Mare basalts: Crystal chemistry, mineralogy, and petrology
Mare basalts: Crystal chemistry, mineralogy, and petrology
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DOI：
10.1029/rg014i004p00475
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发表时间：
1976-11
影响因子：
25.2
通讯作者：
J. Papike;F. Hodges;A. Bence;M. Cameron;J. M. Rhodes
中科院分区：
文献类型：
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作者：
J. Papike;F. Hodges;A. Bence;M. Cameron;J. M. Rhodes
月壳岩石可分为两类：高地型和月海玄武岩。高地样品的解释是复杂的衍生性质，这是由一系列的结晶，冲击和角砾岩事件。相比之下，月海玄武岩似乎不那么复杂，而且自从它们到达月球表面以来就没有受到影响;因此在这个时候合成似乎是可能的。虽然月海玄武岩只占月球地壳的不到1%，但它们包含了许多关于月球热历史和月球内部性质的信息。现在我们知道，阿波罗和月球任务没有对一套完整的玄武岩进行采样，采样了所有化学和时间上不同的单元。已研究的月海玄武岩的年龄在3.15和3.96戈伊之间。然而，摄影地质学证据（陨石坑计数和陨石坑退化研究）表明，月球上存在年轻至2.5戈伊的玄武岩，而且没有取样。返回的样本可分为两大组：老年高钛组（年龄，3.55-3.85戈伊; TiO 2，9-14重量%）和年轻低钛组（年龄，3.15-3.45戈伊; TiO 2，1-5重量%）。阿波罗11号和17号的玄武岩属于较老的高钛组;阿波罗12号和15号以及月球16号的玄武岩属于较年轻的低钛组。玄武岩的两大类可以根据主要和次要元素化学进一步细分。在这些子组中的每一个中，存在由不同冷却历史引起的各种晶粒尺寸和纹理。这些玄武岩的近地表分馏主要涉及低钛玄武岩中的橄榄石和高钛玄武岩中的橄榄石加铁钛氧化物。贫碱的月海玄武岩是在极端还原条件下（1150°C，10 - 13 atm）在月球表面快速冷却而形成的。这种低氧逸度导致Ti（Ti 4 + → Ti 3+）和Cr（Cr 3 + → Cr 2+）的价态降低，这反过来又影响了月海玄武岩矿物的化学性质和稳定性。这些岩石中最重要的矿物种类是硅酸盐（辉石、长石和橄榄石）和铁钛氧化物（钛铁矿、尖晶石和铁铝尖晶石）。月海玄武岩源区的模型仍然存在争议。提出了海玄武岩源区的三种基本模式。这些模型包括堆积物源模型（由早期月球分异导致的堆积物重熔）、原始源模型（深未分异地幔的熔融）和同化模型（原始熔体被同化污染）。所有这些模型都有问题。如果假设至少有一些月球玄武岩样品到达表面时化学性质没有改变，高压实验相平衡方法可以对这些岩石的源区性质提供限制。这些研究的结果表明，低钛和高钛月海玄武岩群来自矿物学上不同的源区。低钛玄武岩可能来自200 - 500 km深度的橄榄石-辉石源岩，而高钛玄武岩可能来自月球外部150 km的橄榄石-辉石-钛铁矿堆晶岩。
Lunar crustal rocks can be divided into two groups: the terra, or highland, types and the mare basalts. Interpretation of the highland samples is complicated by their derivative nature, which resulted from a series of crystallization, shock, and brecciation events. In contrast, mare basalts appear to be much less complicated and to have been rather uncompromised since their arrival at the lunar surface; thus a synthesis appears possible at this time. Although the mare basalts comprise less than 1% of the lunar crust, they contain much information about the thermal history of the moon and the nature of the lunar interior. It is now known that a complete suite of basalts, sampling all of the chemically and temporally distinct units, was not sampled by the Apollo and Luna missions. The mare basalts that have been studied have ages between 3.15 and 3.96 Gy. However, photogeologic evidence (crater counts and crater degradation studies) indicates that basalts as young as 2.5 Gy exist on the moon and were not sampled. The returned samples can be divided into two broad groups: the older, high-titanium group (ages, ∼3.55–3.85 Gy; TiO2, 9–14 wt %) and the younger, low-titanium group (ages, 3.15–3.45 Gy; TiO2, 1–5 wt %). Basalts from Apollo 11 and 17 fall into the older, high-titanium group; basalts from Apollo 12 and 15 and Luna 16 fall into the younger, low-titanium group. The two major groups of basalts can be further subdivided on the basis of major- and minor-element chemistry. Within each of these subgroups a variety of grain sizes and textures, which result from different cooling histories, are present. Near-surface fractionation of these basalts involved mainly olivine in the low-titanium basalts and olivine plus iron-titanium oxides in the high-titanium basalts. The alkali-depleted mare basalts evolved by rapid cooling at the lunar surface under extremely reducing conditions (∼10−13 atm at 1150°C). This low oxygen fugacity resulted in reduced valence states for Ti (Ti4+ → Ti3+) and Cr (Cr3+ → Cr2+), which in turn affected both the chemistry and the stability of the mare basalt minerals. The most important mineralogical species in these rocks are the silicates (pyroxene, feldspar, and olivine) and the Fe-Ti oxides (ilmenite, spinel, and armalcolite). Models for the source regions of the mare basalts remain controversial. Three basic models for mare basalt source regions have been advanced. These include the cumulate source model (remelting of cumulates resulting from early lunar differentiation), the primitive source model (melting of deep undifferentiated mantle), and the assimilation model (primary melts are contaminated by assimilation). All of these models have problems. If one assumes that at least some of the lunar basalt samples arrived at the surface with unaltered chemistry, the high-pressure experimental phase equilibria approach can provide constraints on the nature of the source regions for these rocks. Results of these studies indicate that the low- and high-Ti mare basalt groups were derived from mineralogically distinct source regions. The low-Ti basalts could have been derived from an olivine-pyroxene source rock at depths ranging from 200 to 500 km, while the high-Ti basalts could have been derived from olivine-pyroxene-ilmenite cumulates in the outer 150 km of the moon.