Secondary Plagioclase Research Articles

Nitrogen enrichments in sheeted dikes and gabbros from DSDP/ODP/IODP Hole 504B and 1256D: Insights into nitrogen recycling in Central America and global subduction zones

The fate of subducted crustal nitrogen (N) in modern-style subduction zones remains hotly debated from nearly complete return to the surface via arc volcanoes to net ingassing into the deep mantle beyond the sub-arc depth. One of the major obstacles for this controversy is the lack of constraint on the input N flux from altered oceanic crust (AOC). While notable N enrichment by low-temperature hydrothermal alteration has been widely reported in the upper volcanic section of AOC, N behavior during high-temperature alteration of the underlying intrusive oceanic crust (i.e., sheeted dikes and gabbros) remains poorly understood. Here we examined the bulk-rock N concentrations and isotopic compositions of the AOC (with a main focus on sheeted dikes and gabbros, yet including basalts from sections that have not been examined before) from ODP/IODP Hole 1256D and DSDP/ODP Hole 504B, which are two reference sites for subducting material into the warm Central America (CA) subduction zone. The results show that the intrusive sections have comparable N concentrations and isotopic compositions to their overlying upper volcanic rocks at 1256D (sheeted dikes: 16.4±7.2 ppm and −1.3±1.5‰; gabbros: 11.3±5.6 ppm and +1.3±1.0‰; basalts: 11.8±3.8 ppm and +0.9±1.7‰) and 504B (sheeted dikes: 7.6±2.1 ppm and +2.1±1.7‰; basalts: 8.0±3.8 ppm and +2.4±2.9‰). These data can be readily explained by mixing between inherited mantle N and secondary N mostly derived from seawater/sediments with a minor contribution from abiotic N2 reduction. The secondary N in sheeted dikes and gabbros was mainly incorporated at moderate- to high-temperature (>250 °C) alteration stages and likely resides in chlorite, secondary plagioclase and amphibole. These new data clearly indicate that the intrusive section of AOC is a non-negligible N reservoir. Using these data, together with previously published N concentration data of basalts from Holes 1256D and 504B and of subducting sediments from Hole 1039B offboard the CA trench, we obtained a total N input flux of 3.1±0.6 × 109 mol‧yr−1 to 4.0±0.8 × 109 mol‧yr−1 into the CA trench. Integrating recently published N output flux of 0.58 × 109 to 1.4±0.6 × 109 mol‧yr−1 at the CA arc, an N recycling efficiency of 15±3–45±5% was yielded for the CA margin, indicating that up to 55±5–85±3% of subducted slab N is possibly transported beyond the sub-arc depth in this warm subduction zone. Based on the new finding of comparable N enrichment between basalts and intrusive oceanic crust, our modeling also gave a revised N recycling efficiency of 12±10–17±12% for the cold Izu-Bonin-Mariana (IBM) subduction zone, which is consistent with previous estimate of 4–17%. While these new estimates do not provide a conclusive constraint on the effect of subduction-zone thermal structure on N recycling, they consistently suggest that a large fraction of slab N (>50%) can survive the sub-arc filter and be delivered to the deeper mantle in global subduction zones.

Differential Fluid Activity in a Single Exhumed Continental Subduction Unit from Local P-T-M(H2O) Records of Zoned Amphiboles (North Muya, Eastern Siberia)

The behavior of the continental lithosphere in the Alpine-type subduction zones, which primarily depends on its thickness, thermal regime of subduction and availability of fluids/melts, remains an important issue for both metamorphic petrology and geochemistry as well as for resolving the thermomechanical properties of subduction paleo-interfaces. Rehydrated (amphibole- and zoisite-bearing) eclogites from the Neoproterozoic North Muya high-pressure complex (northern Central Asian Orogenic belt, eastern Siberia) were studied in order to assess their peak burial depths, degree of prograde dehydration, and further retrograde hydration extent within a subducted and exhumed continental unit. Three medium-grained eclogites from different localities of HP complex show similarly dry peak assemblages of pyrope-almandine-grossular garnet (XGrs up to 0.30, XPrp up to 0.25) + Na-rich omphacite (up to 44 mol % of jadeite) + rutile + quartz, which are variably replaced by secondary plagioclase + clinopyroxene ± amphibole symplectitic aggregate. The eclogites were subjected to burial at similar peak depths (up to ~17–21 kbar) but different peak temperatures (within ~600–730 °C) with or without notable heating and re-equilibration due to crustal thickening. Variable degrees of exhumation-induced pervasive rehydration led to growth of individual zoned porphyroblastic barroisite-hornblende amphibole ([B]Na = 0.03–0.45) ± zoisite over the primary eclogitic assemblage or after notable thermally-driven development of symplectitic aggregate after omphacite. Amphibole compositions together with the zoisite presence/absence in different samples reflect continuous rehydration by addition of ~0.5–1.5 wt.% at different exhumation conditions, from nearly peak eclogitic P–T (~17–21 kbar) to granulite- and amphibolite-facies depths within the plagioclase stability field (<14 kbar). This diversity most likely required irregular distribution of internally sourced, low-volume, hydrous metamorphic fluid (i.e., from host felsic rocks or metasediments) acting at different depths of the subduction interface. From the performed PTX calculations, I suggest that nearly isochemical (i.e., without any significant modification of the bulk-rock composition other than incorporation of additional H2O), retrograde hydration by only at lower- to middle-crust conditions did not significantly influence the density and the rheology of the subducted continental slices due to both (1) a limited abundance of dense metabasic rocks, which are commonly more fluid-rich (e.g., due to chlorite or amphibole alteration), and (2) the initially dry nature of mafic and felsic continental rocks. The limited dehydration and rehydration scales exemplified by the North Muya eclogites and therefore low availability of hydrous metamorphic fluids may have accounted for the high buoyancy of the eclogitic crust and explained the absence of contemporaneous suprasubduction magmatism in the regional context at ca. ~630 Ma.

Tectonics of Volcanogenic Massive Sulphide (VMS) Deposits at Flores Back Arc Basin: A Review

The bathymetry, petrology, marine magnetic, and seismic-SBP data have identified the northwest-southeast direction submarine ridge that shows hydrothermal activity. This activity occurred through Mount Baruna Komba, Abang Komba, and Ibu Komba. The volcanic rocks are andesite basaltic lava flows, tuff, and pumice. The andesite basaltic lava shows porphyritic, intergranular, intersertal to glomeroporphyritic textures. The rock composes anhedral minerals of k-feldspar, plagioclase, and pyroxene. These minerals present in small-sized, short prismatic dispersed in very fine groundmass minerals or glasses. Most of the volcanic rocks have experienced various degrees of alteration. The k-feldspar and plagioclase are most dominantly transformed into sericite, clay mineral, carbonate, epidote and oxide mineral, opaque mineral, and secondary plagioclase through the albitization process, while pyroxene replaced by chlorite. Other minerals are biotite and quartz, and base metals are present Cu, Zn, Ag, As, Pb, and gold. Mineralization categorizes as the phyllic zone, sub-prophylithic zone, and phyllic-potassic zone that formed at a temperature range of 250-400oC. The submarine hydrothermal alteration in the Komba Ridge is associated with a volcanogenic sulphide deposit controlled by crust thinning due to the crust rifts in the back-arc tectonic setting.

Petrography, mineral chemistry and shock metamorphism of the Mangui meteorite

The Mangui meteorite, also known as the Xishuangbanna meteorite, fell in Menghai, Xishuangbanna Dai Nationality Autonomous Prefecture, Yunnan Province, China at ~9: 45 pm on June 1, 2018. More than 500 fragments of Mangui were collected a total mass of ~50 kg. The mass of the largest fragment is ~1228 g. Mangui has a black fusion crust, a light gray exposure surface, and contains many black shock melt veins and pockets. In this work, the classification of Mingui and its shock history are investigated by studying its petrology, mineral chemistry, and the nature of its shock assemblages. Mangui is mainly composed of olivine, pyroxene, feldspar, Fe-Ni metal, troilite, chromite, merrillite and apatite, showing typical petrologic features of an ordinary chondrite. Relict chondrules are observed in hand specimens but their boundaries are less clear under FE-SEM. Electron Probe Micro Analysis (EPMA) shows that the chemical compositions of olivine (Fa25.1±0.3 mol%( n =71)), low-Ca pyroxene (Fs21.1±0.3Wo1.5±0.2 mol% ( n =58)) and plagioclase Ab84.1±0.7Or7.3±0.4 mol%( n =49)) in Mangui are relatively homogeneous. The petrographic type of Mangui is classified as type 6 by the coarse grains of secondary plagioclase, the homogeneous compositions of olivine and pyroxene, the recrystallization of matrix and blurry boundaries of relict chondrules. The chemical group of Mangui is L based on the average Fa content of olivine (Fe/(Fe+Mg)×100, atomic percentage, 25.1±0.3, n =71) and the average Fs content of pyroxene (Fe/ (Fe+Mg+Ca)×100, atomic percentage, 21.1±0.3, n =58). Therefore, the Mangui meteorite is classified as an L6 ordinary chondrite. The Mangui parent body experienced strong shock metamorphism with the widespread distribution of shock melt veins with widths up to 600 μm. Shock induced melt pockets were also observed. Micron-sized olivine and pyroxene in the center of the shock melt veins display chemical zonation, suggesting that they recrystallized from the shock induced melt. Plagioclase in the shock melt veins has transformed into maskelynite. Two high pressure polymorphs, jadeite and majorite, occur in the shock melt veins and melt pockets as measured by Raman analysis. Jadeite usually displays granular textures in these assemblages as euhedral to sub-euhedral crystals. Majorite was only found in one jadeite assemblage, occurring as inclusions in outer rim of an assemblage. The co-occurrence of majorite and jadeite indicate shock pressures from 17–20 GPa at temperatures ranging from 1900–2000°C. However, ringwoodite, whitlockite and other high pressure polymorphs have not been identified in the shock melt veins pockets. The occurrences of maskelynite, jadeite and majorite, and widespread distribution of shock melt veins and pockets suggest the shock stage of Mangui as S5. The absence of ringwoodite and tuite probably attribute to slow post-shock cooling rates, suggesting that the Mangui parent body experienced strong shock metamorphism with a subsequent slow post-shock cooling history. TEM and XRD work will be performed to better constrain the formation mechanisms of jadeite and majorite and their P - T - t histories. Furthermore, we aim on modelling the impact conditions on the Mangui parent body such as impact velocity, and parent body-impactor sizes by further study of the high-pressure minerals in shock-melt veins of the Mangui meteorite.

Petrology, mineralogy, porosity, and cosmic‐ray exposure history of Huaxi ordinary chondrite

AbstractA meteorite fall was heard and collected on July 13, 2010 at about 18:00 (local time) in the Shibanjing village of the Huaxi district of Guiyang, Guizhou province, China. The total mass of the fall is estimated to be at least 1.6 kg; some fragments are missing. The meteorite consists mainly of olivine, low‐Ca pyroxene, high‐Ca pyroxene, plagioclase, kamacite, taenite, and troilite. Minor phases include chromite and apatite. Various textural types of chondrules exist in this meteorite: most chondrule textures can be easily defined. The grain sizes of secondary plagioclase in this meteorite range from 2 to 50 μm. The chemical composition of olivine and low‐Ca pyroxene are uniform; Fa in olivine and Fs in low‐Ca pyroxene are, respectively, 19.6 ± 0.2 and 17.0 ± 0.3 (mole%). Huaxi has been classified as an H5 ordinary chondrite, with a shock grade S2, and weathering W0. The weak shock features, rare fractures, and the high porosity (17.6%) indicates that Huaxi is a less compacted meteorite. The preatmospheric radius of Huaxi is ~11 cm, corresponding to ~21 kg. The meteorite experienced a relatively short cosmic‐ray exposure of about 1.6 ± 0.1 Ma. The 4He and 40Ar retention ages are older than 4.6 Ga implying that Huaxi did not degas after thermal metamorphism on its parent body.

Preservation of magmatic signals in metavolcanics from Wedel Jarlsberg Land, SW Svalbard

AbstractThe purpose of this study is to determine the role of metamorphism and thereby identify the preserved magmatic signature in metavolcanics from Wedel Jarlsberg Land in southwestern Svalbard. Samples have been collected from late Precambrian metavolcanics occurring within metasedimentary rocks of the Sofiebogen Group, as well as dikes cutting older metasedimentary rocks of the Deilegga Group. The volcanic rocks were metamorphosed under greenschist facies conditions during the Caledonian Orogeny. To investigate the role of metamorphism, we present petrography, major and trace element geochemistry, and use factor analysis as a tool to identify correlations that correspond to primary magmatic signals.The metavolcanics are classified as subalkaline basalt to basaltic andesite and they contain relicts of primary clinopyroxene and plagioclase. The metamorphic minerals are actinolite, secondary plagioclase, chlorite and minerals belonging to the epidote group. Major element variations are highly scattered with no obvious trends observed. The HFSE and REE show strong trends attributed to fractional crystallization. The LILE, Th and La show elevated contents in some samples.Factor analysis shows that the HFSE and REE are well correlated. The LILE form a separate well correlated group, while the major elements are not correlated, except for Na2O, Fe2O3 and CaO. The lack of correlation for major elements, as well as the lack of observed fractional crystallization trends between these elements suggests that they were modified by metamorphism. The strong correlation of HFSE and REE reflects the original geochemical signal generated by magmatic processes. The correlation of the LILE is consistent with their elevated composition implying the influence of crustal contamination processes, and though some variability is likely superimposed due to metamorphism, the primary magmatic record is not completely destroyed. We conclude that the HFSE and REE are not influenced by metamorphic processes and therefore provide robust records of magmatic processes.

Lithium and Li-isotopes in young altered upper oceanic crust from the East Pacific Rise

Lithium contents and Li-isotope ratios have been measured in a section of young oceanic upper crust formed at the East Pacific Rise and exposed in the Hess Deep rift. The Li contents of both lavas and sheeted dikes are lower than in the fresh protolith (by 43% and 74%, respectively) suggesting that axial hydrothermal systems leach Li from the entire upper crust. The Li-isotopic composition of the lavas (4.1±1.7‰) is similar to the protolith (3.4±1.4‰) whereas the dikes extend to both higher and lower isotopic compositions (δ7Li from −11 to 14‰). Loss of Li from all lava samples was unexpected due to the petrographically fresh appearance of these rocks and the relatively cool fluids generally thought to exist within the lava pile. This Li loss is interpreted as indicating that warm (50–100°C) fluids pervade the lava pile, at least episodically, leaching Li from the lavas without recrystallization of the primary igneous minerals. This hypothesis has been tested experimentally. Hydrothermal experiments at 125°C, using synthetic basalts cooled at different rates to produce different starting material texture (glass to crystalline rock), demonstrate that Li is leached rapidly at this temperature and substantially more rapidly from crystalline rock than from glass. The natural and experimental data are consistent with Li diffusion out of plagioclase into warm hydrothermal fluids acting to leach Li from the lavas without any concomitant mineralogical changes. The wide range of δ7Li in the sheeted dike complex, and the lack of a systematic correlation of δ7Li with the concentration of Li, suggest that more than one process leads to Li depletion. A combination of mineral–fluid reactions that break-down igneous minerals and produce secondary minerals (principally amphibole, chlorite and secondary plagioclase), along with diffusive loss of Li from plagioclase into the fluid, can explain the Li-systematics of the sheeted dike complex at Hess Deep.

Constraints on the evolutionary history of aluminous mafic rocks in the Ronda peridotite massif (Spain) from trace-element compositions of clinopyroxene and garnet

Multiple trace-element compositions of clinopyroxene and garnet in aluminous mafic layers (±corundum and/or sapphirine) alternating with peridotites from the Ronda massif, Spain, were determined by an ArF 193 nm excimer laser system coupled with a quadrupole ICP-MS. The studied aluminous mafic rock is divided into several sublayers based on the modal mineral variations. Trace-element compositions of minerals vary between sublayers. Primitive mantle-normalized trace-element distribution patterns have more significant positive Eu anomaly for both garnet and clinopyroxene in the corundum-bearing sublayers than for those in the other sublayers whereas corundum-bearing sublayers contain more abundant secondary plagioclase than corundum-free sublayers. These geochemical signatures of the corundum-bearing sublayers intercalated with corundum-free sublayers were acquired as plagioclase-rich portions of gabbroic layered sequence originated at the lower crustal to the uppermost mantle sections rather than selective accumulation of metamorphic plagioclase due to deformation during the late stage of exhumation of the massif. Partition coefficients of trace elements between clinopyroxene and garnet show systematic variations with respect to major element composition, indicating local attainment of chemical equilibrium. The trace element compositions of clinopyroxene and garnet were equilibrated with secondary metamorphic plagioclase, i.e., at granulite-facies conditions, because plagioclase is required for the geochemical balance in Sr and Eu contents of the whole-rock trace element compositions. Positive Eu anomaly of clinopyroxene (±garnet) is also found in mantle-derived aluminous eclogitic xenoliths and eclogitic mineral inclusions in diamond captured in kimberlites, and is thus a useful indicator to explore recycled crustal components in the mantle.

Melt–rock reaction in the lower oceanic crust and its implications for the genesis of mid-ocean ridge basalt

Primitive cumulates from a 2–3 Ma old gabbro massif exposed in the Kane Megamullion (23°N, Mid-Atlantic Ridge) contain abundant clinopyroxene with high Mg# (86–91). Such magnesian clinopyroxenes have hitherto been taken to signify crystallization at elevated pressures. Kane clinopyroxenes, however, are dominantly oikocrysts that overgrow olivine and plagioclase, indicating crystallization occurred at low pressure. The oikocrysts have textures and compositions indicative of disequilibrium processes. First, many of the oikocrysts enclose resorbed plagioclase with lower anorthite contents than plagioclase in the host rock, and olivine is notably absent as a chadacryst despite being abundant in the host rock. Second, the oikocryst minor element compositions are inconsistent with equilibrium growth from a MORB melt. These data indicate that high-Mg# clinopyroxene in the Kane gabbros formed as a result of reaction between primitive cumulates and migrating melt in the lower oceanic crust, with clinopyroxene and secondary plagioclase growing at the expense of olivine and primary plagioclase. Thus high-Mg clinopyroxene does not result from high-pressure crystallization as has been inferred previously. Assimilation–fractional crystallization modeling indicates that melts undergoing such reactions are enriched in Al 2O 3 and MgO and depleted in CaO and SiO 2. This effect is similar to that expected for fractional crystallization of MORB at elevated pressures, and reacted melts yield higher calculated pressures than starting melts. This suggests that the CaO–Al 2O 3–MgO–SiO 2 relationships of MORB may result from melt–rock reaction, and that calculated pressures of MORB fractionation are overestimated as a result. Melt–rock reaction in the lower oceanic crust may thus account for both lines of evidence for high-pressure fractionation of MORB.

Submarine Gold Mineralization Near Lihir Island, New Ireland Fore-Arc, Papua New Guinea

Gold-rich, siliceous veins with disseminated polymetallic sulfides and pyritic stockwork mineralization have been recovered from the top of Conical seamount, a shallow (1,050-m water depth) submarine volcano located about 10 km south of Lihir island, Papua New Guinea. Grab samples from the summit of Conical seamount contain the highest concentration of gold yet reported from the modern sea floor (max 230 ppm Au; avg 26 ppm, n = 40). The gold occurs in sulfide-rich veins of black amorphous silica hosted by intensely altered, high K calc-alkaline basalts. Sulfides in the veins consist of sphalerite, galena, pyrite, chalcopyrite, marcasite, and a variety of Cu-Pb-As-Sb sulfosalts. The gold occurs as native gold and electrum in the amorphous silica and as inclusions in the sulfides. The highest gold concentrations are associated with high Ag, As, Sb, and Hg. Zoned alteration adjacent to the veins consists of illite, smectite, amorphous silica, K feldspar, secondary plagioclase, minor chlorite, and trace carbonate. The association of gold with illite, smectite, amorphous silica, and K feldspar indicates deposition from near neutral pH hydrothermal fluids. However, the auriferous polymetallic sulfide veins and the associated alteration are overprinted on stockwork pyrite mineralization that is associated with earlier acid alteration containing alunite, aluminum phosphate sulfates, kaolinite, and other clay minerals. The platy habit of the alunite in this assemblage, the presence of alumium posphate sulfate minerals, and the sulfur isotope ratios of the crystalline pyrite (–8.6 to –0.2‰ δ 34 S, n = 28) and alunite (7.5 and 6.4‰ δ 34 S) are consistent with a contribution of magmatic volatiles in the earliest stages of the hydrothermal system. Framboidal pyrite within and at the margins of the mineralized zone has δ 34 S values suggesting involvement of biogenic activity (–11.6 to –13.9‰ δ 34 S). The gold-rich veins at Conical seamount are distinct from sea-floor massive sulfide deposits and represent a new style of mineralization on the modern sea floor. The mineralogy, alteration, geochemistry, and texture of the veins resemble those of some subaerial epithermal gold deposits and indicate that features long considered to define a subaerial setting can also form in a submarine environment. The proximity of Conical seamount to the giant Ladolam epithermal gold deposit on nearby Lihir island also raises the possibility that both subaerial and submarine gold mineralization in the region may be related to the same district-scale magmatic events.

PX-NOM—an interactive spreadsheet program for the computation of pyroxene analyses derived from the electron microprobe

PX-NOM is a Microsoft Excel™ spreadsheet program that enables the user to calculate structural formulae of pyroxene analyses obtained from the electron microprobe and to determine respective pyroxene names according to the classification scheme of the International Mineralogical Association. The nomenclature used in the program contains only mineral names and adjectival modifiers accepted by the Subcommission on Pyroxenes. Additionally, PX-NOM permits the use of the jadeite+quartz geobarometer for clinopyroxenes containing jadeite as well as various garnet-clinopyroxene Fe–Mg exchange thermometers. The barometer calculates a minimum pressure, if only secondary plagioclase is available in the mineral assemblage and therefore X Ab Pl has to be set to 1. The exchange thermometers show best results in LT-MT eclogites which bear only low contents of high-Si phases (quartz, feldspar). Pyroxene analyses belonging to the Ca–Fe–Mg, Ca–Na or Na group can be plotted in the Wo–Fs–En triangle or the Quad-Ae-Jd diagram, respectively. The diagrams are presented on separate sheets and can be manipulated and printed optionally.

Hydrothermal alteration of plagioclase and growth of secondary feldspar in the Hengill Volcanic Centre, SW Iceland

Dissolution of igneous feldspar and the formation and occurrence of secondary feldspar in tholeiitic basalts from the Hengill volcanic centre, in SW Iceland was studied by microprobe analysis of cuttings from two ca. 2000 m deep geothermal wells. Well NG-7 in Nesjavellir represents a geothermal system in a rift zone where the intensity of young, insignificantly altered intrusions increases with depth. Well KhG-1 in Kolviðarhóll represents the margin of a rift zone where the intensity of intrusives is lower and the intensity of alteration higher. This marginal well represents altered basaltic crust in an early retrograde state. The secondary plagioclase in both wells is mainly oligoclase, occurring in association with K-feldspar and chlorite±actinolite. The texture of this assemblage depends on the lithology and intensity of alteration. In Nesjavellir (NG-7) the composition of secondary albite-oligoclase is correlated with the host-rock composition. This connection is not apparent in more intensely altered samples from Kolviðarhóll (KhG-1). The influence of temperature on composition of secondary Na-feldspar is unclear in both wells although Ca is expected to increase with temperature. Any temperature dependence may be suppressed by the influence of rock composition in Nesjavellir and by retrograde conditions at Kolviðarhóll. The absence of clear compositional gradients between igneous plagioclase and secondary feldspar and between Na-feldspar and K-feldspar suggests that secondary feldspars formed by dissolution precipitation reactions.

Cl-scapolite, Cl-amphibole, and plagioclase equilibria in ductile shear zones at Nusfjord, Lofoten, Norway: implications for fluid compositional evolution during fluid-mineral interaction in the deep crust

Cl-rich scapolite and amphibole formed during ductile shear deformation associated with the infiltration of an externally derived Cl-bearing fluid in a gabbroanorthosite of the Flakstadøy Basic Complex, Lofoten, Norway. Amphibole and scapolite formed along the contacts between incompletely altered igneous mafic minerals (orthopyroxene, clinopyroxene, biotite, and ilmenite) and plagioclase and internally in the grains of primary igneous plagioclase. The secondary minerals show large compositional variations on thin-section scale. The Cl content of scapolite varies between 0.3 and 0.95 apfu (atoms per formula unit), whereas amphibole shows Cl concentrations from 0 to 1.5 apfu. The primary igneous plagioclase (An 50-An 60) underwent extensive recrystallization and alteration during the fluid–rock interactions. Secondary plagioclase shows compositions in the range from An 20 to An 55. In general, plagioclase did not equilibrate with the fluid phase during alteration, due to the sluggishness of the cation exchange reactions between plagioclase and fluid. Occasionally, however, equilibrium among plagioclase, scapolite, and the fluid phase was attained. The compositional variations of amphibole, scapolite, and plagioclase that equilibrated with the fluid are principally related to variations in the fluid activity ratios a Cl − /a (CO 3) 2− and a Cl − /a OH − . The large compositional variations of the minerals on thin-section scale thus indicate steep gradients in the fluid activity ratios a Cl − /a (CO 3) 2− and a Cl − /a OH − . The activity gradients of the fluid phase developed as a result of the preferential extraction of water from the grain-boundary fluid during the formation of hydrous silicates. Scapolite and the most Cl-enriched amphibole formed in equilibrium with an evolved fluid phase, enriched in Cl and CO 2.

Composition and Sm-Nd isotopic data of the lower crust beneath San Luis Potosí, central Mexico: Evidence from a granulite-facies xenolith suite

Quaternary volcanics from San Luis Potosí (SLP) in central Mexico sampled a xenolith suite comprising uppermantle nodules (spinell lherzolites, hornblende pyroxenites, spinel pyroxenites) and lower-crustal xenoliths. The latter consist of granulite-facies, mafic to intermediate metaigneous rocks and high-grade metasediments. Precursors of the metaigneous suite range from gabbroic to tonalitic compositions. Whole-rock REE patterns permit classification of metaigneous rocks into cumulates and fractionated melts.Mafic xenoliths consist of plagioclase ± clinopyroxene ± orthopyroxene ± garnet ± amphibole ± apatite ± rutile ± scapolite. Garnet has reacted extensively with clinopyroxene, forming corona textures of secondary plagioclase, orthopyroxene and amphibole. Intermediate xenoliths are garnet-free and quartz-bearing. All samples display similar Ca-Al zoning patterns documented in orthopyroxene, clinopyroxene and plagioclase which are, along with the presence of coronas around garnet, interpreted as representing partial re-equilibration stages from 940 ± 60°C and 7–11 kbar down to lower temperatures and pressures. There is no indication of a secondary heating event in these xenoliths.Sm-Nd isotopic compositions of the spinel lherzolites indicate a heterogeneous composition of the mantle segment beneath SLP. Similar Sr- and Nd-isotopic signatures reveal that the hornblende pyroxenites are high-pressure cumulates linked to the host basanites. Four mainly intermediate metaigneous granulites from a single vent, which are free of secondary amphibole, and two spinel pyroxenites, regarded as the most primitive member of the metaigneous suite, define a regression line in a Sm-Nd isochron diagram, yielding an age value of 1248 ± 69 Ma and an initial ϵNd-value of +2.3 Metaigneous xenoliths containing secondary amphibole plot significantly above the regression line, which is probably due to the introduction of externally derived Nd during amphibole formation under retrograde p-T conditions. Discussing the geological significance of the calculated age value, there are some arguments for interpreting this age as representing the intrusion age of the magmatic precursors. This gives evidence for formation of lower crust of basic to intermediate compositions during the mid-Proterozoic, now situated beneath SLP.

Evolution of oceanic gabbros from DSDP Leg 82: influence of the fluid phase on metamorphic crystallizations

Gabbro breccias were recovered from an anomalously shallow level of the ocean crust during DSDP Leg 82. The rocks display evidence of metamorphic crystallization related either to localized deformation or to hydrothermal circulation of a seawater-derived fluid under static conditions. Secondary phases consist of plagioclase, amphibole and minor clinopyroxene, ilmenite, sphene and chlorite. Petrological study indicates that deformation took place at high temperature, under anhydrous conditions, and was followed by hydrothermal circulation. The compositions of secondary minerals (i.e. strong zonations, presence of chlorine in amphiboles, varying compositions of secondary plagioclase) indicate that reactions of the gabbros with the fluids occurred at a low water/rock ratio. Relations between Cl, Na and K in amphiboles suggest penetration of at least two distinct fluids of different compositions. Metamorphic crystallization stopped when greenschist facies conditions were reached(∼ 350°C), probably because hydrothermal circulation faded out.

Fabric and origin of gneissic layers in anorthositic rocks of the St. Charles sill, Ontario

The St. Charles sill is located in the Grenville Province and consists of rocks of the anorthosite suite. The sill is a northwesterly trending body, 11 km long and as much as 0.8 km wide, and with a steep dip to the northeast. The sill is characterized by interlayered massive and gneissic rocks metamorphosed under conditions of the amphibolite facies. In the massive rocks plagioclase occurs as strongly twinned laths that range in size from fine-grained crystals to megacrysts. Hornblende, biotite, and garnet occur as subophitic masses and apparently replace original pyroxene. In the gneissic rocks the plagioclase ranges in size from fine to coarse grained and the primary grains are partially replaced by elongate, weakly twinned, anhedral plagioclase. The gneissosity is defined by a dimensional preferred orientation of biotite, hornblende, and secondary plagioclase. The formation of the secondary plagioclase is attributed largely to growth by grain boundary diffusion and, to a lesser extent, by replacement of primary plagioclase by grain boundary migration. In the diffusion mechanism strain rate is inversely proportional to grain size and it is interpreted that the tectonic fabric developed in the finer grained layers of the sill while the coarser grained layers remained essentially undeformed.

Features of secondary transformation in rocks of sedimentary series

Mineral parageneses and distribution of secondary minerals in the sedimentary sections of the western Siberian lowland (Mesozoic), the Kuznetsk basin (coal measures) and the Minusinsk depression (Devonian), in reference to diagenesis, epigenesis, and the initial metageneses, as criteria for identification of large cycles of sedimentation (series) by means of the lithological-formatonal analysis, show among other things that the originally polymictic rocks (continental terrigenic sediments) are rendered practically oligomictic by kaolinization of their feldspars and micas; corrosion of feldspars in carbonate rocks renders them significantly quartzitic; formation of dioctahedral mica and chlorite (deep epigenesis) in all horizons homogenizes mineralogically the clay fraction of the originally clayey rocks. The sequence of changes in the ratio of secondary quartz to secondary plagioclase is essentially the same throughout the profiles as in their terrigenic analogues. An abundance of clayey cementing substan...

The Significance of Primary Scapolite in Granulitic Inclusions from Deep-seated Pipes

Sulphur-rich scapolite is described as a primary constituent of several granulite inclusions occurring, along with eclogitic and rare ultrabasic rocks, in the breccia- and basalt-filled pipes of eastern Australia. The wide distribution of such inclusions in space and time indicates that granulites, some scapolite-bearing, form an important part of the base of the crust of this region. Two inclusions with the assemblage orthopyroxene-clinopyroxene-plagioclase-scapolite, two with the assemblage clinopyroxene-garnet-plagioclase-scapolite, and one with the assemblage orthopyroxene-clinopyroxene-plagioclase, have been analysed and chemical and optical data on the constituent minerals are given. Since it is impossible to adequately separate scapolite from primary and secondary plagioclase, partial analyses of these minerals have been made with an A.R.L. electron probe X-ray microanalyser. From our data and that obtained from the literature, we suggest that scapolites with more than 10 mol per cent sulphur in the anion group are confined to rocks of the granulite meta-morphic facies. Within the granulite facies there appears to be a progressive increase in the sulphur content of scapolite in equilibrium with such sulphur-rich minerals as pyrrhotite, with increasing metamorphic grade.