Compositional Variation in Magmas Supplied to the Southern East Pacific Rise, 17°-19° S: Implications for Magma Reservoir Dynamics

Fine-scale observation and sampling of lavas from the southern EPR 17°-19° S reveal both short- and long-term compositional heterogeneity of flows produced in single eruptive episodes. Located between 17° 24' and 17° 36'S, the 140 x 10⁶ m² Aldo-Kihi flow reaches a maximum width of 2.2 km between 17° 26' and 17° 28'S; the presence of sheet flows, lava channels, and summit collapse troughs imply that the eruption was centered in this area of broad axial morphology. Some lava channels and collapsed lava tubes extend beyond the margins of the recently erupted Aldo-Kihi flow, indicating that lava distribution systems can persist over at least several hundreds of years and multiple separate eruptions were apparently centered in this region. Extensive glass analyses of the Aldo-Kihi flow show that MgO contents range from 7.7-8.4 wt %; all the samples with greater than 8.0 wt % MgO occur south of 17° 30'S. This result is hard to reconcile with along-axis propagation of a single dike, and suggests vertical eruption from a magma chamber that is compositionally zoned along-axis. Twenty-three other samples older than Aldo-Kihi contain > 8.0 wt % MgO; all but two occur south of 17° 28.4'S suggesting that the displacement of eruptive centers from the location of hottest subaxial magma is a long-lived feature of this region. Lack of compositional variation across some contacts indicates that this length of ridge has erupted compositionally similar lavas in separate volcanic episodes. Elsewhere distinctly different lava compositions include the several-hundred-year-old Rehu-Marka Fe-Ti basalt, and local occurrences of incompatible element-enriched T-MORB. The distribution of rock types in this area requires a complex history of mantle melting, recharge, cooling, and eruption that has been spatially systematic over time scales encompassing several eruptive episodes. Between 18° 31.5' and 18° 34.5'S the South Hump lava is distinctly bimodal with highly evolved ferrobasalts (MgO < 6.2 wt %, Na₂O < 3.1 wt %) in the south, and basalts (~7.2 wt % MgO, Na₂O > 3.1 wt %) in the north. Major and trace element data for these lavas indicate that they cannot be related by low-pressure fractional crystallization and that variations in mantle source composition appears to be required to explain the observed compositional variation. Existing geological evidence indicates that either the axial graben here was resurfaced by a single, chemically bimodal eruption, or by two adjacent eruptions that must be close in age. The boundary between the two chemical types corresponds to discontinuities in the axial magma chamber imaged by seismic reflection, as well as to significant differences in the thickness of seismic layer 2A. This result indicates that magma arising from vastly different melting processes can reside in adjacent crustal magma reservoirs and be tapped, either during a single eruption or during closely spaced eruptions in adjacent regions.