Fluid--Rock Interaction during Low-Pressure Polymetamorphism of the Reynolds Range Group, Central Australia

Marbles and metapelites from the Reynolds Range Group (centralAustralia) were regionally metamorphosed at low pressure duringM₂ at 1.6 Ga, M₂ ranged in grade from greenschist to granulitefacies along the length of the Reynolds Range, and overprinted1.78 Ga granites and their contact aureoles in the ReynoldsRange Group metasediments. At all M₂ grades the marbles andmetapelites have highly variable oxygen isotope ratios [marbles:¹⁸O(carb) 14–20%; metapelites: ¹⁸O 6–14%). Similarly, 1.78 Ga granites have highly variable oxygen isotope ratios(¹⁸O 5–13%), with the lowest values occurring at thegranite margins. In all rock types, the lowest oxygen isotopevalues are consistent with the infiltration of channelled magmaticand/or meteoric fluids. The variable lowering of oxygen isotopevalues resulted from pre-M₂ contact metamorphism and fluid—rockinteraction around the 1.78 Ga granites. In contrast, mineralassemblages in the marbles define a trend of increasing X_CO2with increasing grade from <0.05 (greenschist facies) to0.7–1.0 (granulite facies). This, together with the lackof regionally systematic resetting of oxygen isotope ratios,implies that there was little fluid—rock interaction duringprograde regional metamorphism. KEY WORDS: low pressure; polymetamorphism; fluids; stable isotopes; petrology ^*Corresponding author Fax: 61–3–94791272. e-mail: geoisb{at}lure.latrobe.edu.au     

Oxygen Isotope Evidence for Chemical Interaction of Ki lauea Historical Magmas with Basement Rocks

Klauea historical summit lavas have a wide range in matrix ¹⁸O_VSMOWvalues (4·9–5·6) with lower values in rockserupted following a major summit collapse or eruptive hiatus.In contrast, ¹⁸O values for olivines in most of these lavasare nearly constant (5·1 ± 0·1). The disequilibriumbetween matrix and olivine ¹⁸O values in many samples indicatesthat the lower matrix values were acquired by the magma afterolivine growth, probably just before or during eruption. BothMauna Loa and Klauea basement rocks are the likely sources ofthe contamination, based on O, Pb and Sr isotope data. However,the extent of crustal contamination of Klauea historical magmasis probably minor (< 12%, depending on the assumed contaminant)and it is superimposed on a longer-term, cyclic geochemicalvariation that reflects source heterogeneity. Klauea's heterogeneoussource, which is well represented by the historical summit lavas,probably has magma ¹⁸O values within the normal mid-ocean ridgebasalt mantle range (5·4–5·8) based on thenew olivine ¹⁸O values. KEY WORDS: Hawaii; Klauea; basalt; oxygen isotopes; crustal contamination     

The Ni-S System and Related Minerals

The system Ni-S has been studied systematically from 200^? to1, 030^? C by means of evacuated, sealed silica-glass tube experimentsand differential thermal analyses. Compounds in the system areNi₃S₂ (and a high temperature, non-quenchable Ni_3?S₂ phase),Ni₇S₆, Ni₁–S₄ Ni₃S₄, and NiS₂. The geologic occurrenceof the minerals heazlewoodite (Ni₂S₂), millerite (ßSNi₁-₂S),polydymite (Ni₃S₄), and vaesite (NiS₂) can now be describedin terms of the stability ranges of their synthetic equivalents. Hexagonal heazlewoodite, which is stoichiometric within thelimit of error of the experiments, inverts on heating to a tetragonalor pseudotetragonal phase at 556^? C. This high-temperature phase(Ni₃ has a wide field of stability, from 23.5 to 30.5 wt percent sulfur at 600^? C, and melts incongruently at 806^??3^? C.The ßNi₇S₆ phase inverts to Ni₇₈ at 397^? C6 when inequilibrium with Ni₃S₂, and at 400^? C when in equilibrium withNiS. Crystals of Ni₇S₆ break down to Ni₃-S₂+NiS at 573^??3^?C.The low-temperature form of Ni₁-S₁ corresponding to the mineralmillerite, is rhombohedral, and the high-temperature form hasthe hexagonal NiAs structure. Stoichiometric NiS inverts at379^??3^?C, whereas Ni₁-S with the maximum nickel deficiency invertsat 282^??5OC. The Ni₁-alphS-NiS₂ solvus was determined to 985^??3^?C,the eutectic temperature of these phases. Stoichiometric NiSis stable at 600^?C but breaks down to Ni₂-S₂ and Ni₁-S below797^?C, whereas Ni₁-S with 38.2 wt per cent sulfur melts congruentlyat 992^??3^?C. Vaesite does not vary measurably from stoichiometricNiS₂ composition, and melts congruently at 1.007?5^?C. Polydymitebreaks down to aNi-S? vaesite at 356^??3^?C. Differential thermalanalyses showed the existence of a two-liquid field in the sulfur-richportion of the system above 991^?C and over a wide compositionalrange.     

Stable Isotopic Evidence for Fluid-Rock Interactions in the Ivrea Zone, Italy

Stable isotope compositions of Ivrea Zone marbles and associatedlithologies are in general heterogeneous. The oxygen isotopecomposition of quartz in pelites ranges from ¹⁸O +9 to + 17(SMOW) and does not vary systematically with metamorphic grade.Peridotites retain oxygen isotope signatures close to mantlevalues. Marble calcites vary in isotopic composition from ¹³C + 2(PDB),¹⁸0 +24(SMOW)to ¹³C –6(PDB), ¹⁸O + 13 (SMOW).Depletions in ¹⁸O and ¹³C may be explained dominantly by interactionwith fluids derived from within the observed metasedimentarysequence during prograde metamorphism. ¹⁸O and ¹³C show gradients of greater than 5/m across marblemargins and within marbles. The preservation of such isotopicgradients is not consistent with the long-term presence of grain-boundary-scaleinterconnected fluid films in and around marbles. There is ageneral lowering of ¹⁸O within individual marble bodies althoughlarge carbon and oxygen isotopic gradients are present. Calcitein marbles may attain oxygen isotope equilibrium, but rarelycarbon isotope equilibrium, with surrounding metapelites. Infiltrationof marbles must involve a component of channelized fluid flow. The general lack of isotopic equilibration within the sequencerequires channelized fluid flow and limited fluid-rock ratios.Large pervasive mantle to crust fluid fluxes are not consistentwith the observations. ^*Present address: Natural Environment Research Council, Polaris House, North Star Avenue, Swindon SN2 1EU, England     

Melting of a Hydrous Mantle: II. Geochemistry of Crystals and Liquids Formed by Anatexis of Mantle Peridotite at High Pressures and High Temperatures as a Function of Controlled Activities of Water, Hydrogen, and Carbon Dioxide

The system peridotite-H₂O–CO₂ serves as a simplified modelfor the phase relations of mantle peridotite involving morethan one volatile component. Run products obtained in a studyof phase relations of four mantle peridotites in the presenceof H₂O- and (H₂O+CO₂)- bearing vapors and with controlled hydrogenfugacity (f_H2) at high pressures and temperatures have beensubjected to a detailed chemical investigation, principallyby the electron microprobe. Mg/(Mg+Fe) of all phases generally increases with increasingtemperature and with increasing Mg/(Mg+Fe) of the starting material.This ratio appears to decrease with increasing pressure forolivine, and for amphibole coexisting with garnet. Decreasingf_H2 from that of IW buffer to that of MH buffer decreases Mg/(Mg+Fe)of the partial melt from approximately 0-85 to approximately0.50, whereas the Fo content of coexisting olivine increasesslightly less than 3 per cent and the Mg/(Mg+Fe) of clinopyroxeneincreases about 4 per cent. However, the variations in Fo contentof olivines are within those observed in olivines from naturalmantle peridotite. The chemistry of other silicate mineralsdoes not significantly reflect variations of f_H2. Consequently,the peridotite mineralogy and/or chemistry is not a good indicatorfor the f_H2 conditions during crystallization. All crystalline phases, except amphibole, and to some extentgarnet, show increasing Cr content with increasing temperatureand increasing Cr content of the starting material, resultingin a positive correlation with Mg/(Mg+Fe). Partial melts aredepleted in Cr₂O₃ relative to the crystalline phases. High Mg/Mg+Fe)and Cr₂O₃ are thus expected in crystal residues after partialmelting. The absolute values depend on degree of melting andthe composition of the parent peridotite. Liquids formed by anatexis of mantle peridotite are andesiticunder conditions of X_H2O^v > 0.6 to at least 25 kb total pressureand to more than 200?C above the peridotite solidus. This observationsupports numerous suggestions that andesite genesis in islandarcs may result from partial melting of underlying peridotitemantle. In contrast to basaltic rocks, the absence of amphibole(paragasitic hornblende) does not affect the silica-saturatednature of the liquids. Increasing K₂O content of the startingmaterial (up to 1 wt. per cent K₂O) results in increasing potassiumcontent of the amphibole (1 wt. per cent K₂O) as well as theappearance of phlogopite. The liquid under these conditionsis relatively K₂0-poor (less than 1 wt. per cent K₂O). Partial melts are olivine normative with X_H2O 0.5, and initialliquids contain normative ol and ne at X_H2O 0.4. The alkalinityof these liquids increases with decreasing X_H2O below valuesof 0.5. The (ol+opx)-normative liquids resemble oceanic basaltswhereas (ol+ne)-normative liquids resemble olivine nepheliniteand melilite basalt. Low aHlo and high a_Co2 conditions may bethose under which kimberlites and related rocks are formed inthe mantle.     

Early-Middle Jurassic Dolerite Dykes from Western Dronning Maud Land (Antarctica): Identifying Mantle Sources in the Karoo Large Igneous Province

A suite of dolerite dykes from the Ahlmannryggen region of westernDronning Maud Land (Antarctica) forms part of the much moreextensive Karoo igneous province of southern Africa. The dykecompositions include both low- and high-Ti magma types, includingpicrites and ferropicrites. New ⁴⁰Ar/³⁹Ar age determinationsfor the Ahlmannryggen intrusions indicate two ages of emplacementat 178 and 190 Ma. Four geochemical groups of dykes have beenidentified in the Ahlmannryggen region based on analyses of60 dykes. The groups are defined on the basis of whole-rockTiO₂ and Zr contents, and reinforced by rare earth element (REE),⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope data. Group 1 were intrudedat 190 Ma and have low TiO₂ and Zr contents and a significantArchaean crustal component, but also evidence of hydrothermalalteration. Group 2 dykes were intruded at 178 Ma; they havelow to moderate TiO₂ and Zr contents and are interpreted tobe the result of mixing of melts derived from an isotopicallydepleted source with small melt fractions of an enriched lithosphericmantle source. Group 3 dyke were intruded at 190 Ma and formthe most distinct magma group; these are largely picritic withsuperficially mid-ocean ridge basalt (MORB)-like chemistry (flatREE patterns, ⁸⁷Sr/⁸⁶Sr_i 0·7035, Nd_i 9). However, theyhave very high TiO₂ (4 wt %) and Zr (500 ppm) contents, whichis not consistent with melting of MORB-source mantle. The Group3 magmas are inferred to be derived by partial melting of astrongly depleted mantle source in the garnet stability field.This group includes several high Mg–Fe dykes (ferropicrites),which are interpreted as high-temperature melts. Some Group3 dykes also show evidence of contamination by continental crust.Group 4 dykes are low-K picrites intruded at 178 Ma; they havevery high TiO₂–Zr contents and are the most enriched magmagroup of the Karoo–Antarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENE–WSW) and both groups were emplaced at190 Ma in response to the same regional stress field, whichhad changed by 178 Ma, when Group 2 and Group 4 dykes were intrudedalong a dominantly NNE–SSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke     

Petrology of Mafic Plutons Associated with Calc-Alkaline Granitoids, Chilliwack Batholith, North Cascades, Washington

Small (<5 km²), lithologically diverse gabbro and dioritestocks make up 2% of the 34 to 2 Ma Chilliwack batholith, andoverlap in age with associated calc-alkaline granitoids. Thesemafic plutons are similar to those in other I-type bath-oliths,and represent basaltic magmas present during batholith formation.Objectives of this study are: (1) to examine the origins ofboth interpluton and intrapluton petrologic diversity, and (2)to compare chemical and Sr-Nd isotopic traits of these gabbroswith those of Cascade are basalts. Mafic rocks in the Chilliwackare divided into a medium-K series (MKS) and a low-K series(LKS). The former contain 0.7–2.4 wt% K₂0 and are similarin composition to calc-alkaline basalts and basaltic andesites.Inverse REE modeling supports derivation of the MKS by 9–27%melting of a garnet-free, LREE-enriched source (La/b_N 2). ChilliwackLKS gabbros have chemical characteristics of low-K olivine tho-eiites,including low K₂O (0.3–0.5 wt%) and La/b_N (1.7–3.4),and high CaO (8.8–11.3 wt%) and Na₂O/K₂O (6–22).These traits suggest a source with more clinopyroxene and lowerLa/b_N than the MKS source. Differences in _Nd(O) between MKSand LKS gabbros suggest that lower Nd/Sm is a long-lived LKSsource characteristic. Lithologic variation within compositeplutons of both series resulted primarily from multiple intrusionof related magmas, in some cases differentiates of a commonparent. Two contrasting examples were studied in detail. AtMt Sefrit, MKS variation (gabbronorite-quartz diorite) is modeledby low-pressure fractionation (ol + plag + cpx), accompaniedby 10% wallrock assimilation. In contrast, chemical and Sr-Ndisotopic variation among LKS gabbro-quartz diorite at CopperLake points to crystallization dominated by clinopyroxene+plagioclaseCr-spinel,indicative of differentiation at pressures 10 kbar, althoughthe assimilant in this case is poorly constrained. Chemicaland isotopic similarities between these mafic plutons and QuaternaryCascade lavas indicate that mafic magmas present during theproduction of Chilli-wack granitoids were low-and medium-K arebasalts. KEY WORDS: are magmatism; Cascades; gabbro; granitoid; trace element ^*Present address: Department of Physics, Astronomy, & Geology, Valdosta State University, Valdosta, GA 31698, USA. Telephone: (912) 249–4847 or 333–5752 Fax: (912) 333–7389. e-mail: jtepper{at}valdosta.peachnet.edu     

Oxygen and Hydrogen Isotope Geochemistry of the Mourne Mountains Tertiary Granites, Northern Ireland

An oxygen and hydrogen isotopic study of minerals and wholerocks from the granites of the Mourne Mountains Tertiary complex,and related rocks, shows that whereas a significant circulationof meteoric water was associated with the complex, it had onlyminor and localized effects on the granites themselves. TheSilurian slate and greywacke country rocks, which would havehad ¹⁸O_(SMOW) values of +10 to +20 before the Tertiary igneousevents, have been depicted ¹⁸O to values of –40 to –05Tertiary acid minor intrusions outside the main granite massesare also ¹⁸O depleted. ^l8O whole-rock data on the granites showa range of +6.0 to +9.5, and include values significantly higherthan most of those obtained for the granites of the Tertiarycentral complexes of Skye, Mull, and Ardnamurchan. Many of thelowest whole-rock ¹⁸O values are found in samples where theminerals are not in isotopic equilibrium. The mineral oxygenisotopic data can be explained in terms of localized interactionwith meteoric water, resulting in preferential ¹⁸O depletionin feldspar(s) and biotite, with quartz being much less affected.The granites all show low values of D_(SMOW) for biotite andamphibole separates (–137 to –104). The lowest valuesoccur close to the margins of the plutons, near internal contactsor near greisen localities, and these probably reflect limitedinteraction with meteoric water. The higher D values are fromsamples which show evidence of chloritization. This processappears to have occurred both during interaction with meteoricwater, and also during autometasomatism by an exsolved magmaticfluid in other parts of the plutons, including central locationswhere there is little or no evidence for the penetration ofmeteoric water. Granite samples which exhibit near-equilibriumoxygen isotope fractionations for constituent minerals are characterizedby magmatic O-isotopic compositions. The G2 granite, the largestpluton of the eastern centre, has a magmatic ¹⁸O_(SMOW) valueof {small tilde}+95; intrusions G3 (eastern centre) and G4(western centre) both have ¹⁸O_(SMOW) values of {small tilde}+90.The other two main intrusive phases have distinctly lower ¹⁸O_(SMOW)values: {small tilde}+75 for Gl (the least fractionated graniteof the Mourne Mountains central complex), and from +75 to +85for G5. The oxygen isotopic data rule out simple partial meltingof the country rocks as the origin of the granites and alsopreclude an origin by closed-system fractional crystallizationof basaltic magma typical of that represented by Tertiary basicigneous rocks of the region. ^* Present address: NERC Isotope Geosciences Laboratory, Keyworth, Nottingham BG12 5GG, UK Present address: School of Engineering Technology, Georgian College, Barrie, Ontario, L4M 3X9, Canada     

The Gronnedal-Ika Carbonatite-Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility

The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcite–siderite carbonatite. Aegirine–augite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680–910°Cand silica activities of 0·28–0·43 weredetermined for the crystallization of the syenites on the basisof mineral equilibria. Oxygen fugacities, estimated using titanomagnetitecompositions, were between 2 and 5 log units above the fayalite–magnetite–quartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (La_CN–Yb_CN = 10–70). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54–106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicate–carbonate liquid pairs. _Nd(T =1·30 Ga) values of clinopyroxenes from the syenites varybetween +1·8 and +2·8, and _Nd(T) values of whole-rockcarbonatites range from +2·4 to +2·8. Calcitefrom the carbonatites has ¹⁸O values of 7·8 to 8·6and ¹³C values of –3·9 to –4·6. ¹⁸Ovalues of clinopyroxene separates from the nepheline syenitesrange between 4·2 and 4·9. The average oxygenisotopic composition of the nepheline syenitic melt was calculatedbased on known rock–water and mineral–water isotopefractionation to be 5·7 ± 0·4. Nd and C–Oisotope compositions are typical for mantle-derived rocks anddo not indicate significant crustal assimilation for eithersyenite or carbonatite magmas. The difference in ¹⁸O betweencalculated syenitic melts and carbonatites, and the overlapin _Nd values between carbonatites and syenites, are consistentwith derivation of the carbonatites from the syenites via liquidimmiscibility. KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite     

Dehydration--Melting Phenomena in Leptynitic Gneisses and the Generation of Leucogranites: a Case Study from the Kerala Khondalite Belt, Southern India

Pan-African high-grade metamorphism in the Kerala KhondaliteBelt (South India) led to the in situ formation of garnet-bearingleucosomes (L1) in sodic quartz—alkali feldspar—biotitegneisses. Microtextures, mineralogy and the geochemical characteristicsof in situ leucosomes (L1) and gneiss domains (GnD) indicatethat the development of leucosomes was mainly controlled bythe growth of garnet at the expense of biotite. This is documentedby the selective transfer of FeO, MgO, , Sm and the heavy rareearth elements into the L1 domains. P-T constraints (T>800C,P>6kbar, aH₂O0.3) suggest that the leucosomes were formedthrough complete melting of biotite in fluid-absent conditions,following the model reaction Biotite+Alkali feldspar+QuartzlGarnet+Ilmenite+Melt.The fraction of melt generated during this process was low (<10vol.%). The identical size of the leucosomes as well as theirhomogeneous and isotropic distribution at outcrop scale, whichlacks any evidence for melt segregation, suggest that the migmatiteremained a closed system. Subsequent to migmatization, the leptyniticgneisses were intruded by garnet-bearing leucogranitic melts(L2), forming veins parallel and subperpendicular to the foliation.The leucogranites are rich in potassium (K₂O5.5 wt%), (Ba400p.p.m.) and Sr (300 p.p.m.), and exhibit low concentrationsof Zr (40 p.p.m.), Th (<1 p.p.m.) and (<10 p.p.m.). Thechondrite-normalized REE spectra show low abundances (La_N20,Lu_N3) and are moderately fractionated (La_N/Lu_N7). An Eu anomalyis absent or weakly negative. The higher ⁸⁷Sr/⁸⁶Sr ratio at550 Ma (0.7345) compared with the migmatite (0.7164) precludesa direct genetic relationship between leptynitic gneisses andleucogranites at Manali.Nevertheless, the chemical and mineralogicalcompositions of the leuocogranites strongly favour a derivationthrough fluid-absent biotite melting of isotopically distinctbut chemically comparable Manali-type gneisses. The undersaturationof Zr, Th and REE, a typical feature of leucogranitic meltsgenerated during granulite facies anatexis of psammo-peliticlithologies and attributed to disequilibrium melting with incompletedissolution of accessory phases (zircon, monazite), is weaklydeveloped in the leucogranites of Manali.It is concluded thatthis is mainly due to the sluggish migration of the melts instatic conditions, which facilitated equilibration with therestitic gneisses. ^*Fax: 0228-732763; e-mail: ingo.braun{at}uni-bonn.de     

The 1982-83 Eruption at Galunggung Volcano, Java (Indonesia): Oxygen Isotope Geochemistry of a Chemically Zoned Magma Chamber

Mount Galunggung is a historically active volcano in southwesternJava that has erupted four times in the last two centuries.During the most recent event, which occurred during a 9–monthinterval in 1982– 83, some 305 10⁶ m³ of medium–K,calc–alkaline magma was erupted. This eruption was unusualbecause of its duration, the diversity of eruption dynamicsand products, and the range of lava compositions produced. Thecomposition of juvenile material changed gradually during thecourse of the eruption from initial plagioclase (An_60–75)and two–pyrozene bearing andesites with 58% SiO₂ to finalplagioclase (An_85–90), diopside, and olivine (Fo_85–90)bearing primitive magnesium basalts with 47% SiO₂ Mineralogicaland compositional relationships indicate a magmatic evolutioninvolving differentitation of high–Mg parental melt. Theeruptive volumes of 35 10⁶ m³ andesite, 120 10⁶ m³ maficandesite, and 150 10⁶ m³ basalt are consistent with the ideathat the 1982– 83 eruption progressively tapped and draineda magma chamber that had become chemically stratified throughextensive crystal fractionation. Separates of plagioclase and pyroxene have ¹⁸O( SMO W) rangesof + 5. 6 to + 6.0 and + 5.3 to + 5.6, respectively, with ¹⁸O_plag–pxvalues of + 0.4 to + 0.6o, indicating internal O–isotopeequiliburium at temperature of 1100–850 C. The magenesianbasalts have magmatic ¹⁸O/ ¹⁶O ratios similar to those of mid–oceanridge basalt, and the O–isotope ratios of compositionallyevolved derivative melts show no evidence for contaminationof the galunggung magmas by ¹⁸O–rich crust during differentiation.Andesites and transitional mafic and sites have a more variableO–isotope character, with laves and phenocrysts havingboth higher and lower ¹⁸O values than observed in the parentalmagnesium basalts. These features are interpreted to reflectintramagma chamber processes affecting the upper portions ofthe differentiating Galunggung magma body before the 1982–83eruption.     

Liquidus Equilibria in the System K2O-Na2O-Al2O3-SiO2-F2O-1-H2O to 100 MPa: I. Silicate-Fluoride Liquid Immiscibility in Anhydrous Systems

Liquidus relations in the four-component system Na₂O–Al₂O₃–SiO₂–F₂O_–1were studied at 0· 1 and 100 MPa to define the locationof fluoride–silicate liquid immiscibility and outlinedifferentiation paths of fluorine-bearing silicic magmas. Thefluoride–silicate liquid immiscibility spans the silica–albite–cryoliteand silica–topaz–cryolite ternaries and the haplogranite-cryolitebinary at greater than 960°C and 0· 1–100 MPa.With increasing Al₂O₃ in the system and increasing aluminum/alkalication ratio, the two-liquid gap contracts and migrates fromthe silica liquidus to the cryolite liquidus. The gap does notextend to subaluminous and peraluminous melt compositions. Forall alkali feldspar–quartz-bearing systems, the miscibilitygap remains located on the cryolite liquidus and is thus inaccessibleto differentiating granitic and rhyolitic melts. In peralkalinesystems, the magmatic differentiation is terminated at the albite–quartz–cryoliteeutectic at 770°C, 100 MPa, 5 wt % F and cation Al/Na =0· 75. The addition of topaz, however, significantlylowers melting temperatures and allows strong fluorine enrichmentin subaluminous compositions. At 100 MPa, the binary topaz–cryoliteeutectic is located at 770°C, 39 wt % F, cation Al/Na 0·95, and the ternary quartz–topaz–cryolite eutecticis found at 740°C, 32 wt % F, 30 wt % SiO₂ and cation Al/Na 0· 95. Such location of both eutectics enables fractionationpaths of subaluminous quartz-saturated systems to produce fluorine-rich,SiO₂-depleted and nepheline-normative residual liquids. KEY WORDS: silicate melt; granite; rhyolite; fluorine; liquid immiscibility     

A Method for Thermodynamical Calculation of Basicity Indicators of Rocks and Minerals

A new method has been suggested for evaluating the overall basicityof minerals and rocks by using ionization reactions involvingone proton: (sum of cations) + H₂O = mineral + H⁺, (sum of cations) + H₂O = (sum of normative minerals of a rock)+ H⁺. The basicity indicators are expressed as standard free energychanges of these reactions (). At standard water pressure (logP_H2O = 0) and chemical activity of the metal ions ( log _Mⁿ⁺= 0), the relationship between and alkalinity of solutions(pH) becomes: = –2.303 RTlog _H⁺ = 2.303 RT pH. The overall basicities of rock-forming oxides, minerals andmajor rocks were calculated from the thermodynamic data on ionsin water solutions and solid compounds.     

Integrated Models of Basalt Petrogenesis: A Study of Quartz Tholeiites to Olivine Melilitites from South Eastern Australia Utilizing Geochemical and Experimental Petrological Data

The Tertiary to Recent basalts of Victoria and Tasmania havemineralogical and major element characteristics of magmas encompassingthe range from quartz tholeiites to olivine melilitites. Abundancesof trace elements such as incompatible elements, including therare earth elements (REE), and the compatible elements Ni, Coand Sc, vary systematically through this compositional spectrum.On the basis of included mantle xenoliths, appropriate 100 Mg/Mg+ Fe₊₂ (68–72) and high Ni contents many of these basaltsrepresent primary magmas (i.e., unmodified partial melts ofmantle peridotite). For fractionated basalts we have derivedmodel primary magma compositions by estimating the compositionalchanges caused by fractional crystallization of olivine andpyroxene at low or moderate pressure. A pyrolite model mantlecomposition has been used to establish and evaluate partialmelting models for these primary magmas. By definition and experimentaltesting the specific pyrolite composition yields parental olivinetholeiite magma similar to that of KilaeauIki, Hawaii (1959–60)and residual harzburgite by 33 per cent melting. It is shownthat a source pyrolite composition differing only in having0.3–0.4 per cent TiO₂ rather than 0.7 per cent TiO₂, isable to yield the spectrum of primary basalts for the Victorian-Tasmanianprovince by 4 per cent to 25 per cent partial melting. The mineralogiesof residual peridotites are consistent with known liquidus phaserelationships of the primary magmas at high pressures and thechemical compositions of residual peridotite are similar tonatural depleted or refractory lherzolites and harzburgites.For low degrees of melting the nature of the liquid and of theresidual peridotite are sensitively dependent on the contentof H₂O, CO₂ and the CO₂/H₂O in the source pyrolite. The melting models have been tested for their ability to accountfor the minor and trace element, particularly the distinctivelyfractionated REE, contents of the primary magmas. A single sourcepyrolite composition can yield the observed minor and traceelement abundances (within at most a factor of 2 and commonlymuch closer) for olivine melilitite (4–6 per cent melt),olivine nephelinite, basanite (5–7 per cent melt), alkaliolivine basalt (11–15 per cent melt), olivine basalt andolivine tholeiite (20–25 per cent melt) provided thatthe source pyrolite was already enriched in strongly incompatibleelements (Ba, Sr, Th, U, LREE) at 6–9 x chondritic abundancesand less enriched (2.5–3 x chondrites) in moderately incompatible(Ti, Zr, Hf, Y, HREE) prior to the partial melting event. Thesources regions for S.E. Australian basalts are similar to thosefor oceanic island basalts (Hawaii, Comores, Iceland, Azores)or for continental and rift-valley basaltic provinces and verydifferent in trace element abundances from the model sourceregions for most mid-ocean ridge basalts. We infer that thismantle heterogeneity has resulted from migration within theupper mantle (LVZ or below the LVZ) of a melt or fluid (H₂O,CO₂-enriched) with incompatible element concentrations similarto those of olivine melilitite, kimberlite or carbonatite. Asa result of this migration, some mantle regions are enrichedin incompatible elements and other areas are depleted. Although it is possible, within the general framework of a lherzolitesource composition, to derive the basanites, olivine nephelinitesand olivine melilitites from a source rock with chondritic relativeREE abundances at 2–5 x chondritic levels, these modelsrequire extremely small degrees of melting (0.4 per cent forolivine melilitite to 1 per cent for basanite). Furthermore,it is not possible to derive the olivine tholeiite magmas fromsource regions with chondritic relative REE abundances withoutconflicting with major element and experimental petrology argumentsrequiring high degrees (15 per cent) of melting and the absenceof residual garnet. If these arguments are disregarded, andpartial melting models are constrained to source regions withchondritic relative REE abundances, then magmas from olivinemelilitites to olivine tholeiites can be modelled if degreesof melting are sufficiently small, e.g., 7 per cent meltingfor olivine tholeiite. However, the source regions must be heterogenousfrom 1 to 5 x chondritic in absolute REE abundances and heterogerieousin other trace elements as well. This model is rejected in favorof the model requiring variation in degree of melting from 4per cent to 25 per cent and mantle source regions ranging fromLREE-enriched to LREE-depleted relative to chondritic REE abundances.     

Source of the Quaternary Alkalic Basalts, Picrites and Basanites of the Potrillo Volcanic Field, New Mexico, USA: Lithosphere or Convecting Mantle?

The <80 ka basalts–basanites of the Potrillo VolcanicField (PVF) form scattered scoria cones, lava flows and maarsadjacent to the New Mexico–Mexico border. MgO ranges upto 12·5%; lavas with MgO < 10·7% have fractionatedboth olivine and clinopyroxene. Cumulate fragments are commonin the lavas, as are subhedral megacrysts of aluminous clinopyroxene(with pleonaste inclusions) and kaersutitic amphibole. REE modellingindicates that these megacrysts could be in equilibrium withthe PVF melts at 1·6–1·7 GPa pressure. Thelavas fall into two geochemical groups: the Main Series (85%of lavas) have major- and trace-element abundances and ratiosclosely resembling those of worldwide ocean-island alkali basaltsand basanites (OIB); the Low-K Series (15%) differ principallyby having relatively low K₂O and Rb contents. Otherwise, theyare chemically indistinguishable from the Main Series lavas.Sr- and Nd-isotopic ratios in the two series are identical andvary by scarcely more than analytical error, averaging ⁸⁷Sr/⁸⁶Sr= 0·70308 (SD = 0·00004) and ¹⁴³Nd/¹⁴⁴Nd = 0·512952(SD=0·000025). Such compositions would be expected ifboth series originated from the same mantle source, with Low-Kmelts generated when amphibole remained in the residuum. ThreePVF lavas have very low Os contents (<14 ppt) and appearto have become contaminated by crustal Os. One Main Series picritehas 209 ppt Os and has a _Os value of +13·6, typical forOIB. This contrasts with published ¹⁸⁷Os/¹⁸⁸Os ratios for KilbourneHole peridotite mantle xenoliths, which give mostly negative_Os values and show that Proterozoic lithospheric mantle formsa thick Mechanical Boundary Layer (MBL) that extends to 70 kmdepth beneath the PVF area. The calculated mean primary magma,in equilibrium with Fo₈₉, has Na₂O and FeO contents that givea lherzolite decompression melting trajectory from 2·8GPa (95 km depth) to 2·2 GPa (70 km depth). Inverse modellingof REE abundances in Main Series Mg-rich lavas is successfulfor a model invoking decompression melting of convecting sub-lithosphericlherzolite mantle (Nd = 6·4; T_p 1400°C) between90 and 70 km. Nevertheless, such a one-stage model cannot accountfor the genesis of the Low-K Series because amphibole wouldnot be stable within convecting mantle at T_f 1400°C. Thesemagmas can only be accommodated by a three-stage model thatenvisages a Thermal Boundary Layer (TBL) freezing conductivelyonto the 70 km base of the Proterozoic MBL during the 20 Myrtectonomagmatic quiescence before PVF eruptions. As it grew,this was veined by hydrous small-fraction melts from below.The geologically recent arrival of hotter-than-ambient (T_p 1400°C) convecting mantle beneath the Potrillo area re-meltedthe TBL and caused the magmatism. KEY WORDS: western USA; picrites; Sr–Nd–Os isotopes; petrogenetic modelling; thermal boundary layer     

Genesis of Ultramafic Lamprophyres and Carbonatites at Aillik Bay, Labrador: a Consequence of Incipient Lithospheric Thinning beneath the North Atlantic Craton

Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite,damtjernite) and subordinate dolomite-bearing carbonatite withU–Pb perovskite emplacement ages of 590–555 Ma occurin the vicinity of Aillik Bay, coastal Labrador. The ultramaficlamprophyres principally consist of olivine and phlogopite phenocrystsin a carbonate- or clinopyroxene-dominated groundmass. Ti-richprimary garnet (kimzeyite and Ti-andradite) typically occursat the aillikite type locality and is considered diagnosticfor ultramafic lamprophyre–carbonatite suites. Titanianaluminous phlogopite and clinopyroxene, as well as comparativelyAl-enriched but Cr–Mg-poor spinel (Cr-number < 0.85),are compositionally distinct from analogous minerals in kimberlites,orangeites and olivine lamproites, indicating different magmageneses. The Aillik Bay ultramafic lamprophyres and carbonatiteshave variable but overlapping ⁸⁷Sr/⁸⁶Sr_i ratios (0·70369–0·70662)and show a narrow range in initial _Nd (+0·1 to +1·9)implying that they are related to a common type of parentalmagma with variable isotopic characteristics. Aillikite is closestto this primary magma composition in terms of MgO (15–20wt %) and Ni (200–574 ppm) content; the abundant groundmasscarbonate has ¹³C_PDB between –5·7 and –5,similar to primary mantle-derived carbonates, and ¹⁸O_SMOW from9·4 to 11·6. Extensive melting of a garnet peridotitesource region containing carbonate- and phlogopite-rich veinsat 4–7 GPa triggered by enhanced lithospheric extensioncan account for the volatile-bearing, potassic, incompatibleelement enriched and MgO-rich nature of the proto-aillikitemagma. It is argued that low-degree potassic silicate to carbonatiticmelts from upwelling asthenosphere infiltrated the cold baseof the stretched lithosphere and solidified as veins, therebycrystallizing calcite and phlogopite that were not in equilibriumwith peridotite. Continued Late Neoproterozoic lithosphericthinning, with progressive upwelling of the asthenosphere beneatha developing rift branch in this part of the North Atlanticcraton, caused further veining and successive remelting of veinsplus volatile-fluxed melting of the host fertile garnet peridotite,giving rise to long-lasting hybrid ultramafic lamprophyre magmaproduction in conjunction with the break-up of the Rodinia supercontinent.Proto-aillikite magma reached the surface only after coatingthe uppermost mantle conduits with glimmeritic material, whichcaused minor alkali loss. At intrusion level, carbonate separationfrom this aillikite magma resulted in fractionated dolomite-bearingcarbonatites (¹³C_PDB –3·7 to –2·7)and carbonate-poor mela-aillikite residues. Damtjernites maybe explained by liquid exsolution from alkali-rich proto-aillikitemagma batches that moved through previously reaction-lined conduitsat uppermost mantle depths. KEY WORDS: liquid immiscibility; mantle-derived magmas; metasomatism, Sr–Nd isotopes; U–Pb geochronology     

Oxygen Isotope Geochemistry of the Lassen Volcanic Center, California: Resolving Crustal and Mantle Contributions to Continental Arc Magmatism

This study reports oxygen isotope ratios determined by laserfluorination of mineral separates (mainly plagioclase) frombasaltic andesitic to rhyolitic composition volcanic rocks eruptedfrom the Lassen Volcanic Center (LVC), northern California.Plagioclase separates from nearly all rocks have ¹⁸O values(6·1–8·4) higher than expected for productionof the magmas by partial melting of little evolved basalticlavas erupted in the arc front and back-arc regions of the southernmostCascades during the late Cenozoic. Most LVC magmas must thereforecontain high ¹⁸O crustal material. In this regard, the ¹⁸O valuesof the volcanic rocks show strong spatial patterns, particularlyfor young rhyodacitic rocks that best represent unmodified partialmelts of the continental crust. Rhyodacitic magmas erupted fromvents located within 3·5 km of the inferred center ofthe LVC have consistently lower ¹⁸O values (average 6·3± 0·1) at given SiO₂ contents relative to rockserupted from distal vents (>7·0 km; average 7·1± 0.1). Further, magmas erupted from vents situated attransitional distances have intermediate values and span a largerrange (average 6·8 ± 0·2). Basaltic andesiticto andesitic composition rocks show similar spatial variations,although as a group the ¹⁸O values of these rocks are more variableand extend to higher values than the rhyodacitic rocks. Thesefeatures are interpreted to reflect assimilation of heterogeneouslower continental crust by mafic magmas, followed by mixingor mingling with silicic magmas formed by partial melting ofinitially high ¹⁸O continental crust (9·0) increasinglyhybridized by lower ¹⁸O (6·0) mantle-derived basalticmagmas toward the center of the system. Mixing calculationsusing estimated endmember source ¹⁸O values imply that LVC magmascontain on a molar oxygen basis approximately 42 to 4% isotopicallyheavy continental crust, with proportions declining in a broadlyregular fashion toward the center of the LVC. Conversely, the¹⁸O values of the rhyodacitic rocks suggest that the continentalcrust in the melt generation zones beneath the LVC has beensubstantially modified by intrusion of mantle-derived basalticmagmas, with the degree of hybridization ranging on a molaroxygen basis from approximately 60% at distances up to 12 kmfrom the center of the system to 97% directly beneath the focusregion. These results demonstrate on a relatively small scalethe strong influence that intrusion of mantle-derived maficmagmas can have on modifying the composition of pre-existingcontinental crust in regions of melt production. Given thisresult, similar, but larger-scale, regional trends in magmacompositions may reflect an analogous but more extensive processwherein the continental crust becomes progressively hybridizedbeneath frontal arc localities as a result of protracted intrusionof subduction-related basaltic magmas. KEY WORDS: oxygen isotopes; phenocrysts; continental arc magmatism; Cascades; Lassen     

Experimental Petrology of a Highly Potassic Magma

The melting behaviour of a highly potassic biotite mafuriteof the Central African olivine leucitite kindred has been studiedexperimentally as a function of pressure (to 30kb) temperature,and water content (0%, 5%, 15%, 25%, and 40% H₂O). Olivine isthe liquidus phase up to 30 kb for all water contents studiedexcept for anhydrous (clinopyroxene on the liquidus) and 15%H₂O (phlogopite on the liquidus) conditions. Analyses of phasescrystallizing from the biotite mafurite show that pressure hasvery little effect on the composition of clinopyroxene whichis extremely calcium-rich, and low in Al₂O₃ and TiO₂ for allconditions investigated. Phlogopite has low TiO₂ content andtitanphlogopite cannot be a refractory phase in the upper mantlecausing Ti-depletion in partial melts in equilibrium with titanphlogopite.There are apparently no conditions where the extremely potassicbiotite mafurite could be a partial melt from pyrolite but derivationfrom an olivine+clinopyroxene+phlogopite+ilmenite assemblageoccurring as ‘enriched’ patches in the upper mantle,is possible. Liquids in equilibrium with phlogopite as a residualphase at 30 kb would be olivine nephelinites with approximately5% K₂O, Na₂O/K₂O 1 and TiO₂ > 5+. Crystal elutriation withtransported residual phlogopite reacting (phlogopite+liquid1 olivine+liquid 2) at lower pressures provides a mechanismfor K-enrichment and generating Na₂O/K₂O < 1.     

Geochemistry of South African On- and Off-craton, Group I and Group II Kimberlites: Petrogenesis and Source Region Evolution

Bulk-rock geochemical compositions of hypabyssal kimberlites,emplaced through the Archaean Kaapvaal craton and ProterozoicNamaqua–Natal belt, are used to estimate close-to-primarymagma compositions of Group I kimberlites (Mg-number = 0·82–0·87;22–28 wt % MgO; 21–30 wt % SiO₂; 10–17 wt% CaO; 0·2–1·7 wt % K₂O) and Group II kimberlites(Mg-number = 0·86–0·89; 23–29 wt %MgO; 28–36 wt % SiO₂; 8–13 wt % CaO; 1·6–4·6wt % K₂O). Group I kimberlites are distinguished from GroupII by their lower Ba/Nb (<12), Th/Nb (<1·1) andLa/Nb (<1·1) but higher Ce/Pb (>22) ratios. Thedistinct rare earth element patterns of the two types of kimberlitesindicate a more highly metasomatized source for Group II kimberlites,with more residual clinopyroxene and less residual garnet. Thesimilarity of Sr and Nd isotope ratios and diagnostic traceelement ratios (Ce/Pb, Nb/U, La/Nb, Ba/Nb, Th/Nb) of Group Ikimberlites to ocean island basalts (OIB), but more refractoryMg-numbers and Ni contents, are consistent with derivation ofGroup I kimberlites from subcontinental lithospheric mantle(SCLM) that has been enriched by OIB-like melts or fluids. Sourceenrichment ages and plate reconstructions support a direct associationof these melts or fluids with Mesozoic upwelling beneath southernAfrica of a mantle plume(s), at present located beneath thesouthern South Atlantic Ocean. In contrast, the geochemicalcharacteristics of both on- and off-craton Group II kimberlitesshow strong similarity to calc-alkaline magmas, particularlyin their Nb and Ta depletion and Pb enrichment. It is suggestedthat Group II kimberlites are derived from both Archaean andProterozoic lithospheric mantle source regions metasomatizedby melts or fluids associated with ancient subduction events,unrelated to mantle plume upwelling. The upwelling of mantleplumes beneath southern Africa during the Mesozoic, at the timeof Gondwana break-up, may have acted as a heat source for partialmelting of the SCLM and the generation of both Group I and GroupII kimberlite magmas. KEY WORDS: kimberlite; geochemistry; petrogenesis; mantle plumes; South Africa     

A Stable Isotope Study of Anorogenic Magmatism in East Central Asia

A stable isotope study of 168 plutonic igneous rock and fivewater samples from Transbaikalia, East Asia, has been made,including 318 whole-rock and mineral ¹⁸O/¹⁶O analyses and 14D/H analyses. This represents the first detailed isotopic studyof the enormous Phanerozoic K-rich (mostly anorogenic) granitoidbelts of this region, which are thousands of kilometers in lengthand span an age range of >250Ma. Of the five main intrusivesuites, emplacement of the youngest (Permo-Triassic) was accompaniedby intense meteoric-hydrothermal activity, locally causing extreme¹⁸O depletio (¹⁸O _feldspar<–12). This reflects thevery low ¹⁸O of the water involved in these systems, which probablyhad ¹⁸O < – 20 and D < – 150, consistent withthe high paleolatitude of Transbaikalia in the early Mesozoic(80N). Despite local post-emplacement, hydrothermal ¹⁸O-depletioneffects near Permo-Triassic and younger plutons, the variationof magmatic isotopic composition in the five intrusive suites,in space and time, can be clearly discerned using the ¹⁸O/¹⁶Ovariation in phases such as quartz and sphene that are resistantto sub-solidus exchange. A procedure for analyzing ¹⁸O/¹⁶O insphene using a laser fluorination technique is described: analysisof samples as small as 0.4 mg (including single crystals ofsphene from granitoids) is possible and provides an effectiveway to estimate the magmatic ¹⁸O value of plutonic igneous rocks.Most sphene and quartz ¹⁸O values vary by 1.0–2.0% withineach of the five main intrusive suites in Transbaikalia (rangingin age from mid-Paleozoic to Mesozoic), and are uniform bothwithin individual plutons and among plutons of the same suiteseparated by tens or hundreds of kilometers. However, each suitehas a unique range in ¹⁸O/¹⁶O, indicating that, on a regionalscale, the magmatic ¹⁸O values of these granitoids decreasedprogressively in 1% decrements from +10 in the earliest groupto +6 in the youngest. This progression was accompanied by increasesin the concentration of elements such as K and Zr, and decreasesin the concentration of elements such as Sr and Ba. These systematicsrequire large scale deep crustal melting and mixing processesto generate the compositional uniformity of individual plutonsand groups over such wide areas, and also a progressive hybridizationof the crust with alkalic, mantle-derived magmas to generatesyenites and granites with progressively lower ¹⁸O values. Thisprocess may be a hallmark of anorogenic granitoid petrogenesisand the intracontinental cratonization process in general, andalso represents an important (though largely cryptic) crustalgrowth mechanism. KEY WORDS: anorogenic granitoids; crustal growth; hybridization; hydrothermal systems; stable isotopes ¹Present address: Galson Sciences Limited, 5 Grosvenor House, Melton Road, Oakham LE15 6AX, UK.