Fine-grained rims (FGRs) are ubiquitous in chondrites. They consist of unequilibrated mineral assemblages that surround chondrules and refractory inclusions. As such, they carry information about the material that was accreted onto chondrules. To decipher the nature and the formation mechanism of FGRs and compare them to adjacent matrix material, we investigated their composition, mineralogy, density and texture in the pristine Paris CM chondrite. We coupled a new method at the SEM scale (ACADEMY) that allows high-resolution quantitative petrology and an analytical TEM study.

Significant differences in modal abundance, grain size and porosity are observed between the FGRs and their adjacent matrix. Amorphous silicates domains embedding nanosulfides are indicative of a high preservation degree. They are less abundant in the matrix than in the rims. In contrast, secondary alteration phases (phyllosilicates, carbonates and tochilinites) are more abundant in the matrix and associated with larger and fewer sulfides grains. The similar composition of the amorphous silicate in the rims and the matrix attests for a close relationship between the two reservoirs. However, matrix underwent more aqueous alteration. We interpret it as the result of the accretion of material with a higher water/rock ratio in the matrix, leading to a more aqueously altered microenvironment. We also find that coarse-grained anhydrous silicates (olivine and pyroxene) are present in the matrix but not in the FGRs, likely as a result of a chondrule fragmentation episode that occurred after FGR but before matrix accretion.

Most of the time, FGRs display distinct inner and outer layers. The inner part is compact and displays larger sulfide grains than the outer part, which is more porous (porosity ∼ 45%) and altogether more pristine. These mineral and textural differences are not easily explained by differential aqueous alteration. Instead, a pre-accretion thermal process that preferentially affected the inner rim could have induced loss of porosity, compaction of the amorphous silicate domains as well as sulfides growth. We therefore suggest that FGRs acquired their characteristics in the nebula before matrix accretion and discuss possible mechanisms such as dust heating in the chondrule formation environment or secondary heating episode of the previously rimmed chondrule.

粒状边缘普遍存在细密的轮辋（FGR）。它们由围绕软骨和难熔夹杂物的不平衡矿物组合组成。这样，它们会携带有关软骨材料的信息。为了破译FGRs的性质和形成机理，并将其与邻近的基质材料进行比较，我们研究了原始Paris CM球粒陨石的成分，矿物学，密度和质地。我们在SEM规模（ACADEMY）上采​​用了一种新方法，该方法可以实现高分辨率定量岩石学和TEM分析研究。

在FGR及其相邻基质之间观察到模态丰度，晶粒尺寸和孔隙率的显着差异。嵌入纳米硫化物的无定形硅酸盐域表示高保存度。它们在矩阵中的数量要少于在轮辋中的数量。相反，次生相变相（页硅酸盐，碳酸盐和to石）在基质中更为丰富，并与较大和较少的硫化物晶粒有关。边缘和基质中无定形硅酸盐的相似成分证明了这两个储层之间的紧密联系。但是，基质发生了更多的水相变化。我们将其解释为基质中水/岩石比较高的物质积聚的结果，从而导致水环境变化更大。

在大多数情况下，FGR显示不同的内层和外层。内部较致密，与外部相比，硫化物晶粒更大，多孔性更高（孔隙度约45％），原始质素也更多。这些矿物和质地上的差异不易通过水相差异变化来解释。取而代之的是，优先影响内缘的预积热过程可能会导致孔隙率损失，无定形硅酸盐畴的致密化以及硫化物的生长。因此，我们建议FGR在基质增加之前先在星云中获得其特征，并讨论可能的机制，例如在球粒形成环境中粉尘加热或先前边缘球粒的二次加热。