V23B-01
Nature of Phyllosilicates and Magma Types in the Plutonic Complexes of NE Egypt
The Late Proterozoic plutonic complexes of northeastern Egypt belong to a number of magma types of variable tectonic regimes, and exhibit a variety of phyllosilicate minerals. The results show that the phyllosilicates define three compositionally distinct groups. Phyllosilicates in the anorogenic peralkaline granites, which were developed from a hypersolvus felsic melt, are restricted to iron-rich siliceous annites and belong to type A-biotite (referring to biotite in anorogenic alkaline A-type suites). Such Fe-enriched, Al-depleted mica compositions suggest that the substitutions Fe3+ = Al, 3Fe2+ = 2Al, Fe2+ = Mg and (Fe2+ + Si) = 2Al are vital in producing annitic compositions in the peralkaline granites. In sharp contrast, phyllosilicates in the trondhjemitic pluton that crystallized from a peraluminous felsic melt, are siderophyllitic in composition and belong to type P-biotite (referring to biotite in peraluminous, including S-type suites). The substitutions Fe2+ = Mg, along with the Tschermaks substitution (Fe2+, Mg) + Si = 2Al are vital in producing siderophyllitic compositions in the trondhjemite pluton. The substitution 2Al = 3Mg (or Fe2+) also plays a role when referring to a possible solid solution between phlogopite (or annite) end-member and muscovite end-member. This pluton also contains late- to post-magmatic muscovite. Phyllosilicates in the diorite-tonalite and the voluminous granodiorite-adamellite orogenic complexes (crystallized from calcic to calc-alkaline subsolvus magmas), cover a wide range of composition, and belong to type C-biotite (referring to biotite in calcic and calc- alkaline, mostly I-type suites). The nature of micas and the magmatic stages of their development were controlled by the various physiochemical conditions (including the behavior of volatiles) prevailed during the crystallization of these hypersolvus, dry, anorogenic magmas versus subsolvus, wet, orogenic magmatic systems.
V23B-02
Characterization of the Geochemical Reference Material BRP-1 from Brazil: New Data from High Precision LA-ICPMS Analyses
To get a well-characterized Brazilian basaltic reference material Cotta et al. (2007) prepared BRP-1, which was collected from the Cretaceous Parana Province. About 150 kg of the raw material was crushed and pulverized at the USGS in Denver, USA. Finally, the powder was split into 1920 glass bottles containing about 55 g of material. Homogeneity tests using XRF and microbeam-SRXRF showed that the material is homogeneous for most of the investigated elements. Representative minimum masses were 1 mg for major and 1 - 15 mg for trace elements. To get high-precision trace element data and to further test the homogeneity of BRP-1, we have applied a recently developed LA-ICPMS technique using an automated Ir-strip-heater (Stoll et al., 2007). About 40 mg of the rock powder was placed on an iridium strip. The melting of the sample took place in a closed box under an argon atmosphere to suppress oxidation and to limit volatilization of elements with low boiling points (e.g., Cs, Pb). In order to get reproducible conditions an automatic computer control served for 1650 ° C temperature using a pyrometer and heat up of 20 s, melting of 10 s and subside of 2 s. Fast cooling occurred by an Ar flow underneath. Using these conditions homogeneous glasses with no significant loss of volatile elements were obtained. Contamination from the high-purity iridium strip (99.955 %) was not detected with the exception of Ir whose concentration was about 20 ppm. Mass spectrometric analyses were performed with the double focusing Finnigan Element2 ICPMS and a new New Wave 193 nm Nd:YAG laser ablation system (Jochum et al., 2007). Matrix matched calibration was done with the reference glasses KL-2G and BCR-2G. BCR-2 was used as quality control material. We obtained a precision (1 RSD) of about 1 - 3 % for most trace elements. Our results obtained from two different bottles of BRP-1 agree within uncertainty limits confirming the homogeneity of BRP-1 in the mg range.
V23B-03
Variations in Argon Isotopes From Plagioclase and Hornblende Phenocrysts at El Chichon Volcano
The ratios between the isotopes of argon vary greatly between different reservoirs (atmosphere, crust, mantle). Coupled with the fast diffusion of argon at magmatic temperatures, the argon isotopic system could prove highly valuable for the investigation of magmatic processes such as injection and assimilation. In the past 8,000 years, El Chichon volcano has produced at least 11 eruptive events displaying a relatively constant trachyandesitic composition, attesting to the tapping of a long-lived magma chamber held in steady state between the influx of basaltic magma and differentiation of resident magma. Plagioclase phenocrysts from the 1,500 ybp and older eruptions display variable enrichment of 40Ar (excess argon), while hornblende phenocrysts from the same eruptions show similar, but subdued, enrichment. In contrast, both plagioclase and hornblende phenocrysts from the four post-1,500 ybp eruptions have argon isotopic ratios near atmospheric values. Isochron analysis rules out xenocrystic contamination as the source of excess argon. All eruptive units display a positive correlation between 40Ar and Cl, indicating melt inclusions as a major reservoir for argon within plagioclase. Some units show positive correlation between 40Ar/36Ar and Cl/K, which points to inclusion-hosted excess argon, while others display a negative correlation between 40Ar/36Ar and Cl/K, pointing to lattice-dissolved excess argon. Electron microprobe and x-ray fluorescence data supplement our ability to distinguish these different reservoirs of argon within phenocrysts. The subdued behavior of hornblende is due to slower diffusion and minimal melt inclusions. Variations in the amount and reservoir location of excess argon within a phenocryst are a function of crystal residence and the isotopic signature of the magma chamber.
V23B-04
Contrasting compositions of amphiboles in two Neoproterozoic Banded iron formations of Uruguay
Two occurrences of iron formations from the Precambrian of Uruguay were mineralogically and petrographically investigated. One of these deposits, located to the north of Minas City is associated with a low metamorphic grade Ediacaran succession (Minas Formation - Arroyo del Soldado Group) where glacial influence has been suggested. Fine grained quartz, Fe-rich amphibole (grunerite), magnetite porphyroblasts and hematite laths characterize these BIFs. Although an earlier increase of ferrous iron in solution in deeper water through the glacial episode is a probable factor, the identification of volcaniclastic acid rocks in the sequence indicates that Fe-fluids of volcanic origin played also an important role. A Rapitan type for this deposit is indicated by geological features and age constrains. The other analyzed deposit is associated with a highly deformed low- to medium- grade metamorphic supracrustal succession (Zanja del Tigre Formation) occurring as tectonic slices in a pre- Neoproterozoic basement. The age of this unit is not well constrained but field evidences and indications of glacigenic deposits indicate Cryogenian or Tonian age. This iron formation consists of about 2-5 cm thick layers, composed of alternating bands rich in quartz, magnetite and hematite, carbonate (calcite) and Mg-amphibole (cummingtonite). The difference found in the mineral chemistry of the amphiboles between both deposits suggests that, in the latter deposit, metamorphic changes in the paragenesis were associated with higher oxygen fugacity. An alternative explanation is that those rocks are not BIF deposits.
V23B-05
Re-Os Isotope Systematics in Subduction Zones: Izu, Indonesia and Kamchatka
The Re-Os isotope system has the potential to serve as a sensitive tracer of crustal recycling processes during subduction due to the extremely radiogenic signatures of most sediment and altered oceanic crust compared to depleted mantle. Slab-derived fluid-fluxing of the mantle wedge should impart a radiogenic Os isotope signature to the arc mantle and associated arc magmas if Os is sufficiently mobile in slab-derived fluids. Results from several studies of arc mantle xenoliths show that some arc mantle is radiogenic in Os compared to non-arc mantle despite depleted Re/Os ratios, suggesting that radiogenic slab Os may be mobile in some subduction environments. Most arc volcanic rocks also are radiogenic in Os, but determining the cause of the radiogenic Os is complicated by very low Os abundances in most arc magmas, which make them highly susceptible to minor amounts of crustal assimilation. Potential subduction-related Os isotope signatures are thus easily overprinted. Recent studies of arc lavas from numerous subduction systems demonstrate low Os abundances and radiogenic Os in arc magmas globally, interpreted to reflect either crustal assimilation or radiogenic slab-derived Os in the mantle wedge. In order to further address the origin of radiogenic Os in arc lavas, our studies in Izu, Indonesia and Kamchatka focus on the most MgO-rich samples available. In general, Os isotope ratios are negatively correlated with MgO and Os concentrations. In each arc Os abundances and isotope ratios exhibit an extreme range (Izu backarc: 1- 130 ppt Os, 187Os/188Os = 0.131-0.734; Indonesia: 1-360 ppt Os, 187Os/188Os = 0.135 - 0.788; Kamchatka: 0.5-278 ppt Os, 187Os/188Os = 0.138-1.24). The least radiogenic Os isotope signature in each arc is similar (0.131-0.138), and despite relatively high Os concentrations in these magmas, all are more radiogenic than depleted or primitive upper mantle, but within the range found in arc peridotites. In all three arcs, two sample groups can be defined with a gap in 187Os/188Os between groups. We suggest that the Os isotope signatures of the more radiogenic sample groups from Izu, Indonesia and Kamchatka are caused by crustal assimilation, but propose that the less radiogenic sample group from each arc may record radiogenic slab-derived Os in the respective mantle sources.
V23B-06
Speciation of Iron in Silicic Glasses: Inferences From Spectroscopic Methods and TEM
Iron environment in silicic glasses has been investigated using optical absorption spectroscopy (OAS), X-ray Absorption Near Edge Structure (XANES), Electron Paramagnetic Resonance (EPR) and TEM. The samples are calco-alkaline silicic glasses from different localities and containing 0.5 to 2 wt% Fe. The redox state of iron in silicate glasses and the local environment around Fe2+ and Fe3+ is known to influence properties such as color or viscosity and crystal-liquid element partitioning, for glasses and melts, respectively. Considering eruption conditions, this information can give a better understanding of the cooling conditions and setting of lava flows. The spectra of silicic glasses are strongly different from that of synthetic glasses and tektites. All spectroscopic methods show the presence of Fe3+ and Fe2+ cations belonging to the glassy network associated with clustered superparamagnetic Fe-oxides. TEM data indicate that nanometric Fe-oxide clusters are poorly crystallized. The spectroscopic data on obsidians have been compared to those recorded on magnetite and synthetic glasses to assess the importance of Fe oxide clusters. These clusters are responsible for the variety of coloration of these obsidians. The speciation of iron in these glasses and associated clusters will be discussed in terms of formation conditions.
V23B-07
Water Content of Ore Forming Magma From the Porphyry-Copper-Molybdenum Deposit at Butte, Montana, USA
Porphyry copper deposits form where exsolved magmatic volatiles carry metals and sulfur from a large magma volume to a cupola, then ascend and deposit ore. The magma temperature, bulk composition, crystallinity, and volatile contents determine the pressure (and depth) at which the melt liberates ore-forming fluids. Butte fluid inclusion studies indicate that mineralization occurred at pressures of 2 to 3 kbar, which requires a greater depth of formation than most other porphyry deposits. It is possible that the large Butte deposit formed at great depth due to unusually high water content in the parental magma. Estimates of bulk magmatic water contents in porphyry systems range from 1 to 5 wt.% H2O, but have large uncertainties owing to the rarity of fresh porphyry samples, difficulty of analyzing melt inclusions, and difficulty in comparing natural samples with experimental phase equilibria. We estimate the water content of the bulk magma that fed the Butte porphyry deposit using (1) quantitative textural analysis of quartz porphyry dikes, (2) thermodynamic modeling of phase relations (MELTS), and (3) VolatileCalc, a silicate melt-H2O-CO2 solubility model, in combination with existing fluid inclusion data. Counting phenocrysts >0.4 mm, the crystallinity of the Butte quartz porphyry is ~25%, which is smaller than commonly observed in porphyries from other deposits (~50% phenocrysts). MELTS calculations indicate that 25% crystallinity at conditions of deep fluid exsolution, 2 to 3 kbar and 600 to 700°C, requires 6 to 8 wt.% H2O in the coexisting melt. Similarly, VolatileCalc indicates Butte fluid inclusions which contain 95 mol% H2O would be in equilibrium with a silicic melt containing 6 to 8 wt.% dissolved H2O. These H2O contents represent the amount of water dissolved in the interstitial silicate melt within the partly crystallized dikes, and thus the bulk water content of the parental magma before crystallization is likely to be ~5 to 6.5 wt.% H2O. These results suggest that the magma responsible for the Butte porphyry deposit was relatively water-rich and thus was able to saturate with volatiles at greater depths than in other porphyry deposits. High water contents may also explain the large size of the Butte porphyry deposit. An alternative possibility is that pressures used in our models are somewhat too high, calling for a re-evaluation of the interpretations of fluid inclusion data.
V23B-08
Characteristics of Mineralized Volcanic Centers in Javanese Sunda Island Arc, Indonesia
The subduction-related arc magmatism in Java island, Sunda Arc, Indonesia might have started in earliest Tertiary period, but the distinctively recognizable volcanic belts related with Java trench subduction occurred since the Oligocene. We compiled geoinformation on volcanic centers of different epochs, distribution of metallic mineral deposits, petrochemistry of volcanic rocks, geologic structures, and regional gravity image in order to elucidate characteristics of the known mineralized volcanic centers. Metallic deposits are present in various styles from porphyry-related, high-sulfidation, and low-sulfidation epithermal systems; all related with subaerial volcanism and subvolcanic plutonism. Only few and small occurrences of volcanigenic massive sulfides deposits suggest that some mineralization also occurred in a submarine environment. Most locations of mineral deposits can be related with location of Tertiary volcanic centers along the volcanic arcs (i.e. volcanoes whose genetic link with subduction is clear). On the other side there is no mineralization has been identified to occur associated with backarc magmatism whose genetic link with subduction is under debate. There is strong evidence that major metallic deposit districts are located within compressive tectonic regime and bound by coupling major, deep, and old crustal structures (strike-slip faults) that are recognizable from regional gravity anomaly map. So far the most economical deposits and the only existing mines at major industry scale are high-grade epithermal gold deposits which are young (Upper Miocene to Upper Pliocene), concentrated in Bayah dome complex in west Java, and are associated with alkalic magmatism-volcanism. On the other hand, known porphyry Cu-Au deposits are associated with old (Oligocene to Upper Miocene) stocks, and except for one case, all deposits are located in east Java. Petrochemical data suggest a genetic relationship between porphyry mineralization with low- to moderate-K, high Sr/Y, and low K2O/Na2O magmatism. In turn these features suggest genetic relationship between porphyry Cu-Au mineralization and partial melting of subducted slab.