首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
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.  相似文献   

2.
Melt and fluid inclusions have been studied in olivine phenocrysts (Fo 81–79) from trachybasalts of the Southern Baikal volcanic area, Dzhida field. The melt inclusions were homogenized, quenched, and analyzed on an electron and ion microprobe. The study of homogenized glasses of nine inclusions showed that basaltic melts (SiO2 = 47.1–50.3 wt %, MgO = 5.0–7.7 wt %, CaO = 7.1–11.1 wt %) have high contents of Al2O3 (17.1–19.6 wt %), Na2O (4.1–6.2 wt %), K2O (2.2–3.3 wt %), and P2O5 (0.6–1.1 wt %). The volatile contents are low (in wt %): 0.24–0.31 H2O, 0.08 F, 0.03 Cl, and 0.02 S. Primary fluid inclusions in olivines from four trachybasalt samples contain high-density CO2 (0.73–0.87 g/cm3), indicating a CO2 fluid pressure of 4.3–6.6 kbar at 1200–1300°C and olivine crystallization depths of 16–24 km. Ion microprobe analyses of 20 glasses from melt inclusions for trace elements showed that the magmas of the Baikal rift were enriched in incompatible elements, thus differing from oceanic rift basalts and resembling oceanic island basalts. A comparison of our data on melt and fluid inclusions in olivine from trachybasalts of the Dzhida field with preexisting data on the Eastern Tuva volcanic highland in the Southern Baikal volcanic area showed that they had similar contents of volatiles, major, and trace elements.  相似文献   

3.
Oligocene volcanics from Oatlands in Tasmania, Australia, include olivine tholeiites, alkali olivine basalts, nepheline basanites and olivine nephelinites. They have compositional characteristics that are typical of intraplate basalts worldwide. They are generally enriched in incompatible elements relative to the primitive mantle and are strongly enriched in Nb, Ta and light rare earths, but not heavy rare earths. At the same time, they have Sr and Nd isotope compositions that are similar to those in some incompatible-element-depleted mid-ocean ridge basalts (E-type MORB). Experimentally obtained mineral/melt partition coefficients for an Oatlands basanite allow the relative concentrations of incompatible elements in the volcanics to be produced by small degrees of melting (≤1%) of a source similar to the E-type MORB source of Workman and Hart (2005). However, the absolute concentrations that can be achieved in this way are much less than present in the most incompatible-element-enriched basanites and nephelinites at Oatlands. This contradiction can be explained by open-system melting under the influence of a conductive geotherm. This would have involved upwardly migrating near-solidus melts from the asthenosphere cooling along a sub-adiabatic geotherm. Cooling of the melts would have caused them to re-crystallize and accumulate in the overlying mantle, thereby enriching both the new host rocks and any residual melts in incompatible elements. This would also have increased the buoyancy of the host rocks leading to upwelling and further (decompression) melting of incompatible-element-enriched peridotite. We were able to use our partition coefficients to quantitatively model the development of incompatible-element enrichments in the Oatlands magmas by these processes. Our explanation is consistent with the characteristically scattered but widespread distributions and long time scales of intraplate volcanism in a broad variety of tectonic settings. This is because the conditions required to initiate volcanism (i.e. those of near-solidus melting of the asthenosphere) are relatively easy to produce and can therefore be caused by both near-surface tectonics and deeper mantle processes. Furthermore, the super-enrichments of incompatible elements in some intraplate volcanics can be attributed to the influence of normal geothermal gradients on melting processes. Without the very strong fractionation imposed by this combination of factors, the Oatlands volcanics would more closely resemble mid-ocean-ridge basalts.  相似文献   

4.
Continental flood basalts from the Parana plateau are of LowerCretaceous age and are represented by abundant (c. 45 per centby volume) two-pyroxene tholeiites characterized by relativelylow-TiO2 (< 2 wt. percent) and incompatible (e.g., P, Ba,Sr, La, Ce, Zr) element contents. Low-Ti basalts are distributedthroughout the Parana Basin and predominate in the southernregions, where they represent over 90 per cent by volume ofthe basic activity. Major and trace elements and Sr-Nd isotope ratios were analysedin 43 low-Ti basalts selected so as to cover the entire Paranabasin. In general, low-Ti basalts with initial 87Sr86Sr ratios (R0)lower than O7060 may be divided into two groups: (A) those relativelyenriched in incompatible elements (e.g., average K2O = O.85and P2O5 = 0.27 wt. per cent, and Ba = 346, Sr =289, Rb=16;La =18; Zr=132 p.p.m.) and SiO2 (average 51.1 wt. per cent);and (B) depleted in incompatible elements (e.g., average K2O= 0.31, P2O5 =0.17 wt. per cent, and Ba=178, Sr= 179, Rb= 11,La = 9, Zr = 93 p.p.m.) and SiO2 (average 49.7 wt. per cent).Low-Ti basalts of Group A are typical of northern Paran? {Ro= O70550–O70596), but a few are also present in centralParan? (Ro = 070577–0–70591), while those of GroupB are exclusive to central Paran– {Ro = 070463–0–70580) Low-Ti basalts with R0> O7060 are typical of southern Paran?(R0 = O7O639 –O71137), but are also present in centralParana (Ro = 070620–070890). These low-Ti basalts havechemical similarity (e.g., Ti, P, Sr) with low-Ti basalts depletedin incompatible elements (Group B) from which, however, theydiffer-in possessing significantly higher concentrations ofSiO2, K2O, Rb, and Ba. Such chemical diversity, accompaniedby important Ro variations (070463–071137) suggests thatthe low-Ti basalts from southern and part of central Paranamay result from crustal contamination. On the contrary, low-Ti basalts from northern, and part of central, Parana (GroupA) may be considered virtually uncontaminated. Results indicate that crustal contamination by granitic material(s)may be in the range 7–17 per cent. Such contaminationin central Paran? appears compatible with an assimilation-fractionalcrystallization process (AFC), while in southern Parana, othercontamination processes (e.g., mixing of magmasfrom crustaland mantle sources, assimilation of wall rock while magmas flowthrough dykes, etc.) were probably superimposed on AFC. Thedegree of crustal contamination generally decreases from southernto northern Parana. Sr and Nd isotope ratios suggest that mantle source materialfor low-Ti basalts depleted in incompatible elements (GroupB: southern and part of central Parana) had a lower R0 value(c. O.7046) and a higher l43Nd/144Nd ratio (Nd + c. 0.51274)than that for low-Ti basalts enriched in incompatible elements(Group A: northern and part of central Parana), namely R0 c.O.7059 and Nd+ c. 0.51242. These Sr-isotopic differences alsoapply to the northern (incompatible-element rich, R0 c. O.7053)and southern (incompatible-element poor R0 c. 0.7046) basaltprovinces of Karoo, suggesting that both Parana and Karoo basaltmagmas, differing by about 70 m.y. in age, probably originatedin a similar batch of subcontinental lithospheric mantle inpredrift times (cf. Cox, 1986). The extension of the Dupal Sr-anomaly (i.e. Rio Grande Rise+ Wai vis Ridge + Gough and Tristan da Cunha islands: Sr = 46=53;Hart, 1984) inside the Brazilian continent (Sr = 46–59)suggests that the lithospheric mantle of the Parana (and Karoo)provinces was possibly also the local source of oceanic volcanismup to advanced stages of the opening of the South Atlantic. *Reprint requests to E. M. Piccirillo.  相似文献   

5.
Experimental Melting of Carbonated Peridotite at 6-10 GPa   总被引：2，自引：0，他引：2  
Partial melting of magnesite-bearing peridotites was studiedat 6–10 GPa and 1300–1700°C. Experiments wereperformed in a multianvil apparatus using natural mineral mixesas starting material placed into olivine containers and sealedin Pt capsules. Partial melts originated within the peridotitelayer, migrated outside the olivine container and formed poolsof quenched melts along the wall of the Pt capsule. This allowedthe analysis of even small melt fractions. Iron loss was nota problem, because the platinum near the olivine container becamesaturated in Fe as a result of the reaction Fe2SiO4Ol = FeFe–Ptalloy + FeSiO3Opx + O2. This reaction led to a gradual increasein oxygen fugacity within the capsules as expressed, for example,in high Fe3+ in garnet. Carbonatitic to kimberlite-like meltswere obtained that coexist with olivine + orthopyroxene + garnet± clinopyroxene ± magnesite depending on P–Tconditions. Kinetic experiments and a comparison of the chemistryof phases occasionally grown within the melt pools with thosein the residual peridotite allowed us to conclude that the meltshad approached equilibrium with peridotite. Melts in equilibriumwith a magnesite-bearing garnet lherzolite are rich in CaO (20–25wt %) at all pressures and show rather low MgO and SiO2 contents(20 and 10 wt %, respectively). Melts in equilibrium with amagnesite-bearing garnet harzburgite are richer in SiO2 andMgO. The contents of these oxides increase with temperature,whereas the CaO content becomes lower. Melts from magnesite-freeexperiments are richer in SiO2, but remain silicocarbonatitic.Partitioning of trace elements between melt and garnet was studiedin several experiments at 6 and 10 GPa. The melts are very richin incompatible elements, including large ion lithophile elements(LILE), Nb, Ta and light rare earth elements. Relative to theresidual peridotite, the melts show no significant depletionin high field strength elements over LILE. We conclude fromthe major and trace element characteristics of our experimentalmelts that primitive kimberlites cannot be a direct productof single-stage melting of an asthenospheric mantle. They rathermust be derived from a previously depleted and re-enriched mantleperidotite. KEY WORDS: multianvil; carbonatite melt; peridotite; kimberlite; element partitioning  相似文献   

6.
The effects of small amounts of H2O (<4 wt % in the melt)on the multiply saturated partial melting of spinel lherzolitein the system CaO–MgO–Al2O3–SiO2 ±Na2O ± CO2 have been determined at 1·1 GPa inthe piston-cylinder apparatus. Electron microprobe analysisand Fourier transform infrared spectroscopy were used to analysethe experimental products. The effects of H2O are to decreasethe melting temperature by 45°C per wt % H2O in the melt,to increase the Al2O3 of the melts, decrease MgO and CaO, andleave SiO2 approximately constant, with melts changing fromolivine- to quartz-normative. The effects of CO2 are insignificantat zero H2O, but become noticeable as H2O increases, tendingto counteract the H2O. The interaction between H2O and CO2 causesthe solubility of CO2 at vapour saturation to increase withincreasing H2O, for small amounts of H2O. Neglect of the influenceof CO2 in some previous studies on the hydrous partial meltingof natural peridotite may explain apparent inconsistencies betweenthe results. The effect of small amounts of H2O on multiplysaturated melt compositions at 1·1 GPa is similar tothat of K2O, i.e. increasing H2O or K2O leads to quartz-normativecompositions, but increasing Na2O produces an almost oppositetrend, towards nepheline-normative compositions. KEY WORDS: H2O; CO2; FTIR; hydrous partial melting; mantle melting; spinel lherzolite; system CaO–MgO–Al2O3–SiO2 ± H2O ± CO2 ± Na2O  相似文献   

7.
Basalt was successfully cored at Site 54 in the Parace VelaBasin of the Philippine Sea, and at Site 57 on the CarolineRidge as part of Leg 6 of the Deep Sea Drilling Project. Site54 basalts are altered, but selected major and trace elements,particularly REE allow their characterization as high-aluminaolivine tholeiites, with flat REE patterns, no Eu anomalies,and low dispersed trace-element contents. Basalt from Site 57is distinctly different, with higher TiO2, P2O5, Fe/Mg ratio,and dispersed trace elements, low Ni, Mg, and a strongly fractionatedREE pattern. Derivation by fractionation of transitional basaltis suggested. Varying degrees of alteration of Site 54 basalts has causeddepletion in MgO and addition of Rb, Sr, Ba, Ka2O, and Na2O.Mineralogically these changes have resulted in alteration ofolivine and pyroxene, but plagioclase and iron-titanium oxidesremain unaffected. Microprobe data are presented for olivine, clinopyroxene, plagioclase,Fe-Ti oxides, and chrome-spinel in all three basalts. In 54–4,54–8 pyroxenes are zoned from salite to ferroaugite, andshow erratic decrease in A12O3 and TiO2 with increasing ironcontent. In 57–2, pyroxenes are more magnesian than thosein site 54 basalts, and show an increase in A12O3 and TiO2 withiron-enrichment. The anomalous behaviour of Al2O3 and TiO2 insome clinopyroxenes, and the close textural relationship ofpyroxene and magnesian olivine in 57–2 basalt, suggestthese minerals are xenocrystic, and were incorporated duringmagma ascent. Spinels of picotite composition in early-formed,and possibly xenocrystic, olivine, and plagioclase crystallizedprior to magma eruption, and may have begun crystallizationwithin the mantle. Site 54 basalts add further confirmation that inter-are basaltsclosely resemble ocean ridge tholeiites, although there is apaucity of data from inter-are environments. Site 57 basaltis chemically and mineralogically distinct from ocean ridgebasalts and may be related to similar volcanics erupted on theCaroline Islands.  相似文献   

8.
Equilibrium H2O pressure (PeH2O) was fixed at values less thantotal pressure (PT) in melting experiments on mixtures of 1921Kilauea tholeiite, H2O, and CO2 (58.5 mole per cent H2O, 41.5mole per cent CO2), buffered by Ni+NiO. New determinations ofthe beginning of melting of mixtures of 1921 Kilauea tholeiiteand H2O buffered by quartz+fayalite+magnetite were made at 2and 3 kb. Microprobe analyses of coexisting glass, clinopyroxene,?olivine, ?amphibole were determined for several runs. Decreasing H2O fugacity (fH2O) to about six-tenths the fugacityof pure H2O (f?II2O) raises the solidus and the upper stabilitylimit of plagioclase. Plagioclase and clinopyroxene coexistin equilibrium with liquid-a feature not observed in the pureH2O system. Amphibole is stable to about 970 ?C at 2 kb, 1025?C at 5 kb and 1060 ?C at 8 kb. The Al (VI)+Ti contents of theamphibole increase with P, yielding kaersutite at 1050 ?C and8 kb. Calculated modes for the condensed phases reveal large differencesin the amount of glass (liquid) present and large differencesin liquid composition below and above the breakdown temperatureof amphibole at 5 and 8 kb. Liquids coexisting with amphibole,clinopyroxene, olivine, and magnetite are dacitic near the solidusand silica-rich andesites around 1000 ?C at 5 and 8 kb. Theresults of this study substantiate the model for the generationof the calc-alkaline suite by partial melting of H2O-rich basalts.  相似文献   

9.
Mineral and melt inclusions in olivines from the most Mg-richmagma from the southern West Sulawesi Volcanic Province indicatethat two distinct melts contributed to its petrogenesis. Thecontribution that dominates the whole-rock composition comesfrom a liquid with high CaO (up to 16 wt %) and low Al2O3 contents(CaO/Al2O3 up to 1), in equilibrium with spinel, olivine (Fo85–91;CaO 0·35–0·5 wt %; NiO 0·2–0·30wt %) and clinopyroxene. The other component is richer in SiO2(>50 wt %) and Al2O3 (19–21 wt %), but contains significantlyless CaO (<4 wt %); it is in equilibrium with Cr-rich spinelwith a low TiO2 content, olivine with low CaO and high NiO content(Fo90–94; CaO 0·05–0·20 wt %; NiO0·35–0·5 wt %), and orthopyroxene. Boththe high- and low-CaO melts are potassium-rich (>3 wt % K2O).The high-CaO melt has a normalized trace element pattern thatis typical for subduction-related volcanic rocks, with negativeTa–Nb and Ti anomalies, positive K, Pb and Sr anomalies,and a relatively flat heavy rare earth element (HREE) pattern.The low-CaO melt shows Y and HREE depletion (Gdn/Ybn 41), butits trace element pattern resembles that of the whole-rock andhigh-CaO melt in other respects, suggesting only small distinctionsin source areas between the two components. We propose thatthe depth of melting and the dominance of H2O- or CO2-bearingfluids were the main controls on generating these contrastingmagmas in a syn-collisional environment. The composition ofthe low-CaO magma does not have any obvious rock equivalent,and it is possible that this type of magma does not easily reachthe Earth's surface without the assistance of a water-poor carriermagma. KEY WORDS: melt inclusions; mineral chemistry; olivine; syn-collisional magmatism; ankaramites; low-Ca magma  相似文献   

10.
Major and trace elements analysis has been carried out on the Late Ladinian Tabai basalts from Yunnan Province with the aim of studying their petrogenesis. Their SiO2 contents range from 43.63 wt.% to 48.23 wt.%. The basalts belong to the weakly alkaline(average total alkalis Na2 O+K2O=3.59 wt.%), high-Ti(3.21 wt.% to 4.32 wt.%) magma series. The basalts are characterized by OIB-like trace elements patterns, which are enriched in large ion lithosphile elements(LILE) including Rb and Ba, and display negative K, Zr and Hf anomalies as shown on the spider diagrams. The Tabai basalts display light rare-earth elements(LREE) enrichment and are depleted in heavy rare-earth elements(HREE) on the REE pattern. Those dates indicate that the parental magma of the Tabai basalts was derived from low-degree(1%–5%) partial melting of garnet peridotite. The magma underwent olivine fractional crystallization and minor crustal contamination during their ascent. The Tabai basalts were related to a relaxation event which had triggered the Emeishan fossil plume head re-melting in the Middle Triassic.  相似文献   

11.
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 (Fo86–89·5).Their bulk compositions are used to calculate primary melt compositionsin equilibrium with the most magnesian Macquarie Island olivines(Fo90·5). 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  相似文献   

12.
We explore the partial melting behavior of a carbonated silica-deficienteclogite (SLEC1; 5 wt % CO2) from experiments at 3 GPa and comparethe compositions of partial melts with those of alkalic andhighly alkalic oceanic island basalts (OIBs). The solidus islocated at 1050–1075 °C and the liquidus at 1415 °C.The sub-solidus assemblage consists of clinopyroxene, garnet,ilmenite, and calcio-dolomitic solid solution and the near solidusmelt is carbonatitic (<2 wt % SiO2, <1 wt % Al2O3, and<0·1 wt % TiO2). Beginning at 1225 °C, a stronglysilica-undersaturated silicate melt (34–43 wt % SiO2)with high TiO2 (up to 19 wt %) coexists with carbonate-richmelt (<5 wt % SiO2). The first appearance of carbonated silicatemelt is 100 °C cooler than the expected solidus of CO2-freeeclogite. In contrast to the continuous transition from carbonateto silicate melts observed experimentally in peridotite + CO2systems, carbonate and silicate melt coexist over a wide temperatureinterval for partial melting of SLEC1 carbonated eclogite at3 GPa. Silicate melts generated from SLEC1, especially at highmelt fraction (>20 wt %), may be plausible sources or contributingcomponents to melilitites and melilititic nephelinites fromoceanic provinces, as they have strong compositional similaritiesincluding their SiO2, FeO*, MgO, CaO, TiO2 and Na2O contents,and CaO/Al2O3 ratios. Carbonated silicate partial melts fromeclogite may also contribute to less extreme alkalic OIB, asthese lavas have a number of compositional attributes, suchas high TiO2 and FeO* and low Al2O3, that have not been observedfrom partial melting of peridotite ± CO2. In upwellingmantle, formation of carbonatite and silicate melts from eclogiteand peridotite source lithologies occurs over a wide range ofdepths, producing significant opportunities for metasomatictransfer and implantation of melts. KEY WORDS: carbonated eclogite; experimental phase equilibria; partial melting; liquid immiscibility; ocean island basalts  相似文献   

13.
Cenozoic（Miocene to Pleistocene） basaltic rocks in Jiangsu province of eastern China include olivine tholeiite and alkali basalt.We present major,trace element and Sr-Nd isotopic data as well as Ar-Ar dating of these basalts to discuss the petrogenesis of the basalts and identify the geological processes beneath the study area.On the basis of chemical compisitions and Ar-Ar dating of Cenonoic basaltic rocks from Jiangsu province,we suggest that these basalts may belong to the same magmatic system.The alkali basalts found in Jiangsu province have higherΣFeO,MgO,CaO,Na2O, TiO2 and P2O5 and incompatible elements,but lower Al2O3 and compatible elements contents than olivine tholeiite which may be caused by fractional crystallization of olivine,pyroxene and minor plagioclase.In Jiangsu basaltic rocks the incompatible elements increase with decreasing MgO/ΣFeO ratios.The primitive mantle-normalized incompatible elements and chondrite-normalized REE patterns of basaltic rocks found in Jiangsu province are similar to those of OIB.Partial loss of the mantle lithosphere accompanied by rising of asthenospheric mantle may accelerate the generation of the basaltic magma.The 143Nd/144Nd vs.87Sr/86Sr plot indicates a mixing of a depleted asthenospheric mantle source and an EMI component in the study area.According to Shaw’s equation,the basalts from Jiangsu province may be formed by l%-5%partial melting of a depleted asthenospheric mantle source.On the basis of Ar-Ar ages of this study and the fractional crystallization model proposed by Brooks and Nielsen（1982）,we suggest that basalts from Jiangsu province may belong to a magmatic system with JF-2 as the primitive magma which has undergone fractional crystallization and evolved progressively to produce other types of basalts.  相似文献   

14.
Volatiles and major elements in submarine glasses from Loihi seamount and Kilauea volcano. Hawaii were analyzed by high temperature mass spectrometry and the electron microprobe. Loihi glasses are subdivided into three groups: tholeiitic, transitional and alkali basalts. The glasses are evolved: Mg numbers range from 48–58. The alkalic lavas are the most evolved.Total volatiles range from 0.73 to 1.40 wt.%. H2O shows a positive linear correlation with K2O content [H2O = 0.83 (± .09) K2O + 0.08 (± .06)]. Concentrations of H2O are higher in the alkalic lavas, but Cl and F abundances are highly variable. Variations in ratios of incompatible elements (K2O, P2O5, H2O) indicate that each group was derived from a distinct source. CO2 contents range from 0.05 to 0.19 wt.% but show no systematic correlation with rock type or Mg #. A well-defined decrease in glass CO2 content with increasing vesicularity is shown by the alkalic lavas. CO2 may have been outgassed from the tholeiitic and transitional magmas prior to eruption during storage in a shallow magma chamber. Reduced carbon species (CO and CH4) were found in small amounts in most of the alkalic samples. Although the redox histories of Hawaiian lavas are poorly known, these new data indicate the presence of a reduced source for Loihi magmas.The Kilauea tholeiitic glasses are evolved (Mg # 48.3 to 55) and have higher H2O contents (av. 0.54 wt.%) than Loihi tholeiites (av. 0.42 wt.%) at the same Mg # (~55). Cl is distinctly lower in Kilauea glasses (0.01 wt.%) compared to Loihi glasses (0.09 wt.%). The data indicate significant source differences for the two volcanoes, consistent with results of other geochemical studies.Loihi tholeiites have distinctly higher 3He/4He ratios than Kilauea tholeiites and are the highest measured in submarine basalts (KURZ et al., 1983). These high ratios have been used to invoke a primitive source for Loihi basalts. The high Cl content of these basalts, the highest we have ever measured in submarine basalts, may be a fingerprint of this primitive source, as previously noted for Icelandic basalts (Schillinget al. 1980).  相似文献   

15.
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–CrN =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.  相似文献   

16.
Potassic volcanic rocks from the Wudalianchi, Erkeshan and Keluo(WEK) fields in NE China are located between the Mesozoic SongliaoBasin and the Palaeozoic Xing'am Mountains fold belt. Theserocks erupted during three main eruptive episodes-Miocene (9•6–7•0Ma), Pleistocene (0•56–0•13 Ma) and Recent (AD1719–1721)-and are subdivided into three types-olivineleucitite, leucite basanite and trachybasalt—on the basisof modal composition. In comparison with Cenozoic alkaline basaltsfrom East China that are similar to oceanic island basalts (OIBs),WEK volcanic rocks are lower in Al2O3, CaO, Fe2O3 and Sc, buthigher in K2O (3•5–7•1 wt %), K2O/Na2O (>1)and incompatible elements. High 87Sr/86Sr (0•7050–0•7056),low 143Nd/144Nd (0•51238–0•51250) and 206Pb/204Pb(17•06–16•61) ratios also distinguish them fromoceanic and Chinese basalts. Trace element and isotope dataindicate that a post-Archaean subcontinental lithospheric mantlesource similar to the postulated EM1 component (enriched mantlewith low l43Nd/144Nd and moderate high 87Sr/86Sr) must haveplayed a significant role in magma generation. The source rockis considered to be refractory phlogopite-bearing garnet peridotiteheterogeneously enriched in both large ion lithophile elementsand light rare earth elements by ancient metasomatism duringProterozoic times. This source may have mixed recently withOIB-like melts, but has not been modified by subduction of theKula-Pacific plate. Primitive WEK potassic magma was generatedby a low degree of partial melting, initiated by an extensionalphase beginning in the late Tertiary, at pressures of 20–45kbar and in the presence of mixed volatile components of H2O,CO2 and halogens. KEY WORDS: potassic volcanic rocks; NE China; geochemistry; montle sourc *Corresponding author. Present address: Centre for Petrology and Lithoipheric Studies, School of Earth Sciences, Macquarie University, NSW 2109, Australia  相似文献   

17.
Mafic tholeiitic basalts from the Nejapa and Granada (NG) cindercone alignments provide new insights into the origin and evolutionof magmas at convergent plate margins. In comparison to otherbasalts from the Central American volcanic front, these marietholeiitic basalts are high in MgO and CaO and low in Al2Op,K2O1, Ba and Sr. They also differ from other Central Americanbasalts, in having clinopyroxene phenocrysts with higher MgO,CaO and Cr2O3 concentrations and olivine phenocrysts with higherMgO contents. Except for significantly higher concentrationsof Ba, Sr and 87Sr/86Sr, most of the tholeiites are indistinguishable in compositionfrom mid-ocean ridge basalts. In general, phenocryst mineralcompositions are also very similar between NG tholeiites andmid-ocean ridge basalts. The basalts as a whole can be dividedinto two groups based on relative TiO2-K2O concentrations. Thehigh-Ti basalts always have the lowest K2O and Ba and usuallyhave the highest Ni and Cr. All of the basalts have experienced some fractional crystallizationof olivine, plagioclase and clinopyroxene. Relative to otherCentral American basalts, the Nejapa-Granada basalts appearto have fractionated at low PT and PH2O. The source of primarymagmas for these basalts is the mantle wedge. Fluids and/ormelts may have been added to the mantle wedge from hydrothermally-altered,subducting oceanic crust in order to enrich the mantle in Sr,Ba and 87Sr/86Sr, but not in K and Rb. The role of lower crustaicontamination in causing the observed enrichments in Sr, Baand 87Sr/86Sr of NG basalts in comparison to mid-ocean ridgebasalts, however, is unclear. Rutile or a similar high-Ti accessoryphase may have been stable in the mantle source of the low-TiNG basalts, but not in that of the high-Ti basalts. Mafic tholeiiticbasalts, similar to those from Nejapa and Granada, may representmagmatic compositions parental to high-Al basalts, the mostmafic basalts at most Central American volcanoes. The characterof the residual high-Al basalts after this fractionation stepdepends critically on PH2O Both high and low-Ti andesites are also present at Nejapa. Likethe high-Ti basalts, the high-Ti andesites have lower K2O andBa and higher Ni and Cr in comparison to the low-Ti group. Thehigh-Ti andesites appear to be unrelated to any of the otherrocks and their exact origin is unknown. The low-Ti andesitesare the products of fractional crystallization of plagioclase,clinopyroxene, olivine (or orthopyroxene) and magnetite fromthe low-Ti basalts. The eruption that deposited a lapilli sectionat Cuesta del Plomo involved the explosive mixing of 3 components:high-Ti basaltic magma, low-Ti andesitic magma and high-Ti andesiticlava.  相似文献   

18.
The Petrogenesis of the Kirwan Basalts of Dronning Maud Land, Antarctica   总被引：3，自引：3，他引：0  
The 420 m thick sequence of Kirwan basalt crops out along thesouthernmost 50 km of the Kirwanveggen Escarpment (74?S, 6?W).There is little variation in major element chemistry of thesebasalts (SiO2 49?3–51?6 wt.%; MgO 5?1–6?6 wt.%),but the concentrations of certain incompatible elements (e.g.,Zr) vary by factors of approximately two or more. Most interelementplots show rather poor correlation (r<0?78), but rocks fromopposite ends of the data array can be related by 30% fractionationof plagioclase, clinopyroxene, olivine, and magnetite in theproportions 51:35:11:3. Plagioclase is much more abundant inphenocryst assemblages (85%) and it appears that selective transportof plagioclase to the surface occurred. The range in incompatible element concentrations cannot be explainedby crystal fractionation and is most probably a result of theparent liquids of these basalts being derived by slightly differentdegrees of partial melting of a common source, or alternativelyof open-system (RTF) magma processes. The strontium isotopedata for the freshest rocks (R0=0?7049–0?7065) may beexplained by 7% contamination by crustal material with an R0of 0?709 and bulk Sr of800 ppm, but there is little supportingevidence from other trace element variations for this hypothesis.Oxygen isotope determinations on whole-rock-plagioclase pairsshow that alteration has resulted in a 0?5%o shift in (18O.Alteration also appears to have resulted in a greater spreadof data, particularly for the LIL elements and Sr isotopes.The Sr and Nd isotopic composition of the suite is close tobulk Earth at 172 Ma and this, together with REE and other traceelement data, shows these basalts to be similar in compositionto the more primitive basalts among the Karoo basalt lavas.It is suggested that the Kirwan basalts were derived from asource which was similar to that of the southern Lebombo variantof the Sabie River Basalt Formation of the Karoo Volcanic Province.This part of the Karoo was closest to the Kirwanveggen beforethe break-up of Gondwanaland.  相似文献   

19.
 This paper uses the geochemistry of primitive mafic lavas from the Rungwe volcanic province (southwestern Tanzania) to infer the source mineralogy and melting history. Post-Miocene mafic lavas from Rungwe include alkali basalts, basanites, nephelinites and picrites with up to 18.9 wt% MgO; nephelinites (>13.5% normative nepheline) are restricted to Kiejo volcano in the southern portion of the province. Rungwe lavas differ from most Western Rift volcanics in that they are not unusually potassic (K2O/Na2O ca. 0.40). Sparsely phyric mafic lavas contain phenocrysts and xenocrysts of plagioclase (An82–90), clinopyroxene (4.5–9.5 wt% Al2O3), and olivine (Fo79–88); one basanite contains a 1 mm xenocryst of apatite included in magnesian clinopyroxene. All samples have high abundances of incompatible elements (e.g., 0.7–2.2 wt% P2O5) and are enriched in REE relative to HFSE (Hf, Zr, Ti, Y), Cs, Ba, and K. Some incompatible element ratios are constant throughout the Rungwe suite (e.g., Zr/Nb, Sr/Ce, K/Rb), but other ratios are extremely variable and exceed the range measured in global Ocean Island Basalts (OIB) (e.g., Ba/Nb, Sm/Zr, La/Nb, Pb/Ce, Nb/U). The range in degree of silica saturation, and its excellent correlation with P2O5/Al2O3, indicate that the Rungwe suite records variable degrees of melting. Variations of individual incompatible trace element abundances in nephelinite and basanite samples suggest that the source contains metasomatic amphibole, ilmenite, apatite, and zircon. The Rungwe suite is interpreted as a series of low-percentage melts of CO2-rich peridotite at pressures that span the garnet-spinel transition. A geochemical comparison of Rungwe samples to lavas from other Western Rift volcanic centers requires that the source mineralogy varies along the rift axis, although each province is underlain by metasomatized peridotite. The incompatible trace element signatures of Western Rift lavas indicate that the source area is typically homogeneous on the scale of individual volcanoes, although lavas from each volcano reflect a range in degree of melting. Significantly, volcanoes with distinct geochemistry are always separated by major rift faults, suggesting that volcanic and tectonic surface features may correspond to metasomatic provinces within the subcontinental lithospheric mantle. Received: 30 May 1994 / Accepted: 5 April 1995  相似文献   

20.
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.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号