FTIR Spectrum of Phenocryst Olivine as an Indicator of Silica Saturation in Magmas

Fourier Transform infrared (FTIR) absorption spectra of hydroxylwere measured on olivine phenocrysts from hydrous basaltic meltsthat originated in island-arc tectonic settings. The basalticmelts encompass a wide range of silica activities from orthopyroxene-saturatedhypersthene-normative to nepheline-normative compositions. Theintensities and wavenumber placement of hydroxyl absorptionbands correlate with the degree of silica saturation of theparent melt from which the olivine crystallized. Olivines fromsilica-undersaturated nepheline-normative melts absorb IR radiationin the wavenumber range 3430–3590 cm^–1 (Group 1).In contrast, olivines from orthopyroxene-saturated boniniticmelts exhibit hydroxyl absorption bands in the wavenumber range3285–3380 cm^–1 (Group 2). Olivines crystallizedat intermediate silica activities exhibit a combination of thetwo groups of hydroxyl IR bands, where the proportion of Group2 bands increases with increasing silica saturation of the parentmelt. The positions of hydroxyl absorption peaks observed herefor natural samples are consistent with previous measurementson experimentally annealed olivines. Thus protonation experimentscan be employed to make spectroscopically dry olivine structuresvisible by IR, yielding information on the silica saturationof the parental magmas. Hydroxyl concentrations in the studiedolivines were estimated to be 1–2 ppm, corresponding toan olivine–melt partition coefficient of (1·0 ±0·3) x 10^–4. KEY WORDS: nominally anhydrous minerals; olivine; water; mantle; silica activity; melt inclusions     

Effect of Silica Activity on OH- IR Spectra of Olivine: Implications for Low-aSiO2 Mantle Metasomatism

Hydrogen solubility and hydroxyl substitution mechanism in olivineat upper-mantle conditions are not only a function of pressure,temperature, water fugacity and hydrogen fugacity, but are alsoinfluenced by silica activity. Olivine synthesized in equilibriumwith magnesiowüstite displays hydroxyl stretching bandsin the wavenumber range from 3640 to 3430 cm^–1. In contrast,olivine in equilibrium with orthopyroxene shows absorption bandsin a narrower wavenumber range from 3380 to 3285 cm^–1.The two fundamentally different spectra are assigned to hydroxylin tetrahedral and octahedral sublattices, respectively. Olivinein equilibrium with orthopyroxene is also less capable of incorporatinghydroxyl, relative to olivines in equilibrium with magnesiowüstite,by about a factor of ten. A comparison of spectra obtained aspart of this study with hydroxyl spectra of natural mantle olivinesshows that the latter display hydroxyl stretching patterns reminiscentof equilibrium with magnesiowüstite, although undoubtedlyolivine in the Earth’s mantle coexists with orthopyroxene.This may be attributed to a metasomatic overprint by a low-silicafluid and/or melt that was in reaction relationship with orthopyroxene.A likely metasomatic agent is a carbonatitic melt. When carbonatiticmelts decompose to oxides and CO₂, they may temporarily imposea low-aSiO₂ environment inherited by the olivine structure.If this suggestion proves true, Fourier transform IR spectroscopymay be used to fingerprint metasomatic episodes in the lithosphericmantle. KEY WORDS: FTIR spectrometry; olivine; mantle; metasomatism; water     

Magma Mixing and Intraplutonic Quenching in the Wingellina Hills Intrusion, Giles Complex, Central Australia

The Wingellina Hills intrusion is a small composite gabbroic/ultramaficintrusion and forms a tectonically dismembered segment of theUpper Proterozoic Giles complex in central Australia. Its 1600m of exposed magmatic stratigraphy formed in a continuouslyfractionating, periodically replenished magma chamber. Olivinegabbro and gabbronorite units alternate with lenticular strataboundintercalations of ultramafic (peridotite and pyroxenite) cumulates.A well-developed hybrid footwall zone of intermingled gabbroand pyroxenite underlies each ultramafic unit and demonstratesthe intrusive relationships of ultramafics into gabbroic cumulatemembers. The limited range of mg-number [100 ? Mg/(Mg+Fe)] of ferromagnesiansilicates indicates that the magmatic sequence covers a rathersmall spectrum in chemical fractionation and that the WingellinaHills intrusion represents the basal portion of a formerly largerlayered complex. The mg-number of olivine ranges from 89 to77, below which olivine is replaced by cumulus orthopyroxene.Clinopyroxene covers a wider mg-number range from 91 to 77 andis systematically enriched in MgO relative to coexisting orthopyroxeneand olivine. Anorthite content in plagioclase generally correlatespositively with mg-number changes of coexisting ferromagnesiansilicates. Interstitial plagioclase in clinopyroxenites containsexsolution lamellae of pure orthoclase. These antiperthitesare among the most calcic recorded, with plagioclase hosts betweenAn₆₀ and An₈₀. Bulk antiperthite compositions range around An₆₅–Ab₁₅–Or₂₀and straddle a high-temperature (Or₂₀) solvus in the plagioclasetriangle. The extent of former solid solution between calcicplagioclase and orthoclase indicates crystallization and coolingof the cumulates under moderate pressure and anhydrous conditions. Cryptic mg-number variations show that the intrusion experiencedweak iron enrichment with stratigraphic height. Normal fractionationis confined to the gabbroic members of the sequence, whereasultramafic intercalations are associated with sharp chemicalreversals toward more refractory mineral compositions. Reversalsof mg-number are considerably displaced into the underlyinggabbroic units by up to 50 m relative to the basis of ultramaficintercalations, which indicates extensive postcumulus infiltrationmetasomatism following the emplacement of fresh magma. The trivalentoxides in clinopyroxene have retained their pristine stratigraphicvariation patterns through later metasomatic events and stillcoincide with the cumulus layering. Macroscopic and cryptic layering in the Wingellina Hills intrusionare consistent with a continuously fractionating magma chamberwhose differentiation path was repeatedly reset by periodicinfluxes of primitive parent melt. Ultramafic and gabbroic cumulatemembers can be derived from a single olivine-saturated parentmelt by sequential separation of olivine, olivine-clinopyroxene,and finally olivine/orthopyroxene-clinopyroxene-plagioclase.A series of orthopyroxene-rich cumulates in the mixing zonesof the two melts crystallized from hybrids of the most primitiveand most evolved end-member compositions. Liquidus temperatures calculated for the resident and replenishingmelt components yield 1250 and 1350?C, respectively. As a resultof this temperature difference, fresh influxes of hot parentliquid crystallized rapidly under strongly undercooled conditionsas they ponded on, and quenched against,the chamber floor. Rapidcooling caused a temporary acceleration of the crystallizationfront and formation of impure cumulates with high trapped meltproportions, which resulted in a close coincidence of orthocumulateunits with stratigraphic levels of primitive melt addition.Grain sizes in orthocumulates vary with the cooling rate andpass through a maximum as the degree of undercooling increases.High cooling rates also influenced the composition of some cumulusphases. Clinopyroxenes from ultramafics in the mixing zonesare enriched in iron and aluminium (despite a more primitiveparent melt) and fall outside the fractionation path, especiallyif the batch of new hot magma was small compared with the poolof cooler resident liquid. Aluminous cumulus spinel is partof a metastable crystallization sequence and only crystallizedin the most magnesian ultramafics after episodes of intraplutonicquenching.     

Zircons from Syros, Cyclades, Greece--Recrystallization and Mobilization of Zircon During High-Pressure Metamorphism

Zircons were studied from high-pressure/low-temperature metamorphosedmeta-igneous lithologies from Syros. These rocks carry severalzircon generations related to each other by dissolution–reprecipitationprocesses. One generation is pristine zircon that shows growthzoning, relatively elevated contents of trivalent cations andhigh Th/U ratios. The other end-member is a skeletal zircongeneration with negligible trivalent cation contents and lowTh/U ratios (     

Phase Relations in the Fe-Ni-Cu-PGE-S System at Magmatic Temperature and Application to Massive Sulphide Ores of the Sudbury Igneous Complex

Experiments in the Fe–Ni–Cu–S system wereperformed to identify the role of the metal/S atomic ratio onmonosulphide–melt partition coefficients and closed-systemfractionation paths. In accord with previous work, D_Cu is     

Crystallization Pressure and Cooling History of the Giles Layered Igneous Complex, Central Australia

The Giles Complex, central Australia, consists of a series oflarge layered gabbroic/ultramafic intrusions emplaced in acidicand intermediate granulites of the Middle Proterozoic Musgraveblock. Lithologies range from well-layered dunite, wehrlite,and pyroxenite in the lower primitive series, to massive olivinegabbro, gabbronorite, and anorthosite in the main units, andferrodiorites, vanadife-rous magnetite layers, and granophyresin the upper, most fractionated parts. Unlike many layered intrusions,the Giles Complex is tectonically dismembered to an extent thata reconstruction of the original morphology is difficult. The Complex is believed to be a type example for medium- tohigh-pressure differentiation. (1) Chilled margin samples (wherepreserved) are orthopyroxene-phyric, and liquidus olivine isreplaced by liquidus orthopyroxene at an mg-number of 0.77,suggesting a pressure-related expansion of the orthopyroxenestability field (Goode & Moore, 1975). (2) Tschermaks substitutioninto pyroxene and plagioclase-orthoclase solid solution areextensive, indicating unusually high crystallization temperaturerelated to high pressure; antiperthites in the Giles Complexare amongst the most calcic reported for terrestrial rocks.(3) The lower primitive cumulate units of the Complex are coroniticand feature a variety of subsolidus high-pressure reaction textures;olivine and cumulus chromite have reacted with calcic plagioclaseto orthopyroxene-clinopyroxene-spinel, olivine-spinel, and clinopyroxene-spinelsymplectites. The principal reaction mechanism for the symplectites was continuousmass transfer of alumina from plagioclase toward spinel, asthe Complex passed from the olivine-plagioclase stability fieldinto the pyroxene-spinel field during cooling. Geothermometersapplicable to the cumulates record a wide range of equilibrationtemperatures from late-magmatic to granulite-metamorphic conditions.FeMg₁ exchange gives closure temperatures around 600–700?C,whereas Al₂Mg₁Si₁ net-transfer equilibria have preserved highertemperatures around 750–900 ?C. Defocused beam bulk analysesof exsolved cumulus clinopyroxenes and intercumulus plagioclasesrecover magmatic compositions; i. e., two-pyroxene solvus CaMg_-1temperatures plot around 1120?50?C, whereas two-feldspar thermometersgive 1200?C. Pressures are calculated from thermochemical data with the heterogeneousequilibria 2 fo + an = en + di + sp, fo + an = di + Mg-Ts, andfo + an = en + Ca-Ts, after correcting spinel activities forselective retrograde FeMg_-1 exchange during cooling. These equilibria,combined with orthopyroxene-spinel Al₂Mg_-1Si_-1 temperaturesfor metamorphic assemblages and two-pyroxene temperatures forcumulus phases define a medium-pressure cooling path extendingfrom 1150 ?C (at 6?5 kb) to 750 ?C (at 6?2 kb). The resultssuggest an isobaric cooling path for the Giles Complex, withno evidence for a post-intrusive metamorphic overprint. Themagmas intruded at lower to middle crustal levels after thepervasive deformation in the Musgrave block, and probably afterthe peak metamorphic event.     

Noble Metal Enrichment Processes in the Merensky Reef, Bushveld Complex

We have analysed sulphides, silicates, and chromites of theMerensky Reef for platinum-group elements (PGEs), Re and Auusing laser ablation-inductively coupled plasma mass spectrometryand synthetic pyrrhotite standards annealed with known quantitiesof noble metals. Os, Ir and Ru reside in solid solution in pyrrhotiteand pentlandite, Rh and part of the Reef’s Pd in pentlandite,whereas Pt, Au, Re and some Pd form discrete phases. Olivineand chromite, often suspected to carry Os, Ir and Ru, are PGEfree. All phases analysed contain noble metals as discrete micro-inclusionswith diameters typically <100 nm. Inclusions in sulphidescommonly have the element combinations Os–Ir–Ptand Pt–Pd–Au. Inclusions in olivine and chromiteare dominated by Pt ± Au–Pd. Few inclusion spectracan be related to discrete noble metal phases, and few inclusionshave formed by sub-solidus exsolution. Rather, some PGE inclusions,notably those in olivine and chromite, are early-magmatic nuggetstrapped when their host phases crystallized. We suggest thatthe silicate melt layer that preceded the Merensky Reef wasPGE oversaturated at early cumulus times. Experiments combinedwith available sulphide–silicate partition coefficientssuggest that a silicate melt in equilibrium with a sulphidemelt containing the PGE spectrum of the Merensky ore would indeedbe oversaturated with respect to the least soluble noble metals.Sulphide melt apparently played little role in enriching thenoble metals in the Merensky Reef; rather, its role was to immobilizea pre-existing in situ stratiform PGE anomaly in the liquid-stratifiedmagma chamber. KEY WORDS: Bushveld Complex; Merensky Reef; laser-ablation ICP-MS; platinum-group mineralization