Sims, Kenneth W. W.
Hart, Stanley R.
Reagan, Mark K.
Blusztajn, Jerzy S.
Sohn, Robert A.
Layne, Graham D.
Ball, Lary A.
Andrews, J. E.
We report 238U-230Th-226Ra-210Pb-210Po, 232Th-228Ra and 235U-231Pa measurements for a suite of 14 geologically and geochemically well-characterized basaltic samples from the Samoan volcanoes Vailulu'u, Malumalu, and Savai'i. Maximum eruption ages based on the presence of parent-daughter disequilibria indicate that Vailulu'u is magmatically productive with young lavas (<8 Ka) resurfacing both its summit crater and lower flanks. 210Pb and 210Po measurements indicate that several flows have erupted within its summit crater in the past 100 years, with the newest observed flow being erupted in November of 2004. For lavas which have eruption ages that are demonstrably young, relative to the half-lives of 230Th, 231Pa, and 226Ra, we interpret their 238U -230Th, 235U-231Pa and 230Th - 226Ra disequilibria in terms of the magmatic processes occurring beneath the Samoan Islands. (230Th/238U) > 1 indicates that garnet is required as a residual phase in the magma sources for all these lavas. The large range of (238U/232Th) and (230Th/232Th) is attributed to long-term source variation. The Samoan basalts are all alkaline basalts and show significant 230Th and 231Pa excesses but limited variability, indicating that they have been derived by small but similar extents of melting. Their (230Th/238U), (231Pa/235U) and Sm/Nd fractionation are consistent with correlations among other ocean island basalt suites (particularly Hawaii) which show that (230Th/238U) and (231Pa/235U) of many OIBS can be explained by simple time-independent models. Interpretation of the 226Ra data requires time-dependent melting models. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Samoan basalts. Several young samples from the Vailulu'u summit crater also exhibit significant 210Pb deficits that reflect either shallow magmatic processes or continuous magma degassing. In both cases, decadal residence times are inferred from these 210Pb deficits. The young coeval volcanism on Malumalu and Vailulu'u suggests the Samoa hot spot is currently migrating to the northeast due to dynamic interaction with the Tonga slab.
Sims, Kenneth W. W.
Basu, Asish R.
Reagan, Mark K.
The silica-undersaturated Nyiragongo volcanics, located in the East African rift, have
globally unique chemical compositions and unusually low viscosities, only higher than
carbonatite lavas, for terrestrial silicate magmas. We report 238U-230Th-226Ra-210Pb series
disequilibria in 13 recent and prehistoric lava samples from Nyiragongo including those
from the 2002 flank eruption and a 2003 lava lake sample. (230Th/238U) ranges from 0.90-
0.97 in the recent lavas and from 0.94-1.09 in the prehistoric lavas. To explain the variable
230Th and 238U excesses in these lavas, we hypothesize that different processes with
opposite effects in terms of fractionating Th/U in the mantle source are involved. These
processes include: 1) low degree partial melting of a phlogopite-bearing mantle source
(consistent with low K/Rb) with residual garnet (consistent with high chondrite-normalized
Dy/Yb), to produce the observed 230Th excesses; and, 2) carbonate metasomatism for the
238U enrichment, consistent with high Zr/Hf in the Nyiragongo lavas.
The Nyiragongo volcanics have higher (230Th/232Th) values than observed in most
mantle-derived rocks, especially ocean-island basalts, suggesting that their mantle-source
was affected by carbonate metasomatism less than 300 ka ago. Several Nyiragongo
samples display significant 226Ra excesses implying rapid magma transport (less than 8 ka)
from the mantle-source to the surface. Modeling the observed (226Ra/230Th) versus Zr/Hf
correlation in the lavas indicates that the 2002, 2003 and a few pre-historic lavas
incorporated 50-60% of a carbonate-metasomatized mantle source while the other prehistoric lavas show 10-22% contribution of this source. This result indicates that the
Nyiragongo lavas were derived from a heterogeneous, non-uniformly carbonated mantle
source. The 2002 lava shows (210Pb/226Ra) equilibrium, whereas the 2003 lava lake sample
shows initial (210Pb/226Ra) < 1. The latter observation suggests that Nyiragongo magmas
degas as they rise to the surface over years or decades before eruption. (210Pb/226Ra)
equilibrium in the 2002 lava suggests that the 2002 magma may have stagnated for more
than a decade before eruption. The high CO2 content, high emission rates, extreme fluidity,
along with the inferred short residence time and our inferences of rapid magma transport
and high eruptive frequency suggest that the volcanic hazards of Nyiragongo, both from
lava flows and gas emissions, are higher than previously estimated.