Plagioclase Component Research Articles

The Fluid-absent Partial Melting of a Zoisite-bearing Quartz Eclogite from 1middle dot0 to 3middle dot2 GPa; Implications for Melting in Thickened Continental Crust and for Subduction-zone Processes

Fluid-absent melting experiments on a zoisiteand phengite-bearing INTRODUCTION eclogite (omphacite, garnet, quartz, kyanite, zoisite, phengite and Zoisite or clinozoisite is present in many high-pressure rutile) were performed to constrain the melting relations of these eclogites, and many occurrences are known from the hydrous phases in natural assemblages, as well as the melt and Scandinavian Caledonides (e.g. Holsnoy: Austrheim & mineral compositions produced by their breakdown. From 1·0 to Mork, 1988; Jamtveit et al., 1990; Western gneiss region: 3·2 GPa the solidus slopes positively from 1·5 GPa at 850°C to Griffin et al., 1985; Seve Nappe: Kullerud et al., 1990). 2·7 GPa at 1025°C, but bends back at higher pressures to 975°C Indeed, most eclogites worldwide contain minor amounts at 3·2 GPa. The melt fraction is always low and the melt of zoisite or other hydrous phases, according to the compositions always felsic and become increasingly so with increasing compilations of Smith (1988) and Carswell (1990). During pressure. The normative Ab–An–Or compositions of the initial prograde eclogitization, zoisite forms by breakdown of melts vary from tonalites at 1·0 GPa to tonalite–trondhjemites at the anorthite component of plagioclase in the presence 1·5 GPa, adamellites at 2·1 and 2·7 GPa, and to true granites of a hydrous fluid phase. The origin of these hydrous at 3·2 GPa. At pressures 2·5 GPa zoisite and phengite break fluids is controversial. One possibility is that they are down more or less simultaneously. At 3·2 GPa and 1000°C zoisite introduced from below during continental collision, for is unreacted whereas phengite is absent, so that the first formed melt example, if wet continental sedimentary rocks are deeply at these conditions is granitic. Our experiments show that if subducted. During subduction of oceanic crust zoisite sufficiently high temperatures (of the order of 1000°C) are attained, forms by prograde metamorphism of hydrothermally zoisiteand phengite-bearing eclogites can produce small fractions altered oceanic crust, which in its upper levels contains of silicic melts of a wide range of compositions. These melts are low-temperature hydrous Ca-rich phases (e.g. Poli & rich in water and, probably, in Sr and other incompatible elements, Schmidt, 1997). At the gabbro to dyke transition zone so that they can act as metasomatic agents in the mantle wedge. the temperature and pressure are high enough to allow

Incomplete reaction of plagioclase in experimental dehydration melting of amphibolite

Dehydration melting of a hornblende‐plagioclase mixture of amphibolitic composition was investigated at 1000°C and at 800 MPa and 1200 MPa. At 1200 MPa the reaction products are garnet, clinopyroxene, melt and relatively Ab‐rich plagioclase (An47). At 800 MPa the products are orthopyroxene, clinopyroxene, magnetite, amphibole (pargasite) and An‐rich plagioclase (An75). The melts are rich in plagioclase components (especially in Ab) and, when compared to tonalites, relatively poor in silica. The grainsize of the starting materials was ≤⃒5 μm in the 800 MPa and ≤⃒10 μm in the 1200 MPa runs. All run products show unchanged plagioclase cores, which are the remnants of a very sluggish reaction assumed to be controlled by dissolution/precipitation processes at the plagioclase grain boundaries. The results indicate that only local equilibrium could have been obtained in recent investigations on dehydration melting experiments in plagioclase‐bearing systems. The results also suggest that plagioclase compositions once formed may persist for a very long time, even in hot magma chambers, if the prevailing water activity is low.

Magmatic inclusions and felsic clasts in the Dar al Gani 319 polymict ureilite

Abstract— Magmatic inclusions occur in type II ureilite clasts (olivine‐orthopyroxene‐augite assemblages with essentially no carbon) and in a large isolated plagioclase clast in the Dar al Gani (DaG) 319 polymict ureilite. Type I ureilite clasts (olivine‐pigeonite assemblages with carbon), as well as other lithic and mineral clasts in this meteorite, are described in Ikeda et al.(2000). The magmatic inclusions in the type II ureilite clasts consist mainly of magnesian augite and glass. They metastably crystallized euhedral pyroxenes, resulting in feldspar component‐enriched glass. On the other hand, the magmatic inclusions in the large plagioclase clast consist mainly of pyroxene and plagioclase, with a mesostasis. They crystallized with a composition along the cotectic line between the pyroxene and plagioclase liquidus fields.DaG 319 also contains felsic lithic clasts that represent various types of igneous lithologies. These are the rare components not found in the common monomict ureilites. Porphyritic felsic clasts, the main type, contain phenocrysts of plagioclase and pyroxene, and their groundmass consists mainly of plagioclase, pyroxene, and minor phosphate, ilmenite, chromite, and/or glass. Crystallization of these porphyritic clasts took place along the cotectic line between the pyroxene and plagioclase fields. Pilotaxitic felsic clasts crystallized plagioclase laths and minor interstitial pyroxene under metastable conditions, and the mesostasis is extremely enriched in plagioclase component in spite of the ubiquitous crystallization of plagioclase laths in the clasts. We suggest that there are two crystallization trends, pyroxene‐metal and pyroxene‐plagioclase trends, for the magmatic inclusions and felsic lithic clasts in DaG 319. The pyroxene‐metal crystallization trend corresponds to the magmatic inclusions in the type II ureilite clasts and the pilotaxitic felsic clasts, where crystallization took place under reducing and metastable conditions, suppressing precipitation of plagioclase. The pyroxene‐plagioclase crystallization trend corresponds to the magmatic inclusions in the isolated plagioclase clast and the porphyritic felsic clasts. This trend developed under oxidizing conditions in magma chambers within the ureilite parent body.The felsic clasts may have formed mainly from albite component‐rich silicate melts produced by fractional partial melting of chondritic precursors. The common monomict ureilites, type I ureilites, may have formed by the fractional partial melting of alkali‐bearing chondritic precursors. However, type II ureilites may have formed as cumulates from a basaltic melt.

Paleoclimatology and provenance of the glaciogenic Gowganda Formation (Paleoproterozoic), Ontario, Canada: A chemostratigraphic approach

The Gowganda Formation (dated as about 2.3 Ga) is widely (but not universally) interpreted as glaciogenic, and our chemostratigraphic study supports such an interpretation. The bulk compositions of diamictite matrix materials and associated argillites are considered to have resulted from the mixture of two detrital components; one is glacial flour, similar in composition to the unweathered Archean upper crust, whereas the other is chemically weathered detritus resembling average Proterozoic shale in composition. The first component has a chemical index of alteration (CIA) value of ∼50, reflecting the abundance of feldspar and dearth of aluminous clay minerals. The second component has a CIA value of ∼70 and was produced primarily in weathering profiles by chemical weathering, typical of temperate climatic regimes. Diamictite matrix materials (92 samples) have an average CIA value of 57. The argillites have higher values; the average is 62 (97 samples). The lower CIA values of the diamictites reflect a higher proportion of glacial flour, suggesting a stronger influence of frigid climatic conditions than that which prevailed during deposition of the argillites. Systematic trends in CIA values are developed across diamictite-argillite boundaries and within a thick argillite between two major diamictite units. These values reflect climatic variations, with maximum amelioration during deposition of the middle section of the argillite. Rare earth element data and Th/Sc and Ti/Al ratios from the diamictite matrix materials all suggest a provenance that included a large supracrustal component (∼45%), together with lesser amounts of tonalite-trondhjemite-granite and granite. The proportion of supracrustal materials such as volcanic rocks and shales may have been exaggerated by selective comminution during glacial transport. The matrix materials of the Gowganda diamictites are severely depleted in Ca and proportionally enriched in Na, relative to the source. The anorthitic component of plagioclase has been selectively replaced by an albitic component during diagenesis and metamorphism. A chemostratigraphic approach provides high-resolution data for reconstruction of paleoclimatic conditions and valuable information concerning provenance and metasomatic processes.

Dissolution origin and syn-entrapment compositional change of melt inclusion in plagioclase

In the system Di–Ab–An–H 2O, melt inclusions in plagioclase were reproducibly formed in a series of partial dissolution experiments, and the compositional differences from the surrounding matrix melt were evaluated. At 200 MPa and 1050−−1150°C, a seed crystal of An 59 was partially dissolved into the matrix melt, which was in equilibrium with more calcic plagioclase than the seed. The reaction proceeded inward from the crystal surface by a dissolution and recrystallization process to form a reaction zone composed of melt channels and recrystallized calcic plagioclase. The melt channels were choked (necked) off by the recrystallized plagioclase at the crystal surface to create melt inclusions. The experimentally formed inclusions, having square and zigzag shapes up to a few tens of micrometers in size, are similar in texture to some common types of natural melt inclusion. Since the partial dissolution of phenocrysts can occur during various magma mixing processes, it should be taken into consideration as one of the inclusion formation mechanisms when trying to interpret information from natural melt inclusions, especially for those in hydrous arc magmas. The chemical composition of the reproduced melt inclusions deviates from the matrix melt basically along the plagioclase liquidus isotherm. The molar ratio of Ca/(Ca + Na) and the MgO concentration in the synthesized melt inclusion are up to 17 and 61% (relative) lower than those in the matrix melt, respectively. This compositional difference is caused by a combination of factors: (1) supersaturation with dissolved plagioclase component; (2) insufficient diffusive homogenization in the melt channels due to rapid dissolution of plagioclase; and (3) the choking-off effect. On the basis of the experimental results, a model for diffusion in a growing melt channel in dissolving plagioclase was constructed which revealed that the concentration of incompatible elements with low diffusivities can be significantly lower in the melt inclusions than that in the matrix melt. These results demonstrate the importance of petrography for interpretation of inclusion data and of more experimental studies on melt inclusion origin.

Magma evolution observed in the Matsuura basalts in northwest Kyushu, Japan: An example of high-pressure open system fractional crystallization in a refilled magma chamber near the crust–mantle boundary

Formation of an extensive shallow water sedimentary basin before the voluminous Matsuura basalt volcanism in the back arc volcanic belt in northwest Kyushu, Japan, seems to indicate the lower crust erosion due to the upwelling of the mantle, which results in the decompression melting of mantle to form basaltic magma. The Matsuura basalts, 10 to 1 Ma, range in composition from low alkali tholeiitic basalt through alkali olivine basalt to basaltic andesite. Their compositions show a wide variation in MgO from 11.6 wt.% to 2.8 wt.%. Their chemical characteristics are: (1) evolution across the alkali–subalkali boundary, (2) crescent form of the basalt distribution on a SiO 2 vs. MgO diagram, (3) enrichment in normative plagioclase component and a diminution in normative clinopyroxene component with the magma evolution, (4) abrupt termination of this plagioclase-enrichment trend at MgO=3.3 wt.%, (5) marked enrichment in SiO 2 in aphyric basalts with MgO<5.5 wt.%, (6) a limit of magma evolution, and (7) magma mixing. Mineral crystallization sequence is found to be olivine, olivine+clinopyroxene, and then olivine+clinopyroxene+plagioclase. Iron oxide crystallization starts in iron-enriched magmas a little before the joining of plagioclase in crystallization, which results in the rapid enrichment in SiO 2. The chemical characteristics of the Matsuura basalts are well accounted for by assuming open system fractional crystallization in a refilled magma chamber near the crust–mantle boundary. The chamber may have contained small amounts of water. It seems probable that the open system fractional crystallization operative under high pressure conditions can transform low alkali tholeiitic primitive magmas through weakly nepheline-normative basalt compositions finally to the convergent composition of hypersthene-normative basaltic andesite.

The Role of Disseminated Calcite in Rates of Calcium Release and CO2 Consumption During the Weathering of Granitoid Rocks

Although Ca and Na reside predominately in plagioclase feldspar in granitoid rocks, recent studies have demonstrated that excess Ca often exits in surface and ground waters relative to plagioclase stochiometry. Explanations advanced to explain the Ca excess have included selective loss of Ca in soils due to watershed acidification (Hyman et al., 1998), selective weathering of the anorthite component of plagioclase (Clayton 1986), and the rapid weathering of trace mineral phases such as calcite (Mast and Drever 1990). Chemical hydrolysis of Ca-silicate rocks is recognized as impacting long term atmospheric CO2 consumption and global climate through silicate hydrolysis reactions (Bemer and Caldeira 1997). In contrast, carbonate mineral dissolution does not have a corresponding impact because the CO2 consumed in weathering is reintroduced back into the atmosphere by precipitation of carbonates in the ocean. Thus, the solute Ca/Na ratio in watersheds and rivers, reflecting proportions of silicate and carbonate weathering, is important in understanding long term global CO2 balances.

REE-bearing minerals in the albitites of central Sardinia, Italy

AbstractRecent studies on albitite rocks located in the granodiorite complex of Central Sardinia have revealed that epidote has a widespread occurrence as a light rare-earth element (LREE)-bearing accessory common phase. Titanite has been recorded as a heavy rare earth element (HREE)-bearing mineral. The Hercynian granodiorite complex of Central Sardinia is composed chiefly of quartz, Ca-plagioclase, K-feldspar and biotite and of a wide variety of secondary assemblages, mainly allanite, titanite and zircon. Albitic plagioclase and quartz are the main mineral components of the albitites. Additional minerals include, besides allanite and epidote, a more calcic-plagioclase (oligoclase), K-feldspar, chlorite, titanite and more rarely muscovite. The mineral assemblages and REE-bearing minerals of albitites were analysed by wavelength dispersive spectrometry (WDS). Chemical data suggest that there is a near complete solid-solution between epidote and allanite whereas little variations in HREE of titanites were detected. In epidote-group minerals a pronounced zoning in REE was observed while titanite was recorded unzoned. Textural relations were studied by SEM to distinguish primary from secondary epidotes. Chemical criteria to recognize magmatic from alteration epidotes were also applied. The alteration epidotes mainly occur and generally originate from plagioclase alteration and from leaching of magmatic allanite. Comparison of textures using both the SEM technique and EPMA data showed that the characteristic ‘patchy zoning’, observed in epidotes, corresponds with different amounts of REE in these minerals.The schematic model proposed for the epidote-forming reactions during the metasomatic processes that affected the granodiorites involves: (i) the instability of the anorthitic component of plagioclase; (ii) the simultaneous formation of albite; (iii) the leaching of the magmatic allanite with a redistribution of REE in the epidotes of the albitites.

The metamorphosed molybdenum vein-type deposit of the Alpeinerscharte, Tyrol (Austria) and its relation to Variscan granitoids

The molybdenite deposit of the Alpeinerscharte (Austria) is situated in Variscan greenschist- to amphibolite-facies metamorphosed granodiorites and granites of the western Tauern Window. These granitoids represent strongly fractionated calc-alkaline I-type magmas with minor S-type components and reveal post-orogenic affinities. Molybdenum contents (average 4.3 ppm) are slightly above the general background of average granites. Molybdenite mineralization is restricted to narrow quartz veins and quartz vein selvages which are presently composed of biotite and (almandine-grossular) garnet. These selvages show geochemical features typical of intermediate argillic alteration in a hydrothermal system postdating granite intrusion: instability of plagioclase causes removal of Na, Ba, Sr, Pb and Eu, while K and Ca remain nearly constant. Rare earth elements (apart from Eu) and metals are extremely enriched. Application of Fe-Mg exchange (garnet-biotite) and oxygen isotope (quartz-garnet, quartz-plagioclase) geothermometers to vein selvage mineral assemblages reveals temperatures of the late-Alpine (35–55 Ma) metamorphic overprint (∼540°C, 7–10 kbar). Leucocratic rocks composed of mainly orthoclase and plagioclase are occasionally spatially related to molybdenite-bearing veins; they are interpreted as episyenites formed by hydrothermal alteration of the host granite. This episyenitic alteration is characterized by a mass loss of ca. 30%, relative enrichment of plagioclase components, extreme depletion of Si, and minor depletion of Fe, Zn, Cu and Mo.

Melt-Peridotite Interactions in Shallow Mantle at the East Pacific Rise: Evidence from ODP Site 895 (Hess Deep)

AbstractOcean Drilling Program (ODP) Leg 147 recently drilled at Site 895 in Hess Deep (eastern Equatorial Pacific), where a structurally dissected section of the East Pacific Rise (EPR) is preserved, and intersected the mantle-crust transition zone of a fast-spreading centre for the first time. The core from Hole 895D (latitude 2°16.635′N, longitude 101°26.777′W) revealed that harzburgite is predominant over dunite in the top section of the Hole, but the reverse relationship is found lower in the section where dunite is closely associated with gabbroic rocks (gabbro and troctolite). Texture, mineralogy and mineral chemistry suggest a two-stage evolution for harzburgite preserved at the transition zone. Harzburgite with a porphyroclastic texture was produced by partial melting of peridotite to, or beyond the clinopyroxene-out phase boundary before or during asthenospheric (&gt;1000°C) flow, which suggests a higher degree of mantle melting than normally expected below mid-ocean ridges. Subsequently, basaltic melt(s) interacted with this refractory harzburgite (olivine + orthopyroxene + spinel), which resulted in dissolution of orthopyroxene, re-equilibration and formation of olivine and spinel, and formation of clinopyroxene ± plagioclase, this is manifested as a progressive conversion of harzburgite to gabbroic rock through an intermediate dunite. At low melt/peridotite ratios, harzburgite was refertilised as the plagioclase component of the melt completely reacted with the peridotite matrix to produce clinopyroxene-spinel intergrowths and Al enrichment in ferromagnesian minerals. At high ratios, orthopyroxene completely dissolved incongruently, plagioclase appeared, and spinel was partially to completely resorbed; this produced olivine-bearing and olivine-free gabbroic rocks. Residual minerals in peridotites adjacent to gabbroic zones were enriched in Fe and Ti and depleted in Al.

Experimental study of a jotunite (hypersthene monzodiorite): constraints on the parent magma composition and crystallization conditions (P, T, f O 2) of the Bjerkreim-Sokndal layered intrusion (Norway)

The Bjerkreim-Sokndal layered intrusion is part of the Rogaland anorthosite Province of southern Norway and is made of cumulates of the anorthositemangerite-charnockite suite. This study presents experimental phase equilibrium data for one of the finegrained jotunite (Tjorn locality) occurring along its northwestern lobe. These experimental data show that a jotunitic liquid similar in composition to the Tjorn jotunite, but slightly more magnesian and with a higher plagioclase component is the likely parent of macrocyclic units (MCU)III and IV of the intrusion. The limit of the olivine stability field in the experimentally determined phase diagram as well as comparison of the Al2O3 content of low-Ca pyroxenes from experiments and cumulates (≈1.5%) yields a pressure of emplacement ≤5 kbar. Experimentally determined Fe-Ti oxide equilibria compared to the order of cumulus arrival in the intrusion show that the oxygen fugacity was close to FMQ (fayalite-magnetite-quartz) during the early crystallization. It subsequently decreased relative to this buffer when magnetite disappeared from the cumulus assemblage and then increased until the reentry of this mineral. Calculated densities of experimental liquids show a density increase with fractionation at 7, 10 and 13 kbar due to the predominance of plagioclase in the crystallizing assemblage. At 5 kbar and 1 atm (FMQ-1), where plagioclase is the liquidus phase, density first increases and then drops when olivine (5 kbar) or olivine+ilmenite (1 atm: FMQ-1) precipitate. At 1 atm and NNO (nickel-nickel oxide), the presence of both magnetite and ilmenite as near liquidus phases induces a density decrease. In the Bjerkreim magma chamber, oxides are early cumulus phases and liquid density is then supposed to have decreased during fractionation. This density path implies that new influxes of magma emplaced in the chamber were both hotter and denser than the resident magma. The density contrast inferred between plagioclase and the parent magma shows that this mineral was not able to sink in the magma, suggesting anin situ crystallization process.

Geochemical and hydrologic controls on the composition of surface water in a high‐elevation basin. Sierra Nevada, California

Hydrologic, mineralogic, and soil data are used to determine the sources and geochemical controls on the composition of surface water in the Emerald Lake watershed (ELW), a high‐altitude basin located in the southern Sierra Nevada. The solute composition of stream waters at the ELW can be divided into three periods: snowpack runoff, a transition period in summer as snowpack runoff decreases and little precipitation occurs, and a low‐flow period from late summer through winter. Each period has different geochemical controls on the solute composition of surface waters. During snowpack runoff ∼50% of stream flow was from direct surface runoff and ∼50% of stream flow was return flow from subsurface reservoirs. Hydrologic residence time of subsurface water at maximum snowpack runoff was measured directly with a 6LiBr tracer and varied from 9 to 20 h. Three independent measurements show that the acidity in snowpack runoff was neutralized by cation exchange in soils and talus. Discharge from soil reservoirs was the primary source of stream flow during the summer transition period when the composition of stream flow was congruent with the stoichiometry of plagioclase weathering. Processes occurring below the soil zone exerted the dominant geochemical controls on the composition of stream waters during the period of low flow, with preferential weathering of the anorthite component of plagioclase in subsurface rock and further weathering of kaolinite to gibbsite.

Fluid absent melting of a layered crustal protolith: implications for the generation of anatectic granites

We report the result of H2O-undersaturated melting experiments on charges consisting of a layer of powdered sillimanite-bearing metapelite (HQ36) and a layer of powdered tonalitic gneiss (AGC150). Experiments were conducted at 10 kbar at 900°, 925° and 950°C. When run alone, the pelite yielded ∼40 vol% strongly peraluminous granitic melt at 900°C while the tonalite produced only ∼5 vol% weakly peraluminous granitic melt. At 950°C, the pelite and the tonalite yielded ∼50 vol% and ∼7 vol% granitic melt, respectively. When run side by side, the abundance of melt in the tonalite was ∼10 times higher at all temperatures than when it was run alone. In the pelite, the melt abundance increased by ∼25 vol%. When run alone, biotite dehydration-melting in the tonalite yielded orthopyroxene and garnet in addition to granitic melt. When run side by side only garnet was produced in addition to granitic melt. Experiments of relatively short duration, however, also contained Al-rich orthopyroxene. We suggest that the large increase in melt fraction in the tonalite is mainly a result of increased activity of Al2O3 in the melt, which lowers the temperature of the biotite dehydration-melting reaction. In the pelite, the increase in the abundance of melt is caused by transport of plagioclase component in the melt from the tonalite-layer to the pelite-layer. This has the effect of changing the bulk composition of this layer in the direction of “minimum-temperature” granitic liquids. Our results show that rocks which are poor melt-producers on their own can become very fertile if they occur in contact with rocks that contain components that destabilize the hydrous phase(s) and facilitate dehydration-melting. Because of this effect, the continental crust may have an even greater potential for granitoid melt production than previously thought. Our results also suggest that many anatectic granites most likely contain contributions from two or more different source rocks, which will be reflected in their isotopic and geochemical compositions.

Albitization of Detrital Plagioclase in Triassic Reservoir Sandstones from the Snorre Field, Norwegian North Sea

ABSTRACT Detrital plagioclase (An12-An28) in Triassic reservoir sandstones of the Lunde Formation in the Snorre Field, offshore Norway has been partially to completely albitized during burial diagenesis (depths of about 2,500-3,000 m; 75-100°C present temperature). Calcite (CaCO3 > 99 mole %) and kaolinite were formed as by-products of plagioclase albitization. Sodium-rich detrital plagioclase (An2-An11) are usually unalbitized. Detrital K-feldspar is fresh or, rarely, partly albitized. Differences in albitization behavior of detrital plagioclase and K-feldspar are shown to be related to the chemistry of formation waters, reactivity of kaolinite, and temperature. The diagenetically formed albite pseudomorphs are very pure (Ab > 9 ), well-ordered low-albite, dark-luminescent, vacuolized, mostly untwinned, and comprised of numerous euhedral albite crystals. Some of the microporosity within the albite pseudomorphs is caused by volume reduction during albitization of the detrital plagioclase. The amount of albite formed in the pseudomorphs is related to the molar proportion of albite component in the detrital unstable Ca-rich plagioclase. The microporosity formed during albitization is proportional to the anorthite component, which is believed to have dissolved and resulted primarily in the formation of calcite and kaolinite. The albitization of detrital plagioclase in the studied sandstones can thus be explained without assuming the consumption of significant amounts of Na+ from pore solutions.

Melting of plagioclase-quartz assemblages at 2 kbar water pressure

The melting of plagioclase and quartz has been investigated at PH2O =2kb. A single crystal of plagioclase was surrounded by quartz powder and water. A reaction rim consisting of glass and of An-rich plagioclase developed around unchanged starting plagioclase. Microprobe determinations of melt and coexisting plagioclase compositions reveal a strong fractionation of plagioclase components between melt and new plagioclase. For example at 850° C the approximate XAn of melt is 0.3 and that of plagioclase is 0.8. The temperature interval between beginning of melting and complete melting of cotectic compositions is 100° C or more for quartz-plagioclase mixtures with plagioclases between An 40 and An 75. In comparison to the system Ab-An-H2O the plagioclase melting loop is somewhat wider in the investigated system Qz-Ab-An-H2O but the temperature interval is much smaller (100° C) than in the system Ab-An-H2O (200° C). The solidus data indicated by the new plagioclase compositions are practically identical with those observed in beginning of melting experiments. The results show that fractionation of plagioclase components between partial melts and restite plagioclase can be more pronounced in multi-component rocks than in the pure plagioclase system. This finding is important for the development of albite rich rocks from more basic compositions.

Experimental reactions of amphibolite with boron-bearing aqueous fluids at 200 MPa: implications for tourmaline stability and partial melting in mafic rocks

Reactions between hornblende-plagioclase amphibolite and acidic and alkaline B-bearing aqueous fluids have been investigated by experiments at 475°–600° C and 200 MPa. At 600° C, hornblende+calcic plagioclase react to form tourmaline+danburite+clinopyroxene+quartz in acidic fluids containing ≥0.5–1.0 wt% B2O3.Tourmaline is precipitated directly from acidic fluids, and the reaction is driven by neutralization of fluids by Na±Ca derived from the breakdown of reactant solids. The concentration of B2O3 in fluids needed to stabilize tourmaline increases as pH increases (above approximately 6.0), and tourmaline is unstable in alkaline fluids (pH > approximately 6.5–7.0) regardless of B concentration. In addition to acid-base relations, tourmaline stability is favored by comparatively higher activity coefficients for Al species in acidic fluids. The concentrations of Al and Si in fluid increase with alkalinity, with the eventual production of felsic borosilicate melts through partial melting of the plagioclase component of the amphibolite. In seeded experiments, tourmaline also contributes components to melt. Partial melting is evident in the range 500°–525° C at 200 MPa in experiments with ≥8wt% B2O3 in fluid as Na2B4O7. The experimental results are applied primarily to metasomatic reactions between mafic rocks and borate fluids derived from granitic magmas, but tourmaline stability and partial melting in mafic regional metamorphic systems are also discussed briefly.

Mineral Reactions in Altered Sediments From the California State 2–14 Well: Variations in the Modal Mineralogy, Mineral Chemistry and Bulk Composition of the Salton Sea Scientific Drilling Project Core

X ray diffraction‐reference intensity method (XRD‐RIM) modes, whole rock chemistry, and mineral chemistry were determined on 36 samples from the Salton Sea Scientific Drilling Project (SSSDP) core. These samples display a wide variation in grain size (shale to sandstone), bulk composition (SiO2 = 36.2–81.2 wt %; Al2O3 = 5.1–17.1 wt %; CaO = 1.2–21.1 wt % and (total Fe as Fe2O3) Fe2O3/(Fe2O3 + MgO) wt ratio = 0.48–0.98) and mineral assemblage. Seven mineral assemblages have been identified in this sample suite: (1) chlorite + calcite + quartz + plagioclase ± K‐feldspar ± illite (476–2559 m), (2) chlorite + quartz +plagioclase + K‐feldspar (918.7–1984.3 m), (3) chlorite + epidote + quartz ± plagioclase ± calcite ± Kfeldspar ± illite ± magnetite (905.6–2954.8 m), (4) chlorite + biotite ± calcite + quartz + plagioclase + K‐feldspar (2700.0–2819.2 m), (5) epidote + quartz ± plagioclase (1424.2–2954.8 m), (6) epidote + actinolite + quartz ± biotite ± chlorite ± plagioclase ± K‐feldspar ± anhydrite (2880.2–3021.2 m), and (7) epidote + clinopyroxene + actinolite + quartz (2484.7 m). These assemblages define three metamorphic zones in the SSSDP well: calcite‐chlorite zone, biotite zone, and actinolite zone. Accompanying these transitions in mineral assemblages are changes in the Si‐Al ordering in K‐feldspar and an increase in the anorthite component in plagioclase. Fe/(Fe+Al) in epidote shows very little systematic variation downhole (in the sediments) but can be highly zoned and shows considerable systematic variation between vein systems and sedimentary pore space. Quantitative modal mineralogy and whole rock chemistry can be utilized in determining mineral reactions and their importance and in evaluating the evolution of mineral assemblages. For example, epidote‐forming reactions precede actinolite‐epidote‐ and biotite‐epidote‐forming reactions in numerous bulk compositions and result in the formation of high modal abundances of epidote. The production of fluids with high XCO2 from epidote‐, actinolite‐, biotite‐, and diopside‐forming reactions in rocks in equilibrium with fluids with CO2 &lt; 0.1 indicates extensive interaction between fluid and the shales. This is also indicated by the leaching of Cu, Zn, and Mn and the enrichment of Sr. Numerous elements, however, do not show evidence of exchange between sediments and fluid.

Laumontite mineralization in rocks exposed north of San Andreas Fault at Cajon Pass, southern California

Laumontite of hydrothermal origin occurs in veins and as a replacement of the anorthite component of plagioclase in granitic and gneissic basement and overlying arkosic sandstone and conglomerate of the San Francisquito(?), Vaqueros and Cajon formations exposed in Cajon Pass north of the San Andreas fault. Accompanying alteration includes exsolution of albite from plagioclase, conversion of orthoclase to microcline and perthite, and oxidation of iron in biotite and magnetite to form a disseminated hematite stain. These changes are pervasive within 1 km of the fault but diminish to no alteration at 4 km. Granitic rocks close to the fault are crushed to highly fractured and contain a mosaic of laumontite veins. Veins occur along joints and minor faults in sedimentary rocks. Most veins are nearly vertical and trend northwest at a more northerly angle than the San Andreas. This mineralization is attributed to deep circulation of ground water in fractured and frictionally heated rocks along the San Andreas fault.

Chrome spinels from the Ōura layered igneous complex, central Japan

Chrome spinel is disseminated as a minor cumulus phase in olivine-rich ultramafic cumulates in the Ōura igneous complex, a series of layered mafic and ultramafic plutonic rocks. The Al 2O 3 content of the spinel increased markedly with proceeding crystallization of the magma. The Al spinel A magma distribution coefficient seems to have increased with the crystallization of chrome spinel. In view of experimental results obtained in the system forsterite-anorthite-diopside-quartz with MgCr 2O 4 spinel, the increase in the distribution coefficient may be interpreted in terms of enrichment of plagioclase component in the magma by olivine fractionation. The Cr (Cr + Al) ratio of late-separated chrome spinel is presumably an indicator of the chemical conditions of parental magma: the ratio decreases with increasing the plagioclase component content of magma on the normative mineral diagram plagioclase-clinopyroxene-orthopyroxene. The presence of highly aluminous chrome spinel in the Ōura igneous complex suggests that the magma was enriched in plagioclase and clinopyroxene components and depleted in orthopyroxene component. It was apparently similar to abyssal tholeiite magma and markedly different from the Great “Dyke” and Bushveld magmas which were rich in orthopyroxene component.

The subsolidus segregation of layer‐parallel quartz‐feldspar veins in greenschist to upper amphibolite facies metasediments

Abstract Layer‐parallel (i.e. parallel to foliation or bedding) vein formation in the graywackes and pelites of the Quetico Metasedimentary Belt occurred during synchronous prograde metamorphism and regional (D2) compression. In a traverse across metasediments which change in metamorphic grade from greenschist to upper amphibolite (migmatite) facies, layer‐parallel veins show the following trends: (1) an increase in thickness and internal complexity, the latter due to successive boudinage; (2) low‐grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks, but at higher grades other sites are also used and (3) a systematic increase in plagioclase/quartz ratio in the veins towards higher grade, adjacent mafic selvedges first exhibit quartz depletion then, in the amphibolite facies, plagioclase depletion. Mineralogical zoning is often preserved in a single vein, older parts are more quartz‐rich than younger.Mass balance calculations and whole‐rock geochemistry based on veins, mafic selvedges and country rock are consistent with a closedsystem subsolidus segregation origin. The layerparallel veins are syntectonic, and migration of the mobile components required to form their mineralogy is a stress‐induced mass transfer. The source of these components appears to be dominantly pressure solution of the same minerals in the host rocks, although metamorphic reactions may also have contributed. Veins nucleated first at those sites where initial sedimentary heterogeneites, such as fine‐scale graded bedding, provided gradients of normal stress across grain boundaries, and hence of chemical potential, necessary to drive the subsolidus segregation process. The earliest veins are thus parallel to bedding. Later, nucleation of the veins could also occur along more randomly distributed sites within the metasediments, and these veins grew parallel to the schistosity rather than bedding, if the two were distinct. Once formed, the veins themselves, which are more competent than the surrounding rock, provide the stress heterogeneity required for their further growth. The increasing plagioclase/quartz ratio in the veins may be due to a temperature dependent increase in plagioclase component mobility relative to quartz. Alternatively, the increasing transfer distances for silica, resulting from prior quartz depletion in the inner parts of the mafic selvedge, may increase the relative mobility of plagioclase component.