Uranium‐lead ages for lunar zircons: Evidence for a prolonged period of granophyre formation from 4.32 to 3.88 Ga
Uranium‐lead ages for lunar zircons: Evidence for a prolonged period of granophyre formation from 4.32 to 3.88 Ga
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DOI：
10.1111/j.1945-5100.1996.tb02075.x
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发表时间：
1996-05
影响因子：
2.2
通讯作者：
C. Meyer;I. Williams;W. Compston
中科院分区：
文献类型：
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作者：
C. Meyer;I. Williams;W. Compston
用灵敏的高质量分辨离子探针(SHRIMP I)对锆石进行了原位U-Th-Pb同位素分析，确定了一些月球花岗岩小碎片的年龄。与基性月岩中的锆石相比，月球花岗岩中的锆石具有较高的U和Th含量(大多数分别为200-500ppm和100-300ppm)。一些月球花岗岩的碎片被发现的年龄高达4.32Ga，这支持了其他证据，即原始月球岩浆海洋在月球本身形成的∼200Ma内完全结晶。其他碎片的年龄只有3.88Ga，比大部分月球地壳的形成时间要晚得多。较老的月球花岗岩具有类似于月球Kreep的稀土元素(REE)配分模式，而较年轻的花岗岩具有弓形的REE配分模式，其特征是重稀土元素的相对富集程度更高。众多月球锆石、月球花岗岩和其他岩石的广泛年龄范围表明，月球高地形成锆石的岩浆活动在4.3Ga之前最为活跃，但至少持续到3.88Ga。许多月球锆石的U-Pb同位素组成接近一致，但在一些锆石中观察到了直到∼1.0Ga的同位素扰动效应，无论是在重新加热重结晶的花岗斑岩碎片中，还是在保存完好的原始硅钾长花岗斑共生的碎片中。因此，必须对单个的锆石颗粒进行多种分析，最好是对月球火成岩中的锆石进行成套分析，才能可靠地用U-Pb法测定它们的年龄。一些较年轻的月球花岗岩可能是晚期分异作用中大规模硅酸盐-液体不混溶的产物，但这一点仍未得到证实，直到确定了明显的共生、富铁、不混溶液体的残留物。
The ages of a number of small fragments of lunar granophyre have been determined by the in situ U-Th-Pb isotopic analysis of zircon using a sensitive high mass-resolution ion microprobe (SHRIMP I). The zircon from lunar granophyre is characterized by consistently high U and Th contents (most 200–500 ppm and 100–300 ppm, respectively) compared to zircon from mafic lunar rocks. Some fragments of lunar granophyre are found to be as old as 4.32 Ga, supporting other evidence that the original lunar magma ocean crystallized completely within ∼200 Ma of the formation of the Moon itself. Other fragments are as young as 3.88 Ga, which is much later than the time of formation of most of the lunar crust. The older lunar granophyres have rare-earth-element (REE) patterns that are similar to lunar KREEP, whilst the younger granophyres have bow-shaped REE patterns that feature a greater relative enrichment in the heavy REE. The wide range of ages of numerous lunar zircons, lunar granophyres and other rocks indicates that zircon-forming magmatism in the lunar highlands was most active prior to 4.3 Ga but continuous until at least 3.88 Ga. The U-Pb isotopic composition of much lunar zircon is near concordant, but the effects of isotopic disturbance as late as ∼1.0 Ga are observed in some zircon, both within granophyre fragments recrystallized by reheating and within fragments in which the original delicate silica-K-feldspar granophyric intergrowth is well preserved. It is therefore essential to make multiple analyses of individual zircon grains, and preferably analyses of suites of zircons from lunar igneous rocks if they are to be dated reliably by the U-Pb method. It is possible that some of the younger lunar granophyres are the product of large-scale silicate-liquid immiscibility within late-stage differentiates, but this remains unproven until remnants of demonstrably cogenetic, Fe-rich, immiscible liquid are positively identified.