Abstract It has long been inferred that mantle metasomatism and the incompatible element enrichment of the continents both require movement of melts formed by very low degree melting of the mantle. Yet establishing the presence of these melts and whether this process is on-going and continuous, or spatially and temporally restricted, has proved difficult. Here we report large U-Th-Ra disequilibria in metasomatised, mantle xenoliths erupted in very young lavas from the Newer Volcanics Province in southeastern Australia. The 226Ra-230Th disequilibria appear to require reappraisal of previous estimates for the age of eruption that now seems unlikely to be more than a few kyr at most. We propose that infiltration of carbonatitic melts/fluids, combined with crystallization of pargasite, can account for the first order U-series disequilibria observations. Irrespective of the exact details of the complex processes responsible, the half-lives of the nuclides require that some of the chemical and isotopic disturbance was extremely young (« 8 kyr) and potentially on-going at the time of incorporation into the alkali basalts that transported the xenoliths to the surface. This provides evidence for the presence and possibly continuing migration of small melt fractions (~0.02%) in the upper convecting mantle that may contribute to the seismic low velocity zone. By implication, it appears that the asthenosphere must lie close to its solidus, at least in this region. Pressure-temperature estimates indicate that the small degree melts identified could infiltrate as far as 25 km upwards into the sub-continental lithospheric mantle leading to strong incompatible element enrichment and the recent timing of this event this urges a reappraisal of the meaning of 300–500 Ma Nd model ages in mantle xenoliths from this region. In principle, the resultant metasomatised mantle could provide a component for some ocean island basalts, should the sub-continental lithospheric mantle be returned to the asthenosphere by convective removal at some later time.

中文翻译：

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次大陆岩石圈地幔的极年轻熔体入渗

摘要 长期以来人们推测，地幔交代作用和大陆不相容元素富集都需要地幔极低程度熔融形成的熔体运动。然而，确定这些熔体的存在以及这个过程是持续和连续的，还是在空间和时间上受到限制，已被证明是困难的。在这里，我们报告了在澳大利亚东南部新火山省的非常年轻的熔岩中喷发的交代地幔包体中的大量 U-Th-Ra 不平衡。226Ra-230Th 的不平衡似乎需要重新评估之前对喷发年龄的估计，现在似乎最多不超过几个 kyr。我们建议碳酸岩熔体/流体的渗透，结合帕加石的结晶，可以解释一阶 U 系列不平衡观察。不管负责的复杂过程的确切细节如何，核素的半衰期要求一些化学和同位素干扰非常年轻（« 8 kyr），并且在掺入碱性玄武岩时可能正在进行将捕虏体运送到地表。这为上对流地幔中可能有助于地震低速带的小熔体部分 (~0.02%) 的存在和可能的持续迁移提供了证据。言外之意，软流圈似乎必须靠近固线，至少在这个区域是这样。压力-温度估计表明，确定的小程度熔体可以向上渗透到次大陆岩石圈地幔 25 公里，导致强烈的不相容元素富集，而这一事件的最近时间促使人们重新评估 300-500 Ma Nd 模型在该地区的地幔包体中老化。原则上，如果次大陆岩石圈地幔在稍后通过对流去除而返回软流圈，合成的交代地幔可以为一些海洋岛屿玄武岩提供一个组成部分。