Testing rheological models on Central Andean volcanic geoforms as analogues for Mars

1. Introduction
 
The dynamic of lava flows as inferred from rheological models is characterized by physical parameters, such as viscosity and yield strength (assuming the flow is a Non-Newtonian fluid). Historically, the rheological behavior of lava flows is controlled, and determined (when samples are available), from intrinsic parameters, e.g chemical composition of the melt, crystal fraction rates, outgassing. Rheology is also reflected in outer characteristics such as the 3D morphology [1], whose analysis is the most common approach in planetary science.

To study extraplanetary volcanic features, terrestrial analogue sites are useful. The Altiplano Puna Volcanic Complex (APVC), a volcano-tectonic province located at the Central Andes, has been suggested as an excellent site for the combined use of satellite imagery and ground truthing studies, being extraordinarily well-exposed and preserved due to the aridity of the Atacama Desert [2]. Its volcanic units vary in composition and geomorphology [3], representing potential analogs to a range of Mars volcanic features [4].

In this work, we focus on Andean volcanic geoforms from the APVC and apply two main approaches: chemical and morphological, to constrain the rheological behavior and viscosity estimates by testing several existing models [e.g., 1] in the Atacama Desert site, as an analog for Mars lava flows.

2. Methods

About 30 terrestrial lava flows of age < 1 Ma from stratovolcanoes and monogenetic centres from the APVC (21°S 69°W to 24°S 67°W) have been study from orbit and field (Table 1). The composition of these lava samples ranges from basaltic-andesite (53.07 wt.% SiO₂) to dacite (64.62 wt.% SiO₂) [5,6,7].

2.1 Rheological models

Lava flow viscosities are estimated using two different, but complementary approaches.
First, the melt viscosity (η_G) is calculated using the geochemical composition of the lava samples [8]. Then, the lava flow geometry is used as an input in existing models [9,10,11] to determine the apparent viscosity (η_A)from orbit. Morphological dimensions of the lava flows are measured using topographic data to determine the flow length, height, width, and slope angle from a 12.5 m/pixel Alos Palsar digital elevation model.

The ratio of the apparent viscosity to the melt viscosity (relative viscosity η_R) is known to increase with the presence of crystals. Existing models rely on the Rosco-Einstein equation to quantify the crystal effects on viscosity, assuming a theoretical maximum packing fraction (φ) at which magmas essentially become solid [10]. We further evaluate the amount and shape of phenocrysts in the lava flows through petrographic analyses.

3. Results

Results for a first selection of lava flows are presented in Table 1. Calculated melt viscosities (η_G) for basaltic andesites and dacites range from 5.8E+04 to 3.58E+08 Pa.s, which is in agreement with the theoretical viscosity values of intermediate silicic flows. However, apparent viscosities (η_A) estimated from orbit show values ranging from 8.0E+07 to 2.30E+13 Pa s (Table 1). This subset of lava flows returns high relative viscosity (η_R) values in the order of 10E+03 to 10E+07, implying packing fractions close to the theoretical maximum. Petrological analyses confirm that all these samples contain phenocrysts with abundances comprised between 7.09 and 19.67%, as obtained using the modal counting method.

4. Discussion and perspectives

Different approaches have been used to estimate lava viscosities, with large gaps observed between remote sensing estimates and actual melt viscosities. With a complete range of geochemical compositions and crystal abundances, our catalog should help to note some limitations of existing rheological models. An inherent limitation is the use of single values for viscosity and yield strength for each lava flow because these parameters may indeed vary within the lava flow, both parallel to the flow direction, and vertically. Some authors agree that crystallinity is likely to play a more important role in viscosity changes in less evolved lavas [1]. On the contrary, for more evolved compositions, the crystallization sequence might not be the principal or decisive factor affecting the viscosity, compared to degassing and glass content [12]. Combining different rheological models for a wider compositional (i.e., SiO₂ content) and textural (i.e., more aphanitic) range may allow us to better understand complex rheological scenarios of extra-terrestrial lava flows. On Mars, previous estimates of lava viscosities seem to be more consistent with basaltic to andesitic compositions [1], however, more evolved rocks (e.g., trachyandesite and trachyte) have been described recently based on orbital, in situ and meteorite data [13], and could be identified with our approach.

Volcano	TAS composition	L (m)	H (m)	W (m)	Apparent viscosity (ηA)	Melt viscosity (ηG)	Relative viscosity (ηR)	Packing fraction (φ)
El Negrillar	Basaltic andesite [12]	1809	53	827	3.10E+11	3.58E+08	4.5E+05	0.74
La Poruña	Basaltic andesite	7670	59	1778	8E+07	5.8E+04	1.5E+03	0.70
Azufre	Andesite	6740	336	3227	2.29E+13	1.3E+06	2.8E+07	0.73
Paniri	Trachyte	8140	199	2013	1.02E+12	1.34E+06	7.6E+05	0.73
San Pedro	Dacite	11800	153	2526	3.15E+11	1.12E+06	2.8E+05	0.73
El Negrillar	Dacite [12]	1679	54	509	2.30E+13	2.00E+06	4.6E+03	0.72

Table 1. Summary of rheological parameters calculated for selected lava flows in study. η is in Pa.s.

5. Acknowledgements

This work is funded by ANID - Fondecyt de Iniciación N° 11200293 and N° 11200013, Otelo JCJC and the ANR Mars-Spec grants.

6. References

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