Toshiaki Hasenaka

The cinder cones of Michoacán-Guanajuato, Central Mexico: their age, volume and distribution, and magma discharge rate

The Michoacan—Guanajuato Volcanic Field (MGVF) in central Mexico contains over 1000 late Quaternary volcanic centers, of which approximately 90% are cinder cones. This area is distinct from other parts of the Mexican Volcanic Belt (MVB), where composite volcanoes predominate. Other volcanic forms in this field include lava cones, lava domes, maars, tuff rings, small shield volcanoes, and coneless lava flows. Most of the shield volcanoes are eroded and predate the currently observable cinder cones. Within the MGVF, cinder cones are situated between 200 km and 440 km from the Middle America Trench. Nearly 75% of the volcanoes are distributed between 200 km and 300 km from the trench, and cone density is highest at 250 km. Overall cone density is 2.5 cones/100 km2, and median separation distance is 2 km. The median cinder cone has a height of 90 m, a basal diameter of 800 m, a crater diameter of 230 m, and a volume of 0.021 km3. The cinder cones typically erupted olivine-basalt or basaltic andesite; these rock types are less silicic than those of composite volcanoes in the MVB. In general, samples from the MGVF show higher MgO, Cr, and Ni and lower K2O, P2O5, and Zr than those farther from the trench. Cinder cones show various stages of degradation, from which relative ages can be estimated; radiocarbon dates of seven cinder cones were obtained for calibration. Of several morphological indices of age, gully density and surface morphology of associated lava flows are the most sensitive. The morphological classification, based on gully density and lava flow surface features, revealed that 78 volcanoes are younger than 40,000 years. All of them are situated in the south, and some have a rough NE alignment, parallel to the relative motion vector between the Cocos and North America plates. Such NE alignments are also found locally for older cones, although in general cones are randomly spaced. Local cinder cone alignments are E—W in the northern part of the volcanic field, where E—W normal faults also occur. Despite the large number of scattered cinder cones and other small volcanoes in the MGVF, total erupted volume suggests a low magma supply rate. The estimated total volume of lava flows, ash, and cones erupted during the last 40,000 years for an area of 15,000 km2 is 31 km3. The calculated magma eruption rate of 0.8 km3/1000 years is small in comparison to a single composite volcano in the MVB.

Size, distribution, and magma output rate for shield volcanoes of the Michoacán-Guanajuato volcanic field, Central Mexico

The Michoacan-Guanajuato Volcanic Field (MGVF), in the west-central Mexican Volcanic Belt (MVB), contains nearly 400 medium-sized volcanoes in addition to 1000 small monogenetic cones. The area is distinct from other parts of the MVB, where large composite volcanoes predominate. Shield volcanoes are dominant among medium-sized volcanoes, which also include minor lava domes and composite volcanoes. The location, height, basal diameter, and crater diameter (when applicable) of 378 medium-sized volcanoes were catalogued, and the slope angles and volumes were calculated from these data. The median shield volcano has a height of 340 m, a basal diameter of 4100 m, an average slope angle of 9.4°, and a volume of 1.7 km3. The shield volcanoes in the MGVF are similar in size to Icelandic-type shield volcanoes, but the former have much higher slope angles and smaller basal diameters than the latter. Within the MGVF, these medium-sized volcanoes are located between 190 km and 430 km from the Middle America Trench; the distribution area is similar to that of small cones, but clusters and alignments are not as obvious for the medium-sized cones. The shield density is highest between 270 and 280 km from the Middle America Trench, 20 km farther than the density maximum for small cones. Ten medium-sized volcanoes were considered younger than 40,000 yr B.P. from lava flow morphology. The slope angle and a ratio of height to basal diameter are not useful age indicators, because they seem to reflect difference in original shield shape. K-Ar ages of shield volcanoes reveal that the volcanoes located north of latitude 19°55′N were active between 1 Ma and 3 Ma, whereas those south of latitude 19°55′N were active since 1 Ma. The average volcanic output rate estimated for the last 1 Ma is 0.7 km3/1000 yr., whereas that for the period of 1–3 Ma is 0.2 km3/1000 yr. An increase in magma production occurred around 1 Ma in response to migration of volcanism and probably to a change in tectonics and the thermal structure of the magma source region.

On B/Be ratios in the Mexican Volcanic Belt

Boron and beryllium concentrations were measured in a diverse suite of well-characterized rocks from the Mexican Volcanic Belt (MVB). Low B and high Be result in relatively low B/Be ratios in the MVB, compared to other arcs. Nevertheless, B systematics resemble those of other arcs and provide insights into mantle processes. In the MVB, B enrichment depends first on magma type, and second on edifice type and location. B/Be values are highest (5–15) in andesites and dacites erupted from calcalkaline strato-volcanoes located along the volcanic front, such as Volcan Colima and V. San Juan. Rocks from strato-volcanoes located behind the volcanic front generally have lower values (1–5). B/Be values are also elevated in differentiated members of rock suites that show evidence for significant crustal assimilation. In the westernmost MVB, west of the Michoacan-Guanajuato Volcanic Field (MGVF), cindercone ejecta, including basalts, lamprophyres, and basanites, contain low B/Be values (<5). The lamprophyres and basanites have very low B/Be, despite high Ba/Ce and other common measures of subduction signature. In the MGVF, where cinder cones occur exclusively, B/Be values in primitive calc-alkaline basalts are distinctly higher than those from alkaline basalts (3–8 vs. 1–3), indicating that high B/Be is a mantle-derived feature and not an artifact of crustal assimilation. Comparison among various elemental ratios indicates that Cs and U show enrichment patterns similar to B; all are enriched in calc-alkaline rocks, but not in lamprophyres or basanites. In contrast, Ba, K, and Sr, are enriched in both calc-alkaline rocks and the lamprophyres and basanites. Multi-stage processes and differing melting mechanisms are inferred to explain the variable characteristics of MVB volcanic rocks. First, slab-derived fluids, rich in fluid-mobile elements including B and Ba, infiltrate the mantle wedge. These fluids cause fluid-fluxed melting that produces calc-alkaline magmas enriched in all fluid-mobile elements. These lavas erupt from large, volcanic-front strato-volcanoes. The slab-derived fluids also metasomatize portions of the mantle wedge, producing phlogopite and/or amphibole. These phases have high partition coefficients for Ba-Sr-K, but may have low partition coefficients for B-Cs-U. Accordingly, subduction-zone metasomatism produces a mantle wedge enriched in Ba-Sr-K, but not necessarily in B-Cs-U. Decompression melting of this type of metasomatized mantle will consume the hydrous phases and produce magmas such as lamprophyres or basanites with high Ba/Ce, Sr/La, and K/La, but low B/Be, Cs/La, and U/La. This interpretation implies two types of subduction-zone signatures: one involving enriched Ba-Sr-K, elements that have longer residence times in the mantle wedge, and another involving enriched B-Cs-U, which all partition so strongly into fluids or melts that they have short residence times in the mantle, and are only enriched in magmas generated by fluid-fluxed melting or that have assimilated crustal material. Assimilation of granites and crustal rocks can also enrich differentiated lavas in B.

Volcano-tectonic interactions during rapid plate-boundary evolution in the Kyushu region, SW Japan

Evolution of the local plate tectonic and volcanic system relationship at Kyushu Island is defined by major changes in tectonics and volcanic style at ca. 15, 10, 6, and 2 Ma. Plate reconstructions presented here suggest that prior to 15 Ma, the Pacific plate subduction dominated Kyushu tectonics. From 15 to 6 Ma, the evolving relative plate motions shifted the triple junction between the Pacific plate, Philippine Sea plate, and southwest Japan northwards, so that the Philippine Sea plate was subducted beneath Kyushu. We suggest that a lack of subduction-related volcanism from 10 to 6 Ma is due to shallow subduction of the young Shikoku Basin lithosphere. By 6–5 Ma, changes in the Philippine Sea plate motion led to more rapid, nearly trench-normal, subduction of the Eocene west Philippine Basin crust beneath Kyushu. This model is supported by an increase in arc-like geochemistry of lavas since ca. 6.5 Ma. Subduction of fluid-rich features such as the Kyushu-Palau ridge introduced large volumes of fluids into the Kyushu arc system, leading to voluminous volcanism across Kyushu, focused particularly in areas where the ridge subduction occurs in tandem with local extensional tectonics. Key issues, such as the timing of Izu arc collision with central Japan and the history of motion of the Philippine Sea plate, are reassessed here, resulting in a model that favors Izu arc–central Japan collision at ca. 8–6 Ma, rather than the more widely accepted date of ca. 15 Ma.

Boron contents of Japan Trench sediments and Iwate basaltic lavas, Northeast Japan arc: estimation of sediment-derived fluid contribution in mantle wedge

Mantle wedge below the Northeast Japan arc contains hydrous fluids that dehydrated from two parts of the descending slab: a sediment layer and an altered oceanic crust (AOC). In order to estimate the weight percent of a sediment-derived fluid in the mantle wedge, we determine boron and other element contents of Japan Trench sediments and Iwate basaltic lavas on the Northeast Japan volcanic front. Trace element contents of the trench sediments are used to estimate those of the sediment-derived fluid, and previously reported trace element contents of the AOC are used to estimate those of the AOC-derived fluid. Examination of mobile/immobile element ratios (B/Sm, Ba/Sm, K2O/Sm, B/Zr, Ba/Zr, K2O/Zr) of the two slab-derived fluids and Iwate basaltic lavas, indicates a mixing ratio between the AOC-derived fluid and the sediment-derived fluid is 90:10 in the mantle wedge beneath the Northeastern Japan arc. The mobile/immobile element ratios also suggest that the subducted sediment recycle to the mantle wedge beneath Northeast Japan as sediment-fluid rather than sediment-melt. Since the weight fraction of the total slab-derived fluid in the mantle wedge is estimated to be less than 1 wt%, an addition of a small amount (<0.1 wt%) of the sediment-derived fluid to the mantle wedge fulfills the boron and other trace element contents in the Iwate basaltic lavas.

Accurate and efficient determination of boron content in volcanic rocks by neutron induced prompt gamma-ray analysis

An accurate and efficient analytical method using neutron-induced prompt γ-ray was developed for the determination of boron contents in volcanic rocks. We corrected the effect of sample geometry and flux fluctuation by using silicon as an internal standard. However, we found that the slopes of the calibration line vary among volcanic samples with different matrix. Because the increase of boron activity correlates positively with γ-ray count rate of hydrogen (water), we call this as the hydrogen effect. The hydrogen effect was confirmed by our experiment in which the boron activities showed systematic increase with the amount of added hydrogen (water). Most volcanic rocks, however, contain little water (<2 wt.%) to show this effect. We determined boron contents in various volcanic rocks in order to confirm the validity of the procedure that we established. The analyzed boron contents agreed well with the previous reported values. For efficient PGA of boron in volcanic rocks, we recommend JB-2 (GSJ standard rock) as a single geochemical standard, because of its high boron content (31.2 ppm).

Chemical Characteristics of Obsidian from Archaeological Sites in Western Mexico and the Tequila Source Area: Implications for Regional and Pan-Regional Interaction Within the Northern Mesoamerican Periphery

A total of 51 obsidian samples from archaeological sites in western Mexico (La Quemada, Totoate, Las Ventanas, Laguna San Marcos) and from the Tequila source area were analyzed chemically by direct-current plasma atomic emission spectrometry (DCP) and instrumental neutron activation analysis (INAA) in an effort to identify the sources of the archaeological obsidian by step-wise discriminant analysis of the data. Surprisingly, only 3 of the 39 archaeological samples (2 from Laguna San Marcos and 1 from Las Ventanas) could be correlated with a Volcan Tequila source (Teuchitlan). Four other groups of archaeological obsidian were recognized. The largest, Group Y, was found to be derived from the La Lobera source located near the Jalisco–Zacatecas border. This source accounted for 12% of La Quemada obsidian and a higher proportion for Las Ventanas and Totoate. Source locations for the remaining three groups could not be determined from the existing chemical data base. These results could indicate that a minimum of interaction took place between La Quemada and the Teuchitlan cultural tradition. It shows that one focus of La Quemadas trade endeavors was in the Rio Bolanos/Tlaltenango valleys. This study also indicates that organized obsidian trade between the Mexican core and its outer periphery was probably not a factor between a.d. 400 and 800.

Efficient inversion and uncertainty quantification of a tephra fallout model: TEPHRA INVERSION

An efficient and effective inversion and uncertainty quantification approach is proposed for estimating eruption parameters given a data set collected from a tephra deposit. The approach is model independent and here is applied using Tephra2, a code that simulates advective and dispersive tephra transport and deposition. The Levenburg-Marquardt algorithm is combined with formal Tikhonov and subspace regularization to invert eruption parameters; a linear equation for conditional uncertainty propagation is used to estimate posterior parameter uncertainty. Both the inversion and uncertainty analysis support simultaneous analysis of the full eruption and wind field parameterization. The combined inversion/uncertainty quantification approach is applied to the 1992 eruption of Cerro Negro and the 2011 Kirishima-Shinmoedake eruption. While eruption mass uncertainty is reduced by inversion against tephra isomass data, considerable uncertainty remains for many eruption and wind field parameters, such as plume height. Supplementing the inversion data set with tephra granulometry data is shown to further reduce the uncertainty of most eruption and wind field parameters. The eruption mass of the 2011 Kirishima-Shinmoedake eruption is 0.82 × 1010 kg to 2.6 × 1010 kg, with 95% confidence; total eruption mass for the 1992 Cerro Negro eruption is 4.2 × 1010 kg to 7.3 × 1010 kg, with 95% confidence. These results indicate that eruption classification and characterization of eruption parameters can be significantly improved through this uncertainty quantification approach.