data may be judiciously applied to some contact metamorphic systems, laboratory-based kinetic predictions clearly misrepresent regional metamorphism. To explain this kinetic discrepancy, regional metamorphic reaction rates may be limited by slow intergranular transport due to comparatively limited (or transient) availability of aqueous fluid in the intergranular medium. The general field-based rate law may be applied to regional metamorphic, and other environments (i.e. ultrahigh pressure or ultrahigh temperature metamorphism), if similar system characteristics (mainly, low aqueous fluid content) can be inferred.

Phase transformations of continental crust during subduction and exhumation: Western Gneiss Region, Norway

Whether quartzofeldspathic rocks transform to (U)HP minerals during subduction - and back to low-pressure minerals upon exhumation - remains one of the more profound questions pertaining to collisional orogenesis. Garnet-bearing quartzofelds- pathic gneisses from the Western Gneiss Region, Norway provide an opportunity to answer this question. High-precision Sm-Nd garnet geochronology of these gneisses documents garnet growth from 418 to 398 Ma. Garnet zoning in two samples implies growth during subduction-related increase in pressure from 0.5 GPa and 550 � C to 1.7 GPa and 700 � C. Zoning in all other garnets suggests growth over rather narrow P-T ranges from 1.0-1.6 GPa and 725-800 � C during decompression, possibly accompanying melting. If these samples constitute a representative suite, (i) the dearth of (U)HP garnets suggests that most of the quartzofeldspathic gneisses in the Western Gneiss Region did not transform to eclogite-facies parageneses during subduction; (ii) the abundance of garnets grown during decompression indicates that the major period of densification was during exhumation. The widespread metastability of quartzofeldspathic rocks during subduction is substantially different from the findings of previous work and suggests commensu- rately less subduction of continental crust before the slab is positively buoyant.

Garnet growth as a proxy for progressive subduction zone dehydration

The release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism, seismicity, the growth and maturation of continents, and the global geological water-CO 2 cycle. While models exist to predict slab volatile release from hydrous phases, it is challenging to reconstruct and test these fluid fluxes in nature. Here we show that the growth of garnet may be used as a proxy for devolatilization at blueschist to lower eclogite facies conditions in subduction zones. Using thermodynamic analysis including the effects of garnet fractionation and fluid removal, we show the proportional relationship between garnet and water production in two end-member crustal lithologies (pelitic sediment and hydrated mid-oceanic-ridge basalt [MORB]) in three representative subduction geotherms. Dehydrating minerals such as lawsonite, chlorite, amphibole, and epidote contribute to garnet growth, especially between ∼1.4 and 3.0 GPa where geophysical models and observations predict dehydration. The average production ratio for altered MORB compositions is 0.52 (wt% water as fluid per vol% garnet) in cooler geotherms (Honshu [Japan] and Nicaragua) and 0.27 in hotter geotherms (Cascadia [North America]), whereas for pelite the production ratios are about half (0.24 and 0.13, respectively). Garnet growth correlates with production of 3.3–5.9 wt% water in hydrated MORB and 1.8–3.1 wt% water in pelite, representing 42%–100% of the water lost between 0.5 and 6.5 GPa from a fully saturated starting material. Garnet abundance, its pressure-temperature growth span, and its growth chronology may be used to recognize, reconstruct, and test models for progressive subduction zone dehydration.

Quantification of the factors controlling the presence of excess 40Ar or 4He

Abstract A quantitative physical model is presented which includes the factors that control the presence, or absence, of internally derived excess 40Ar or excess 4He in geological systems. In particular, the model incorporates the transport and partitioning properties of the rock surrounding the mineral of thermochronologic interest and illuminates the related effects on the amount of excess 40Ar or 4He preserved in the system. Modeling of a simplified 1-D rock column bounded by an external sink for 40Ar or 4He shows that a steady-state excess 40Ar or 4He profile develops, the magnitude of which is determined by a system parameter called the ‘transmissive timescale’, τT. The characteristic time required to reach this steady state depends upon τT and the ‘total local sink capacity’, TLSC, wherein the important role of local matrix mineral and fluid phases is incorporated. Together, these two system parameters (τT and TLSC) determine the evolution of excess 40Ar or 4He buildup within a system above the closure temperatures of all minerals involved. An analytical expression for the 1-D system describing the evolution of excess 40Ar (or by analogy 4He) in a particular potassium-bearing (or U–Th-bearing) mineral located at a distance, L, from an external sink has been derived empirically from model results: 40 Ar age - equivalent (L,t)≅ τ T Ar 2 1− exp 5 2 · t τ T Ar (1+ TLSC Ar ) Local matrix minerals, perhaps most notably quartz, may act as important sinks for 40Ar (except in the most fluid-rich systems where fluids dominate) and thus are fundamental in controlling, and limiting, thermochronologically problematic excess 40Ar in neighboring potassium-bearing minerals. In general, the model provides a rigorous means of predicting excess noble gas content, residence, release, and transport within the compositionally variable and thermally evolving crust.

Discovery of ultrahigh-temperature metamorphism in the Acadian orogen, Connecticut, USA

We report the discovery of granulite facies gneisses that attained ultrahigh temperatures (UHT) above those predicted by typical models of conductive thermal relaxation of over-thickened crust during exhumation. The rocks, which form part of the Acadian (Devonian) metamorphic belt in Connecticut (United States), reached ∼1000 °C and minimum pressures of ∼1 GPa based on Zr-in-rutile thermometry, ternary feldspar compositions, and pseudosection analysis. This is the first regional UHT locality in the United States of which we are aware, and one of relatively few post-Gondwana assembly UHT localities known worldwide. The UHT metamorphism requires heretofore unrecognized contributions to the regional thermal budget. Some possibilities include rapid exhumation from the mantle, underthrusting of extremely radiogenic crust, mechanical strain heating, asthenospheric upwelling, and/or heat input from mantle-derived magmas. The rocks are fairly ordinary looking in outcrop, raising the possibility that other UHT domains remain undiscovered in the orogen.