Kenneth Rubin - Po dating

Ken Rubin/SOEST Isotope Lab Research

----____ ²¹⁰Po-²¹⁰Pb Dating of Very Young Submarine Basalts ____----

The ²¹⁰Po-²¹⁰Pb dating technique provides ages of eruptions having occurred within about 2 years of sample collection (Rubin et al., 1994). With this method one monitors the rate of change of ²¹⁰Po with time in a lava once it is sampled, to determine the date of eruption. Upon eruption, ²¹⁰Po is degassed by the lava while its radioactive grandparent ²¹⁰Pb is not significantly volatilized. Following eruption, this Po isotope "grows back" toward radioactive equilibrium with ²¹⁰Pb over a 138.4 day half-life. This phenomenon is illustrated in Figure 1.
We developed this dating method back in 1991 after divers in the manned submersible Alvin appeared on the scene soon after a submarine eruption at 9° 50'N on the East Pacific Rise (EPR), to determine when the eruption had occured. We have also applied this technique to lavas collected after suspected submarine eruptions at NW Rota-1 seamount, the Northeast Lau Spreading Center, Axial Seamount (project underway), the 2005-06 eruption at 9° 50'N and 2004 eruption 10° 45'N on the EPR, the Gorda Ridge, Boomerang Caldera (near South East Indian Ridge), and the Loihi Eruption in Aug 1996.

Figure 1. Changes in ²¹⁰Po in a single lava revealed by repeated measurements over time. Regression of the data to an ingrowth curve allows one to date the time since degassing (eruption).

Our best-estimate of the timing of three example eruptions .
Figure 2. ²¹⁰Po-²¹⁰Pb dating results from 3 eruptions on the EPR and Gorda Ridges. Ages are given as eruption windows (black bars), which represent unavoidable uncertainty in the extent of initial Po degassing. Heavy dashed lines show likely eruption intervals. Po is completely degassed upon eruption at subaerial and shallow submarine volcanoes (see review by Rubin, 1997). We are not certain this occurs at oceanic depths of 2-3 km, so we report an “eruption window” based on a conservative minimum degassing estimate and 100% degassing. The 2-month window we currently report is based on observations at 9°50'N EPR (panel A), although the still preliminary data from 10°44'N EPR (panel c) may allow us to narrow eruption window widths because 3 of 4 samples were >75% degassed of Po upon sample collection. Errors in maximum age (gray horizontal bars) reflect data regression and analytical errors (see the eruption window inset). Sample collection dates are depicted by a --|, coinciding with cruises to those areas (vertical gray bars). Absolute and relative times are given at the base of the plot. Note the difference in error on maximum ages for Gorda samples taken in April and August 1996 (ages overlap for all samples in panel B but early sampling led to much higher resolution ages). The young lava flow image is a video frame grab from Alvin dive 3935 of the FIELD expedition (provided by Robert Zierenberg). An example ingrowth curve and the seismic record that led to the Gorda event response are also shown, tied to calendar time and sample ages.

This figure is from an article in the NSF Ridge2000 program newsletter (Spring 2005, which is available in pdf-format)

For more info please see these papers, and the references therein:

KH Rubin, SA Soule, WW Chadwick, DJ Fornari, DS Clague, RW. Embley, ET Baker, MR Perfit, DW Caress, RP Dziak (2012) Volcanic Eruptions in the Deep Sea, Oceanography, 25(1), 142–157, http://dx.doi.org/10.5670/oceanog.2012.12.

ET Baker, WW Chadwick, JP Cowen, RP Dziak, KH Rubin, DJ Fornari (2012) Hydrothermal discharge during submarine eruptions: The importance of detection, response, and new technology, Oceanography, 25(1), 128–141, http://dx.doi.org/10.5670/oceanog.2012.11.

Rubin, K.H., Macdougall, J.D. and Perfit, M.R. (1994) ²¹⁰Po-²¹⁰Pb dating of recent volcanic eruptions on the sea floor, Nature, 368, 841-844.

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