Calcic Amphibole Research Articles

Makarochkinite, Ca2Fe42+Fe3+TiSi4BeAlO20, a new beryllosilicate member of the aenigmatite-sapphirine-surinamite group from the Il'men Mountains (southern Urals), Russia

Makarochkinite, [VII](Ca1.64Na0.25Mn0.11)[VI](Fe2+3.56Fe3+1.46Ti0.61Mg0.25Mn0.02Nb0.038Ta0.007)[IV](Si4.48Be0.91Al0.54 Fe0.07O18)O2 (end-member Ca2Fe42+Fe3+TiSi4BeAlO20) from electron-microprobe data, Mossbauer spectroscopy, and single-crystal structure refinement, occurs in a granitic pegmatite near Lake Ishkul., Il’men Mountains (Southern Urals), Il’men State Reserve, Chelyabinsk Oblast., Russia. Associated minerals include danalite, phenakite, titanite, potassian calcic amphibole (ferro-edenite and hastingsite), biotite, ilmenite, magnetite, ferrocolumbite, fergusonite-(Y), and samarskite-(Y). Makarochkinite forms equant masses 5–50 mm across, black in hand specimen; luster vitreous; it is opaque except in slivers <1 μm thick. It is brittle; Moh’s hardness 5.5–6, and has no discernable cleavage; fracture is uneven. Twinning is absent. The measured density is 3.93(1) g/cm3; calculated density 3.933 g/cm3. It is optically biaxial, α = 1.835 ± 0.014 and γ = 1.865 ± 0.015 from reflectance data, pleochroism marked: X = greenish brown, Y = yellowish brown, Z = reddish brown; Y ≤ X < Z . Single-crystal X-ray diffraction gives triclinic symmetry, space group P 1, Z = 2, wR ( F 2) (all) = 0.096 for 6497 reflections, a = 10.355(2), b = 10.751(3), and c = 8.873(2) A, α = 105.707(8), β = 96.227(6), γ = 124.861(6)°, V = 735.7(3) A3. The eight strongest lines in the powder pattern [ d -spacing (A),( I ),( hkl )] are 7.997(57)(100), 4.779(29)(011), 3.120(32)(012), 2.924(69)(013), 2.676(77)( 203), 2.530(100)( 213), 2.410(28)(223), 2.075(39)(411). Of the aenigmatite-sapphirine-surinamite group minerals, makarochkinite is compositionally closest to rhonite and hogtuvaite. It is distinguished from rhonite by Fe2+ > Mg and by the presence of 0.91–0.98 Be per 20 cations; Be occupies the two most polymerized T sites in roughly equal amounts. Distinction from hogtuvaite is based on occupancy of the M7 site, which is dominated by Fe3+ in hogtuvaite and by Ti in makarochkinite.

Quartz exsolution in clinopyroxene is not proof of ultrahigh pressures: Evidence from eclogites from the Eastern Blue Ridge, Southern Appalachians, U.S.A.

Oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahighpressure (UHP) metamorphism. The presence of apparently exsolved quartz is taken as evidence of decompression of a non-stochiometric Ca.Eskola component (Ca 0.5 ⃞ 0.5 AlSi 2 O 6 , CaEs) that is presumed to be stable only at UHP conditions. Eclogite from the Eastern Blue Ridge, North Carolina, contains clinopyroxene (Jd 20 CaTs 5 Ac 5 CaEs 0 Di 65 Hd 5 ) with oriented needles of quartz and calcic amphibole that appear to have exsolved together. The quartz + amphibole intergrowths are surrounded by 1.5 µm haloes of neoformed pyroxene (Jd 10 CaTs 10 Ac 5 CaEs 0 Di 70 Hd 5 ). The modes of quartz, amphibole, and clinopyroxene haloes were determined using BSE images, and reintegrated with the host clinopyroxene. Viewing the quartz and amphibole needles down the c-axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections. Reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no CaEs component. Clinopyroxene with CaEs solid solution has been repeatedly synthesized at UHP conditions. However, examination of the phase equilibria usually cited as evidence for CaEs stability at conditions of ≥25 kbar shows that clinopyroxene with 10 mol% CaEs is stable well within the quartz field, and provides a pressure minimum similar to the albite = jadeite + quartz barometer. Exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the Blue Ridge eclogite that lacks coesite or other evidence for UHP metamorphism. The presence of a diluted (5.10%) CaEs component in clinopyroxene does not require UHP conditions.

Determination of Lithium, Beryllium and Boron at Trace Levels by Laser Ablation‐Inductively Coupled Plasma‐Sector Field Mass Spectrometry

The analytical capabilities of laser ablation (LA)‐ICP‐MS in determining Li, Be and B at trace levels in geological samples have been tested on a series of glass reference materials and natural samples. The LA‐ICP‐MS instrument used consisted of a sector‐field ICP‐MS coupled with a laser ablation microprobe operating at either 266 or 213 nm wavelength. Reference glasses from NIST (SRM 612, 614 and 616) and MPI‐DING (KL2‐G, ML3B‐G, StHs6/80‐G, GOR128‐G, GOR132‐G, T1‐G and ATHO‐G) were selected to develop the analytical method and to assess the best instrumental configuration. A series of calcic amphiboles with different Li, Be and B concentrations were also analysed using both LA‐ICP‐MS and SIMS to test the applicability of the method to natural minerals. Results indicated that with a spot size of 40 μm the agreement between measured and reference values of Li, Be and B is generally better than 10% for NIST SRM 612 and 20% for NIST SRM 614. Average reproducibility at the 2s level was 10% for Li, 20% for Be and 15% for B. Limits of detection were approximately 100 ng g‐1 for Be and B and 200 ng g‐1 for Li. These results were confirmed by analyses carried out on natural amphiboles and compared well in terms of precision and accuracy with those commonly achieved by SIMS.

Determination of the local chemistry of iron in inorganic and organic materials

The systematic EELS analysis of a series of naturally occurring, synthetic and biological samples has provided a framework from which Fe valence of unknown materials can be determined and the relative ratios of the valences present can be calculated. The quantification of the relative ratios of Fe 3+/Fe 2+ is possible via the fitting of Gaussian or Voigt (with 89.3% Gaussian contribution) line profiles to the Fe L 3-edge. The ratio of Fe 3+/ΣFe is determined from the areas under the fitted peaks. The method is robust and has an error in the range of <10% on Fe 3+/ΣFe for samples with >2 atom% Fe. We present applications of this method including the investigation of polaron hopping in calcic amphibole, the quantification of Fe 3+/ΣFe in clay minerals from deep water sediments, quantification of Fe valence in human liver tissue and the determination of Fe valence in airborne particulate matter.

Magnesiosadanagaite, a new member of the amphibole group from Kasuga-mura, Gifu Prefecture, central Japan

Magnesiosadanagaite, ideally NaCa 2 [Mg 3 (A1, Fe 3+ ) 2 ]Si 5 A1 3 O 22 (OH) 2 , is a new member of the amphibole group occurring as rims of zoned amphibole in rock samples from the contact aureole in Kasuga-mura, central Japan. The amphibole is associated with phlogopite, titanite, calcite, pyrrhotite and chalcopyrite, and composed of an [4] A1-poorer core and [4] A1-richer rim. The core and rim are classified into pargasite [ [4] A1 = 1.84–2.19 atoms per formula unit (apfu); for O = 23] and pargasite-magnesiosadanagaite ( [4] A1 = 2.45–2.84 apfu), respectively. Both have relatively high [A] Na (0.67–0.96 apfu) and low K content ( [A] Na (0.70–0.92 apfu) and low K (0.09–0.28 apfu) values of magnesiosadanagaite refute the view that high K substitution at the A site is an essential feature of Si-poor calcic amphiboles. The crystal structure of magnesiosadanagaite was refined for two compositionally different samples. Magnesiosadanagaite has the lowest O(5)-O(6)-O(5) angles (160.4° and 161.8°) ever reported in C 2 m amphiboles, indicating maximal kinking of the double-chain of tetrahedra.

Computer simulation of Al-Mg ordering in glaucophane and a comparison with infrared spectroscopy

TheorderingofMgandAlovertheoctahedralsitesinglaucophane, (A) (8) Na2 (6) (Mg3Al2) (4) Si8O22(OH)2,hasbeenstudied by Monte Carlo simulation using a model Hamiltonian parameterised using both empirical interatomic interactions and ab initio calculations. It is found that Al is fully ordered at the M(2) site, with disorder beginning to appear for temperatures above ~1000 K. Infrared spectra of threesynthetichigh-PT glaucophane-nybo ite amphiboles were also collected and theOH-stretching frequencies usedtoinferthestateofAl-Mgordering.Thespectraofallthreeamphibolescompriseonlytwopeaksat ~3662cm -1 and~3720cm -1 , correspondingtoMgMgMg- OH- (A) andMgMgMg- OH- (A) Na,respectively.TheseinfraredspectrashowunequivocallythatM(1) andM(3) sites are fully occupied by Mg, and, therefore, (6) Al isfully orderedat M(2),in agreement withthebehaviour predicted by the computational studies and bond-valence considerations. Such a highly (6) Al-ordered state for alkali amphiboles contrasts starkly with calcic amphiboles synthesized under similar pressure-temperatureconditions,whichhaveahighdegreeof (6) AldisorderoverM(2)andM(3)sites.Thisdifferencebetweenalkali and calcic amphibolesshows the major influence that the M(4) cation (monovalent versus divalent) has, via its bonding relations to O(4), in controlling the ordering of trivalent cations over the octahedral sites in amphiboles.

Phase equilibria in subducting basaltic crust: implications for H 2O release from the slab

Fluids released from subducting slabs induce partial melting of the mantle wedge above the slab, which in turn is responsible for arc volcanism at the Earth’s surface. The partially hydrated basaltic layer of the slab is a potential source of these fluids and a major reservoir for H 2O at depth. Constraining the stability domains of hydrous phases and the position of the dehydration reactions in this system in pressure–temperature ( P– T) space is essential to describe and quantify the fluid release from subducting oceanic crust into the overlying mantle wedge. Experiments were conducted in the ranges of 2.2–3.4 GPa and 625–750°C to determine phase equilibria in an H 2O-saturated natural basalt at conditions relevant to subduction zones. The experimental duration was typically 1 month, although some experiments were replicated with a shorter run duration (1–2 weeks) in order to identify potentially metastable phases. A mixture of a natural mid-ocean ridge basalt (MORB) glass and mineral seeds was used as the starting material. Oxygen fugacity was buffered within ±1.3 log units of nickel-bunsenite (NiNiO). The results obtained show that a calcic amphibole (barroisite) is stable from 2.2 to about 2.4 GPa. At 2.6 GPa, it is replaced by a sodic amphibole (near end-member glaucophane), which is stable up to 3 GPa at 625°C. This high-pressure assemblage constitutes a true analog of a natural amphibole-bearing eclogite and the first synthesis of glaucophane from a rock of basaltic composition. As opposed to the results of previous studies on basaltic compositions [A.R. Pawley, J.R. Holloway, Science 260 (1993) 664–667; S. Poli, Am. J. Sci. 293 (1993) 1061–1107; S. Poli, M.W. Schmidt, J. Geophys. Res. 100 (1995) 22299–22314; M.W. Schmidt, S. Poli, Earth Planet. Sci. Lett. 163 (1998) 361–379], chloritoid is present only as a metastable phase in the pressure–temperature range investigated here. Metastability of chloritoid in earlier experiments, due to short run duration, is the most likely explanation for this difference, and suggests that chloritoid does not play an important role in the overall dehydration process of the basaltic layer in subduction zones. At pressures above the stability field of amphibole, zoisite/clinozoisite becomes the stable hydrous phase at temperatures above 645°C, whereas lawsonite is stable at lower temperatures. The positions of the zoisite-out and lawsonite-out reactions determined in this study indicate that, for an intermediate temperature subduction zone, the basaltic layer of the slab would be completely dehydrated between 90 and 110 km depth.

PATTERNS OF TRACE-ELEMENT DISTRIBUTION IN CALCIC AMPHIBOLES AS A FUNCTION OF METAMORPHIC GRADE

The distribution of the rare earths and other trace elements in calcic amphiboles from the Njurundukan mafic complex, northwestern Baikal region, in Russia, depends on metamorphic grade. Both total REE and La/Yb in amphiboles decrease with decreasing temperature of metamorphism. Consistent partition-coefficients for trace elements in amphibole and coexisting garnet, suggestive of equilibrium, are best developed in granulite-facies rocks. Such a consistent distribution of the REE is disturbed in amphibolite-facies rocks owing to incipient metasomatism. Amphiboles from metasomatic rocks show REE distributions that differ from those in amphiboles belonging to the metamorphic culmination, and commonly are inherited from precursor minerals. This property leads to the establishment of the sequence of mineral formation in metasomatically altered rocks. Comparison with amphiboles from the high-pressure Lapland granulite complex, Baltic Shield, suggests that pressure does not affect REE abundance and distribution in amphiboles.

Intergrowths between anthophyllite, gedrite, calcic amphibole, cummingtonite, talc and chlorite in a metamorphosed ultramafic rock of the KTB pilot hole, Bavaria

A metamorphosed ultramafic rock, penetrated by the KTB pilot hole at a depth of 1382.36 m, contains the assemblage Ca-amphibole — anthophyllite — chlorite — talc, formed at about 630°C/10 kbar under conditions of the high-P amphibolite facies. EMP analyses of the Ca-amphibole yielded magnesiohastingsite (to pargasite) compositions with (Al+Fe 3+ +Cr+Ti) [6] of 1.0-1.2 p.f.u. and (Fe 2+ +Mn)/(Mg+Fe 2+ +Mn) ratios of 0.10-0.16, while coexisting anthophyllite has (Al+Fe 3+ +Cr+Ti) [6] ≤ 0.16 and (Fe 2+ +Mn)/(Mg+Fe 2+ +Mn) ratios of 0.21-0.29. Chlorite is clinochlore with Al [4] /(Al [4] +Si) of 0.27-0.30 and Fe 2+ /(Fe 2+ +Mg) of 0.12-0.14. TEM investigations revealed, for the first time, a complex lamellar intergrowth in the sequence talc → anthophyllite → anthophyllite/cummingtonite intercalations → anthophyllite → Ca-amphibole. This lamellar intergrowth is about 15 μm in width, but individual cummingtonite lamellae intercalated with dominant anthophyllite are only 1 μn wide. All the amphiboles share the directions b * and a * ; anthophyllite and the monoclinic amphiboles are intergrown along (100); anthophyllite and talc are intergrown with (100) Ath //(001) Tlc . In other areas anthophyllite and chlorite are intergrown with a * Ath // c * Chl and b * Ath // b * Chl and Ca-amphibole exsolves lamellae of cummingtonite //(100). With one possible exception, cummingtonite has the space group C 2/ m . Anthophyllite displays chain multiplicity faults //(010), anthophyllite and cummingtonite chain arrangement faults //(100). Microstructures suggest that anthophyllite was formed at the expense of cummingtonite that is interpreted as a high-T precursor phase testifying to a possible earlier, granulite-facies metamorphic stage. During the retrograde P-T path, on cooling presumably below 500-550°C, anthophyllite exsolves platelets or lamellae of gedrite with {hk0} composition planes. Orientations close to {230} and {110} have been recorded, so far not described in the literature. Alteration products of biotite are composed of clinochlore and a sheet silicate intermediate in composition between chlorite and biotite, i.e. , with partly high K contents, but with chlorite metric.

Trace-element partitioning between synthetic potassic-richterites and silicate melts, and contrasts with the partitioning behaviour of pargasites and kaersutites

Solid/liquid partition coefficients for large ion lithophile elements (Ba, Rb, Sr), high field strength elements (Zr, Hf, Nb, Ta, Ti), rare earth elements (La-Yb), Pb, Th, U and selected transition elements (Sc, V) were determined by means of Secondary Ion Mass Spectrometry on potassic-richterites synthesised at upper mantle conditions ( P = 1.4 GPa and T = 850-1020°C) from silica-rich lamproites. Most trace elements display an incompatible behaviour in potassic-richterites; only Sr, Ti, Sc and V show strong positive anomalies in the partitioning pattern. When S/L D for potassic-richterites are compared with those for calcic amphiboles (pargasites and kaersutites) several differences become evident. In general, S/L D are lower in potassic-richterites; also, different partitioning patterns are apparent for RE and LIL elements. These differences are discussed in terms of the distinct crystal-chemical behaviour of the involved amphibole end-members, with particular emphasis to the available charge-balance mechanisms and to the site dimensional constraints ruling incorporation of trace elements in the various sites. The distinct partitioning behaviours of trace elements in potassic-richterites and pargasites and kaersutites imply that melts produced from amphibole-bearing sources may differ markedly depending on the type of amphibole crystallised. Therefore, the new partitioning data are used to discuss the role of potassic-richterite in its principal modes of occurrence, namely in lamproites, in peralkaline ultramafic veins in the lithospheric mantle, and in the deeper parts of subduction zones.

Cation distribution in calcic amphiboles from the Nuggihalli Greenstone belt, S. India and its geological significance

Amphiboles from Nuggihalli schist belt are analyzed and found to be mainly magnesio-hornblende, ferro-hornblende and actinolitic hornblende. Mossbauer study of actinolitic hornblende shows presence of iron in the Ml, M2, M3 and M4 sites. A lower QS value (1.66 mm/s) at M4 site for Fe 2 + indicates a faster rate of crystallization of the studied sample whereas Fe 3 + /Fe 2 + ratio (0.14) indicates that the amphiboles were formed in an oxidizing environment (high fO 2 ). Based on plagioclase-amphibole geothermometry, equilibration temperature of meta-anorthosite is seen to range between 648 °C (921 K) and 704 °C (977K). The amphibolite is found to be equilibrated in a water rich environment (X H 2 O ranging from 0.81 to 0.89). Considering the pressures as 5 and 7kbar, and the amphibolite subassemblages such as, calcite-rutile-quartz and epidote-plagioclase-calcite, in the T-X C O 2 section (derived from THERMOCALC program) the invariant points are seen to be at (i) X C O 2 = 0.11, T = 493 °C and (ii) X C O 2 = 0.19, T = 562 °C at the respective pressures. The calculated M2-M4 distribution coefficient (K D ) in Nuggihalli actinolitic hornblende is found to be 0.0034, which corresponds to a temperature of equilibration at around 435 °C.

Fayalite, pyroxene, amphibole, annite and their decay products in mafic clots within Younger Granites of Nigeria: Petrography, mineral chemistry and genetic implications

The Younger Granites of Nigeria, emplaced in the basement complex 213–141 Ma ago, occur in the form of ring complexes. About 50 complexes are known with the town of Jos approximately in the center of the area. Apart from biotite granite (main rock type) granites and porphyries occur that contain olivine, pyroxene, amphibole, and mica. Granites and porphyries of five ring complexes were investigated in this study. These are the Jos-Bukuru, Shere, Ganawuri, Rop, and Ririwei complexes. Among the granitic minerals (feldspar and quartz) mafic clots were observed being mainly rounded to oval in shape (300 μm to 3 mm) often showing reaction rims and corroded outlines. These clots contain extremely iron-rich minerals: Olivine, exclusively found in the granites, is represented by nearly pure fayalite; Ca–Fe pyroxene, restricted to rocks (granites and porphyries) of the Shere complex, consist of hedenbergite and iron-rich augite; calcic amphiboles (occurring in all complexes) have Mg/(Fe 2+ + Mg)-ratios mainly below 0.2 and vary from hastingsite via ferro-edenite to ferro-hornblende and ferro-actinolite; and mica (not observed in the Rop complex) consist nearly of the annite endmember. With decreasing temperature (lying in the range 850–600 °C) the following mineral succession was found: fayalite, Ca–Fe pyroxene, calcic amphibole, and annite. These minerals were directly formed from the melt. Due to cooling and resulting disequilibrium conditions, the primary minerals became unstable and were replaced by a variety of minerals of sub-solidus origin and a temperature lower than 600 °C. Fayalite decomposes into grunerite + magnetite or alters into ferro-saponite; pyroxene is rarely replaced by calcic amphibole and often by an unidentified water-rich mineral; calcic amphibole may be replaced by titanite and ilvaite or decomposes into grunerite, chlorite and stilpnomelane; and annite into chlorite (iron-rich chamosite or delessite). Different in composition are pyroxene and amphibole of the albite-rich granite of the Ririwei complex where fayalite and annite do not occur. Pyroxene is dominated by the aegirine endmember and amphibole is arfvedsonite. The formation of these minerals is attributed to sodic metasomatism and higher oxygen fugacity conditions than observed in the other rocks.

Tectonometamorphic evolution of the Acatlan Complex eclogites (southern Mexico)

The Acatlan Complex of southern Mexico is linked to the evolution of the Appalachian–Caledonian chains and records events related to the Taconian, Acadian, and Alleghanian orogenies of northeastern North America. Mafic eclogites and garnet amphibolites from two selected localities are used to partially reconstruct the tectonometamorphic evolution of this complex. Eclogites contain garnet (almandine) + Ca–Na pyroxene + phengitic mica + zoisite–clinozoisite + quartz ± Ca–Na amphibole (barroisite, katophorite) ± albitic plagioclase ± rutile. Phase and textural relationships, thermobarometric determinations, and available radiometric ages indicate that eclogite-facies metamorphism took place during the Ordovician at temperatures around 560 ± 60°C and pressures between 11 and 15 kbar (1 kbar = 100 MPa). Eclogites underwent widespread retrogression to epidote-amphibolite then greenschist facies during exhumation, most probably during Devonian times. Epidote–amphibolite facies include the critical assemblage calcic pyroxene + calcic amphibole (magnesiohornblende and pargasite) + muscovite + garnet + plagioclase + epidote ± quartz, whereas greenschist facies is defined by the assemblage actinolite + albitic plagioclase + epidote + chlorite. Thermobarometric data suggest that retrogression occurred at temperatures between 510 ± 20°C and 300 ± 25°C and pressures ranging from 6 to 3.5 kbar. The obtained P–T (pressure–temperature) path suggest that the Acatlan Complex evolved in a more complex continental collisional setting, including intraoceanic arcs, than shown in previously proposed models.

Triassic blueschists and eclogites from northwest Turkey: vestiges of the Paleo-Tethyan subduction

Triassic eclogite and blueschist facies rocks occur as a thrust sheet, 25-km long and over 2-km thick, in an Eocene fold-and-thrust belt in northwest Turkey along the İzmir–Ankara suture. The thrust sheet consists mainly of metabasites with minor marble, phyllite and metachert, and rare lenses of serpentinite. The common blueschist facies mineral assemblage in the metabasites is sodic amphibole+epidote+albite+chlorite+phengite±garnet. Sodic amphibole commonly shows replacement by barroisite, and there is continuous petrographic transition from blueschist–metabasites to barroisite-bearing epidote–amphibolites. Eclogite with the mineral assemblage of garnet+sodic pyroxene+sodic–calcic amphibole+epidote is found only in one locality. P– T conditions of the epidote–blueschist facies metamorphism are estimated as 450±50 °C and 11±2 kbar. The blueschist formation was followed by a decrease in pressure and increase in temperature, leading to the development of barroisite-bearing epidote–amphibolites. Phengite, sodic amphibole and barroisite Ar/Ar ages from three metabasic rocks range between 215 and 205 Ma, and indicate Late Triassic high-pressure metamorphism. The Triassic blueschists in northwest Turkey constitute part of a much larger allochthonous tectonic unit of Triassic mafic volcanic rocks. They probably represent the upper layers of a Triassic oceanic plateau, which was accreted to the Laurasian margin during the latest Triassic. The close spatial association of the Triassic and Cretaceous blueschists along the İzmir–Ankara suture suggests that the suture represents a long-lived plate boundary of Late Palaeozoic to early Tertiary age.

Magmatic versus metamorphic origin of olivine-plagioclase coronas

An SEM, REE, and mineral Sm–Nd isotope study on olivine-plagioclase coronas in the Vestre Dale gabbro, Norway, was carried out in order to solve the temporal relationships within these coronas. It is demonstrated that in contrast to common models, corona formation is a multistage, late-magmatic process, starting with the formation of orthopyroxene by (partial) dissolution of olivine, followed by the nucleation and rapid outward growth of orthopyroxene+spinel symplectites at its outer margin, and concluded by replacement of this precursory outer shell by calcic amphibole. Isotopic equilibrium between inner shell orthopyroxene and the magmatic assemblage, and the REE contents of orthopyroxene and outer shell amphibole also provide strong arguments for a magmatic origin, with the local availability of fractionated magma acting as a limiting factor. The absence of replacement of intercumulus clinopyroxene by amphibole also favours a late stage magmatic rather than a metamorphic origin for corona amphibole.

MENSHIKOVITE, Pd3Ni2As3, A NEW PLATINUM-GROUP MINERAL SPECIES FROM TWO LAYERED COMPLEXES, RUSSIA

Menshikovite, ideally Pd3Ni2As3, is a new mineral species found in two localities in Russia. It occurs in a pod of coarsegrained, entirely altered gabbronorite rich in sulfides (up to 30 vol.%), hosted by a microgabbronorite, in the Vostok deposit, Lukkulaisvaara intrusion, in Karelia. There, menshikovite (up to 0.2 mm) is associated with chalcopyrite, pentlandite, calcic amphibole (mostly actinolite), clinochlore, merenskyite, kotulskite, sobolevskite, michenerite, hollingworthite, hessite, and a Re-rich sulfide. Menshikovite (up to 0.1 mm) also occurs in mineralized quartz–feldspar sandstones, which are metasomatically altered and recrystallized. They are located at the lower (exo)-contact of the Chiney intrusion in Siberia, and contain amphibo le(s), mica(s), chlorite, disseminated or massive sulfides (mostly chalcopyrite), Pd-rich maucherite, nickeline, sperrylite, mertieite -II or stibiopalladinite (or both), isomertieite, majakite, and michenerite. We also report a third occurrence of menshikovite from th e Oktyabr’ deposit, Noril’sk complex, in Siberia. In reflected light, menshikovite is pink and opaque. A cleavage is not observed. There is no evidence of bireflectance. The anisotropy is weak, from light gray to brownish gray. Reflectance percentages in air (and in oil) are, for R 1 and R2, at 470 nm 48.4, 50.2 (38.45, 39.3), at 546 nm 51.2, 53.2 (41.0, 41.8), at 589 nm 53.2, 55.3 (42.3, 43.3), and at 650 nm 56.6, 58.7 (46.6, 47.8). Fine twins are present. The average value of microhardness VHN 40, 50, 65 is 517.1 kg/ mm 2 ; it has Mohs hardness of ~5 and is brittle. The average of two sets of multiple wavelength-dispersion electron-microprobe analyses of menshikovite from Lukkulaisvaara gave Pd 48.18, Pt 0.07, Ni 17.77, Fe 0.07, As 33.91, the total is 100.0 wt.% (not normalized), corresponding to Pd2.99(Ni2.00Fe0.01)2.01As2.99 (basis: atoms = 8). Compositions of menshikovite from Chiney vary from Pd3.00(Ni1.98Co0.01)1.99As2.99 to Pd3.15(Ni1.97Co0.01)1.98As2.86. A solid-solution series probably exists between menshikovite and majakite [i.e., (Pd,Ni)2As of the -Pd2As type]. The powder pattern of menshikovite from Lukkulaisvaara was indexed on a hexagonal cell with a 8.406(4), c 6.740(4) A, and V 412.4(6) A 3 . The strongest eight lines in this pattern [ d in A (I)(hkl)] are 2.626(10)(112), 2.477(10)(202), 2.429(8)(300), 2.283(7)(301), 2.150(6)(103), 1.978(7)(113), 1.818(7)(400) and 1.757(6)(401). Menshikovite formed in volatile-rich environments relatively late in the crystallization history of the layered complexes. The name honors Yurii P. Men’shikov, of the Kola Science Center, Apatity, Russia.

Gradients in fluid composition across metacarbonate layers of the Wepawaug Schist, Connecticut, USA

Metamorphic index mineral zones, pressure–temperature (P–T) conditions, and CO2–H2O fluid compositions were determined for metacarbonate layers within the Wepawaug Schist, Connecticut, USA. Peak metamorphic conditions were attained in the Acadian orogeny and increase from ~420 °C and ~6.5 kb in the low-grade greenschist facies to ~610 °C and ~9.5 kb in the amphibolite facies. The index minerals oligoclase, biotite, calcic amphibole, and diopside formed with progressive increases in metamorphic intensity. In the upper greenschist facies and in the amphibolite facies, prograde reaction progress is greatest along the margins of metacarbonate layers in contact with surrounding schists, or in reaction selvages bordering syn-metamorphic quartz veins. New index minerals typically appear first in these more highly reacted contact and selvage zones. It has been postulated that this spatial zonation of mineral assemblages resulted from infiltration, largely by diffusion, of water-rich fluids across lithologic contacts or away from fluid conduits like fractures. In this model, the infiltrating fluids drove prograde CO2 loss and were derived from surrounding dehydrating schists or sources external to the metasedimentary sequence. The model predicts that significant gradients in the mole fraction of CO2 ( $$X_{CO_2 } $$ ) should have been present during metamorphism, but new estimates of fluid composition indicate that differences in $$X_{CO_2 } $$ preserved across layers or vein selvages were very small, ~0.02 or less. However, analytical solutions to the two-dimensional advection–dispersion-reaction equation show that only small fluid composition gradients across layers or selvages are needed to drive prograde CO2 loss by diffusion and mechanical dispersion. These gradients, although typically too small to be measured by field-based techniques, would still be large enough to dominate the effects of fluid flow and reaction along regional T and P gradients. Larger gradients in fluid composition may have existed across some layers during metamorphism, but large gradients favor rapid reaction and would, therefore, seldom be preserved in the rock record. Most of the H2O needed to drive prograde CO2 loss probably came from regional dehydration of surrounding metapelitic schists, although H2O-rich diopside zone conditions may have also required an external fluid component derived from syn-metamorphic intrusions or the metavolcanic rocks that structurally underlie the Wepawaug Schist.

Thermodynamics of the amphiboles: Ca-Mg-Fe2+quadrilateral

A thermodynamic model has been developed and calibrated for monoclinic and orthorhombic amphiboles compositionally contained within the Ca-Mg-Fe 2 + amphibole quadrilateral. The model incorporates the energetic consequences of cation ordering of Fe 2 + and Mg over nonequivalent sites in the crystal structure, and accounts for the temperature, pressure, and compositional dependence of the orthorhombic-monoclinic phase transition. Calibration is based on previously published work on the thermodynamic properties of Fe 2 + -Mg amphiboles and tremolite, and experimental cation-ordering data, along with solvus width and tieline orientations of natural coexisting amphiboles in the quadrilateral. Among derived parameters is the enthalpy of formation of end-member ferroactinolite (-10534.966 kJ/mol). A calculated FeMg - 1 isopotential solvus at mid-composition in the quadrangle agrees well with a revised calibration of experimental data from Cameron (1975). The solvus is not strongly asymmetric and narrows with increasing Fe/Mg ratio and temperature. Phase relations along the Mg and Fe sides of the quadrilateral are compared and contrasted. Cummingtonite-anthophyllite phase relations are shown in the T-X F e plane, with and without coexisting calcic amphibole. The breakdown reactions of quadrilateral amphiboles to assemblages of two pyroxenes, olivine, quartz, and H 2 O are depicted as functions of temperature, pressure, composition, H 2 O activity, and oxygen fugacity. Some highlights of their relevance to metamorphic and igneous systems are discussed.

The Candelaria-Punta del Cobre Iron Oxide Cu-Au(-Zn-Ag)Deposits, Chile

Several iron oxide-rich Cu-Au(-Zn-Ag) deposits define an approximately 5-km-wide and at least 20-km-long belt along the eastern margin of the coastal batholith near Copiapo, Chile. This belt includes the large Candelaria mine and a group of middle- and small-sized mines in the Punta del Cobre district, which is located about 3 km northeast of the Candelaria deposit. Estimated geologic resources of the belt are on the order of 700 to 800 million metric tons (Mt) at 1.0 percent Cu. The ore occurs in veins, breccia, and stringer bodies, and in replacement bodies that are roughly concordant to bedding. The orebodies are hosted mainly by volcanic and volcaniclastic rocks of the Punta del Cobre Formation and, in places, also occur in volcaniclastic intercalations in the lower part of the overlying Early Cretaceous Chanarcillo Group. Most of the larger orebodies in the belt are located where northwest-trending brittle faults intersect the contact between massive volcanic and volcaniclastic rocks. These northwest faults and a major northeast-trending ductile shear zone control portions of the ore of the Candelaria deposit. Chalcopyrite is the only hypogene Cu mineral. The Cu-Au ore is characterized by abundant magnetite and/or hematite and by locally elevated concentrations of Ag, Zn, Mo, and light rare earth elements. The ore is hosted mainly in zones with biotite-potassium feldspar ± calcic amphibole ± epidote alteration at Candelaria. In the Punta del Cobre district, ore in the deeper parts of the deposits is similarly associated, whereas at shallow levels it occurs in zones of biotite-potassium feldspar, or albite-chlorite ± calcite alteration. Mineralization at Candelaria-Punta del Cobre took place under relatively oxidized conditions manifested by the formation of specular hematite. In parts of the district, the pseudomorphic replacement of early specular hematite by magnetite during the main iron oxide formation marks a shift toward more reduced conditions or higher temperatures. The bulk of the magnetite probably formed at temperatures of about 500° to 600°C. The main sulfide stage followed with formation of pyrite and chalcopyrite at temperatures of >470° to 328°C. Subsequent martitization of the magnetite points to a temperature decrease. Cooling of the hydrothermal system is also indicated by the homogenization temperatures of <236°C of saline fluid inclusions in late-stage calcite. Oxygen isotope combined with microthermometric data suggest that magmatic fluids or nonmagmatic fluids equilibrated with magmatic silicates were dominant during the main copper mineralization. Relatively light oxygen isotope signatures of fluids in equilibrium with late calcite suggest mixing with a nonmagmatic fluid (e.g., basinal brines or meteoric waters) during the late stages of hydrothermal activity. Sulfur isotope ratios of chalcopyrite, pyrite, pyrrhotite, and sphalerite from the Bronce, Candelaria, Las Pintadas, Santos, and Socavon Rampa deposits range from δ 34SCDT values of –0.7 to +3.1 per mil. This narrow range of sulfur isotope ratios near 0 per mil is consistent with sulfur of magmatic origin. Anhydrite from the Candelaria mine paragenetically overlaps with chalcopyrite. Fluid inclusions in this anhydrite homogenize between 340° and 470°C and it has δ 34SCDT values between 14.5 and 17.5 per mil. A Δ sulfate-sulfide value of 13.4 per mil for a sample with coexisting anhydrite and chalcopyrite is consistent with sulfide-sulfate fractionation at temperatures on the order of 400°C. Ore lead isotope signatures are homogeneous and similar to those of least altered volcanic host rocks and nearby intrusive rocks. Radiometric ages, including new 40Ar/39Ar ages for hydrothermal alteration at Candelaria, point to a main Cu-Au mineralization event at Candelaria-Punta del Cobre at around 115 Ma. The ages indicate that ore formation was broadly coeval with batholithic granitoid intrusions and with regional uplift. They further imply that the Cu-Au(-Zn-Ag) deposits formed at shallow crustal levels (<3 km). The stable isotope data, the presence of previously reported hypersaline CO2-rich fluid inclusions in the main Cu ore stage and saline fluid inclusions in late-stage calcite, the oxidized character of the first ore-fluid pulses, and the mineralization age coeval with nearby intrusive activity are consistent with, but not unequivocally evidence of, magmatic fluid contribution into the hydrothermal system.

Strontian fluoro-magnesiohastingsite in Alban Hills lavas (Central Italy): constraints on crystallization conditions

AbstractA magmatic calcic amphibole (M4Ca≥1.50) characterized by large Sr and F contents (SrO&gt;1 wt.%, F&gt;2.5 wt.%) is described. According to the crystal-chemical formula, the amphibole can be classified as a strontian fluoro-magnesiohastingsite and the presence of Sr cations in theAsites is suggested. The amphibole occurs in the groundmass of ultrapotassic lavas from the Alban Hills Volcanic District (Central Italy). This peculiar chemical composition is due to the lavas' groundmass compositional trend occurring under low silica-activity conditions. We suggest that the occurrence of F amphiboles allows us to consider the Alban Hills Volcanic District magmas to be as rich in F as other ultrapotassic magmas.