Volatiles in glasses from Mauna Loa Volcano,Hawai'i: implications for magma degassing and contamination,and growth of Hawaiian volcanoes

Glasses from Mauna Loa pillow basalts, recent subaerial vents, and inclusions in olivine were analyzed for S, Cl, F, and major elements by electron microprobe. Select submarine glasses were also analyzed for H₂O and CO₂ by infrared spectroscopy. The compositional variation of these tholeiitic glasses is dominantly controlled by crystal fractionation and they indicate quenching temperatures of 1,115-1,196 °C. Submarine rift zone glasses have higher volatile abundances (except F) than nearly all other submarine and subaerial glasses with the maximum concentrations increasing with water depth. The overwhelming dominance of degassed glasses on the submarine flanks of Mauna Loa implies that much of volcano's recent submarine growth involved subaerially erupted lava that reached great water depths (up to 3.1 km) via lava tubes. Anomalously high F and Cl in some submarine glasses and glass inclusions indicate contamination possibly by fumarolic deposits in ephemeral rift zone magma chambers. The relatively high CO₂ but variable H₂O/K₂O and S/K₂O in some submarine rift zone glasses indicates pre-eruptive mixing between degassed and undegassed magma within Mauna Loa's rift system. Volatile compositions for Mauna Loa magmas are similar to other active Hawaiian volcanoes in S and F, but are less Cl-rich than Ll'ihi glasses. However, Cl/K2O ratios are similar. Mauna Loa and Ll'ihi magmas have comparable, but lower H₂O than those from Kilauea. Thus, Kilauea's source may be more H₂O-rich. The dissimilar volatile distribution in glasses from active Hawaiian volcanoes is inconsistent with predictions for a simple, concentrically zoned plume model.     

The Jinchuan Ultramafic Intrusion: Cumulate of a High-Mg Basaltic Magma

The Jinchuan intrusion, situated in Gansu province, China, isan ultramafic dyke-like body emplaced in the Longshoushan upliftedterrain on the southwest margin of the Sino-Korea platform.The intrusion is 6 km long, 350 m wide and hosts a major Ni-Cusulfide deposit. It comprises three subchambers: the west, west-central,and east. The two western subchambers are narrow and deep, andboth are laterally zoned from dunite in the core through Iherzoliteto olivine pyroxenite toward the margins. The eastern subchamberis shallow and wide, and it shows vertical stratification gradingfrom dunite at the base upward into Iherzolite and plagioclaseIherzolite, then back to Iherzolite at the top. Sixty rock samples from the Jinchuan intrusion have been analyzedfor major and trace elements, and 54 samples were also analyzedfor the REE. All samples contain > 24 wt.% MgO, with themajority having > 35% when recalculated to 100% anhydrous.Negative linear correlations are observed between MgO and mostother constituents (except for a few such as Na₂O, K₂O, Sr,and Rb, which may have been affected by alteration), and itappears that the rocks were essentially formed as mixtures ofcumulus olivine and primary magma. Electron microprobe analyses show olivine compositions fromFo₇₉ to Fo₈₆, with most between Fo₈₃ and Fo₈₅. The MgO/(MgO+ FeO) value of the primary magma is calculated to have been 0.64, and its MgO content is estimated to have been 12 wt.%.Thus, the Jinchuan igneous body is probably the ultramafic cumulateportion of an intrusion of a high-magnesium basaltic magma relatedto continental rifting. We suggest that the two western subchambers of the Jinchuanintrusion represent the main conduit to the original magma chamberand that their zoning was formed by flow differentiation. Theeastern subchamber probably represents a higher level of themagma chamber, where crystallization was marked by convectionand periodic replenishment. After consolidation, the Jinchuanintrusion was tilted to the east so the deeper parts of thewestern subchambers are now exposed to the same erosion levelas the shallower part of the eastern subchamber.     

Potassium and Associated Elements in Tholeiitic Basalts

Selected average analyses of tholeiitic basalts, which havemarkedly differing contents of K₂O, show a broad coherence betweenK, P, Ti, Ba, Rb, Sr, and Zr. The content of K₂O in the averageanalyses of 43 tholeiitic provinces shows a wide range (0.07–1.61per cent) and has a positively skewed frequency distribution.K₂O in individual tholeiitic basalts and dolerites has a similardistribution pattern. The relationship between K₂O and MgO in the average analysesindicates that the range of K₂O content cannot be solely theresult of near-surface differentiation, nor, indeed, any differentiationprocess which is indexed by the MgO content of the differentiates. Processes which could effect an enrichment of K and associatedelements in tholeiites are assessed and it is concluded thatdegree of melting, eclogite fractionation at high pressure,and mantle wall-rock reaction are likely to be significant factors.Crustal contamination is considered not to be a generally significantfactor in the development of continental tholeiites. The concept that all other tholeiites are derived from a parentalmagma with the composition of low-K deep oceanic tholeiitesis rejected.     

Experimental Phase Relations in the System Tonalite-Peridotite-H2O at 15 kb; Implications for Assimilation and Differentiation Processes near the Crust-Mantle Boundary

Experiments at 15 kb in the tonalite-peridotite-H₂O system provideinformation on some of the phase equilibrium factors that mayinfluence reaction and assimilation processes between quartznormativemagmas and ultramafic rocks in the deep crust and upper mantle.Experiments were done with 5 or 10 wt.% H₂O added to powderednatural samples of tonalite, and mixtures of tonalite with 5or 10 wt.% peridotite added (TP5 and TP10, respectively). Theliquidus phase relations of these starting compositions wereinvestigated between 850 and 1100?C at 15 kb, using gold capsulesso that iron loss to the sample containers was not a problemand meaningful glass and mineral analyses could be obtained.Experiments on the tonalite alone show either liquidus garnet,for samples with 5% H₂O added, or liquidus hornblende, for sampleswith 10% H₂O. In contrast, orthopyroxene is the sole liquidusphase, irrespective of water content, in experiments using startingmixtures of 5 or 10 wt.% peridotite added to tonalite. Glassanalyses of partially crystallized tonalite define a crystallizationpath diverging significantly from the calc-alkaline trend towardshigher Ca/(Mg + Fe) in the CaO–(MgO + FeO)–?SiO₂triangle. In contrast, glasses from partially crystallized mixturesof tonalite with 5 or 10 wt.% peridotite added define a liquidtrend close to natural calc-alkaline compositions in terms ofCa/(Mg + Fe). Of more general significance, the proximity ofa field ofliquidus orthopyroxene on the high (Mg + Fe) sideof compositions along the calc-alkaline trend serves to limitthe Mgenrichment of such melts by interaction with ultramaficrocks. Unless heat is added to the system, reaction of tonaliticcomposition melts with ultramafic rocks will produce only slightlyMg-enriched melts: increasing degree of reaction simply resultsin further precipitation of orthopyroxene + garnet ? clinopyroxeneonce melt compositions reach the orthopyroxene field boundary.     

Enriched End-member of Primitive MORB Melts: Petrology and Geochemistry of Glasses from Macquarie Island (SW Pacific)

Macquarie Island is an exposure above sea-level of part of thecrest of the Macquarie Ridge. The ridge marks the Australia–Pacificplate boundary south of New Zealand, where the plate boundaryhas evolved progressively since Eocene times from an oceanicspreading system into a system of long transform faults linkedby short spreading segments, and currently into a right-lateralstrike-slip plate boundary. The rocks of Macquarie Island wereformed during spreading at this plate boundary in Miocene times,and include intrusive rocks (mantle and cumulate peridotites,gabbros, sheeted dolerite dyke complexes), volcanic rocks (N-to E-MORB pillow lavas, picrites, breccias, hyaloclastites),and associated sediments. A set of Macquarie Island basalticglasses has been analysed by electron microprobe for major elements,S, Cl and F; by Fourier transform infrared spectroscopy forH₂O; by laser ablation–inductively coupled plasma massspectrometry for trace elements; and by secondary ion mass spectrometryfor Sr, Nd and Pb isotopes. An outstanding compositional featureof the data set (47·4–51·1 wt % SiO₂, 5·65–8·75wt % MgO) is the broad range of K₂O (0·1–1·8wt %) and the strong positive covariation of K₂O with otherincompatible minor and trace elements (e.g. TiO₂ 0·97–2·1%;Na₂O 2·4–4·3%; P₂O₅ 0·08–0·7%;H₂O 0·25–1·5%; La 4·3–46·6ppm). The extent of enrichment in incompatible elements in glassescorrelates positively with isotopic ratios of Sr (⁸⁷Sr/⁸⁶Sr= 0·70255–0·70275) and Pb (²⁰⁶Pb/²⁰⁴Pb =18·951–19·493; ²⁰⁷Pb/²⁰⁴Pb = 15·528–15·589;²⁰⁸Pb/²⁰⁴Pb = 38·523–38·979), and negativelywith Nd (¹⁴³Nd/¹⁴⁴Nd = 0·51310–0·51304).Macquarie Island basaltic glasses are divided into two compositionalgroups according to their mg-number–K₂O relationships.Near-primitive basaltic glasses (Group I) have the highest mg-number(63–69), and high Al₂O₃ and CaO contents at a given K₂Ocontent, and carry microphenocrysts of primitive olivine (Fo_86–89·5).Their bulk compositions are used to calculate primary melt compositionsin equilibrium with the most magnesian Macquarie Island olivines(Fo_90·5). Fractionated, Group II, basaltic glasses aresaturated with olivine + plagioclase ± clinopyroxene,and have lower mg-number (57–67), and relatively low Al₂O₃and CaO contents. Group I glasses define a seriate variationwithin the compositional spectrum of MORB, and extend the compositionalrange from N-MORB compositions to enriched compositions thatrepresent a new primitive enriched MORB end-member. Comparedwith N-MORB, this new end-member is characterized by relativelylow contents of MgO, FeO, SiO₂ and CaO, coupled with high contentsof Al₂O₃, TiO₂, Na₂O, P₂O₅, K₂O and incompatible trace elements,and has the most radiogenic Sr and Pb regional isotope composition.These unusual melt compositions could have been generated bylow-degree partial melting of an enriched mantle peridotitesource, and were erupted without significant mixing with commonN-MORB magmas. The mantle in the Macquarie Island region musthave been enriched and heterogeneous on a very fine scale. Wesuggest that the mantle enrichment implicated in this studyis more likely to be a regional signature that is shared bythe Balleny Islands magmatism than directly related to the hypotheticalBalleny plume itself. KEY WORDS: mid-ocean ridge basalts; Macquarie Island; glass; petrology; geochemistry     

Petrology and Geochemistry of Boninites from the North Termination of the Tonga Trench: Constraints on the Generation Conditions of Primary High-Ca Boninite Magmas

We report here a detailed mineralogical, geochemical, and experimentalstudy of a high-Ca boninite suite from the northern terminationof the Tonga trench. Most samples are strongly olivine porphyriticand show a significant range of phenocryst compositions includinga very refractory olivine-spinel assemblage Fo₉₄–Cr_N =87. They are also characterized by a wide range of incompatible-elementcontents, e.g., (La/Yb)_N varies from 0.5 to 16, whereas compatiblemajor-element concentrations (Al₂O₃, FeO, CaO, SiO₂, and MgO)remain essentially the same. Primary melt compositions for thesuite were established on the basis of an experimental studyof melt inclusions in phenocrysts and numerical modelling ofthe reverse of fractional crystallization. Tongan primary meltsare characterized by high MgO contents (22–24 wt.%) andoriginated in the mantle wedge at pressures of 20–25 kbarand temperatures of 1450–1550 C. H₂O contents in primarymelts were estimated from direct measurements of melt inclusionsby ion probe, and range from 2.0 to 1.0 wt.%, and a strong correlationexists between H₂O and other incompatible element contents.The primary melts crystallized in the presence of an H₂O-richfluid in the temperature range 1390–1150 C and pressuresof 1.7–0.15 kbar. Continuous degassing of melts took placeduring crystallization. Trace-element concentrations in primarymelts were estimated using proton- and ion-probe analyses ofmelt inclusions in olivine, and whole-rock analyses. Our datasuggest that three independent components (D, E1, and E2) wereinvolved. Component D was a refractory mantle depleted in incompatibleelements, likely to be hot ‘dry’ Iherzolite producedby previous melting within a mantle plume. Component E1 wasan H₂O-rich fluid containing LILE and Th, and had an H₂O/K₂Ovalue of 20; it was probably produced by dehydration of thesubducted slab. Component E2 is thought to have been an incompatible-element-enrichedsilicate melt of plume origin. Formation of high-Ca boninitesrequires interaction of hot ‘dry’ residual mantle,associated with plumes, with a subduction-related H₂O-bearingcomponent.     

Standard chemical‐based tephra extraction methods significantly alter the geochemistry of volcanic glass shards

The chemical compositions of tephra shards are widely utilised in a myriad of disciplines, including volcanology, petrology, tephrochronology, palaeoecology and climate studies. Previous research has raised concerns over the possible chemical alteration of microscopic (<100 µm) volcanic glass shards through standard extraction procedures, such as the widely used acid digestion method. This study subjects 10 samples of well‐characterised volcanic glasses ranging from basalt to rhyolite to three common methods used in the extraction of volcanic material from lake sediments and peats. The major element geochemistry of each sample was analysed and compared with a control group. The results of this test indicate that basaltic and andesitic glasses are highly susceptible to chemical alteration, particularly to the concentrated corrosive materials used in acid and base digestion techniques. PERMANOVA analysis of the variation within groups suggests that the oxides most susceptible to variation are alkalis from groups I and II (K₂O, Na₂O, CaO, MgO) and SiO₂, and the most stable oxides are Al₂O₃ and FeO. Felsic glasses are considerably less susceptible to alteration by both acidic (HCl, HNO₃, H₂SO₄) and alkaline (KOH) digestions. Our findings have important implications for interpreting the geochemistry of volcanic glasses. Copyright © 2019 John Wiley & Sons, Ltd.     

Inference of a detailed P–T path from P–T pseudosections using metapelitic rocks of variable composition from a single outcrop, Shackleton Range, Antarctica

Generally, P–T pseudosections for reduced compositional systems, such as K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O, Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O and MnO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O, are well suited for inferring detailed P–T paths, comparing mineral assemblages observed in natural rocks with those calculated. Examples are provided by P–T paths inferred for four metapelitic samples from a 1 m² wide outcrop of the Herbert Mountains in the Shackleton Range, Antarctica. The method works well if the bulk composition used is reconstituted from average mineral modes and mineral compositions (AMC) or when X‐ray fluorescence (XRF) data are corrected for Al₂O₃ and FeO. A plagioclase correction is suitable for Al₂O₃. Correction for FeO is dependent on additional microscopic observations, e.g. the kind and amount of opaque minerals. In some cases, all iron can be treated as FeO^tot, whereas in others a magnetite or hematite correction yields much better results. Comparison between calculated and observed mineral modes and mineral compositions shows that the AMC bulk composition is best suited to the interpretation of rock textures using P–T pseudosections, whereas corrected XRF data yield good results only when the investigated sample has few opaque minerals. The results indicate that metapelitic rocks from the Herbert Mountains of the Northern Shackleton Range underwent a prograde P–T evolution from about 600 °C/5.5 kbar to 660 °C/7 kbar, followed by nearly adiabatic cooling to about 600 °C at 4.5 kbar.     

A Study of Phase Equilibria in a Metamorphic Iron Formation

The paper is based on a study of minerl assemblages in metamorphiciron formations in northere Quebec. Thirty-four mineral analyseshave been performed, using photometric methods of silicate analysis. Three facies of iron formation are represented, namely, quartz-speculariteiron formation, quartz-magnetite iron formation, and quartz-silicateiron formation. The silicate iron formation is made up mainlyof quartz, ferrohypersthene, ferroaugite, cummingtonite, andcalcite. Arfvedsonite and aegirine-hedenbergite occur in magnetite-richrocks. The analysed coexisting minerals are from the silicate ironformation and from overlying mafic rocks of igneous origin.The analysed assemblages include ferrohypersthene-ferro-augite-cummingtonite,ferrohypersthen-ferroaugite, cummingtonite-ferroaugite, diopside-hornblende,anthophyllite-hornblende, hypersthene-diopside-hornblende-garnet,and olivine-hypersthene-chlorite. The analyses show that (1) the minerals are nearly in equilibriumas shown by parallel tie lines, (2) cummingtonite generallyhas a higher MgO/FeO ratio than coexisting ferrohypersthene,(3) the phase higher in calcium has the higher MgO/FeO ratio,(4) the solubility gap between ferrohypersthene and ferroaugiteis 43 mole percent. The stability relationships of pyroxenes and amphiboles arediscussed in view of the new data. It is shown that FeO-CaO-SiO₂systems often behave differently from MgO-CaO-SiO₂ systems.Hedenbergite is probably not stable in the presence of excessCO₂ in the area. The low variance ferrohypersthene-ferroaugite-cummingtonite-calcite-quartzassemblage indicates that the rocks were closed to H₂O and CO₂.The behaviour of the volatile components is discussed in viewof this fact. The source of the silicate minerals is assumed to be hydrousiron silicates and carbonates. A model of the sequence of chemicalreactions taking place in carbonate-silicate iron formationsduring metamorphism is suggested. ¹Present address: Bolidens Gruvaktiebolag, Boliden, Sweden.     

Temporal, Spatial and Geochemical Discriminations of Granitoids in South Korea

Abstract: Five groups of the Phanerozoic granitoids in South Korea can be deduced from their temporal and spatial distributions: (1) Jurassic granitoids in the Gyeonggi massif, (2) Permo-Jurassic granitoids in the Ogcheon belt, (3) Permo-Jurassic granitoids in the Yeongnam massif, (4) Cretaceous granitoids in the Ogcheon belt, and (5) Cretaceous granitoids in the Gyeongsang basin. Though the granitoids of each group generally show calc-alkaline and orogenic natures, the petrological, geochemical and genetical features are different with each other. The Permo-Jurassic granitoids in the Ogcheon belt have lower contents of Al₂O₃, Fe₂O₃, CaO, P₂O₅, but higher of FeO, FeOT, MgO, K₂O than those in the Yeongnam massif. From higher feature of K₂O, Na₂O+K₂O and K₂O/Na₂O, the Ogcheon belt seems to have been located at closer continent side relative to the Yeongnam massif during Permo-Jurassic time. From lower values of Fe₂O₃/FeO and magnetic susceptibility the granitoids of the Ogcheon belt had been solidified under more reducing environment than those of the Yeongnam massif. The Cretaceous granitoids in the Ogcheon belt have lower contents of TiO₂, Fe₂O₃, FeO, FeOT, CaO and P₂O₅, but higher of MgO, K₂O, Na₂O+K₂O and K₂O/Na₂O than those in the Gyeongsang basin. This feature indicates that the Ogcheon belt would correspond to the continental environment of magma genesis during Cretaceous time. Higher values of Fe₂O₃/FeO and magnetic susceptibility in the Cretaceous granitoids in the Gyeongsang basin suggest that the granitoids had been solidified under highly oxidizing environment. From the particular chemical features of K₂O, Na₂O+K₂O and K₂O/Na₂O, the Permo-Jurassic granitoids in the Ogcheon belt, the Yeongnam massif as well as the Cretaceous ones in the Gyeongsang basin can be categorized to the continental margin type granite. The Jurassic granitoids in the Gyeonggi massif are possibly of collision type, and the Cretaceous granitoids in the Ogcheon belt of post–orogenic, intra–conti–nent type. The Jurassic granitoids in the Gyeonggi massif had been possibly generated by crustal melting during the collision of Gyeonggi massif to the northern Pyeongnam basin block. The Cretaceous granitoids in the Ogcheon belt had been emplaced at the hinterland of the continental margin during post-orogenic stage of the Honam Shear Zone. The Cretaceous granitoids in the Gyeongsang basin are often compared with Japanese Cretaceous?Paleogene granitoids in their geochemical and genetical natures. For the granitoid composition, the granitoids in the Gyeongsang basin are higher in Fe₂O₃, Fe₂O₃/FeO, Na₂O, K₂O, Na₂O+K₂O and K₂O/Na₂O, but lower in Al₂O₃, FeO, MnO, CaO and P₂O₅ than the Japanese granitoids. The contents of TiO₂, FeOT and MgO are similar in both granitoids. This geochemical contrast would imply that the Cretaceous granitoid magmas in the Gyeongsang basin had been originated at closer place to the continent side under more tensional field, and solidified under more oxidizing environment than the coeval Japanese granitoid magmas.     

Kimberlite-like Metasomatism and 'Garnet Signature' in Spinel-peridotite Xenoliths from Sal, Cape Verde Archipelago: Relics of a Subcontinental Mantle Domain within the Atlantic Oceanic Lithosphere?

A mantle xenolith suite from two Late Tertiary necks on SalIsland (Cape Verde Archipelago) consists of nearly equivalentamounts of anhydrous spinel-bearing lherzolites and harzburgites,in which secondary metasomatic textural domains are superimposedon the original protogranular textures. Detailed petrographicstudies, coupled with in situ major and trace element analysesof the constituent minerals and interstitial glasses, revealthe complex evolutionary history of the Cape Verde lithosphericmantle, from depletion in the garnet facies to re-equilibrationand re-enrichment in the spinel stability field. Low CaO (16·4–18·0wt %) and heavy rare earth element (HREE; Yb_n = 2·4–4·8),and high Cr₂O₃ (1·06–1·84 wt %) contentsin the clinopyroxenes of the lherzolites can be quantitativelyaccounted for by (1) low-degree (4%) partial melting of a PrimitiveMantle-like garnet lherzolite followed by (2) partial re-equilibrationof the melting residuum from the garnet to the spinel stabilityfield. This model is further supported by thermobarometric estimates(T = 975–1210°C; P = 1·3–2·1 GPa),which cluster around the spinel–garnet boundary in theperidotite system. Secondary parageneses, regardless of theprimary lithologies, are characterized by (1) two clinopyroxenes,cpx2-O and cpx2-C, respectively related to orthopyroxene andclinopyroxene destabilization after reaction with metasomaticfluids, and (2) glasses with anomalously high, even for continentalsettings, K₂O contents (up to 8·78 wt %), together withK-feldspar. Major and trace element mass balance calculationsbetween the primary and secondary parageneses suggest infiltrationof a kimberlite-like metasomatizing agent (on volatile-freebasis, MgO 17–27 wt %; K₂O/Na₂O 1·6–3·2molar; (K₂O + Na₂O)/Al₂O₃ 1·1–3·0 molar;Rb 91–165 ppm; Zr 194–238 ppm). The kimberlite-likemetasomatism in the Cape Verde lithospheric mantle, togetherwith the presence of lherzolitic domains, partially re-equilibratedfrom the garnet to the spinel stability field, may suggest thepresence of subcontinental mantle lithosphere relicts left behindby drifting of the African Plate during the opening of the CentralAtlantic Ocean. KEY WORDS: Cape Verde; mantle metasomatism; garnet signatures; clinopyroxenes; kimberlites     

Imnaha Basalt, Columbia River Basalt Group

The Columbia River volcanic episode began with the eruptionof the coarsely porphyritic Imnaha Basalt between 17.0 and 16.5m.y. B.P. Lava poured from NNW trending vertical fissures andlocal vents north and south of the Seven Devils-Wallowa Mountainsdivide, covering a deeply dissected topographic surface of morethan 30, 000 km², with an estimated volume of 6000 km³. A minimumof 26 flows or flow units are represented in 14 or 15 members.These include 11 chemical types and are exposed in sectionsranging to 577 m in thickness. All flows have normal polaritywith the exception of the youngest and oldest whose polarityis either reversed or transitional. The petrologic and majorelement chemical features of the Imnaha Basalt have much incommon with those of the Picture Gorge Basalt exposed in theJohn Day Basin of north-central Oregon, but the latter is younger,equivalent in age to part of the Grande Ronde Basalt formation. Using major and trace elements, the flows of Imnaha Basalt areclearly distinguished from those of all other formations ofthe Columbia River Basalt Group. Imnaha Basalt has lower SiO₂,K₂O, Ba and Rb than does Grande Ronde Basalt and differs frommost Wanapum and Saddle Mountains Basalt flows in its lowerTiO₂ and P₂O₅ contents. The 11 Imnaha chemical types fall into two subgroups, the AmericanBar (AB) and Rock Creek (RC) subgroups, which differ in thecoarseness of their groundmass, the abundance of olivine, theirphenocryst assemblages, their SiO₂ contents, CaO/Al₂O₃ ratio,and in their Sc, V, Sr, and Ni contents. Flows of the two subgroupsinterdigitate, but AB flows are predominant at the base of thesequence and RC flows at the top. One flow is a hybrid of thetwo magma types. Neither subgroup displays a significant variation in SiO₂ content,but each does show systematic variation in K₂O, P₂O₅, TiO₂,Ba, Zr, Rb, and the REE, all of which vary inversely with MgO/(MgO+ FeO + Fe₂O₃). AB flows show a systematic increase in contentsof the incompatible elements upward in the succession, whileRC flows show a less obvious decrease upwards. Modelling of major and trace elements indicates that the chemicalvariations within each subgroup can be explained by simultaneouscrystal fractionation and assimilation of lower crustal material,in which the mass assimilated is only slightly less than thatlost by crystal fractionation; the mass fractionated varyingup to 50 per cent and the mass assimilated up to 42 per centof the original magma mass. These processes alone cannot explainthe relationships between the two Imnaha subgroups, nor thatbetween either subgroup and the overlying aphyric Grande Rondebasalt. The value of more complex quantitative models, in whichrecharge by more primitive magma, a variable composition forthe lower crustal contaminant, and the partial melting of aheterogeneous source, is limited by lack of data. Some suchprocess, or combination of processes in addition to a combinationof crystal fractionation and lower crustal assimilation, wouldseem to be required to account for the diversity in the earliestColumbia River basalts.     

The Volume and Composition of Melt Generated by Extension of the Lithosphere

Calculation of the volume and composition of magma generatedby lithospheric extension requires an accurate mitial geotherm,and knowledge of the variation and composition of the melt fractionas a function of pressure and temperature. The relevant geophysicalobservations are outlined, and geotherms then obtained fromparameterized convective models. Experimental observations whichconstrain the solidus and liquidus at various pressures aredescribed by simple empirical functions. The variation in meltfraction is then parameterized by requiring a variation from0 on the solidus to 1 on the liquidus. The composition of the melts is principally controlled by themelt fraction, though those of FeO, MgO, and SiO² in additionvary with pressure. Another simple parameterization allows theobserved compositions of major elements in 91 experiments tobe calculated with a mean error of 1.1%, and those of TiO₂ andNa₂O to 0.3%. These expressions are then used to calculate theexpected compositions of magma produced by adiabatic upwelling.The mean major element composition of the most magnesium-richMORB glasses resemble the mean composition calculated for amantle potential temperature T_p of 1280?C. Adiabatic meltingduring upwelling of mantle of this temperature generates a meltthickness of 7 km. The observed variations of the MgO and TiO₂concentrations in a large collection of MORB glass compositionssuggest that extensive low pressure fractional crystallizationoccurs, but that its effect on the concentrations of SiO₂, Al₂O₃,and CaO is small. There is no evidence that normal oceanic crustis produced from magmas containing more than 11% MgO. The mantlepotential temperature within hot rising jets is about 1480?Cand can generate 27 km of magma containing 17% MgO. Extension of the continental lithosphere generates little meltunless ß> 2 and T_p> 1380?C. The melts generatedby larger values of ß or of T_p are alkali basalts,and change to tholeiites as the amount of melting increases.Large quantities of melt can be generated, especially at continentalmargins, where estimates of ß obtained from changesin crustal thickness will in general be too small.     

安徽新桥块状硫化物矿床地球化学特征

新桥块状硫化物矿床发育双层结构,地球化学特征也呈现出明显的“二元性”和垂向变化。下部网脉状蚀变矿化岩石SiO₂、Al₂O₃、K₂O和Na₂O等含量较高,上部层状块状矿石和含矿岩石Fe₂O₃、FeO、CaO、MgO及SiO₂等明显富集。稀土含量相对较低,上部层状块状矿层平均值为10.73×10^-6,下部通道相蚀变矿化岩石平均值为126.1×10^-6。重晶石δ³⁴S值为+16.2‰,硬石膏δ³⁴S值为+11.2‰,黄铁矿δ³⁴S值为+1.5‰～+4.7‰。含矿硅质岩δ¹⁸O为+12.0‰～+13.9‰,下部通道相含黄铁矿石英脉δ¹⁸O值为+13.3‰～+18.6‰。自下部网脉状矿化到上部层状块状矿层,从粗晶细晶到变胶状胶状黄铁矿,δ³⁴S、δ¹⁸O和δ30Si值逐渐降低,²⁰⁶Pb/²⁰⁴Pb、²⁰⁷Pb/²⁰⁴Pb和²⁰⁸Pb/²⁰⁴Pb平均值逐渐增高。     

Unusual acid melts in the area of the unique Rosia Montana gold deposit, Apuseni Mountains, Romania: Evidence from inclusions in quartz

Crystalline and melt inclusions were studied in large (up to 2 cm across) dipyramidal quartz phenocrysts from Miocene dacites in the area of the Rosia Montana Au-Ag deposit in Romania. Data were obtained on the homogenization of fluid inclusions and the composition of crystalline inclusions and glasses in more than 40 melt inclusions, which were analyzed on a electron microprobe. The minerals identified in the crystalline inclusions are plagioclase (An 51–62), orthoclase, micas (biotite and phengite), zircon, magnetite (TiO₂ = 2.8 wt %), and Fe sulfide. Two types of the melts were distinguished when studying the glasses of the melt inclusions. Type 1 of the melts is unusual in composition. The average composition of 20 inclusions is as follows (wt %): 76.1 SiO₂, 0.39 TiO₂, 6.23 Al₂O₃, 4.61 FeO, 0.09 MnO, 1.64 MgO, 3.04 CaO, 2.79 Na₂O, 3.79 K₂O (Na₂O/K₂O = 0.74), 0.07 P₂O₅, 0.02 Cl. The composition of type 2 of the melts is typical of acid magmas. The average of 23 inclusion analyses is (wt %) 79.3 SiO₂, 0.16 TiO₂, 10.27 Al₂O₃, 0.63 FeO, 0.08 MnO, 0.29 MgO, 1.83 CaO, 3.56 Na₂O, 2.79 K₂O (Na₂O/K₂O = 1.28), 0.08 P₂O₅, 0.05 Cl. The compositions of these melts significantly differ in concentrations of Ti, Al, Fe, Mg, Ca, Na, and K. The high analytical totals of the analyses (close to 100 wt %, more specifically 98.9 and 99.0 wt %, respectively) testify that the melts were generally poor in water. Two inclusions of type 1 and two inclusions of type 2 were analyzed on an ion probe, and their analyses show remarkable differences in the concentrations of certain trace elements. These concentrations (in ppm) are for the melts of types 1 and 2, respectively, as follows: 10.0 and 0.69 for Be, 29.3 and 5.7 for B, 6.4 and 1.4 for Cr, 146 and 6.9 for V, 74 and 18 for Cu, 92 and 29 for Rb, 45 and 15 for Zr, 1.7 and 0.6 for Hf, 10.3 and 2.3 for Pb, and 52 and 1.3 for U. The Th/U ratio of these two melt types are also notably different: 0.04 and 0.19 for type 1 and 2.0 and 2.9 for type 2. These data led us to conclude that the magmatic melts were derived from two different sources. Our data on the melts of type 1 testify that the magmatic chamber was contaminated with compositionally unusual crustal rocks (perhaps, sedimentary, metamorphic, or hydrothermal rocks enriched in Si, Fe, Mg, U, and some other components). This can explain the ore-forming specifics of magmatic chambers in the area.     

The Cinder Cones of Michoac?n-Guanajuato, Central Mexico: Petrology and Chemistry

The Micho?ch-Guanajuato Volcanic Field (MGVF) of central Mexicocontains 900 cinder and lava coes but lacks the large activecomposite volcanoes found in other portions of the Mexican VolcanicBelt (MVB). Scoriae and lavas from these cinder cones are primarilyolivine-basalts and olivine-andesites containing phenocrystsof olivine (plus Cr-rich spinel inclusions), plagioclase, and,less frequently, augite; pyroxene- and hornblende-andesitesare subordinate. Most samples are calcalkaline; however, alkalineand transitional rocks are also found. Compositional variationat individual cones is usually less than 5 per cent SiO₂ andat Volc?n Paricutin (1943–1952) and Volc?n Jorullo (1759–1774),lava compositions have become more silica-rich with time. Alkaline cinder cones are generally older, but in the late Quaternary,both calc-alkaline and alkaline magmas erupted in the southernpart of the MGVF. Positive correlations between K, Zr, and Baand distance from the Middle America trench are distinct forevolved lavas; no correlations are found for less differentiatedlavas. In contrast, a correlation between decreasing Mg, Ni,and Cr and distance from the trench is found. In comparison to composite volcanoes in the MVB, the cinder-conelava are typically more basic. Four samples have mg-numbersand Ni contents which indicate possible mantle source regions.These samples include calc-alkaline, transitional and alkalinelavas, but all contain phenocrysts and/or microphenocrysts ofolivine, augite, and plagioclase; in these high-Mg lavas, spinelinclusions in olivine are Cr-rich. Those high-Mg lavas withsmall amounts of coexisting olivine, augite, and plagioclasephenocrysts plot close to a high-pressure (8 kb ? H₂O) 0l-Aug-Plcotectic. Others project between this high-pressure clusterand the 1 atm. cotectic, indicating polybaric fractionation.Low-Mg lavas in the northern part of the MGVF result from fractionationat relatively shallow depths. Estimated olivine equilibrium temperatures decrease from about1200?C with increasing FeO/FeO + MgO, which is also accompaniedby an increase in H²O. Relative oxygen fugacities (relativeto NNO) calculated for lavas with Fe₂O₃+FeO show that NNO increasessystematically during an eruption, and this is well displayedat both Paricutin and Jorullo. The more oxidized lavas may containhornblende, and do so at Colima. The calc-alkaline lavas fromthroughout the MGVF only span the redox state of the Jorulloeruption, and all these continental magmas are 2–3 ordersof magnitude more oxidized than their submarine counterparts. Petrographic and mineralogical evidence supports the absenceof long-lived shallow magma reservoirs, consistent with theobserved small magma output rate in the MGVF.     

Metasomatism in the Generation of Granulite Veins: Mass Balance, Mass Transfer, and Reference Frames

In the Seguin subdomain, Grenville Province, dark hornblende–garnet-richquartz-absent metagabbro is transected by anastomosing light-colouredveins rich in orthopyroxene, clinopyroxene, plagioclase, and,in some cases, quartz. Three types of veins form a smoothlytransitional series from fine-grained diffuse veins and patchesto a more defined intermediate vein type, and finally to coarsegrainedtonalitic veins with abrupt contacts to the metagabbro host.The textures and contact relations of diffuse and intermediateveins are suggestive of channelized subsolidus dehydration ofthe metagabbro. Mass-balance calculations show that dehydrationwas accompanied by metasomatism; all reasonable solutions requirethe addition of SiO₂, Na₂O, and large ion lithophile elements(LILE) to the host rock to produce the veins. Reference framesfor the evaluation of mass transfers can be represented by twocases: (1) constant volume, and (2) constant FeO, MgO, V, andZr requiring volume increases by factors of 13—33. Thesimplest model (minimizing the number of mobile constituentsand giving progressively larger volume increases for more leucocraticveins) is based on constant FeO, MgO, V, and Zr. In this case,model mineral reactions and mass transfers for two types ofveins, distinguished on the outcrop as (1) diffuse and (2) intermediate,are: These mass balances show that SiO₂, Na₂O, and Al₂O₃ were added,and H₂O lost, in producing the vein assemblages. Minimum fluid/rockvolume ratios of 3–20 imply flow-through. The fluid didnot induce melting of hornblende+plagioclase, and hence it musthave had relatively low a_H2O. The open-system transformation of hornblende–garnet-richmetagabbro to pyroxene-rich veins was driven primarily by theintroduction of silica and feldspar components in a low-a_H2Osupercritical fluid at granulite-facies P–T conditions.The application of quantitative mass balance, taking into accountpropagated errors and a critical appraisal of the possible referenceframes, enhances our ability to resolve open-system processes.     

Minor-element and Revised Major-element Contents of Some Mediterranean Pantellerites and Comendites

Optical emission spectrographic analysis of three pantelleritesfrom Pantelleria and two comendites from Sardinia show highconcentrations of B, Be, Ga, La, Mo, Nb, Sn, Y, Yb, and Zt andlow contents of Ba, Co, Cr, Ni, Sr, and V. Minor-element trendsof these specimens are very similar to those of pantelleritesfrom southern Nevada and New Zealand and other peralkaline silicicrocks from various localities. Fluorine contents of the Pantellerianspecimens range from 0.19 to 0.32 per cent by weight resultingin abnormally high Cl to F ratios. New analyses for MgO, Na₂O,and K₂O are almost identical with the values obtained by Zies.Revised major-element compositions and calculated glass-phasecompositions for the three Pantellerian specimens are given.     

Quantitative phase petrology of cordierite–orthoamphibole gneisses and related rocks

Cordierite–orthoamphibole gneisses and rocks of similar composition commonly contain low‐variance mineral assemblages that can provide useful information about the metamorphic evolution of a terrane. New calculated petrogenetic grids and pseudosections are presented in the FeO–MgO–Al₂O₃–SiO₂–H₂O (FMASH), Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) and Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO) chemical systems to investigate quantitatively the phase relations in these rocks. Although the bulk compositions of cordierite–orthoamphibole gneisses are close to FMASH, calculations in this system do not adequately account for the observed range of mineral assemblages. Calculations in NCKFMASH and NCKFMASHTO highlight the role of minor constituents such as Ca, Na and Fe³⁺ in the mineral assemblage evolution of such rocks and these systems are more appropriate for interpreting the evolution of natural examples.     

Lineages and Fractionation Trends in Undersaturated Volcanic Rocks from the East Otago Volcanic Province (New Zealand) and Related Rocks

The petrology of the East Otago Volcanic Province (late Miocene),including the Dunedin Complex, is dominated by undersaturatedsodic lavas and shallow intmsives whose compositions range fromalkali basalt, basanite, and nephelinite to trachyte and phonolite.A range of relatively potassic volcanic rocks is also developedin the Province. The degree of undersaturation and ratios suchas (FeO+Fe₂O₃):MgO and K₂O:Na₂O vary widely in rocks with comparabledifferentiation indices. Chemical and mineralogical data onwhole rock-glass pairs and variants of shallow intrusives inthe Province, assessed in conjunction with similar data fromother volcanic provinces, demonstrate the production of trachyticliquids from alkali basaltic parents and of phonolitic liquidsfrom basanitic parents. Fractionation trends in both sodic andpotassic series, defined by variation in differentiation indexand normative nepheline, indicate that the degree of undersaturationof the derivative salic liquids is dependent on that of theparent; the slope of the fractionation trend lines is influencedby additional factors, including Po₂. The unequivocal productionof phonolite from trachyte in the East Otago Province has notbeen demonstrated. The mafic variants of theralites from Waihola and a basanite-pegmatoidassociation from Omimi are similar in composition, but the mostsalic differentiates from these differentiation sequences displaypronounced differences in their KO:Na₂O ratios, tending towardsleuco-theralitic (cf. lugarite) and malignitic compositionsrespectively. A multicomponent spectrum of fractionation lineages is suggestedfor individual differentiated bodies and for lava series. Sodiclineages include: (I) alkali basalt–hawaiite–mugearite–benmoreite–trachyte;(2) a more undersaturated series from basanite through nephelinehawaiite, nepheline mugearite, and nepheline benmoreite, tophonolite; (3) a nephelinite series, more restricted in compositionalrange. The relatively high Fe: Mg ratios characterizing themugearitic variants are sometimes exhibited by members of themore potassic series in which, however, Fe:Mg ratios tend tobe characteristically lower. This feature is correlated withthe frequent occurrence of kaersutitic amphibole or its resorptionproducts, indicative of intratelluric fractionation under relativelyhydrous conditions and probably relatively high Po₂. Certainnepheline trachyandesites from East Otago are described in detail.Trachy-basalt-trachyandesite-tristanite-trachyte and sanidinebasanite-nepheline trachyandesite-nepheline tristanite-phonolitelineages are also proposed, and the nomenclature of these moreseries discussed.