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1.
We used neutron activation to characterize the metal of 33 main-group pallasites (PMG), widely held to be samples of a core-mantle interface. Most PMG cluster in a narrow range of metal and silicate compositions, but 6 are assigned to an anomalous subset (PMG-am) because of their deviant metal compositions, and 4 others to another anomalous subset (PMG-as) because of their appreciably higher olivine Fa contents. Metal compositions in all PMG are closely related to those in evolved IIIAB irons, and are generally consistent with their formation in the IIIAB parent asteroid. On element-Au diagrams for incompatible elements the normal PMG plot near an extrapolation of IIIAB trends to higher Au concentrations. On element-Au plots of compatible elements such as Ir or Pt the loci of PMG spread out over a broader region explainable by mixing of evolved IIIAB magma with early-crystallized core or mantle-residue solids.Two features of PMG require special models: (1) Ga and Ge contents are generally high (≈1.5×) compared to the IIIAB-based mixing model: and (2) the FeO/(FeO + MgO) ratios span a surprisingly wide range, from 0.11-0.14 in normal PMG to 0.16 to 0.18 in PMG-as This range is larger than expected in a cumulate layer at the base of a mantle. We suggest that both features may be related to the interaction of PMG precursors with a highly evolved magmatic gas phase, and that some or all of these anomalies may have resulted from vapor deposits in voids near the core-mantle interface.An important boundary condition for understanding the detailed PMG origin at the core-mantle interface is the large difference between the solidus temperature of Fa11 olivine (≈2000 K) and the liquidus temperature of an evolved IIIAB melt containing >100 mg/g S and some P (≈1600 K). Following the mixing event that formed the PMG it is therefore reasonable that there would have been olivine rubble floating on top of the IIIAB-like magma, but with appreciable void space present just above the upper level reached by the magma. These voids would have contained gases released from the magma during its flow into the PMG region. We suggest that Ga and Ge, the two most volatile siderophiles in our element set, were added to PMG metal from the magmatic gas. We also suggest that the magmatic gas was oxidizing and that the PMG having high olivine fayalite contents formed in regions where the ratio of void to olivine was high, and that some metallic Fe was oxidized and entered the olivine (or the phosphoran olivine). In support of the latter idea is the observation that both Ni and Co are elevated in the PMG-as (Fa≥16) compared to values predicted by IIIAB trends.We analyzed two Eagle-Station pallasites (PES); after correction for weathering effects in Cold Bay, its composition is found to closely resemble that of Eagle Station but to represent a more evolved composition (i.e., lower Ir, higher Au). Vermillion and Yamato 8451 have been called pyroxene pallasites but have metal compositions (unrelated to those of the PMG or PES) that are too different from each other to even allow assignment to the same grouplet.  相似文献   

2.
Using improved analytical techniques, which reduce the Re blanks by factors of 8 to 10, we report new Re-Os data on low Re and low PGE pallasites (PAL-anom) and IIIAB irons. The new pallasite samples nearly double the observed range in Re/Os for pallasites and allow the determination of an isochron of slope 0.0775 ± 0.0008 (T = 4.50 ± 0.04 Ga, using the adjusted λ187Re = 1.66 × 10−11 a−1) and initial (187Os/188Os)0 = 0.09599 ± 0.00046. If the data on different groups of pallasites (including the “anomalous” pallasites) are considered to define a whole-rock isochron, their formation would appear to be distinctly younger than for the iron meteorites by ∼60 Ma. Five IIIAB irons (Acuna, Bella Roca, Chupaderos, Grant, and Bear Creek), with Re contents ranging from 0.9 to 2.8 ppb, show limited Re/Os fractionation and plot within errors on the IIAB iron meteorite isochron of slope 0.07848 ± 0.00018 (T = 4.56 ± 0.01 Ga) and initial (187Os/188Os)0 = 0.09563 ± 0.00011. Many of the meteorites were analyzed also for Pd-Ag and show 107Ag enrichments correlated with Pd/Ag, requiring early formation and fractionation of the FeNi metal, in a narrow time interval, after injection of live 107Pd (t1/2 = 6.5 Ma) into the solar nebula. Based on Pd-Ag, the typical range in relative ages of these meteorites is ≤10 Ma. The Pd-Ag results suggest early formation and preservation of the 107Pd-107Ag systematics, both for IIIAB irons and for pallasites, while the younger Re-Os apparent age for pallasites suggests that the Re-Os system in pallasites was subject to re-equilibration. The low Re and low PGE pallasites show significant Re/Os fractionation (higher Re/Os) as the Re and PGE contents decrease. By contrast, the IIIAB irons show a restricted range in Re/Os, even for samples with extremely low Re and PGE contents. There is a good correlation of Re and Ir contents. The correlation of Re and Os contents for IIIAB irons shows a similar complex pattern as observed for IIAB irons (Morgan et al., 1995), and neither can be ascribed to a continuous fractional crystallization process with uniform solid-metal/liquid-metal distribution coefficients.  相似文献   

3.
We report structural and compositional data leading to the classification of 41 iron meteorites, increasing the number of classified independent iron meteorites to 576. We also obtained data on a new metal-rich mesosiderite and on two new iron masses that are paired with previously studied irons. For the first time in this series we also report concentrations of Cr, Co, Cu, As, Sb, W, Re and Au in each of these 44 meteorites. We determined 7 of these elements (all except Sb) in 30 previously studied ungrouped or unusual irons, and obtained Cu data on 104 irons, 21 pallasites, and 3 meteorite phases previously studied by E. Scott. We show that Cu possesses characteristics well suited to a taxonomic element: a siderophile nature, a large range among all irons, and a low range within magmatic groups. For the first time we report the partial resolution of the C-rich group IIIE from its populous twin group IIIAB on element-Ni diagrams other than Ir-Ni. Cachiyuyal previously classified ungrouped and Armanty (Xinjiang) previously classified IIIAB are reclassified IIIE. Despite the addition of 3 new irons and the reanalysis of 3 previously studied irons the members of the set of 15 ungrouped irons having very low Ga (<3 μg/g) and Ge (<0.7 μg/g) contents remain individualists. The same is generally true for irons having 100 ≤ Ni ≤ 180 mg/g and compositional similarities to IIICD, but A80104 increases the Garden Head trio to a quartet. Algoma is reclassified from ungrouped to IIICD-an and Hassi-Jekna and Magnesia from IIICD to IIICD-an. The metal of Horse Creek and Mount Egerton is compositionally closely related to metal from EH chondrites. We suggest that the P-rich Bellsbank trio irons formed in the IIAB core in topographic lows filled with an immiscible, P-rich second liquid.  相似文献   

4.
We have determined metallographic cooling rates below 975 K for eight main group (MG) pallasites from Ni profiles across taenite lamellae of known crystallographic orientation in metallic regions with Widmanstätten patterns. Comparison with profiles generated by modeling kamacite growth gave cooling rates ranging from 2.5 to 18 K/Myr. Relative cooling rates were also inferred from the sizes of cloudy zone particles in 28 MG pallasites (86-170 nm) and tetrataenite bandwidths in 20 MG pallasites (1050-2170 nm), as these parameters are positively correlated with each other and negatively correlated with the metallographic cooling rates. These three different techniques show that MG pallasites cooled below 975 K at significantly diverse rates. Since samples from the core-mantle boundary should have indistinguishable cooling rates, MG pallasites could not have cooled at this location. Group IIIAB irons, which were previously thought to be core samples from the MG pallasite body, have faster cooling rates (∼50-350 K/Myr) and smaller cloudy zone particle sizes and tetrataenite bandwidths. This shows that IIIAB irons cooled faster than MG pallasites and could not plausibly be from the same body. The absence of related iron meteorites and achondrites and our thermal constraints suggest that MG pallasites cooled at diverse depths in a pallasitic body consisting of well-mixed olivine and metallic Fe-Ni. Such a body may have formed during an impact on a differentiated asteroid or protoplanet that mixed olivine mantle fragments with residual Ir-poor molten metal from the outermost part of a core that chemically resembled the IIIAB core and was ∼80% fractionally crystallized. Separation of the solid core and most of the associated mantle may have resulted from a grazing hit-and-run impact with a larger protoplanet or asteroid. Thermal calculations suggest that the radius of the pallasitic body was 400 km but the likely presence of a regolith would reduce this estimate considerably.  相似文献   

5.
An improved computer simulation program has been developed and used to re-measure the metallographic cooling rates of the IIIAB irons, the largest iron meteorite chemical group. The formation of this chemical group is attributed to fractional crystallization of a single molten metallic core during solidification. Group IIIAB irons cooling rates vary by a factor of 6 from 56 to 338 °C/My. The cooling rate variation for each meteorite is much smaller than in previous studies and the uncertainty in the measured cooling rate for each meteorite is greatly reduced. The lack of correction for the orientation of the kamacite-taenite interface in the cooling rate measurement of a given meteorite in previous studies not only leads to large cooling rate variations but also to inaccurate and low cooling rates. The cooling rate variation with Ni content in the IIIAB chemical group measured in this study is attributable, in part, to the variation in nucleation temperature of the Widmanstatten pattern with Ni content and nucleation mechanism. However, the factor of 6 variation in cooling rate of the IIIAB irons is hard to explain unless the IIIAB asteroidal core was exposed or partially exposed in the temperature range in which the Widmanstatten pattern formed. Measurements of the size of the island phase in the cloudy zone of the taenite phase and Re-Os data from the IIIAB irons and the pallasites make it hard to reconcile the idea that pallasites are located at the boundary of the IIIAB asteroid core.  相似文献   

6.
王梦玺  王焰 《矿床地质》2012,31(2):179-194
扬子地块北缘~635 Ma周庵超镁铁质岩体是一个新发现的含铜镍硫化物矿化的隐伏岩体,主要由二辉橄榄岩组成。文章通过研究周庵岩体中橄榄石、铬铁矿和辉石的矿物成分变化探讨了岩浆演化过程和含矿岩体成因。根据岩石的矿物组合和蚀变程度,岩体从上到下分为3个部分:上部绿泥石-蛇纹石化二辉橄榄岩相带、中部二辉橄榄岩相带和下部绿泥石-角闪石化二辉橄榄岩相带。根据岩体中部带橄榄石和铬铁矿的成分,计算得到母岩浆的Mg#值为0.63,MgO/FeO摩尔比值为1.72,w(Al2O3)为10.2%~11.7%,w(Ni)为476×10-6,说明其为高镁玄武质岩浆;岩体中部带原生铬铁矿和粒间相铬铁矿核部的Cr2O3和Al2O3呈正相关关系,说明铬铁矿与粒间硅酸盐熔体发生了平衡交换,铬铁矿的高w(TiO2) 和Cr#值与拉张环境中层状岩体的铬铁矿特征一致;根据辉石温压计得到岩体中部单斜辉石和斜方辉石的共结温度为1 017~1 077℃,压力为(3.6~4.5)×108 Pa,暗示形成岩体的浅部岩浆房深度约为12 km。岩体上部和中部带的橄榄石Fo值大部分集中在80 mol%~85 mol%,w(Ni)介于2 255×10-6~4 455×10-6,说明这些橄榄石是从没有经过强烈分离结晶和硫化物熔离的岩浆中结晶出来的。岩体下部带橄榄石的Fo值(67 mol%~68 mol%)和w(Ni) (1 500×10-6~2 000×10-6)都低于岩体上部和中部带的橄榄石相应值,说明岩体下部带的橄榄石可能形成于演化程度较高、并经历了硫化物熔离的岩浆。因此,笔者认为周庵岩体是由相对原始的和演化了的高镁玄武质岩浆两期侵位形成的。  相似文献   

7.
Pallasites are highly differentiated meteorites and provide a unique sample from the deep interiors of solar system parent bodies. They contain evidence of the former existence of one or more residual melts. Olivine is a major phase. Its primary shape is rounded; the angular crystals in many pallasites are secondary. Tubular inclusions are widespread. They perhaps are the residence of CO2, released during laboratory heating experiments. Phosphoran olivine, a new variety of olivine containing 4–5 wt% P2O5, occurs in a few pallasites. Its Fe/Mg ratio is apparently independent of the host olivine composition.Pyroxene (not previously described from pallasites) occurs in symplectic intergrowths in seven meteorites. Compositionally, it lies in the gap between pyroxenes in chondrites and most irons. There are two groups: Fs11.6 ± 0.2 and Fs16.7 ± 0.2 The pyroxene contains exceptionally low Ca (< 0.1–0.2 wt%) and there is an indication of an inverse relation between Fe and Ca.Modal analyses and density measurements were made on all available specimens and bulk compositions were calculated. The ‘average’ pallasite contains 65 vol. % olivine and 50.5 wt % total Fe. Many of the densities of pallasites cluster around that calculated for close-packed olivine.Pallasites are exotic cumulates. Their textures resemble terrestrial cumulates, as does the presence of olivine and chromite. The metal texture resembles a solidified intercumulus liquid. Those pallasites containing olivine in excess of close-packing were subjected to adcumulus growth, thereby also explaining the widespread mutual borders.There is abundant evidence of deformation. For olivines this includes their fragmental shape and kink banding. Troilite formed a eutectic-like melt with kamacite: pieces of spalled olivine and schreibersite were injected into and captured by this melt. Troilite polycrystallinity resulted from the deformation. This deformation occurred while the pallasites were still deeply buried, resulting in incipient spheroidization of olivine fragments, including the formation of elongate, rounded crystals. A later, lower temperature deformation disrupted plessite.Pallasites formed in multiple parent bodies by processes that recurred in several places within the solar system, as shown by the mineralogical and textural similarities between pallasites that differ in their isotopic and trace element compositions. Type IIIB irons still seem the most likely associated meteorites.Two new pallasites, Dora and Rawlinna, are described briefly.  相似文献   

8.
叶大年 《地质科学》1980,15(1):1-10
现在岩石学、矿床学的实验一般用样品是几十毫克、甚至十几毫克,其目的是要精确化。但是许多事实说明,这种精密的实验工作并不能完全解决矿床的成因。要解决矿床成因的问题,至少必须回答下面三个问题:(1)有用金属矿物是在什么条件下形成的;(2)有用金属矿物是如何富集为矿石的;(3)矿石如何具有一定的量,并达到经济上可开采的要求。实验岩石学和实验矿床学的资料一般只说明了第一个问题。  相似文献   

9.
Calc-alkaline andesites and olivine tholeiitic basalts are widely distributed on Shodo-Shima island, southwest Japan. The Fo content of olivine phenocrysts in the andesite is higher than in the basalt. The primary magma of the andesite, estimated on the basis of the olivine fractional crystallization model, is not basaltic but andesitic. The basalt contains both chromite and titanomagnetite as inclusions in olivine phenocrysts, while only chromite appears in the andesite. The Cr content of chromite in the andesite is higher than in the basalt. These facts again indicate that the andesite cannot be a fractionation product of the basalt, and that andesitic and basaltic primary magmas were generated independently.  相似文献   

10.
The compositional variation of chromite and associated olivine in chromite-rich and chromitepoor cumulus layers of the Panton Sill is described and a diffusion-controlled crystallization mechanism is proposed to explain this variation. By this mechanism, chromite initially precipitates with a fairly uniform composition, irrespective of the relative proportions of coprecipitating olivine and chromite, and is modified by continued growth during the postcumulus stage. The effect of postcumulus overgrowth of chromite, K d =(Mg/Fe2+)liquid/(Mg/Fe2+) chromite6, is to deplete the surrounding magma in chromium and decrease Fe2+ relative to Mg such that a chemical gradient exists between the overlying magma, through which the cumulus grains settled, and the magma in contact with settled chromite grains near the magma/crystal pile interface. Postcumulus equilibration of olivine and chromite with the surrounding magma results in higher Mg/(Mg + Fe2+) ratios of both olivine and chromite and higher Al content of chromite. The extent of this postcumulus modification is directly related to the proportion of chromite to olivine in a particular layer. This model can be extended to stratiform intrusions elsewhere in which chromite coprecipitates with olivine, orthopyroxene or plagioclase and displays similar compositional trends.  相似文献   

11.
Group IIAB is the third largest group of iron meteorites and the second largest group that formed by fractional crystallization; many of these irons formed from the P-rich portion of a magma consisting of two-immiscible liquids. We report neutron-activation data for 78 IIAB irons. These confirm earlier studies showing that the group has the largest known range in Ir concentrations (a factor of 4000) and that slopes are steeply negative on plots of Ir vs. Au or As (or Ni). High negative slopes imply relatively high distribution coefficients for Ir, Au, and As (but, with rare exceptions, remaining less than unity for the latter). IIAB appears to have had the highest S contents of any magmatic group of iron meteorites, consistent with its high contents of other volatile siderophiles, particularly Ga and Ge. Large fractions of trapped melt were present in the IIAB irons with the highest Au and As and lowest Ir contents. As a result, when these irons crystallized, the DAu and DAs values can, with moderate accuracy, be estimated to have been roughly 0.53 and 0.46, respectively. These low values imply that the initial nonmetal (S + P) content of the magma was much lower than 170 mg/g, as estimated in earlier studies; our estimate is 75 mg/g. Our results are consistent with an initial P/S ratio of 0.25, similar to the ratio estimated for other magmatic groups. There is little doubt that incompatible S-rich and P-rich metallic liquids were involved during the formation of group IIAB. After 20% crystallization of our assumed starting composition the two-liquid boundary is encountered (at 72 mg/g S and 18 mg/g P). Initially the volume of S-rich liquid is very small, but continued crystallization increased the volume of this phase and decreased its P/S ratio while increasing this ratio in the P-rich liquid. Most crystallization of the IIAB magma would have occurred in the lower, P-rich portion of the core. However, metal was still a liquidus phase at the top of the core and, because both the immiscible liquids would have convected, they may have approached equilibrium throughout the very limited crystallization of the magma recorded in group IIAB. All IIAB irons contain trapped melt, and this melt will have had very different compositions depending on whether the liquid is S-rich (at the outer solid/liquid interface) or P-rich (at the inner interface). The P/S ratio in the melt trapped in the Santa Luzia iron is about 0.6 g/g, consistent with our modeling of Ir-Au and Ir-As trends implying that Santa Luzia formed in the lower, P-rich portion of the core after about 48% crystallization of the magma. Because the liquids were in equilibrium, the point at which immiscibility first occurred is not recorded by a dramatic change in the trends on element-Au diagrams; the main compositional effect is recorded in the P/S ratio of the trapped melt. The high-Au (>0.8 μg/g) irons for which large sections are available all contain skeletal schreibersite implying a relatively high (>0.3 g/g) P/S ratio; none of these irons could have crystallized from the S-rich upper layer of the core.  相似文献   

12.
The metal from 17 mesosiderites has been analyzed for Ni, Ga, Ge and Ir by the techniques of atomic-absorption spectrometry and neutron activation. Most mesosiderite metal samples fall in a narrow compositional range: Ni, 7·0–9·0 per cent; Ga, 13–16 ppm; Ge, 47–58 ppm; and Ir, 2·4–4·4 ppm. Most of those falling outside these ranges belong to Powell's (1971) least-metamorphosed type. Mesosiderite metal falls in the same general composition range as IIIAB irons, IIIE irons, pallasites and H-group chondrite metal. There are distinct differences in detail, however, and firm evidence for a close genetic relationship between any of these groups and the mesosiderites is lacking. Metallic portions of Weekeroo-type irons tend to have slightly higher Ni, Ga, Ge and Ir contents than found in mesosiderite metal, and the two groups tend to form a single trend on all plots. The Weekeroo-type silicates closely resemble mesosiderites in terms of orthopyroxene composition and oxygen-isotope ratio. We interpret these similarities to indicate that the silicate and metallic portions of these two groups are closely related; if the mesosiderite silicates and metal were initially formed in separate parent bodies, these were of similar composition and formed at about the same distance from the Sun.  相似文献   

13.
Chromite deposits in the northern Oman ophiolite: Mineralogical constraints   总被引：1，自引：0，他引：1  
Chromite deposits in the northern Oman ophiolitic complex occur in three structural contexts, i.e., (1) at the base of the cumulate series, (2) in the top kilometer of the mantle sequence, and (3) in the deeper parts of the mantle. Types 1 and 2 are characterized by the diversity of interstitial silicates where in decreasing order of abundance olivine, clinopyroxene, orthopyroxene, plagioclase, and amphibole occur, as opposed to type 3 which contains only olivine. They differ however in ore texture. Similar silicates also occur as euhedral inclusions in chromite crystals, but their proportions are reversed. The composition of the interstitial silicates is comparable to that found in early cumulates. Type-1 and type-2 chromite deposits crystallized from a magma similar to that from which the basal cumulates formed (Al2O3, 15.1–16.1 wt%; FeO/MgO, 0.55–0.60). The type-3 chromites were derived from a magma of much lower Al2O3 content (12.5 wt%). It is considered that they belong to an older episode in the magmatic evolution of the complex.  相似文献   

14.
The exsolutious of diopside and magnetite occur as intergrowth and orient within olivine from the mantle dunite, Luobusa ophiolite, Tibet. The dunite is very fresh with a mineral assemblage of olivine （〉95%） ＋ chromite （1%-4%） ＋ diopside （〈1%）. Two types of olivine are found in thin sections： one （Fo = 94） is coarse-grained, elongated with development of kink bands, wavy extinction and irregular margins; and the other （Fo = 96） is fine-grained and poly-angied. Some of the olivine grains contain minor Ca, Cr and Ni. Besides the exsolutions in olivine, three micron-size inclusions are also discovered. Analyzed through energy dispersive system （EDS） with unitary analytical method, the average compositions of the inclusions are： Na20, 3.12%-3.84%; MgO, 19.51%-23.79%; Al2O3, 9.33%-11.31%; SiO2, 44.89%-46.29%; CaO, 11.46%-12.90%; Cr2O3, 0.74%-2.29%; FeO, 4.26%- 5.27%, which is quite similar to those of amphibole. Diopside is anhedral f＇dling between olivines, or as micro-inclusions oriented in olivines. Chromite appears euhedral distributed between olivines, sometimes with apparent compositional zone. From core to rim of the chromite, Fe content increases and Cr decreases; and A! and Mg drop greatly on the rim. There is always incomplete magnetite zone around the chromite. Compared with the nodular chromite in the same section, the euhedral chromite has higher Fe3O4 and lower MgCr2O4 and MgAI2O4 end member contents, which means it formed under higher oxygen fugacity environment. With a geothermometer estimation, the equilibrium crystalline temperature is 820℃-960℃ for olivine and nodular chromite, 630℃-770℃ for olivine and euhedral chromite, and 350℃-550℃ for olivine and exsoluted magnetite, showing that the exsolutions occurred late at low temperature. Thus we propose that previously depleted mantle harzburgite reacted with the melt containing Na, Al and Ca, and produced an olivine solid solution added with Na^＋, Al^3＋, Ca^2＋, Fe^3＋, Cr^3＋. With temperature d  相似文献   

15.
芬兰科密铬铁矿床是欧洲规模最大的铬铁矿床。对该矿床的地质背景、矿床特征、矿床成因及找矿标志进行了总结。结果表明,该矿床与古元古代层状超镁铁质杂岩体具有密切的时空分布关系,杂岩体内的铬铁矿床具典型的层状堆积特征,矿层延伸稳定。由于Cr/Fe值较低,该矿床矿石品位在世界同类矿床中属偏低水平。同位素年代学证据表明,其形成于古元古代早期(2.44 Ga),是由卡累利阿造山作用诱发多期次岩浆侵入活动并与新太古代基底岩层发生混染作用形成的。科密铬铁矿床属于层状铬铁矿床,其地质特征与中国目前已发现的豆荚状或似层状铬铁矿床存在一定区别。  相似文献   

16.
产于层状镁铁质-超镁铁质岩体中的太和岩浆型Fe-Ti氧化物矿床是峨眉山大火成岩省内带几个超大型Fe-Ti氧化物矿床之一。太和岩体长超过3km,宽2km,厚约1.2km。根据矿物含量和结构等特征,整个岩体从下向上可划分为下部岩相带、中部岩相带、上部岩相带。下部岩相带主要以(橄榄)辉长岩和厚层不含磷灰石的块状Fe-Ti氧化物矿层组成。中部岩相带韵律旋回发育,(磷灰石)磁铁辉石岩主要位于旋回的底部,旋回上部为(磷灰石)辉长岩。上部岩相带主要是贫Fe-Ti氧化物的磷灰石辉长岩。太和中部岩相带磷灰石磁铁辉石岩含有5%~12%磷灰石、20%~35%Fe-Ti氧化物、50%~60%硅酸盐矿物,且硅酸盐矿物与磷灰石呈堆积结构。磷灰石磁铁辉石岩中磁铁矿显示高TiO2、FeO、MnO、MgO,且变化范围与趋势接近于攀枝花岩体。钛铁矿FeO分别与TiO2、MgO显示负相关,而FeO分别与Fe2O3、MnO显示正的相关,且TiO2、FeO、MnO、MgO含量变化较大,这些特征都暗示磁铁矿和钛铁矿是从富Fe-Ti-P岩浆中分离结晶。因此,可以推断太和磷灰石磁铁矿辉石岩形成于矿物重力分选和堆积。太和下部岩相带包裹在橄榄石中磁铁矿含有相对较高Cr2O3(0.07%~0.21%),而中部岩相带包裹在橄榄石中磁铁矿Cr2O3(0.00%~0.03%)显著降低,且这些磁铁矿Cr2O3含量变化与单斜辉石Cr含量和斜长石An牌号呈正相关。这些特征印证了形成中部岩相带的相对演化的富Fe-Ti-P母岩浆可能是源自中部岩浆房的混合岩浆。上部岩相带磁铁矿和中部岩相带顶部少量磁铁矿显示较低Ti+V可能是由于岩浆房中累积的岩浆热液对磁铁矿成分进行了改造。  相似文献   

17.
单斜辉石是坡北镁铁-超镁铁杂岩体的主要造岩矿物之一,在各岩相中普遍存在。本文对岩体各岩相中单斜辉石的矿物化学组成进行了电子探针和LA-ICPMS分析,以探明岩体性质、形成的构造背景及演化过程。分析表明,单斜辉石主要氧化物含量为:SiO21.05%~54.00%、MgO 13.95%~17.64%、CaO 19.87%~23.56%、FeO3.44%~6.58%、TiO20.13%~1.36%、Al2O32.17%~4.21%、Na2O 0.17%~0.50%、Cr2O30.01%~1.27%。岩石判别图解表明寄主岩浆为亚碱性拉斑玄武岩,形成于岛弧环境。单斜辉石的结晶温度范围介于1 141~1 221℃。∑REE和(Ce/Yb)N值表明从(异剥)橄榄岩和橄榄单辉辉石岩到橄榄辉长岩和角闪辉长岩,岩浆结晶作用增强。与岩浆正常结晶演化相比,(异剥)橄榄岩和橄榄单辉辉石岩中单斜辉石的SiO2、Na2O含量有所增大,角闪辉长岩和橄榄辉长岩中单斜辉石的CaO含量略有富集,结合前人研究成果和围岩性质,初步认为成岩过程中,可能发生过地壳同化混染作用,其中大理岩为角闪辉长岩和橄榄辉长岩提供了部分Ca元素,而黑云母石英片岩则为(异剥)橄榄岩和橄榄单辉辉石岩贡献了Si、Na等。(异剥)橄榄岩和橄榄单辉辉石岩具较低的FeO含量,此外黑云母石英片岩混入其中,这可能是引起硫化物熔离的重要因素。  相似文献   

18.
T. Agata 《Lithos》1994,33(4):241-263
The Asama igneous complex comprises layered mafic and ultramafic plutonic rocks exposed over about 500×6000 m in the Mikabu greenstone belt, Sambagawa metamorphic terrain of Mie Prefecture; its margins terminate by faults, and there is no trace of chilled rocks. The exposed layered sequence is about 460 m thick, and includes dunite, plagioclase wehrlite, olivine gabbro and two-pyroxene gabbro. The crystallization sequence of essential cumulus minerals is olivine, followed by plagioclase and clinopyroxene together, and finally the appearance of orthopyroxene. Olivine systematically varies in composition from Fo89 to Fo78 with stratigraphic height in the lower to middle portion of the layered sequence. The composition of clinopyroxene changes from Ca49Mg46Fe5 to Ca40Mg47Fe13 upward in the layered sequence; cumulus orthopyroxene, which occurs at the top of the exposed layered sequence, has a composition of Ca2Mg74Fe24. Cumulus chromite occurs as disseminated grains in peridotitic rocks, and tends to increase its Fe3+/(Cr+Al+Fe3+) ratio with stratigraphic height. The most aluminous chromite [Cr/(Cr+Al) = 0.48] occurs in dunite that crystallized shortly before plagioclase began to separate as an essential phase. The Cr/(Cr+Al) ratio of the most aluminous chromite, coupled with the crystallization order of essential minerals, suggests that the Asama parental magma was moderately enriched in plagioclase and clinopyroxene components in the normative mineral diagram plagioclase-clinopyroxene-orthopyroxene. It was similar to a Hawaiian tholeiite and different from the Bushveld and Great “Dyke” parental magmas that were more enriched in orthopyroxene component; it also differed from mid-oceanic ridge basalts that are more depleted in the orthopyroxene component. The Asama clinopyroxene and chromite show characteristically high TiO2 contents and are also similar to those in Hawaiian tholeiites. The Asama igneous complex probably resulted from the crystallization of a magma of a Hawaiian (oceanic-island) tholeiite composition and formed in an oceanic island regime.  相似文献   

19.
ABSTRACT

A chromite deposit was discovered in the Kudi ophiolite in the Palaeozoic western Kunlun orogenic belt. Chromite forms elongated (<2 m in width) and banded chromitite bodies (<0.1 m in width for each band) in dunite and podiform chromitite bodies (<1.5 m in width) in harzburgite. Dunite is classified into two types. Type I dunite hosting massive and banded chromitites shows low Fo in olivine (88.1–90.9), moderate Cr# [=Cr/(Cr + Al), 0.47–0.56] in chromite, and a positively sloped primitive mantle-normalized platinum group elements (PGE) pattern, suggesting that it is a cumulate of a mafic melt. Harzburgite and type II dunite show olivine with high Fo (>91.1) and chromite with moderate to high Cr# (0.44–0.61), and flat to negatively sloped primitive mantle-normalized PGE patterns, indicating that they are residual mantle peridotite after partial melting. Chromite in all three types of chromitites has relatively uniform moderate values Cr# ranging from 0.43 to 0.56. Massive chromitite contains euhedral chromite with high TiO2 (0.40–0.43 wt.%) and has a positively sloped primitive mantle-normalized PGE pattern, suggesting that it represents a cumulate of a melt. Rocks containing disseminated and banded chromite show overall low total PGE, < 117 ppb, and a negatively sloped primitive mantle-normalized PGE pattern. Chromite grains in these two types of occurrences are irregular in shape and enclose olivine grains, suggesting that chromite formed later than olivine. We suggest that chromite-oversaturated melt penetrated into the pre-existing dunite and crystallized chromite. The oxygen fugacity (fO2 values of chromitites and peridotites are high, ranging from FMQ+0.8 (0.8 logarithmic unit above the fayalite-magnetite-quartz buffer) to FMQ+2.3 for chromitites and from FMQ+0.9 to FMQ+2.8 for peridotites (dunite and harzburgite). The mineral compositions and high fO2 values as well as estimated parental magma compositions of the chromitites suggest that the Kudi ophiolite formed in a sub-arc setting.  相似文献   

20.
The addition of two meteorites to the iron meteorite grouplet originally known as the Bellsbank trio brings the population to five, the minimum number for group status. With Ga and Ge contents in the general “II” range, the new group has been designated IIG. The members of this group have low-Ni contents in the metal and large amounts of coarse schreibersite ((Fe,NI)3P); their bulk P contents are 17-21 mg/g, the highest known in iron meteorites. Their S contents are exceptionally low, ranging from 0.2 to 2 mg/g. We report neutron-activation-analysis data for metal samples; the data generally show smooth trends on element-Au diagrams. The low Ir and high Au contents suggest formation during the late crystallization of a magma.Because on element-Au or element-Ni diagrams the IIG fields of the important taxonomic elements Ni, Ga, Ge and As are offset from those of the IIAB irons, past researchers have concluded that the IIG irons could not have formed from the same magma, and thus that the two groups originated on separate parent bodies. However, on most element-Au diagrams the IIG fields plot close to extensions of IIAB trends to higher Au concentrations.There is general agreement that immiscibility led to the formation of an upper S-rich and a lower P-rich magma in the IIAB core. We suggest that the IIG irons formed from the P-rich magma, and that schreibersite was a liquidus phase during the final stages of crystallization. The offsets in Ni and As (and possibly other elements) may result from solid-state elemental redistribution between metal and schreibersite during slow cooling. For example, it is well established that the equilibrium Ni content is >2× higher in late-formed relative to early-formed schreibersite. It is plausible that As substitutes nearly ideally for P in schreibersite at eutectic temperatures but becomes incompatible at low temperatures.[Wasson J. T., Huber, H. and Malvin, D. J. (2007) Formation of IIAB iron meteorites. Geochim. Cosmochim. Acta71, 760-781] argued that, in the most evolved IIAB irons, the amount of trapped melt was high. The high P contents of IIG irons also require high contents of trapped melt but the local geometry seems to have allowed the S-rich immiscible melt to escape as it formed. The escaping melt may have selectively depleted elements such as Au and Ge.  相似文献   

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