Ofu and Olosega

Ta'u and Ofu/Olosega volcanoes: The" Twin Sisters" of Samoa, their Samoa enamel regimes and major consequences - Hart - 2014 - Geochemistry, Geophysics, Geosystems

Samoan Ta'u and Ofu/Olosega (Ofol in the following) are individual peak volcanos that have broken out in the last 70 and 440 kilograms of alkaline base. Scientists are publishing isotope information for these islets; here we are reporting the first main and microelement information for Ofol. Her tall 3He/4He designates her as a FOZO coat finish member.

Through the comparison of the volcanoes' results, we test the effectiveness of melt patterns in limiting the shell composition and its P-T of the smelt. Let us show that the coat springs are similar, with spidergrammes culminating in Ta (4× BSE) and Lu ?. 5× BSE. Melt and cladding wells both have Lu/Hf rates that are too low to sustain measurable 176Hf/177Hf rates, indicating a recent accumulation in the cladding well.

The degree of smelting is 6. Five percent for Ta'u and five. Two percent for Ofol. P-T of the melt show a broad, overlay area, but give an accurate matrix; the mean temperatures are 1475°C-77 km for Ta'u and 1550°C-110 km for Ofol. Lowest P-T estimation is 4.2 Ga and 1550°C. P-T arrays are either a fusing adiabatic or a mixed line of molten masses balanced at different dephts.

P-T estimations for smelting can usually be minimal levels. Ta'u and Ofol are the two most eastern volcanoes of the Samoa flag line, see Fig. 1. While the overall nature of the necklace is strongly EM2[Wright and White, 1987; Workman et al., 2004; Jackson et al., 2007b; Hart and Zindler, 1989], these two isles are free of GM2 and have a FOZO logo that extends to very high 3He/4He[Jackson et al., 2007a].

It is a one-of-a-kind way to limit the geochemistry of the FOZO cladding spring and to study the pressures, temperatures and types of smelting regimes and systems of smelting transportation in these two Samoan volcanos. Notice that this document represents the first main and microelement information for Ofol; examples are shown in Figure 1 and their position and descriptions are given in Annex Table A1.

Principal and micronutrient datasets for Ta'u and all isotope datasets have already been released. Section 3 reviews the existing K-Ar aging figures showing that the aging for Ofol is 440 kilograms and younger, versus an aging of less than 70 kilograms for Ta'u.

Part 5 shows the resulting clear but very small isotope difference between Ta'u and Ofol. Part 6 contrasts micronutrient samples, both volcanos being similar and both having the significant Nb, Ta and Ti positives that are the trademarks of the high 3He/4He basalt and FOZO cladding final members[Jackson et al. 2007a, 2008].

In Section 7 Mantel spring music is modelled using the Minster and Allegre[1978] technique, and Ta'u and Ofol give unexpectedly similar wells. Looking at a possible long-term isotope development of these resources in Section 8 indicates that these shell composition have been recently modified. Part 9 uses the petrological models of Lee et al. [2009] to deduce P-T estimations for the smelting of Ta'u and Ofol cladding.

The kinetic modelling is used in Section 10 to evaluate the possible impact of the rebalancing of melt wall rocks on these P-T-estimations. On the first geological survey of these archipelagos Daly[1924] followed by Stearns[1944], and the most recent is the extensive account of Stice and McCoy[1968], which defines five vulcanic batches on Ta'u and three on Ofol (Ofu and Olosega are parts of the same volcano).

While Stice and McCoy judge the form of these two volcanos as a result of the calendara formations, we concur with Daly[1924]'s previous assertion that the form of Ta'u was caused by a great lateral collapse (failure) on the north. We believe that the Ofol Volcano, from what used to be a form similar to today's Ta'u, to its present form, which is split into two neighbouring islets, has been shaped by a longer story of repeated flanking falls andvalanches.

Exposition of the large end wall on the southern side of Ta'u was recently set at 22 kyr[Williams et al., 2014], indicating that its origin is related to the low ocean floor during the last glacier apex. Ofol volcano's lower degree of erosion is also indicated by the existence of a large dyke system in the narrow reaches between Ofu and the Olosega Islands (samples 04-02,-03,-05,-06).

The analysed specimens have less than 49% silicon dioxide, the Mg value is 35. K-Ar Era are now available for Ta'u, Ofu and Olosega Islands[McDougall, 2010]. There are four specimens of the oldest formations on Ta'u (Lata) ranging from 20 to 70 Kyr (±10-20 Kyr). On Ofu/Olosega (Tuafanua), seven specimens of the major sign formations vary from 240 to 440 Kyr (±10 Kyr).

Whilst this is complete K-Ar merger information, it is said that the rock is not weathered and there is no evidence of excessive Ar-problem. In two specimens from the western crack of Ofu give 40Ar/39Ar platform aging of 270 ?±COPY11±? kyr and 560 ?±COPY11 2011_yr, while one specimen from the southern crack gives a platform aging of 490?±? kyr[Koppers et al., 2011].

This information provides convincing proof that Ta'u is generally younger than Ofu/Olosega, which is also strongly confirmed by the two volcanos' relatively high morphology. That is remarkable near the mean of the 275 kilogram specimens of McDougall[2010]. This is not likely to be more than a general indication of the relatively old volcanos, just as we make the same computation for Vailulu'u (a U-boat volume that is the flag's working location) versus Ta'u (45 km distance), we would anticipate an aging gradient of 620kyr whereas the oldest dating specimen from Ta'u is only 70kyr (but there are clearly older basalt on Ta'u at greater lows in the shield).

Proof of young post-glacial volcanic activity (