Perspectives on 40ar 39ar dating

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Open symbols are whole-rock data; filled symbols are electron microprobe data for interstitial glasses. Ilm, Ilmenite; Tmt, Titanomagnetite; n. White and grey pumice from the MTP are similar in composition Fig. Amphibole is edenite—hornblende Table 5. Ilmenite and lesser amounts of titanomagnetite are present as single crystals, attached to rims, or as inclusions in other phenocrysts biotite, orthopyroxene and hornblende. Both ilmenite and titanomagnetite are the same composition in both white and grey pumice Table 6. Juvenile UTP samples consist of white pumice, grey pumice, and fragments of the Ombligo central dome, all of which are dacitic in composition, with very little variation amongst them Fig.

All samples are porphyritic, with pumice containing 31—37 vol. Pumice is highly vesiculated with rhyolitic glass Table 2 forming the bubble walls, whereas the dome rock has a microlite-rich matrix and only minor glass Fig. In order of abundance, phenocrysts are plagioclase, orthopyroxene and hornblende, with minor Fe—Ti oxides, biotite and rare apatite as inclusions in orthopyroxene.

Biotite appears either rimmed with small intergrowths of amphibole, orthopyroxene, Fe—Ti oxides, and plagioclase, or with resorbed margins Fig. Plagioclase varies in abundance between UTP samples Table 1.

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Euhedral, subhedral, and resorbed margins are observed in all samples. Orthopyroxene represents 4—5 vol. Amphibole phenocrysts are euhedral and range in size significantly. In addition, some of the larger phenocrysts have inner dissolution zones Fig. Compositions vary little between cores and rims Table 5 and are generally edenite to pargasite—hornblende. Ilmenite and lesser titanomagnetite are present attached to phenocrysts mainly of hornblende and orthopyroxene or as individual crystals in the matrix.

All samples contain two populations of phenocrysts: Biotite is present only as large phenocrysts and is always partially resorbed or rimmed by reaction products.

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The lack of euhedral microphenocrystic biotite suggests that it is not stable in the three magmas. Crystallinity varies amongst the samples; the Ombligo dome contains 43 vol. This temperature for the dome reflects the different composition of the titanomagnetite Table 6. To constrain the extent of Fe—Ti oxide re-equilibration, we calculated maximum oxygen fugacities and temperatures for the three magmas utilizing the compositions of orthopyroxene, magnetite, and ilmenite in our samples, and assuming that the silica activity was nearly unity.

Interestingly, the temperatures and oxygen fugacities obtained are very similar to those calculated with the two oxides, even for the dome see Fig. This indicates that orthopyroxene, titanomagnetite, and ilmenite are in equilibrium in all rocks, except for the dome, which displays a large temperature range Fig. The presence of large phenocrysts and microlites in the dome samples suggest that the magma underwent slow ascent and cooling, promoting rapid re-equilibration of the Fe—Ti oxides as proposed elsewhere e.

Frost, ; Lindsley et al. This process modified the Fe—Ti oxide compositions resulting in an apparently higher temperature. Microphenocrysts of plagioclase, orthopyroxene, hornblende, and Fe—Ti oxides are euhedral and appear to be in equilibrium with the surrounding matrix. Biotite has reaction rims of hornblende, orthopyroxene, and plagioclase Fig.

Instead, it probably occurred while the magma was stored in a deep reservoir. To explore the possibility that the biotite is xenocrystic, we determined its Ar isotopic composition, because this technique has been used successfully to identify xenocrysts in other magmatic systems e. Lo Bello et al. The detailed analyses of these four grains are reported in Appendix B and the age spectra are shown in Fig. The poor precision of the analyses provides no additional information about the age of eruption or the potential presence of xenocrysts; consequently, the data are not included in the paper.

Grey boxes denote non-zero age fractions. Diffusion loss profiles for each sample are denoted by the bold line. Thus, our modeling implies an amount of argon loss that prevents us from determining the true source or age of the xenocrysts based on the age spectra alone. As biotite xenocrysts would probably be highly reactive in the host dacitic magma they are likely to have only short residence time. The presence of reaction rims on the biotite crystals may have acted to reduce their contact with the melt, and hence extend their lifetimes. It is likely, however, that because biotite is present in all three of the eruptions, and those eruptions span about years, contamination by xenocryst incorporation was continual, and the biotite crystals in each of the deposits were introduced just prior to each eruption.

It is, however, possible that some of the larger biotite xenocrysts would retain a trace of their original radiogenic argon, such as seen in the four crystals analyzed Fig. Relatively short times between incorporation and eruption have also been proposed for xenocrysts in other magmatic systems e. It is noteworthy that the pumice clasts in the three deposits have a population of large phenocrysts of plagioclase, pyroxene, and hornblende with disequilibrium textures.

These mineral phases may also represent xenocrysts incorporated into the magma, but because the latter was saturated with these phases, they did not completely dissolve. To constrain the stability field of this mineral assemblage, we performed hydrothermal experiments using natural samples of UTP see Appendix A for methods.

All experiments were run water-saturated and above the solidus all experimental conditions are listed in Table 8 , and so all runs contain melt and water vapor. Orthopyroxene and Fe—Ti oxides were present in all the charges.

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The stabilities of plagioclase and hornblende vary with temperature and pressure Fig. In contrast, hornblende is stable at higher temperatures as pressure increases. In contrast, abundant hornblende and plagioclase exist together in the dome sample. Thus, those higher temperatures are not considered further. Despite the common presence of biotite in the natural samples, biotite did not crystallize in any of our experimental runs, and considering our petrographic observations that biotite is always resorbed or in reaction with the melt, the experimental results support the conclusion that biotite is xenocrystic.

It could be argued that biotite might be stable under more oxidized conditions e. We note, however, that even in our most oxidized sample BAF biotite is in reaction with the melt, and thus if biotite was originally phenocrystic it would require all of the magmas to have been much more oxidized up to one log unit greater. Electron microprobe analyses of biotite crystals in the Toluca samples did not detect sulfur, suggesting that its presence was not important in these magmas.

We believe, therefore, that our experimental results are consistent with the conclusion that biotite is xenocrystic in the three magmas. The arrowheads represent melting pointing to the right and crystallization pointing to the left experiments. All run conditions are listed in Table 8.

Mineral-in curves are shown for orthopyroxene Opx , amphibole Amph , and plagioclase Plg. Residual melt quenched to glass is the most abundant phase in all the experimental runs and varies systematically in composition as a function of pressure and temperature Fig.

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Variation of experimental glass compositions as a function of pressure and temperature. Continuous lines represent isobaric curves. We did not perform experiments on MTP samples, but their bulk compositions are similar enough to the UTP pumice Table 7 that it is unlikely that the results would be different. Again, assuming that biotite is xenocrystic, the similarities in glass composition and phase compositions to UTP samples, and the only slightly lower temperature, suggest that there was little difference in water pressures between the MTP and UTP magmas.

We lack compositional data for matrix glass in BAF, but its similar phase assemblage and bulk composition to the MTP and UTP samples suggest that its storage conditions were also similar to those for the other magmas. All three magmas have similar whole-rock compositions, mineral assemblages, mineral compositions and pre-eruptive conditions, and contain biotite xenocrysts. There are two possibilities to explain how the plumbing system could have produced compositionally identical magmas over years of activity.

This would require that each partial melting event occurred to essentially the same extent each time, involved the same crustal substrate, and that the resultant magma ascended to the same shallow level before erupting.

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Although we cannot rule out the first hypothesis, we believe that the second is more likely, given that each magma appears to come from the same storage level, at about the same temperature, and that all contain biotite xenocrysts. Given our hypothesis that the same magma body was tapped three times, minor differences in crystal content and glass composition between the grey and white pumice of the three eruptions could reflect their origin from different levels in the magma storage reservoir.

In the case of the UTP samples, the grey pumice is slightly cooler than the white pumice Table 7 , suggesting that the grey pumice could have come from the upper, cooler part of the magma reservoir whereas the white pumice was derived from the main part of the magma body. The stratigraphic section through the UTP Fig. This may represent slight warming of the magma reservoir with time, probably by injections of mafic magma at the base of the chamber. The observed disequilibrium textures of the large phenocrysts may originate from such injections.

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Founded in , we are the oldest geological society in the world. Argon extraction from geological samples by CO 2 scanning laser step-heating. Ar diffusion and solubility measurements in plagioclases using the ultra-violet laser depth-profiling technique. Cooling, uplift and implications for orogeny dynamics. Observation of centimetre-scale argon diffusion in alkali feldspars: When did the first flowers bloom? Timing of geothermal activity in an active island-arc volcanic setting: Issues in dating young rocks from another planet: Geoscientists vol 1 Feb All sections of the volume are well written and edited, concisely laid out with clear and appropriate figures, photographs and data-tables — features that one has come to expect from the Geological Society Special Publication series.

In summary, the volume is a comprehensive and significant contribution to the field, and the editors are to be congratulated. I expect this will become a valuable complementary volume to the established principal reference work for the technique. American Mineralogist, vol February-March …the real strength of this book lies in the overall scientific quality of the chapters and diversity of the subject matter: In addition, the chapters that highlight sub-grain sample characterization and analysis provide a glimpse into research directions that should be more accessible with improved technology.