Non-detrital Gallium and Aluminum in Early Precambrian Marine Chemical Sediments and the Potential Use of the Ga/Al Ratio as a Geochemical Proxy for Metal Sources and Relative Fluxes to the Early Ocean

在“前寒武纪早期海洋化学沉积物中的非碎屑镓和铝以及Ga/Al比值作为金属来源和早期海洋相对通量的地球化学指标的潜在用途”项目中，将重点研究微量元素镓（Ga）及其地球化学相似的“伪同位素”铝（Al）。虽然这两种金属在火成岩和碎屑沉积岩中紧密结合，但它们可能在自然水体中，如河水、海水和热液流体中相互分离。因此，从大陆岩石经河水到海水，Ga/Al比值显著增加。然而，最高的Ga/Al比率是海洋高温热液流体的特征。假设这种系统关系在早前寒武纪海洋中也是一样的，那么在地质档案中，如前寒武纪海洋化学沉积岩（如带状铁地层、燧石岩、石灰岩和白云岩）中的Ga/Al比值应该是这些金属的来源和相对于周围前寒武纪海水通量的地球化学代表。因此，Ga-Al系统可以用来验证其他地球化学指标的结果，如稀土元素（REE）和Ge-Si关系。在拟议的项目中，重点将放在地质特征良好的海洋化学沉积物的超纯（即无碎屑）样品中的Ga-Al分布，这些样品将与侧重于同位素代用物的研究小组密切合作进行研究。来自沉积年龄在3.8 - 0.6 Ga之间的沉积岩的样品集将提供有关海水Ga-Al系统随时间演变的信息，而同时沉积但在不同地点的样品所确定的数据将提供有关早期地球海洋Ga/Al比率空间异质性的信息。这不仅将扩大我们对早期地球表面系统中金属来源和通量的认识，而且还将提高我们对其他地球化学代理的潜力和局限性的理解。

In the proposed project "Non-detrital Gallium and Aluminum in Early Precambrian Marine Chemical Sediments and the Potential Use of the Ga/Al Ratio as a Geochemical Proxy for Metal Sources and Relative Fluxes to the Early Ocean" research will focus on the trace element gallium (Ga) and its geochemically similar „pseudo-isotope“ aluminium (Al). While these two metals are rather closely coupled in igneous and clastic sedimentary rocks, they may be fractionated from each other in natural waters, such as river water, seawater and hydrothermal fluids. Hence, Ga/Al ratios increase substantially from continental rocks via river water to seawater. Highest Ga/Al ratios, however, characterize marine high-temperature hydrothermal fluids. Assuming this systematic relationship was the same in the Early Precambrian oceans, the Ga/Al ratio in geoarchives such as Precambrian marine chemical sedimentary rocks (e.g., banded iron formations, cherts, lime- and dolostones) should be a geochemical proxy for the sources and relative fluxes of these metals to ambient Precambrian seawater. Hence, Ga-Al systematics may be used to verify the results derived from other geochemical proxies such as rare earth elements (REE) and Ge-Si relationships. In the proposed project, the focus will be on the Ga-Al distribution in ultrapure (i.e. detritus-free) samples from geologically well-characterized marine chemical sediments which will be studied in close collaboration with research groups which focus on isotope proxies. While sample sets that originate from sedimentary rocks with depositional ages from 3.8 to 0.6 Ga will provide information on the evolution of the Ga-Al systematics of seawater through time, data determined for samples deposited contemporaneously but at different locations will provide information on the spatial heterogeneity of the Ga/Al ratios in the Early Earth’s oceans. This will not only expand our knowledge of metal sources and fluxes in the Early Earth’s surface system, but will also improve our understanding of the potential and the limits of other geochemical proxies.