Evidence for Oceans pre-4300 Ma Confirmed by Preserved Igneous Compositions in Hadean Zircon
Evidence for Oceans pre-4300 Ma Confirmed by Preserved Igneous Compositions in Hadean Zircon
复制标题
DOI：
10.2138/am-2023-9180
复制
发表时间：
2024-03
影响因子：
3.1
通讯作者：
Emilia M. Cameron;Tyler Blum;A. Cavosie;K. Kitajima;Lutz Nasdala;I. Orland;C. Bonamici;J. Valley
中科院分区：
文献类型：
--
作者：
Emilia M. Cameron;Tyler Blum;A. Cavosie;K. Kitajima;Lutz Nasdala;I. Orland;C. Bonamici;J. Valley
来自杰克山的碎屑锆石是今天可供研究的冥古宙（前4000马）陆地材料的主要来源。许多锆石的δ 18 O值（6.0 ~ 7.5‰）高于地幔平衡值。20年来，这些轻微升高的值一直是主要证据，表明形成锆石的岩浆的原岩在埋藏和熔化之前在低温下与液态水相互作用，这意味着地球表面在核心和月球形成后迅速冷却，并且在地球形成后的2.5亿年内存在生命的可居住条件，在最古老的公认微体化石之前超过8亿年。这些结论是基于氧同位素分析的锆石域具有明确的生长分区和几乎一致的U-Pb年龄的锆石颗粒内的低磁化率，进一步推断是不变的各种测试。然而，没有研究杰克山锆石直接相关的氧同位素比和辐射损伤，这有利于锆石的蚀变。先前的几项研究选择了显示出辐射损伤、不协调和/或含水区域的锆石，并表明这种蚀变物质作为火成岩成分的记录是不可靠的。与此相反，这项研究针对的是被解释为原始和没有改变的锆石，并证明了测试锆石的辐射损伤和蚀变的重要性，作为任何地球化学研究的一部分，无论年龄。本研究在现有资料的基础上，首次对杰克山锆石的δ 18 O、OH/O、CL成像、U-Pb一致性和辐射损伤状态进行了综合评价。在这项研究中，共有115个冥古宙锆石颗粒的水含量类似于标称无水标准参考锆石，并被解释为原始的。原位拉曼谱带展宽数据与δ 18 O分析文件低水平的辐射损伤，表明显着的退火。目前的有效剂量（Deff）均小于第一渗滤点（在较低损伤状态下隔离的损伤域重叠以形成通过晶体的连续路径的剂量，约2×1015 α-衰变/mg，Ewing等人，2003年），大多数锆石的Deff<1×1015 α衰变/mg。代表性的α-反冲损伤和退火历史的建模表明，在这项研究中，大多数锆石一直低于Deff的第一个渗流点在整个历史。这些原生锆石的δ 18 O值包括许多高于在岩浆温度下与地幔平衡的值，在冥古宙平均为6.32 ± 1.3‰，在太古代平均为6.26 ± 1.6‰。在我们的一套原始冥古宙锆石中，δ 18 O与OH/O、Deff、年龄或U-Pb年龄一致性之间没有相关性。这些仔细记录的Hadean时代锆石在δ 18 O分析取样的区域中具有少量的辐射损伤，是贫水的。δ 18 O值的轻度升高是原生岩浆的地球化学特征。这些结果加强了在冥古宙期间存在轻度升高的-δ 18 O岩浆的结论，支持了在4300 Ma之前存在海洋和可居住地球的假设。
Detrital zircons from the Jack Hills are the dominant source of Hadean (pre-4000 Ma) terrestrial material available for study today. Values of δ18O in many of these zircons (6.0 to 7.5‰ are above the mantle-equilibrated value. For two decades, these mildly elevated values have been the primary evidence that protoliths of the zircon-forming magmas interacted at low temperature with liquid water before burial and melting, implying that the surface of Earth cooled quickly after core and moon formation, and that habitable conditions for life existed within 250 Myr of the formation of Earth, over 800 Myr before the oldest generally accepted microfossils. These conclusions are based on oxygen isotope analyses of zircon domains with well-defined growth zoning and nearly concordant U-Pb ages within zircon grains with low magnetic susceptibility, which are further inferred to be unaltered by various tests. However, no studies of Jack Hills zircons have directly correlated oxygen isotope ratios and radiation damage, which facilitates alteration in zircon. Several previous studies have selected zircons that show radiation damaged, discordant and/or hydrous domains, and have shown that such altered material is not reliable as a record of igneous composition. In contrast, this study targeted zircons that are interpreted to pristine and not altered, and demonstrates the importance of testing zircons for radiation damage and alteration as part of any geochemical study, regardless of age. This study expands on existing data, and presents the first comprehensive evaluation of δ18O, OH/O, CL imaging, U-Pb concordance and radiation-damage state within Jack Hills zircons. A total of 115 Hadean zircon grains in this study have water contents similar to nominally anhydrous standard reference zircons and are interpreted as pristine. In situ Raman data for band broadening correlated with δ18O analyses document low levels of radiation damage, indicating significant annealing. The present-day effective doses (Deff) are uniformly less than the first percolation point (dose where damage domains, that are isolated at lower damage state, overlap to form a continuous pathway through the crystal, ~2×1015 α-decays/mg, Ewing et al., 2003) and most zircons have Deff<1×1015 α-decays/mg. Modeling of representative alpha-recoil damage and annealing histories indicates that most zircons in this study have remained below the Deff of the first percolation point throughout their history. The δ18O values for these primary zircons include many that are higher than would be equilibrated with the mantle at magmatic temperatures and average 6.32 ± 1.3‰ in the Hadean and 6.26 ± 1.6‰ in the Archean. There is no correlation in our suite of pristine Hadean zircons between δ18O and OH/O, Deff, age, or U-Pb age-concordance. These carefully documented Hadean-age zircons possess low amounts of radiation damage in domains sampled by δ18O analysis, are water-poor. The mildly elevated δ18O values are a primary-magmatic geochemical signature. These results strengthen the conclusion that mildly elevated-δ18O magmas existed during the Hadean, supporting the hypothesis that oceans and a habitable Earth existed before 4300 Ma.