Sodic Pyroxene Research Articles

The Beacon Heights “Tinguaite” (Phonolite): A recrystallised agpaitic ignimbrite in the Mesoproterozoic Pilanesberg alkaline complex, South Africa

AbstractAn extrusive rock unit in the Mesoproterozoic Pilanesberg alkaline complex that has been referred to as the “Beacon Heights Tinguaite” since the work of Shand (1928) has been reexamined in the field and studied by petrographic microscopy, scanning electron microscopy, and whole-rock major and trace element analysis. The new observations indicate that the unit (here informally renamed the “Beacon Heights Phonolite” to comply with current petrographical nomenclature) is a laminated, volcanic rock in which mafic layers rich in sodic pyroxene alternate on any scale with discontinous, felsic layers consisting of alkali feldspar, nepheline, sodalite and their alteration products natrolite and analcime. Highly strontian apatite, fluorite, a manganiferous pectolite-group mineral, sphalerite and pyrrhotite are minor to accessory minerals. In addition, the rock carries indicator minerals typical of agpaitic rocks (including lamprophyllite and eudialyte-group minerals and their replacement products). The rock contains abundant crystal fragments of felsic minerals (alkali feldspar, nepheline and sodalite), fragments and elongated lenses of nepheline syenite, some of which contain primary magmatic lamprophyllite, and rare armoured lapilli. The overall structure of the rock is that of a welded ashflow tuff (igmimbrite) affected by post-magmatic recrystallisation processes. Whole-rock major and trace element compositions suggest a compositional affinity with members of the agpaitic, intrusive Green Foyaite Suite of the Pilanesberg Complex, and probably also a genetic relationship. The Beacon Heights Phonolite is a rare, possibly unique, example of an ashflow deposit of agpaitic, highly silica-undersaturated composition.

Interaction of seawater with (ultra)mafic alkaline rocks—Alternative process for the formation of aegirine

Abstract Submarine mafic and relatively Na-poor alkaline rocks in the Outer Carpathians often contain aegirine, a sodic pyroxene usually found in differentiated alkaline rocks. Its presence in rocks that are too basic and Na-poor for its conventional magmatic appearance is linked to sodic alteration of submarine alkaline rocks. Aegirine crystals grow on altered rims of diopside, commonly with crystal-lographic unconformity, suggesting that their growth was related to alteration and that aegirine does not represent a late stage of continuous clinopyroxene crystallization. The U-shaped REE patterns in the studied aegirine lack Eu anomaly, characteristic for aegirine from differentiated alkaline rocks. Therefore, the involvement of chemically more evolved magma is unlikely to have played any role in the formation of aegirine in ijolites and essexites. Formation of aegirine in submarine alkaline rocks may thus represent an alternative process to spilitization. However, this process is strongly limited by the availability of Fe3+ oxidized and mobilized by hydrothermal alteration, which may explain a relative scarcity of aegirine observed in submarine alkaline rocks compared to near-complete albitization of spilites, and its absence in high-MgO rocks (>10 wt%). Due to the blocking effect related to Fe3+ unavailability, ijolites, and essexites do not display significant Na enrichment. We posit that Na incorporated in aegirine was mainly sourced from the zeolitized interstitial glass.

Experimental Synthesis of the Rare-Earth Fluoride Gagarinite-(Ce) by Fluid-Induced Alteration of Chevkinite-(Ce)

Abstract Gagarinite-(Ce) [Na(Ca,Ce)2F6] has been synthesized as a product in experiments designed to examine the fluid-induced alteration of chevkinite-(Ce). The experiments were conducted at 600 °C and 400 MPa for 21 days and at 550 °C and 200 MPa for 63 days. At 600 °C, a rim of gagarinite-(Ce) was seen to develop around chevkinite-(Ce), which was itself enclosed in a glassy, amorphous material containing inclusions of albite. At 550 °C, the chevkinite-(Ce) was observed to be surrounded by a rim of gagarinite-(Ce) and fluorbritholite-(Ce), which in turn was enclosed in massive narsarsukite with inclusions of albite. This assemblage was associated with a sodic pyroxene. In the 550 °C run both rims with gagarinite-(Ce) were subsequently overgrown by both Ce-bearing frankdicksonite and a lamprophyllite group mineral identified as delindeite. Electron probe microanalyses are given of all reactant phases. The stability conditions of gagarinite-(Ce) are in accord with those inferred from the only natural occurrence, Strange Lake, Canada. The formation of gagarinite-(Ce) is ascribed to NaF being the only component added in the experiments, resulting in a very high Na/Ca ratio in the system. New parageneses for narsarsukite, frankdicksonite, and delindeite are reported.

Review on the Compositional Variation of Eudialyte-Group Minerals in the Ilímaussaq Complex (South Greenland)

We review the compositional variation of eudialyte-group minerals (EGM) from the Ilímaussaq complex in South Greenland. Investigated samples cover all major rock units and associated pegmatites and aplites. The whole data set (>3000 analyses from >250 samples) exhibits variable XMn (0.1–0.5), REE (0.2–1.7 apfu), Nb (0.1–0.4), and Cl contents (0.4–1.6 apfu). Most EGM compositions are Na-rich (13–15 apfu), while deviations to Na-rich but also to Na-poor compositions occur because of a combination of primary features (peralkalinity, water activity) and secondary alteration. During magma evolution, REE contents in EGM cores generally increase and reach their highest contents in the most evolved rock units of the complex. This points to the moderate compatibility of REE in EGM and a bulk D (cEGM/cmelt) value of <1 during magma differentiation. Chlorine contents in EGM cores continuously decrease, and are lowest at the rims of individual crystals, suggesting a continuous decrease of Cl activity in the magmas by large-scale EGM and sodalite extraction during the orthomagmatic stage and water enrichment during the late-magmatic stage. The overall variations of XMn across stratigraphy are only minor and likely influenced by the co-crystallization of sodic pyroxene and amphibole (c.f. aegirine, arfvedsonite) and local phase proportions. Similarly, Nb and Ti contents are influenced by co-crystallizing aenigmatite, rinkite, and others. Their presence buffers Ti and Nb contents to rather constant and low values, while their absence may cause variable enrichment on a local scale. Very low Sr contents (<0.1 apfu) in magmatic EGM from Ilímaussaq are related to the basaltic nature of the parental magmas of the complex, as large-scale plagioclase fractionation occurred prior to the formation of the Ilímaussaq magmas, effectively removing Sr from the system. This is in line with very strong negative Eu anomalies in EGM from Ilímaussaq. Consistently, Sr contents in EGM from alkaline complexes, for which foiditic parental magmas are assumed, are much higher and, in such cases, negative Eu anomalies are generally absent.

Petrogenesis and tectonic implications of the Maladob ring complex in the South Eastern Desert, Egypt: new insights from mineral chemistry and whole-rock geochemistry

Alkaline ring complexes are widely distributed in the south Eastern Desert (SED) of Egypt, but their petrogenesis and geodynamic settings are a matter of many discussions. In this contribution, mineral chemistry, and whole-rock geochemical data of the Maladob intrusive rocks were used to investigate their petrogenesis. The Maladob ring complex is a small alkaline intrusion in the SED of Egypt, composed of oversaturated syenite, quartz syenites and peralkaline granite. The rocks consist of K-feldspar (Or94–97) and albite (An0-1), alkali amphiboles (arfvedsonite and ferro-katophorite), sodic pyroxene (aegirine), with accessory aenigmatite, zircon, apatite, ilmenite, magnetite and titanite. Geochemically, the rocks are peralkaline, show distinctive geochemical characteristics such as low contents of CaO, MgO, Sr and high contents of alkalis, Rb, Nb, Y, and REE, typical of A-type granites. The rocks are weakly to moderately fractionated with a general enrichment in LREE [(La/Sm)N = 2.96–3.64]. Typically, the Maladob rocks are classified as within plate granites and exhibit A1 subtype characteristics. We suggest that the Maladob rocks were formed from a similar OIB-like mafic magma by fractional crystallization. The fractionation of feldspars and mafic minerals play a significant role during magma evolution. During the Mesozoic intraplate magmatism, the upwelling of asthenosphere causes underplating of lithospheric OIB-like magma which was subjected to prolonged fractional crystallization to produce the Maladob syenites and peralkaline granite.

The Petrological and Geochemical Evolution of Ediacaran Rare‐Metal Bearing A‐type Granites from the Jabal Aja Complex, Northern Arabian Shield, Saudi Arabia

AbstractNew fieldwork, mineralogical and geochemical data and interpretations are presented for the rare‐metal bearing A‐type granites of the Aja intrusive complex (AIC) in the northern segment of the Arabian Shield. This complex is characterized by discontinuous ring‐shaped outcrops cut by later faulting. The A‐type rocks of the AIC are late Neoproterozoic post‐collisional granites, including alkali feldspar granite, alkaline granite and peralkaline granite. They represent the outer zones of the AIC, surrounding a core of older rocks including monzogranite, syenogranite and granophyre granite. The sharp contacts between A‐type granites of the outer zone and the different granitic rocks of the inner zone suggest that the AIC was emplaced as different phases over a time interval, following complete crystallization of earlier batches. The A‐type granites represent the late intrusive phases of the AIC, which were emplaced during tectonic extension, as shown by the emplacement of dykes synchronous with the granite emplacement and the presence of cataclastic features. The A‐type granites consist of K‐feldspars, quartz, albite, amphiboles and sodic pyroxene with a wide variety of accessory minerals, including Fe‐Ti oxides, zircon, allanite, fluorite, monazite, titanite, apatite, columbite, xenotime and epidote. They are highly evolved (71.3–75.8 wt% SiO2) and display the typical geochemical characteristics of post‐collisional, within‐plate granites. They are rare‐metal granites enriched in total alkalis, Nb, Zr, Y, Ga, Ta, REE with low CaO, MgO, Ba, and Sr. Eu‐negative anomalies (Eu/Eu* = 0.17–0.37) of the A‐type granites reflect extreme magmatic fractionation and perhaps the effects of late fluid‐rock interactions. The chemical characteristics indicate that the A‐type granites of the AIC represent products of extreme fractional crystallization involving alkali feldspar, quartz and, to a lesser extent, ferromagnesian minerals. The parent magma was derived from the partial melting of a juvenile crustal protolith with a mantle contribution. Accumulation of residual volatile‐rich melt and exsolved fluids in the late stage of the magma evolution produced pegmatite and quartz veins that cut the peripheries of the AIC. Post‐magmatic alteration related to the final stages of the evolution of the A‐type granitic magma, indicated by alterations of sodic amphibole and sodic pyroxene, hematitization and partial albitization.

The Growth of Sodic Amphibole at the Greenschist- to Blueschist-facies Transition (Dent Blanche, Western Alps): Bulk-rock Chemical Control and Thermodynamic Modelling

Abstract The sodic amphibole glaucophane is generally considered as indicative of blueschist-facies metamorphism. However, sodic amphiboles display a large range in chemical compositions, owing principally to the Fe2+Mg–1 and Fe3+Al–1 substitutions. Therefore, the whole-rock composition (namely its Na2O and FeO* content, and the Fe2+–Fe3+ ratio), strongly controls the stability field of the sodic amphiboles at the transition from greenschist- to blueschist-facies conditions. Neglecting these variables can lead to erroneous estimates of the metamorphic conditions and consequently the tectonic framework of the rocks. This paper explores the mechanisms that control the development of sodic amphibole and sodic pyroxene within the basement of the Dent Blanche Tectonic System (Western Alps), as a result of the Alpine metamorphic history. Field, petrographic and geochemical data indicate that sodic amphibole and sodic pyroxene form in different rock types: (1) in undeformed pods of ultramafic cumulates (hornblendite), sodic amphibole (magnesioriebeckite) forms coronas around magmatic pargasite; (2) metatonalite displays patches of radiating sodic (magnesioriebeckite) and calcic (actinolite) amphiboles; (3) sodic amphibole (magnesioriebeckite–glaucophane) occurs with high-Si potassic white mica (phengitic muscovite) in fine-grained (blue) schists; (4) in mylonitized granitoids (amphibole-gneiss) metasomatized along the contact with ultramafic cumulates, sodic amphibole (magnesioriebeckite–winchite) mainly forms rosettes or sheaves, generally without a shape-preferred orientation. Only locally are the needles aligned parallel to the stretching lineation. Pale green aegirine–augite is dispersed in an albite–quartz matrix or forms layers of fine-grained fibrous aggregates. The bulk-rock chemical composition of the different lithologies indicates that sodic amphibole and sodic pyroxene developed in Na- and Fe-rich systems or in a system with high Fe3+/Fe*. Thermodynamic modelling performed for different rock types (taking into account the measured Fe2O3 contents) reveals that sodic amphibole appears at ∼8 ± 1 kbar and 400–450 °C (i.e. at the transition between the greenschist- and blueschist-facies conditions) about 5 kbar lower than previous estimates. To test the robustness of our conclusion, we performed a review of sodic amphibole compositions from a variety of terranes and P–T conditions. This shows (1) systematic variations of composition with P–T conditions and bulk-rock chemistry, and (2) that the amphibole compositions reported from the studied area are consistent with those reported from other greenschist- to blueschist-facies transitions.

Mineralogy and geochemistry studies on the Nusab El Balgum granitic batches, South Western Desert, Egypt

Igneous rocks of Nusab El Balgum are formed as an elongated complex mass covering an area of about 4 km × 12.5 km (50 km2), in the NNE-SSW direction of the Tarfawi-Qena-South Sinai trend, which is a branch of the Trans-African shear zone at the intersection with the Kalabsha fault, which is a branch from Guinean-Nubian lineaments. The continuous reactivation of these two major weakness zones from the late Triassic to recent times has created many generations of the magma batches. The exposed granitic rocks of these batches at Nusab El Balgum were represented by the fresh peralkaline granite (youngest) and hydrothermally altered granites (oldest). The fresh peralkaline granite takes the form of a small stock composed essentially of perthites, quartz, sodic pyroxenes, amphiboles (secondary), and rare albite according to the proportion of presence, respectively. The accessory minerals are zircon, bastnaesite-(Ce), columbite-(Fe), magnetite, barite, and sphalerite. The geochemical study indicated that this granite is peralkaline, ferroan, A-type (specifically belongs to the A1-subgroup), anorogeny, emplaced in a within-plate, and crystallized at relatively shallow depth from the alkali basaltic magma similar to the OIBs. Furthermore, it is enriched in the HFSE (e.g., Th, U, Nb, REE, and Zr). The hydrothermally altered granites are formed as an incomplete ring shape and a small stock. They were formed during the late Cretaceous age and were altered due to the hydrothermal solutions from the continuous reactivation affected weakness zones and the new magmatic batches. The hydrothermally altered granites are extremely rich in HFSE found in the accessory minerals such as zircon (different in shape, size, and contains inclusions of bastnaesite and columbite), columbite-(FeM transportation and redeposition of the HFSE in these rocks, in addition to the clay minerals and K-metasomatism, were formed. The relations between the silicification index (SI = SiO2/(SiO2 + Al2O3) and Zr, Nb, Th, U, LREE, and HREE are positive but they become negative with the K-metasomatism.

High pressure minerals in the Ch\xe2teau-Renard (L6) ordinary chondrite: implications for collisions on its parent body

We report the first discoveries of high-pressure minerals in the historical L6 chondrite fall Château-Renard, based on co-located Raman spectroscopy, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy and electron backscatter diffraction, electron microprobe analysis, and transmission electron microscopy (TEM) with selected-area electron diffraction. A single polished section contains a network of melt veins from ~40 to ~200 μm wide, with no cross-cutting features requiring multiple vein generations. We find high-pressure minerals in veins greater than ~50 μm wide, including assemblages of ringwoodite + wadsleyite, ringwoodite + wadsleyite + majorite-pyropess, and ahrensite + wadsleyite. In association with ahrensite + wadsleyite at both SEM and TEM scale, we find a sodic pyroxene whose Raman spectrum is indistinguishable from that of jadeite but whose composition and structure are those of omphacite. We discuss constraints on the impact record of this meteorite and the L-chondrites in general.

Emplacement and geochemical evolution of highly evolved syenites investigated by a combined structural and geochemical field study: The lujavrites of the Ilímaussaq complex, SW Greenland

Structural mapping and the combined study of magmatic to solid-state deformation textures and mineral compositions in highly evolved nepheline syenites (lujavrites) of the alkaline to peralkaline Ilímaussaq complex (South Greenland) reveal detailed insight into the emplacement and geochemical evolution of the melts they crystallized from.Based on magmatic to solid-state flow textures such as foliations and lineations, we propose that the investigated rock sequence forms a sill-like structure with a steep feeder zone that flattens out over a short distance and intrudes into less evolved overlying units as sub-horizontal sheets by roof uplift. Systematic compositional variation of early-magmatic eudialyte-group minerals (EGM) in the investigated rock sequence monitors the geochemical evolution of the lujavrite-forming melt(s). The chlorine contents of EGM decrease successively upwards within the rock sequence, which probably indicates a successive increase of water activity during differentiation, consistent with a change from sodic pyroxene (aegirine) to sodic amphibole (arfvedsonite) in the mineral assemblage. Both REE contents and Fe/Mn ratios of EGM are promising differentiation indicators, which increase and decrease, respectively, upwards within the sequence due to fractional crystallization. This closed-system evolution is interrupted by a shift towards less evolved melt compositions in one lujavrite unit, for which we assume magma recharge.Our study demonstrates the strength of a combined structural and petrological approach to understand the petrogenesis of an igneous body in more detail and highlights their close connection.

Petrogenesis of Late Permian silicic rocks of Tu Le basin and Phan Si Pan uplift (NW Vietnam) and their association with the Emeishan large igneous province

Major and trace elements and whole rock Sr–Nd isotope data of Late Permian silicic plutonic and volcanic rocks from the Phan Si Pan–Tu Le region in NW Vietnam were collected in order to establish their petrogenetic relationship with the magmatic rocks of the Song Da zone (NW Vietnam) and the alkaline silicic rocks of the Panxi area of the Emeishan large igneous province (ELIP) in SW China. The granites and rhyolites have geochemical characteristics of anorogenic granites (e.g. high Fe# and high Ga/Al) and are further subdivided based on mineralogy. The Phu Sa Phin and Muong Hum granites contain sodic to sodic–calcic amphiboles and sodic pyroxene, while the Phan Si Pan granite does not. The Phu Sa Phin and Muong Hum granites occasionally show peralkaline to metaluminous compositions, while the Phan Si Pan granites and the Tu Le rhyolites have metaluminous to peraluminous compositions. The chondrite-normalized REE patterns [(La/Yb)N=3.6–51.3, (Gd/Yb)N=1.4–3.4] and the primitive mantle-normalized spidergrams (enrichment in high field strength elements) are similar to those of the Song Da silicic rocks and the Panzhihua and Taihe granites in the Panxi area. Rhyolite-MELTS modeling suggest that the silicic magmas were likely generated by fractionation of the Song Da high-Ti basalts. The εNd(t) values range from weakly negative to moderately positive values (−2.2 to +2.2), suggesting the silicic rocks may be assimilated by basement rocks during magma emplacement. Permian magmatic rocks in the Phan Si Pan–Tu Le region are petrologically, geochemically and geochronologically comparable to those of the inner zone of ELIP. Although Permian lithospheric mantle and lower crustal structure of the Phan Si Pan–Tu Le region may be destroyed by a lithospheric removal in response to Cenozoic India–Eurasia collision, our results along with recent geophysical analyses in NW Vietnam suggest that the upper to middle crust of Phan Si Pan–Tu Le region represent a remnant upper to middle crust of inner zone of ELIP.

The Axum–Adwa basalt–trachyte complex: a late magmatic activity at the periphery of the Afar plume

The Axum–Adwa igneous complex consists of a basalt–trachyte (syenite) suite emplaced at the northern periphery of the Ethiopian plateau, after the paroxysmal eruption of the Oligocene (ca 30 Ma) continental flood basalts (CFB), which is related to the Afar plume activity. 40Ar/39Ar and K–Ar ages, carried out for the first time on felsic and basaltic rocks, constrain the magmatic age of the greater part of the complex around Axum to 19–15 Ma, whereas trachytic lavas from volcanic centres NE of Adwa are dated ca 27 Ma. The felsic compositions straddle the critical SiO2-saturation boundary, ranging from normative quartz trachyte lavas east of Adwa to normative (and modal) nepheline syenite subvolcanic domes (the obelisks stones of ancient axumites) around Axum. Petrogenetic modelling based on rock chemical data and phase equilibria calculations by PELE (Boudreau 1999) shows that low-pressure fractional crystallization processes, starting from mildly alkaline- and alkaline basalts comparable to those present in the complex, could generate SiO2-saturated trachytes and SiO2-undersaturated syenites, respectively, which correspond to residual liquid fractions of 17 and 10 %. The observed differentiation processes are consistent with the development of rifting events and formation of shallow magma chambers plausibly located between displaced (tilted) crustal blocks that favoured trapping of basaltic parental magmas and their fractionation to felsic differentiates. In syenitic domes, late- to post-magmatic processes are sometimes evidenced by secondary mineral associations (e.g. Bete Giorgis dome) which overprint the magmatic parageneses, and mainly induce additional nepheline and sodic pyroxene neo-crystallization. These metasomatic reactions were promoted by the circulation of Na–Cl-rich deuteric fluids (600–400 °C), as indicated by mineral and bulk rock chemical budgets as well as by δ18O analyses on mineral separates. The occurrence of this magmatism post-dating the CFB event, characterized by comparatively lower volume of more alkaline products, conforms to the progressive vanishing of the Afar plume thermal effects and the parallel decrease of the partial melting degrees of the related mantle sources. This evolution is also concomitant with the variation of the tectono-magmatic regime from regional lithospheric extension (CFB eruption) to localized rifting processes that favoured magmatic differentiation.

Towards a Model for Albitite-Type Uranium

Albitite-type uranium deposits are widely distributed, usually of low grade (<1% U3O8), but are often large and collectively contain over 1 million tonnes of U3O8. Uranium is hosted in a wide range of metamorphic lithologies, whose only common characteristic is that they have been extensively mylonitised. Ore minerals are disseminated and rarely in megascopic veins, within and adjacent to albitised mylonites. Grain size is uniformly fine, generally less than 50 microns. Scanning electron microscopy reveals that spatial association between uranium and various Ti-bearing phases is common. Gangue minerals include albite, carbonates (calcite and dolomite), and sodic pyroxene and amphibole. The ore rarely contains economic metals apart from uranium, phosphorous at Itataia being an exception. There is widespread evidence of hydrothermal zirconium mobility and hydrothermal zircon and other Zr phases are frequent and in some cases abundant gangue minerals. Positive correlations are noted between uranium and various high field strength elements. The group remains poorly described and understood, but a link to iron-oxide copper-gold (IOCG) deposits and/or carbonatite and/or alkaline magmatism is plausible.

Calculated stabilities of sodic phases in the Sambagawa metapelites and their implications

Pressure (P)–temperature (T) pseudosection analyses were carried out on metapelites from Sambagawa belt by using Perple_X 07 so as to determine mineral equilibria and the stability of sodic phases, in the model system MnO–Na2O–K2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–(CO2) under high-pressure (HP) conditions (0.5–2.5 GPa/400–600 °C). A pressure–XNA [=Na/(Na+Al–2K–1.5Ca)] pseudosection at 500 °C is also calculated to examine the effect of Na/Al value of the bulk-rock composition on the stabilities of sodic minerals. The bulk-rock compositions of Sambagawa metapelite are variable in XNA values. The calculation results of stable assemblages in metapelites under the blueschist and eclogite facies conditions indicate that: (i) paragonite and glaucophane are stable throughout the wide XNA range of bulk-rock compositions of host rocks; (ii) stable P–T conditions of sodic pyroxene enlarge with increasing XNA value; and (iii) the stability field of paragonite enlarges with the presence of CO2 in the metamorphic fluid. The suggested wide stability of paragonite in metapelites and the relationships between the stability of sodic pyroxene and the bulk-rock compositions explain the reasons why (i) the occurrence of omphacite in metapelites from several subduction-related terranes is rare; and (ii) paragonite commonly occurs as inclusions in garnet of metapelites from the Besshi region of the Sambagawa belt. Paragonite is an important sodic phase of HP metapelites, and a combination of paragonite and quartz with high residual pressure included in garnet may be a useful indicator to verify the evidence for the eclogite facies metamorphism recorded in metapelites.

Coexistence of pyroxenes jadeite, omphacite, and diopside/hedenbergite in an albite-omphacite rock from a serpentinite melange in the Kurosegawa Zone of Central Kyushu, Japan

An albite-omphacite rock, containing the three sodic pyroxenes of the jadeite-omphacite-diopside/ hedenbergite ternary system, has been investigated in detail to better characterize the phase relationships in this system. The rock is from a serpentinite mélange in the Kurosegawa Zone of Central Kyushu, Japan, and exhibits an omphacite overgrowth over diopside/hedenbergite and the further overgrowth of jadeite over omphacite, indicative of the successive replacement of pyroxenes during metamorphism. Partial replacement of omphacite by diopside/hedenbergite at the margins of K-feldspar veins suggests that all three pyroxenes were stable locally at the same stage of metamorphism. The peak temperature and pressure conditions are roughly estimated to be 350 °C and 5-10.8 kbar. Partitioning of Fe2+-Mg between pyroxene pairs is consistent with a recent thermodynamic model, indicating the attainment of local equilibrium for pyroxene pairs. The observed compositional gap is also consistent with the miscibility gap reported in many previous studies. The compositional field of omphacite in the ternary jadeite-aegirine-diopside/hedenbergite diagram extends from the jadeite-diopside/hedenbergite line toward the aegirine-rich field, maintaining approximately constant the jadeite content, resulting in an asymmetric phase diagram.

Phase relations and dehydration behaviour of psammopelite and mid-ocean ridge basalt at very-low-grade to low-grade metamorphic conditions

P–T pseudosections were calculated in the system Na–Ca–K–Fe–Mg–Al–Si–Ti–H–O with the PERPLE_X software package for the pressure–temperature range 1–25 kbar and 150–450 °C to gain a better understanding of the phase relations and the dehydration behaviour of psammopelite and mid-ocean ridge basalt (MORB) during prograde metamorphism at very-low-grade and low-grade. For this purpose, the thermodynamic data set of Holland & Powell was enlarged by end-member data for Fe2+- and Fe3+-pumpellyite, Fe2+- and Mg-stilpnomelane, actinolite, and magnesioriebeckite. In addition, a three-component solid-solution model for pumpellyite, a two-component model for stilpnomelane, and four-component models for amphibole and sodic pyroxene were created. Studied metamorphosed MORB and psammopelite contain around 6 wt.% structural H2O bound in minerals at 150 °C and pressures up to 5 kbar. Prograde metamorphism causes dehydration patterns, which are, for example, important for an understanding of the formation of accretionary-wedge systems: along a relatively high geotherm of 15 °C/km, significant dehydration (1.5 wt.% H2O release) of metapsammopelite can be noted in the temperature range 220–240 °C. We believe that this process leads to softening of the sedimentary cover of oceanic crust during early subduction so that this material can be scraped off the basic crust, which then would dehydrate at higher T , to form frontal accretionary prisms. Basal accretionary prisms are generated at lower geotherms ( e.g ., 12 °C/km) by dehydration of metapsammopelite between 260 and 300 °C. Again, metabasic material would dehydrate at significantly higher T and is, therefore, only subordinately involved in accretionary wedges. Along geotherms lower than 7 °C/km, almost no water is released up to temperatures of 400 °C and more. Thus, the corresponding material is subducted to mantle depths as in the subduction/exhumation channels of collision zones. We also hypothesize that accretionary-wedge complexes of the hot subduction zones in Precambrian times should mainly have formed by frontal accretion.

Metamorphic evolution of the Susunai metabasites in southern Sakhalin, Russian Republic

ABSTRACT The Susunai Complex of southeast Sakhalin represents a subduction-related accretionary complex of pelitic and basic rocks. Two stages of metamorphism are recognized: (1) a local, low-P/T event characterized by Si-poor calcic amphiboles; (2) a regional, high-P/T event characterized by pumpellyite, actinolite, epidote, sodic amphibole, sodic pyroxene, stilpnomelane and aragonite. The major mineral assemblages of the high-P/T Susunai metabasites contain pumpellyite + epidote + actinolite + chlorite, epidote + actinolite + chlorite, epidote + Na-amphibole + Na-pyroxene + chlorite-(-haematite. The Na-amphibole is commonly magnesioriebeckite. The Na-pyroxene is jadeite-poor aegirine to aegirine-augite. Application of empirically and experimentally based thermobarometers suggests peak conditions of T= 250–300C, P= 4.7–6 kbar. Textural relationships in Susunai metabasite samples and a petrogenetic grid calculated for the Fe3+-rich basaltic system suggest that pressure and temperature increased during prograde metamorphism.

Sodic Pyroxene and Sodic Amphibole as Potential Reference Materials for In Situ Lithium Isotope Determinations by SIMS

Two large pegmatitic crystals of sodic pyroxene (aegirine) and sodic amphibole (arfvedsonite) from the agpaitic igneous Ilímaussaq Complex, south Greenland were found to be suitable as reference materials for in situ Li isotope determinations. Lithium concentrations determined by SIMS and micro‐drilled material analysed by MC‐ICP‐MS generally agreed within analytical uncertainty. The arfvedsonite crystal was homogeneous with [Li] = 639 ± 51 μg g−1 (2s, n = 69, MC‐ICP‐MS and SIMS results). The aegirine crystal shows strongly developed sector zoning, which is a common feature of aegirines. Using qualitative element mapping techniques (EPMA), the homogeneous core of the crystal was easily distinguished from the outermost sectors of the crystals. The core had a mean [Li] of 47.6 ± 3.6 μg g−1 (2s, n = 33) as determined by SIMS. The seven micro‐drilled regions measured by solution MC‐ICP‐MS returned slightly lower concentrations (41–46 μg g−1), but still overlap with the SIMS data within uncertainty. Based on MC‐ICP‐MS and SIMS analyses, the variation in δ7Li was about 1‰ in each of the two crystals, which is smaller than that in widely used glass reference materials, making these two samples suitable to serve as reference materials. There was, however, a significant offset between the results of MC‐ICP‐MS and SIMS. The latter deviated from the MC‐ICP‐MS results by −6.0 ± 1.9‰ (2s) for the amphibole and by −3.9 ± 1.9‰ (2s) for the aegirine. This indicates the presence of a significant matrix effect in SIMS determinations of Li isotopes for amphibole and pyroxene relative to the basalt glasses used for calibration. Based on the MC‐ICP‐MS results, mean δ7Li values of +0.7 ± 1.2‰ (2s, n = 10) for the arfvedsonite crystal and of −3.7 ± 1.2‰ (2s, n = 7) for the core of the aegirine crystal were calculated. Adopting these values, SIMS users can correct for the specific IMF (instrumental mass fractionation) of the ion probe used. We propose that these two crystals serve as reference materials for in situ Li isotope determinations by SIMS and pieces of these two crystals are available from the first author upon request.

An order-disorder model for omphacitic pyroxenes in the system jadeite-diopside-hedenbergite-acmite, with applications to eclogitic rocks

A new thermodynamic model for sodic pyroxenes involving jadeite-diopside-hedenbergite-acmite is presented. This model allows for ordering of Mg, Al, Fe2+, and Fe3+ on the M1 sites, with coupled Na and Ca ordering on the M2 sites. It is calibrated on the basis of experiments in three chemical subsystems together with available information on ordering in different pyroxenes and on the limited calorimetric data. Central to the determination of the parameters of the model is the use of relationships among the end-member Gibbs energies and the interaction energies in the various possible non-independent sets of end-members. An important aspect of this model, which uses the symmetric formalism, is that Fe-Mg (FM) and Al-Fe3+ (AF’) mixing is not assumed to be ideal. The model accounts successfully for the experiments at both 600 °C and at higher temperatures involving ordered and disordered pyroxenes coexisting with albite and quartz in NCMAS, NCFAS, NF’AS systems as well as the available calorimetry. It is also able to predict the positions and slopes of tielines in coexisting jadeite + omphacite and omphacite + augite found in nature at lower temperatures. Although the model requires a large number of energy parameters, some of these are not critical to the behavior of the model, while for others the constraints from experiment, calorimetry, ordering state and solvi lead to very limited allowable combinations. The model places some restrictive constraints on the shape of the phase relations in the jadeite–augite–acmite system. Petrological applications of the model are illustrated via a phase diagram example for a MORB bulk composition eclogite.

Coexisting retrograde jadeite and omphacite in a jadeite-bearing lawsonite eclogite from the Motagua Fault Zone, Guatemala

Coexisting jadeite and omphacite were found as retrograde minerals in a jadeite-bearing lawsonite-eclogite from the Motagua Fault Zone, Guatemala. The lawsonite-eclogite is characterized by the occurrence of garnet porphyroblasts up to 2.5 cm in size, and the eclogite-facies parageneses, almandine-rich garnet + impure jadeite + lawsonite + rutile + quartz; garnet contains inclusions of impure jadeite, phengite, ferroglaucophane, chlorite, lawsonite, rutile, ilmenite, and quartz. Textural relations and parageneses and compositions of minerals indicate that the lawsonite-eclogite experienced two stages of metamorphism: prograde eclogite-facies stage (M 1 ) and retrograde stage (M 2 ). The impure jadeite (Jd-I) of the M 1 eclogite-facies occurs in both the matrix and as inclusions in garnet, and contains considerable amounts of augite and aegirine components (Jd 6 1 - 7 5 Aug 1 6 - 2 4 Ae 0 - 1 8 ). It is partly recrystallized to retrograde M 2 jadeite (Jd-II) (Jd 7 4 - 8 7 Aug 9 - 1 6 Ae 0 - 1 1 ) and omphacite (Jd 4 2 - 5 0 Aug 3 6 - 4 6 Ae 7 - 1 6 ); some of these two sodic pyroxenes may have crystallized from fluids. Both M 2 jadeite and omphacite show textural equilibrium and are believed to have grown concurrently. Based on the observed compositions and the phase relations of sodic pyroxenes from Carpenter (1980), the M 1 impure jadeite (Jd-I) may have had a disordered C2/c symmetry at T=ca. 450 °C and P = ca. 1.8-2.4 GPa, and was subsequently crystallized into jadeite (Jd-II) plus ordered P2/n omphacite during retrogression with infiltration of fluids at T < ca. 300 °C and P = ca. 0.7 GPa (M 2 ). The extreme low-T conditions during retrogression may have prevented reaction between eclogitic jadeite and adjacent minerals. Instead, eclogitic impure jadeite (plus fluid) has recrystallized into the retrograde jadeite + omphacite pair with a wide compositional gap.