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1.
Experimental Petrology of Melilite Nephelinites   总被引：3，自引：1，他引：3  
Experimental study of natural melilite nephelinite lavas ofintermediate K/Na ratio at low pressure (fo2 reveals the presenceof a peritectic ‘point’ of distributary type (1090?C)for liquids saturated with leucite, nepheline, and spinel. Withdecreasing temperature on the olivine + melilite cotectic, botholivine and melilite react with such liquids to produce high-calciumpyroxene at the peritectic. Both the olivine + high-calciumpyroxene and melilite + high-calcium pyroxene cotectics arestable at temperatures below the peritectic. Olivines coexistingwith such liquids are much more magnesian than those in comparabletholeiitic liquids. The olivine-liquid Fe-Mg distribution coefficient is a monotonically increasing function of silica activity over the composition range spannedby melilite nephelinite, ugandite, alkali basalt, and tholeiitebasalt liquids. The analogous Fe-Mg distribution coefficientfor melilite and liquid is effectively constant , while that for high-calcium pyroxene and liquidis highly dependent on the chemistry of high-calcium pyroxene(cf., Sack & Carmichael, 1984). Pseudoternary liquidus projectionsof multiply saturated liquids coexisting with nepheline, leucite,and spinel (?olivine?high-calcium pyroxene?melilite) have beenprepared to facilitate graphical analysis of the evolution oflava compositions during hypabyssal cooling. Major element chemicalanalyses and petrographic features of lavas from Mt. Nyiragongo,East Africa and Oahu, Hawaii (e.g., Denaeyer et al., 1965; Wilkinson& Stolz, 1983) confirm the validity of these diagrams andthe systematics established from the experimental data. *Reprint requests to R.O. Sack  相似文献   

2.
Progress () of the infiltration-driven reaction, 4olivine +5CO2 + H2O = talc + 5magnesite, that occurred during Barrovianregional metamorphism, varies at the cm-scale by a factor of3·5 within an 3 m3 volume of rock. Mineral and stableisotope compositions record that XCO2, 18Ofluid, and 13Cfluidwere uniform within error of measurement in the same rock volume.The conventional interpretation of small-scale variations in in terms of channelized fluid flow cannot explain the uniformityin fluid composition. Small-scale variations in resulted insteadbecause (a) reactant olivine was a solid solution, (b) initiallythere were small-scale variations in the amount and compositionof olivine, and (c) fluid composition was completely homogenizedover the same scale by diffusion–dispersion during infiltrationand subsequent reaction. Assuming isochemical reaction, spatialvariations in image variations in the (Mg + Fe)/Si of the parentrock rather than the geometry of metamorphic fluid flow. Ifinfiltration-driven reactions involve minerals fixed in composition,on the other hand, spatial variations in do directly imagefluid flow paths. The geometry of fluid flow can never be determinedfrom geochemical tracers over a distance smaller than the oneover which fluid composition is completely homogenized by diffusion–dispersion. KEY WORDS: Alpine Barrovian metamorphism; diffusion; metamorphic fluid composition; metamorphic fluid flow; reaction progress  相似文献   

3.
The Ni-S System and Related Minerals   总被引：1，自引：0，他引：1  
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 Ni3?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.  相似文献   

4.
BREY  G. P.; K?HLER  T. 《Journal of Petrology》1990,31(6):1353-1378
On the basis of experiments presented in Part I of this series,most of the published thermobarometers relevant to four-phaseperidotites are tested here for their ability to reproduce experimentalconditions. They were rejected if any systematic discrepancyin either pressure or temperature was discernible. This testcautions against the use of all published versions of thermometersbasad on the compositions of coexisting ortho- and clinopyroxenesand the use of existing barometers based on the Al content oforthopyroxene axxisting with garnet. Therefore, we formulatednew versions of the two-pyroxene thermometer and the Al-in-opxbarometer: with and is in degress Kelvin and P is in kilobars. Our new barometer is of the form (C1–C3) and site occupancies are given in the text. Temperatures may also be calculated from the Ca content of opxalone: This thermometer can be applied both to the CMS and the naturalsystem experiments, which may indicate that Fe and Na have counter-balancingeffects on the Ca content of opx. The partitioning of Na between opx and cpx can also serve asa useful thermometer, and was calibrated from natural rock data: where T is in degrees Kelvin, P is in kilobars, and DNa=Naopx/Nacpx. The following three published thermobarometers based on furtherexchange reactions are capable of reprducing experimental conditions:
  1. exchangeof Ca between olivine and clinopyroxene as a barometer(PKB),
  2. exchange of Fe and Mg between garnet and clinopyroxene asathermometer (TKrogh),
  3. exchange of Fe and Mg between garnetand olivine as a thermometer(TO'Neiii).
Our tests also show that the most accurate pressure and temperatureestimates arc obtained from the following combinations of thermometersand barometers:
  1. TBKN+PBKN,
  2. TBKN+PKB,
  3. TKrogh+PBKN,
  4. TO'Ne$$$ll+PBKN.
  相似文献   

5.
ZINGG  A. J. 《Journal of Petrology》1996,37(3):471-496
The literature data of nine different occurrences of coexistingmineral pairs of Ca-amphibole have been studied and the bulkvectors, spanning the miscibility gap, derived. The additivecomponent is always impure Mg-tremolite accompanied by someglaucophane and cummingtonite component. The four major exchangecomponents required to describe the compositional variationin coexisting mineral pairs are the edenite (ED), tschermak's(TS), FeMg–1 and Fe3+-tschermak's (FeTs) vector. Trivalentiron is postulated on the basis of excess charges in the bulkvector the size of which coincides with residuals in Altet,–Si, Fe and –Mg. The four cations have equal sizes,forming the vector Fe3+ Altet Mg–1Si–1. This distributionscheme is consistent for all the different occurrences and setsthe basis for a comparison. Deviations from the scheme wouldradically complicate the proposed exchange pattern. The ratioTS:ED in most mineral samples fluctuates between one and two.Projection of the data points in the vector space TS–EDonto the line 1ED: 2TS (Tr–Hbl) or 1ED:1TS (Tr–Prg)provides the projected tremolite content (= 1–XHbl or = 1–Xprg). This parameter,applied to coexisting pairs, and plotted against the ratio Mg/(Mg+ Fe) shows some characteristic features about the miscibilitygap. In the Mg-pure system the solvus is almost symmetric andlocated in the temperature range between 800 and 870C. Smallamounts (0.10 pfu) of Fe2+ in the M(4) -sites and replacingCa have a dramatic effect, forcing the solvus to much lowertemperatures of 650C. An increase in the ratio Fe/(Fe + Mg)causes a shift of the solvus towards more tremolitic compositionswith temperatures 500–650C. The maximum asymmetry ofthe solvus is reached where the Al-poor member (tremolite) hasa composition of =1.0 and Mg/ (Mg + Fe) 0.6. The corresponding Al-rich member has =0.5 and Mg/ (Mg + Fe) 0.4. An anomalyof the solous is observed at Mg/ (Mg + Fe)=0.8. It manifestsas a kind of highly asymmetric ‘sub-gap’ in thetremolite-rich composition range. This is explained by the partitioningof Fe2+ into the single M(3) -site and is characterized by athermal hump to 650–700C. KEY WORDS: tremolite; hornblende; pargasite; immiscibility; solous  相似文献   

6.
Within the western Sierra Nevada metamorphic belt, linear bodiesof alpine-type ultramafic rock, now composed largely of serpentineminerals, parallel the regional strike and commonly coincidewith major fault zones. Within this metamorphic belt, east ofSacramento, California, ultramafic rocks near a large maficintrusion, the Pine Hill Intrusive Complex, have been emplacedduring at least two separate episodes. Those ultramafic rocks,evidently unaffected by the Pine Hill Intrusive Complex andcomposed largely of serpentine minerals, were emplaced alonga major fault zone after emplacement of the Pine Hill IntrusiveComplex. Those ultramafic rocks, contact metamorphosed by thePine Hill Intrusive Complex, show a zonation of mineral assemblagesas the igneous contact is approached: olivine+antigorite+chlorite+tremolite+Fe-Cr spinel olivine+talc+chlorite+tremolite+Fe-Crspinel olivine+anthophyllite+chlorite+tremolite+Fe-Cr spinel olivine+orthopyroxene+aluminous spinel+hornblende+Fe-Cr spinel.Superimposed on these mineral assemblages are abundant secondaryminerals (serpentine minerals, talc, chlorite, magnetite) whichformed after contact metamorphism. Correlation of observed mineralassemblages with the experimental systems, MgO-SiO2-H2O andMgO-Al2O3-SiO2-H2O suggests an initial contact temperature of775±25 °C for the Pine Hill Intrusive Complex assumingPtotal Pfluid PH2O. The pressure acting on the metamorphic rockduring emplacement of the intrusion is estimated to be a minimumof 1.5 kb.  相似文献   

7.
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 (logPH2O = 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.  相似文献   

8.
Field, petrologic and geochemical data were used to characterizefluid infiltration and partial melting during metamorphism ofpelitic rocks in the contact aureole of the Onawa pluton, centralMaine, USA. Mineral assemblages delineate five metamorphic zoneswithin the contact aureole: chlorite zone, andalusite–cordierite(a–c) zone, alkali feldspar zone, sillimanite zone andleucocratic-vein (l–v) zone. The sequence of observedmineral assemblages and mineral–fluid reactions calculatedby mass balance is similar to those observed in other contactaureoles. Pressure of contact metamorphism is 3 kbar, on thebasis of optimum geothermobarometry calculations. Metamorphictemperatures vary from 500C in the andalusite–cordieritezone to 65OC in the leucocratic-vein zone. Data from fieldobservations, mineral textures, observed reaction stoichiometry,geothermometry and major-element geochemistry suggest that theleucocratic veins of the l-v zone represent crystallized, partialmelts. Two overall calculated mineral reactions are responsiblefor vein formation: which can be modeled as combinations of two NKFMTASH meltingreactions: Progress of (M1) and (M2) was measured in eight samples, andreaction (M1) is the dominant melt-forming reaction in all samples.Partial melting (and vein formation) was therefore driven byinfiltration of the l-v zone by H2O-rich fluids. Calculatedtime-integrated fluid fluxes for l-v zone samples range from09 104 to 31 104 mol/cm2, and flow was in the directionof increasing temperature. KEY WORDS: pelites; contact metamorphism; fluid infiltration; partial melting; Onawa Pluton; Maine; USA *Corresponding author. Telephone:(516) 632–8192. Fax (516)632–8240 e-mail: gsymmes{at}ccmail.sunysb.edu  相似文献   

9.
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  相似文献   

10.
Klauea historical summit lavas have a wide range in matrix 18OVSMOWvalues (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  相似文献   

11.
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 XCO2with 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  相似文献   

12.
Significant petrogenetic processes governing the geochemicalevolution of magma bodies include magma Recharge (includingformation of ‘quenched inclusions’ or enclaves),heating and concomitant partial melting of country rock withpossible ‘contamination’ of the evolving magma body(Assimilation), and formation and separation of cumulates byFractional Crystallization (RAFC). Although the importance ofmodeling such open-system magma chambers subject to energy conservationhas been demonstrated, the effects of concurrent removal ofmagma by eruption and/or variable assimilation (involving imperfectextraction of anatectic melt from wall rock) have not been considered.In this study, we extend the EC-RAFC model to include the effectsof Eruption and variable amounts of assimilation, A. This model,called EC-E'RAFC, tracks the compositions (trace elements andisotopes), temperatures, and masses of magma body liquid (melt),eruptive magma, cumulates and enclaves within a composite magmaticsystem undergoing simultaneous eruption, recharge, assimilationand fractional crystallization. The model is formulated as aset of 4 + t + i + s coupled nonlinear differential equations,where the number of trace elements, radiogenic and stable isotoperatios modeled are t, i and s, respectively. Solution of theEC-E'RAFC equations provides values for the average temperatureof wall rock (Ta), mass of melt within the magma body (Mm),masses of cumulates (Mct), enclaves (Men) and wall rock () and the masses of anatectic melt generated () and assimilated (). In addition, t trace element concentrations and i + s isotopic ratios inmelt and eruptive magma (Cm, m, m), cumulates (Cct, m, m), enclaves(Cen, , ) and anatectic melt (Ca, , ) as a function of magma temperature (Tm) are also computed. Input parametersinclude the (user-defined) equilibration temperature (Teq),a factor describing the efficiency of addition of anatecticmelt () from country rock to host magma, the initial temperatureand composition of pristine host melt (, , , ), recharge melt (, , , ) and wall rock (, , , ), distribution coefficients (Dm, Dr, Da) and their temperaturedependences (Hm, Hr, Ha), latent heats of transition (meltingor crystallization) for wall rock (ha), pristine magma (hm)and recharge magma (hr) as well as the isobaric specific heatcapacity of assimilant (Cp,a), pristine (Cp,m) and recharge(Cp,r) melts. The magma recharge mass and eruptive magma massfunctions, Mr(Tm) and Me(Tm), respectively, are specified apriori. Mr(Tm) and Me(Tm) are modeled as either continuous orepisodic (step-like) processes. Melt productivity functions,which prescribe the relationship between melt mass fractionand temperature, are defined for end-member bulk compositionscharacterizing the local geologic site. EC-E'RAFC has potentialfor addressing fundamental questions in igneous petrology suchas: What are intrusive to extrusive ratios (I/E) for particularmagmatic systems, and how does this factor relate to rates ofcrustal growth? How does I/E vary temporally at single, long-livedmagmatic centers? What system characteristics are most profoundlyinfluenced by eruption? What is the quantitative relationshipbetween recharge and assimilation? In cases where the extractionefficiency can be shown to be less than unity, what geologiccriteria are important and can these criteria be linked to fieldobservations? A critical aspect of the energy-constrained approachis that it requires integration of field, geochronological,petrologic, and geochemical data, and, thus, the EC-ERAFC ‘systems’approach provides a means for answering broad questions whileunifying observations from a number of disciplines relevantto the study of igneous rocks. KEY WORDS: assimilation; energy conservation; eruption; open system; recharge  相似文献   

13.
The mutual solubility in the system corundum–hematite[-(Al, Fe3+)2O3] was investigated experimentally using bothsynthetic and natural materials. Mixtures of -Al2O3 and -Fe2O3(weight ratios of 8:2 and 10:1) were used as starting materialsfor synthesis experiments in air at 800–1300°C withrun times of 7–34 days. Experiments at 8–40 kbarand 490–1100°C were performed in a piston-cylinderapparatus (run times of 0·8–7·4 days) usinga natural diasporite consisting of 60–70 vol. % diasporeand 20–30 vol. % Ti-hematite. During the diasporite–corunditetransformation, the FeTiO3 component (12–18 mol %) ofTi-hematite only slightly increased, implying that oxygen fugacitywas maintained at high values. Run products were studied byelectron microprobe and X-ray diffraction (Rietveld) techniques.An essentially linear volume of mixing exists in the solid solutionwith a slight positive deviation at the hematite side. Up to1000°C, corundum contains <4 mol % Fe2O3 and hematite<10 mol % Al2O3; at 1200°C these amounts increase to9·3 and 17·0 mol %, respectively. At 1300°Chematite was no longer stable and coexists with the orthorhombic phase . The present results agree with corundum (solvus) compositions obtained inprevious studies but indicate a larger solubility of Al in hematite.The miscibility gap in the solution can be modelled with anasymmetric Margules equation with interaction parameters (2uncertainties): ; ; ; . Application of the corundum–hematite solution as a solvus geothermometer is limited because of thescarcity of suitable rock compositions. KEY WORDS: corundum; hematite; corundum–hematite miscibility gap; experimental study; Margules model; metabauxite  相似文献   

14.
BAKER  A. J. 《Journal of Petrology》1990,31(1):243-260
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  相似文献   

15.
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/86Sri 0·7035, Ndi 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  相似文献   

16.
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 (An60–75)and two–pyrozene bearing andesites with 58% SiO2 to finalplagioclase (An85–90), diopside, and olivine (Fo85–90)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 18Oplag–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.  相似文献   

17.
The pressure-temperature-compositional (P-T-X) dependence ofthe solubility of Al2O3 in orthopyroxene coexisting with garnethas been experimentally determined in the P-T range 5–30kilobars and 800–1200 ?C in the system FeO—MgO—Al2O3—SiO2(FMAS). These results have been extended into the CaO—FeO—MgO—Al2O3—SiO2(CFMAS) system in a further set of experiments designed to determinethe effect of the calcium content of garnet on the Al2O3 contentsof coexisting orthopyroxene at near-constant Mg/(Mg + Fe). Startingmaterials were mainly glasses of differing Mg/(Mg + Fe) or Ca/(Ca+ Mg + Fe) values, seeded with garnet and orthopyroxene of knowncomposition, but mineral mixes were also used to demonstratereversible equilibrium. Experiments were performed in a piston-cylinderapparatus using a talc/pyrex medium. Measured orthopyroxene and corrected garnet compositions werefitted by multiple and stepwise regression techniques to anequilibrium relation in the FMAS system, yielding best-fit,model-dependent parameters Goy= –5436 + 2.45T cal mol–1,and WM1FeA1= –920 cal mol–1. The volume change ofreaction, Vo, the entropy change, So970 and the enthalpy changeHo1,970, were calculated from the MAS system data of Perkinset al. (1981) and available heat capacity data for the phases.Data from CFMAS experiments were fitted to an expanded equilibriumrelation to give an estimate of the term WgaCaMg = 1900 ? 400cal/mole cation, using the other parametric values already obtainedin FMAS. The experimental data allow the development of a arnet-orthopyroxenegeobarometer applicable in FMAS and CFMAS: where This geobarometer is applicable to both pelitic and metabasicgranulites containing garnet orthopyroxene, and to garnet peridoditeand garnet pyroxenite assemblages found as xenoliths in diatremesor in peridotite massifs. It is limited, however, by the necessityof an independent temperature estimate, by errors associatedwith analysis of low Al2O3 contents in orthopyroxenes in high-pressureor low-temperature parageneses, and by uncertainties in thecomposition of garnet in equilibrium with orthopyroxene. Ananalysis of errors associated with this formulation of the geobarometersuggests that it is subject to great uncertainty at low pressuresand for Fe-rich compositions. The results of application ofthis geobarometer to natural assemblages are presented in acompanion paper.  相似文献   

18.
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/86Sri 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 13CPDB between –5·7 and –5,similar to primary mantle-derived carbonates, and 18OSMOW 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 (13CPDB –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  相似文献   

19.
Four natural peridotite nodules ranging from chemically depletedto Fe-rich, alkaline and calcic (SiO2=43?7–45?7 wt. percent, Al2O3=1?6O–8?21 wt. per cent, CaO=0?70–8?12wt. per cent,alk=0?10–0?90 wt. per cent and Mg/(Mg+Fe2+)=0?94–0?85)have been investigated in the hypersolidus region from 800?to 1250?C with variable activities of H2O, CO2, and H2. Thevapor-saturated peridotite solidi are 50–200?C below thosepreviously published. The temperature of the beginning of meltingof peridotite decreases markedly with decreasing Mg/(Mg+Fe)of the starting material at constant CaO/Al2O3. Conversely,lowering CaO/Al2O3 reduces the temperature at constant Mg/(Mg+Fe)of the starting material. Temperature differences between thesolidi up to 200?C are observed. All solidi display a temperatureminimum reflecting the appearance of garnet. This minimum shiftsto lower pressure with decreasing Mg/(Mg+Fe) of the startingmaterial. The temperature of the beginning of melting decreasesisobarically as approximately a linear function of the mol fractionof H2O in the vapor (XH2O). The data also show that some CO2may dissolve in silicate melts formed by partial melting ofperidotite. Amphibole (pargasitic hornblende) is a hypersolidus mineralin all compositions, although its P/T stability field dependson bulk rock chemistry. The upper pressure stability of amphiboleis marked by the appearance of garnet. The vapor-saturated (H2O) liquidus curve for one peridotiteis between 1250? and 1300?C between 10 and 30 kb. Olivine, spinel,and orthopyroxene are either liquidus phases or coexist immediatelybelow the temperature of the peridotite liquidus. The data suggest considerable mineralogical heterogeneity inthe oceanic upper mantle because the oceanic geotherm passesthrough the P/T band covering the appearance of garnet in variousperidotites. The variable depth to the low-velocity zone is explained byvariable aH2O conditions in the upper mantle and possibly alsoby variations in the composition of the peridotite itself. It is suggested that komatiite in Precambrian terrane couldform by direct melting of hydrous peridotite. Such melting requiresabout 1250?C compared with 1600?C which is required for drymelting. The genesis of kimberlite can be related to partial meltingof peridotite under conditions of (). Such activities of H2Oresult in melting at depths ranging between 125 and 175 km inthe mantle. This range is within the minimum depth generallyaccepted for the formation of kimberlite.  相似文献   

20.
The Mboutou complex is one of a line of early Tertiary ringcomplexes which runs from Lake Chad to the Gulf of Guinea, noneof which has hitherto been described in detail. The main rocktypes are layered gabbros and gabbronorites, with minor bodiesof quartz-syenodiorite, quartz-syenite and hypersolvus granite.Feldspars form a continuum with exceptional compositional range,from An85Ab13Or2 to around An1Ab46Or55, and form an entirelyhypersolvus sequence with very strong zoning in the syenodiorites.Ca-rich clinopyroxenes (salite and calcic augite) and olivines(Fo78–62) have restricted range. Orthopyroxene-bearingleucogabbros and syenodiorites contain minor orthopyroxene (En62Fs35Wo3)and quartz; olivine and orthopyroxene never coexist. In moreevolved rocks amphibole (magnesio-hornblende to ferroedenite)and minor biotite, showing progressive Fe-enrichment, are theonly mafic silicates.Major-element rock chemistry, minor elementsin clinopyroxenes and biotite chemistry show that, notwithstandingits thoroughly anorogenic setting, Mboutou was, at the outset,only very mildly alkaline. Its more evolved members embarkedon a line of evolution with some calc-alkaline characteristics,probably because of ingress of water into residual batches ofmagma, a possibility supported by stable isotope data. Thischange in behaviour corresponded with the sudden appearanceof quartz and orthopyroxene, which was not in equilibrium withclinopyroxene on the two-pyroxene surface. Amphibole then becamethe main mafic silicate with further increase in . The more evolved rocks are relatively highly altered,but Fe-Ti oxide pairs suggest that was maintained near to and above the QFM buffer and the rangeof biotite compositions further suggests crystallization undera regime of decreasing . Biotites maintain alkaline characteristics throughout the sequence. Zoningpatterns in the ternary feldspars in the syenodiorites, andthe hypersolvus character of the final granite, limit maximumvalues of to < 1 kb, and suggest minimum temperatures for the end of crystallizationin the syenodiorites of{small tilde} 850 ?C.  相似文献   

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