美国南卫理公会大学Jordan K. Wright课题组的最新研究成果报道了地球上最早枯竭的地幔储层遗迹。相关论文发表在2024年3月13日出版的《地质学》杂志上。

据介绍，在岩石记录中保存的关于地球最早富集地壳和冥古代期间互补亏损地幔的证据很少。近年来，通过对锆石Hf同位素系统的研究，研究推断出了这些早期储层的遗迹。例如，印度东部的Singhbhum克拉通仅保存了冥古代—太古代期间的一个富集的(ε_Hf<0)地壳储层。直到ca.3.5Ga才出现明显缺乏亏损的地幔储层特征 (ε_Hf>0)。

本文报道了来自Singhbhum克拉通西部铁矿群的下部熔岩绿岩带中，一个新的Sm-Nd等时线，研究确定了其结晶年龄为3.42±0.14Ga，初始ε_Nd为+5.7±2.5。这是从该年龄的等时线得出的最高正ε_Nd值。研究人员推断，这种亏损的地幔源是在行星分异后与原始地壳互补的遗迹。

此外，研究人员给出了3.39±0.02Ga凝灰岩的U-Pb锆石年龄，该凝灰岩地层位于下层熔岩之上，位于大面积整合的条带状铁矿层(BIF)之下30cm。该年龄意味着西部铁矿群的BIF是古太古代时期最大的经济级铁建造，表明在这个时候大气中的游离氧不仅仅只是零星存在。

附：英文原文

Title: Vestiges of Earth’s earliest depleted mantle reservoir

Author: Jordan K. Wright, Asish R. Basu

Issue&Volume: 2024-03-13

Abstract: There is a paucity of evidence preserved in the rock record regarding Earth’s earliest enriched crust and its complementary depleted mantle during the Hadean. In recent years, vestiges of these early reservoirs have been inferred by examination of Hf isotope systematics compiled from zircons. The Singhbhum craton of Eastern India, for example, preserves only the existence of an enriched (εHf <0) crustal reservoir during the Hadean—Eoarchean, with the notable absence of a depleted mantle reservoir signature (εHf >0) until ca. 3.5 Ga. Here we report a new Sm-Nd isochron for the Lower Lava greenstones of the western Iron Ore Group from the Singhbhum craton, confirming a 3.42 ± 0.14 Ga crystallization age with an initial εNd of +5.7 ± 2.5. This is the highest positive εNd value derived from an isochron of this age. We infer that this depleted mantle source is a vestige complementary to the primary crust following planetary differentiation. Furthermore, we present U-Pb zircon ages for a 3.39 ± 0.02 Ga tuff that lies stratigraphically above the Lower Lava and <30 cm below an extensive conformable banded iron formation (BIF). This age implies that the western Iron Ore Group’s BIF is the largest economic-grade iron formation of its Paleoarchean age, suggesting that free atmospheric oxygen existed as more than just whiffs at this time.

DOI: 10.1130/G51936.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G51936.1/636306/Vestiges-of-Earth-s-earliest-depleted-mantle