The Origin of Transparent and Non-Transparent White Pumice: A Case Study of the 52ka Maninjau Caldera Forming Eruption, Indonesia

The 52ka eruption of Maninjau caldera produced two distinctive type of white pumices: transparent (TWP) and non-transparent (NTWP). Both pumice types are crystal-poor (avg. 3.3 %), having similar mineralogy (pl > qz > bt > px > opq), similar glass compositions (avg. 78.5 wt. % SiO2), and similar plagioclase core compositions (avg. An20-30). We found that the abundance of TWP decrease towards the upper stratigraphic positions, together with the increase in NTWP, grey pumice, banded pumice, and lithic contents. Vesicles in TWP are typically dominated by large vesicles, while NTWP is characterized by abundant-small vesicles. Large vesicle corresponds to the preexisting bubble which formed in magma chamber (pheno-bubble, > 0.1 mm). On the other hand, small vesicle in groundmass (matrix-bubble, < 0.1 mm) is attributed to second nucleation in the conduit during the eruption. We performed quantitative comparison using pheno- and matrix-bubble number densities (PBND and MBND) for these two white pumice types. The correlation between PBND and MBND result in two regimes: (1) decompression-controlled regime, showing nearly constant-PBND correlation for TWP, and (2) phenobubble-controlled regime, showing steeply-decreasing PBND correlation for NTWP. In the first regime, MBNDs value varies dramatically, suggesting the variation of decompression rate by two to three orders of magnitudes. While in the second regime, the slight increase of MBNDs is considered as the effect of the decrease in PBND within the nearly constant decompression rate.

Br diffusion in phonolitic melts: Comparison with fluorine and chlorine diffusion

Bromine diffusion was measured in two natural phonolitic melts: (1) a K2O-rich (~10 wt%) one synthesized from the white pumice phase of the 79 AD eruption of Vesuvius (Italy), and (2) a Na2O-rich (~10 wt%) one corresponding to the most differentiated melt of the 12 000 BC eruption of the Laacher See (Germany). Experiments were performed at 0.5 and 1.0 GPa, 1250 to 1450 °C, at anhydrous and hydrous (2.65 ± 0.35 wt% of dissolved water) conditions. Experiments conducted with the diffusion-couple technique in the piston cylinder were performed with only bromine diffusing and with the simultaneous diffusion of a halogen mixture (F, Cl, Br) to evaluate the interactions between the halogens during diffusion. The diffusion profiles of Br were measured by X-ray fluorescence using synchrotron radiation microprobe (SYXRF), ID18F, at the European Synchrotron Radiation Facility (ESRF, France). Bromine diffusion displays Arrhenian behavior under anhydrous conditions that is similar when it diffuses alone and when it diffuses with F and Cl. The Br diffusion coefficients range between 2 × 10–12 m2/s at 1250 °C and 1.5 × 10–11 m2/s at 1450 °C for the Na-rich melt and between 3 × 10–12 m2/s at 1250 °C and 2.5 × 10–11 m2/s at 1450 °C for the K-rich melt, at 1.0 GPa. Although Br mobility is independent of F and Cl in anhydrous phonolitic melts, its behavior may be dependent on the dominant alkali in the melt, as previously observed for Cl, but not F. For hydrous experiments, although the data are scattered, the Br diffusivity increases slightly with water and the Na/K ratio seems to influence Br diffusivity. Similarly to noble gases, halogen diffusivity at a given temperature in the phonolitic melts appears related to the ionic porosity of the silicate structure. Compared to basaltic melt, Br diffusivities are approximately one order of magnitude lower in the Na-phonolite melt, because of the difference of the pre-exponential factor. Br mobility appears to be decoupled from melt viscosity, considering the results here.

Pómez Bosque de Tlalpan, producto de una erupción de gran magnitud en el margen suroeste de la cuenca de México

In this contribution we describe a white pumice fall deposit, informally named pómez Bosque de Tlalpan (PBT), found in several outcrops in the southern Mexico basin. The most representative sequence occurs at the Bosque de Tlalpan park, Delegación Tlalpan, in Mexico City. In this site the deposit is 3 m thick, massive, and contains 80–90 vol. % of pumice clasts, predominantly of coarse lapilli-sized and some block-sized fragments. The PBT has a dacitic composition (64.9–66 wt. % SiO2, on anhydrous basis) of calc-alkaline affinity, and a mineral assemblage represented by plagioclase > amphibole > orthopyroxene > biotite, ±Fe-Ti oxides, with quartz in lesser proportions, and zircon as accesory mineral, in a vesicular and glassy matrix. According to data collected from seven outcrops, thickness and clast size of the deposit decrease towards the NE and therefore the vent source should be located to the SW of Bosque de Tlalpan. Possible sources are the volcanoes Ajusco and San Miguel, which are part of the Sierra de Las Cruces volcanic range, and are located in this direction at ~11 km from Bosque de Tlalpan. The age of the PBT deposit was determined by radiocarbon dating of underlying paleosoils that yielded 25,730 ± 130 to 37,450 ± 330 yrs B.P. (Late Pleistocene). Additionally, thermoluminscence dating of one sample yielded an age of 30,300 ± 5,000 years, similar to the radiocarbon ages. Hence, the PBT represents the youngest reported deposit from the Sierra de Las Cruces volcanic range. Although only seven outcrops were described in this work, we interpret the PBT deposit as produced by a plinian-type eruption, based on its massive structure, its ca. 3 m thickness, and the presence of pumice clasts in coarse lapilli to block sizes, as well as 3.5 cm sized lithics, at 11 km from the possible vent source, characteristic of this kind of deposits. We discard the monogenetic volcanoes from the Chichinautzin volcanic field as the vent source of the PBT deposit, because this kind of volcanoes is relatively mafic in composition and do not produce plinian-type deposits.

Submucous implant of abrasive dental materials in rat tongues: clinical and histological evaluation

This study evaluated the clinical and histological manifestations produced by the submucuous implant of abrasive dental materials in the rat tongue. A total of 128 rats were divided into 4 groups of 32 rats each, according to the material selected for the implant: Herjos-F prophylactic paste both in its normal composition as well as lacking its abrasive components, SS White pumice stone and the abrasive powder of the 3M finishing and polishing sandpaper. The specimens were submitted to clinical and histological analyses at 30, 60, 90 and 120 days. After 90 days, the formation of nodular lesions in animals implanted with materials containing abrasive substances was observed. Histologically, these materials produced marked chronic granulomatous reactions. Herjos-F prophylactic paste produced the greatest reaction. However, without the abrasive components, this paste caused the mildest tissue reactions, with no inflammatory response, that was seen in a progressively greater number of cases after 90 days.