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1.
The Neogene-Quaternary Harrat Rahat volcanic field is part of the major intercontinental Harrat fields in western Saudi Arabia.It comprises lava flows of olivine basalt and hawaiite,in addition to mugearite,benmorite,and trachyte that occur mainly as domes,tuff cones and lava flows.Based on opaque mineralogy and mineral chemistry,the Harrat Rahat volcanic varieties are distinguished into Group I(olivine basalt and hawaiite) and Group II(mugearite,benmorite and trachyte).The maximum forsterite content(~85) is encountered in zoned forsteritic olivine of Group I,whereas olivine of Group II is characterized by intermediate(Fo=50),fayalitic(Fo=25) and pure fayalite in the mugearite,benmorite and trachyte,respectively.The more evolved varieties of Group II contain minerals that show enrichment of Fe2+,Mn2+and Na+that indicates normal fractional crystallization.The common occurrence of coarse apatite with titanomagnetite in the benmorite indicates that P5+becomes saturated in this rock variety and drops again in trachyte.Cr-spinel is recorded in Group I varieties only and the Cr#(0.5) suggests lherzolite as a possible restite of the Harrat Rahat volcanics.The plots of Cr# vs.the forsterite content(Fo) suggest two distinct trends,which are typical of mixing of two basaltic magmas of different sources and different degrees of partial melting.The bimodality of Harrat Rahat Cr-spinel suggests possible derivation from recycled MORB slab in the mantle as indicated by the presence of high-Al spinel.It is believed that the subcontinental lithospheric mantle was modified by pervious subduction process and played the leading role in the genesis of the Harrat Rahat intraplate volcanics.The trachytes of the Harrat Rahat volcanic field were formed most probably by melting of a lower crust at the mantle-crust boundary.The increase in fO2 causes a decrease in Cr2 O3,and Al2 O3,and a strong increase in the proportion of Fe3+and Mg# of spinel crystallizing from the basaltic melt at T ~1200°C.The olivine-pyroxene and olivine-spinel geothermometers yielded equilibrium temperature in the range of 935-1025°C,whereas the range of <500-850°C from single-pyroxene thermometry indicates either post crystallization reequilibrium of the clinopyroxene,or the mineral is xenocrystic and re-equilibrated in a cooling basaltic magma.  相似文献   

2.
Integrated zircon–olivine O–Hf isotope data have been successfully used to unravel the nature of the source mantle for the early Permian post-collisional mafic–ultramafic intrusive rocks in the southern margin of the Central Asian Orogenic Belt in NW China. Olivine crystals with forsterite (Fo) contents varying from 91 to 87 mol% from the Permian Pobei mafic–ultramafic complex in the region yield highly elevated δ18O from 6.0 to 7.2‰. These values are much higher than typical mantle values (~?5.3‰) and are apparently at odds with the mantle-like εNd(t) values of whole rocks (4.9–5.4). Magmatic zircon crystals from troctolite and gabbroic rocks show divergent oxygen and hafnium isotopic compositions: mantle-like εHf(t) values from 5.1 to 11.9 and crust-like δ18O values from 7.6 to 10.1‰. The observed increase of δ18O values from olivine (an early crystallizing phase) to zircon (a late crystallizing phase) in the mafic–ultramafic rocks is generally consistent with an AFC process. However, this process cannot fully explain the highly elevated δ18O values (6–7‰) for the most primitive olivine containing Fo as high as mantle olivine (>?90 mol%) and the mantle-like Hf isotope composition of zircon. Mixing calculation indicates that such highly unusual isotope compositions can be explained by the previous source mantle contamination with subducted sediment-derived melts and slab-derived fluids. Our results show that the combination of zircon O–Hf isotopes and olivine oxygen isotopes is more effective than the data of zircon or olivine alone to distinguish the effect of AFC process from source contamination. The results from this study provide a new line of evidence that the sub-arc mantle is not homogeneous in oxygen isotopes.  相似文献   

3.
The sintering behavior of olivine and olivine-basalt aggregates has been examined at temperatures near 1,300° C. Experimental factors contributing to rapid sintering kinetics and high-density, fine-grained specimens include: (i) the uniform dispersion of basalt throughout the specimen, (ii) a very fine, uniform particle size for the olivine powder, (iii) oxygen fugacities near the high P O2 end of the olivine stability field, and (iv) rapid heating to the sintering temperature. Olivine-basalt specimens prepared from olivine particles coated with a synthetic basalt achieve chemical and microstructural equilibrium more rapidly, as well as produce higher density and finer grain-sized aggregates, than do specimens prepared by mechanical mixing of olivine and natural basalt powders. The grain boundary mobility for olivine, measured for olivine-basalt aggregates which have undergone secondary recrystallization, is on the order of 2×10?15 (m/s)/(N/m2) in the temperature range 1,300–1,400° C. Solution-precipitation (pressure-solution) processes make an important contribution to the development of the microstructure in olivine-basalt aggregates.  相似文献   

4.
结晶分异和同化混染是侵入岩体形成过程中控制岩相成分和分异程度的主导因素。探讨这两个因素在岩浆演化过程中的作用阶段及对矿物成分变化的影响对揭示岩体成因和成矿具有重要意义。新疆北山地区的红石山镁铁-超镁铁岩体钻孔剖面上岩相和各种矿物含量的渐变特征反映了岩浆演化的结晶分异过程。长石中Si、Na、Al、Ca和单斜辉石中Si、Al、Ti、Ca、Mg的系统变化揭示不同程度的同化混染作用。橄榄石和尖晶石对同化混染作用的反应较为迟缓,但其Fo、Ni和Cr#、Ti对新鲜岩浆注入较为敏感。这些矿物化学对岩浆演化过程的推断与全岩成分变化的指示相一致。因此,矿物化学在研究岩浆演化方面将会发挥重要作用。  相似文献   

5.
We report a study of the oxygen isotope ratios of chondrules and their constituent mineral grains from the Mokoia, oxidized CV3 chondrite. Bulk oxygen isotope ratios of 23 individual chondrules were determined by laser ablation fluorination, and oxygen isotope ratios of individual grains, mostly olivine, were obtained in situ on polished mounts using secondary ion mass spectrometry (SIMS). Our results can be compared with data obtained previously for the oxidized CV3 chondrite, Allende. Bulk oxygen isotope ratios of Mokoia chondrules form an array on an oxygen three-isotope plot that is subparallel to, and slightly displaced from, the CCAM (carbonaceous chondrite anhydrous minerals) line. The best-fit line for all CV3 chondrite chondrules has a slope of 0.99, and is displaced significantly (by δ17O ∼ −2.5‰) from the Young and Russell slope-one line for unaltered calcium-aluminum-rich inclusion (CAI) minerals. Oxygen isotope ratios of many bulk CAIs also lie on the CV-chondrule line, which is the most relevant oxygen isotope array for most CV chondrite components. Bulk oxygen isotope ratios of most chondrules in Mokoia have δ18O values around 0‰, and olivine grains in these chondrules have similar oxygen isotope ratios to their bulk values. In general, it appears that chondrule mesostases have higher δ18O values than olivines in the same chondrules. Our bulk chondrule data spread to lower δ18O values than any ferromagnesian chondrules that have been measured previously. Two chondrules with the lowest bulk δ18O values (−7.5‰ and −11.7‰) contain olivine grains that display an extremely wide range of oxygen isotope ratios, down to δ17O, δ18O around -50‰ in one chondrule. In these chondrules, there are no apparent relict grains, and essentially no relationships between olivine compositions, which are homogeneous, and oxygen isotopic compositions of individual grains. Heterogeneity of oxygen isotope ratios within these chondrules may be the result of incorporation of relict grains from objects such as amoeboid olivine aggregates, followed by solid-state chemical diffusion without concomitant oxygen equilibration. Alternatively, oxygen isotope exchange between an 16O-rich precursor and an 16O-poor gas may have taken place during chondrule formation, and these chondrules may represent partially equilibrated systems in which isotopic heterogeneities became frozen into the crystallizing olivine grains. If this is the case, we can infer that the earliest nebular solids from which chondrules formed had δ17O and δ18O values around -50‰, similar to those observed in refractory inclusions.  相似文献   

6.
The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock 87Sr/86Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial 87Sr/86Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An72 plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An43; Mg# 5 clinopyroxene; Fo1 olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V2O5 of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO2 and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO2 and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation  相似文献   

7.
《Chemical Geology》2007,236(1-2):167-179
Unusual mineral structures have recently been found in a sandstone-hosted roll-type uranium deposit in the Middle Jurassic Zhiluo Formation in the Shashagetai deposit, the northern Ordos basin, NW China. The structures possess a chemical composition and crystal structure characteristic of mineral coffinite [(USiO4)1−x(OH)4x], which occurs as nanoparticles with size ranging from 5 to 25 nm. These structures are interpreted to be fossilized microorganisms, based on mineralogical biosignatures including morphology, size, occurrence of biogenic coffinite as nano-crystals, and biological elements such as P. The intimate intergrowth of coffinite with secondary pyrite of bacterial origin, as defined by low δ34S values, and calcite cements with petroleum-derived carbon supports its biogenic origin. Oil inclusions in the host sandstone are characterized by abundant n-alkanes, slightly increased Pr/nC17 and Ph/nC18 ratios, significant amounts of demethylated hopanes and tricyclic terpanes, and the existence of unresolved complex mixtures. These characteristics are interpreted to be a result of mixing of an earlier, heavily degraded oil with a later charged fresh oil; subsequently the oils were slightly degraded. These lines of evidence lead to the proposal that the reduction of sulfate and oxidization of petroleum are likely synchronous with reduction of hexavalent [U(VI)] to tetravalent [U(IV)] uranium by sulfate-reducing bacteria (SRB). The discovery of a natural association of microorganism-like structures, a uranium mineral, and biodegraded petroleum has implications for uranium biomineralization and fossil fuel exploration.  相似文献   

8.
Rubidium-strontium and samarium-neodymium isotopes of lunar meteorite LaPaz Icefield (LAP) 02205 are consistent with derivation of the parent magma from a source region similar to that which produced the Apollo 12 low-Ti olivine basalts followed by mixing of the magma with small amounts (1-2 wt%) of trace element-enriched material similar to lunar KREEP-rich sample SaU 169. The crystallization age of LAP 02205 is most precisely dated by an internal Rb-Sr isochron of 2991 ± 14 Ma, with an initial 87Sr/88Sr at the time of crystallization of 0.699836 ± 0.000010. Leachable REE-rich phosphate phases of LAP 02205 do not plot on a Sm-Nd mineral isochron, indicating contamination or open system behavior of the phosphates. Excluding anomalous phases from the calculation of a Sm-Nd isochron yields a crystallization age of 2992 ± 85 (initial ε143Nd = +2.9 ± 0.8) that is within error of the Rb-Sr age, and in agreement with other independent age determinations for LAP 02205 from Ar-Ar and U-Pb methods. The calculated 147Sm/144Nd source ratios for LAP 02205, various Apollo 12 and 15 basalts, and samples with strong affinities to KREEP (SaU 169, NWA 773, 15386) are uncorrelated with their crystallization ages. This finding does not support the involvement of a common KREEP component as a heat source for lunar melting events that occurred after crystallization of the lunar magma ocean.  相似文献   

9.
甘肃北山黑山岩浆铜镍硫化物矿床橄榄石特征及成因意义   总被引：7，自引：2，他引：5  
黑山铜镍硫化物矿床是近年在甘肃北山发现的大型岩浆铜镍硫化物矿床,含矿岩体主要由含矿橄榄岩相和南部边缘的角闪辉长岩相构成。研究发现含矿岩体中的橄榄石属贵橄榄石(Fo值为81.54~86.87),其w(Ni)介于(801.53~2 703.19)×10-6;利用橄榄辉长岩中最高Fo值和主量元素反演,表明原始岩浆属高镁玄武质岩浆,w(MgO)=11.65%,w(FeO)=10.12%;橄榄石分离结晶模拟计算结果表明,橄榄石结晶过程中伴随有0.12%~0.17%硫化物熔离,深部岩浆房中橄榄石分离结晶程度小于3%,橄榄石与硫化物最小质量比约14∶1;隙间硅酸盐熔浆和硫化物熔浆作用明显,是造成早期结晶橄榄石成分变化的重要原因。  相似文献   

10.
The oxygen-isotope compositions (obtained by laser fluorination) of hand-picked separates of isolated forsterite, isolated olivine and chondrules from the Tagish Lake carbonaceous chondrite describe a line (δ17O = 0.95 * δ18O − 3.24; R2 = 0.99) similar to the trend known for chondrules from other carbonaceous chondrites. The isolated forsterite grains (Fo99.6-99.8; δ18O = −7.2‰ to −5.5‰; δ17O = −9.6‰ to −8.2‰) are more 16O-rich than the isolated olivine grains (Fo39.6-86.8; δ18O = 3.1‰ to 5.1‰; δ17O = −0.3‰ to 2.2‰), and have chemical and isotopic characteristics typical of refractory forsterite. Chondrules contain olivine (Fo97.2-99.8) with oxygen-isotope compositions (δ18O = −5.2‰ to 5.9‰; δ17O = −8.1‰ to 1.2‰) that overlap those of isolated forsterite and isolated olivine. An inverse relationship exists between the Δ17O values and Fo contents of Tagish Lake isolated forsterite and chondrules; the chondrules likely underwent greater exchange with 16O-poor nebular gases than the forsterite. The oxygen-isotope compositions of the isolated olivine grains describe a trend with a steeper slope (1.1 ± 0.1, R2 = 0.94) than the carbonaceous chondrite anhydrous mineral line (CCAMslope = 0.95). The isolated olivine may have crystallized from an evolving melt that exchanged with 16O-poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite. The primordial components of the Tagish Lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups, especially CMs. Its alteration history has its closest affinities to CI carbonaceous chondrites.  相似文献   

11.
The results of experimental studies and examination of variations in major elements, trace elements and Sr isotopes indicate that fractionation, assimilation and magma mixing combined to produce the lavas at Medicine Lake Highland. Some characteristics of the compositional differences among the members of the calc-alkalic association (basalt-andesite-dacite-rhyolite) can be produced by fractional crystallization, and a fractionation model reproduces the major element trends. Other variations are inconsistent with a fractionation origin. Elevated incompatible element abundances (K and Rb) observed in lavas intermediate between basalt and rhyolite can be produced through assimilation of a crustal component. An accompanying increase in 87Sr/86Sr from ∼ 0.07030 in basalt to ∼0.7040 in rhyolite is also consistent with crustal assimilation. The compatible trace element contents (Ni and Sr) of intermediate lavas can not be produced by fractional crystallization, and suggest a magma-mixing origin for some lavas. Unusual phenocryst assemblages and textural criteria in these lavas provide additional evidence for magma mixing. A phase diagram constructed from the low pressure melting experiments identifies a distributary reaction point, where olivine+augite react to pigeonite. Parental basalts reach this point at low pressures and undergo iron-enrichment at constant SiO2 content. The resulting liquid line of descent is characteristic of the tholeiitic trend. Calc-alkalic differentiation trends circumvent the distributary reaction point by three processes: fractionation at elevated pH2O, assimilation and magma mixing.  相似文献   

12.
On the basis of the previous quantitative modeling of trace element behavior in magmatic pro-cesses such as fractional crystallization ,batch melting and magma mixing,the author has shown the covariance relationship between compatible and incompatible elements. The discussion of the covariance relationship among trace elements indicates that the covariance relationship can be used to study the petrogenesis of igneous rocks and that the relationship be-tween compatible and incompatible elements are most important in distinguishing equilibrium process(melting and crystallization )from other magmatic processes, i.e.,the former shows a hyperbola trend on C1^i vs .C1^i diagrams,and however, the fractional process gives an exponent curve and magma mixing presents a straight line .On the other hand ,log C1^i vs .C1^i diagrams are also discussed here. Alkaline rocks from Zijinshan, Lin County, Shanxi Province are taken for example in our study ,indicating that the rocks were derived from batch melting of the upper mantle.  相似文献   

13.
Primitive andesites from the Taupo Volcanic Zone formed by magma mixing   总被引：1，自引：0，他引：1  
Andesites with Mg# >45 erupted at subduction zones form either by partial melting of metasomatized mantle or by mixing and assimilation processes during melt ascent. Primitive whole rock basaltic andesites from the Pukeonake vent in the Tongariro Volcanic Centre in New Zealand’s Taupo Volcanic Zone contain olivine, clino- and orthopyroxene, and plagioclase xeno- and antecrysts in a partly glassy matrix. Glass pools interstitial between minerals and glass inclusions in clinopyroxene, orthopyroxene and plagioclase as well as matrix glasses are rhyolitic to dacitic indicating that the melts were more evolved than their andesitic bulk host rock analyses indicate. Olivine xenocrysts have high Fo contents up to 94%, δ18O(SMOW) of +5.1‰, and contain Cr-spinel inclusions, all of which imply an origin in equilibrium with primitive mantle-derived melts. Mineral zoning in olivine, clinopyroxene and plagioclase suggest that fractional crystallization occurred. Elevated O isotope ratios in clinopyroxene and glass indicate that the lavas assimilated sedimentary rocks during stagnation in the crust. Thus, the Pukeonake andesites formed by a combination of fractional crystallization, assimilation of crustal rocks, and mixing of dacite liquid with mantle-derived minerals in a complex crustal magma system. The disequilibrium textures and O isotope compositions of the minerals indicate mixing processes on timescales of less than a year prior to eruption. Similar processes may occur in other subduction zones and require careful study of the lavas to determine the origin of andesite magmas in arc volcanoes situated on continental crust.  相似文献   

14.
Alkali basaltic rocks from the Southern Highlands, N.S.W., contain oxide phases of both high and low pressure origin. The two phases are readily distinguished using chemical and textural criteria.

Chemical data for low‐pressure Fe‐Ti oxides indicate that oxygen fugacities of the host lavas range from 10‐12.8 to 10‐8 atm at 950° to 1110°C. In most cases, the oxygen fugacities of the individual lava flows appear to be principally a function of temperature and intrinsic chemical equilibria existing at the time of formation of the basaltic liquid. However, some relatively differentiated flows shows a high degree of oxidation due to volatile enrichment with fractionation. Rare glassy flows show dendritic crystallization of Fe‐Ti oxides. Most flows in which abundant olivine was the first phase to be precipitated also contain Cr‐rich spinels associated, and apparently coeval, with the earliest‐crystallizing olivine.  相似文献   

15.
Deep saline aquifers in sedimentary basins are considered to have the greatest potential for CO2 geological storage in order to reduce carbon emissions. CO2 injected into a saline sandstone aquifer tends to migrate upwards toward the caprock because the density of the supercritical CO2 phase is lower than that of formation water. The accumulated CO2 in the upper portions of the reservoir gradually dissolves into brine, lowers pH and changes the aqueous complexation, whereby induces mineral alteration. In turn, the mineralogical composition could impose significant effects on the evolution of solution, further on the mineralized CO2. The high density of aqueous phase will then move downward due to gravity, give rise to “convective mixing,” which facilitate the transformation of CO2 from the supercritical phase to the aqueous phase and then to the solid phase. In order to determine the impacts of mineralogical compositions on trapping amounts in different mechanisms for CO2 geological storage, a 2D radial model was developed. The mineralogical composition for the base case was taken from a deep saline formation of the Ordos Basin, China. Three additional models with varying mineralogical compositions were carried out. Results indicate that the mineralogical composition had very obvious effects on different CO2 trapping mechanisms. Specific to our cases, the dissolution of chlorite provided Mg2+ and Fe2+ for the formation of secondary carbonate minerals (ankerite, siderite and magnesite). When chlorite was absent in the saline aquifer, the dominant secondary carbon sequestration mineral was dawsonite, and the amount of CO2 mineral trapping increased with an increase in the concentration of chlorite. After 3000 years, 69.08, 76.93, 83.52 and 87.24 % of the injected CO2 can be trapped in the solid (mineral) phase, 16.05, 11.86, 8.82 and 6.99 % in the aqueous phase, and 14.87, 11.21, 7.66 and 5.77 % in the gas phase for Case 1 through 4, respectively.  相似文献   

16.
Eight C3 chondrites were examined by the I129Xe129 dating method, to see whether their IXe “ages” (better, initial I129I127ratios ≡ R0) correlate with any other properties. The R0's range from 1.60 × 10?4 to 1.09 × 10?4, corresponding to IXe ages from 2.0 Myr before to 6.7 Myr after Murchison magnetite. Three C3O's (Lancé, Felix, Ornans) have essentially indistinguishable R0's of (1.41 ± 0.13) to (1.17 ± 0.10) × 10?4; the fourth C3O, Warrenton, is undatable owing to homogenization of radiogenic and trapped Xe.Four C3V's show a distinct spread: Vigarano and Grosnaja are highest [R0 = (1.60 ± 0.07) and (1.57 ± 0.14) × 10?4], Mokoia is intermediate, and Kaba is lowest [R0 = (1.38 ± 0.06) and (1.09 ± 0.10) × 10?4]. Literature values for Allende place it near Kaba. These R0's correlate inversely with 4 other properties: I-, Br-, and Cd-content, and olivine composition, both percent mean deviation (PMD) and proportion of iron-poor olivine grains (≤2% fayalite).It is difficult to accept the ～9 Myr spread in R0 as a true age, reflecting either nebular or parent-body processes. This time span is more than an order of magnitude longer than the lifetime of the solar nebula inferred from astronomical evidence. Nor does the degree of thermal metamorphism, which is slight for C3's anyway, correlate with R0. A more plausible interpretation is that the variations in R0 reflect mainly isotopic heterogeneity of iodine. The simplest model that accounts for the correlations with R0 involves mixing of two iodine components in the solar nebula, associated with gas and grains, respectively. The second, of lower I129I127 ratio, predominated at later times and thus became enriched in late-formed meteorites, along with other volatiles such as Cd and Br. The low Fe content and large PMD of olivine may reflect either less metamorphism owing to shallow location in the parent body, or greater reduction of Fe2+ during chondrule formation.  相似文献   

17.
Systematic variations of the mineral chemistry of ferroan anorthosite 60025, which is probably a mixture of closely related materials, suggest that lunar anorthosites formed by strong fractional crystallization and near-perfect adcumulate growth, without trapping liquid. The parent liquid for the most primitive samples was saturated with olivine, plagioclase, pigeonite, and chromite, and evolved to one saturated with plagioclase, pigeonite, high-Ca pyroxene, and ilmenite. The parent liquid also had a very low Na2O content, and combined with strong fractional crystallization this explains the steep trend of anorthosites on an Mg1 (atomic 100 × Mg/(Mg + Fe)) v. An diagram. The mineral and chemical data for other anorthosites are consistent with such a model. Near-perfect adcumulation can occur if growth takes place at the crystal-liquid interface without the physical accumulation of crystals grown elsewhere, and is encouraged by the shifts in phase boundaries with pressure.Anorthosites are probably the remnants of a crust floating on, and crystallizing at the surface of, a magma ocean originally of bulk Moon composition. Mineralogical and trace element data suggest that the parental liquid for the most primitive anorthosites had previously crystallized no plagioclase and some but perhaps very little pyroxene. Hence the bulk Moon appears to be similar to that proposed by Ringwood (1976) but to have even lower alkalis, a subchondritic CaAl ratio, and REE abundances and patterns close to chondritic. The mare basalt sources are not directly complementary to the feldspathic crust, because experimental and trace element data indicate that they are too magnesian and contain too much high-Ca pyroxene. Other crustal rocks, such as the Mg-suite samples, are not closely related to anorthosites; in addition to their chemical differences they have a different crystallization sequence: ol → plag → px, in contrast with the ol → px → plag inferred for anorthosite parental liquid evolution.  相似文献   

18.
Isotopic compositions of He (27 samples) and Ar (7 samples) as well as major and trace elements have been determined in Cenozoic effusive rocks and hosted olivine and pyroxene phenocrysts from northern and central Mongolia. The R = 3He/4He values are within (0.1–13)⋅10−6. Abnormally high R value, 13⋅10−6, atypical of Cenozoic basaltic-volcanism areas in Mongolia, has been first revealed at one of the sites of the Hangayn upland. In composition the rocks under study correspond to tephrites, trachybasalts, and subalkalic andesite-basalts. Analysis of their REE patterns and spidergrams shows that the elements participating in the formation of basalt fields of the Hangayn upland were supplied from the enriched mantle (EM1). These patterns are similar to the OIB ones; (La/Yb)n = 9–53. The R values in the olivine phenocrysts are higher as compared with the pyroxene phenocrysts and the bulk rock compositions of the same samples. Based on the elemental composition of the rocks, their contents of radiogenic 4He and 3He were calculated. The rate of 3He formation is 5.65⋅10−2 at/(g⋅year). The calculated and measured R values in the rock samples point to the presence of trapped mantle helium.  相似文献   

19.
Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock δ18O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO2, Al2O3). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock δ56Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134‰ to 0.217‰ and 0.038‰ to 0.104‰, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions.The δ18O and δ56Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average δ56Fe values of low-Ti basalts (0.073 ± 0.018‰, n = 8) and high-Ti basalts (0.191 ± 0.020‰, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine δ18O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock δ18O values for source regions, 5.57‰ for low-Ti and 5.30‰ for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The δ18O and δ56Fe values of the lunar upper mantle are estimated to be 5.5 ± 0.2‰ (2σ) and 0.085 ± 0.040‰ (2σ), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2‰), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030‰).  相似文献   

20.
Oxygen-isotopic compositions in conjunction with petrologic investigation have been determined for a coarse-grained type B2 Ca, Al-rich inclusion (CAI) from the reduced CV3 Vigarano using secondary ion mass spectrometry. The oxygen-isotopic compositions of minerals are distributed along the carbonaceous chondrite anhydrous mineral line indicating mixing between 16O-rich and 16O-poor nebular components. The O-isotopic heterogeneities among and within minerals in the CAI interior indicate that CAI formation started in an 16O-rich nebula and subsequently continued in an 16O-poor nebula. 16O-rich signatures of melilite and fassaite in the Wark-Lovering rim and of olivine of the accretionary rim indicate that the nebular environment during formation of the CAI returned to an 16O-rich composition after processing in an 16O-poor nebula. These O-isotopic variations in the CAI support multiple heating events in the solar nebula and indicate that the nebular environments fluctuated from 16O-rich to 16O-poor and back to 16O-rich compositions during the formation of a single CAI.  相似文献   

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