Compositional Zoning in Pyroxenes from Lunar Rock 12021, Oceanus Procellarum
Compositional Zoning in Pyroxenes from Lunar Rock 12021, Oceanus Procellarum
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月岩 12021（Oceanus Procellarum）中辉石的成分分区
DOI：
10.1093/petrology/12.3.439
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发表时间：
1971
影响因子：
3.9
通讯作者：
["F. R. Boyd
中科院分区：
文献类型：
--
作者：
["F. R. Boyd
对阿波罗12号岩石样品12021中的辉石斑晶的电子探针研究揭示了异常多样的分带现象。这些斑晶具有相对均匀的鸽质岩心，并被富钙辉石包裹；地幔从岩心-地幔边界的约20%的fesio3向外分带至斑晶边缘的70%的fesio3。正常分区的特征在每个增长部门是不同的。描述了在（110）、（100）、（010）和与c平行的四种不同的增长趋势。所有部门的正常分区都受到明显但不稳定的振荡影响。月球辉石的正常分带通常比陆地辉石更广泛，可能是因为月球熔岩比陆地玄武岩更具流动性一个数量级。月球熔岩中的湍流可能造成了一些振荡效应。辉石中辉石和鸽石结晶的循环性质可能是动力学成因；可以想象，这可能反映了奥辉鸽质包晶的存在。很可能需要进一步了解高温下鸽子石的晶体结构，然后才能制定结构模型，令人满意地解释扇形分区的细节。粗粒焦铁铁矿和亚钙铁辉铁矿在12021地层形成的最后阶段以明显的“平衡”状态结晶。然而，相对于铁辉石+铁辉石+铁辉石的组合，焦铁辉石的结晶可能是亚稳的。要证实这一点，还需要有关镁取代对焦铁酸盐稳定性影响的数据。我们认为12021岩石中斑晶的鸽质岩心是在静止条件下结晶的，可能是在月球表面以下的岩浆室中，而强烈分带的辉长岩幔是在熔岩侵位的过程中结晶的。
Electron probe study of the pyroxene phenocrysts in rock 12021 of the Apollo 12 sample reveals an extraordinary variety of zoning phenomena. These phenocrysts have cores of relatively homogeneous pigeonite which are mantled by Ca-rich pyroxenes; the mantles are zoned outward from about 20 mol per cent FeSiO3at the core-mantle boundaries to 70 mol per cent FeSiO3at the margins of the phenocrysts. The character of the normal zoning differs in each growth sector. Four different trends characterizing growth on (110), (100), (010), and parallel to c are described. Pronounced but erratic oscillatory effects are impressed on the normal zoning in all sectors.Normal zoning in lunar pyroxenes is generally more extensive than in terrestrial pyroxenes, probably because the lunar lavas were an order of magnitude more fluid than terrestrial basalts. Turbulent flow in lunar lavas may have caused some of the oscillatory effects. The cyclic nature of augite and pigeonite crystallization in these pyroxenes is probably kinetic in origin; conceivably it could reflect the presence of an augite-pigeonite peritectic. It is likely that further knowledge of the crystal structure of pigeonite at high temperatures will be required before a structural model can be formulated that will satisfactorily explain the details of the sector zoning.Coarse-grained pyroxferroite and subcalcic ferroaugite have crystallized in apparent ‘equilibrium’ in the final stages of formation of the groundmass in rock 12021. Nevertheless, the crystallization of the pyroxferroite was probably metastable relative to the assemblage ferroaugite+ fayalite+ tridymite. Data on the effect of Mg substitution on the stability of pyroxferroite will be needed to establish this point.We suggest that the pigeonite cores of the phenocrysts in rock 12021 crystallized under quiescent conditions, probably in a magma chamber below the lunar surface, and that the strongly zoned augite mantles crystallized during emplacement of the lava on the surface.