Tracy Rushmer

Partial melting of subducting oceanic crust

The conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two-dimensional thermal models of subduction zones. For porosities of ~ 1% containing H20 the amount of partial melt generated at the wet basaltic solidus is limited to ~ 100 MPa) can be maintained by rocks close to, or above, their melting temperatures. In the absence of high shear stresses, substantial melting of the oceanic crust will only occur during subduction of very young (< 5 Ma) oceanic lithosphere. Partial melting of hydrated basalt (amphibolites) derived from the mid-ocean ridge has been proposed [e.g., 1-3] as being responsible for the generation of certain recent high-Al andesitic to dacitic volcanic rocks (adakites). Three of these volcanic suites (Mount St. Helens, southern Chile, and Panama) occur in volcanic arcs where oceanic crust < 25 Ma is being subducted at rates of 1-3 cm/yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments. However, oceanic lithosphere is not currently being subducted beneath Baja and New Guinea, where recent adakites are also present, suggesting that some adakite magmas may form by water-undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust. Further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately.

An experimental deformation study of partially molten amphibolite: Application to low-melt fraction segregation

An amphibolite has been experimentally deformed under subsolidus and partially molten conditions to evaluate the influence of melt on the mechanical behavior of a natural mafic rock and to assess low-melt fraction segregation processes. Experiments were performed under fluid-absent conditions at 1.8 GPa, between temperatures of 650°C and 1000°C. These conditions are similar to those of thickened lower continental crust or intermediate depths in subducting oceanic lithosphere. At ≥850°C, melt is granitoid in composition, and its viscosity is that of “wet granite”, 103–105 Pa s. The results can be summarized as follows: (1) Under subsolidus conditions (tests at 650°C and 750°C the amphibolite is macroscopically ductile and deformation is homogeneously distributed throughout the sample. (2) At near solidus conditions (≥800°C, ∼ 0–5 vol % melt), fractures (∼1–10 μm in width) displace hornblende and plagioclase grains, and melt, formed in situ, is found in some of these cracks. The formation of a ductile shear zone in one sample is attributed to the presence of very fine grained reaction products from combined dehydration/hydration reactions that involve plagioclase. The dehydration reaction products have apparently changed the deformation mechanism and even overrode the melt-embrittlement process, trapping melt in pockets of lower strain. With higher melt fractions (∼10–15 vol %), broad, melt-bearing shear zones form and grains within these zones are brittlely deformed. (3) At ∼20 vol % melt (1000°C), additional weakening occurs but fractures are not observed and the overall deformation is by viscous flow. The results show that at low melt fractions (<15 vol %) fluid-absent melting reactions can induce fracture in previously ductile rocks. This suggests that the percentage at which melt may escape by fracture is lower than the theoretical critical melt fraction or CMF of 26–40 vol %. In general, estimating the fraction of melt at the onset of segregation cannot be predicted by the CMF. Melt segregation models have to be adapted to variations in such major factors as pressure, temperature, type of melting reaction, rate of melting, and strain rate. Such models will greatly assist our understanding of continental growth and evolution during orogenesis.

Experimental high-pressure granulites: Some applications to natural mafic xenolith suites and Archean granulite terranes

High-pressure granulite assemblages have been produced as residues in partial melting experiments on a natural (alkalic basalt) amphibolite between pressures of 12 and 18 kbar. In particular, at 18 kbar, partial melting of the hornblende + plagioclase ± quartz assemblage under fluid-absent conditions produces garnet + clinopyroxene + new albitic plagioclase + melt. Seismic velocities ( V P ) are estimated from the modal data for the experimental assemblages and range from 6.90 km/s for hornblende-bearing residues to 7.62 for the dominantly garnet- clinopyroxene residues. These values are typical for rock types in the lowermost crust, transitional to mantle. The experimental results help place additional pressure-temperature- a (H 2 O) constraints on the source region for the natural high-pressure granulites. The experimental residue assemblage formed at 18 kbar has been described in natural xenolith suites from the Delegate Pipes in Australia, where the pipes intrude Upper Ordovician and Lower Devonian continental crust, and in the Bearpaw Mountains, Montana, where the xenolith- bearing magmas intrude older crust of Archean age. The combination of these data shows that the xenoliths may indeed represent lowermost continental crust and furthermore helps interpret the nature of the crust-mantle boundary in these areas. The Delegate Pipes xenoliths suggest that the crust-mantle boundary may be the site for partial melting and assimilation, whereas the Bearpaw Mountains samples indicate that magmatic underplating may have been a major process in generating thick continental Archean crust. The experimental data and Bearpaw Mountains xenoliths suggest that the large range of rock types found in Archean granulite terranes may not be representative of the lowermost continental crust.

Heading down early on? Start of subduction on Earth

How the Earth’s earliest crust was formed and when present-day plate tectonics (i.e., subduction) and life commenced remain fundamental questions in Earth sciences. Whereas the bulk composition of the crust is similar to that of rocks generated in subduction settings, it does not necessarily follow that melting and crust formation require subduction. Many workers suggest that subduction may have only commenced toward the end of the Archean or later. Here we observe that both the stratigraphy and geochemistry of rocks found in Quebec, Canada, that have been variously argued to be 4.4 or 3.8 Ga in age, closely match those from the modern-day Izu-Bonin-Mariana forearc. We suggest that this geochemical stratigraphy might provide a more robust test of ancient tectonic setting than individual chemical or isotopic signatures in rocks or detrital minerals. If correct, the match suggests that at least some form of subduction may have been operating as early as the Hadean or Eoarchean. This could have provided an ideal location for the development of first life.

Volume change during partial melting reactions: implications for melt extraction, melt geochemistry and crustal rheology

Abstract The volume change associated with dehydration melting has been investigated experimentally in muscovite and biotite-bearing assemblages because it is a possible driving force for melt segregation during orogenesis. Experiments have been performed on cores of a muscovite+biotite-bearing pelite and on a biotite+plagioclase+quartz gneiss. The muscovite+biotite-bearing pelite produced a similar set of melt-filled cracks to that observed in muscovite-bearing quartzite under partial melting conditions of 700 MPa, and 850 and 900 °C. However, no cracking was observed in the biotite gneiss under a range of temperature conditions (700 MPa, 800–900 °C). The textures of the partially melted rock samples suggest the volume change and associated dilational strain accompanying melting in assemblages with only biotite is insignificant or negative. This is confirmed by calculations of the dilational strain in the biotite gneiss experiments and other experiments in the literature. For example, the dilational strain associated with partial melting of biotite-bearing metagreywacke assemblages ranges from +1.90% to −12.24% (given 30% modal biotite, calculated on a 1-oxygen basis), becoming negative when garnet is produced at higher pressure. In contrast, the dilational strain associated with melt-induced cracking in a muscovite-bearing metapelite is higher, +6.76%, for the same modal abundance. These results suggest that volume change alone is not an important driving force for melt segregation in biotite-only-bearing assemblages, and external deformation at higher melt fractions may be required to segregate melt from the lower crust during partial melting. Reaction-controlled segregation is possible in muscovite-bearing rocks and melt will be more easily expelled in the upper to mid levels of the crust because of rapid pore pressure development during early stages of melting. Major element chemistry of melt in the two-mica assemblage is dominated by muscovite melting, even when assemblage contains reacting biotite. Some implications of these results are that: (1) the melt that escapes at low melt volumes from the mid-crust is likely to have a muscovite-melting chemical signature; and (2) in the lower portions of the crust where melting is controlled by biotite stability, melt may become trapped within and along grains and remain distributed, pervasively, at the grain scale until greater melt fractions are generated. Recent modeling of orogenic belts shows that the evolution of collisional belts likely involves the prolonged presence of a weak crustal layer. Melt trapped along grain boundaries from low dilational strain melting reactions may be a mechanism for keeping melt in the crust and weakening it during active orogenesis.

Reaction-induced microcracking: An experimental investigation of a mechanism for enhancing anatectic melt extraction

Melting reactions can create melt overpressure that may induce microcracking. To determine whether such microcracking can enhance rock permeability and melt extraction, we have studied the partial melting of a muscovite-bearing metaquartzite at 800 MPa and 950–1126 K. Melting begins at muscovite-quartz grain boundaries and results in progressive replacement of muscovite by melt pools containing mullite and biotite. The volume change for the reaction (0.021 m 3 per m 3 of original rock) generates randomly oriented microcracks that emanate from melting sites. The mean crack length in two-dimensional sections is 151 ± 5 µm and reflects the spacing between melting sites. Experiments in which quartz sand was loaded with the metaquartzite to act as a drain verified that the microcracks, together with the melt pools, form a connected network. The estimated network permeability is 10 −14 ± 1 m 2 , at least four orders of magnitude greater than permeabilities characteristic of regional metamorphic environments. For reaction-induced microcracking to occur, the reaction must take place on a time scale such that creep cannot accommodate the associated volume change. Our analysis suggests that that requirement can be met on regional metamorphic time scales and that reaction-induced microcracking is a feasible mechanism of permeability enhancement during partial melting and devolatilization.

Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the Earth's early continental crust

To investigate formation of the Earth9s earliest continental crust, partial-melting experiments were conducted (at 900–1100 °C and 0.5–3.0 GPa) on two greenstones from the 4.3 Ga Nuvvuagittuq complex of Quebec, Canada. For comparison, experiments were also conducted on a compositionally similar but modern arc volcanic (a Tongan boninite). At 1.5–3.0 GPa and 950–1100 °C, the experimentally produced melts are compositionally similar to the tonalite-trondhjemite-granodiorite (TTG) granitoids that compose most of Earth9s early continental crust, including a 3.66 Ga tonalite that encloses the Nuvvuagittuq Complex. Because the degree of melting needed to produce the TTG-like melts is comparatively high (>30%), the relative concentrations of most incompatible elements in the melts are similar to those in their greenstone parent rocks. These greenstones have compositional affinities with modern subduction zone magmas and do not resemble mid-oceanic ridge basalts. That arc-like mafic rocks could have been selectively involved in TTG formation (in spite of their volumetrically subordinate status in most greenstone terrains) must reflect tectonic circumstances that were specific to their generation. These must have enabled accumulations sufficiently deep to melt at the 1.5–3.0 GPa needed to generate TTG magmas from eclogitic sources. They are also likely to have been related to some form of crustal recycling whereby mafic crust and water were returned to the mantle and arc-like mafic magmas generated as a consequence. To what degree these circumstances replicated modern plate tectonics is difficult to say, but it seems likely that, as in the modern Earth, the Hadean crust was organized into different tectonic environments and that one of these gave rise to the first continental crust.

Introduction to Special Section: Mechanisms and Consequences of Melt Segregation From Crustal Protoliths

… much of petrological effort has been concentrated on attempts to discern, define and interpret regularly recurring patterns of igneous and metamorphic petrogenesis… I do not wish to deny the existence of discernable order in petrological phenomena… but… every geological event… is unique. If we concentrate too heavily on discernment of order and pattern, may we perhaps overlook or underrate the unique quality of each igneous or metamorphic episode? Possibly uniqueness may have as great significance as conformity to pattern when we attempt to fit the phenomena of petrology into the broad framework of geology. Francis J. Turner Presidential Address to the Mineralogical Society of America November 11, 1969

Hydration crystallization reactions between anhydrous minerals and hydrous melt to yield amphibole and biotite in igneous rocks : description and implications

The crystallization of some hydrous magmas can be modeled, in part, via the reaction between hydrous melt and anhydrous minerals to yield hydrous minerals. The reactions (herein termed “hydration crystallization”) are equilibrium, incongruent, vapor‐absent crystallization reactions, the reverse of dehydration‐melting reactions. The principal petrographic evidence for hydration crystallization is partially reacted and resorbed pyroxene and oxide minerals mantled by amphibole and biotite. Hydration crystallization can buffer the water content of mildly hydrous (up to ∼2 wt% H2O) magmas at values below saturation, conceivably to the completion of crystallization, thus contradicting the maxim that “all magmas go to water saturation.” Even if the buffering effect does not persist throughout the crystallization history, water contents of magmas in which these incongruent reactions occur will remain lower than magmas in which they have not. Whether they preclude vapor saturation, equilibrium hydration crystallization reactions control the late magmatic evolution of many hydrous plutons.

Melt segregation in the lower crust: how have experiments helped us?

The rheological and chemical behaviour of the lower crust during anatexis has been a major focus of geological investigations for many years. Modern studies of crustal evolution require significant knowledge, not only of the potential source regions for granites, but also of the transport paths and emplacement mechanisms operating during granite genesis. We have gained significant insights into the segregation and transport of granitoid melts from the results of experimental studies on rock behaviour during partial melting. Experiments performed on crustal rock cores under both hydrostatic conditions and during deformation have led, in part, to two conclusions. (1) The interfacial energy controlling melt distribution is anisotropic and, as a result, the textures deviate significantly from those predicted for ideal systems—planar solid-melt interfaces are developed in addition to triple junction melt pockets. The ideal dihedral angle model for melt distribution cannot be used as a constraint to predict melt migration in the lower crust. (2) The ‘critical melt fraction’ model, which requires viscous, granitic melt to remain in the source until melt fractions reach >25 vol%, is not a reliable model for melt segregation. The most recent experimental results on crustal rock cores which have helped advance our understanding of melt segregation processes have shown that melt segregation is controlled by several variables, including the depth of melting, the type of reaction and the volume change associated with that reaction. Larger scale processes such as tectonic environment determine the rate at which the lower crust heats and deforms, thus the tectonic setting controls the melt fraction at which segregation takes place, in addition to the pressure and temperature of the potential melting reactions. Melt migration therefore can occur at a variety of different melt fractions depending on the tectonic environment; these results have significant implications for the predicted geochemistry of the magmas themselves.