DIVERSITY OF IGNEOUS ROCKS AT GALE CRATER, MARS

Curiosity rover has observed a total of 67 igneous rocks up to sol 800. We first identified these rocks by textural analyses from the MastCam and ChemCam images. The objective of this study is to show that Early Mars presents an interesting diversity of igneous products, some of them being evolved and not observed with previous in situ missions.

First, the rocks were classified as effusive or intrusive based on textural analyses. Among the effusive rocks, two groups show an aphanitic texture: one with dark rocks while the other one contains leucocrate rocks. These leucocrate aphanitic rocks are characterized by the presence of potassic feldspars. These rocks are also enriched on Ba compared to other igneous targets, as well as in Rb (~ 110 ppm median). Some points from these rocks have sampled a mixture with Ti-oxide grains. We also observed porphyritic rockss with cm-size light-toned minerals that correspond mainly to oligoclase/andesine feldspars. As the laser sampled both the matrix and the light-toned minerals, a nice mixing trend is observed between these feldspars and more primitive material.

The two groups of intrusive rocks are either dark and fine-grained with similar proportions of dark and light-toned minerals, or coarse-grained dominated by light-toned minerals. These coarse-grained leucocrate rocks are enriched in andesine/oligoclase with some mixture with a Si-rich component.

Most of the igneous rocks have been observed as float rocks. Some “ambiguous” textures are also reported. These rocks seem to be igneous from their texture, this one is different from the five distinctive groups and also these rocks present a different chemistry.

The combination of textural and whole-rock analysis revealed a nice alkali-trend up to trachytes, observed for the first time on Mars. This suggests a low-degree of partial melting in the Martian mantle at relatively low pressure. Moreover, the light-toned coarse-grained intrusive rocks correspond to diorite/granodiorite with normative quartz, suggesting that Si-rich igneous rocks may constitute a significant portion of the martian crust [4].

We have shown that felsic rocks seem to have been transported from the northern part of the crater rim whereas the dark aphanitic ones are observed at the end of the traverse, probably coming from another source.