Elemental composition of individual chondrules from ordinary chondrites     

T.W. Osborn  R.H. Smith  R.A. Schmitt 《Geochimica et cosmochimica acta》1973,37(8):1909-1942

Sequential non-destructive neutron activation analysis was used to determine the bulk abundance of Fe, Al, Na, Mn, Or, Sc, Co and Ir in approximately 300 individual chondrules from 16 chondrites representing the H (3–5), L4 and LL(3–6) compositional and petrologic classes. For some of the chondrules, Si, Ni, Ca and V were also determined. The histograms indicate that the most probable abundances for lithophilic elements, except Cr, are enriched in the chondrules, while the siderophilic elements are depleted in the chondrules compared to the whole chondrite. Some of the abundance populations, such as Al and Fe, appear to be multimodal. Systematic variations in the composition of the chondrules with increasing petrologic type were observed; most consistent are an increasing Na-Al and Cr-Al correlation, a decreasing Na-Mn correlation, increasing Na abundance and decreasing Na and Mn dispersions among chondrules. The systematic compositional variations with increasing petrologic type are consistent with an increasing approach to equilibrium between chondrules and matrix.Observed elemental correlations are generally consistent with mineralogical controls expected on the basis of geochemical affinities suggested by the mineral assemblages present in the chondrules. However, a prevalent Al-Ir correlation was observed, and is most pronounced for a group of chondrules belonging to a population high in Al. A Sc-Ir correlation was observed. Also, an anti-correlation between chondrule masses and Al (and Ir for some chondrules) content of the chondrules was observed. These correlations are attributed to a fractionation during condensation or chondrule formation and cannot be attributed to classical geochemical similarities i.e. these correlations result from a cosmochemical fractionation. From the compositional evidence, it is suggested that there may be two mechanisms for chondrule production. Some high Al chondrules which exhibit the Al-Ir correlation are believed to be remelted primitive high-temperature aggregates. The elemental composition of the chondrules from the lower Al abundance population is consistent with a preferential remelting of pre-existing silicates.  相似文献   

Calorimetric study of high pressure polymorphism in FeTiO3: Stability of the perovskite phase     

Apurva Mehta  Kurt Leinenweber  Alexandra Navrotsky  Masaki Akaogi 《Physics and Chemistry of Minerals》1994,21(4):207-212

A calorimetric study of the ilmenite and lithium niobate polymorphs of FeTiO₃ was undertaken to assess the high-pressure stabilities of these phases. Ilmenite is known to be the stable phase at ambient pressure, but the lithium niobate form may be a quench phase from a perovskite form which has been previously observed in situ at high pressure.In this study, the lithium niobate phase of FeTiO₃ was synthesized from an ilmenite starting material at 15– 16 GPa and 1473 K, using a uniaxial split-sphere high-pressure apparatus (USSA 2000). The energetics of the ilmenite to lithium niobate transformation were investigated through transposed-temperature drop calorimetry. The heat of back-transformation of lithium niobate to ilmenite was measured by dropping the sample in argon from ambient conditions to a temperature where the transformation occurs spontaneously. In drops made at 977 K, an intermediate x-ray amorphous phase was encountered. At 1273 K, the transformation went to completion. A value of -13.5±1.2 kJ/mol was obtained for the heat of transformation.  相似文献   

Chemical evidence for the genesis of the ureilites, the achondrite Chassigny and the nakhlites   总被引：1，自引：0，他引：1  

W.V Boynton  P.M Starzyk  R.A Schmitt 《Geochimica et cosmochimica acta》1976,40(12):1439-1447

Major element and REE, Cr, Sc, V, Ni, Co, Ir, Au, Sr, Ba abundances were determined in three ureilites and the unique achondrite, Chassigny. Chondritic-normalized REE abundance patterns for the ureilites are v-shaped, similar to pallasites, indicating a possible deep-seated origin. The lithophile trace element abundances and v-shaped REE patterns of the ureilites are consistent with a two-stage formation process, the first of which is an extensive partial melting of chondrite-like matter to yield ureilite precursors in the residual solid, which is enriched in Lu relative to La. The second step consists of an admixture of small and variable amounts of material enriched in the light REE. Such contaminating material may be magmas derived from the first formed melt of partial melting of chondrite-like matter.

In contrast to the ureilites, Chassigny has a chondritic-normalized REE pattern which decreases smoothly from La(1.8 × ) to Lu(0.4 × ) and is parallel to and ˜0.25 × the REE pattern in the nakhlitic achondrites. The composition of the magma from which Chassigny crystallized was highly enriched in the light REE; e.g. chondritic normalized La/Lu ˜ 7. The similarity in the fractionated REE patterns (no Eu anomalies) for the olivine-pyroxene Chassigny and for the nakhlites suggests a genetic relationship.

Siderophile trace element relationships in ureilites can be interpreted by three components: (1) ultramafic silicates enriched in Co relative to Ni, (2) an indigenous metal phase remaining after the partial melting event, and (3) a component of the carbon-rich vein material added after the partial melting.  相似文献   

Co²⁺ as a chemical analogue for Fe²⁺ in high-temperature experiments in basaltic systems     

William E. Coons  John R. Holloway  Alexandra Navrotsky 《Earth and Planetary Science Letters》1976,30(2):303-308

High-temperature experiments on ferromagnesian compositions have been hampered by the rapid absorption of up to 95% of the original iron by platinum and 40% by silver-palladium capsules. Molybdenum or iron capsule materials can decrease or alleviate iron loss, but restrict oxygen fugacities to values near the iron-wustite buffer. Because Co²⁺ is stable at f_O2 =HM and because the solubility of Co in platinum in this range of f_O2 is ～0.05% at temperatures to 1350°C, its use as an analogue for Fe²⁺ is possible. In addition, experiments simulating various Fe²⁺ ratios can be easily performed by choosing appropriate Co²⁺/Fe³⁺ ratios. The cobalt phases produced possess brilliant and distinctive colors which are valuable aids in optical identification of minute phases. The cobalt analogue hypothesis was tested with atmospheric pressure experiments in air on the cobalt analogue of the 1921 Kilauea basalt at three simulated Fe²⁺/Fe³⁺ ratios. The results were compared with those of R.E.T. Hill (1969) for the natural 1921 basalt. The phase relations were the same, with the cobalt system stability fields systematically shifted by about +50°C. Microprobe analysis of olivines and the coexisting glasses indicate that the distribution of Co²⁺ between olivine and melt is independent of temperature and liquid composition. Although the analogue liquid composition differs from the equilibrium composition of the natural system, it may be corrected be employing distribution coefficients (K_D = 0.61 for the Co system; K_D = 0.33 for the Fe system) to closely approximate what the natural system would yield if iron loss did not occur.  相似文献   

Derivation of a heterogeneous lithic fragment in the Bovedy L-group chondrite from impact-melted porphyritic chondrules     

Alan E Rubin  Klaus Keil  G.Jeffrey Taylor  M.-S Ma  R.A Schmitt  D.D Bogard 《Geochimica et cosmochimica acta》1981,45(11):2213-2228

The Bovedy L-group chondrite contains a light-colored poikilitic lithic fragment with olivine, low-Ca pyroxene and kamacite compositions characteristic of porphyritic chondrules from unequilibrated ordinary chondrites. Its texture, compositional similarities to porphyritic chondrules, and low Na₂O, K₂O and P₂O₅ content indicate that the fragment represents a solidified, slightly fractionated impact melt formed from a source that was rich in porphyritic chondrules. The fragment is heterogeneous, with a progressive increase in the bulk $\text{MgO}\text{FeO}$ ratio and in MgO content of olivines and low-Ca pyroxenes across its length. ${}^{39}\text{Ar}{}^{40}\text{Ar}$ analyses of the fragment and host indicate that the meteorite experienced extensive degassing due to reheating. The approximate age of 0.5–0.94 Byr dates the reheating event and not the formation of the lithic fragment or the Bovedy breccia. This reheating event renders the fragment's and host's metallographic cooling rate of ~ 5 C/Myr (through 500°C) imprecise. However, the absence of martensite and the presence of kamacite. zoned taenite and tetrataenite in the fragment and host are consistent with such slow cooling through 500°C. This cooling rate must have resulted from burial of the fragment-host assemblage beneath insulating material on the Bovedy parent body. If the thermal diffusivity (κ) of this overburden was approximately comparable to that of the lunar regolith (10^?4cm²/sec), then the fragment was buried at a depth ≌ 6.5 km; if K = 10^?2 cm²/sec (similar to chondritic material), then the fragment was buried at a depth ?65 km.  相似文献   

Genesis of the Angra dos Reis and other achondritic meteorites     

M.-S. Ma  A.V. Murali  R.A. Schmitt 《Earth and Planetary Science Letters》1977,35(2):331-346

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   

Calculation of subsolidus phase relations in carbonates and pyroxenes     

Alexandra Navrotsky  Douglas Loucks 《Physics and Chemistry of Minerals》1977,1(1):109-127

This formulation of the free energy of mixing in a binary system takes as parameters a Bragg-Williams type cooperative disordering energy and the difference in free energy between different structures for the end-members. Subsolidus phase relations in carbonate systems such as CaCO₃—MgCO₃ and CdCO₃—MgCO₃ are calculated. Similar equations also reproduce the topologies of subsolidus phase relations in pyroxenes, including orthopyroxene-clinopyroxene phase boundaries in the enstatitediopside and ferrosilite-hedenbergite systems, the pigeonite-augite solvus, and the stability field of iron-free pigeonite.  相似文献   

Petrology and geochemistry of alkali basalts and ultramafic inclusions from the palei-aike volcanic field in Southern Chile and the origin of the patagonian plateau lavas     

Milka Alexandra Skewes  Charles R. Stern   《Journal of Volcanology and Geothermal Research》1979,6(1-2)

The Palei-Aike volcanic field, the southernmost unit of the Patagonian plateau lavas, consists of Pleistocene to Recent alkali olivine basalts petrologically and geochemically similar to alkali basalts from diverse tectonic environments. The Palei-Aike basalts have lower SiO₂ and Al₂O₃ and higher TiO₂ and P₂O₃ than published analyses of other Patagonian plateau basalts. Garnet, garnet + spinel-, and phlogopite-peridotites, not reported from other Patagonian plateau lavas or from elsewhere in South America, are common inclusions within Palei-Aike lavas along with spinel-lherzolite, dunite, granulites, and aluminous clinopyroxene megacrysts. The inclusion of these high-pressure assemblages indicates a mantle origin for the Palei-Aike lavas. The Patagonian plateau lavas are located in a tectonic position similar to back-arc basins, and their origin may be a consequence of subduction. The origin and distinct chemical features of the Palei-Aike basalts may be due in part to thermal or mechanical perturbations of the mantle related to changes in plate boundaries and motions in the vicinity of the unstable trench-transform triple junction formed by the South American, Antarctic and Scotia plates.  相似文献   

The petrogenesis of L-6 chondrites: insights from the chemistry of minerals     

David B. Curtis  R.A. Schmitt 《Geochimica et cosmochimica acta》1979,43(7):1091-1103

Major, minor and trace element concentrations have been measured in all the volumetrically significant minerals from the L-6 chondrites Alfianello, Colby (WI) and Leedey. Most of the elements show the compositional uniformity observed in other studies. However, REE, Co and Ni are distinctly different in their abundances and their phase distributions in each of the three meteorites. Such differences must manifest differing physical and chemical conditions involved in the formation of each of the three rocks. The distributions of REE and Ni between various phases are compatible with models which assume equilibrium between presently coexisting minerals. Such a model demonstrates how plausible differences in temperature and Ni content of metallic minerals could produce the observed differences in Ni abundances in olivine and clinopyroxene from each of the three chondrites.REE are too abundant in the minerals to have been produced by crystal-liquid fractionation from a magma of whole rock composition.Alkali element abundances are incompatible with a model of closed system equilibration of the meteorites. Rb is underabundant in feldspar relative to Na when compared to the equilibrium model predictions. A portion of the Cs seems to be included in feldspar by diadochic substitution, however, significant amounts are randomly associated with other minerals, perhaps by incorporation into lattice defects and at interstices. Partial loss of this nonfeldspathic Cs may be responsible for the large range of Cs abundances observed in chondrites.  相似文献   

Empirical estimation of magnetite/liquid distribution coefficients for some transition elements   总被引：2，自引：0，他引：2  

W. P. Leeman  M. -S. Ma  A. V. Murali  R. A. Schmitt 《Contributions to Mineralogy and Petrology》1978,65(3):269-272

Magnetite/liquid distribution coefficients have been calculated that are consistent with observed variations in contents of V, Sc, Cr, and Ti in lavas from Craters of the Moon lava field, Idaho. In particular, our average distribution coefficient for V (27±8) is in close agreement with that determined experimentally at the temperature and oxygen fugacity appropriate for crystallization of these lavas. Although this value is not very precise, it shows that V contents of at least some calcalkaline series are consistent with generation of andesitic and related magmas from basaltic parental magmas by crystal fractionation, involving removal of appreciable amounts of magnetite.Current address: Geology Department, Rice University. Houston, Texas 77001, USA  相似文献