Electron Microprobe Data Research Articles

Progressive aqueous alteration of CM carbonaceous chondrites

CM chondrites are aqueously altered rocks that contain ∼9 wt% H 2O + (i.e., indigenous water) bound in phyllosilicates; also present are clumps of serpentine-tochilinite intergrowths (previously called “poorly characterized phases” or PCP), pentlandite and Ni-bearing pyrrhotite. We studied 11 CM chondrites that span the known range from least altered to most altered. We used various petrologic properties (many previously identified) that provide information regarding the degree of aqueous alteration. There are no known unaltered or slightly altered CM chondrites (e.g., rocks containing numerous chondrules with primary igneous glass). Some CM properties result from processes associated with early and intermediate stages of the alteration sequence (i.e., hydration of matrix, alteration of chondrule glass, and production of large PCP clumps). Other petrologic properties reflect processes active throughout the alteration sequence; these include oxidation of metallic Fe–Ni, alteration of chondrule phenocrysts, changes in PCP composition (reflecting an increase in the phyllosilicate/sulfide ratio), and changes in carbonate mineralogy (reflecting the development of dolomite and complex carbonates at the expense of Ca carbonate). On the basis of these parameters, we propose a numerical alteration sequence for CM chondrites. Because there are no known CM samples that display only incipient alteration, the least altered sample was arbitrarily assigned to subtype 2.6. The most altered CM chondrites, currently classified CM1, are assigned to subtype 2.0. These highly altered rocks have essentially no mafic silicates; they contain chondrule pseudomorphs composed mainly of phyllosilicate. However, their bulk compositions are CM-like, and they are closer in texture to other C2 chondrites than to CI1 chondrites (which lack chondrule pseudomorphs). Using several diagnostic criteria, we assigned petrologic subtypes (±0.1) to every CM chondrite in our study: QUE 97990, CM2.6; Murchison, CM2.5; Kivesvaara, CM2.5; Murray, CM2.4/2.5; Y 791198, CM2.4; QUE 99355, CM2.3; Nogoya, CM2.2; Cold Bokkeveld, CM2.2; QUE 93005, CM2.1; LAP 02277, CM2.0; MET 01070, CM2.0. The proposed CM numerical alteration sequence improves upon the existing scheme of Browning et al. (1996) in that it does not require a complicated algorithm applied to electron-microprobe data to determine the average matrix phyllosilicate composition. The new sequence is more comprehensive and employs petrologic subtypes that are easier to use and remember than mineralogic alteration index values. New neutron-activation analyses of QUE 97990, QUE 93005, MET 01070, Murchison and Crescent, together with literature data, confirm the compositional uniformity of the CM group; different degrees of alteration among CM chondrites do not lead to resolvable bulk compositional differences. This suggests that the textural differences among individual CM chondrites reflect progressive alteration of similar hypothetical CM3.0 starting materials in different regions of the same parent body, with minimal aqueous transport of materials over appreciable (e.g., meters) distances.

Significance of monazite EPMA ages from the Quamby Conglomerate, Queensland

Th – U – Pb electron microprobe (EPMA) dating of mainly detrital monazite from the Quamby Conglomerate in the Eastern Succession of the Mt Isa inlier reveals three distinct monazite growth/recrystallisation events at around 1640, 1580 and 1490 Ma. These ages are particularly significant with respect to the timing of deposition, iron and gold mineralisation, and deformation in the Mt Isa inlier. The oldest age probably represents provenance from igneous rocks. In the sample, the majority of monazite growth occurred at 1580 Ma, coeval with peak metamorphism in the Eastern Succession. The low metamorphic grade of the conglomerate and wide compositional range of monazite bearing this age indicates that the monazite grew elsewhere and was later deposited in the conglomerate. Purple bands in the rock are composed mainly of coarse specular hematite with recrystallised margins that contribute to high (up to 20%) Fe2O3 contents in the conglomerate. Gold is also present in some of the samples. Some of the monazite grains contain small, younger (ca 1490 Ma) domains that may have grown/recrystallised in situ during a lower grade syn- or post-diagenetic metamorphic/hydrothermal event that may have been related to hematite (re)crystallisation. Together, these ages bracket deposition of the Quamby Conglomerate to between ca 1580 and 1490 Ma, the latter age most likely representing diagenesis. This depositional age also represents a maximum age for north – south-striking, upright folds of the Quamby Conglomerate and implies that significant ductile deformation has affected parts of the Mt Isa inlier after 1580 Ma and probably after 1490 Ma.

Discovery of sapphirine-bearing Mg–Al granulites in the North China Craton: Implications for Paleoproterozoic ultrahigh temperature metamorphism

We report here for the first time the occurrence of Mg–Al granulites within the khondalite belt of the North China Craton and provide robust evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions in this region. The UHT indicator assemblages in these rocks include equilibrium sapphirine + quartz, high alumina orthopyroxene + sillimanite + quartz and high temperature perthites. The surrounding khondalites also carry spinel + quartz equilibrium assemblage. The sapphirines occurring in various microstructural associations are markedly magnesian ( X Mg = 0.69–0.77) with a composition varying from 7:9:3 to 2:2:1 end members. Spinel is a solid solution of Mg-spinel and hercynite ( X Mg = 0.41–0.54) with ZnO content varying from 0.48 to 1.79 wt.%. Garnet shows a compositional range between pyrope and almandine ( X Mg = 0.39–0.48) with minor grossular (2.6–3.8 mol%) and spessartine (0.5–1.8 mol%). The orthopyroxenes display marked compositional variations in terms of Al 2O 3 content, with the core of the medium-grained orthopyroxene adjacent to sillimanite in the matrix showing the highest Al 2O 3 content (8.4 wt.%). The history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine, spinel, quartz and sillimanite within garnet. Sapphirine formed during the peak stage through the reaction: Spl + Qtz + Sil → Spr. Occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction: Spr + Qtz + Grt → Opx + Sil. A series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse-grained minerals such as: Opx + Sil + Qtz → Crd, Grt + Sil + Qtz → Crd, Grt + Qtz → Crd + Opx, Grt + O 2 → Crd + Qtz + Fe 2O 3 in Mag or Ilm, and finally Opx + Kfs + H 2O → Bt + Qtz. Whereas the stability of sapphirine + quartz and orthopyroxene + sillimanite + quartz in the matrix assemblage suggests T > 1000 °C and P > 10 kbar peak metamorphism, conventional geothermobarometric estimates confirm the ultrahigh-temperature nature and also trace the retrograde conditions (930–970 °C at P > 10 kbar — Al in Opx; 900–1000 °C — perthite; 930–990 °C — Grt–Opx; and 910–940 °C — sapphirine–spinel). Electron microprobe dating of monazites in textural association with the UHT assemblages yield a sharply defined isochron age of 1917 ± 48 Ma, which correlates with the 1927 ± 11 Ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel + quartz assemblage. An evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions, the rocks underwent initial isobaric cooling and subsequent isothermal decompression, and we interpret these trajectories to be part of an overall anti-clockwise P– T evolution. The study area, located within the northern margin of the North China Craton, defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen. Evidence for the extensional phase is provided by the several mafic magmatic intrusives of Paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle-derived magmas. The UHT metamorphism in North China Craton coincided with the assembly of the Paleoproterozoic supercontinent Columbia.

Zr-bearing minerals as indicators for the polymetamorphic evolution of the eastern, lower Austroalpine nappes (Stubenberg Granite contact aureole, Styria, Eastern Alps, Austria)

Contact metamorphism during emplacement of the Permian Stubenberg Granite has led to the formation of the assemblage forsterite + calcite + titanian clinohumite ± phlogopite ± chlorite in the adjacent marbles. During intrusion of the granite, veins, rich in Ti, Zr, REE, and actinides (U + Th) formed. These veins show a distinct mineralogical zoning-sequence with four zones. Going from the center of the vein to the margin, these zones include the assemblages (1) geikielite + baddeleyite + zirconolite + apatite + calcite + chlorite ± magnetite ± pyrrhotite assemblage, (2) calcite + chlorite, (3) forsterite + titanian clinohumite + chlorite + calcite ± phlogopite and (4) calcite ± forsterite. Baddeleyite is always replaced by zirconolite, possibly via the model reaction baddeleyite + 2 geikielite + 3 calcite + CO 2 = zirconolite + 2 dolomite. Zirconolite (Zirc I) shows a strong internal oscillatory zoning and distinct overgrowths (Zirc II), which have a different chemical composition. The chemical variation between the cores (Zirc I) and the rims (Zirc II) can be explained by using the substitutions: Me 5+ + Me 2+ = Ti 4+ + Me 3+ and REE 3+ + Me 5+ + Me 2+ = Ca 2+ + 2Ti 4+. In contrast to zirconolite from metacarbonates associated with contact aureoles, these analyses show elevated Nb contents of up to 4.5 wt.% Nb 2O 5 and unusually high W contents of 1–2 wt.% WO 3. A strong Eo-Alpine metamorphic overprint led locally to the formation of the assemblage chlorite + dolomite + calcite + ilmenite ± zirconolite II ± geikielite + Fe-sulfides. Late zircon grew locally, presumably as the last Zr-bearing mineral in the carbonates which formed during Permian contact metamorphism. The Zr-mineral sequence baddeleyite–zirconolite–zircon implies an increasing a(SiO 2) and fCO 2 during the growth of these minerals. Electron-microprobe dating of zirconolite (Zirc I) yields a weighted average age of 263 ± 16 Ma and indicates that the HFSE-and REE-rich assemblages formed during the Permian emplacement of the Stubenberg granite. As a result of the subsequent high- P Eo-Alpine metamorphic overprint (111 ± 15 Ma), HFSE and REE were locally re-mobilized leading to dissolution of Zirc I and reprecipitation of the REE and Nb-rich overgrowths of Zirc II.

Fluoro-magnesiohastingsite from Dealul Uroi (Hunedoara county, Romania): Mineral data and crystal structure of a new amphibole end-member

The new mineral fluoro-magnesiohastingsite, ideally (Na,K,Ca)Ca2(Mg,Fe3+,Al,Ti)5(Si,Al)8O22F2, a member of the calcic-amphibole-group, occurs in small cavities of an altered hematite-rich xenolith in the quaternary trachyandesite at Dealul Uroi, Hunedoara county, Romania. Associated minerals are: titaniferous hematite, augite, phlogopite, enstatite, feldspar, tridymite, titanite, fluorapatite, ilmenite and pseudobrookite. The mineral is reddish-brown to yellowish, with a light reddish-brown streak, and has a perfect {110} cleavage. The tenacity is brittle and the Mohs hardness is 6. Optically the mineral is biaxial (+) and weakly pleochroic, α = 1.642 (yellow-brown), β = 1.647 (light brown), γ = 1.662 (light brown) at 589 nm 2V (meas. and calc.) is 61°, dispersion was not observed. The orientation of β is parallel to b. The axial plane is (010) (γ ^ c = 26o). It is monoclinic, C2/m, with the unit-cell parameters a = 9.871(1) A, b = 18.006(2) A, c = 5.314(1) A, β = 105.37(1)°, V = 910.7(2) A3, Z = 2. The calcu- lated density is 3.18 g/cm3. The strongest lines in the X-ray powder diffraction pattern (dobs in A, (hkl), I) are: 9.008, (020), 27; 8.421, (110), 61; 3.377, (131), 44; 3.271, (240), 61; 3.124, (310), 100; 2.932, (221), 35; 2.805, (330), 28; 2.700, (151), 54. The crystal structure of fluoro-magnesiohastingsite was refined to R(F) = 0.054 using reflection intensities collected with MoKα X-radiation. Refined site-scattering values and considerations of mean bond lengths demonstrated an ordering of Fe3+ at M2. Electron-microprobe data and the A site-scattering values show significant Ca (0.19 apfu) together with K and Na at the Am and A2 sites. The site-scattering value and bond-length of O3 to cations indicate that this site is occupied by F, which was indi- rectly confirmed by infrared spectroscopy. The emp-analyses correspond to the empirical formula: A(Na0.50K0.22Ca0.17)0.89 BCa2.00 C(Mg4.03Fe3+ 0.70Al0.13Ti0.13)4.99 T(Si5.89Al2.11)8.00O22.00F2.00. Most companion minerals of fluoro-magnesiohastingsite containing fluorine, like phlogopite, fluorapatite and chondrodite, have a XF = F/(F+OH) > 0.79. Amphiboles of the hosting trachyandesite are fluoro-members (fluoro-edenite to fluoro-magnesio- hastingsite) as well but they have a more variable Si/Al-ratio as the xenolith-hosted fluoro-magnesiohastingsites.

U-Th-total Pb dating of monazite from orthogneisses and their ultra-high temperature metapelitic enclaves: implications for the multistage tectonic evolution of the Madurai Block, southern India

Electron microprobe dating of monazite was performed on enclaves of sapphirine-bearing granulites and their host granites from the southeastern part of the Madurai Block in India. The granulites are strongly migmatitic and comprise quartzofeldspathic leucosome veins and restitic domains of sapphirine, garnet, orthopyroxene, sillimanite and cordierite. Inclusions of sapphirine-quartz intergrowths in garnet testify to an UHT stage of metamorphism which was followed by a stage of decompression during which garnet was replaced by coronitic opx-sill intergrowths. Monazite inclusions in garnet, orthopyroxene and quartz often yield a bimodal age distribution of Mid- to early Neoproterozoic (1500-850 Ma) ages in the cores which are rimmed by late Neoproterozoic age domains (630-580 Ma). Monazites in the granite and the charnockite exclusively yield Pan-African values. The occurrence of sapphirine-quartz intergrowths and the early Neoproterozoic age population in the same petrographical position infers that the crystalline basement of the Madurai Block already experienced a stage of UHT metamorphism prior to the Pan-African orogenic event.

Phyllosilicate Alteration Mineral Assemblages in the Active Subsea-Floor Pacmanus Hydrothermal System, Papua New Guinea, ODP Leg 193

Pacmanus is an active submarine hydrothermal system in the Manus back-arc basin, Papua New Guinea, located at 1,600 to 1,700 m below sea level on the crest of a dacitic volcano. It is inferred to represent a modern analogue of ancient mineralizing sea-floor hydrothermal systems that produced volcanogenic polymetallic massive sulfide deposits. Ocean Drilling Program (ODP) Leg 193 drilled the subsea-floor hydrothermal alteration zone of Pacmanus in a high-temperature discharge area (Roman Ruins) and a low-temperature discharge area (Snowcap), reaching a maximum depth of 380 m below the sea floor. Evidence from short-wavelength infrared (SWIR) spectroscopy, X-ray diffraction (XRD) analyses, calculated normative mineral abundances, and electron microprobe data show that there are substantial variations in the hydrothermal phyllosilicate assemblages within the Pacmanus hydrothermal system. The altered dacite at Roman Ruins contains 10 to 25 wt percent normative chlorite + smectite and up to 15 wt percent normative illite. In contrast, altered dacite below Snowcap contains up to 50 wt percent combined normative illite + paragonite + chlorite + smectite + pyrophyllite. SWIR data show that pyrophyllite is particularly abundant between 50 and 120 and between 220 and 270 m below sea floor, indicating localized interaction of dacite with acidic hydrothermal fluids. Variations in the Na/K ratio of dioctahedral phyllosilicate, determined by electron microprobe and spectral analyses, confirm the presence of a paragonitic component at Snowcap. A model is suggested to explain the differences in the distribution of hydrothermal phyllosilicate assemblages in the two areas in terms of variations in fluid composition and changes to the subsea-floor hydrology in a dynamically evolving volcanic-hydrothermal environment.

Electron microprobe dating of monazite from high- T shear zones in the São José de Campestre Massif, NE Brazil

The easternmost domain of the Borborema Province, northeastern Brazil, presents widespread, extensional-related high-temperature metamorphism during the Brasiliano (=Pan-African) orogeny. This event reached the upper amphibolite to granulite facies and provoked generalized migmatization of Proterozoic metapelitic rocks of the Seridó Group and tonalitic to granodioritic orthogneisses of the Archean to Paleoproterozoic basement. We report new geochronological data based on electron microprobe dating of monazite from metapelitic migmatite and leuconorite within the high- T shear zones that make up the eastern continuation of the huge E–W Patos shear belt. These data were also constrained by using the Sm–Nd isotopic systematic on garnet from a syntectonic alkaline granite and two garnet-bearing leucosomes. The results suggest an age of about 578 to 574 Ma for the peak of the widespread high- T metamorphism. This event is best recorded by Sm–Nd garnet-whole rock ages. The U–Th–Pb isotopes on monazite of the metapelitic migmatite show a younger thermal event at 553 ± 10 Ma. When compared to the Sm–Nd garnet-whole rock ages, the U–Th–Pb electron probe monazite ages seem to record an event of slightly lower temperatures after the peak of the high- T metamorphism. This may reflect the difference in the isotopic behavior of the geochronological methods employed. Otherwise, the U–Th–Pb ages on monazites could indicate an event not yet very well defined. In anyway, this paper reveals the partial or even complete re-opening and resetting of the U–Th–Pb isotopic system produced by the action of low- T Ca-rich fluid.

Timing of metamorphism and formation of garnet granite in the Kontum Massif, central Vietnam: Evidence from monazite EMP dating

Garnet-bearing granite (Grt-granite) and high-grade pelitic gneisses occur in the Kannak Complex, southern Kontum Massif, central Vietnam. Electron microprobe (EMP) dating of monazite in Grt-granite and garnet-orthopyroxene gneiss (Grt-Opx gneiss) gave ages of ca. 260 Ma from cores and ca. 230 Ma from thin rims. Since most monazite grains are predominately of core composition, the ca. 260 Ma age indicates the timing of high-temperature metamorphism and the formation of Grt-granite. Grt-granite is locally accompanied by coeval intrusions of fine-grained gabbro, which is a candidate for the heat source leading to high-grade metamorphism and partial melting of the crust during the Late Permian.

Timing of metamorphism and formation of garnet granite in the Kontum Massif, central Vietnam: Evidence from monazite EMP dating

Garnet-bearing granite (Grt-granite) and high-grade pelitic gneisses occur in the Kannak Complex, southern Kontum Massif, central Vietnam. Electron microprobe (EMP) dating of monazite in Grt-granite and garnet-orthopyroxene gneiss (Grt-Opx gneiss) gave ages of ca. 260 Ma from cores and ca. 230 Ma from thin rims. Since most monazite grains are predominately of core composition, the ca. 260 Ma age indicates the timing of high-temperature metamorphism and the formation of Grt-granite. Grt-granite is locally accompanied by coeval intrusions of fine-grained gabbro, which is a candidate for the heat source leading to high-grade metamorphism and partial melting of the crust during the Late Permian.

The Xiaojiao high-grade eclogite-type rutile deposit in Jiangsu, China: Geology, geochemistry and metallogenesis

The Xiaojiao rutile deposit with stratiform, banded and lens-shaped rutile-bearing eclogite in the Sulu ultra-high pressure metamorphic zone represents the largest and richest primary rutile mineralization in the zone. Four kinds of rutile can be recognized. They are: (1) inclusions in garnet and omphacite; (2) intergranular infilling between garnet and omphacite; (3) altered remnant; and (4) hydrothermal infilling in veinlets. Electron microprobe data show that sodic metasomatism probably from metamorphic fluid could play an important role in rutile mineralization during high and ultra-high pressure meta-morphism. The present paper concludes that according to geochemical characteristics, the eclogite originated from mafic and ultramafic rocks in the upper mantle and crust-mantle boundary and the Xiaojiao eclogite-type rutile deposit is genetically closely related to ultra-high pressure metamorphism.

Archean nucleii and the distribution of kimberlite and related rocks in the São Francisco Craton, Brazil

Available gravimetric data allow defining the low Bouguer gravimetric anomaly associated with the Paleoproterozoic continental plate, which contains the Sao Francisco Craton. The high anomalies that mark the border of the relative low gravimetric anomaly are related probably to hidden sutures and dense rocks remains formed during sea-floor spreading and deformed later on along Neoproterozoic colisional belts. Kimberlite mineral analyses allied to interpretation of magnetic, geochronological and surface geology data suggest the presence of thick lithosphere below Archean terrains observed in the northern and southern portions of the craton. Important volcanism represented by kimberlite, kamafugite and alkaline intrusive complexes of Early and Late Cretaceous ages is recorded at the south-southwest border of the Sao Francisco Craton, along the Neoproterozoic Brasilia fold belt. Morphostructural and magnetic data suggest that besides the NW-SE structural control of the Alto Paranaiba high, a NE-SW extensional system is associated with igneous intrusions in inner Minas Gerais. Electron microprobe data on minerals from kimberlite sampled in northern and southern Sao Francisco Craton indicate that they contain temperatures belonging to the diamond stability field were recorded in some specific areas. Recovery of micro- and macrodiamonds confirm the assumption.

Genesis of monazite and Y zoning in garnet from the Black Hills, South Dakota

The paragenesis of monazite in metapelitic rocks from the contact aureole of the Harney Peak Granite, Black Hills, South Dakota, was investigated using zoning patterns of monazite and garnet, electron microprobe dating of monazite, bulk-rock compositions, and major phase mineral equilibria. The area is characterized by low-pressure and high-temperature metamorphism with metamorphic zones ranging from garnet to sillimanite zones. Garnet porphyroblasts containing euhedral Y annuli are observed from the garnet to sillimanite zones. Although major phase mineral equilibria predict resorption of garnet at the staurolite isograd and regrowth at the andalusite isograd, textural and mass balance analyses suggest that the formation of the Y annuli is not related to the resorption-and-regrowth of garnet having formed instead during garnet growth in the garnet zone. Monazite grains in Black Hills pelites were divided into two generations on the basis of zoning patterns of Y and U: monazite 1 with low-Y and -U and monazite 2 with high-Y and -U. Monazite 1 occurs in the garnet zone and persists into the sillimanite zone as cores shielded by monazite 2 which starts to form in the andalusite zone. Pelites containing garnet porphyroblasts with Y annuli and monazite 1 with patchy Th zoning are more calcic than those with garnet with no Y annuli and monazite with concentric Th zoning. Monazite 1 is attributed to breakdown of allanite in the garnet zone, additionally giving rise to the Y annuli observed in garnet. Monazite 2 grows in the andalusite zone, probably at the expense of garnet and monazite 1 in the andalusite and sillimanite zones. The ages of the two different generations of monazite are within the precision of chemical dating of electron microprobe. The electron microprobe ages of all monazites from the Black Hills show a single ca. 1713 Ma population, close to the intrusion age of the Harney Peak Granite (1715 Ma). This study demonstrates that Y zoning in garnet and monazite are critical to the interpretation of monazite petrogenesis and therefore monazite ages.

A natural scandian garnet

Garnet from an aposkarn achtarandite-bearing rodingite-like rock in Sakha-Yakutia, Russia, has a Sc content close to 6 wt% Sc 2 O 3 (~0.45 apfu). The scandian garnet is a relict mineral from a hightemperature, shallow-level melilite skarn. Structural and electron microprobe data for a crystal of the scandian garnet with cell parameter a = 12.331(1) Å, Ia3̅d allows refinement of the structural formula (Ca 2.97 Mg0.02Y0.01) Σ3 (Fe 3+ 0.663 Zr 0.584 Ti 4+ 0.294 Sc 0.153 Cr 0.152 Mg 0.094 Fe 2+ 0.04 Hf 0.008 V 0.003 ) Σ2 (Si 1.898 Al 0.420 Ti 4+ 0.359 Fe 3+ 0.323 ) Σ3 O 12 . Investigation of the composition of many of the scandian garnets reveals the existence of a solid-solution between kimzeyite-schorlomite Ca 3 (Zr,Ti) 2 (Al,Fe) 2 SiO 12 and the scandium analog of andradite Ca 3 Sc 2 Si 3 O 12 . This is the first report of a natural scandian garnet.

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.

Depositional constraints and age of metamorphism in southern India: U–Pb chemical (EMPA) and isotopic (SIMS) ages from the Trivandrum Block

We report U–Pb electron microprobe (zircon and monazite) and Secondary Ion Mass Spectrometry (SIMS) U–Pb (zircon) ages from a granulite-facies metapelite and a garnet–biotite gniess from Chittikara, a classic locality within the Trivandrum Block of southern India. The majority of the electron-microprobe data on zircons from the metapelite define apparent ages between 1500 and 2500 Ma with a prominent peak at 2109±22 Ma, although some of the cores are as old as 3070 Ma. Zircon grains with multiple age zoning are also detected with 2500–3700 Ma cores, 1380–1520 mantles and 530–600 Ma outer rims. Some homogeneous and rounded zircon cores yielded late Neoproterozoic ages that suggest that deposition within the Trivandrum Block belt was younger than 610 Ma. The outermost rims of these grains are characterized by early Cambrian ages suggesting metamorphic overgrowth at this time. The apparent ages of monazite grains from this locality reveal multiple provenance and polyphase metamorphic history, similar to those of the zircons. In a typical case, Palaeoproterozoic cores (1759–1967 Ma) are enveloped by late Neoproterozoic rims (562–563 Ma), which in turn are mantled by an outermost thin Cambrian rim (∼515 Ma). PbO v. ThO*2 plots for monazites define broad isochrons, with cores indicating a rather imprecise age of 1913±260 Ma (MSWD=0.80) and late Neoproterozoic/Cambrian cores as well as thin rims yielding a well-defined isochron with an age of 557±19 Ma (MSWD=0.82). SIMS U–Pb isotopic data on zircons from the garnet–biotite gneiss yield a combined core/rim imprecise discordia line between 2106±37 Ma and 524±150 Ma. The data indicate Palaeoproterozoic zircon formation with later partial or non-uniform Pb loss during the late Neoproterozoic/Cambrian tectonothermal event. The combined electron probe and SIMS data from the metapelite and garnet–biotite gneiss at Chittikara indicate that the older zircons preserved in the finer-grained metapelite protolith have heterogeneous detrital sources, whereas the more arenaceous protolith of the garnet–biotite gniess was sourced from a single-aged terrane. Our data suggest that the metasedimentary belts in southern India may have formed part of an extensive late Neoproterozoic sedimentary basin during the final amalgamation of the Gondwana supercontinent.

Mineralogy of the U - Se-polymetallic deposit Niederschlema-Alberoda, Erzgebirge, Germany. IV. The continuous clausthalite-galena solid-solution series

In a sample from the Niederschlema-Alberoda U - Se-polymetallic deposit, western Erzgebirge, Germany, the entire PbSe - PbS solid-solution series was observed associated with uraninite, coffinite, hematite, acanthite, sphalerite, chalcopyrite, pyrite, and lollingite. Early deposited, Se-rich members of the Pb(Se, S) series occur as fracture fillings inside spherical uraninite or on its surface or form anhedral to subhedral grains precipitated in the immediate neighbourhood of the U minerals. Later crys- tallized, S-rich members of the series are affiliated with the sulfide minerals. The solid-solution series covers the range PbS1.00 - Pb(S0.04Se0.96)Σ1.00 virtually free of gaps, consistent with a temperature of formation of ≥ 100 uC. The PbSe - PbS solid solutions were likely deposited from hydrothermal fluids that became successively depleted in Se and enriched in S. The fugacities of sele- nium and sulfur covered the range -17 < logƒSe2 < - 26 and -17 < logƒS2 < - 22, respectively, implying ƒSe2/ƒS2 ≤ 1. The spher- ical texture of the uraninite, as well as its U-Th - total Pb age (192 ± 21 Ma), imply deposition of the Pb(Se, S) series during the Jurassic, contemporaneous with the formation of the bulk of the other selenium minerals. The electron-microprobe data from this study confirm earlier inferences on complete miscibility between clausthalite and ga- lena deduced from X-ray patterns of PbSe - PbS solid solutions from different uranium-vanadium deposits of the Colorado Pla- teau (Coleman 1959). In Niederschlema-Alberoda, the entire clausthalite - galena series occurs in a single section.

Equilibrium mineral–fluid calculations and their application to the solid solution between alunite and natroalunite in the El Indio–Pascua belt of Chile and Argentina

The effects of temperature and bulk-fluid K/Na on the alunite−natroalunite solid solution have been determined at 200 bar and 100–300 °C. These are conditions typical of magmatic-hydrothermal alteration and precious-metal mineral deposition in high-sulfidation environments, which are characterized by the presence of alunite-bearing alteration zones. Our model is based on published experimental parameters, which indicate that both alunite and natroalunite are stable over this temperature range, and that increased Na substitution is favored at higher temperature. Our calculation results predict that, at all temperatures, fluids in equilibrium with natroalunite are significantly more Na-rich than those typical of modern volcanic-hydrothermal environments. The results also indicate that a large variation in fluid K/Na is required to precipitate both K- and Na-rich alunite at high temperature (assuming near-equilibrium conditions). At lower temperature, much less variation in fluid composition can yield compositions near those of the end-members. Controls on alunite–natroalunite solid solution in natural systems were evaluated using electron-microprobe data from high-sulfidation gold deposits of the El Indio–Pascua belt. The results indicate that higher temperatures of deposition correlate with a greater range of Na substitution in alunite. Elevated Na content in alunite is also locally attributed to higher concentrations of Na in the source fluid as a result of the leaching and alteration of andesitic host rocks. Dacitic to granitic host lithologies in other regions of the belt are correlative with more K-rich alunite compositions. The results from this study suggest that the use of alunite K–Na composition as a guide to mineral exploration should be treated with caution.

Constraints on clinopyroxene/melt partitioning of REE, Rb, Sr, Ti, Cr, Zr, and Nb during mantle melting: First insights from direct peridotite melting experiments at 1.0 GPa

Earlier piston-cylinder experiments in our laboratory produced a collection of mantle melting run products at 1.0 GPa that have now been analyzed by ion probe for selected REE, Ti, Cr, Rb, Sr, Y, Zr, and Nb. Natural starting materials were used and experiments were run in graphite-lined Pt capsules with the melt separated from the residual minerals into a layer of vitreous carbon spheres (VCS) to circumvent quench modification. The glass phase in 18 run products, representing melt percentages of ∼2–20 wt%, yielded excellent data that were inverted to yield the first estimates ever of clinopyroxene/melt distribution coefficients, Ds, derived from direct peridotite partial melting experiments. Uncertainties were estimated with a Monte Carlo method. For the REE and Y, these Ds were then compared to Ds calculated with the widely-used model of Wood and Blundy (1997) and the two sets overlap at the ±2σ level in 123 of 128 cases (∼96%). This indicates to us that: 1) the experiments analyzed here are well equilibrated with respect to major and trace element distributions, thus supporting the efficacy of the VCS technique and its variation involving diamond (e.g., Baker and Stolper, “Determining the composition of high-pressure mantle melts using diamond aggregates” [1994], Geochim. Cosmochim. Acta 58, 2811–2827); 2) the model of Wood and Blundy (1997), calibrated largely on the basis of large melt fraction, inverse- or sandwich-type experiments, describes REE and Y partitioning during peridotite melting well, even very near the solidus; and it suggests that the cpx/melt Ds derived here for other elements, not modeled by the Wood and Blundy formulation, are probably also correct for peridotite melting to within their ±2σ uncertainties. D sp/liq and D cpx/liq values for Cr calculated directly from electron microprobe data decrease by about a factor of five with increasing temperature and melt percentage. The degree to which our experiments appear to have equilibrated seems at odds with recent measurements of the diffusivities of REE in diopside which suggest that relatively small percentages of our starting mineral grains should have equilibrated diffusively. Instead, we suggest that equilibration occurs much more rapidly through the processes of recrystallization and grain coarsening, accomplished through dissolution and reprecipitation. This suggestion is supported by the observation that our final grain sizes are typically 5–10 times larger than the ∼10 μm starting sizes, indicating that substantial mass transfer occurred in our experiments, probably mediated by the melt phase in which diffusion is faster.

Accurate quantification of melt inclusion chemistry by LA-ICPMS: a comparison with EMP and SIMS and advantages and possible limitations of these methods

Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) has recently emerged as a powerful in situ microanalytical technique for major to trace elements in heterogeneous samples such as fluid and melt inclusions. Here, a rigorous comparison of melt inclusion (MI) data acquired by electron microprobe (EMP), ion microprobe (the secondary ion mass spectrometry, SIMS) and LA-ICPMS is used to evaluate the applicability and advantages/drawbacks of these approaches. We are specifically interested in determining if LA-ICPMS data on entire, unexposed, crystallized MI that cannot be homogenized in the lab are accurate and of a useful precision. Quantification of LA-ICPMS MI signals requires the use of an internal standard, i.e., the concentration of one element, or an element ratio, at the time of MI entrapment must be known independently, in order to derive the pure MI composition from the MI plus host mixed signal. Analysis of plagioclase-hosted glassy MI of a mid-ocean ridge basalt (MORB) sample from the East Pacific Rise illustrates that melt inclusion chemistry can be accurately quantified by LA-ICPMS, including the correction for postentrapment sidewall crystallisation of the host mineral without prior reheating in the lab. The LA-ICPMS data obtained on crystallized MI demonstrate agreement with the EMP and SIMS data on exposed glassy MI at the 1 standard deviation uncertainty level except for a few elements close to their limits of detection. LA-ICPMS data reduction schemes include the quantification of analytical uncertainty on each element of single MI. Therefore, weighted average element concentrations can be obtained for MI assemblages, at precisions that compare well with those of average element concentrations obtained by EMP and SIMS. Simple sample preparation minimizing inclusion loss through polishing combined with the analytical efficiency of 50 inclusions plus neighbouring host mineral at up to 40 elements per day enable the collection of statistically relevant datasets by LA-ICPMS. These allow to recognize nonrepresentative MI (e.g., heterogeneous entrapment). Application to individual clinopyroxene crystals from the AD79 pumice horizon of Mt. Somma-Vesuvius reveals chemical variability that exceeds the analytical precision on single melt inclusions. This variability was not obvious from the limited data set obtained by SIMS and EMP. The largest source of nonquantifiable error for EMP and SIMS data stems from the requirement of reheating the melt inclusions in the lab in order to reverse postentrapment crystallisation onto inclusion walls or growth of crystallites. For LA-ICPMS analysis of unexposed MI, the reliability with which the internal standard (IS) element concentration is known determines the quality of the data. LA-ICPMS, however, cannot analyse H 2O, F, S and Cl reliably, has higher limits of detection (LODs) than SIMS for some elements for MI below ∼25 μm, has lower spatial resolution than both EMP and SIMS and consumes much more sample per analysis. Therefore, EMP, SIMS and LA-ICPMS are complementary in MI research, and the type of application will determine the analytical method or methods of choice.