In the high mountain rock walls of the Mont Blanc massif, changes in the granite surface colour are related to its exposure age. The light grey colour of fresh rock surfaces turns orange when is long exposed to weathering. In order to study this colour/age relationship, reflectance spectroscopy was performed on 73 samples, and Terrestrial Cosmogenic Nuclide (TCN) dating was used to obtain their surface exposure age. The standard deviation of the reflectance values was calculated for each wavelength of the visible spectrum to study the behaviour of each spectral region. The aim was to find two colour regions that showed opposite behaviour, and once they are combined, they could provide a representative index of the rock colour changes that are linked to the degree of weathering.

As an adaptation of the Green Red Vegetation Index used to measure colour changes in vegetation, the GReen-Infrared GRanite Index (GRIGRI), a normalized difference between the granite 770 nm and 530 nm reflectance values, was developed. The GRIGRI value of a weathered granite surface has a close relationship with its exposure age (R² = 0.85). The reflectance spectra of seven samples for which TCN dating failed and two samples for which the TCN age was considered to be an outlier were used to calculate their GRIGRI value to assess the colour-based ages, that were plausible according to rock wall morphology and the TCN exposure ages of the surrounding surfaces. We propose a new method of surface dating for the rock walls of the Mont Blanc massif using reflectance spectroscopy.

在勃朗峰地块的高山岩壁上，花岗岩表面颜色的变化与其暴露年龄有关。当长时间暴露在风化下时，新鲜岩石表面的浅灰色变为橙色。为了研究这种颜色/年龄关系，对73个样品进行了反射光谱分析，并使用了陆地宇宙成因核素（TCN）年代来获得其表面暴露年龄。针对可见光谱的每个波长计算反射率值的标准偏差，以研究每个光谱区域的行为。目的是找到表现出相反行为的两个颜色区域，一旦将它们组合在一起，它们就可以提供与风化程度相关的岩石颜色变化的代表性指标。

为了适应用于测量植被颜色变化的绿色红色植被指数，开发了GReen-IR GRanite Index（GRIGRI），即花岗岩770 nm和530 nm反射率值之间的归一化差异。风化花岗岩表面的GRIGRI值与其暴露年龄密切相关（R ²  = 0.85）。根据岩石壁的形态和特征，可以将7个TCN测年失败的样品和2个TCN年龄被认为是异常值的样品的反射光谱用于计算其GRIGRI值，以评估基于颜色的年龄。周围表面的TCN暴露时间。我们提出了一种使用反射光谱法对勃朗峰地块的岩壁进行表面测年的新方法。