Comparison of Magma Residence, Magma Ascent and Magma-Hydrothermal Interaction at EPR 9°N and Endeavour Segment
Abstract
We compare magmas’ temperatures (Mg#s), their degree of crustal assimilation (“excess” Chlorine) and their residence depth and ascent speed (dissolved CO2 content) at similar scales, using new data for Endeavour and new and published [1] data for EPR 9°N. We relate differences between the two segments to other differences, e.g., depth and width of the AMC reflector. Cl in glasses, and Cl/K or Cl/Nb ratios, are indicators of magma’s interaction with altered crust, probably at the roof of the AMC [1,2]. An excess Cl (in ppm) value for each glass can be calculated by subtracting mantle-derived Cl from measured Cl. 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 [1]. Endeavour magmas on-axis have lower Mg# than EPR, while their 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. Excess Cl is similar on-axis between the two ridges. At both ridges, assimilation has a stochastic distribution, such that high- and low-Cl glasses are found in most locations. Because CO2 exsolution and bubble formation is slow compared to magma ascent and surface flow, many glasses are oversaturated compared to their eruption depth. Dissolved CO2 contents thus provide information about the duration of a magma’s transit between its last stopping point and final lava emplacement. If magma erupts and cools quickly, its dissolved CO2 should correspond to its last resting point, possibly the AMC. At EPR 9°N, maximum CO2 contents would be in equilibrium at the AMC roof, while minimum CO2 contents are nearly in equilibrium with collection depths. Glasses have high CO2 on-axis and low CO2 off-axis, and there is a negative correlation between CO2 and distance off-axis [1]. This is partly due to post-eruptive flow away from the axis, as seen in the 2005-6 flow where CO2 decreases and bubble size increases away from the eruptive vent [3]. CO2 contents of Endeavour glasses are lower in general than EPR 9°N glasses despite their deeper AMC [4], suggesting that they had more time to exsolve. Highest values are lower than the CO2 content corresponding to the AMC roof, while lowest values are in equilibrium with their seafloor depths. 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 that cannot be ascribed to post-eruptive flow away from the axis, because they occur outside an enclosed axial valley. The comparison of the CO2 and Cl data from the two ridges does not support a simple interpretation in which fast-rising magmas are less likely to interact with hydrothermally altered crust. [1] leRoux et al. (2003) EPSL 251, 209-231. [2] Michael & Schilling (1989) GCA 53, 3131-3143. [3] Michael et al. (2008) Fall AGU #V21B-2106. [4] vanArk et al., (2007) JGR 112, doi:10.1029/2005JB004210;
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V11A2241M
- Keywords:
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- 1032 GEOCHEMISTRY / Mid-oceanic ridge processes;
- 8416 VOLCANOLOGY / Mid-oceanic ridge processes