Department of Earth and Planetary Sciences, Harvard University.
Department of Earth Science, Rice University.
Sci Rep. 2014 Jun 17;4:5314. doi: 10.1038/srep05314.
Major terrestrial reservoirs have Pb isotopes more radiogenic than the bulk silicate Earth. This requires a missing unradiogenic Pb reservoir, which has been argued to reside in the lower continental crust or dissolved in the core. Chalcophile element studies indicate that continent formation requires the formation of sulfide-bearing mafic cumulates in arcs. Because Pb, but not U, partitions into sulfides, we show that continent formation must have simultaneously generated time-integrated unradiogenic Pb reservoirs composed of sulfide-bearing cumulates, now recycled back into the mantle or stored deep in the continental lithosphere. The generation of such cumulates could also lead to coupled He-Pb isotopic systematics because (4)He is also produced during U-Th-Pb decay. Here, we show that He may be soluble in sulfide melts, such that sulfide-bearing cumulates would be enriched in both Pb and He relative to U and Th, "freezing" in He and Pb isotopes of the ambient mantle at the time of sulfide formation. This implies that ancient sulfide-bearing cumulates would be characterized by unradiogenic Pb and He isotopes (high-(3)He/(4)He). These primitive signatures are usually attributed to primordial, undifferentiated mantle, but in this case, they are the very imprint of mantle differentiation via continent formation.
主要的陆地储层的铅同位素比整个硅酸盐地球的更具放射性。这需要一个缺少的非放射性铅储层，有人认为它存在于下地壳或溶解在核心中。亲硫元素的研究表明，大陆的形成需要在弧中形成含硫化物的镁铁质堆积物。由于铅而不是铀分配到硫化物中，因此我们表明，大陆的形成必须同时产生由含硫化物的堆积物组成的时间积分的非放射性铅储层，现在这些储层又被循环回地幔或储存在大陆岩石圈的深处。这种堆积物的形成也可能导致氦-铅同位素的耦合系统，因为（4）氦也是铀-钍-铅衰变过程中产生的。在这里，我们表明，（4）氦可能在硫化物熔体中是可溶的，因此含硫化物的堆积物相对于铀和钍来说，富含铅和氦，在硫化物形成时，环境地幔中的（4）氦和铅同位素“冻结”。这意味着，古代含硫化物的堆积物的铅和氦同位素特征为非放射性（高（3）氦/（4）氦）。这些原始特征通常归因于原始的、未分化的地幔，但在这种情况下，它们是通过大陆形成进行地幔分化的印记。
