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1.
Major, minor and trace element abundances were determined in seven Angra dos Reis samples including whole rocks, fassaite (clinopyroxene), olivine and whitlockite separates via sequential instrumental neutron activation analysis. The chondritic normalized rare earth element (REE) abundance pattern for the Angra dos Reis clinopyroxene separates shows a concave downward shape with a small negative Eu anomaly. The strong fractionation between the light and the heavy REE in olivine separates could be attributed to the presence of islands of kirschsteinite in the olivines. The large-ion lithophile trace elements were highly enriched in the whitlockite separate as expected (e.g. La ≈ 370 ppm). The lower Hf and Sc abundances in whitlockite compared to that in the equilibrium “magma” could be the result of favorable partitioning of Hf and Sc in baddeleyite, which may have crystallized prior to or with whitlockite in the interstitial liquid. Comparison of whole rock with mineral separate data shows the presence of ～3% olivine, ～2.6% spinel and small amounts of metallic Ni-Fe and troilite in the whole rock.The trace element abundances in the derivative magma from which the Angra dos Reis clinopyroxene crystallized were estimated from the clinopyroxene data and the clinopyroxene mineral-liquid partition coefficients. From the derivative magma, the trace element abundances in the possible parent magmas were calculated by assuming that these parent magmas have undergone different degrees of clinopyroxene fractional crystallization to yield the Angra dos Reis derivative magma. Using the trace element abundances in these possible parent magmas, a two-stage crystal-liquid fractionation model with source material containing olivine, orthopyroxene and clinopyroxene is presented for the genesis of Angra dos Reis. Possible combinations of the degree of equilibrium non-modal partial melting, the source mineral composition and the initial element abundances required to generate possible Angra dos Reis parent magmas are calculated by the multilinear regression analysis method. Favorable solutions for this two-stage crystal-liquid fractionation model could be that Angra dos Reis crystallized at ～70% fractional crystallization of clinopyroxene from magmas generated by reasonable degrees of equilibrium partial melting (～7–10%) of deep-seated primitive source materials (olivine ～54–30%, orthopyroxene ～33–53%, and clinopyroxene ～13–17%) with trace element (Ba, Sr, REE and Sc) abundances ～3.5–4.7 × chondrites. These calculated REE abundances in the Angra dos Reis parent body are very similar to those suggested for the primordial moon (～3–5 × chondrites).Possible genetic relationships between Angra dos Reis and other achondrites, especially cumulate eucrites and nakhlites, are studied. Apparently, the unique Angra dos Reis could not be related to those achondrites by crystal-liquid fractionation of the same parent body.  相似文献   

2.
We have studied SmNd systematics in pyroxene and phosphate mineral separates of Angra dos Reis. A pyroxene-phosphate internal isochron age ofT2 = 4.55 ± 0.04AE is obtained, in excellent agreement with reported Pb-Pb ages.142Nd/144Nd ratios in pyroxene samples are systematically larger than those in phosphates by 6 parts in 105. This variation is tentatively assigned to a radiogenic contribution from extinct146Sm. Fission xenon components in pyroxene and phosphate separates are characterized by discrete ratios of fission/spallation and evidence is presented for a third ratio in celsian. It is shown that this characteristic is due to a close association of244Pu with the light REE. Computed ratios244Pu/Nd are the same in pyroxene and phosphate separates, but244Pu/238U and244Pu/232Th ratios are not. Taking the fission xenon retention age to be 4.55 AE, we obtain an abundance ratio244Pu/Nd= 1.5 × 10?4 (or an atomic ratio244Pu/150Nd= 1.6 × 10?3) at this time and in the region of the solar system where the Angra dos Reis parent body formed. The exposure age of Angra dos Reis, as obtained by the81Kr-83Kr method is55.5 ± 1.2m.y. Neutron capture during the 55.5-m.y. exposure to cosmic rays increased the ratio150Sm/149Sm in Angra dos Reis by 6 parts in 104.  相似文献   

3.
Fassaitic pyroxene from the Angra dos Reis meteorite has striking spectral properties. The57Fe Mössbauer spectra show no Fe3&+;, and thus the absorption is thought to originate from a complex charge-transfer process. Intense absorption at 480 nm dominates the spectrum of the meteorite and may be important in the interpretation of telescope spectra of objects in space.  相似文献   

4.
The Angra dos Reis meteorite fell in 1869 and is a unique achondrite. It is an ultramafic igneous rock, pyroxenite, with 93% fassaite pyroxene which has 15.7% Ca-Tschermak's molecule, plus calcic olivine (Fo53.1; 1.3% CaO), green hercynitic spinel, whitlockite (merrillite), metallic Ni-Fe, troilite, as well as magnesian kirschsteinite (Ks62.3Mo37.7), within olivine grains, and celsian (Cs90.2An7.7Ab1.7Or0.4) which are phases reported in a meteorite for the first time, and plagioclase (An86.0), baddeleyite, titanian magnetite (TiO2, 21.9%), and terrestrial hydrous iron oxide which are phases reported for the first time in this meteorite. Petrofabric analysis shows that fassaite has a preferred orientation and lineation which is interpreted as being due to cumulus processes, possibly the effect of post-depositional magmatic current flow or laminar flow of a crystalline mush. The mineral chemistry indicates crystallization from a highly silica-undersaturated melt at low pressure. Since the meteorite formed as a cumulate, pyroxene crystals may have gravitationally settled from a melt which crystallized melilite first. Plagioclase would be unstable in such a highly undersaturated melt, and feldspathoids would be rare or absent due to the very low alkali contents of the melt. The presence of rare grains of plagioclase and celsian may be the result of late-stage crystallization of residual liquids in local segregations. Thus, the Eu anomaly in Angra dos Reis may be the result of pyroxene separation from a melt which crystallized melilite earlier, rather than plagioclase as previously suggested.  相似文献   

5.
The preatmospheric mass of Angra dos Reis is estimated to be about 80 kg, based on cosmic ray track densities of 1?3 × 106 cm?2 in feldspars, olivines, and pyroxenes. The244Pu-fission track densities in pyroxenes and adjacent olivine-feldspar track detectors, as well as in whitlockites and adjacent pyroxene-olivine detectors are about 20 and 280 × 106 cm?2 respectively. These values correspond exactly to the244Pu-fission xenon contents in the two minerals. This suggests an almost instantaneous cooling (>70 K per m.y.) of the Angra dos Reis material 4.55 b.y. ago. This fast cooling is the reason for a quantitative4He retention since that time. If the4He excess found in Angra dos Reis is a real effect, the source of this excess remains unknown.  相似文献   

6.
Extraterrestrial calcium phosphates (“whitlockites”) have the anhydrous β-Ca3(PO4)2 structure, which is different from that of hydrous terrestrial whitlockite. This has been confirmed by X-ray refinement of the structure of a phosphate from the achondrite Angra dos Reis. In the β-Ca3(PO4)2 structure, there is one crystallographic site, Ca(IIA), which is half-occupied by calcium, and which seems to have an energetically unfavorable configuration; natural phosphates with this configuration (including Angra dos Reis) have composition Ca19(Mg,Fe)2(PO4)14. Stability of the structure is probably increased by substitution of Na for Ca in Ca(IIA) giving composition Ca18 (Mg,Fe)2Na2(PO4)14, which occurs in chondrites; by vacancy of Ca(IIA), with rare earths and yttrium substituting for calcium in other sites for charge balance, giving composition Ca16(Y,RE)2(Mg,Fe)2(PO4)14, found in lunar rocks; or by replacing Ca with hydrogen, giving composition Ca18(Mg,Fe)2H2(PO4)14, which is terrestrial whitlockite. Lack of the favorable substitutions of Na, (Y, RE) or H in Angra dos Reis phosphate implies that these elements were relatively scarce in its environment of formation.  相似文献   

7.
The Muntsche Tundra basic pluton is well known for its high K-A ages (4 to 10 by). Using a precise87Rb-87Sr technique an age of 2.02 by has been measured. The (87Sr/86Sr) initial determined for this pluton is 0.702, higher than the predicted values using modern tholeiites and closed system extrapolation to BABI or ADOR.  相似文献   

8.
The five diogenites, Johnstown, Roda, Ellemeet, Shalka and Tatahouine, give scattered data in the87Rb/86Sr,87Sr/86Sr diagram. This can result from a disturbance which occurred later than 4.45 Ga ago. However, it is shown that if samples of sufficient size were analyzed, there meteorites could plot on the eucrite isochron and are thereby in agreement with a genetic relation between eucrites, howardites and diogenites. The age of eucrite differentiation from diogenites has been computed using data from the two families yielding an age of 4.47±0.1Ga(2σ) (λ=1.42×10?11a?1), the initial87Sr/86Sr ratio being BABI.  相似文献   

9.
The87Sr/86Sr ratio of seawater strontium (0.7091) is less than the87Sr/86Sr ratio of dissolved strontium delivered to the oceans by continental run-off (～0.716). Isotope exchange with strontium isotopically lighter oceanic crust during hydrothermal convection within spreading oceanic ridges can explain this observation. In quantitative terms, the current87Sr/86Sr ratio of seawater (0.7091) may be maintained by balancing the continental run-off flux of strontium (0.59 × 1012 g/yr) against a hydrothermal recirculation flux of 3.6 × 1012 g/yr, during which the87Sr/86Sr ratio of seawater drops by 0.0011. A concomitant mean increase in the87Sr/86Sr ratio of the upper 4.5 km of oceanic crust of 0.0010 (0.7029–0.7039) should be produced. This required87Sr enrichment has been observed in hydrothermally metamorphosed ophiolitic rocks from the Troodos Massif, Cyprus.The post-Upper Cretaceous increase in the strontium isotopic composition of seawater(～0.7075–0.7091) covaries smoothly with inferred increase in land area. This suggests that during this period the main factor which has caused variability in the87Sr/86Sr ratio of seawater strontium could have been variation in the magnitude of the continental run-off flux caused by variation in land area. Variations in land area may themselves have been partly a consequence of variations in global mean sea-floor spreading rate.  相似文献   

10.
Measurements of the oxygen fugacity (?O2) as a function of temperature (T) were made on an interior bulk sample of the cumulate achondrite, Angra dos Reis. Data clustered between the?O2-T relationship of the iron-wüstite assemblage and 1.2 log atm units above iron-wüstite. Interpretation of the data indicates that, throughout most of the cooling history of the meteorite, ?O2 values were defined by equilibria involving iron-bearing species at values close to the ?O2 of the assemblage iron-wüstite. Measured ?O2 data are compatible with crystallization and cooling at pressures greater than 50 bars.  相似文献   

11.
A modified AEI-IM20 ion microprobe has been used to measure87Sr/86Sr ratios in carbonates. A suite of carbonates with varying major elements (Ca, Mg, Fe, Mn) was studied at low and high (M/ΔM ? 3000) mass resolution to determine the types and intensities of molecular species isobaric with Sr peaks; Sr data collected at low mass resolution must be corrected for Ca2 and CaMgO species. Rb/Sr ratios are extremely low, and correction for87Rb is not required (< 0.1‰ of87Sr).Usable Sr isotopic data may be obtained from calcite givenSr≥ 400ppm, and for Sr > 5000 ppm a precision of ～ ± 1‰ (± 0.0007) in87Sr/86Sr (2σ mean) can be achieved under optimum conditions. The corrections for Ca2 and CaMgO are smaller than the within-run precision in calcite, but in dolomite the correction for CaMgO is + 1.5%. Mass fractionation corrections to87Sr/86Sr (based on86Sr/88Sr= 0.1194) are typically +8 to +10‰. Good agreement between ion probe and solid source mass spectrometer results was found for calcites of known Sr isotopic composition: ST4a (～ 400ppm Sr), average ion probe87Sr/86Sr= 0.7267 ± 0.0015, solid source87Sr/86Sr= 0.72680 ± 0.00008 [14]; JCG36 (～ 6000ppm Sr), average ion probe87Sr/86Sr= 0.7056 ± 0.0009, solid source87Sr/86Sr= 0.70588 ± 0.00009 [16]; JCG44 (～ 6000ppm Sr), average ion probe87Sr/86Sr= 0.7057 ± 0.0006, solid source87Sr/86Sr= 0.70540 ± 0.00008 [16]. The ability of the ion microprobe to measure87Sr/86Sr for 10-μm spots in calcite was used: (1) to measure variation in87Sr/86Sr of ～ 0.01 on a centimetre scale in a hydrothermally altered basalt from the Isle of Skye, northwestern Scotland; and (2) to determine the Sr isotopic composition of tiny (< 35 μm) calcite grains in a veined mantle lherzolite from Bultfontein, South Africa. Because of calcite's ubiquitous occurrence in many parageneses this technique may offer many opportunities for the measurement of fine scale heterogeneities in87Sr/86Sr.  相似文献   

12.
K, Rb, Sr contents and Sr isotopic composition are reported for (1) the coexisting silicate minerals of a spinel lherzolite xenolith, (2) the whole rock xenolith, and (3) the host alkali basalt from the Pleistocene-Recent San Quintin volcanic field in Baja California. The data also include major element chemistry of the four mineral phases of the xenolith. The olivine-spinel and pyroxene thermometers indicate that the last temperature of equilibration of the xenolith in the upper mantle was about 1100°C.K, Rb, and Sr abundances are extremely low in the minerals of the xenolith, in contrast with the general enrichment of Ca, Al, Fe and Na in the whole xenolith. Furthermore, the abundance levels and the87Sr/86Sr ratios of the minerals are greatly lowered by surface washing of the minerals in2N cold HCl for three minutes. It is suggested that this is due to grain boundary and surface contamination of the minerals which took place in the upper mantle by a vapor phase deposition, prior to the inclusion of the xenolith in the basalt. The source of the vapor phase must have a87Sr/86Sr ratio greater than 0.7070, the highest measured ratio in the unwashed orthopyroxene. Sr in the host alkali basalt has a87Sr/86Sr ratio of 0.7031, unrelated to the grain boundary material.The acid-washed minerals, the unwashed minerals and the acid-washed whole rock xenolith show a scatter on a87Rb/86Sr versus87Sr/86Sr diagram. However, the surface-washed minerals and the whole rock alone define a straight-line relationship with a positive slope, which corresponds to an age3.4 ± 0.3AE(2σ) for the rock and an initial87Sr/86Sr of0.70057 ± 0.0004 (2σ). The age of 3.4 AE for the lherzolite is interpreted as its last involvement in a small degree of partial melting and the consequent extreme depletion of the large ion lithophile elements from the constituent minerals.  相似文献   

13.
New strontium isotopic data of calc-alkaline Pliocene-to-Quaternary lavas (southern Peru) confirm their anomalous isotopic composition compared to those of calc-alkaline rocks from active margins where continental crust is not involved. Gradual enrichment of radiogenic Sr occurs during fractional crystallization of calc-alkaline magma. The variation of the isotopic composition of these lavas as well as87Sr/86Sr versus 1/(86Sr) diagrams form the basis for a model involving processes of fractional crystallization combined with mixing and addition of radiogenic Sr that originated in continental crust and was transported by a fluid phase.  相似文献   

14.
Tholeiitic basalts and sulfide deposits from the “Cyana” and “Alvin” diving programs (RITA project) on the East Pacific Rise were analyzed for Pb and Sr isotopes. The basalt data plot within the field defined previously by other East Pacific Rise basalts (206Pb/204Pb: 18.35–18.58;207Pb/204Pb: 15.48–15.53;208Pb/204Pb: 37.8–38.1;87Sr/86Sr: 0.7022–0.7025). Pb, U and Sr contents (～0.5, ～0.05 and ～110 ppm, respectively) and μ values (～6) are typical of MORB, whereas Th/U ratios (～3.5) are significantly higher.The Pb isotopic ratios of the sulfide samples are very homogeneous (206Pb/204Pb～18.47207Pb/204Pb～15.49208Pb/204Pb～37.90), and plot in the middle of the basalt field. This indicates that (1) the sulfide Pb was derived from the basaltic crust without any significant contribution from either seawater or hemipelagic sediments, and (2) the solutions from which the sulfides were deposited had uniform Pb isotopic composition. The Pb contents of three sulfide samples is relatively high (170–1310 ppm).The Sr contents of five sulfide samples are widely scattered from 12 to 210 ppm, with87Sr/86Sr ratios intermediate between basaltic and seawater values (0.70554±0.00005 to0.70795±0.00011). Leaching experiments show that both basalt-derived Sr and seawater Sr were present in the solutions which deposited the sulfides. In some cases, Sr was also adsorbed from seawater onto the sulfides following their deposition. Basalt-derived Sr and seawater Sr are also present in associated non-sulfide phases.  相似文献   

15.
Abstract Interstitial pore waters from Ocean Drilling Program Site 1150, where ～1200 m of sub‐sea‐floor sediment from the upper Japan Trench forearc were recovered, were analyzed for element concentrations and Cl, Sr and B isotopes. Although chlorinity showed profound down‐hole freshening to values as low as ～310 mm (0.55 × seawater) in the deeper part of the claystone‐dominated succession, both Sr and B concentrations showed an overall increase. Sr reached concentrations of up to >250 µm (～3.00 × seawater), whereas B‐enrichment was even stronger (3920 µm; i.e. 9.30 × seawater). The strong variations in concentration correspond to fractionation reactions in the deep, tectonically deformed part of the forearc. The heavily fractured portion of Site 1150 (from ～700 m to the total depth of the hole) has two shear zones that very likely act as conduits that expel deep‐seated fluids to the sea floor. These fluids not only showed the strongest freshening of Cl, but were also characterized by low δ37Cl measurements (down to ?1.1‰), the heaviest δ11B measurements (～40–46‰) and the least radiogenic 87Sr/ 86Sr measurements. The profound isotope anomalies together with the excursions in element concentrations suggest that diagenetic processes operate at that depth. These include clay mineral diagenesis, alteration of tephra from the Japan and Izu Arcs, and possibly transformation of biogenic silica from abundant diatoms. Given the strong enrichment of some mobile elements (e.g. Sr, B, Li), enhanced fluid flow through permeable penetrative faults through the forearc (like the shear zones at Site 1150) could be an efficient mechanism for back‐flux of those elements from the deep forearc into the hydrosphere.  相似文献   

16.
A re-investigation of the Rb-Sr isotope systematics of early Archaean granodiorite gneisses from Hebron, northern Labrador, confirms the existence of early Archaean crust in that area, but has failed to corroborate both the high degree of coherence and the high initial87Sr/86Sr reported by Barton [1]. Instead, marked geological scatter is observed in the data for both slabbed gneisses and large bulk samples, a scatter unequivocally due to the development of secondary whole-rock isochrons at ～ 1800 Ma.Regression analysis of the new data gives very large uncertainties in the age and initial87Sr/86Sr because of this secondary disturbance, viz. 3645?470+875 Ma and 0.702+0.005?0.009. We interpret the previous indications of high initial87Sr/86Sr at 3600 Ma as due to chance.The Hebron data cannot be distinguished from similar populations of geologically-disturbed Rb-Sr results from the Uivak I gneisses to the north [5] and the Amîtsoq grey gneisses [6]. This supports a field-based contention that the so-called “Hebron gneisses” are both lithologic and stratigraphic equivalents of the Uivak I gneisses.  相似文献   

17.
87Rb87Sr analysis of the Norton County achondrite has been achieved with special attention to the rubidium analysis. Enstatite crystals and polycrystalline material give an “age” of 4.48 ± 0.04 × 109 years and an initial ratio 87Sr/86SrI= 0.7005 ± 0.0004 (λ = 1.39 × 10?11yr?1, maximum errors). The feldspar component of the meteorite contains about 70% of the strontium and 30% of the rubidium of the whole sample, and does not lie on the isochron. Its model age relative to the strontium initial ratio of Allende is 4.6 × 109 years. The data are consistent with a complex history dealing with an incomplete isotopic reequilibration of the meteorite, 120 m.y. after its formation at 4.6 × 109 years, with an initial ratio similar to that of Allende.  相似文献   

18.
40Ar-39Ar and Rb-Sr ages have been measured on separated minerals from the potassic volcanics of the Roman Comagmatic Region to test the ability of these methods to accurately data Quaternary geological events. The very high K and Rb contents of the Roman magmas present particularly favorable situations in which the very high concentrations of the radioactive nuclides40K and87Rb result in well resolved in situ enrichments of the daughter isotopes despite the very young ages. Six leucite separates contained Ar with very high bulk40/36 ratios (above 1000) and in which the40Ar and the39Ar were very well correlated, yielding well-defined ages averaging3.38±0.08×105 years. Two leucites contained Ar with lower bulk40/36 ratios (～400), and in at least two release steps from these leucites the40Ar/36Ar ratio was significantly lower than atmospheric. Despite the uncertainty in the composition of the trapped component, these two leucites have ages that do not differ significantly from the ages of the other leucites. For the biotites, it was not possible to obtain through stepwise degassing a good separation of in situ radiogenic40Ar from trapped40Ar and therefore the calculated ages are not as precise as those of the leucites. In three cases the biotite age agrees with the age of the cogenetic leucite, but in the remaining two cases discordant ages are obtained, suggesting caution when using biotites as Quaternary age indicators.Rb-Sr measurements on leucite, biotite, and pyroxene separates hand-picked from each of three tuff samples yielded a dispersion in87Sr/86Sr as large as 16 parts in 104 and87Rb/86Sr as high as 218 for leucites, and permitted the determination of internal isochron ages. The ages obtained range from3.8±0.2×105to3.3±0.2×105 years and are in good agreement with the40Ar-39Ar ages on the leucites. The data for each tuff sample yield a well-defined uniform initial87Sr/86Sr. However, different tuffs show small differences in initial87Sr/86Sr pointing to distinct sources or to assimilation of different materials during the extrusion of the tuffs. These measurements demonstrate the possibility of dating Quaternary materials by both the40Ar-39Ar method and the Rb-Sr method. The observation of concordant ages with a precision of a few percent represents a powerful tool in Quaternary stratigraphy.  相似文献   

19.
The Pampean Ranges of northwest Argentina are a basin-and-range tectonic province with a late Precambrian to Paleozoic basement and extensive Miocene-Recent calc-alkaline volcanism. The volcanoes include the large resurgent Cerro Galan caldera, and Recent scoria cones and lava flows. Miocene-Recent volcanic rocks of basalt to dacite composition from the Cerro Galan area exhibit a range of Rb/Sr ratios of 0.043–1.092 and initial87Sr/86Sr ratios of 0.7057–0.7115 with a clear positive correlation between87Sr/86Sr and87Rb/86Sr, indicating an apparent age of ca. 130 Ma. This relationship is interpreted to indicate that the Sr isotope variation in the Cerro Galan volcanic rocks results from mixing of a mantle-derived component with low87Sr/86Sr (<0.7057) and high Sr (>700 ppm) with a crustal component characterized by higher87Sr/86Sr (>0.7115) and lower Sr (<240 ppm). It is concluded that the mixing is best explained as a result of a small degree of selective crustal Sr contamination (ca. 10%) of a range of subsequently erupted magmas produced largely by fractional crystallization within the continental crust. We propose that the mantle-derived end-member is derived by partial melting of sub-Andean mantle with an87Sr/86Sr ratio of ca. 0.704, and that such an Sr isotope ratio characterizes the source region for calc-alkaline volcanic rocks throughout the Andes.  相似文献   

20.
The crystal structure of fassaite from the Angra dos Reis meteorite has been determined by least-squares refinement of three-dimensional X-ray data to anR value of 3.3%. The pyroxene is monoclinic, space groupC2/c, with unit-cell dimensionsa = 9.738(1),b = 8.874(2),c = 5.2827(5)Å, β = 105.89(1)°, andV = 439.1(1)Å3. Average bond lengths are (Si,Al)-O = 1.651, M1-O = 2.061, and M2-O = 2.489Å. The distribution of iron and magnesium between M1 and M2 suggests a temperature of equilibration greater than 1000°C.Electron microprobe analysis of several fassaite grains reveals small but statistically significant variations of (Mg + Si) versus (Al - Ti). The range of fassaite composition may be represented byEn3Hd22TiCpx6(Di53±2CaTs16?2) whereEn=Mg2Si2O6,Hd=CaFeSi2O6,TiCpx=CaTiAl2O6,Di=CaMgSi2O6,CaTs=CaAl2SiO6. Most fassaite analyses calculated on the basis of four cations yielded greater than six anions, suggesting that part of the titanium or chromium might be reduced to Ti3+ or Cr2+.  相似文献   

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