Influx Of H2O Research Articles

Self-induced incipient 'eclogitization' of metagranitoids at closed-system conditions.

The incipient development of diagnostic high‐pressure assemblages—the ‘eclogitization’—of granitoids, such as plagioclase breakdown and small‐scale formation of garnet and phengite does not require exogenous hydration because unlike dry protoliths like basalt/gabbro or granulite, granitoids s.l. contain crystallographically bound H2O in biotite. During high‐pressure overprint, partial biotite breakdown causes a localized increase in the chemical potential of H2O (μH2O). Transport of H2O into nearby plagioclase induces the formation of diagnostic eclogite facies assemblages of jadeite–zoisite–K‐feldspar–quartz ± kyanite ± phengite that pervasively replace former cm‐sized plagioclase without requiring the participation of free H2O. Depending on pressure–temperature evolution, similar textures may involve albite instead of jadeite, consistent with the general absence of Na‐clinopyroxene in high‐pressure metagranitoids and kindred gneisses. Plagioclase breakdown may also occur due to simple burial because compression leads to an increase of μH2O, without requiring additional influx of H2O at the texture scale. However, the addition of biotite‐derived H2O into plagioclase sites likely increases reaction rates. In parallel, ∼100‐μm‐thick complementary coronae involving garnet | phengite–quartz develop at former biotite–plagioclase/K‐feldspar interfaces due to the coupled diffusion of FeO–MgO–H2O from biotite towards feldspars and minor CaO in the opposite direction. The reaction textures likely create structural weaknesses and preferential fluid pathways that facilitate further hydration and/or deformation along the prograde path, thereby obliterating the textures. If exogenous H2O is introduced, it is accommodated in phengite growing at the expense of igneous K‐feldspar and possibly in epidote‐group minerals. Upon decompression, such hydrated rocks would dehydrate, thereby favouring fluid‐assisted retrogression and loss of diagnostic eclogite facies assemblages at lower pressure. Whereas the prograde reaction textures are only preserved at closed‐system conditions and in the absence of deformation, they are suggested to commonly form during orogenic metamorphism of granitoids and quartzofeldspathic gneisses that dominate the continental crust in high‐pressure terranes such as the Western Italian Alps and the Western Gneiss Region (Norway).

The role of mantle and the ancient continental crust in the generation of post-collisional high-K calc-alkaline and alkaline granites, with main reference to the Arabian-Nubian Shield

Continental-scaled magmatic activity occurs after the end of collision-orogenic stages of the Wilson cycle. This post-collisional magmatism is widely exhibited, from 635 to 540 Ma, in a series of large Neoproterozoic (Pan-African) orogenic belts throughout North Africa. Special reference is given to the Arabian-Nubian Shield (ANS) and the LATEA metacraton (Tuareg shield). For comparison, two Late Paleozoic-Early Mesozoic transcontinental belts, the Variscan in Western Europe and the east Central Asian Orogenic Belt, are briefly discussed. Two main types of consecutive magmatic suites characterize the Late Neoproterozoic (Ediacaran) and Late Paleozoic-Early Triassic (Variscan) post-collisional magmatic events despite their different formation age and geographic location: (a) Early post-collisional suites composed of dominant HKCA monzogranite, with secondary quartz monzonite, granodiorite, and related high-K calc-alkaline gabbro, diorite; (b) Late post-collisional to intra-continental suites of Alk syenogranite, alkali feldspar granite and quartz syenite, in places with Na-rich pyroxene and amphibole; volcanic activity and numerous dike swarms are abundant. The mafic rocks related to Alk granite suites are monzogabbro and monzonite (shoshonitic series). Granitoids of the same type, HKCA or Alk, are very similar in mineral and chemical composition throughout the belts and in different belts. This similarity is observed in regions with very distinct types of crust, e.g. the dominant juvenile oceanic island arc crust in the ANS, and Archean-Paleoproterozoic continental crust in the LATEA metacraton. The isotope U-Sr-Nd-Hf-O signatures suggest that HKCA granite magmas in these regions were produced from different crustal sources with varying contribution of mantle-derived basic materials to the granitoid magma generation. For HKCA granites, a model of crustal anatexis, accompanied with a limited influx of H2O, K, LILE, HFSE from underplated basalt magmas (convective diffusion in the contact of silicic and mafic liquids) is suggested. The most likely model of the Alk silicic magma generation is partial melting of related mantle-derived mafic rocks (monzogabbro, monzonite in the lower crust. Data on the enhanced temperature of A-type silicic magmas, more than 1000 °C, suggest that magma-generation could occur even at the depth of uppermost lithospheric mantle.

Mineral equilibrium constraints on the feasibility of diffusive H2O‐fluxed melting in the continental crust

AbstractGeneration of granitic melt is believed to occur predominantly by melting through the breakdown of hydrous minerals. However, melting due to the influx of H2O has been recognized in anatectic amphibolite facies tonalitic grey gneisses, metagreywackes and low‐P metapelites, and has consequently been proposed as an alternative mechanism for the generation of granitic melt. Melting induced by H2O addition is recognized from voluminous melt production at relatively low temperature, where hydrous minerals are stable and anhydrous minerals are preferentially consumed during melting. Mineral equilibrium modelling to determine the P–T conditions, melt volumes, melting reactions and viable H2O sources reveals that the process is not restricted to specific compositions or P–T conditions, although lower pressure and lithologies with a low hydrous mineral content are more favourable. Melting reactions in all lithologies primarily consume quartz and feldspars to yield 5–6 mol.% melt for each mol.% of H2O added. remains constant at ~0.70 to 0.77 during progressive melting as long as alkali feldspar is present. Once alkali feldspar is exhausted, plagioclase becomes the main reactant, producing more tonalitic melt compositions with gradually higher . Our results demonstrate that, at the site of melting, melting is driven by diffusion of H2O into the target rock along chemical potential gradients, rather than the advective flow of a mechanically distinct water‐rich fluid phase. Melting will initiate and proceed as long as a gradient exists between the H2O source and target lithology. Our calculations show that an ordinary magma, such as an I‐type magma with typical H2O content, has a high enough to be a viable H2O source, allowing diffusive H2O‐fluxed melting to produce melt proportions and fertility comparable to that of dehydration melting. However, high degrees of partial melting require a considerable amount of H2O, which necessitates a continuously advecting H2O source such as a magma conduit or melt‐bearing shear zone. A magmatic H2O source at emplacement level will undergo a similar amount of crystallization as the melt fraction produced in the target rock such that there will be no net melt production. Considering that shear‐zone hosted magma conduits are localized features, diffusive H2O‐fluxed melting is likely to only be viable in a small fraction of the anatectic orogenic crust. Although it may play an important role in locally raising melt volumes and modifying magma chemistry through mingling and hybridization, it does not appear to, of itself, be able to generate significant volumes of granitic melt.

The initial garnet‐in reaction involving siderite–rhodochrosite, garnet re‐equilibration and P–T–t paths of graphitic schists in the Black Hills orogen, South Dakota, USA

AbstractIt is generally thought that garnet in metapelites is produced by continuous reactions involving chlorite or chloritoid. Recent publications have suggested that the equilibrium temperatures of garnet‐in reactions may be significantly overstepped in regionally metamorphosed terranes. The growth of small spessartine–almandine garnet crystals on Mn‐siderite at the garnet isograd in graphitic metapelites in the Proterozoic Black Hills orogen, South Dakota, demonstrates that Mn‐siderite was the principal reactant that produced the initial garnet in the schists. Moreover, the positions of garnet compositions in isobaric, T–(C/H) pseudosections for the schists show that the temperature of the garnet‐in reaction from Mn‐siderite was overstepped minimally at the most. In the Black Hills, garnet was initially produced during regional metamorphism beginning at c. 1755 Ma due to the collision of Wyoming and Superior cratons, and was subsequently partially or fully re‐equilibrated at more elevated temperatures and pressures during intrusion of the Harney Peak Granite (HPG) at c. 1715 Ma. Garnet occurs in graphitic schists in garnet, staurolite and sillimanite zones, the latter being a product of contact metamorphism by HPG. During metamorphism, coexisting fluid contained both CO2 and CH4. In the garnet zone, garnet crystals contain petrographically distinct cores with inclusions of quartz, graphite and other minerals. Centres of the cores have distinctly elevated Y concentrations that mark the positions of garnet nucleation. The elevated Y is thought to have come from the Mn‐siderite onto which Y was probably absorbed during precipitation in an ocean. In the upper garnet and staurolite zones, the cores were overgrown by inclusion‐poor mantles. Mantles are highly zoned and have more elevated Fe and Mg and lower Mn and Ca than cores. The growth of mantles is attributed to late‐orogenic heating by leucogranite magmas and attendant influx of H2O that caused consumption of graphite in rock matrices. A portion of the Proterozoic terrane that includes the HPG is surrounded by four large faults. In this ‘HPG block’, garnet is inclusion‐poor and its composition does not preserve its early growth history. This garnet appears to have re‐equilibrated by internal diffusion of its major components and/or recrystallization of an earlier inclusion‐rich garnet. It has equilibrated within the kyanite stability range, and together with remnant kyanite in the high‐strain aureole of the HPG, indicates that the HPG block had a ≥6 kbar history. The HPG block has undergone decompression during emplacement of the HPG. The decompression is evident in occurrences of retrograde andalusite and cordierite in the thermal aureole of the HPG. The data support a polybaric metamorphic history of the Black Hills orogen with different segments of the orogen having their own clockwise P–T–t paths.

Hydration vs. oxidation: Modelling implications for Fe–Ti oxide crystallisation in mafic intrusions, with specific reference to the Panzhihua intrusion, SW China

Recent work on the Panzhihua intrusion has produced two separate models for the crystallisation of the intrusion: (1) low-Ti, high CaO and low H2O (0.5 wt.%) parent magma (equivalent to Emeishan low-Ti basalt) at FMQ; and (2) high-Ti, low CaO and higher H2O (>1.5 wt.%) parent magma (equivalent to Emeishan high-Ti basalt) at FMQ + 1.5. Modelling of these parent magma compositions produces significantly different results.We present here detailed f(O2) and H2O modelling for average compositions of both Emeishan high-Ti and low-Ti ferrobasalts in order to constrain the effects on crystallisation sequences for Emeishan ultramafic–mafic layered intrusions. Modelling is consistent with numerous experimental studies on ferrobasaltic magmas from other localities (e.g. Skaergaard intrusion). Modelling is compared with the geology of the Panzhihua intrusion in order to constrain the crystallisation of the gabbroic rocks and the Fe–Ti oxides ore layers. We suggest that the gabbroic rocks at the Panzhihua intrusion can be best explained by crystallisation from a parent magma similar to that of the high-Ti Emeishan basalt at moderate H2O contents (0.5–1 wt.%) but at the lower end of TiO2 content for typical high-Ti basalts (2.5 wt.% TiO2). Distinct silicate disequilibrium textures in the Fe–Ti oxide ore layers suggest that an influx of H2O may be responsible for changing the crystallisation path. An increase in H2O during crystallisation of gabbroic rocks will result in the depression of silicate liquidus temperatures and resultant disequilibrium with the liquid. Continued cooling of the magma with high H2O then results in precipitation of Mt–Uv alone.The H2O content of parent magmas for mafic layered intrusions associated with the ELIP is an important variable. H2O alters the crystallisation sequence of the basaltic magmas so that at high H2O and f(O2) Mt–Uv crystallises earlier than plagioclase and clinopyroxene. Furthermore, the addition of H2O to an anhydrous magma can explain silicate disequilibrium texture observed in the Fe–Ti oxide ore layers.

Phase equilibria in quartzitic garnet-kyanite-chloritoid micaschist from the Eclogite Zone, Tauern Window, Eastern Alps

In a quartzitic garnet-kyanite-chloritoid micaschist garnet is present as compact, partly euhedral grains and as web-like types with numerous inclusions of quartz. Compact types overgrown by web garnets or alternating layers of both types are also common. Continuous zoning is observed with increase of pyrope and decrease of grossular and spessartine component towards garnet rim. A first stage of garnet growth together with chloritoid is recognized, followed by a garnet rim stage including kyanite. Calculated P-T pseudosections in the KNCFMMnASTH system suggest a prograde path with garnet growth starting in the assemblage chloritoid-chlorite-lawsonite-glaucophane (muscovite-quartz-rutile-H2O excess phases). Later breakdown of chloritoid +chlorite is followed by the reaction chloritoid+quartz = garnet+kyanite+H2O. The calculations suggest that glaucophane was present along a major portion of the prograde P-T path. Conditions of ~20.5 kbar, 540 °C are suggested for growth of garnet core and ~19–21 kbar, 580–590 °C for garnet rim coexisting with the assemblage garnet-kyanite-chloritoid-paragonite. During exhumation, cooling and influx of H2O took place. As a result high-XMg chloritoid was formed in the matrix and lower-XMg chloritoid in garnet inclusions. In a later event at lower P-T conditions, minor amounts of low-Si phengite, biotite, chlorite and clinozoisite replaced high-Si phengite, kyanite, garnet and chloritoid.

Nanoinclusions of high-pressure hydrous silicate, Mg3Si4O10(OH)2 · nH2O (10Å-phase), in mantle olivine: Mechanisms of formation and transformation

The 10A-phase, Mg3Si4O10(OH)2 · nH2O, where n = 0.65÷2, belongs to the group of dense hydrous magnesium silicates (DHMS), which were produced in experiments and are regarded as hypothetical mineral carriers for H2O in the mantle. However, DHMS were almost never observed in nature. The only exception is the finding of the 10A-phase as nanoinclusions in olivines from mantle nodules in kimberlites. The inclusions with sizes of a few ten nanometers have a pseudohexagonal habit and are characterized by the presence of voids free of solids. The 10A-phase fills the equatorial parts of the inclusions, and, in the majority of inclusions, it is replaced by the low-pressure serpentine + talc assemblage. Based on the analysis of electron microscope images, a model was proposed for the solid-state formation of inclusions, the precursory material of which was transformed to the 10A-phase with the liberation of a water fluid. According to this model, the formation of hydrous nanoinclusions and their subsequent autoserpentinization occurred without the influx of H2O from the external medium through the mobilization of intrinsic hydroxyl-bearing point defects trapped during olivine crystallization. The subsequent autoserpentinization of the inclusions occurred during decompression owing to interaction between the inclusion material and the host olivine matrix. The process was accompanied by the partial exhaustion of the fluid phase and the replacement 10A-phase + H2O = Serp + Tc. The criterion for the credibility of the model is the conservation of the volume of material during the reaction at P = const and T = const.

Progress of actinolite‐forming reactions in mafic schists during retrograde metamorphism: an example from the Sanbagawa metamorphic belt in central Shikoku, Japan

AbstractHydration reactions are direct evidence of fluid–rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite → hornblende → actinolite in the high‐grade zone, and winchite → actinolite in the low‐grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass‐balance relations suggest that: (1) the actinolite‐forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H2O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of actinolite to total amphibole, Yact, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low (Yact < 0.5), but it increases drastically in the high‐grade part of the garnet zone (Yact > 0.7); and (3) the extent of the winchite breakdown reaction is commonly high (Yact > 0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H2O and the surface area of hornblende. In contrast to high‐grade rocks, the winchite breakdown reaction is well advanced in the low‐grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low‐grade rocks may be due to small size of winchite before the reaction.

Partial Melting of Metapelitic Rocks Beneath the Bushveld Complex, South Africa

Metapelitic rocks in the aureole beneath the Bushveld Complex preserve evidence for both high- and low-aH2O anatexis. The aureole is characterized by an inverted thermal structure in which suprasolidus rocks potentially interacted with an H2O-rich volatile phase derived from underlying, dehydrating rocks. At lower grade (T < 700°C) the rocks contain fibrolite mats and seams that record local redistribution of volatiles. Incongruent reactions consuming biotite produced small quantities (<1 mol %) of liquid and peritectic cordierite that remained trapped within the mesosome. Larger volumes of melt (3–4%), preserved as coarse-grained discordant leucosomes, were produced by congruent melting following a structurally focused influx of H2O. Subhorizontal volatile-phase flow was concentrated within thin (∼10 mm) metapsammite horizons that are preserved as stromatic quartz–sillimanite veins. Upward migration occurred along steep fibrolite seams that are subparallel to a variably inclined foliation. Discordant leucosomes are concentrated within antiformal fold closures of quartz–sillimanite veins and along the axial planar schistosity. Closer to the contact (T > 725°C), volatile-phase-absent, biotite-consuming melting and melt extraction produced coarse-grained garnet–cordierite granofels. At the contact, leucodiatexites devoid of peritectic phases suggest effective segregation of melt from an underlying source. Migmatitic metapelites and their lower-grade stratigraphic equivalents have similar bulk-rock oxygen isotope values, consistent with very limited volatile-phase infiltration and precluding the Bushveld Complex magmas as the source of the volatiles.

P-T Paths, H2O recycling, and depth of crystallisation for crustal melts

The P-T (pressure-temperature) paths taken by the high-grade metamorphic rocks during orogenesis govern which melting and crystallisation reactions are encountered and hence the location and amount of melt. Small differences in rock fertility, water amount and migmatite deformability influence the amount and distribution of anatectic partial melt on an outcrop scale. Layer-scale migration of H2O in response to gradients in μH2O controls when melting in one layer occurs while partial melt in a nearby layer crystallises. Suprasolidus decompression - dehydration reactions (SDDR) can occur patchily at mid-crustal depths generating feldspathic segregations with alumino-silicates, and releasing H2O. Quite different mineralogies and textures are diagnostic of the possible crystallisation reactions at different crustal depths. Local mineralogical variations in anatectites can reveal whether melting occurred in response to decompression through a melting reaction or to access of H2O. This is particularly important to help decide whether anatexis required a localised heat supply or an influx of H2O.

Voyager electron density measurements on Saturn: Analysis with a time dependent ionospheric model

We have used a one‐dimensional chemical diffusive model of the ionosphere, in conjunction with the measured Voyager ultraviolet spectrometer (UVS) upper atmospheric temperature and composition structure, to analyze the Voyager measurements of Saturn's upper ionospheric electron densities. Electron density measurements are available from the analysis of the radio science (RSS) experiments. In addition, if interpreted as an atmospheric phenomenon the Saturn electrostatic discharges (SED), detected by the Voyager planetary radio astronomy instrument, the measurements also yield important information on electron densities at the peak. These latter data suggest a strong day‐to‐night variation in the peak electron densities with a maximum of ∼5 × 105 cm−3 near noon and a minimum of ∼103 cm−3 near midnight. The analysis of RSS data yields a peak of ∼104 cm−3 in the electron density profiles near the local terminators. As with simpler ionospheric models, we find serious difficulties in interpreting these electron density measurements because time constants in the model for the chemical sinks in the ionosphere, of mostly H+ ions, are &gt;106 s much longer than a Saturnian day, suggesting little diurnal variation. We have investigated the effects of H2 vibrational excitation (characterized by a single vibrational temperature) and a gaseous H2O influx from Saturn's rings, as enhancers of plasma recombination, since they result in the conversion of slowly recombining H+ ions to more rapidly recombining molecular ions. This does introduce a larger diurnal variation in the peak electron densities, but not as strong as that deduced from the analysis of SED measurements. Furthermore, it also reduces the magnitude of the electron densities at the peak well below that inferred from the SED data. Based on the model results and extrapolation of calculations by Kim and Fox [1991] from Jupiter, we suggest that SEDs may not be associated with atmospheric storm systems. In our attempt to interpret the RSS measurements of electron density profile at sunrise, we have invoked an upward field‐aligned drift, H2 vibrational excitation and an influx of H2O to explain the results, and, in particular, maintain plasma peak at the measured altitude.

Cotransport of K+, Cl- and H2O by membrane proteins from choroid plexus epithelium of Necturus maculosus.

1. The interaction between K+, Cl- and H2O fluxes was studied in the ventricular membrane of the choroid plexus epithelium from Necturus maculosus by means of ion-selective microelectrodes. 2. Three experimental strategies were adopted: the osmolarity of the ventricular solution was increased abruptly by addition of (i) mannitol or (ii) KCl; (iii) Na+ in the ventricular solution was replaced isosmotically by K+. 3. The mannitol experiments showed that H2O had two pathways across the ventricular membrane. One was purely passive, with a water permeability, L'p, of 0.64 x 10(-4) cm s-1 (osmol l-1)-1. This operated in parallel with an ion-dependent pathway of similar magnitude which was abolished in Cl(-)-free solutions. 4. When KCl was added there was a flow of H2O into the cell. Surprisingly, this took place despite the osmotic gradient which favoured an efflux of H2O. The effect was blocked by frusemide (furosemide), in which case KCl had the same effects as applications of NaCl or mannitol. 5. Replacement of Na+ with K+ caused an influx of H2O. This flux could proceed against osmotic gradients implemented by mannitol. 6. The present data and those of earlier publications show that the interdependence of the fluxes of K+, Cl- and H2O in the exit membrane can be described as cotransport. The fluxes have a fixed stoichiometry of 1:1:500, the flux of one species is able to energize the flux of the two others, and the transport exhibits saturation and is specific for K+ and Cl-. 7. A molecular model based upon a mobile barrier in a membrane spanning protein gives an accurate quantitative description of the data.

Interdependence of deformation, fluid infiltration and reaction progress recorded in eclogitic metagranitoids (Sesia Zone, Western Alps)

ABSTRACTThe effects of high‐strain deformation and fluid infiltration during Alpine eclogite facies metamorphism have been studied across ductile shear zones in relatively undeformed metagranitoids at Monte Mucrone (Sesia Zone, Western Alps, Italy). Microfabrics together with bulk rock and stable isotope data indicate that the mineralogical and chemical variations are related to the degree of deformation, rather than to changes in P‐T conditions or tectonic position. Transformation of meta‐quartz diorite to recrystallized eclogitic mylonites involved the breakdown of biotite and plagioclase and required the influx of H2O. Bulk‐rock geochemical data show that ductile deformation to form eclogitic mylonites involved an increase in volume with a weight percent gain in H2O and Si and variable loss of K, Na, Ca and Al. δ18O changes systematically across ductile shear zones into the undeformed country rocks. Constant values in shear zone centres indicate advection parallel to the shear zone and within 10 cm of the mylonites. A dominant component of diffusive oxygen exchange perpendicular to the shear zones produced isotopic fronts, evident from a gradual increase in δ18O values to the reference values of the country rocks. The degree of isotopic shift within the shear zones reflects increasing deformation and degree of reaction progress.Multiple phases of Alpine deformation and mineral growth are recognized in the Monte Mucrone metagranitoids, and in some cases, eclogite facies shear zones were reactivated under greenschist facies conditions. The results of this study suggest that high‐strain deformation provided pathways for both synkinematic and post‐kinematic metamorphic fluids which were necessary for complete reactions. Relict igneous fabrics, as well as the presence of corona textures around biotite and pseudomorphs after primary igneous plagioclase in the least deformed rocks, indicate a paucity of hydrous fluids and support the conclusion that fluid movement was channelled rather than pervasive.

Metamorphic evolution of exhumed middle to lower crustal rocks in the Mojave Extensional Belt, southern California, USA

The Waterman Metamorphic Complex of the central Mojave Desert was exposed as a consequence of early Miocene detachment‐dominated extension. However, it has evidence consistent with a more extensive geological history that involves collision of a crustal fragment(s), tectonic thickening by overthrusting and two periods of extension. The metamorphic complex contains granitoid intrusives and felsic mylonitic gneisses as well as polymetamorphic rocks that include marble, calc‐silicate, quartzite. mafic granulite, pyribolite, amphibolite, migmatite and biotite schist. The latter group of rocks was affected by an initial series of high‐grade metamorphic events (M1 and M2) and a localized lower grade overprint (M3). The initial metamorphism (M1) can be separated into two stages along its high‐grade P–T path: M1a, a granulite facies metamorphism at 800–850° C and 7.5–9 kbar and Mlb, an upper amphibolite facies overprint at 750–800° C and 10–12 kbar. M1a developed mineral assemblages and textures consistent with granulite facies conditions at a reduced activity of H2O and is associated with intense ductile deformation (D1) and minor local partial melting. M1b overprinted the granulite assemblages with a series of hydrous phases under conditions of increasing pressure and H2O activity and is accompanied by little or no deformation. M2 developed at lower pressures and temperatures (650–750° C, 4.5–5.5 kbar) and is distinguished by a second local overprint of hydrous phases that reflects an input of aqueous fluids probably associated with the intrusion of a series of granitic dykes and veins. Effects of M3 are confined to the Mitchel detachment zone, an anastomosing early Miocene detachment fault, and are characterized by local ductile/brittle deformation (D2) of the pre‐existing high‐grade rocks and granitoid intrusives and by the production of mylonites and mylonitic gneisses under greenschist facies conditions (300–350° C, 3–5 kbar). The initial overprint (M1a) represents metamorphism, devolatilization and minor partial melting of supracrustal rocks under granulite facies conditions as a consequence of tectonic and, possibly, magmatic thickening. The increasing pressure transition of M1a to M1b reflects a period of continued compressional tectonism, thrusting and influx of H2O, in part, locally related to crystallization of partial melts. The near isothermal decompression between M1b and M2 probably represents a pre‐112‐Ma extensional episode that may have been the result of a decompressional readjustment of a thickened crust. Following the initial extensional event, the metamorphic complex remained at depths of 10–17 km for at least 90 Ma until it was uplifted following Miocene extension. M3 develops locally in response to this second extensional period resulting from the early Miocene detachment faulting.

Secondary active transport of water across ventricular cell membrane of choroid plexus epithelium of Necturus maculosus.

1. The interaction between Cl-, K+ and H2O fluxes were studied in the ventricular membrane of the choroid plexus epithelium from Necturus maculosus by means of ion-selective microelectrodes. The flux of H2O was measured by means of K+ electrodes as the dilution or concentration of intracellular choline ions, Ch+i. 2. In one series of experiments Cl- was readministered to the ventricular solution of tissues incubated in media with low Cl- concentrations. The resulting influx of Cl- was associated with an instantaneous influx of K+ and H2O. 3. Both the Cl- and the K+ influxes were reduced by the diuretic furosemide but were unaffected by inhibitors of Na+, K(+)-ATPase or changes in membrane potentials induced by Ba2+. Since the influx of K+ proceeds against its electrochemical gradient and is unaffected by changes in membrane potentials, the membrane exhibits secondary active, electroneutral transport of K+. 4. The influx of water, initiated simultaneously with the influx of K+ and Cl-, commenced before these ions had changed the osmolarity of the intracellular solution significantly. The influx of H2O could proceed against an osmotic gradient. The influx stopped when 100 mmol l-1 of mannitol was added to the ventricular solution at the same time as the Cl- ions. The influx of H2O was inhibited by K+ removal, furosemide or high external Ba2+ (10 mmol l-1), but not by strophanthidin, ouabain or low concentrations of Ba2+ (0.5 mmol l-1). The influx could not continue with other permeable anions, NO3-, acetate- or SCN-, replacing Cl-. 5. In another series of experiments Cl- was removed from the ventricular solution of tissues bathed in saline solutions with normal concentrations of Cl-. The resulting efflux of Cl- was associated with an instantaneous efflux of K+ and H2O. This efflux of H2O could proceed against an osmotic gradient of up to 70 mosmol l-1. This effect was inhibited by furosemide, in which case the water fluxes were entirely dependent on the osmotic gradients and the osmotic water permeability Lp of the ventricular membrane. 6. The data suggest that there is a coupling between the flux of KCl and of water in the ventricular membrane, which implies that the reflection coefficient sigma for KCl under the given circumstances is less than one. I suggest that the ability of leaky epithelia to transport against osmotic gradients depends on such a coupling, which derives from the properties of the proteins through which K+, Cl- and H2O leave the cell.

Water permeability of ventricular cell membrane in choroid plexus epithelium from Necturus maculosus.

1. The osmotic water permeability Lp and the relations between the flows of H2O, K+ and Cl- were studied in the ventricular membrane of the epithelium from the choroid plexus of Necturus maculosus. 2. The flows were induced by abrupt changes in external osmolarity of the ventricular solution. Solution changes were convective and no effects of unstirred layers could be detected on measured parameters. 3. The initial rate of change in intracellular concentrations of K+ and Cl- was monitored by double-barrelled ion-selective microelectrodes. 4. The initial rate of flux of H2O could be monitored as the changes in the concentration of intracellular choline ions (Ch+i). When 0.5 mmol l-1 of choline chloride was added to the external solutions, Ch+i attained values of 1-5 mmol l-1. The dilution or concentration of Ch+i could be recorded by K+ electrodes since the sensitivity of these to Ch+ is more than 50 times greater than to K+. 5. The Lp of the ventricular membrane of the epithelium was 1.4-2.1 x 10(-4) cm s-1 (osmol l-1)-1 and independent of the direction of the induced water flux. Lp was unchanged in tissues adapted to osmolarities of half the physiological value. 6. The efflux of H2O induced by mannitol was associated with an instantaneous efflux of K+ which was inhibited by furosemide. The fluxes had a ratio of 40 mmol l-1. The influx of H2O induced by the removal of NaCl from the ventricular solution was associated with an instantaneous influx of K+. The H2O influx had a ratio to the flux of K+ of 70 mmol l-1. 7. The efflux of H2O induced by mannitol was associated with an efflux of Cl- which was inhibited by furosemide. The ratio of the two fluxes was in the range 15-44 mmol l-1. 8. The conclusion is that the Ch+ method gives a reliable measure of the movement of H2O across the ventricular membrane. The magnitude of the Lp and its relevance to transepithelial transport are discussed. The osmotically induced H2O movement is accompanied by furosemide-sensitive fluxes of K+ and Cl- of the same magnitude. This suggests that co-transport between H2O and KCl can take place in the membrane.

751 PROTEIN-ENERGY MALNUTRITION (PEM) EFFECTS ON THE INTESTINAL ABSORPTION OF WATER AND ELECTROLYTES

We have previously determined the optimal conditions for H2O and Na absorption by the jejunum of the normal and tissue damaged rat using various formulae of oral hydration solutions (OHS). H2O absorption was inversely proportional to osmolality and Na absorption was directly proportional to its concentration. In the present study we investigated the characteristics of experimental hydration in an animal model of PEM. This was achieved with a low protein (4%), reduced carbohydrate diet fed to juvenile rats for 3 weeks. Absorption was assessed by an in vivo perfusion procedure using an OHS with (in mEq/1) 60 Na, 20 K, 50 Cl, 30 HCO3, and 111 mM glucose (Glu). Phenol red and 3H2O were used as markers. The net H2O fluxes in PEM (M) rats were lower than in controls (C) (1.78±0.10 vs 3.11±0.06 μl/min x cm, P<0.001). This difference was due both to a decrease in the mucosa-to-serosa influx of H2O in M rats (M= 5.99±0.16; C= 6.43±0.14, P<0.05) as well as to a greater efflux of fluid (M= 4.22±0.12; C= 3.52±0.13, P<0.001). PEM also resulted in failure to absorb Na from that type of OHS (M= -1.4±5.2; C= 52.2±9.4 nEg/min x cm, P<0.001), as well as in a reduced absorption of K (M= 65.9±2.4; C= 75.1±3.7 nEq/min x cm, P<0.05), HCO3 (M= 165±5; C= 181±5 nEq/min x cm, P<0.05), and Glu (M= 404±23; C= 479±22 nmoles/min x cm, P<0.05). The data suggest that an improvement of H2O and solute absorption in PEM could be achieved by specially formulated OHS with reduced osmolality and increased Na concentration, at the expense of K and Glu.

Salt and Water Balance in Frogs

The accepted model for regulation of salt and water in frogs and other aquatic vertebrates(1) has been modified for frogs to accord with results of measurements of fluxes of sodium ion and water using methods in which the effects of measuring procedures on normal function of control mechanisms is minimized. The model includes two control loops. One, activated by an increase in concentration of sodium ion in plasma, acts through increased secretion of the hormone Arg Vasotocin, and other effects tending to conserve H2O and increase influx of H2O. The other, activated by an increase in water potential (decrease in osmotic pressure or concentration) of the plasma, or by increased extracellular fluid volume, acts through increased secretion of the hormone Aldosterone and other actions tending to conserve Na+ and promote uptake of this ion, thus decreasing the water potential of the blood plasma.