GRAVITY AND MAGNETIC CONSTRAINTS ON THE MAGMA CHAMBER STRUCTURE OF APOLLINARIS PATERA, MARS

Apollinaris Patera is 280 by 190 km across and 5.4 km high, with an estimated formation age of 3.7 Ga (Noachian/Hesperian). Based on its morphology, it has long been accepted that pyroclastic volcanism played an important role at Apollinaris. Numerous valleys incised into the flanks of the volcano indicate that large volumes of liquid water were present at some time.

Gravity and magnetic observations place significant constraints on the structure of the Apollinaris Patera magma chamber. The free-air gravity anomaly is too large to be explained by flexurally supported topography and thus requires the presence of high density material buried beneath the volcano. The inferred cumulate chamber is 160 km in radius and 4-8 km thick; a narrower and thicker chamber is also consistent with the gravity observations. The magnetic anomaly requires a magnetized disk roughly 180 km across, located in nearly the same location as the gravity anomaly (Hood et al., Icarus 2010). The close correlation between the gravity and magnetic anomalies and the peak of the volcanic topography strongly suggests that both the gravity and magnetic signatures are due to a single structure formed as a result of the emplacement of the volcano. A likely interpretation is that the dense material is olivine and pyroxene cumulates in a now-solidified magma chamber. Such cumulates are similar to those observed in the nakhlite and chassignite martian meteorites, and a similar cumulate chamber occurs at Syrtis Major on Mars (Kiefer, Earth Planet. Sci. Lett. 2004). A possible explanation for the magnetic anomaly is solidification of Fe-rich basalt in a highly oxidizing environment, producing titanomagnetite. An alternative is later hydrothermal alteration of the basalt. Either possibility is consistent with liquid water in the magma chamber and thus with the observed pyroclastic activity. A similar mechanism may have operated at Hadriaca Patera, another martian pyroclastic volcano, for which gravity evidence requires a large cumulate chamber and for which electron reflectometry observations imply a significant magnetic anomaly in nearly the same location. Our results also imply that the core dynamo survived at least until the major construction of these two volcanos took place.