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首页> 外文期刊>Contributions to Mineralogy and Petrology >Correlated δ~(18)O and [Ti] in lunar zircons: a terrestrial perspective for magma temperatures and water content on the Moon
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Correlated δ~(18)O and [Ti] in lunar zircons: a terrestrial perspective for magma temperatures and water content on the Moon

机译：月球锆石中相关的δ〜（18）O和[Ti]：月球上岩浆温度和含水量的地球透视

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摘要

Zircon grains were separated from lunar reg-olith and rocks returned from four Apollo landing sites, and analyzed in situ by secondary ion mass spectrometry. Many regolith zircons preserve magmatic δ~(18)O and trace element compositions and, although out of petrologic context, represent a relatively unexplored resource for study of the Moon and possibly other bodies in the solar system. The combination of oxygen isotope ratios and [Ti] provides a unique geochemical signature that identifies zircons from the Moon. The oxygen isotope ratios of lunar zircons are remarkably constant and unexpectedly higher in δ~(18)O (5.61 ± 0.07 ‰ VSMOW) than zircons from Earth's oceanic crust (5.20 ± 0.03 ‰) even though mare basalt whole-rock samples are nearly the same in δ~(18)O as oceanic basalts on Earth (~5.6‰). Thus, the average fractionation of oxygen isotopes between primitive basalt and zircon is smaller on the Moon [Δ~(18)O(WR-Zrc) = 0.08 ± 0.09 ‰] than on Earth (0.37 ± 0.04 ‰). The smaller fractionations on the Moon suggest higher temperatures of zircon crystallization in lunar magmas and are consistent with higher [Ti] in lunar zircons. Phase equilibria estimates also indicate high temperatures for lunar magmas, but not specifically for evolved zircon-forming melts. If the solidus temperature of a given magma is a function of its water content, then so is the crystallization temperature of any zircon forming in that melt. The systematic nature of O and Ti data for lunar zircons suggests a model based on the following observations. Many of the analyzed lunar zircons are likely from K, rare earth elements, P (KREEP)-Zr-rich magmas. Zircon does not saturate in normal mafic magmas; igneous zircons in mafic rocks are typically late and formed in the last most evolved portion of melts. Even if initial bulk water content is moderately low, the late zircon-forming melt can concentrate water locally. In general, water lowers crystallization temperatures, in which case late igneous zircon can form at significantly lower temperatures than the solidus inferred for a bulk-rock composition. Although lunar basalts could readily lose H_2 to space during eruption, lowering water fugacity; the morphology, large size, and presence in plutonic rocks suggest that many zircons crystallized at depths that retarded degassing. In this case, the crystallization temperatures of zircons are a sensitive monitor of the water content of the parental magma as well as the evolved zircon-forming melt. If the smaller Δ~(18)O(zircon-mare basalt) values reported here are characteristic of the Moon, then that would suggest that even highly evolved zircon-forming magmas on the Moon crystallized at higher temperature than similar magmas on Earth and that magmas, though not necessarily water-free, were generally drier on the Moon.
机译：锆石与月球沉积石分离，岩石从四个阿波罗登陆点返回，并通过二次离子质谱仪进行原位分析。许多长石锆石保留了岩浆δ〜（18）O和微量元素的组成，尽管从岩石学的角度出发，却代表着一个相对未开发的资源，可用于研究月球以及太阳系中的其他物体。氧同位素比率和[Ti]的组合提供了独特的地球化学特征，可以识别月球上的锆石。尽管母马玄武岩全岩样品几乎接近月球锆石的氧同位素比，但在δ〜（18）O（5.61±0.07‰VSMOW）中，月锆石的氧同位素比仍然显着高于地球大地壳的锆石（5.20±0.03‰）。在δ〜（18）O中与地球上的玄武岩相同（〜5.6‰）。因此，月球上原始玄武岩和锆石之间氧同位素的平均分馏比地球上的平均分数小[Δ〜（18）O（WR-Zrc）= 0.08±0.09‰]。月球上较小的分馏表明，月岩浆中锆石的结晶温度较高，并且与月锆石中较高的[Ti]一致。相平衡估计还表明，月岩浆温度较高，但并非专门用于形成锆石的熔体。如果给定岩浆的固相线温度是其含水量的函数，那么在该熔体中形成的任何锆石的结晶温度也是如此。月球锆石的O和Ti数据的系统性质表明，该模型基于以下观察结果。许多分析过的月球锆石可能来自K，稀土元素，富含P（KREEP）-Zr的岩浆。锆石在正常的镁铁质岩浆中不会饱和。镁铁质岩石中的火成锆石通常较晚，并形成于熔体的最后发展最快的部分。即使最初的大块水含量适度较低，锆石形成后期的熔体也会使局部水浓缩。通常，水会降低结晶温度，在这种情况下，晚期火成锆石的形成温度可能比推断的块岩组合物的固相线低得多。尽管月球玄武岩在喷发时很容易将H_2损失到太空中，从而降低了水的逸度。形态，大尺寸和存在于深成岩中表明许多锆石在阻止脱气的深度结晶。在这种情况下，锆石的结晶温度可以很好地监测母岩浆的水含量以及析出的锆石形成熔体。如果这里报道的较小的Δ〜（18）O（锆石-玄武岩玄武岩）值是月球的特征，那么这表明与月球上类似的岩浆相比，即使月球上高度演化的锆石形成岩浆也要在更高的温度下结晶。岩浆虽然不一定无水，但在月球上通常比较干燥。

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