**4. Geochemistry**

To properly associate radiometric response to rock composition, we need to perform a geochemical analysis on the samples where reflectance was measured, particularly on those identified as basalts, since they are the most common. Ten geochemical analyses throughout the island have been published [1]; we deemed sufficient to analyze only three additional ones, on samples collected at Stations 1, 2, and 5.

The elemental compositions obtained by means of X-ray fluorescence (XRF) appear in **Table 2**. Their basic composition is readily appreciated from their low silica content. After data normalization, the results were processed with the IGPET version 2005 software [9] to get the total alkali silica (TAS; [10]) and Alkali FeOMgO (AFM; [11]) diagrams are shown in **Figures 18** and **19**. Results are quite similar to the elemental data for the ten whole-rock samples across the island reported in [1].


**Table 2.** 

*Elemental composition of three samples from Isla Isabel from X-ray fluorescence (wt%); they correspond to basalt.* 

#### **Figure 18.**

*Total alkali silica (TAS) diagram of samples ISA-01-03 (red) classified as basalt and tephrite-basanite from the elemental compositions in* **Table 2***.* 

*Radiometric Mapping of Hydrothermal Alterations in Isla Isabel, Mexico DOI: http://dx.doi.org/10.5772/intechopen.80530* 

**Figure 19.**  *Alkali FeOMgO (AFM) diagram of samples ISA-01-03 (red) from the elemental compositions in* **Table 2***.* 

 Mineral content of basalts: groundmass generally of pyroxene (augite), plagioclase and olivine, possibly with minor glass; if porphyritic, the phenocrysts will be any of olivine, pyroxene or plagioclase. Silica (SiO2) content—45–52%.
