Arrays Of Fluid Inclusions Research Articles

The interaction of migrating grain boundaries and fluid inclusions in naturally deformed quartz: A case study of a folded and partly recrystallized quartz vein from the Hunsrück Slate, Germany

We studied the microstructure of a folded and partly recrystallized quartz vein from the Hunsrück Slates in Germany, focusing on the morphology and distribution of fluid inclusions in the old and new grains and along the different types of grain boundaries. Blocky vein quartz grains show undulose extinction and develop subgrains. New grains form by subgrain rotation and grain boundary migration, with a bimodal size distribution. The old, deformed grains contain numerous, complex, H 2O–CO 2–graphite inclusions, with significant differences in fluid inclusion setting along subgrain boundaries. The new grains have a lower content of H 2O-rich inclusions than the old grains and do not contain graphite, and there is a significant difference in volume and density of fluid inclusions between the large and small new grains. Grain boundaries between old and new grains are irregular, containing similar fluid inclusions as the old grains, but no enrichment in graphite, while grain boundaries between new grains are smooth and can be inclusion-free or contain arrays of fluid inclusions. We interpret these structures to have formed by a series of complex interactions between grain boundaries migrating at different velocity and the fluid inclusions. Differences in mobility and grain boundary velocity can result in different variations of drag and drop- interaction, while chemical differences lead to phase separation during grain boundary–fluid interaction. Migration of grain boundaries into the old grains was accompanied by significant redistribution of fluids and graphite along the grain boundary together with oxidation of graphitic inclusions to CO 2.

Pattern formation during healing of fluid‐filled cracks: an analog experiment

AbstractThe formation and subsequent healing of cracks and crack networks may control such diverse phenomena as the strengthening of fault zones between earthquakes, fluid migrations in the Earth’s crust, or the transport of radioactive materials in nuclear waste disposal. An intriguing pattern‐forming process can develop during healing of fluid‐filled cracks, where pockets of fluid remain permanently trapped in the solid as the crack tip is displaced driven by surface energy. Here, we present the results of analog experiments in which a liquid was injected into a colloidal inorganic gel to obtain penny‐shaped cracks that were subsequently allowed to close and heal under the driving effect of interfacial tension. Depending on the properties of the gel and the injected liquid, two modes of healing were obtained. In the first mode, the crack healed completely through a continuous process. The second mode of healing was discontinuous and was characterized by a ‘zipper‐like’ closure of a front that moved along the crack perimeter, trapping fluid that may eventually form inclusions trapped in the solid. This instability occurred only when the velocity of the crack tip decreased to zero. Our experiments provide a cheap and simple analog to reveal how aligned arrays of fluid inclusions may be captured along preexisting fracture planes and how small amounts of fluids can be permanently trapped in solids, modifying irreversibly their material properties.

Microtextural constraints on the interplay between fluid–rock reactions and deformation

Schists from two mylonitic localities in the footwall of a low-angle normal fault in the eastern Alps record different degrees of embrittlement during exhumation, depending on the extent to which fluid–rock reactions proceeded. At one site, graphitic schists preserve textural evidence for two metamorphic reactions that modified \( {\rm X}_{\mathop {\rm CO}\nolimits_2 } \) and/or fluid volume: (1) reaction between graphite and aqueous fluid that increased \( {\rm X}_{\mathop {\rm CO}\nolimits_2 } \) without changing the molar amount of fluid, and (2) replacement of titanite by rutile, calcite, and quartz. The latter reaction involved net consumption of increasingly CO2-rich fluid. Areas where the first reaction proceeded are associated with abundant Mode I microcracks. Fluid inclusion arrays within the microcracks show that \( {\rm X}_{\mathop {\rm CO}\nolimits_2 } \) increased from ∼0.1 to 0.6 during decompression from 4.75 to 2 kbar at a reference temperature of 500°C. Titanite consumption is most pronounced within transgranular Mode I microcracks, but microcracks do not crosscut products of this reaction; fluid consumption during reaction was coeval with the end of microcracking, at least on a local scale. At the other site, graphitic schists lack small-scale Mode I cracks as well as evidence for graphite consumption during decompression. SEM imaging shows that graphite is anhedral and pitted at the first site, but occurs in clusters of euhedral grains at the second site. Mass balance calculations demonstrate that rocks with partially consumed graphite were infiltrated by an externally derived, H2O-rich fluid that drove subsequent graphite-fluid reaction. Evidence for similar fluid infiltration is absent at the second site. Variations in the degree of reaction progress indicate that fluid pathways and deformation style were heterogeneous on the scale of millimeters to kilometers during exhumation from mid-crustal depths.

Permeability evolution in quartz fault gouges under hydrothermal conditions

The permeability (k) of fine‐grained quartz aggregates were measured in situ during hot pressing (HPing) experiments to explore the evolution of fluid transport properties of fault zones during the interseismic period. Experiments were conducted at temperatures of 150°C and between 700 and 850°C, with confining and pore water pressures of 250 and 150 MPa, respectively. Significant permeability reduction was observed between 700 and 850°C, with permeability reduction rates (r = (1/t) ln (kto/kt)), ranging from approximately 6 × 10−5 s−1 at 700°C to a maximum of approximately 7.4 × 10−4 s−1 at 850°C. Permeability decreased exponentially with time, and the permeability reduction rate increased with increasing temperature, increasing differential stress, and decreasing grain size. Analysis of the permeability‐porosity relationships indicates that permeability in the simulated gouge at high temperature shuts off at a critical porosity of 0.045 ± 0.004. The presence of microstructures, such as grain interpenetration, grain shape truncation, arrays of fluid inclusions, and development of quartz overgrowths on grains, indicate that k reduction was controlled by dissolution‐precipitation creep processes. Extrapolation of the permeability reduction rates, measured in this study, to temperatures typical of the continental seismogenic regime highlights the strongly time‐dependent nature of permeability in natural fault wear products at depths of nucleation of major earthquakes. Within the recurrence time of large earthquakes, quartz‐rich fault zones in the fluid‐active midcrustal to lower continental crustal regimes can evolve from high‐permeability conduits to low‐permeability seals. Episodic changes in the fluid transport properties of faults during the interseismic period are likely to impact on the pore pressure evolution of fault wear products.

PRESERVATION OF MICROBORINGS AS FLUID INCLUSIONS

In this study, we identify endolithic microbial borings preserved as fluid inclusions in Pennsylvanian and Permian brachiopods from the subsurface of western Kansas. Endolithic microbial boring is a new biologically controlled mode of formation for inclusion vacuoles. The microborings are preserved as three types of linear, curved, and branched arrays of tubular fluid inclusions, each with a distinct diameter. Each of the thousands of microborings observed in 60 brachiopod fragments has changed shape by forming calcite partitions that have divided the original tubules into separate aligned fluid inclusions. On average, only 35% of the original lengths of the microborings are preserved as fluid inclusions; values range from less than 5% to about 85%. Enlarged diameters of inclusions, variable vapor-to-liquid ratio in paired inclusions, and low salinities of inclusions indicate that closed-system necking down is one of the mechanisms responsible for alteration of the microborings. Preservation of parallel walls, fillings with luminescent calcite and high salinity of included fluid indicate that open-system cementation is the other mechanism responsible for alteration of the microborings. As most inclusions have leaked and refilled or stretched during thermal re-equilibration, these fluid inclusions are not particularly reliable as records of diagenetic history. The linear arrays of fluid inclusions that remain after alteration of the original microboring are excellent as trace fossils of microbial activity. The record preserved here indicates how fluid-inclusion techniques can be applied to identify and determine the timing of entrapment of microbial trace fossils in ancient Earth and extraterrestrial materials.

The mechanism of fluid infiltration in peridotites at Almklovdalen, western Norway

AbstractA major Alpine‐type peridotite located at Almklovdalen in the Western Gneiss Region of Norway was infiltrated by aqueous fluids at several stages during late Caledonian uplift and retrogressive metamorphism. Following peak metamorphic conditions in the garnet–peridotite stability field, the peridotite experienced pervasive fluid infiltration and retrogression in the chlorite–peridotite stability field. Subsequently, the peridotite was infiltrated locally by nonreactive fluids along fracture networks forming pipe‐like structures, typically on the order of 10 m wide. Fluid migration away from the fractures into the initially impermeable peridotite matrix was facilitated by pervasive dilation of grain boundaries and the formation of intragranular hydrofractures. Microstructural observations of serpentine occupying the originally fluid‐filled inclusion space indicate that the pervasively infiltrating fluid was characterized by a high dihedral angle (θ > 60°) and ‘curled up’ into discontinuous channels and fluid inclusion arrays following the infiltration event. Re‐equilibration of the fluid phase topology took place by growth and dissolution processes driven by the excess surface energy represented by the ‘forcefully’ introduced external fluid. Pervasive fluid introduction into the peridotite reduced local effective stresses, increased the effective grain boundary diffusion rates and caused extensive recrystallization and some grain coarsening of the infiltrated volumes. Grain boundary migration associated with this recrystallization swept off abundant intragranular fluid inclusions in the original chlorite peridotite, leading to a significant colour change of the rock. This colour change defines a relatively sharp front typically located 1–20 cm away from the fractures where the nonreactive fluids originally entered the peridotite. Our observations demonstrate how crustal rocks may be pervasively infiltrated by fluids with high dihedral angles (θ > 60°) and emphasize the coupling between hydrofracturing and textural equilibration of the grain boundary networks and the fluid phase topology.

The Aureole of the Traigh Bhan na Sgurra Sill, Isle of Mull: Reaction-Driven Micro-cracking During Pyrometamorphism

The intrusion of a Tertiary gabbroic sill of 6 m thickness, in which itself is dependent on the ease with which partial melts can segregate from their source and this process remains magma flow was locally turbulent, into garnet-grade pelitic gneiss an important subject of research. Complete underand psammite at 600 bars at Traigh Bhan na Sgurra on the Ross standing of the segregation process is dependent on of Mull, Scotland, led to the development of a contact aureole of constraining both the distribution of melt in the source 3 m width. Observed reactions include muscovite breakdown, melting region and the mechanisms by which it can move at low on quartz–feldspar grain boundaries, and isochemical breakdown volume percent (<10%). of biotite and garnet. A simple, two-stage thermal model fitted to Experimental determination of equilibrium melt geothe profile of maximum temperature is consistent with a 5 month metries under hydrostatic conditions in crustal materials period of turbulent magma flow. Five generations of micro-cracks demonstrates that melt-filled pores should form an inoccur in the psammite. The oldest pre-dates contact metamorphism terconnected network (e.g. Jurewicz & Watson, 1985; and is marked by sub-parallel fluid inclusion arrays. The next Wolf & Wyllie, 1991; Laporte, 1994). Such a network resulted from anisotropic thermal expansion caused by magma should, in principle, permit the segregation of all but a intrusion. Internally generated stresses related to an increase in very small proportion of melt from the source region volume associated with muscovite breakdown formed two further (McKenzie, 1984). However, the high viscosity of crustal sets of cracks associated with release of H2O and melting. Melting melts is believed to require deformation in a deviatoric on quartz–feldspar grain boundaries also resulted in crack formation. stress field to ‘squeeze’ out the melts before they solidify The final stage of cracking was the result of anisotropic thermal (e.g. Sawyer, 1991; Brown, 1994; Rutter & Neumann, contraction. Despite the high crack density at the metamorphic peak, 1995). Recently it has been pointed out that it is probable little or no melt segregation occurred, demonstrating that microthat pore topologies controlled entirely by textural equicracking alone is not sufficient (at least on this time scale) for melt librium will only very rarely be attained in fluid-producing segregation in static anatectic environments. environments. This is because the rate of reaction and fluid production is generally greater than that of textural adjustment driven by minimization of surface energies

石英における微構造の発達

The statically recrystallized quartz grains, formed by the heating of the intrusion, show several characteristic microstructures associated with the grain boundary migration. Most of the new grains were formed by the strain-induced migration (bulging) of the original grain boundaries. The new grains contain much fewer fluid inclusions than the old ones, and the boundaries of the new grains contain fluid inclusion arrays. Thus the migrating boundaries result in a change in distribution of the inclusions, may make the picture even more complicated, such as “left over grains” and a cloud of fluid inclusions.

Relationships between gas geochemistry and release rates and the geomechanical state of igneous rock massifs

In contrast to sedimentary sequences, the relationships between the stressed state of igneous rocks and the chemistry and physical properties of gases contained within them are not well known. Here, we attempt to fill this gap by using, as an example, the apatite–nepheline and rare-metal ore deposits hosted within the Khibiny and Lovozero alkaline nepheline–syenite complexes of the Kola Peninsula, NW Russia. These massifs are characterized by unusually high, for igneous rocks, contents of multi-component, essentially hydrogen–hydrocarbon, gases and also by high hardness, elasticity and unevenly distributed, subhorizontal tectonic stresses. Relationships between the chemical and dynamic characteristics of the gases and the geomechanical properties of the host rocks have been examined using field observations and laboratory experiments. Patterns of gas release variations in time and space, gas emissions from rock pillars during artificial loading, variations of gas pressure in sealed shot-holes and changes in liberation rates of gaseous components during experimental rock loading are suggested to result from changes in rock stress and deformation state. Gas compositions in sealed shot-holes in stressed rocks change with time. Partly, this is due to belated release of gases held in fluid inclusions and isolated voids and their subsequent mixing with gases held in interconnected fracture systems as the included gases are preferentially released as fluid inclusion arrays are opened during later stages of stress build-up. Partly, it may also be because released gases may react with new fracture surfaces to generate enhanced levels of reduced H2 gases.

Characteristics of barren quartz veins in the Proterozoic La Ronge Domain, Saskatchewan, Canada; a comparison with auriferous counterparts

The Star and Island Lake plutons of the Central metavolcanic belt, La Ronge domain, host a variety of barren and auriferous quartz veins in northeast-trending shear zones (040 degrees 80 degrees NW). Shear zones are coincident with competency contrasts in the plutons, which may be attributed to the northeast-trending boundary of igneous phase changes, or the presence of northeast-trending dikes. Veins range from 30 cm to 2 m wide, sup to hundreds of meters long, and exhibit a discontinuous pinch and swell nature common to most shear zone-hosted mesothermal quartz veins. Barren veins are paragenetically simple, quartz being the dominant hydrothermal mineral with sparse microcline of variable origin. Muscovite, a common gangue mineral in auriferous veins, is rare in barren veins; in barren veins its occurrence is correlated with slightly elevated gold values.Quartz microtextures indicate incremental vein emplacement and deformation in a dynamic environment. Dynamic recrystallization (subgrain formation > recovery rate) is the dominant ductile deformation mechanism. Macro-and microfractures, preserved as veins and planar arrays of fluid inclusions, are a result of episodic brittle failure. The cyclic nature of these events suggests in turn fluid pressure cycling in the brittle-ductile transition.Regionally, veins have a bimodal distribution of delta 18 O quartz values with barren and auriferous veins occupying both populations. Excepting the Jasper mine, on both a regional and hand sample scale, there is no shift in isotopic composition of vein quartz with increased ductile deformation. In fact the majority of the veins are remarkably homogeneous with respect to delta 18 O quartz. Petrographically, fluid inclusions in barren veins are identical to those described in auriferous veins and exhibit temperatures of total homogenization similar to those of inclusions in auriferous veins. Compositionaly, fluid inclusions in barren veins have similar wt percent NaCl and bulk CO 2 as auriferous veins; however, an important distinction is that auriferous veins may have up to 30 mole percent CH 4 , a phase absent in barren veins. Similarities between auriferous and barren veins such as field relationships, microstructures, stable isotopes, and fluid inclusions are interpreted to indicate a broad temporal contemporanity of auriferous and barren fluids. Distinct mineral paragenesis and CH 4 content may represent different redox states of barren and auriferous fluids. Lack of reduced carbon species in barren veins may reflect an oxidizing fluid with few reduced sulfur ligands capable of transporting gold.

Three-dimensional imaging of arrays of fluid inclusions in fluorite by high-resolution X-ray CT

Three-dimensional imaging of arrays of fluid inclusions in fluorite by high-resolution X-ray CT

A TEM investigation of shock metamorphism in quartz from the Sudbury impact structure (Canada)

The Sudbury basin in Canada is an elliptical feature (approx. 60 × 27 km in size) that is now widely believed to be part of a large (approx. 200 km diameter) meteorite impact structure which formed about 1.85 Ga ago and was subsequently deformed and metamorphosed. Despite prolonged debate over the origin and original size of the Sudbury structure, strong evidence for meteorite impact has been provided by the discovery in its rocks of a wide range of shock-metamorphic features, especially the optically observable traces (fluid inclusion arrays) of former Planar Deformation Features (PDFs) in quartz parallel to {10 1 3} planes. In this new examination of Sudbury samples, no preserved original PDFs (glassy lamellae) were observed optically in quartz grains from basement rock fragments in the Onaping Formation, a unit interpreted as a ‘fallback breccia’ deposited within the original crater. However, TEM revealed other thin lamellar features preserved in the quartz; these are identified as Brazil twin lamellae parallel to the basal plane (0001). These lamellae are 15–200 nm thick, show a typical spacing of 30 nm to several microns apart, and are occasionally decorated with fluid inclusions (≤ 0.5 μm in size), which probably formed during post-shock alteration and annealing. Numerous subgrain boundaries (SGBs) were also detected, many of them oriented roughly parallel to the basal plane (0001). The SGBs apparently formed during post-shock recrystallization, leading to the local disappearance of Brazil twin lamellae. Experimental evidence suggests that such basal Brazil twins are the unique product of high-pressure shock waves. The features in the Sudbury samples are identical to those observed in the Vredefort structure, South Africa, which is also widely accepted as an ancient impact structure about 2.0 Ga old. The recognition of basal Brazil twins in quartz at Sudbury provides additional evidence for meteorite impact origin and also emphasizes the high value of these durable shock features for identifying old and tectonically metamorphosed impact structures.

Processes of brine generation and circulation in the oceanic crust: Fluid inclusion evidence from the Troodos Ophiolite, Cyprus

Detailed temporal, thermal, and compositional data on aqueous fluid inclusions from a suite of plutonic and diabase samples from the Troodos ophiolite, Cyprus provide the first documentation that generation of high‐temperature brines may be common at depth in the oceanic crust. Anastomosing arrays of fluid inclusions in rocks of the upper intrusive sequence record episodic fracturing events. The earliest fracturing event, at temperatures &gt;450–600°C resulted in entrapment of brine‐rich aqueous fluids with salinities of 36–61 wt % NaCl equivalent. Homogenization of the brine inclusions by haute dissolution, the virtual absence of vaporrich fluid inclusions throughout the upper level plutonic sequence, and the restriction of brine inclusions to the most evolved plutonic rocks suggests that exsolution of brines off of the late stage gabbro and plagiogranite melts played a significant role in generating the quartz‐hosted, high‐salinity inclusions. Cooling of the fluids during pulses of fluid migration associated with episodic fracturing events, resulted in entrapment of the brines in the deep‐seated, high‐temperature portion of the hydrothermal system. In localized areas, the high‐temperature brines (NaCl±KCl±CaCl2) caused extreme alteration of the plagiogranite bodies and in the formation of podiform epidosites. Arrays of low‐temperature, low‐salinity fluid inclusions, which in some samples crosscut fractures dominated by brine inclusions, indicate downward propagation of a cracking front subsequent to collapse of the high‐temperature magmatic system, resulting in penetration of seawaterlike fluids into the plutonic sequence at temperatures &gt;200–400°C. Hydration reactions under greenschist facies conditions, or limited mixing with brine‐rich fluids, may have resulted in salinity variations from 70% below to 200% above seawater concentrations. Temperatures and compositions of the low‐salinity inclusions are similar to those found in stockwork systems beneath Troodos ore deposits and to those of fluids exiting active submarine hydrothermal vents at mid‐ocean ridge spreading centers. The low‐temperature fracture networks may represent an extensive deep‐seated feeder system which coalesced to form zones of concentrated hydrothermal upflow.

Microstructures in water‐weakened single crystals of quartz

Models of hydrolytic weakening based on the influence of chemical environment on the concentrations of point defects stimulated a series of experiments by Ord and Hobbs [1986] in which single crystals of natural quartz were deformed in a solid medium apparatus under a range of buffered oxygen, water, and hydrogen fugacities. Microstructures in these specimens have been examined using visible light and transmission electron microscopy in an attempt to identify processes responsible for the observed relationships between chemical environment and strength. Preheating treatments, conducted for ∼20 hours at the pressure, temperature, and chemical conditions of each ensuing deformation experiment, modified each specimen microstructurally such that the quartz was not in the form of a low‐dislocation‐density single crystal when deformation testing began. Specimens fractured axially during pretreatment, particularly those in chemical environments for which water fugacity remained high. Fractures later healed giving rise to arrays of fluid inclusions in regions with moderate densities of dislocations. Some deformation and recrystallization, concentrated at healed fractures and specimen end zones, also accompanied pretreatment. “Mn‐buffered” specimens, for which f O2 and f H2O.were high, were densely fractured; in less strained regions, subsequent deformation was restricted to the vicinity of healed fractures, while regions of higher plastic strain and recrystallization show evidence from inclusions for abundant early fractures, now obliterated. “Ta‐buffered” specimens, of low f O2 and f H2O, are also deformed close to axial fractures, but here fractures are comparatively rare. The marked variation in the degree of fracturing seems to be an example of stress‐corrosion cracking in response to chemical environment. Dislocations are developed in material away from healed fractures in one deformed Ta‐buffered specimen. If point defects introduced by diffusion are responsible for water weakening of quartz crystals, the complex microstructures seen in the deformed specimens of Ord and Hobbs [1986] require that point defect distributions were neither uniform nor controlled by bulk diffusion from the sample edges. The clear association between plastic strain and healed fracture does point to important local effects either of “water” upon dislocation activity or of healed‐fracture structure having been involved in the nucleation of dislocations. Regardless of the details, we infer from the dependence of the degree of fracturing upon chemical environment, together with the high correlation between locations of plastic strain and regions of earlier fracture, that the relationship between chemical environment and macroscopic strength of natural quartz crystal is not simply a matter of water weakening as a direct result of high point defect concentrations. Instead, fracture, inferred to be important in natural fluid‐rock interactions, also seems to be an experimentally critical process which might nucleate the dislocations necessary to initiate plastic deformation in the laboratory.

Geological link between fluid inclusions, dilatant microcracks, and paleostress field

Seismic investigations using three‐component instruments have demonstrated shear wave splitting in widely different geological and tectonic environments. Shear wave analysis suggests that the inferred anisotropy can best be explained if the rocks contain distributions of subparallel, subvertical, fluid‐filled microfractures which are aligned according to the current stress field (the Crampin “extensive dilatancy anisotropy” (EDA) model). Geological evidence for widespread fluid activity in the Earth's crust supports this hypothesis and suggests that the myriads of planes of microscopic secondary fluid inclusions which characterise most geological materials represent fossil EDA cracks. Field studies confirm a strong spatial relationship between the orientation of fluid inclusion arrays and the paleodirections of maximum compressive stress. At elevated temperatures and pressures, the healing of fluid‐filled cracks in the crust, of whatever origin, is considered to be a fairly rapid process. Moreover, the inclusions which form as a result of crack healing respond to changes in temperature, pressure, and stress by changing shape and/or volume. This indicates a high degree of fracture compliance; sufficient perhaps to react to rapid changes in the current stress field and associated shear wave splitting. Though individual inclusions (typically &lt;20 μm) will have no effect on the propagation of seismic waves with wavelengths of tens to hundreds of meters, the rock volume sampled by the waves will be rendered anisotropic if the overall average of the inclusions shows a preferred orientation. Thus fluid inclusions may constitute one of the principal types of fluid‐filled microcrack described by the EDA hypothesis.

Microfracture and crack healing in experimentally deformed peridotite

Secondary CO2 fluid inclusion arrays in peridotite xenoliths from basalt are formed by the healing of CO2‐filled microcracks. We have produced healed microcracks and deformation textures comparable to those in xenoliths by deforming peridotite ifi the presence of excess CO2 fluid at a constant strain rate of 10−5 s−1, a confirming pressure of 1.0–1.5 GPa, and a temperature of 800°–1050°C. Plastic and micro‐clastic deformation textures coexist, and healed microcracks extending out of (100) and (011) olivine deformation lamellae have the crystallographic orientation of microcracks nucleated on pileups of (100)[010] and (011)[100] edge dislocations. Healed microcracks in olivine grains without deformation lamellae tend to be in these orientations or in the orientation of microcracks nucleated on pileups of (110)[001] and (010)[001] edge dislocations. The same crystallographic orientation of healed microcracks is present in peridotite xenoliths from basalt. The steady flow stress data of the experiments without added CO2 fluid fit a power law with a stress exponent n of 3 and a thermal activation coefficient of 410 ± 50 kJ mol−1. The maximum stress that can be supported by samples with added CO2 fluid is 100–300 MPa less than the flow stress of samples without added CO2 fluid. The reduction in niechanical strength correlates with an increase in the concentration of intragranular healed microcracks and an increase in the proportion of healed microcracks inclined to the maximum compressive stress direction. Strength and microcrack characteristics of experiments with CO2 performed at 1.5 GPa are similar to experiments without CO2 at 1.0 GPa. The dependence of strength and microcrack concentration on confining pressure suggests that the principal effect of the CO2 fluid is to reduce the effective confining pressure and to promote microclastic deformation mechanisms. The similarity of experimental textures to xenoliths suggests that CO2 fluid may produce semibrittle mechanical behavior in the regions of the lower lithosphere where xenoliths originate.

Two-phase separation and fracturing in mid-ocean ridge gabbros at temperatures greater than 700°C

Microthermometric analyses of fluid inclusions on a suite of hydrothermally altered gabbros recovered just south of the eastern intersection of the Kane Fracture Zone and the Mid-Atlantic Ridge, record the highest homogenization temperatures yet reported for mid-ocean ridge hydrothermal systems. Fluid salinities in the high temperature inclusions are more than ten times that of seawater. Multiple generations of fluid inclusions entrapped along healed microfractures exhibit three distinct temperature-compositional groups. We interpret these populations as having been trapped during three separate fracturing events. The earliest episode of brittle failure in the gabbros is represented by coplanar, conjugate vapor-dominated and brine-dominated fluid inclusion arrays in primary apatite. Vapor-dominated inclusions exhibit apparent homogenization temperatures of 400°C and contain equivalent salinities of 1–2 wt.% NaCl. These inclusions are interspersed with liquid-dominated, sulfide-bearing inclusions containing salinities of 50 wt.% NaCl equivalent. These high salinity inclusions remain unhomogenized at temperatures greater than 700°C. Compositional and phase relationships of the fluid inclusions may be accounted for by two-phase separation of a fluid under 1000–1200 bars pressure. These pressures require that fluid entrapment occurred under a significant lithostatic component and indicate a minimum entrapment depth of 2 km below the axial valley floor. This depth corresponds to a minimum tectonic uplift of 3 km, in order to emplace the samples at the 3100 m recovery depth. The microfracture networks within magmatic apatites represent fluid flow paths for either highly modified, deeply penetrating seawater or a late stage magmatic aqueous fluid. The inclusions may have formed close to the brittle-ductile transition zone adjacent to an active magma chamber. Following collapse of the high temperature front, lower temperature fluids of definite seawater origin circulated through the open fracture networks, pervasively altering portions of the gabbros. This stage is represented by low-to-moderate (1–7 wt.% NaCl equivalent) salinity inclusions in plagioclase, apatite, epidote, and augite, which homogenize at temperatures of approximately 200–300°C and 400°C. Formation of hydrous mineral assemblages, under greenschist to lower amphibolite facies conditions, resulted in sealing of the vein system and may have resulted in modification of seawater salinities by as much as a factor of two. During or following these later stages of hydrothermal activity the gabbros were emplaced high on the axial walls by differential uplift attending formation of the flanking mountains.

A record of high‐temperature embrittlement of peridotite in CO2 permeated xenoliths from basalt

Four ultramafic xenoliths recovered from Hawaiian basalts contain CO2‐fluid inclusion arrays which originated as healed microcracks. These healed microcracks have been used to study the microcracking mechanisms in the xenolith source region. Fluid inclusion arrays in olivine have a preferred crystallographic orientation which is consistent with a hypothesis of microcrack nucleation upon dislocation pileups by the Stroh mechanism on the following olivine slip systems: (0kl)[100], (110)[001], and (010)[001]. Previous investigators have shown that all these slip systems of olivine are active at high stress or low temperature. Olivine neoblasts occur as small grains in local shear zones (0.04 mm diameter and less than 3% of total volume) and larger grains which embay porphyroclasts throughout the specimens (0.22‐mm diameter and approximately 30% percent of total volume). Olivine neoblast diameters provide estimates of the deviatoric stress of 160–200 MPa using the smaller grains and 50–75 MPa using the larger grains. The absence of clinopyroxene mechanical twinning in the xenoliths provides an independent paleopiezometer that limits the maximum deviatoric stress in the peridotite source region to 200 MPa. Fluid inclusion arrays are parallel to each other, are perpendicular to penetrative foliation planes produced by intragranular plasticity, and have an average spacing between arrays of 0.5–2.0 mm. Recrystallized olivine grains (neoblasts) are free of fluid inclusions. Estimates of the contribution of Stroh cracks to the deformation of the xenolith source region indicate a maximum decrease of a few percent in the net energy required to produce an increment of strain, dependent upon confining pressure.