A Comparison of Magma Residence, Magma Ascent and Magma-Hydrothermal Interaction Between EPR 9°N and Endeavour ISS

P. Michael¹*, J. Gill², F. Ramos³, & J. Woodcock²

Corresponding author: pjm@utulsa.edu
¹Department of Geosciences, The Univ. of Tulsa, Tulsa, OK 74104
²Earth and Planetary Sciences, UCSC, Santa Cruz, CA 95064
³Dept. of Geological Sciences, New Mexico State U., Las Cruces, NM, 88003

Abstract:
We explore magmatic behavior using new volatile and major element data for Endeavour ISS basalt glass. Mg# indicates a magma’s temperature and the amount of crystallization it has experienced. Chlorine is a sensitive indicator of magma’s interaction with seawater-altered crust, probably at the roof of the AMC (Michael & Schilling, 1989). Cl/K or Cl/Nb are good indices of assimilation because they should not change due to crystallization, and mantle values are fairly constant or predictable. The excess Cl for magmas can thus be calculated. Typically, assimilation has a stochastic distribution, such that high- and low-Cl glasses are found in most locations.

During magma ascent and decompression, MORB magmas become oversaturated with CO2. Exsolution (bubble fmn) is slow compared to magma ascent, so many glasses are oversaturated compared to their depth of eruption. Dissolved CO2 contents thus provide information about the amount of time that a magma was in transit between its last stopping point and its final emplacement. If a magma erupts and cools quickly enough so that it produces no bubbles, its dissolved CO2 content should correspond to its last resting point, which may have been the AMC.

Contributions to Integration and Synthesis:
We can compare magmas’ temperatures (Mg#s), their degree of crustal assimilation (“excess” Chlorine) and their residence depth and ascent speed (dissolved CO2 content) with the EPR 9°N at similar scales (data of le Roux et al.,2003 ; Hauri and Saal,2009). At 9°N, excess Cl is negatively correlated with Mg#. Mg# is lower and excess Cl is higher off-axis (up to 4 km). At a given Mg#, Cl is higher off-axis (leRoux et al., 2003). Endeavour magmas on-axis have lower Mg# than EPR, while Mg# ranges are similar off-axis. At Endeavour, there is no good correlation of excess Cl with Mg#, although glasses with high Mg# are found mostly on-axis. There is no trend of Mg# or excess Cl with distance from the axis. The excess Cl is similar on-axis between the two ISS. At EPR 9°N, maximum CO2 contents correspond to the pressure of the roof of the AMC, while minimum CO2 contents are almost in equilibrium with their collection depth. Glasses have high CO2 on-axis and low CO2 off-axis, and there is a negative correlation between CO2 and distance off-axis (leRoux et al., 2003). This can be attributed at least in part to post-eruptive flow of lavas away from the axis, which is supported by study of the 2006 flow that shows that CO2 decreases and bubble sizes increase with distance from the eruptive vent (Michael et al., 2008). CO2 contents of Endeavour glasses are lower in general than EPR 9°N glasses despite the deeper AMC there (vanArk et al., 2007), suggesting that they have had more time to degas. The highest values are significantly lower than the CO2 content that would be in equilibrium with the roof of the AMC, while the lowest values are in equilibrium with their collection depth on the seafloor. Either the lavas took longer to ascend from depth or they flowed longer at the surface, or both. Young off-axis lavas on Endeavour have low CO2 which cannot be ascribed to post-eruptive flow away from the axis, because they occur outside of the enclosed axial valley. The comparison of the CO2 and Cl data from the two ISS do not support a simple interpretation in which fast-rising magmas are less likely to interact with the hydrothermally altered crust. We explore more complicated scenarios.

We relate the differences between the two ISS to the depth and width of the AMC reflector. We also explore for small-scale correspondence between magma behavior and the presence and characteristics of hydrothermal behavior and the characteristics of the AMC within Endeavour ISS.