Enrichment in H2O and elevated Fe oxidation states are linked to material recycling in Izu-Bonin-Marianas lavas
Abstract
Distinct geochemical signatures of subduction related lavas include enrichments in H2O and incompatible trace elements, and higher Fe3+/Fe* ratios. Water and trace element enrichments in subduction lavas clearly reflect the influence of fluids and melts released from subducting slabs, but whether Fe oxidation is similarly linked remains debated. Here, we summarize measurements of major, trace, and volatile contents and Fe3+/Fe* ratios of submarine glasses and olivine hosted melt inclusions from the Izu-Bonin-Mariana (IBM) to test several recent hypotheses for the cause of Fe oxidation in subduction related lavas. The dataset includes volcanic rocks spanning the life of the arc from 52 ma to the present, and ranging spatially from the fore-arc to the back-arc and along strike from IODP Expedition 352 (c. 28°N) to dredged pillows from the southernmost Mariana trough (c. 12°N). Measurements for all elements are made in situ, and Fe3+/Fe* ratios are measured via m-x-ray absorption near edge structure spectroscopy (in situ) or micro-colorimetry (bulk glass).
Glasses include basalts, basaltic andesites, and boninites with 1-8 wt% MgO, 0.16-6.0 wt% H2O, <50-1450 ppm CO2, and <50-2100 ppm S. The Fe3+/Fe* range from 0.136-0.250 (mid ocean ridge basalts = 0.124-0.156), with Ba/La between 3.4 and 62. Independent of age or sample type (i.e., melt inclusion or submarine glass), Fe3+/Fe* are relatively invariant with decreasing MgO, H2O, or S, indicating that low-pressure differentiation does not significantly modify the oxidation state of Fe in subduction related lavas. Erupted samples with Fe3+/Fe* > 0.18 (i.e., the most oxidized glasses) have from <50 ppm to >1400 ppm S, indicating that S contents at the time of quench does not generate the high Fe3+/Fe* of arc lavas. Oxidation also appears to be unrelated to crustal thickness. Samples with H2O >2 wt% and Ba/La >10 always have Fe3+/Fe* >0.18. Both modern and Eocene-aged samples define a relationship whereby Fe3+/Fe* increasing nonlinearly by a factor of ~2.3 as Ba/La increases by a factor of ~13. The coherence of elevated Fe3+/Fe* ratios with elevated H2O and Ba/La in both space and time strongly links the production of oxidized, hydrous lavas with the release of aqueous slab fluids into the mantle in the IBM system.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V54B..05B
- Keywords:
-
- 3613 Subduction zone processes;
- MINERALOGY AND PETROLOGY;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8185 Volcanic arcs;
- TECTONOPHYSICS;
- 8413 Subduction zone processes;
- VOLCANOLOGY