186 / 2025-04-14 13:04:53

Apatite transformations in banded iron formations from the Kongling Group: Insights into metamorphism and ancient seawater conditions

apatite,Banded iron formations,metamorphic processes,mesoarchaean

Theme 1: Novel Tools and Techniques in Geobiology > Session 1b: Advances in Geobiology Catalysed by Correlated Microscopy at the Nanoscale

Apatite, a key phosphate mineral, is important for understanding geological processes and ancient sedimentary environments due to its ability to incorporate various trace elements. Despite extensive studies on apatite in Precambrian settings, the effects of metamorphic processes on its geochemical and petrographic characteristics in banded iron formations (BIFs) are still poorly understood. Here, we present results of petrographic and geochemical studies of apatite in BIFs from the Mesoarchaean Yemadong and Huanglianghe formations of the Kongling Group in the northern Yangtze Craton, South China. We employed a range of analytical techniques, including micro-Raman spectroscopy, scanning electron microscopy (SEM), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), U-Pb dating, TESCAN Integrated Mineral Analyzer (TIMA) and cathodoluminescence (CL) to characterize the apatite samples, identifying three distinct types (A, B and C) of apatite. Type A apatite has a large grain size and is surrounded by quartz and magnetite, occurring in bands within the BIF. It exhibits positive Eu anomalies, a REE distribution pattern similar to Archaean seawater, and is rich in hydroxyl groups compared to other types of apatite. Type B shows moderate alteration has a relatively small grain size and exhibits anhedral shapes, characterized by negative Eu anomalies, enrichment in fluorine relative to other types of apatite and presence of cracks and inclusions. Type C exhibits high manganese content, relative enrichment of chlorine contrast to other types of apatite and depletion of heavy rare earth elements (HREE), pointing to metamorphic recrystallization in equilibrium with garnet under granulite-facies conditions. Our observations may reveal how metamorphic processes affect the mineralogy and geochemistry of sedimentary apatite, providing insights into the evolution of BIFs and the nature of ancient marine environments.