A Study of Mesoproterozoic Iron Cosmic Micro-spherules from 1.8 Ga and 1.6 Ga Old Strata in the Ming Tombs District, Beijing

• Published in: 地质学报（英文版） 2007, Vol.81 (4), p.649-657
• Main Authors: SONG Tianrui, HE Zhengjun, WAN Yusheng, LIU Yanxue
• Format: Article
• Language: English
• ISSN: 1000-9515
• DOI: 10.3321/j.issn:1000-9515.2007.04.013

P5; Numerous iron cosmic micro-spherules have been discovered from Mesoproterozoic strata including the Changzhougou Formation (1.8 Ga) and the Dahongyu Formation (1.6 Ga) of the Ming Tombs district, Beijing. There are 1 to 30 grains of cosmic spherules per 2 kg of a sandstone sample taken from the bottom of a coarse sandstone bed of the Changzhougou Formation and 56 grains per 3.69 kg of a rock sample from silicified carbonate rocks of the Dahongyu Formation. The surface textures of cosmic spherules analyzed by means of the secondary electron imagery are identical with those reported from references either domestic or abroad. So far the geo-ages of 1.8 Ga and 1.6 Ga of cosmic spherules from the Changzhougou and Dahongyu formations might be older than those reported in the world. Table 1 gives the electron probe analysis data of cosmic spherules for 30 spherule grains and 44 testing points as follows (%): FeO, 80-95; Cr2O3; 0-9.56; NiO, 0-0.78; CoO, 0-0.46;indicating that the Cr2O3 content is higher and FeO content lower in the Changzhougou Formation than in the Dahongyu Formation. The helium isotopic data of cosmic spherules as well as their host rocks vary greatly between the Changzhougou and the Dahongyu formations as shown in Table 2. The data of cosmic spherules of the Changzhougou Formation vs the Dahongyu Formation are 57.5/1.23 in 3He/4He (10-8); and 55.54/809.60 in 4He (10-6cm3STP/g); those of coarse sandstone of the Changzhougou Formation vs silicified carbonate of the Dahongyu Formation are 3.39/2.59 in 3He/4He (10-8) and 4.56/2.34 in 3He (10-6cm3STP/g). The ratio of analytic data of helium isotopes are different for cosmic spherules and their host rocks; for example, the 3He/4He (10-8) values are 16.96 and 0.48, and the 4He(10-6 cm3STP/g) are 12.18 and 345.98 for the Changzhougou and Dahongyu formations respectively. It was reported that the world's oldest micrometeorites had been found in the Meso-Proterozoie Satakunta Formation, Finland. However, the cosmic spherules from the Meso-Proterozoic Changzhougou and Dahongyu formations are 200 to 400 Ma older than those from the Satakunta Formation. Besides, one carbonaceous chondrite grain was discovered for the first time as the earliest remain formed in the solar nebula from the Dahongyu Formation.