Geochemical and Volcanological Investigations of the East Pacific Rise


P.I. Michael R. Perfit


NSF funded project in collaboration with D. Fornari (Woods Hole), J.Bender (UNCC), R. Haymon (UCSB), M. Lilley (UW), K VonDamm (UNH), P. Shanks and I. Ridley (USGS), R.Lutz and T. Shank (Rutgers), M. Edwards (UH-SOEST)


We have been part of a multidiscipinary research team investigating the volcanic, tectonic, hydrothermal and biologic development of this fast-spreading East Pacific Rise between 9 degrees 17' and 10 degrees N since 1991. In April, 1991, we completed the AdVenture I Program, a 25-dive Ocean Drilling Program (ODP) site survey along the axial zone of the East Pacific Rise (EPR) between 9 degrees16'N and 9 degrees 54'N. This was a multi-institutional, multi-disciplinary study of the axial summit caldera (ASC) and active hydrothermal vents discovered during the Haymon/Fornari near-bottom ARGO survey in late 1989 (Haymon et al., 1991a). Those dives resulted in a comprehensive dataset on Mid-Ocean Ridge (MOR) volcanic and hydrothermal vent processes, as well as the identification of a promising spud-in site for the initiation of bare-rock drilling on Leg 142. During the AdVenture I program in April, 1991, we found that the distribution and settings of vents and vent biota had been significantly altered by volcanic eruptions that occurred during the 15 months between a 1989 ARGO survey (Haymon et al., 1991a) and the AdVenture I cruise. The AdVenture I dives revealed that the ASC is volcanically and hydrothermally active, principally between 9 degrees 45'N and 9 degrees 52'N. Our observations indicated that the EPR crest at 9 degres 45'-52'N was the site of vast, disorganized hydrothermal venting that represented the initial state of a ridge-crest system during and/or immediately following an episode of eruption, drainback, and collapse. These observations and measurements provide an excellent baseline dataset with which one can track with time the development of magmatic/hydrothermal systems and the associated fauna on a fast-spreading MOR crest. During our most recent cruise, detailed mapping and additional sampling with ALVIN concentrated on the section of the ASC that experienced an eruption during 1991 and early 1992 between 9 degrees 45' and 9 degrees 52' N and along off-axis traverses outside of the ASC in 5 different locales. In addition, we have extended our coverage in the 9 degree 37' N area where a small OSC exists and the EPR axis is propagating southward.

Visual observations and samples collected during the ALVIN dives confirm that a significant terrain up to 2.5 km away from the ASC on the crestal plateau has been affected by recent volcanism. Similar to what exits in the 9 degree 31' region, the youngest examples of off-axis volcanism are expressed as elongate pillow mounts (< 20 m high) principally focused along ridge parallel (NNW trending) fissures with no indication of prolonged hydrothermal activity. These pillow lavas are in stark contrast to the dominant sheet and lobate flows that comprise the terrain in and around the ASC.

Considerable effort has gone into compiling all of the ALVIN dive data from the various cruises where off-axis dives were made. This includes parsing the 2 second ALVIN attitude data and integrating it with interpolated, and smoothed 2 second navigation for the sub. Detailed bottom profiles for each dive have been made and the geology and sample locations for each dive are currently being added to provide the geological context for interpretation of the geochemical data and observational information recovered from the 35mm and video photography.

We have completed microprobe analyses of nearly all of the rock core glasses, XRF trace element analyses of about two-thirds of the ALVIN samples and ICP-MS REE analyses of a representative group of 10 basalts. Matt Smith ( UF Ph.D. student) has started processing representative samples for Sr and Nd isotopes. Analyses to date suggest a relatively homogeneous source for the N-MORB on this section of the ridge. J. Bender ( co-I at UNCC) has separated the REE in a dozen glasses and plans to analyzed them by TIMS at Stony Brook this summer. We have been working at the USGS in Denver in collaboration with I. Ridley and F. Lichtey analyzing some of the rock core glasses for trace element abundances by laser ablation ICP-MS. This technique is ideally suited for analyzing small quantities of homogeneous materials such as the glasses. The quantitative results we obtained suggest there is very good agreement with abundances obtained by other methods, the method is relatively quick and easy, and inter-sample comparisons are excellent. J. Dixon (RSMAS) has analyzed some of the samples for volatile contents and has found rather high values in some of the most mafic samples. The initial geochemical results indicate the most recent magmatic event associated with the present ASC erupted relatively homogeneous and mafic (> 8.0 wt.% MgO) basalts compared to the surrounding, older lavas (mostly less than 8 wt.%). The relatively primitive nature of the basalts is in agreement with Ra and Pa disequilibria data which indicate they have the shortest crustal residence ages along this segment of the EPR (Goldstein et al., in prep). The distribution of basalt types is not symmetric across the crestal plateau. Indeed, even the most mafic lavas appear to have a more extensive distribution to the east of the ASC; a feature in accord with the observation of lava breakouts in the 9 degrees 50.5' area. T- and E-type MORB only exist in small areas (< 1 km2) on the crestal plateau and appear to be related to prominent scarps or fissures from which pillows were erupted. Our most recent data suggest that eruptions on this active, fast-spreading portion of the EPR occur throughout the crestal region but possibly not as far as noted around 9 degrees 31' N. Relatively voluminous and mafic volcanism appears to be related to new magmatic episodes whereas off-axis volcanism is characterized by more evolved (and sometimes enriched) lavas fed from smaller and cooler magma bodies.

A cm-sized gabbro xenolith in a lava from the ASC has been the focus of our recent attention (Ridley et al., in press). It has a granular texture and is composed of approximately 80% crystals and 20% basaltic glass, the latter showing a range of compositions that are different from the host basalt lava. The homogeneous interstitial glass patches appear to be slightly more evolved than the basalt that hosts the microgabbro xenolith, having higher TiO2, FeO, K2O and Na2O and lower mg* values (0.61 versus 0.64). Thus the overall mesh-like texture of minerals + glass cannot result simply from disaggregation during transport in the host basaltic magma. Instead, we interpret the microscopic texture to represent a mineral-dominated mush in which interstitial melt may have mixed with surrounding basalt magma during transport to the ocean floor.

Publications related to the above award:

Haymon, R.M., D.J. Fornari, M.H. Edwards, S.C. Carbotte, D. Wright, and K.C. Macdonald, 1991, Hydrothermal Vent Distribution Along the East Pacific Rise Crest (9 degrees 09'-54'N) and its Relationship to Magmatic and Tectonic Processes on Fast-Spreading Mid-Ocean Ridges, Earth Planet. Sci. Lett. 104, p. 513-534.

Fornari, D., Perfit, M., Haymon, R., Edwards, M.H. and O'Brien, T., Morphology and Structure of the Axial Summit Caldera of the East Pacific Rise between 9 degrees-10 degrees N: ARGO and ALVIN Observations, (submitted)

Goldstein, S.J., Perfit, M.R., Batiza, R., Fornari, D.J., & Murrell, M.T., 1994. Off-axis Volcanism at the East Pacific Rise Based on Uranium-Series Dating of Basalts, Nature, 367, p. 157-159.

Mejia, V., Opdyke, N.D., and Perfit, M.R.,1996, Paleomagnetic field intensity study recorded in submarine basaltic glass from the East Pacific Rise, the last 69 Ka. Geophysical Research Lett., in press.

Perfit, M., Fornari, D., Smith, M., Bender, J., Langmuir, C., & R. Haymon, 1994, Small-Scale Spatial and Temporal Variations in MORB Geochemistry and Implications for Ridge Crest Magmatic Processes. Geology.

Perfit, M.R., Fornari, D., Smith, M., Bender, J., Langmuir, C. and Haymon R., 1994, Small-scale spatial and temporal variations in mid-ocean ridge crest magmatic processes., Geology, 22, 375-379.

Rubin, K.H., Macdougall, J.D. and Perfit, M.R., 1995, 210Po-210Pb dating of recent volcanic eruptions on the sea floor. Nature, 368, 841-844.

Shank, T.M., Fornari, Lutz, Haymon, Von Damm, Lilley, Perfit, Edwards, and Black, Geologic Setting of Hydrothermal Vents and Early Development of Biological Community Structure on the East Pacific Rise Crest at 9 degrees 49'-50'N.(in prep.)

Ridley W.I., Perfit, M.R., and Matthew Smith, 1995, Magmatic processes at spreading centers reflected in a microgabbro from the East Pacific Rise. EOS Transactions, 76, p. F694.

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Last update: July 19, 2000