Electron Microprobe Data Research Articles

A semi-empirical manganese-in-garnet single crystal thermometer

Kimberlite-hosted garnet xenocrysts, derived from the disaggregation of garnet peridotite, retain a record of their mantle source rock, including the temperature of its last equilibration. This information is locked into the garnet during its rapid ascent to the Earth's surface in kimberlite magmas. With the proper geochemical tools, diamond explorers can access the information stored within garnet xenocrysts and find economic diamond deposits. Presented here is a semi-empirical thermometer for peridotitic garnet xenocrysts based on the temperature-dependent exchange of manganese (Mn) between garnet and olivine with the formula: T Mn ( ° C ) = − 1635.48 − 184.8 × ( ln a spess − ln a pyr ) calibrated as a single crystal Mn-in-garnet thermometer. The improved accuracy (± 150 °C) of this new calibration allows for the calculation of mantle residence temperatures for large databases of garnet xenocrysts from electron microprobe data when trace-element analyses (LA-ICPMS or PIXE) required for the nickel (Ni)-in-garnet thermometer are not available.

THE CRYSTAL CHEMISTRY OF THE KORNERUPINE-PRISMATINE SERIES. IV. COMPLETE CHEMICAL FORMULAE FROM ELECTRON-MICROPROBE DATA AND X-RAY POWDER DIFFRACTION

A procedure is developed whereby complete chemical formulae and chemical compositions of kornerupine may be derived from electron-microprobe data and unit-cell dimensions. The boron content (as B in apfu ) can be expressed as a linear function of the a and c cell parameters and the Al 2 O 3 and FeO contents (as wt.% oxide). The content of ferric iron (as Fe 3+ in apfu ) can be expressed as a linear function of the B content (as B apfu ) and the Al 2 O 3 and FeO contents (as wt.% oxide). The hydrogen content (in H apfu ) can be expressed as 1 – F ( apfu ). Standard errors of estimates for B and Fe 3+ are both 0.05 apfu .

Chevkinite-group minerals from granulite-facies metamorphic rocks and associated pegmatites of East Antarctica and South India

Electron microprobe data are presented for chevkinite-group minerals from granulite-facies rocks and associated pegmatites of the Napier Complex and Mawson Station charnockite in East Antarctica and from the Eastern Ghats, South India. Their compositions conform to the general formula for this group, viz. A4BC2D2Si4O22 where, in the analysed specimens A = (rare-earth elements (REE), Ca, Y, Th), B = Fe2+, Mg, C = (Al, Mg, Ti, Fe2+, Fe3+, Zr) and D = Ti and plot within the perrierite field of the total Fe (as FeO) (wt.%) vs. CaO (wt.%) discriminator diagram of Macdonald and Belkin (2002). In contrast to most chevkinite-group minerals, the A site shows unusual enrichment in the MREE and HREE relative to the LREE and Ca. In one sample from the Napier Complex, Y is the dominant cation among the total REE + Y in the A site, the first reported case of Y-dominance in the chevkinite group. The minerals include the most Al-rich yet reported in the chevkinite group (≤9.15 wt.% Al2O3), sufficient to fill the C site in two samples. Conversely, the amount of Ti in these samples does not fill the D site, and, thus, some of the Al could be making up the deficiency at D, a situation not previously reported in the chevkinite group. Fe abundances are low, requiring Mg to occupy up to 45% of the B site. The chevkinite-group minerals analysed originated from three distinct parageneses: (1) pegmatites containing hornblende and orthopyroxene or garnet; (2) orthopyroxene-bearing gneiss and granulite; (3) highly aluminous paragneisses in which the associated minerals are relatively magnesian or aluminous. Chevkinite-group minerals from the first two parageneses have relatively high FeO content and low MgO and Al2O3 contents; their compositions plot in the field for mafic and intermediate igneous rocks. In contrast, chevkinite-group minerals from the third paragenesis are notably more aluminous and have greater Mg/Fe ratios.

BUSSYITE-(Ce), A NEW BERYLLIUM SILICATE MINERAL SPECIES FROM MONT SAINT-HILAIRE, QUEBEC

Bussyite-(Ce), ideally (Ce,REE,Ca) 3 (Na,H 2 O) 6 MnSi 9 Be 5 (O,OH) 30 (F,OH) 4 , a new mineral species, was found in the Mont Saint-Hilaire quarry, Quebec. The crystals are transparent to translucent, pale pinkish orange in color, with a white streak and vitreous luster. Bladed crystals are prismatic, having forms {111} and {101}; they are elongate on [101], up to 10 mm in length. Associated minerals include aegirine, albite, analcime, ancylite-(Ce), calcite, catapleiite, gonnardite, hydrotalcite, kupletskite, leucophanite, microcline, nenadkevichite, polylithionite, serandite and sphalerite. Bussyite-(Ce) is monoclinic, space group C 2/ c , with unit-cell parameters refined from powder-diffraction data: a 11.654(3), b 13.916(3), c 16.583(4) A, β 95.86(2)°, V 2675.4(8) A 3 and Z = 4. Electron-microprobe and secondary-ion mass spectrometric analyses give the average and ranges: Na 2 O 7.63 (9.40–6.62), K 2 O 0.05 (0.13–0.0), BeO 8.33 (SIMS), CaO 5.35 (5.95–4.17), MgO 0.03 (0.11–0.00), MnO 2.49 (3.00–1.71), Al 2 O 3 0.82 (1.54–0.39), Y 2 O 3 1.97 (2.68–1.51), La 2 O 3 2.65 (3.11–2.16), Ce 2 O 3 9.77 (11.22–8.15), Pr 2 O 3 1.23 (1.49–0.74), Nd 2 O 3 4.54 (5.13–3.91), Sm 2 O 3 0.99 (1.25–0.68), Eu 2 O 3 0.010 (0.25–0.0), Gd 2 O 3 1.03 (1.23–0.81), SiO 2 38.66 (39.94–37.66), ThO 2 3.31 (4.59–2.12), F 3.67 (6.39–2.54), S 0.03 (0.08–0), H 2 O 4.12 (determined from crystal-structure analysis), for a total of 95.21 wt.%. The empirical formula based on the crystal-structure analysis, ideally showing 34 anions, is: 4{(Ce 0.823 Nd 0.373 Y 0.242 Th 0.173 Pr 0.103 Sm 0.079 Gd 0.078 Eu 0.008 ) ∑1.879 (Ca 0.775 La 0.225 ) ∑1 [Na 3.000 (H 2 O) ∑2.500 Ca 0.544 K 0.015 ] ∑6.055 (Mn 0.485 Na 0.402 Mg 0.012 ) ∑0.899 (Si 8.897 Be 4.605 Al 0.222 ) ∑13.724 O 30 [F 2.67 (OH) 1.33 ] ∑4 }. The structure has been refined to an R index of 4.0% for 1134 unique, observed reflections. The structure has two chemically distinct layers parallel to (101): (1) a layer of [(Si,Be)O 4 ] tetrahedra, and (2) a layer of Ce–, Ca–, Mn–, Na–(O,F) polyhedra. Layers are cross-linked through shared O and F atoms. Significant amounts of OH and H 2 O are present, as indicated in the IR spectrum and crystal-structure analysis for the Na-poor hydrated phase. One instance of a rarer Na-rich phase is observed. Based on the electron-microprobe data, the formula for the rarer Na-rich anhydrous phase is 4{(Ce 1.047 Nd 0.466 La 0.323 Y 0.306 Pr 0.131 Sm 0.102 Gd 0.084 Eu 0.009 Th 0.042 ) ∑2.510 (Na 6.645 Ca 0.701 ) ∑7.346 (Si 9.521 Be 4.772 ) ∑14.293 O 29.251 F 4.749 }. Comparing the structure of bussyite-(Ce) to that of other beryllium silicates, the topology of the layer of tetrahedra most strongly resembles that of semenovite-(Ce) and the closely related mineral harstigite.

OH in zoned amphiboles of eclogite from the western Tianshan, NW-China

Chemically-zoned amphibole porphyroblast grains in an eclogite (sample ws24-7) from the western Tianshan (NW-China) have been analyzed by electron microprobe (EMP), micro Fourier-transform infrared (micro-FTIR) and micro-Raman spectroscopy in the OH-stretching region. The EMP data reveal zoned amphibole compositions clustering around two predominant compositions: a glaucophane end-member (BNa2CM2+3 M3+2TSi8(OH)2) in the cores, whereas the mantle to rim of the samples has an intermediate amphibole composition (A0.5BCa1.5Na0.5CM2+4.5 M0.53+TSi7.5Al0.5(OH)2) (A = Na and/or K; M2+ = Mg and Fe2+; M3+ = Fe3+ and/or Al) between winchite (and ferro-winchite) and katophorite (and Mg-katophorite). Furthermore, we observed complicated FTIR and Raman spectra with OH-stretching absorption bands varying systematically from core to rim. The FTIR/Raman spectra of the core amphibole show three lower-frequency components (at 3,633, 3,649–3,651 and 3,660–3,663 cm−1) which can be attributed to a local O(3)-H dipole surrounded by M(1) M(3)Mg3, M(1) M(3)Mg2Fe2+ and M(1) M(3) Fe2+3, respectively, an empty A site and TSi8 environments. On the other hand, bands at higher frequencies (3,672–3,673, 3,691–3,697 and 3,708 cm−1) are observable in the rims of the amphiboles, and they indicate the presence of an occupied A site. The FTIR and Raman data from the OH-stretching region allow us to calculate the site occupancy of the A, M(1)–M(3), T sites with confidence when combined with EPM data. By contrast M(2)- and M(4) site occupancies are more difficult to evaluate. We use these samples to highlight on the opportunities and limitations of FTIR OH-stretching spectroscopy applied to natural high pressure amphibole phases. The much more detailed cation site occupancy of the zoned amphibole from the western Tianshan have been obtained by comparing data from micro-chemical and FTIR and/or Raman in the OH-stretching data. We find the following characteristic substitutions Si(T-site) (Mg, Fe)[M(1)–M(3)-site] → Al(T-site) Al[M(1)–M(3)-site] (tschermakite), Ca(M4-site)□ (A-site) → Na(M4-site) Na + K(A-site) (richterite), and Ca(M4-site) (Mg, Fe) [M(1)–M(3)-site] → Na(M4-site) Al[M(1)–M(3)-site] (glaucophane) from the configurations observed during metamorphism.

Crystal structure of a synthetic tin-selenium representative of the cylindrite structure type

The synthetic tin-selenium member of the cylindrite structural family, with the empirical formula Sn31.52Sb6.23Fe3.12S59.12 based on electron-microprobe data, has a triclinic crystal structure composed of two alternating layer types, both with a pronounced one-dimensional modulation, and with a noncommensurate layer match in two dimensions. The pseudotetragonal (Q) layer is a MeSe layer twoatomic planes thick with lattice parameters a = 5.969(2) Å, b = 6.004(1) Å, and the layer-stacking vector c = 12.238(1) Å, α = 87.98(4)°, β = 83.14(3)°, and γ = 90.01(4)°. The pseudohexagonal (H) layer is a single-octahedral MeSe2 layer with a = 3.831(1) Å, b = 6.580(3) Å, c = 12.151(5) Å, α = 87.79(4)°, β = 90.59(3)°, and γ = 89.99(3)°; the a and b vectors of the two subsystems are parallel, the c vectors diverge. The transversal wave-like modulation has the wave-normal parallel to b, so that the modulation vector q is 0.0001(3) a* + 0.1921(4) b* - 0.0119(3) c* in terms of the pseudohexagonal subsystem. Superspace structure refinement in the superspace group X1̄ where X stands for non-primitive centering vectors (½,½,0,0,0), (0,0,0,0,½), (½,½,0,0,½) in five-dimensional superspace, and based on 2128 observed reflections, resulted in R1 = 0.038 for all reflections. Composition of the H layer has been modeled as Sn4+240Fe2+54Se588, that of the Q layer as Sn2+306Sb3+108Se414. The cation-anion distances in the Q layer vary between 2.63 and 3.30 Å, indicating that the cations present are primarily Sn2+ (and Sb3+), whereas distances in the H layer lie between 2.665 and 2.721 Å and correspond to Sn4+ with admixture of Fe2+. The shortest cation-anion distance across the interlayer space is 3.24 Å. Relations between layer match and the modulation vector, divergence of layer stackings of the two components, and reasons for the modulation and for the pronounced disorder of the Q component, as well as the differences and similarities with levyclaudite, franckeite, and synthetic layer-misfit compounds are discussed in detail. In its structural principles, although not in numerical values, the Sn-Se cylindrite corresponds fully to the natural Pb-Sn-S cylindrite previously described.

TOURCOMP: A program for estimating end-member proportions in tourmalines

AbstractA Visual Basic program (TOURCOMP) has been written to recast the tourmaline composition into end-member components from electron microprobe data or more complete tourmaline analyses. TOURCOMP is a program based on a linear algebraic model that directly calculates the end-member proportions of tourmalines from their structural formulae. The program is developed for IBM-compatible personal computers running under the Windows™ operating system. The source code has been also translated and compiled in order to run on an Apple computer. Analytical problems, uncertainties concerning site occupancies, and the normalization procedure to determine the structural formula are the main error sources. However, the method of recalculating tourmaline end-members presented in this paper is considered to provide reasonably good results, bearing in mind the chemical complexity of tourmaline.

Geochemistry and geothermobarometry of the Um Eleiga Neoproterozoic island arc intrusive complex, SE Egypt: genesis of a potential gold-hosting intrusion

Gabbro-diorite-tonalite±granodiorite complexes, derived from subduction-related, calc alkaline magmas represent a distinct unit in the Precambrian basement of Egypt. These intrusive complexes are commonly intruded into ophiolitic mélange, schistose or gneissic rock assemblages. Well-constrained geological mapping of the Um Eleiga complex, combined with geochemical studies, should enable a better understanding of its genesis, especially its potential as a host for gold mineralisation. This complex represents a composite pluton intruded into the allochthonous, ophiolitic Abu Dahr massif and its associated undifferentiated metavolcano-sedimentary mélange matrix in the southern Eastern Desert of Egypt. Field and petrographic investigations have revealed a compositional continuum from gabbro to granodiorite through diorite and subordinate tonalite. Petrography and bulk rock geochemical data have been used to determine the magma type and tectonic setting of the Um Eleiga complex, whereas the electron microprobe data were valuable for the purpose of determining the pressure–temperature and oxygen fugacity conditions. The compositional continuum, pattern of zonation, shallow magma origin and marked common negative Nb anomalies suggest that a close petrogenetic relationship between the gabbro, diorite, tonalite and granodiorite units of the Um Eleiga complex. Subducted upper mantle wedge is proposed as being the source region for the gabbroic magma, whereas zoning of the complex is attributed to interplay of fractional crystallisation and assimilation. Pressure estimates based on the Al(iv)-in-hornblende geobarometry indicate that the gabbroic and dioritic rocks crystallised under 4–5 kbar, whereas tonalite and granodiorite were formed at 3–4 kbar. The hornblende-plagioclase thermometer indicates a formation temperature range of about 720 to 880°C for the complex. Varied oxidation states are inferred from the mineral assemblages, including magnetite, titanite and ilmenite and checked by Fe/(Fe+Mg) ratios of biotite in the granitoid rocks. The tectonic setting, mineralogical, geochemical characteristics and varied redox state of the Um Eleiga intrusive complex are typical for arc-related intrusives potential for hosting intrusion-related gold in a collisional environment.

The metamorphic evolution of migmatites from the Ötztal Complex (Tyrol, Austria) and constraints on the timing of the pre-Variscan high-T event in the Eastern Alps

Within the Otztal Complex (OC), migmatites are the only geological evidence of the pre-Variscan metamorphic evolution, which led to the occurrence of partial anatexis in different areas of the complex. We investigated migmatites from three localities in the OC, the Winnebach migmatite in the central part and the Verpeil- and Nauderer Gaisloch migmatite in the western part. We determined metamorphic stages using textural relations and electron microprobe analyses. Furthermore, chemical microprobe ages of monazites were obtained in order to associate the inferred stages of mineral growth to metamorphic events. All three migmatites show evidence for a polymetamorphic evolution (pre-Variscan, Variscan) and only the Winnebach migmatite shows evidence for a P-accentuated Eo-Alpine metamorphic overprint in the central OC. The P-T data range from 670–750 °C and < 2.8 kbar for the pre-Variscan event, 550–650 °C and 4–7 kbar for the Variscan event and 430–490 °C and ca. 8.5 kbar for the P-accentuated Eo-Alpine metamorphic overprint. U-Th-Pb electron microprobe dating of monazites from the leucosomes from all three migmatites provides an average age of 441 ± 18 Ma, thus indicating a pervasive Ordovician-Silurian metamorphic event in the OC.

Micro-tephra in the West Runton Freshwater Bed: Preliminary results

A volcanic mineral assemblage is present throughout the 20–120μm size fraction of the Middle Pleistocene West Runton Freshwater Bed, Norfolk, UK. Glass shards are extremely rare but are present mainly at one horizon. This micro-tephra assemblage is indicative of a potassic mineral suite, possibly from an evolved phonolitic or trachytic melt. Mineralogical and electron microprobe data are inconsistent with a tephra source from mid Atlantic ridge sites and the amphibole and alkali feldspar components are more evolved than early Quaternary mineral assemblages from West Eifel volcanism in Germany. The mineral assemblage and geochemical compositions are most similar to those from the Mont Dore area (Massif Central, France) and the Italian volcanic complex of Mount Vulture in the southern Apennines. Taken alone these preliminary data are not strong enough to infer a deposit age. However, the age ranges for volcanism at both Mont Dore and Mount Vulture are similar (500–800ka); the presence of micro-tephra from either of these sources is wholly consistent with other age estimates for the West Runton Freshwater Bed (>500ka and <780ka) supporting deposition during either the interglacials of marine isotope stages 15 or 17.

Tectonic discrimination of peridotites using fO 2–Cr# and Ga–Ti–Fe III systematics in chrome–spinel

The accessory mineral chrome–spinel, [(Mg,Fe) (Cr,Al,Fe) 2O 4], is well-established as a useful, alteration-resistant petrogenetic and tectonic setting indicator for mafic–ultramafic rocks. This study of unaltered spinels from 58 peridotites from oceanic and ophiolitic settings further develops the tectonic discrimination of peridotites from mid-ocean ridges (MOR) and supra-subduction zone (SSZ) settings. The geochemical data include the standard suite of elements, together with accurate ferric iron obtained by correcting electron microprobe data with Mössbauer spinel standards, and gallium (Ga) obtained by Laser Ablation-ICP-MS. The results show that discrimination using the established oxygen fugacity ( fO 2)–Cr# [Cr/(Cr + Al)] diagram remains effective, but does depend on melt–rock reaction and magmatic differentiation of the reacting melt, in addition to tectonic setting. In particular, the MOR–SSZ boundary for dunites is displaced to higher oxygen fugacities than that for residual harzburgites, with some partially reacted harzburgites proving hard to fingerprint. Ga–Ti–Fe 3# [Fe 3/(Fe 3 + Cr + Al)] systematics provide a potential way to improve the discrimination. Significantly, Ga and Fe 3+ have similar ionic radii but only Fe 3+ is redox-dependent, so Ga/Fe 3# is largely independent of magmatic differentiation while retaining the capability to separate MOR peridotites from the higher Fe 3# SSZ peridotites. Whereas Ga/Fe 3# ratios decrease during melt–rock reaction, Ti/Fe 3# ratios stay constant or increase according to the relative compositions of the mantle lithosphere and the interacting melt. In consequence, the Ti/Fe 3# vs. Ga/Fe 3# diagram requires only a single boundary to separate MOR harzburgites and dunites (with high ratios) from SSZ harzburgites and dunites (with low ratios). Podiform chromitites can only be fingerprinted if they share a similar history of olivine–spinel subsolidus re-equilibration as the peridotites. Unlike the oxygen fugacity plot, the Ti/Fe 3# vs. Ga/Fe 3# diagram can also fingerprint peridotites that have no co-existing olivine or host rock information, such as serpentinites and detrital spinels. Testing these methods using spinels in mantle peridotites from the northern Oman–U.A.E. ophiolite confirms independent indicators that the ophiolite records the switch in tectonic setting from MOR to SSZ and hence magma genesis during the initiation of subduction.

Chemical composition of tourmaline from the Asarcık Pb–Zn–Cu ± U deposit, Şebinkarahisar, Turkey

Abundant tourmaline, commonly as rosettes up to 1.5 cm in diameter, is present in the Upper Cretaceous Asarcik granitoid (Şebinkarahisar), Northeastern Turkey. The tourmalines also occur in quartz–tourmaline veins up to 4 m thick that cut the Asarcik granitoid. This granitoid was emplaced in the eastern Pontide Metallogenic Belt, where accompanying tourmalines are found in close association with a vein-type, uranium-bearing Pb–Zn–Cu deposit. Tourmaline crystals are generally optically zoned from core to rim with a bluish green core. Electron-microprobe studies show that tourmalines from the Asarcik granitoid and quartz–tourmaline veins display mainly dravite–schorl solid solutions with a tendency to schorl compositions. The tourmaline from the Asarcik granitoid has Fe/(Fe + Mg) ratios from 0.28 to 0.69 (mean = 0.52) and Na/(Na + Ca) ratios from 0.57 to 0.93 (mean = 0.76). In contrast, tourmaline in the quartz–tourmaline veins has Fe/(Fe + Mg) ratio from 0.14 to 0.92 (mean = 0.59) and Na/(Na + Ca) ratio from 0.78 to 1.00 (mean = 0.96). Tourmaline in the veins has a more alkali-deficient trend than that in the granitoid. Substitution mechanisms for the chemical evolution of tourmalines at Asarcik are the MgFe−1, (Fe2+Fe3+)(MgAl)−1, □Al(NaR)−1, AlOR2+−1(OH)−1, □Al2ONa−1R2+−2(OH)−1, Ca0.5□0.5Na−1, CaMg2□−1Al−2, CaMgO□−1Al−1(OH)−1, CaMg3OH□−1Al−3O−1, CaMgNa−1Al−1, CaONa(OH)−1, CaMg2OHNa−1Al−2O−1, CaMg2□−1Al−2, and CaMg3OH□−1Al−3O−1 exchange vectors. Chemical zoning in the tourmalines is consistent with these substitutions. Stoichiometric calculations of electron-microprobe data in the Asarcik granitoid suggest a relatively oxidizing environment during tourmaline crystallization.

Petrological evolution of silica-undersaturated sapphirine-bearing granulite in the Paleoproterozoic Salvador–Curaçá Belt, Bahia, Brazil

The Salvador–Curaçá Belt, located in São Francisco Craton, Brazil, was subjected to granulite facies metamorphism during the Paleoproterozoic orogeny ( c. 2.0 Ga). Well preserved in enclaves of silica-undersaturated sapphirine-bearing granulite occur in a charnockite outcrop located along a kilometric-scale shear zone. The sapphirine-bearing granulite preserves domains with distinct mineral assemblages that record interactions between melt and peritectic phases (orthopyroxene 1 + spinel 1 + biotite 1). Sapphirine was crystallized in the Si-poor cores of the enclaves, sillimanite and spinel–cordierite symplectites in the intermediate Si-rich domains between cores and margins, and garnet and quartz-bearing cordierite/biotite symplectites in Si-rich margins of the enclaves. Melt-rock interactions and metamorphism occurred at ultrahigh temperatures of 900–950 °C at 7.0–8.0 kbar pressures. The mineralogical evolution of the domains reflects not only the influence of changes in bulk composition in the equilibrium volume of the reactions but also P– T changes during orogeny evolution. Electron microprobe dating of monazite both in the sapphirine-bearing granulite and charnockite indicates UHT metamorphism timing at c. 2.08–2.05 Ga that is related to global Paleoproterozoic UHT metamorphic events that occurred during the Columbia supercontinent assembly.

Oxygen isotope heterogeneity and disequilibria of olivine crystals in large volume Holocene basalts from Iceland: Evidence for magmatic digestion and erosion of Pleistocene hyaloclastites

This work considers petrogenesis of the largest Holocene basaltic fissure eruptions of Iceland, which are also the largest in the world: Laki (1783–84 AD, 15 km 3), Eldgjá (934 AD, 18 km 3), Veidivötn (900, 1480 AD, multiple eruptions, >2 km 3), Núpahraun (ca. 4000 BP, >1 km 3) and Thjórsárhraun (ca 8000 BP, >20 km 3). We present oxygen isotope laser fluorination analyses of 55 individual and bulk olivine crystals, coexisting individual and bulk plagioclase phenocrysts, and their host basaltic glasses with average precision of better than 0.1‰ (1SD). We also report O isotope analyses of cores and rims of 61 olivine crystals by SIMS with average precision on single spots of 0.24‰ (1SD) in 13 samples coupled with electron microprobe data for major and trace elements in these olivines. Within each individual sample, we have found that basaltic glass is relatively homogeneous with respect to oxygen isotopes, plagioclase phenocrysts exhibit crystal to crystal variability, while individual olivines span from the values in equilibrium with the low-δ 18O matrix glass to those being three permil higher in δ 18O than the equilibrium. Olivine cores with maximum value of 5.2‰ are found in many of these basalts and suggest that the initial magma was equilibrated with normal-δ 18O mantle. No olivines or their intracrystalline domains are found with bulk or spot value higher than those found in MORB olivines. The δ 18O variability of 0.3–3‰ exists for olivine grains from different lavas, and variable core-to-rim oxygen isotopic zoning is present in selected olivine grains. Many olivines in the same sample are not zoned, while a few grains are zoned with respect to oxygen isotopes and exhibit small core-to-core variations in Fe–Mg, Ni, Mn, Ca. Grains that are zoned in both Mg# and δ 18O exhibit positive correlation of these two parameters. Electron microprobe analysis shows that most olivines equilibrated with the transporting melt, and thin Fe-richer rim is present around many grains, regardless of the degree of olivine-melt oxygen isotope disequilibrium. The preservation of isotopic and compositional zoning in selected grains, and subtle to severe Δ 18O (melt-olivine) and Δ 18O (plagioclase-olivine) disequilibria suggests rather short crystal residence times of years to centuries. Synglacially-altered upper crustal, tufaceous hyaloclastites of Pleistocene age serve as a viable source for low-δ 18O values in Holocene basalts through assimilation, mechanical and thermal erosion, and devolatilization of stoped blocks. Cumulates formed in response to cooling during assimilation, and xenocrysts derived from hyaloclastites, contribute to the diverse δ 18O crystalline cargo. The magma plumbing systems under each fissure are likely to include a network of interconnected dikes and sills with high magma flow rates that contribute to the efficacy of magmatic erosion of large quantities (10–60% mass) of hyaloclastites required by isotopic mass balance. Olivine diversity and the pervasive lack of phenocryst-melt oxygen isotopic equilibrium suggest that a common approach of analyzing bulk olivine for oxygen isotopes, as a proxy for the basaltic melt or to infer mantle δ 18O value, needs to proceed with caution. The best approach is to analyze olivine crystals individually and demonstrate their equilibrium with matrix.

TRACE-ELEMENT EVIDENCE FOR VOLATILE-INFLUENCED DIFFERENTIATION IN A FLOW OF ALKALI BASALT, PENG HU ISLAND, TAIWAN

A 20-m-thick alkali basalt flow on the Peng Hu Island, Taiwan, shows three well-developed segregation veins in its upper 8.5 meters. Each pegmatitic and “vesicular” (ocelli-bearing?) segregation displays distinct whole-sample chemical traits that are shared with the enclosing basalt. Augite, plagioclase and olivine exhibit major-element (electron-microprobe data) and trace element compositions (laser ablation microprobe – inductively coupled plasma – mass spectrometry) that reflect this whole-rock chemical stratification. Thus the chemical stratification is a product of igneous processes and is not the result of secondary alteration. Elements defining the stratification (K, Rb, Li, Na, Zn in minerals and whole-rock data, but Cl, S, As, Pb and Sr are also important based on whole-rock data) tend to be complexed and moved by volatiles in various geological environments. Conventional crystal-fractionation models do not reproduce the observed variations in these elements up through the flow. The patterns of data suggest that rising plumes of vesicles carried volatile-complexed (scavenged) elements to high levels in the flow at the time the segregation veins were forming and the interior of the flow was largely molten.

Electron microprobe monazite geochronological constraints on the Late Palaeozoic tectonothermal evolution in the Chinese Tianshan

Electron microprobe monazite data from the early Palaeozoic granitoid gneisses in the Central Tianshan Tectonic Zone provide two main ages: (1) an age of 355 ± 12 Ma obtained on the chemically complex grains, which is good agreement with the high-pressure metamorphic age obtained on eclogite and blueschist rocks from the south Tianshan Tectonic Zone; (2) an age of 323 ± 7 Ma obtained on the complex and the monogenetic grains, which is also closely consistent with the age for a major episode of cooling–recrystallization of these high-pressure rocks. Monazite from the schist of the Xingxingxia complex yields a younger Th/Pb–U/Pb centroid age of 282 ± 9 Ma. This younger age is interpreted as being linked to a magmatic episode, possibly related to underplating–extension, and/or shear activities. These data provide timing constraints on the episodes of metamorphism and high-pressure metamorphism and evidence for transition from compression to extension during the Carboniferous to early Permian, for the Chinese Tianshan region. Moreover, this study shows that electron microprobe dating of monazite, together with recent statistical treatment, can provide a reliable method to reveal a sequence of events within an orogenic cycle.

Electron microprobe dating of monazites from an ultrahigh-temperature granulite in Southern India: Implications for the timing of Gondwana assembly

We report here monazite geochronology on a newly discovered high pressure (HP) and ultrahigh-temperature (UHT) granulite locality at Thoppur within the Palghat-Cauvery Shear Zone system in southern India. In situ dating of monazites associated with the UHT minerals using electron probe technique yielded PbO vs. ThO2* apparent isochron age of 544 ± 5 Ma. This is the first study from southern India where older monazite cores or complex age patterns are virtually absent, and all the monazite population in the studied samples crystallized/recrystallized during a single thermal maximum which we correlate with the timing of extreme crustal metamorphism during the final assembly of the Gondwana supercontinent.

Age constraints on the tectonothermal evolution of the Selwyn Zone, Eastern Fold Belt, Mount Isa Inlier

The Selwyn Zone is characterized by multiple ductile deformation and metamorphic events. There is general consensus that early in the Isan Orogeny there were both extensional and thrust events that produced bedding-parallel foliation (S bp). We propose that these were followed by the D 1 event that resulted in a regional steep east–west foliation, and that the Snake Creek Anticline and the dominant S 2a regional foliation formed as upright north–south structures. Local overturning of D 2 structures was due to subsequent localized deformation (D 2b) that resulted in structures with flat-lying axial planes. D 3 and D 4 produced steep upright folds and crenulations with north–northwest and northeast trending axial surfaces. Metamorphism, largely controlled by the emplacement of mafic rocks, is dominantly low pressure, but the P–T–t path is complex. Early growth of cordierite (M 1) followed by andalusite, was succeeded by a clockwise medium pressure loop of ca. 500–600 MPa in the northern Snake Creek Anticline, as indicated by early D 2a kyanite growth. The metamorphic peak during D 2 was around 580 °C and 400 MPa at the sillimanite isograd and reached a maximum of around 670 °C and 400–500 MPa in the migmatitic sillimanite K-feldspar zone at the Cannington and Osborne mines. Previous age determinations for the metamorphic peak of ∼1580–1600 Ma are confirmed with new electron microprobe dating of monazite, whereas an age grouping of 1600–1630 Ma may represent the D 1 (M 2) event. Additional ages in the range 1640–1680 Ma are recorded in albitites from the Soldiers Cap Group, and it is suggested that they formed by infiltration of hydrothermal fluids during extension and the emplacement of mafic sills. This is supported by SHRIMP dating of zircon from a trondhjemite lens in a largely mafic sill complex in the Snake Creek Anticline that provided an age of 1686 ± 8 Ma, also indicating that at least part of the Soldiers Cap Group is older than previously indicated. A zircon SHRIMP age of 1527 ± 4 Ma was obtained for a sample of albitized Saxby Granite. This age also represents the D 4 (M 6) event, and is the timing of the extensive brecciation and albitization of calc-silicate rocks of the Corella Formation in the Selwyn Zone.

Th-, U- AND LREE-BEARING GROSSULAR, CHROMIAN FERRIALLANITE-(Ce) AND CHROMIAN CERITE-(Ce) IN SKARN XENOLITHS EJECTED FROM THE GOLCUK MAAR CRATER, ISPARTA, ANATOLIA, TURKEY

Thorium-, U- and LREE-bearing skarn xenoliths have been found in the volcaniclastic products of the Golcuk explosive maar crater, Isparta, southwestern Turkey. The samples predominantly consist of clinopyroxene, garnet, plagioclase, wollastonite, esseneite, olivine, calcite and dolomite, with accessory ferriallanite-(Ce), phlogopite, amphibole, quartz, apatite, spinel, cerite-(Ce) and monazite. We report the whole-rock geochemistry and electron-microprobe data for related Th-, U- and LREE-bearing garnet, ferriallanite-(Ce) and cerite-(Ce). Extreme enrichments in the LREE (1600–9400 ppm La, 2200–13500 ppm Ce, 270–900 ppm Pr, 400–2000 ppm Nd), and in Th (200–300 ppm), U (20–400 ppm), Y (50–150 ppm), Be (13–108 ppm), Cu (14–1600 ppm), Zn (50–600 ppm) and Au (up to 9.5 ppb) are observed. A strong correlation between U + Th abundance and ∑REE (5900–26000 ppm) exists in whole-rock samples. Among the minerals, grossular (Th + U: 0.10–3.97 wt.%, ∑LREE: 4.40–7.80 wt.%), chromian ferriallanite-(Ce) (Th + U: 0.02–0.20 wt.%, ∑LREE: 19.30–24.50 wt.%) and a chromian variety of cerite-(Ce) recorded here for the first time (Th + U: 0.97–1.35 wt.%, ∑LREE: 56.83–60.64 wt.%) are the principal hosts of the Th, U and the LREE. The fluids responsible for the geochemically enriched zones are most likely of contact-metasomatic origin, affecting the interface between a shallow magma-chamber and a carbonate wallrock. High concentrations of gold (up to 4.2 ppb) in the magmatic rocks of the Golcuk district demonstrate an important economic potential.

Aureoles of Pb(II)-enriched feldspar around monazite in paragneiss and anatectic pods of the Napier Complex, Enderby Land, East Antarctica: the roles of dissolution-reprecipitation and diffusion

Extraordinarily high Pb content in K-feldspar and plagioclase has been found contiguous to monazite in two occurrences in the ultrahigh-temperature Napier Complex of Antarctica. Monazite shows a variety of textures and compositions. In a garnet-sillimanite-orthopyroxene paragneiss at Mount Pardoe (Amundsen Bay), grains range 80–150 μm across and are anhedral; two grains are Th- and Si-dominant. In pods that crystallized from anatectic melts at 2500 Ma at Zircon Point, Casey Bay, monazite grains range 0.05 mm–1 cm in length and are highly variable in texture. The coarsest grains (>0.7 cm) are skeletal and euhedral, whereas the smallest grains are anhedral and associated with fine- to medium-grained quartz, K-feldspar, plagioclase, garnet, sillimanite and rutile in aggregates that form interstitial veinlets interpreted to be a second generation of anatexis during an event at 1100 Ma. The huttonite component (ThSiO4) reaches 30 mole% in the cores of the coarsest skeletal grains, whereas other grains, particularly smaller ones, show complex and irregular zoning in Th and U. The latter zoning is attributed to dissolution-reprecipitation, which also resulted in complete Pb loss during the 1100 Ma event. In the paragneiss at Mount Pardoe, K-feldspar and myrmekitic plagioclase (An16) are found in a 70–80 μm band between monazite and orthopyroxene and contain up to 12.7 wt.% and 2.7 wt.% PbO, respectively, corresponding to 18.5% and 3.4% PbAl2Si2O8 component, respectively. Cathodoluminescence of both feldspars increases with distance from a nearby monazite grain and is not correlated with Pb content. Incorporation of Pb in K-feldspar and plagioclase could be a result of diffusion, even though the monazite adjacent to feldspar apparently lost little Pb, i.e., Pb could have been transported by fluid from the Th-rich grains, which did lose Pb. In contrast to the paragneiss, cathodoluminescence correlates with Pb content of K-feldspar in aureoles surrounding skeletal monazite grains 0.7–1 cm across in anatectic pods at Zircon Point. Pb content of K-feldspar decreases monotonically to near detection limits within several millimetres of monazite grains; the greatest PbO concentration is attained in K-feldspar inliers and embayments in monazite, 8.8 wt.%, corresponding to 11.7% PbAl2Si2O8 component. Fine-grained quartz in the K-feldspar suggests that the mechanism for Pb incorporation involved breakdown of feldspar: Pb2+ + 2(K,Na)AlSi3O8 → PbAl2Si2O8 + 4SiO2 + 2(K,Na)+ . The smooth decrease of Pb in the aureoles is not characteristic of dissolution-reprecipitation, which is characterized by abrupt changes of composition, and it seems more likely that Pb was incorporated in K-feldspar by diffusion at 1100 Ma. We suggest a model whereby fluid introduced during the 1100 Ma event flowed along grain boundaries and penetrated mineral grains. Temperatures were sufficiently high, i.e., 700°C, assuming burial in the mid-crust, for the fluid to induce localized melting of quartzofeldspathic matrix of the anatectic pods. Loss of radiogenic Pb was complete. Some penetration of K-feldspar by aqueous fluid is suggested by the presence of scattered galena specks and by rays of turbidity emanating from monazite. Aqueous fluid or water-rich granitic melt may have mediated the diffusion of Pb in feldspar, but it did not cause dissolution-reprecipitation. Although Pb was mobilized by aqueous fluid or water-rich granitic melt, it was not entirely flushed from the immediate vicinity of the monazite, but nearly half was incorporated in adjacent feldspar. Fluid activity that could cause Pb loss in monazite does not always leave an obvious trace, i.e., hydrous minerals, such as sericite, are very sparse, and biotite is absent in the anatectic pods at Zircon Point. Nonetheless, electron microprobe dating of monazite from the pods could not detect the 2500 Ma age of original crystallization determined by isotopic dating.