H.L.M. van Roermund

Majoritic garnets monitor deep subduction fluid flow and mantle dynamics

The ultradeep mantle rocks of western Norway display three generations of majoritic garnet. The first two derive from incompatible element–depleted transition-zone mantle and exsolved pyroxene components during Archean upwelling, accretion to subcratonic lithosphere (M1 stage), and isobaric cooling until the Middle Proterozoic (M2). A subsequent Scandian (430–390 Ma) subduction cycle initiated diamond crystallization (M3). Here we report a third majoritic garnet crystallized at grain boundaries and in microfractures, and stable with pyroxene, phlogopite, and spinel in the M3 assemblage. The trace element signatures of M3 minerals indicate crustal metasomatism, phlogopite being the main large ion lithophile element repository. These features imply majorite crystallization from crust-derived subduction fluids at 200 km depth. Our finding fixes the deepest occurrence of free subduction fluid phases and indicates that garnet is a reliable monitor of deep mantle evolution and fluid-mediated chemical recycling.

The pressure path of solid inclusions in minerals: the retention of coesite inclusions during uplift

Abstract An inclusion model allows calculation of the stresses in and around minerals included in other minerals: during changes in pressure and temperature. The equations are applied to illustrate cooling and uplift histories of quarzo-feldspathic rocks from 500°C and various pressures to ambient conditions. Even in the absence of pore-fluid pressure, microfractures may open at external pressures of 200–400 MPa and temperatures of 200–400°C due to differential volume changes of the constituent minerals. Coesite included in garnet cannot have formed during progressive metamorphism from quartz at lithostatic pressures below the coesite stability field because of differential volume changes. Coesite inclusions are captured by their host minerals at ultra-high pressures and they persist to lower pressures because of the large volume increase occurring at the coesite to α-quartz transition. The Pi-T path followed by the SiO2 inclusion traces the boundary between the stability fields of coesite and α-quartz until radial fractures develop in the host at low external pressure.

The dislocation substructures of naturally deformed omphacites

Abstract A transmission electron microscopy study of naturally deformed omphacite has revealed twins, antiphase boundaries and a variety of dislocation substructures consisting of curved free dislocations, loops, isolated nodal points and networks. Both free dislocations and dislocation boundaries contain nodes that involve dislocations with the following Burgers vectors: [001], 1 2 〈110〉 and 1 2 〈112〉 . This implies a reaction of the type [001] + 1 2 〈110〉 → 1 2 〈 112〉 . By analogy with quartz and olivine the dislocation substructures in the omphacites are considered to be indicative of dislocation creep mechanisms and responsible for most of the optical defect-structures. Using microstructural and geochemical data the metamorphic conditions during the creep deformation are estimated to be: minimum —8 kbar, 550°C; maximum —15 kbar, 625°C.

Mechanisms of exsolution in omphacites from high temperature, type B, eclogites

The microstructural development during the transformation of omphacite into pyroxene-plagioclase symplectites has been studied in some eclogites from the Seve nappe, Central Scandinavian Caledonides. The omphacite transformation can be described as a discontinuous precipitation reaction that occurred in two clearly defined stages to produce a coarser type A, followed by a finer, type B symplectite. Each type has its distinctive chemistry. The combination of microstructural and chemical characteristics of the transformation is used to reconstruct the early stages of the cooling history of the eclogites. In addition, based on a classification of phase transformations according to growth processes, the continuous exsolution reaction reported in high-temperature omphacites is combined with the discontinuous reaction in a time-temperature transformation (TTT) diagram to produce a more unified view of the exsolution in the omphacites.

Scandian Ultrahigh-Pressure Metamorphism of Proterozoic Basement Rocks on Fjørtoft and Otrøy, Western Gneiss Region, Norway

Electron microprobe mineral composition data are presented for samples of exsolved and recrystallized garnet websterites within the mantle-derived peridotite bodies and of external orthopyroxene eclogite lenses directly enclosed within the gneisses on the islands of Fjørtoft and Otrøy in the Western Gneiss Region of Norway. These data are utilized to obtain P-T estimates that lie within the coesite stability field, and are mostly also compatible with rare observations of microdiamond formation and preservation for the deformed and re-equilibrated garnet websterite assemblages in the peridotite bodies included within the basement gneiss sequences on both islands. Equivalent P-T estimates for external orthopyroxene eclogite lenses within the basement gneisses on both islands are mostly not well constrained due to fairly extensive modification of orthopyroxene compositions during amphibolite-facies retrogression. Nonetheless, P-T estimates for the least retrogressed samples again indicate the likelihood that these rocks underwent Scandian ultrahigh-pressure metamorphism. We thus conclude that, contrary to a previously published interpretation, both the Caledonian Blåhø nappe sequence and the underlying Proterozoic Baltica basement gneisses on Fjørtoft experienced Scandian ultrahigh-pressure metamorphism, thus denying a previous suggestion that a major, greater than normal continental crust thickness, tectonic break exists between these rock sequences on this island. Similarly, the Baltica basement gneiss sequence exposed on the northern side of Otrøy, with its comparable mantle-derived peridotite bodies and external orthopyroxene eclogite lenses, is also shown to have been subjected to the Scandian ultrahigh-pressure metamorphic event.

Metasomatic origin for Fe-Ti-rich multiphase inclusions in olivine from kimberlite xenoliths

Because solid-solution trace elements in olivine have significant influence on upper mantle rheology, it is important to establish whether inclusions in olivine are exsolved from an intrinsic solid solution or are introduced by metasomatism. In a study of olivine in hot deformed peridotite xenoliths from kimberlites, we have found a new type of multiphase inclusion that contains Fe-Ti-rich and Si-Fe-Al-rich phases. Microstructural relations indicate that these inclusions form by a process involving metasomatic fluid infiltration along a fine network of cracks shortly before entrainment of the xenoliths, followed by crack healing during xenolith transport in the magma. The high trace-element content of Ti, Ca, Al, and Na found in Fe-Ti-rich, hot deformed olivine is related to metasomatic enrichment that occurred shortly before kimberlite eruption. Assessment of the effects of the Fe-Ti metasomatism indicates that the Cr-C-rich inclusions found in olivine from other hot deformed xenoliths are formed by precipitation from an intrinsic solid solution in olivine that has not been affected by Fe-Ti-rich metasomatism.

An optical and electron microscopy study of defect structures in naturally deformed orthopyroxene

Abstract Naturally deformed Mg-rich orthopyroxenes ((Mg, Fe)2Si2O6), deformed at a temperature of 500–600°C and a pressure of 5–9 kbar, have been studied using optical and transmission electron microscopy techniques. Optical and universal-stage measurements reveal well-ordered, deformation-induced subgrain boundaries (sub)parallel to (100), (010) and (001) with an angular mismatch of 3–12° between adjacent subgrains. Transmission electron microscopy shows free dislocations, (isolated) (100) -stacking faults (with R = 1 4 [001] ), tilt walls parallel to (001) and more complex dislocation configurations. All dislocations have Burgers vector b = [001] . The dominant slip system is (100)[001] with subordinate (010)[001] and dissociation of unit 〈c〉 dislocations takes place in (100) and (010). A comparison of the optical and TEM data indicates that the optically visible subgrain boundaries parallel to [001] remain unexplained.

Deformation and recrystallization mechanisms in naturally deformed sillimanites

Abstract Prismatic sillimanite (Al 2 SiO 5 ), with a length between 0.3 and 2.5 mm, was obtained from a garnet migmatite. The sillimanite, naturally deformed at a temperature of 750 ° C and a confining pressure of 6 kbar, has been studied using optical and transmission electron microscopy techniques. Optical and universal stage measurements reveal undulatory extinction, “sharp” deformation-induced subgrain boundaries (subparallel to (001) and (010)) and minor recrystallization. Transmission electron microscopy shows free dislocations, dislocation loops, (110) planar defects and tiltwalls parallel to (001). Dislocations have Burgers vectors of [001] and [100]. All isolated 〈c〉 dislocations are dissociated. The dominant slip system is (100) [001] with subordinate (001) [100]. The microstructure of sillimanite indicates that recrystallization has occurred by a rotation mechanism (around [010]), where single crystals become polycrystals by the progressive development of numerous internal high-angle boundaries. The latter have been interpreted as originating from low-angle (001) tilt- and (010) twistwalls. Rotation recrystallization was followed by grain boundary migration.

An investigation into the genesis of an erratic (retro) eclogite block from Haren, Groningen, the Netherlands

In boulder clays and glacial deposit sands, exposed in the northern part of the Netherlands, erratic blocks of (ultra)high pressure (UHP) metamorphic rocks may be found that originate from the Baltic Shield (Scandinavia). The occurrence of (U)HP metamorphic rocks in Scandinavia is limited to: (1) isolated occurrences within the Scandinavian Caledonides (Western part of Scandinavia); (2) Sveconorvegian rocks from the Halland area, Southwest Sweden; and (3) Kola Peninsula (Northern Scandinavia). For this reason (U)HP rocks form excellent indicator pebbles/rocks that may be used to trace back the source area from where the erratic blocks, found in the Netherlands, were derived. An example of this, an erratic (retro) eclogite block found in Haren, is investigated in the present study using naked eye, light-optical and electron microprobe (EMP) techniques. EMP mineral analyses were used to reconstruct the PT conditions under which the (retro) eclogite was formed (T = 756 °C/min. P = 16,2 kb). This result, in combination with the mineral chemistry of the major rock forming minerals, provides evidence that this erratic block originates from the upper HP tectonic lens exposed in the Caledonian Seve Nappe Complex of Northern Jamtland, Sweden.