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1.
R.K Mark C Lee Hu H.R Bowman F Asaro E.H McKee R.R Coats 《Geochimica et cosmochimica acta》1975,39(12):1671-1678
Olivine tholeiites, the youngest Tertiary units (about 8–11 m.y. old) at five widely spaced localities in northeastern Nevada, are geologically related to the basalts of the Snake River Plain, Idaho, to the north and are similar in major element and alkali chemistry to mid-ocean ridge basalts (MORB) and island arc tholeiites. The measured K (1250–3350 ppm), Rb (1·9–6·2 ppm) and Sr (140–240 ppm) concentrations overlap the range reported for MORB. Three of the five samples have low, unfractionated rare earth element (REE) patterns, the other two show moderate light-REE enrichment. Barium concentration is high and variable (100–780 ppm) and does not correlate with the other LIL elements. The rocks have 87Sr/86Sr = 0·7052–0·7076, considerably higher than MORB (~0·702–0·703). These samples are chemically distinct (i.e. less alkalic) from the olivine tholeiites from the adjacent Snake River Plain, but their Sr isotopic compositions are similar. They contain Sr that is distinctly more radiogenic than the basalts from the adjacent Great Basin. About 10 b.y. would be required for the mean measured Rb/Sr (~ 0·02) of these samples to generate, in a closed system, the radiogenic Sr they contain. The low alkali content of these basalts makes crustal contamination an unlikely mechanism. If the magma is uncontaminated, the time-averaged Rb/Sr of the source material must have been ~0·04. A significant decrease in Rb/Sr of the source material (a factor 2?) thus most probably occurred in the relatively recent (1?09 yr) past. Such a decrease of Rb/Sr in the mantle could accompany alkali depletion produced by an episode of partial melting and magma extraction. In contrast, low ratios indicate that the source material of the mid-ocean ridge basalts may have been depleted early in the Earth's history. 相似文献
2.
Tertiary-Recent Tasmanian and Newer (Victoria/South Australia) basalts range from quartz tholeiite to olivine melilitite and show systematic increases in their incompatible element abundances with increasing degree of silica undersaturation. These two basalt provinces show similar relative abundances of rare earth elements (REE), differences in the relative concentrations of Rb, Ba, Th, K and Nb, and distinct, restricted isotopic compositions. The Tasmanian basalts have from 0.7026 to 0.7034, and ?Nd from + 7.5 to + 5.8; the Newer basalts have higher from 0.7038 to 0.7045, and lower ?Nd from +4.2 to + 1.7. The range in Sr and Nd isotope compositions can be denned by primary magma compositions for both provinces, using Mg-values, Ni content and the presence of spinel lherzolite nodules. Major and trace element and Sr, Nd and Pb isotope compositions are uniform on a scale of up to 50 km for four separate Newer basanite centers. The chemical and isotopic data are consistent with a model whereby tholeiitic basalts are derived by large degrees of partial melting from a chemically uniform but isotopically variable source, and generation of undersaturated, alkaline basalts by smaller degrees of partial melting of the same source. No isotopic or geochemical evidence was found which would suggest that the more evolved basalts have been contaminated by continental crust.In contrast to tholeiitic and alkalic basalts from Hawaii, there is a continuous spectrum of isotope compositions for the Newer tholeiitic to alkalic basalts. A model is proposed for the generation of these basalts involving mixtures of hotspot mantle plume-derived melt and lithospheric mantle-derived melt, where observed differences between ocean island and continental alkaline basalts are attributed to differences between the sub-oceanic and sub-continental lithospheric mantles. Isotopic differences between tholeiitic and alkalic basalts are interpreted to be due to varying degrees of exchange and mixing between the hotspot plume and lithospheric mantle melt components. The model is consistent with the generation of these basalts from a source which has been recently enriched in the LREE. 相似文献
3.
Major-element, trace-element and isotopic compositions of approximately 1200 basalts (< 53 wt. % SiO2) from intra-oceanic island arcs have been compiled to assess the nature and possible sources of primitive island-arc basalts (IAB). The chemical characteristics of IAB are examined with reference to those of mid-ocean ridge basalts (MORB) and intraplate oceanic basalts (IPB). Major-element compositions of primitive [] IAB and MORB are similar, but differ significantly from IPB. In general, IAB do not have higher Al2O3, lower TiO2 or a lack of Fe enrichment compared to primitive MORB but many do have greater K2O contents. Differences in major- and minor-element contents between more evolved IAB and MORB result from the dominance of plagioclase + olivine crystal fractionation in MORB magmas vs. clinopyroxene + olivine controlled fractionation in IAB suites. This difference in crystallization history may be related to the higher PH2O or greater depth of crystallization of IAB magmas compared to those inferred for MORB.IAB are characteristically enriched in large-ion-lithophile (LIL) elements and depleted in high-field-strength ions (e.g., Zr, Nb and Hf) relative to normal MORB (N-type) and IPB. The enrichment of some LIL elements (e.g., Sr, Rb, Ba and Pb) relative to the rare-earth elements in IAB is difficult to explain by simple partial melting alone and suggests a multistage petrogenesis involving an LIL-enriched component. Low abundances of high-field-strength ions in evolved IAB are explicable in terms of fractional crystallization, but the cause for consistently low abundances in primitive IAB remains problematic.Island-arc lavas contain greater concentrations of volatiles and have higher and Cl/F ratios than either MORB or IPB, suggesting involvement of a slab-derived volatile component. However, this is not consistent with data which indicate that only near-trench volcanics have been significantly affected by dehydration of the oceanic crust.Sr-, Nd-, Pb- and O-isotopic data, in conjunction with the trace-element data, clearly indicate that IAB are derived from heterogeneous, LIL-depleted mantle sources most similar to those which give rise to enriched MORB (E-type). The marked shift towards higher in IAB compared to oceanic lavas with similar values cannot be explained simply by the addition of radiogenic Sr from the slab. Variable degrees of contamination from a crustally-derived sedimentary component is consistent with the isotopic and trace-element data from a number of arcs. However, the lack of correlation between LIL/REE ratios and more radiogenic isotopic ratios suggests that this enrichment/contamination process is complex. A multi-stage petrogenetic model involving subducted oceanic crust (± sediments), dehydration/volatile transfer, and partial melting of metasomatized mantle beneath island arcs is considered the most reasonable, although least constrained, method to generate a variety of primitive IAB. 相似文献
4.
Seventy-two basalts from 58 dredge stations located along the Mid-Atlantic Ridge from 29°N to 59°N have been analyzed for and for K, Rb, Cc, Sr and Ba. The Sr-isotope profile along the ridge has three distinct maxima, one coinciding with the Azores platform (0.70345), one at 45°N (0.70340) and the third at 35°N, in the vicinity of the Oceanographer Fracture Zone. Basalts from ridge segments between 29°N and 33°N, and 49°N and 59°N have ratios typical of ‘normal’ mid-ocean ridge basalts (0.70230–0.70280). Profiles of K, Rb, Cs, Sr, Bz, Rb/Sr and Ba/Sr are similar to the profile, but Rb/K, Cs/K and Ba/K show broad maxima between 35°N and 45°N.These variations result from chemical and isotopic heterogeneity in the mantle, and are interpreted as caused by a mantle plume beneath the Azores which mixes with the LIL-element-depleted asthenosphere. Additional plumes may exist beneath 45°N and 35°N.Compared to the LIL-element-depleted asthenosphere, the Azores mantle plume is 10 to 30 times enriched in LIL elements with very small (? 0.1) bulk crystal/melt partition coefficients (Rb, Cs, Ba, La). Mildly incompatible elements (0.1 < D < 1) (Sr, Sm, Yb) are only 0.8–3 times enriched. These, observations suggest that LIL element differences between these two mantle reservoirs resulted from processes involving solid-liquid equilibria and not vapor-solid or vapor-liquid equilibria. Isotope systematics indicate that neither mantle reservoir remained a closed system since the formation of the Earth, but it is not possible to determine the time at which heterogeneity first developed. 相似文献
5.
Asish R. Basu 《Geochimica et cosmochimica acta》1978,42(6):659-668
New analyses of K, Rb, Sr and Ba contents and the ratios of eight amphiboles, one phlogopite, two diopsides and one host alkalic basalt for an amphibole are reported: The samples are mostly inclusions in alkalic basalts and occur in association with peridotite inclusions. Two of the samples are from alpine-type peridotite bodies — one from the Etang de Lhers massif in the French Pyrenees and the other from the Finero massif in the Ivrea zone in northern Italy. The kaersutites come from the following localities: Hoover Dam, Arizona; Deadman Lake, California; Massif Central, France; Queensland; Spring Mountain, New South Wales.The data indicate that kaersutitic amphiboles are genetically unrelated to their host basalts. The isotopic and trace element data of these amphiboles further strengthens the suggestion of BASU and MURTHY (1977) that kaersutites play a significant role in ocean ridge basalt genesis. In addition, pargasitic amphibole with higher ratios, if present, may be important in the source regions of alkalic basalts.The bulk amphibole lherzolite from Lherz has the ratio and ratio appropriate for source material of ridge tholeiites. If the diopside and the amphibole in this rock had isotopically equilibrated under upper mantle conditions, the data show the time of last equilibration to be approximately 735 m.y., in contrast to the young emplacement age of the ultramafic massif.The coexisting phlogopite and diopside in the spinel lherzolite inclusion from Kilbourne Hole, New Mexico, show, surprisingly, isotopic equilibration under upper mantle conditions despite their drastically different ratios. The data show that the phlogopite must have formed very recently in the upper mantle. This phlogopite also has a high ratio (1133), contrary to the commonly held view that mantle phlogopites have low ratios. The coexisting diopside shows high K content (778 ppm) and a lower ratio than the phlogopite. This phlogopite lherzolite has trace elemental and isotopic characteristics that may be adequate for the origin of alkalic basalts upon partial melting. 相似文献
6.
John S. Dickey Frederick A. Frey Stanley R. Hart E.Bruce Watson Geoffrey Thompson 《Geochimica et cosmochimica acta》1977,41(8):1105-1118
Basalts dredged from the Bouvet triple junction (South Atlantic), from the Mid-Atlantic Ridge near the triple junction, and from a spreading center east of Bouvet Island differ from normal mid-ocean ridge tholeiites by having higher concentrations of K and other large-ion-lithophile elements, higher ratios, and rare earth element distributions which show relative enrichment in the lighter rare earths. The basalts are more fractionated than typical oceanic tholeiites, however, fractional crystallization does not fully account for their chemical characteristics, and it appears that they were derived from special source materials, contaminated perhaps by a mantle plume rising beneath Bouvet Island. 相似文献
7.
The Archean Yellowknife Supergroup (Slave Structural Province. Canada) is composed of a thick sequence of supracrustal rocks, which differs from most Archean greenstone belts in that it contains a large proportion ( ~ 80%) of sedimentary rocks. Felsic volcanics of the Banting Formation are characterized by HREE depletion without Eu-anomalies, indicating an origin by small degrees of partial melting of a mafic source, with minor garnet in the residua. Granitic rocks include synkinematic granites [HREE-depleted; low ()I], post-kinematic granites [negative Eu-anomalies, high ()I] and granitic gneisses with REE patterns similar to the post-kinematic granites. Sedimentary rocks (turbidites) of the Burwash and Walsh Formations have similar chemical compositions and were derived from 20% mafic-intermediate volcanics, 55% felsic volcanics and 25% granitic rocks. Jackson Lake Formation lithic wackes can be divided into two groups with Group A derived from 50% mafic-intermediate volcanics and 50% felsic volcanics and Group B, characterized by HREE depletion, derived almost exclusively from felsic volcanics.REE patterns of Yellowknife sedimentary rocks are similar to other Archean sedimentary REE patterns, although they have higher . These patterns differ significantly from typical post-Archean sedimentary REE patterns, supporting the idea that Archean exposed crust had a different composition than the present day exposed crust. 相似文献
8.
9.
G.G. Pe 《Chemical Geology》1975,15(1):53-60
ratios have been determined in fifteen volcanic rocks from the northwestern part of the Hellenic arc. They range from 0.7041 to 0.7134. There is no apparent correlation of strontium isotope values with any major chemical component or with Rb/Sr ratios. The ratios appear to increase in a general way with increasing depth to the Benioff zone. The strontium isotope ratios are higher than from most island arcs; this is believed to be due to contamination. 相似文献
10.
The South Mountain batholith of southwestern Nova Scotia is a large, peraluminous, granodiorite-granite complex which intrudes mainly greenschist facies metasediments of the Cambro-Ordovician Meguma Group. Using Rb-Sr isochrons constructed from whole rocks and mineral separates, the present study shows a variation in age and initial ratios of the intrusive phases of the batholith as follows: biotite granodiorite (371.8 ± 2.2 Ma, ( ranges from 0.7076 ± 0.0003 to 0.7090 ± 0.0003, with the average = 0.7081); adamellite (364.3 ± 1.3 Ma, ); porphyry (361.2 ± 1.4 Ma, ); using λ87Rb = 1.42 × 10?11yr?1.A suite of Meguma country rock samples showed a variation of at the time of intrusion of the batholith. A number of xenoliths of this material occurring in the marginal granodiorite had partially equilibrated isotopically with the granodiorite at a higher ratio than elsewhere in the granodiorites. This evidence demonstrates that isotopic (and probably some accompanying bulk chemical) contamination by the Meguma rocks has been an important factor in determining the ultimate chemical composition and mineralogy of the South Mountain batholith.The of the early granodiorites indicates that the parent magma of the South Mountain batholith was derived from a source unlike the Meguma Group. The precise nature of the source region cannot be determined by Rb-Sr work unless the degree of contamination with Megumalike material is known. 相似文献
11.
Clinopyroxenes in the metamorphic alpine peridotites from Ronda, Béni Bouchera, Lanzo and Othris have ratios in the range of 0.70228 – 0.70370, similar to ocean ridge tholeiitic rock. Insofar as these lherzolites represent mantle sources, the present data allows simple evolutionary models relating basalt genesis and alpine peridotite. The highly radiogenic Sr reported in many whole rock alpine peridotites may be due to contamination in olivine and thus, earlier models that deny a simple relationship between alpine peridotite and the oceanic gabbros and basalts need a re-evaluation. 相似文献
12.
D.J Whitford 《Geochimica et cosmochimica acta》1975,39(9):1287-1302
Pleistocene and Recent lavas from the Sunda arc range from those showing affinities with the island arc tholeiitic series, through a spectrum of calc-alkaline to high-K alkaline rocks. The tholeiitic rocks have relatively low ratios averaging 0–7043; the calc-alkaline rocks show a wide range (from 0.7038 to 0.7059, averaging 0.7048); the high-K alkaline rocks average 0.7045. A rhyolitic ignimbrite from Sumatra has an ratio of 0.7139.The relationship between and major and trace element geochemistry is variable and complex. Lavas from the same volcano sometimes show significant differences in despite close geochemical relationships. Rocks of the calc-alkaline suite show a regular decrease in from West Java to Bali and there is some evidence for increasing with increasing depth to the Benioff zone. Calc-alkaline and tholeiitic rocks from the Sunda arc have significantly higher ratios than those from other island arcs, except from those arcs where continental crustal involvement has been inferred (e.g. New Zealand).A model of 87Sr enrichment due to isotopic equilibration of oceanic crust with sea water and disequilibrium melting in the slab and/or mantle is favoured to explain the Sr isotopic composition of the tholeiitic and normal calc-alkaline lavas. Calc-alkaline lavas with high ratios are best explained by either sialic contamination, or the presence of alkali basalt as a component of the downgoing slab. The Sr isotopic data for the high-K alkaline lavas suggest a mantle origin. The high ratio in the Lake Toba rhyolite implies a crustal origin. 相似文献
13.
Basalts from the Columbia River flood basalt province of the northwestern U.S.A. show a large diversity in chemical and Nd and Sr isotopic compositions. ranges from 0.51303 to 0.51208 and is strongly correlated with variations in . This correlation suggests mixing between two end member compositions, one characterized by and , and the other with and . The more radiogenic component could be mantle enriched in incompatible elements during the Precambrian, or Precambrian materials of the continental crust. A quartz-rich xenolith found in the Columbia lavas has Rb-Sr and Sm Nd model ages of ≈ 1.4Æ, implying the existence of old, isotopically evolved crustal basement which could serve as contaminant. Nevertheless, crustal contamination alone cannot explain the chemical variation of the samples studied, and other fractionation processes must have occurred simultaneously. A model involving combined assimilation and crystal fractionation reproduces the chemical and isotopic characteristics of the volumetrically dominant Grande Ronde unit for an assumed crystallizing component of plagioclase, low calcium pyroxene and minor olivine. The data are not consistent with the suggestion that a ‘primordial’ mantle is the source for this continental flood basalt province. Rather they suggest that the main volume of these lavas was originally derived from a mantle similar in isotopic composition to island arc and ocean island basalts of the north Pacific. The primary magma was modified chemically and isotopically by crystal fractionation and assimilation of sialic crustal materials during its transport through, or storage in the continental crust. 相似文献
14.
In the Sunda arc, only the Bali—Lombok—Sumbawa sector is apparently flanked both north and south by oceanic crust. South of Lombok Island the oceanic crust is probably of Early Cretaceous or Late Jurassic age, whereas the oldest rocks known from Lombok and Sumbawa islands are the Lower Miocene to Pliocene sediments and volcanics of the basement beneath the Quaternary—Recent volcanic centres.Three large active volcanoes form the northern parts of Lombok and Sumbawa. The volcanic rocks of Rindjani on Lombok belong to a basalt—andesite—dacite association, rich in plagioclase and hy- and Q-normative. East of Lombok, the volcanic rocks of Tambora and Sangeang Api on Sumbawa belong to a potassic ne—trachybasalt—trachy-andesite association. All three volcanoes occur only 150–190 km above the active north-dipping Benioff zone.Extinct Quaternary centres occur south of the active volcanoes on Sumbawa. Two of these centres, Soromundi and Sangenges, erupted markedly ne- and lc-normative leucitites together with andesites, dacites and trachybasalts.The volcanic composition—space—time relations in the Lombok—Sumbawa sector of the Sunda arc are not in accordance with the generalized island-arc schemata. Conventionally, potassic ne-mnormative island-arc associations are supposed to occur over the deep part of the Benioff zones, far from the trenches of mature island arcs. The SiO2|K2O relations of the Rindjani association are reasonably appropriate for a volcano overlying intermediate Benioff-zone depths, but both the Tambora and the Sangeang Api associations are far more potassic than would be predicted by generalized schemata, and also occur in a relatively young arc sector that apparently has developed only since Miocene time.Basalts, trachybasalts and leucitites from the Lombok—Sumbawa sector have been compared: at similar MgO contents and Mg/(Mg+Fe), the progression from hy- and Q-normative to ne- and lc-normative magmas is not marked by significant enrichment in TiO2, Na2O, Zr, Nb and P, but is accompanied by a substantial increase in K2O, Rb, Sr and LREE, by increasing and by decreasing K/Rb. ratios from Rindjani (0.70386–0.70402) and Tambora (0.70385–0.70389) are very similar and among the lowest for the Sunda arc, but from Sangeang Api (0.70460–0.70500) are significantly higher and more variable in spite of the similar tectonic setting and petrological affinities. ratios of leucities tend to be higher (0.70488–0.70529).The petrogenesis of the volcanic associations of Lombok and Sumbawa cannot be readily explained. Although even the leucitites display the poverty in TiO2 that generally characterizes volcanics from simple island-arc tectonic settings, there is very obvious uncoupling within the “incompatible elements”: enrichment in the LIL group (K, Rb, Sr but not Na) is not accompanied by similar behaviour in the group of small highly-charged ions (Ti, Zr, Nb, P). It has proved impossible to model this behaviour without invoking inhomogeneities in the source regions, both in mineralogy and in chemical composition. Similar uncoupling within the incompatible elements has also been reported from basalt groups from the Mid-Atlantic Ridge, may also occur in the Birunga province, and might not arise from processes unique to the island-arc environment.We suggest that a LIL-rich component is being progressively added to the source regions. This component could be incorporated by the crystallization of additional phases such as phlogopite or paragasite. If this component occurs deep within the mantle, it might gain passage to shallower regions either by percolating up the downgoing slab to yield the familiar arc magma zonation, or up substantial cross-arc fractures. 相似文献
15.
ratios measured in Quaternary lavas from Java and the Banda arc of Indonesia range from 0.51242 to 0.51280 and exhibit an inverse correlation with . Isotopically, the Indonesian samples resemble Andean rather than island arc lavas. The samples from Java plot either within, or adjacent to the mantle array, towards higher ratios. Samples from the Banda arc and the anomalous calc-alkaline volcano Papandajan are characterized by relatively low and high ratios. These characteristics are consistent with the interpretation that subducted terrigenous material was involved in the genesis of these lavas. Furthermore the Banda arc samples appear to lie on a mixing line between isotopic compositions characteristic of the mantle and upper continental crust. A high-K trachyte from the alkaline volcano Muriah, Java, has isotopic characteristics of the mantle (, ), which implies that the extreme enrichment in large-ion-lithophile elements in its source must have occurred only shortly before its formation. The inferred ratio of the unmodified mantle beneath Java and the Banda arc is lower than that observed in mid-ocean ridge basalt, which may have important implications for a better understanding of the geochemical structure of the mantle. 相似文献
16.
Isotopic, major and trace element studies of loess deposits from America, China, Europe and New Zealand show general uniformity of composition. Silica, Zr and Hf are enriched relative to estimates of bulk composition of the upper continental crust. The REE data are indistinguishable from those of average shales, confirming the concept that these REE patterns () represent the upper crustal average. Sm-Nd model ages are variable but <1700 m.y. They reflect derivation from younger elevated erogenic areas subject to Pleistocene glaciation. Although Sm-Nd model ages vary by a factor of two, the REE patterns remain constant. This indicates that processes responsible for formation of the upper crust have produced no secular change in composition since the mid-Proterozoic. 相似文献
17.
Nd and Sr isotopic compositions as well as trace element concentrations have been determined on a suite of alkali basalts from the Massif Central, in France. Samples show a typical enrichment in incompatible elements. In particular, the REE patterns exhibit a strong fractionation characterized by a ( ratio of about 20. The YbN content is about 10 times chondrite. The ratios exhibit a range from 0.512775 to 0.512989, values quite comparable to those from oceanic island basalts. The ratios vary between 0.70338 and 0.70458 and are anti-correlated with the Nd isotopic ratio.The isotopic and the trace element (in particular REE) data have been used in order to quantitatively model the genesis of the alkali basalts. Among the several types of models tested here, the most likely one appears to be the model of mantle metasomatism. A semi-quantitative approach shows that the source of alkali basalts from the Massif Central was metasomatized prior to melting. In such a model, the basalts could be produced by rather high degrees of partial melting (such as 10 or 15%) of the metasomatically enriched mantle. 相似文献
18.
Richard J. Williams 《Geochimica et cosmochimica acta》1974,38(9):1415-1433
The distribution coefficients of Eu and Sr for plagioclase-liquid and clinopyroxene-liquid pairs as a function of temperature and oxygen fugacity were experimentally investigated using an oceanic ridge basalt enriched with Eu and Sr as the starting material. Experiments were conducted between 1190° and 1140°C over a range of oxygen fugacities between 10?8 and 10?14 atm.The molar distribution coefficients are given by the equations: logPLSr = 7320/T ? 4.62logKCPXSr = 18020/T ? 13.10. Similarly, the weight fraction distribution coefficients are given by the equations: logDPLSr = 6570/T ? 4.30logDCPXSr = 18434/T ? 13.62.Although the mole fraction distribution coefficients have a smaller dependence on bulk composition than do the weight fraction distribution coefficients, they are not independent of bulk composition, thereby restricting the application of these experimental results to rocks similar to oceanic ridge basalts in bulk composition.Because the Sr distribution coefficients are independent of oxygen fugacity, they may be used as geothermometers. If the temperature can be determined independently — for example, with the Sr distribution coefficients, the Eu distribution coefficients may be used as oxygen geobarometers. Throughout the range of oxygen fugacities ascribed to terrestrial and lunar basalts, plagioclase concentrates Eu but clinopyroxene rejects Eu. 相似文献
19.
Surendra P. Verma 《Chemical Geology》1984,46(1):37-53
Twelve samples of mid-Tertiary felsic volcanic rocks from Zacatecas and San Luis Potosí (both belonging to the Sierra Madre Occidental) and one sample of Lower Tertiary porphyritic andesite from Zacatecas are analyzed for , K, Rb, and Sr. Eight selected samples are also analyzed for . A linear regression of the present-day of the felsic volcanic rocks in Zacatecas gives an approximate date of 30 ± 8 Ma. The initial ratios are high and widely distributed (from 0.705 to 0.712 or higher) whereas the initial ratios are somewhat low and show a narrow range (0.5125–0.5127). The available isotopic and trace-element data are best explained in terms of a binary mixing model in which the magmas derived from a slightly depleted-mantle fractionate and mix with varying proportions of the overlying middle/upper continental crust and undergo further shallow-level fractional crystallization before eruption. This model is also compatible with the trace-element and Sr isotopic data published from other areas of the Sierra Madre Occidental for which a purely mantle origin has been proposed. 相似文献
20.
Louise S. Powers Hannes K. Brueckner David H. Krinsley 《Geochimica et cosmochimica acta》1979,43(1):137-146
Weathered quartz grus and stream transported quartz of the Harney Peak Granite, Black Hills, South Dakota, contain low concentrations of Rb (generally 0.3–6.8 ppm) and Sr (0.2–2.0 ppm) and variable Sr isotopic ratios (0.759–1.070).Six of seven single grains of large composite quartz grus which recently entered the weathering environment define an apparent isochron age (about 1800 Myr) and initial ratio (0.7066) that approximate the whole-rock isochron age (1707 Myr) and initial ratio (0.7143) of the Harney Peak Granite. Apparently the Rb-Sr systematics of these grains were not significantly altered during initial weathering. Leached fluid inclusion material from a ca. 2 g aggregate of composite quartz grains contains very little Rb or Sr (0.019 and 0.17 μg, respectively) and has a very low ratio (0.739). The Rb and Sr content of the quartz grains appears to be concentrated in minute, heterogeneously-distributed mineral inclusions.Five aggregates of more completely weathered, small non-composite quartz grains produce a widely scattered pattern on an isochron diagram with all samples plotting below the 1707 Myr isochron. Examination by SEM of these grains shows solution and precipitation features on their relatively large effective surface areas. The differential precipitation of Rb is believed to have been the major perturbating chemical process during weathering.Three aggregates of stream quartz grains define an apparent isochron age of 1777 Myr and an initial ratio of 0.720 that suggest the initial ‘igneous’ Rb-Sr characteristics of the stream quartz were re-attained during their transportation, probably as a result of removal of the outer weathered surface by abrasion. The apparent resistance to chemical weathering of stream quartz and quartz which has just entered the weathering environment suggests that this mineral may be extremely useful in studies of provenance and the geochronology of strongly weathered terranes. 相似文献