Deuteric Fluids Research Articles

Trough structures in the Western syenite of Kûngnât, S Greenland: mineralogy and mechanism of formation

In the Western layered syenite stock of the Kungnât complex, close to the intrusion wall, there are two groups of regularly spaced, horizontally aligned, stacks of infilled troughs. These features resemble, to some extent, the trough layering within the UZa trough zone of the gabbroic Skaergaard1 intrusion. Such features have never before been described in syenitic rocks. The lower group comprises six stacks of infilled troughs; the upper group comprises seven. The two groups are vertically separated by about 10 m of sparsely layered syenite. The infilled troughs in the upper group are stacked vertically; those in the lower group gradually shift laterally and appear to converge over an antiformal structure. It is not possible to measure the plunge of the troughs due to outcrop geometry. Troughs are up to about 80 cm deep. The material filling the troughs is more melanocratic than the syenite that surrounds them. Individual troughs exhibit normal grading. The syenite within the troughs contains large amphibole and biotite oikocrysts. Faintly layered syenite occurs laterally between stacks. Minerals in the host syenite are more ferroan than those in the troughs, this is thought to represent re-equilibration with residual melt. Feldspar textures, irregular pyroxene and apatite zoning, and the presence of irregular shaped amphibole patches within pyroxene grains all imply alteration by a pervasive deuteric fluid in both troughs and host syenite. We suggest the following hypothesis for the formation of the troughs. A mafic crystal sheath which grows on the walls of the magma chamber becomes detached and flows downwards. The descending slurry forms a series of regularly spaced streams which erode trough shaped channels into the crystal pile at the base of the chamber. Uneroded crystal material separates the channels. Crystals are deposited from these streams. Successive streams flow over each other to generate stacks of troughs.

Biotites as Indicators of Fluorine Fugacities in Late-Stage Magmatic Fluids: the Gardar Province of South Greenland

The major element chemistry and fluorine contents of biotites from the Gardar alkaline province of South Greenland provide an insight into the F contents of late-stage fluids associated with the magmatism. Biotites were taken from composite intrusions ranging from alkali gabbro to syenites, nepheline syenites and alkali granites. In each complex they show a large range of Fe/ (Fe + Mg) (from 02 to ~1) and exhibit strong Fe-F avoidance. There is considerable variation in Ffor any value of Fe/(Fe + Mg) but for each centre maximum F values plotted against Fe/(Fe + Mg) define a nearly straight line of characteristic slope towards low or zero F at pure annite. Micas in the SiO2 undersaturated centres have higher F contents than those from oversaturated centres. Cl is low (<0-69 wt%) except in the Kungnat intrusion, where it reaches 1-4 wt%. Phase equilibrium and textural considerations suggest that most or all biotites grew subsolidus in a pervasive deuteric fluid. 5i8O values suggest that these fluids were largely magmatic in character and that extensive reactions with envelope fluids did not occur. Metasomatic resetting of F in biotites in early intrusive units in the aureole of later units can be demonstrated. Experimental data of Munoz (Mineralogical Society of America, Reviews in Mineralogy 13, 469-494, 1984) were used to calculate families of curves showing variation in F with Fe/(Fe + Mg) for biotites in equilibrium with fluids of fixed fugacity ratio, i(HF)/i(H2O), at fixed P andT. The resulting curves cut sharply across the maximum fluorine lines observed in the natural examples. As it seems highly unlikely that changes in fluid composition andT, acting together, could produce the observed linear relationship, we conclude that the partitioning of F between fluid and mica in the plutonic environment is not well modelled by the experiments. Possible explanations are short-range order (SRO) of Fe and Mg on octahedral sites in biotite (Mason, Canadian Mineralogist 30, 343-354,1992) or effects resulting from different F speciation in alkaline fluids. If perfect Fe-F avoidance is assumed, Fe-Mg SRO can increase maximum F content. The F levels seen in the silica-saturated centres are broadly consistent with a cooling-rate related control. It is possible that the higher F in biotites in the undersaturated centres reflects different spedation in the fluid rather than higher F contents, with strongly bound SiF/ complexes more common.

Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite

AbstractAlkali feldspar phenocrysts (bulk composition Or75.0Ab24.6An0.4) in the subsolvus Shap granite comprise a fine-scale mixture of subregular pristine crypto- and micro-perthites with altered, micropore-rich feldspar with irregular microstructures. The regular perthites are strain-controlled intergrowths of Albite and/or Periclinetwinned albite exsolution lamellae within tweed orthoclase. The microperthites formed at ⩽ 590°C by heterogeneous nucleation of thin albite films which coarsened to &gt; 1 µm length. Cryptoperthites developed at &lt; 400°C by homogeneous nucleation of sub-µm long platelets between films. Platelets are coherent, but the coarser microperthite lamellae are semi-coherent, with pairs of misfit dislocations sub-regularly spaced along the albite-orthoclase interface. As much as 30% of any one feldspar crystal is turbid, a result of the formation of numerous µm to sub-µm sized micropores during deuteric alteration. In some areas, deuteric fluids gained access to the interior of feldspar crystals by exploiting semi-coherent film lamellae. Albite was selectively dissolved and micropore-rich irregular microcline was reprecipitated in its place. In other parts of the feldspars deuteric recrystallization completely cross-cuts the pristine microtextures and patch perthites have formed. These are coarse, incoherent to semi-coherent intergrowths of irregular microcline (replacing tweed orthoclase) and Albite-twinned albite. The deuteric reactions occurred at &lt; 400°C; the main driving force for dissolution and reprecipitation was decrease in the elastic strain energy at the coherent interfaces of crypto-and micro-perthite lamellae, and the recrystallization of tweed orthoclase to irregular microcline.

MINERALOGY OF NEPHELINE SYENITES FROM THE POÇOS DE CALDAS ALKALINE MASSIF, SE BRAZIL: CHEMISTRY, X-RAY DATA AND MICROTEXTURES OF FELDSPARS

Nepheline syenites (NeS) of the Pocos de Caldas Alkaline Massif constitute mostly small bodies of miaskitic-intermediate or agpaitic rocks. The alkali feldspars of most NeS are highly potassic (OR > 80-85) and are either orthoclase or highly ordered microcline. The microcline shows a multiple that resembles that of chess-board albite. This may appear in isolated turbid spots in otherwise clean monoclinic crystals or occupy the whole grain (incipient, poor, good twinning); microperthites, when present, occur in the twinned areas. In some NeS the K-feldspars are clean, wholly twinned with larger twin lamellae (very good twinning). Secondary albite (tabular albite) is seen in some facies either included in, or bordering, the K-feldspars. In the singlemost important NeS intrusion, the Pedreira NeS, the alkali feldspars are richer in Na, Ca and Sr, very turbid and show a diffuse twinning resembling a cross-hatched pattern. The Xray diffractograms show a mixture of orthoclase and intermediate microcline. Estimated geothermometric temperatures indicate subsolidus reequilibration between feldspar and nepheline, particularly when the Kfeldspars show good or very good twinning. The microtextures of the K-feldspars are the result of a continuous process of recrystallization of non-twinned earlier crystals, controlled by late magmatic fluids. The turbid patches probably represent regions of abundant micropores facilitating fluid circulation. The styles of the K-feldspars may be due to the composition of the fluid phase and of the feldspars. Sodium redistribution by deuteric fluids may have produced the secondary and perthitic albite.

Mineralogy of nepheline syenites from the Poços de Caldas alkaline massif, SE Brazil: chemistry, x-ray data and microtextures of feldspars

Nepheline syenites (NeS) of the Poços de Caldas Alkaline Massif constitute mostly small bodies of miaskitic-intermediate or agpaitic rocks. The alkali feldspars of most NeS are highly potassic (OR &gt; 80-85) and are either orthoclase or highly ordered microcline. The microcline shows a multiple twinning that resembles that of "chess-board" albite. This twinning may appear in isolated turbid spots in otherwise clean monoclinic crystals or occupy the whole grain (incipient, poor, good twinning); microperthites, when present, occur in the twinned areas. In some NeS the K-feldspars are clean, wholly twinned with larger twin lamellae (very good twinning). Secondary albite (tabular albite) is seen in some facies either included in, or bordering, the K-feldspars. In the singlemost important NeS intrusion, the Pedreira NeS, the alkali feldspars are richer in Na, Ca and Sr, very turbid and show a "diffuse twinning" resembling a cross-hatched pattern. The Xray diffractograms show a mixture of orthoclase and intermediate microcline. Estimated geothermometric temperatures indicate subsolidus reequilibration between feldspar and nepheline, particularly when the Kfeldspars show good or very good twinning. The microtextures of the K-feldspars are the result of a continuous process of recrystallization of non-twinned earlier crystals, controlled by late magmatic fluids. The turbid patches probably represent regions of abundant micropores facilitating fluid circulation. The twinning styles of the K-feldspars may be due to the composition of the fluid phase and of the feldspars. Sodium redistribution by deuteric fluids may have produced the secondary and perthitic albite.

Geology and petrogenesis of the Straumsvola nepheline syenite complex, Dronning Maud Land, Antarctica

AbstractThe 170 Ma Straumsvola nepheline syenite complex in western Dronning Maud Land, Antarctica, is located on the eastern edge of the Penck-Jutul Trough, a major tectonic feature which may be a Palaeozoic-Mesozoic rift system. The 5 km diameter pluton consists entirely of nepheline syenite and can be divided into two volumetrically important units: a relatively structureless outer zone which is overlain by a layered zone. The latter exhibits continuous rhythmic alternations of layers containing different proportions of alkali feldspar to amphibole+Na-rich pyroxene+biotite+nepheline throughout its 350 m thickness. The mafic zone is a volumetrically minor unit which unconformably overlies the layered zone and consists almost entirely of mafic minerals and nepheline. The layered zone shows no systematic stratigraphic variation in major or trace composition or mineral chemistry which is interpreted as being due to a combination of migration of intercumulus liquid and action of deuteric fluids. The remarkably constant thickness of successive layers throughout the layered zone suggests that layering resulted from an internally self-regulating process(es). The layering is defined by changing proportions of alkali feldspar to mafic minerals+nepheline which is consistent with layering being caused by differences in nucleation rate during eutectic crystallization. Dykes associated with the complex show wide-ranging compositions and include both over-and undersaturated types. Low σ18O values of the nepheline syenites (mean 5.9%, n = 9) suggest that the magma from which the nepheline syenites crystallized was mantlederived. Peralkaline microgranite dykes have higher σ18O values (mean 7.3%, n = 4) and it is suggested that they are either undersaturated liquids contaminated by siliceous crust, or derived by partial melting of partly fenitized gneiss. Oxygen isotope ratios of the surrounding gneiss indicate that the intrusion of the nepheline syenite did not cause extensive circulation of meteoric or magmatic hydrothermal fluids.

Biotite Equilibria and Fluid Circulation in the Klokken Intrusion

Chemical variation in biotite from the KJokken gabbro-syenite intrusion in the Proterozoic Gardar province in South Greenland has been investigated by electron probe and, for F and Li, ion microprobe. Most mica occurs in small amounts as fringes on ilmenomagnetite or fayalite, rarely as an intercumulus or poikilitic phase. The micas range continuously from Phlog70Ann30 in a gabbro, to Phlog4Ann96 in the most evolved (slightly persilicic and peralkaline) syenite. In the syenites Fe-Mg partitioning between biotite and olivine can be described by a single distribution coefficient, K6 = Xfi X%? /X^-Xf°'~3, suggesting that these reactant phases mix ideally at the reaction T. Experimental data for Fe-Mg exchange via aqueous chloride solutions (Schulien, 1980) imply low T (~ 320 °Q. F was absent in the experiments and may significantly affect the exchange equilibrium. K6 in the gabbros is ~ 1, consistent with equilibrium via a fluid depleted in F because of crystallization of large amounts of amphibole. Al, Mn, and Ti vary regularly throughout the series and can be used as markers of cryptic variation in the layered syenites. (Al + Si): 22 O is always in the range 7-7—7-85. A1/(A1 + Si) decreases from ~O31 in gabbros to 0-25 in the most Fe-rich micas. Li is always 300 °C subsolidus even when texturally intercumulus. Stable isotope data are consistent with the separation and retention of a deuteric fluid during the final stage of magmatic crystallization. Klokken was not generally subject to the pervasive, long-range (in both distance and time) dydrothermal interactions demonstrated in

A review of the compositional variation of amphiboles in alkaline plutonic complexes

Compositional data for amphiboles occurring in alkaline plutonic complexes are reviewed and a standard procedure for plotting these data in an isometric prism is proposed. The main compositional trend found in both oversaturated and undersaturated complexes of either miascitic or peralkaline affinity is referred to as the primary magmatic trend. Amphiboles range in composition from magnesian hastingsitic hornblende and ferro-edenitic hornblende through katophorite to ferro-richterite and arfvedsonite. Individual complexes differ with respect to the amphibole Mg Fe and Si Al ratio and the extent of Na-enrichment. Extensive or limited ranges in the composition of amphiboles may occur in a given complex. A subtrend found only in oversaturated complexes is from ferro-edenitic hornblende to ferro-actinolite. This trend termed the ferro-actinolitic subtrend is found only in low temperature non-peralkaline residua. Some aluminous nepheline syenites and associated alkali gabbros contain amphiboles which range in composition from kaersutite through ferroan pargasitic hornblende to hastingsite. This trend termed the primary miascitic magmatic trend is is one of decreasing Mg Fe , at essentially constant Si Al and Ca content. Na-enrichment does not occur. Amphiboles formed by reactions of preexisting phases with hydrothermal or deuteric fluids are termed the late stage reaction assemblage. Amphibole compositional trends from calcic through sodic-calcic to sodic amphiboles reflect decreasing temperature and oxygen fugacity at or below the QFM oxygen buffer. The compositional trends are of use in determining petrogenetic relationships between apparently consanguineous syenites.

Argon-loss by alkali feldspars

Ar-loss is generally thought to preclude the use of alkali feldspars for dating old rocks by the K-Ar method, and has been thought to occur via structural features such as the boundaries of perthite lamellae or twins. We have done a 40Ar- 39Ar step-heating study of a suite of alkali feldspars from the Klokken syenite intrusion (U-Pb age 1166 ± 4 Ma) which have been fully characterized by transmission electron microscopy. All are low albite-low microcline intergrowths, but some are non-turbid, ‘strain-controlled’ cryptoperthites, while others are turbid, coarse microperthites. Geological and textural evidence suggests that the turbidity developed by fluid-rock interactions during the initial cooling of the pluton, in the stability field of low microcline (<500°C), 10 5 a after crystallization. The finest cryptoperthites have numerous fully coherent perthite lamellar boundaries (lamellar periodicity ~ 40 nm) and twin composition planes. The have retained radiogenic 40Ar and yield total degassing ages as high as 1125 ± 16 Ma and maximum ages on the age spectra of 1162 ± 16 Ma. The coarse turbid microperthites, with patchy intergrowths and twins on the mm-scale, gave total degassing ages as low as 650 Ma (maximum age 750 Ma). Argon leakage occurs least in feldspars with the most lamellar interfaces, but in proportion to the degree of turbidity, which is thought to be caused by large numbers of micropores. Strain-controlled intergrowths in perthites allow the recognition of volumes of alkali feldspar crystals which have not interacted with deuteric fluids. These retain argon and can give reliable K-Ar ages in old rocks. Saddle-shaped age spectra appear to result from the preferential degassing, at 1000°C, of part of the feldspar structure which has lost more of its argon in nature. The laboratory degassing does not occur in a manner analogous to that which has occurred naturally.

Uranium Occurrence in Major Rock Types by Fission-Track Mapping: ABSTRACT

Microscopic occurrence of uranium has been determined in about 50 igneous rocks from various locations, and in a genetically unrelated sandstone from south Texas. Precambrian granites from the Llano uplift of central Texas contain from a few ppm uranium (considered normal) to over 100 ppm on a whole-rock basis. In granite, uranium is concentrated in: (1) accessory minerals including zircon, biotite, allanite, Fe-Ti oxides, and altered sphene, (2) along grain boundaries and in microfractures by precipitation from deuteric fluids, and (3) as point sources (small inclusions) in quartz and feldspars. Tertiary volcanic rocks from the Davis Mountains of west Texas include diverse rock types from basalt to rhyolite. Average uranium contents increase from 1 ppm in basalts to 7 ppm in rhyolites. Concentration occurs: (1) in iron-titanium-oxides, zircon, and rutile, (2) in the fine-grained groundmass as uniform and point-source concentrations, and (3) as late uranium in cavities associated with banded, silica-rich material. Uranium in ore-grade sandstone is concentrated to more than 3%. Specific occurrences include (1) leucoxene and/or anatase, (2) opaline and calcite cements, (3) mud clasts and altered volcanic rock fragments, and (4) in a few samples, as silt-size uranium- and molybdenum-rich spheres. Uranium content is quite low in pyrite, marcasite, and zeolites. End_of_Article - Last_Page 499------------

Magnetite Veins in Diabase of Laurel Hill, New Jersey

A network of magnetite veins occurs in a Triassic diabase pluton at Laurel Hill, near Jersey City, New Jersey. The veins are typically less than three cm thick and occupy a steeply dipping planar joint system. The diabase adjacent to the veins has been altered and leached of most of the iron that was originally present in the ilmeno-magnetite and pyroxene of the rock. The magnetite veins provide field evidence supporting the hypothesis that iron-bearing deuteric fluids existed in the Triassic diabase. It is proposed that the deuteric fluids transferred iron from the wall rock to the joints at Laurel Hill and may also have formed the Cornwall-type magnetite deposits.

Massive sulfide–late diabase relationships, Home mine, Quebec: Genetic and chronological Implications

At the Home mine, Noranda, Quebec, evidence of penetration and apparent replacement of the 'Late Diabase' dikes by massive sulfides formerly led several workers to believe that these dikes were older than the orebodies. However, a glassy selvage of the diabase against a sulfide wall, in contrast with a microcrystalline one against adjoining rhyolite, demonstrates a different rate of cooling against materials of different thermal diffusivity. Also, dissociation of pyrite to magnetite, increase in the monoclinicity of pyrrhotite, copious development of chalcopyrite blebs in sphalerite, and some interesting sulfide-sulfide and sulfide-silicate reactions in the contact zone record unmistakable imprints of thermal metamorphism of the sulfide wall.Microscopic evidence indicates that, an early stage of dissociation of Fe-sulfides to magnetite was followed by pyritization of pyrrhotite and ferromagnesian silicates, and migration of chalcopyrite along and into the contact zone. These suggest a persistent pressure gradient towards the 'hot-wall' side of the reaction zone, created presumably by the fracturing of the chilled edge and by early dissociation of pyrite.Uninterrupted extensions of tabular, 'vein-like', chalcopyrite bodies from the massive sulfides protrude well into the diabase. Absence of chilling of the diabase along the walls and terminations of these protuberances, and the deformed and recrystallized fabric of these bodies point to their late emplacement by plastic flowage along post-intrusion shear planes. A similar mode of origin is proposed for the very small sulfide veins in 'D' dikelets because the two types of veins are similar.Sulfide patches within the diabase wall, with intensive but localized wall-rock alteration, retain evidence of more thorough dissociation of iron sulfides and other high-temperature mineralogical reconstitution. These masses are therefore inferred to represent clusters of detached or semi-detached portions of the sulfide wall that lost their identity due to the action of deuteric fluid. The fluid, enriched in sulfur, accounted for pyritization and other replacement and alteration features in the surrounding diabase that impart the deceptive appearance of an encroaching sulfide front.Galena-marcasite veinlets on either side of the contact in places indicate a much later solution activity along a potentially weak zone.

Formation of aegirine‐augite and arfvedsonite by reaction

On the northern border of the granite mass of Ear Mountain, Seward Peninsula, Alaska, a thin sill of alaskitic composition has intruded a diopside‐bearing metamorphosed sediment. The diopside has been taken up by the sill, and has reacted with it to form aegirine‐augite. The aegirine‐augite has been preserved unchanged in the fine‐grained margin of the sill, but in the coarser part it has been attacked, presumably by deuteric fluids, to form arfvedsonite.