Recent Volcanic Rocks Research Articles

Raman spectroscopy of volcanic lavas and inclusions of relevance to astrobiological exploration

Volcanic eruptions and lava flows comprise one of the most highly stressed terrestrial environments for the survival of biological organisms; the destruction of botanical and biological colonies by molten lava, pyroclastic flows, lahars, poisonous gas emissions and the deposition of highly toxic materials from fumaroles is the normal expectation from such events. However, the role of lichens and cyanobacteria in the earlier colonization of volcanic lava outcrops has now been recognized. In this paper, we build upon earlier Raman spectroscopic studies on extremophilic colonies in old lava flows to assess the potential of finding evidence of biological colonization in more recent lava deposits that would inform, first, the new colonization of these rocks and also provide evidence for the relict presence of biological colonies that existed before the volcanism occurred and were engulfed by the lava. In this research, samples were collected from a recent expedition to the active volcano at Kilauea, Hawaii, which comprises very recent lava flows, active fumaroles and volcanic rocks that had broken through to the ocean and had engulfed a coral reef. The Raman spectra indicated that biological and geobiological signatures could be identified in the presence of geological matrices, which is encouraging for the planned exploration of Mars, where it is believed that there is evidence of an active volcanism that perhaps could have preserved traces of biological activity that once existed on the planet's surface, especially in sites near the old Martian oceans.

U–Th dating of zircons from Holocene potassic andesites (Maanshan volcano, Tengchong, SE Tibetan Plateau) by depth profiling: Time scales and nature of magma storage

Throughout the Cenozoic, post-collisional potassic volcanism has been widespread in the Tibetan Plateau, but Holocene (< 10 ka) volcanism is rare and has only been reported from three volcanoes near Tengchong (Yunnan Province, China) within the southeastern Tibetan Plateau. Some of the Holocene volcanic rocks are differentiated lavas with SiO 2 contents of 58 wt.% and contain zircon crystals. These zircon-bearing differentiated lavas provide a unique opportunity for studying magma chamber processes in an environment of thickened continental crust by using short-lived uranium–thorium (U–Th) isotopes. Here we report U–Th zircon ages from Maanshan volcano (one of the three Holocene volcanoes from Tengchong) measured by secondary ion mass spectrometry ion microprobe (SIMS) techniques. A novel part of the zircon U–Th dating is the use of shallow depth profiling of the outermost rims and its comparison to traditional “core & rim” spot analyses. The U–Th ages of zircon interiors from Maanshan volcano show bi-modal distributions. The zircon interiors from Maanshan yield an apparent U–Th isochron age of 83.5 − 9.0 + 9.8 ka (2 sigma), with a mean square of weighted deviates (MSWD) of 5.1 that is outside the 95% confidence interval expected for a single age population. Based on mixture modeling of zircon age data, the Maanshan zircons may represent a bi-modal population with ages of 55 ± 7 ka and 91 ± 6 ka in proportions of 23% and 77%, respectively. SIMS shallow depth profiling of unpolished euhedral zircon outermost rims yield a U–Th isochron age of 53.6 − 5.0 + 5.3 ka (MSWD of 0.92). Thus depth profiling of unpolished zircon grains to discern the age of the outermost rims is consistent with the younger of the two peaks on the bi-modal distribution. The older ∼ 91 ka peak represents remobilized zircon antecrysts derived from an earlier episode of andesitic magmatism. The younger 55 ka peak represents zircon phenocrysts that grew in the eruptible magma body itself in the build-up to the eruption. The zircon storage time in the eruptible magma body beneath Tengchong is thus about 45 ka. Almost comparable temperatures and compositions between cores and rims indicate closed system evolution, rather than polybaric crystallization.

Geodynamic setting of recent volcanism in North Eurasia

A GIS layout of the map of recent volcanism in North Eurasia is used to estimate the geodynamic setting of this volcanism. The fields of recent volcanic activity surround the Russian and Siberian platforms—the largest ancient tectonic blocks of Eurasia—from the arctic part of North Eurasia to the Russian Northeast and Far East and then via Central Asia to the Caucasus and West Europe. Asymmetry in the spatial distribution of recent volcanics of North Eurasia is emphasized by compositional variations and corresponding geodynamic settings. Recent volcanic rocks in the arctic part of North Eurasia comprise the within-plate alkaline and subalkaline basic rocks on the islands of the Arctic Ocean and tholeiitic basalts of the mid-ocean Gakkel Ridge. The southern, eastern, and western volcanic fields are characterized by a combination of within-plate alkaline and subalkaline basic rocks, including carbonatites in Afghanistan, and island-arc or collision basalt-andesite-rhyolite associations. The spatial distribution of recent volcanism is controlled by the thermal state of the mantle beneath North Eurasia. The enormous mass of the oceanic lithosphere was subducted during the formation of the Pangea supercontinent primarily beneath Eurasia (cold superplume) and cooled its mantle, having retained the North Pangea supercontinent almost unchanged for 200 Ma. Volcanic activity was related to the development of various shallow-seated geodynamic settings and deep-seated within-plate processes. Within-plate volcanism in eastern and southern North Eurasia is controlled, as a rule, by upper mantle plumes, which appeared in zones of convergence of lithospheric plates in connection with ascending hot flows compensating submergence of cold lithospheric slabs. After the breakdown of Pangea, which affected the northern hemisphere of the Earth insignificantly, marine basins with oceanic crust started to form in the Cretaceous and Cenozoic in response to the subsequent breakdown of the supercontinent in the northern hemisphere. In our opinion, the young Arctic Ocean that arose before the growth of the Gakkel Ridge and, probably, the oceanic portion of the Amerasia Basin should be regarded as a typical intracontinental basin within the supercontinent [48]. Most likely, this basin was formed under the effect of mantle plumes in the course of their propagation (expansion, after Yu.M. Pushcharovsky) to the north of the Central Atlantic, including an inferred plume of the North Pole (HALIP).

Mineralogy of recent volcanic plains in the Tharsis region, Mars, and implications for platy-ridged flow composition

Volcanism on Mars occurred until recently, but the mineralogy of recent lava plains is poorly known because few regions display fresh outcrops devoid of dust. Using visible and near infrared data of the Mars Express probe, two new volcanic plains in Noctis Labyrinthus have been identified, and the existence of a volcanic plain on the floor of Echus Chasma has been confirmed. Crater retention ages estimated for these three plains range between 50 and 100 My, corresponding to the Late Amazonian. These plains represent an excellent opportunity to constrain the mineralogy of recent volcanic rocks. Results show that basaltic compositions with plagioclase and high calcium pyroxene are predominant. The low olivine proportion suggests that the apparent fluidity of these flat plains is not related to magmas being ultramafic. In addition, a platy-ridged texture is observed in two of the studied regions. Our study shows, for the first time, that this texture is associated with volcanic rocks, and that these rocks are of typical basaltic mineralogy. Finally, these volcanic plains are located more than 1000 km east of previously known Late Amazonian volcanic centers of the Tharsis region, an observation to be taken into account when considering models of recent volcanism on Mars.

Nature of mantle heterogeneity and its role in the short-term geochemical and volcanological evolution of Mt. Etna (Italy)

Clues for investigating on the origin and short-term geochemical changes of erupted magmas at Mt. Etna have arisen from an extensive sampling of historic (pre-1971) and recent (post-1971) volcanic rocks, mostly carried out on the southern flank of the volcano. Major and trace element data, obtained by WD-XRF and ICP-MS respectively, display rather similar abundances in both the historic and recent products; only K 2O (mean 1.43 and 1.93 wt.% respectively) and Rb (mean 32 and 50 ppm respectively) exhibit a significant time-correlated increase, whereas Th (mean 10.1 and 9.4 ppm respectively) gradually decreases. Radiogenic isotope ratios, obtained by TIMS, increase through time for 87Sr/ 86Sr (from 0.703343 to 0.703624), whereas for Pb (i.e., 206Pb/ 204Pb from 19.93 to 19.73) and 143Nd/ 144Nd (from 0.512908 to 0.512852) the ratios gradually decrease. Finally, δ 18O values, obtained by dual inlet mass spectrometry, slightly decrease from the historic to recent products (mean + 6.6‰). In order to interpret the observed trends, the role of magma evolution processes in the shallow feeding system was first tested by numerical modeling. Results show that both fractional crystallization and crustal assimilation cannot fully account for the observed variations of some trace elements (particularly Rb and Th) as well as of 87Sr/ 86Sr and δ 18O. On the other hand, partial melting modeling, performed with pre-1971 lava compositions equilibrated to those of mantle melts, supports the occurrence of accessory hydrous phases, such as amphibole and phlogopite, in the source of Etnean magmas. Partial melting of a source characterized by increased amounts of hydrous phases appears consistent with the peculiar geochemical features of the post-1971 magmas. Our findings imply that metasomatic influxes could have heterogeneously affected the pristine mantle, and that the degree of partial melting of the volatile-enriched portions increased. In accordance with trace element and isotope ratios, the model proposed for the genesis and evolution of magmas takes into account melting of a depleted mantle re-fertilized (hundreds of Ma ago) by plume-related silicate melts/fluids, inheriting a signature likely derived from ancient hydrothermally altered oceanic crust stored for long times in the mantle. A limited mantle upwelling, due to the complex recent geodynamics of Southern Italy, could have easily triggered partial melting of this heterogeneously metasomatized mantle, which produced magmas differing in their composition depending on the involved mantle domain. Since the increased extent of partial melting results in a larger melt production, our data are also consistent with the observed volcanic behavior, and could account for the increased frequency of eruptive events and emission rates of volatile-rich magmas observed since 1971, and especially after the 2001 eruption.

Fractional crystallization models and B–Be–Li systematics at Mt Somma-Vesuvius volcano (Southern Italy)

A compilation of B–Be–Li data on rocks that cover the entire eruptive history of Somma-Vesuvius is presented and interpreted in the light of evolution models for the Somma-Vesuvius rocks. Using major and trace element data, fractional crystalllization models are presented for different geochemical units. These data were used to constrain the source mineralogy of the Somma-Vesuvius rocks (ol-opx-cpx-gar-amp of 0.4-0.3-0.1-0.1-0.1), the amount of sediment added (5–10%) and the melt fraction from batch partial melting computations (0.05–0.1). From the B–Li data it is inferred that the main process responsible for the B isotopic signature is sediment recycling. However, the B–Li data show a major variation in Li abundances respect to B which is explained with Li dehydration before the fluid enriched the mantle wedge that produced the arc magmas. The Somma-Vesuvius B isotope composition is intermediate between that of the Campi Flegrei and the broad field of the Eolian Island arc. A low Be isotopes in the recent volcanic rocks can be explained as: (a) the top 1–22 m of the incoming sediment is accreted, (b) large amounts of sediment erosion, (c) a slow rate of subduction which have provoked a long magmatic history for the Vesuvius magma, (d) the sediment component takes several Myr longer than the subducting plate to reach the magma source region beneath Italy.

Butalón igneous rocks, Neuquén, Argentina: Age, stratigraphic relationships and geochemical features

Three main groups of igneous rocks of different age are recognized in the Butalón area, including volcanic rocks of the Choiyoi Group (Permian–Triassic), stocks and dikes of the Granodiorita Varvarcó (Upper Cretaceous–Paleocene) and stocks and dikes of the Tonalita Butalón (Paleocene–Eocene?). The Choiyoi Group consists of ignimbrites, breccias, tuffs and silicic lavas. Most are subalkaline and four groups can be distinguished on the basis of their trace element patterns. Most samples are enriched in Ba, but depleted in Nb, Sr, P, Ti and Eu. The subvolcanic rocks of the Granodiorita Varvarcó and Tonalita Butalón are mainly calc-alkaline, metaluminous to peraluminous, with low to medium potassium contents. They have similar subparallel REE and multielement patterns with low La/Yb and Eu/Eu ∗ ratios. These patterns mimic those observed in Pleistocene–Holocene volcanic rocks, of similar SiO 2 content, from the CSVZ of the Andes. They are depleted in Nb and Ti, have normal to low P, and are enriched in Sr and Zr. Hornfels-skarn type hydrothermal alteration is produced by Tonalita Butalón in the adjacent igneous rocks. Magmatic-hydrothermal breccias are developed on top of some of the stocks. While some of the breccias exhibit Mo anomalies, Au and Ag anomalies are present in the Choiyoi Group.

Holocene volcanic rocks in Jingbo Lake region Deversity of magmatism*

Abstract During the time from 5500 a to 5200 a BP more than 10 Holocene volcanoes in Jingbo Lake region erupted and the volcanic rocks covered an area of about 500 km2. Holocene volcanic rocks in Jingno Lake region Belong to the potassium-rich rocks and contain three rock types: trachybasalts, basanites and phonotephrites. Various types of magmatism formed in a small area and in a sort period of time came from partial melting of patossically-metasomatised lithospheric mantle. The diversity of magmatism can be xplained by that from mantle wedge or dehydration of subducted slab can result in a high heterogeneity of the mantle beneathy this region. Based on the pressure estimation of clinopyroxene megacrysts, we estimate that phonotephrite magma fractionally crystallize at ca. 52-54 km down the earth. Supported by the National Natural Science Foundation of China (Grant Nos. 40372044) and the Ministry of Science and Technology of China (2002DIA20009)

The petrogenesis of volcanics from Mt. Bulusan and Mt. Mayon in the Bicol arc, the Philippines

Pliocene to recent volcanic rocks from the Bulusan volcanic complex in the southern part of the Bicol arc (Philippines) exhibit a wide compositional range (medium- to high-K basaltic-andesites, andesites and a dacite/rhyolite suite), but are characterised by large ion lithophile element enrichments and HFS element depletions typical of subduction-related rocks. Field, petrographic and geochemical data indicate that the more silicic syn- and post-caldera magmas have been influenced by intracrustal processes such as magma mixing and fractional crystallisation. However, the available data indicate that the Bicol rocks as a group exhibit relatively lower and less variable 87Sr/86Sr ratios (0.7036–0.7039) compared with many of the other subduction-related volcanics from the Philippine archipelago. The Pb isotope ratios of the Bicol volcanics appear to be unlike those of other Philippine arc segments. They typically plot within and below the data field for the Philippine Sea Basin on 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams, implying a pre-subduction mantle wedge similar to that sampled by the Palau Kyushu Ridge, east of the Philippine Trench. 143Nd/144Nd ratios are moderately variable (0.51285–0.51300). Low silica (<55 wt%) samples that have lower 143Nd/144Nd tend to have high Th/Nd, high Th/Nb, and moderately low Ce/Ce* ratios. Unlike some other arc segments in the Philippines (e.g. the Babuyan-Taiwan segment), there is little evidence for the involvement of subducted terrigenous sediment. Instead, the moderately low 143Nd/144Nd ratios in some of the Bicol volcanics may result from subduction of pelagic sediment (low Ce/Ce*, high Th/Nd, and high Th/Nb) and its incorporation into the mantle wedge via a slab-derived partial melt.

226Ra or 226Ra/Ba dating of Holocene volcanic rocks: application to Mt. Etna and Merapi volcanoes

This paper shows how 226Ra– 230Th disequilibria can be used to date Holocene volcanic rocks from some well selected volcanoes. A systematic study of these disequilibria on historical or well-dated volcanic samples is indeed first required to test the applicability of this method. Two examples are described here to illustrate its potential. In the case of Mt. Etna, the good correlation observed between ( 226Ra) 0 activities at the time of eruption and Th contents in lava flows from the last two millennia [M. Condomines, J.C. Tanguy, V. Michaud, Magma dynamics at Mt. Etna: constraints from U–Th–Ra–Pb radioactive disequilibria and Sr isotopes in historical lavas, Earth Planet. Sci. Lett. 132 (1995) 25–41] is used to infer the ages of several newly analysed lava flows. The calculated ages are in good agreement with those deduced from the archaeomagnetic curve describing the variation of the geomagnetic field direction in southern Italy [J.C. Tanguy, I. Bucur, J.F.C. Thompson, Geomagnetic secular variation in Sicily and revised ages of historic lavas from Mt. Etna, Nature 318 (1985) 453–455, J.C. Tanguy, M. Le Goff, V. Chillemi, A. Paiotti, C. Principe, S. La Delfa, G. Patane, Variation séculaire de la direction du champ géomagnétique enregistrée par les laves de l'Etna et du Vésuve pendant les deux derniers millénaires, C. R. Acad. Sci. Paris 329 (1999) 557–564, J.C. Tanguy, M. Le Goff, C. Principe, S. Arrighi, V. Chillemi, A. Paiotti, S. La Delfa, G. Patane, Archaeomagnetic dating of Mediterranean volcanics of the last 2100 years: validity and limits. Earth Planet. Sci. Lett. 211 (2003) 111–124]. We also present a whole set of new U-series data on historical, recent, and older samples from Merapi (Indonesia), and show that the ( 226Ra)/Ba ratio has probably maintained a quasi-steady state value during at least the past four millennia, and can be used to infer the ( 226Ra) 0/Ba ratio of old volcanics at the time of eruption, and thus their ages. Comparison with 14C ages available on three samples [R. Gertisser, J. Keller, Temporal variations in magma composition at Merapi volcano (Central Java, Indonesia): magmatic cycles during the past 2000 years of explosive activity, J. Volcanol. Geotherm. Res. 123 (2003) 1–23] shows an excellent agreement. These dating methods, based on the post-eruptive decrease of 226Ra excesses can be confidently used to date young rocks on both volcanoes, an important step to infer their recent eruptive history and magmatic evolution. It also opens the possibility to extend the geomagnetic field variation curve back into the past few millennia. The promising results obtained in this work should encourage new systematic U-series studies to test the applicability of such methods to other permanently active volcanoes showing 226Ra excesses.

Volcanism on the Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71°N): age, source, and relationship to the Iceland and putative Jan Mayen plumes

The Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71°N) is a topographically anomalous shallow area with scattered volcanic peaks extending between the island of Jan Mayen and East Greenland and straddling the northern segment of the mid-Atlantic Kolbeinsey Ridge axis. Basalts dredged from the Eggvin Bank range from variably depleted, tholeiitic, near-axis lavas to enriched, transitional-to-alkaline, off-axis seamount lavas. In terms of normalised incompatible element patterns, the most depleted, near-axis tholeiite is similar to neighbouring Kolbeinsey Ridge basalts, whereas the off-axis, transitional-to-alkaline lavas are similar to other alkaline basalts occurring close to the Eggvin Bank region, e.g., those of Jan Mayen. 40Ar/39Ar step heating data indicate that the off-axis seamount lavas are coeval with other alkaline lavas erupted in the Central Norwegian-Greeland Sea at ca. 0.6–0.7 Ma. In contrast, the Eggvin near-axis tholeiites are <0.1 Ma. Volcanic peaks west and north of Jan Mayen show no indication of a systematic age progression. Therefore, the Jan Mayen hot spot hypothesis is not supported by the available radiometric age data. Sr, Nd, and Pb isotope compositions of near-axis and off-axis Eggvin Bank lavas are distinct, implying differences in their mantle sources. Isotope ratios of the off-axis basalts (87Sr/86Sr=0.70344–0.70352, 143Nd/144Nd=0.51283–0.51288, 206Pb/204Pb=18.82–18.85) resemble those of neighbouring alkali basalt occurrences, however, isotope ratios of the near-axis tholeiites correspond to lavas erupting in the south-eastern volcanic zone of Iceland, e.g., at Vestmannaeyjar. The near-axis tholeiites are generated by an unusual source with highly radiogenic Pb (206Pb/204Pb=18.95) together with relatively radiogenic Nd (143Nd/144Nd=0.51295) and low-radiogenic Sr (87Sr/86Sr=0.70314), respectively, representing an unique composition in the mantle north of central Iceland. The overlap in isotope compositions between Eggvin Bank near-axis tholeiites and south-east Iceland alkaline lavas could be an indication that the Eggvin Bank tholeiite source was derived from the Iceland plume and that it was emplaced in the upper mantle by the original Iceland plume head during the Early Tertiary as suggested by Trønnes et al. [Trønnes, T., Planke, S., Sundvoll, B., Imsland, P., 1999. Recent volcanic rocks from Jan Mayen: low degree melt fractions of enriched north-east Atlantic mantle. J. Geophys. Res. 104, 7153–7167]. Isotope and trace element data indicate an abrupt change in source composition along the Kolbeinsey Ridge axis at latitude ca. 70.6°N, apparently reflecting a boundary between two chemically distinct mantle domains with limited interaction. Based on Pb versus Pb isotope diagrams, no dispersion of enriched material is observed adjacent to the hypothetical Jan Mayen/Jan Mayen Platform plume, neither to the north-east along the Southern Mohns Ridge nor to the south along the Central Kolbeinsey Ridge.

Paleomagnetic study of Deception Island, South Shetland Islands, Antarctica

SUMMARY A paleomagnetic study was carried out on recent volcanic rocks exposed on Deception Island (63.0°S, 60.6°W), Antarctica. Sampling comprised all stratigraphic units exposed on the island, which include basaltic, andesitic and trachytic lavas, basaltic dykes and pyroclastic flows. Following stepwise thermal and alternating field demagnetization procedures, consistent characteristic remanence directions were determined at 21 sites, using principal-component analysis. The overall mean remanence direction for the Deception Island rocks is dec. 348.8°, inc. −73.7°, α95= 4.4°, N= 21, and is consistent within error with the geocentric axial dipole direction at the study locality. All of the studied rocks show normal polarity, indicating a Brunhes Chron age. The only available radiometric date of 153 ± 46 kyr agrees with this and suggests a minimum chronostratigraphic span of 100 kyr for the sampled rocks. The site mean directions show a Fisherian distribution and dispersion values compatible with current palaeosecular variation models. No evidence of the far-sided or right-handed effect is found in our data.

The age of Orrorin tugenensis, an early hominid from the Tugen Hills, Kenya

Throughout the history of the East African Rift valley, tectonic depressions and volcanic dams formed within the graben in which Middle Miocene to Recent volcanic rocks and lacustrine to fluvial sediments accumulated. During the Late Miocene, voluminous trachytes erupted in the vicinity of Kabarnet and almost filled the valley to its brim. Continued tectonic activity formed new basins floored by the Kabarnet Trachyte, one of which was located in the region immediately east of the present day Tugen Hills. The Lukeino sediments that accumulated in this basin crop out over an area of 44 km × 13 km. In 2000, Orrorin tugenensis, which is important for understanding the earliest stages of human evolution, was found in the Lukeino Formation 〚12, 13〛. It is concluded that the Lukeino formation accumulated between 6.0 and 5.7 Ma.

Secular variation in Mexico during the last 40,000 years

Paleomagnetic data are available for Mexico from various archives, covering irregularly the last 40,000 years. Data obtained from lake sediments are affected by ambiguous conversion of age to depth, and the paleomagnetic data themselves are not of high quality, as samples were only demagnetized at one field intensity. These lake sediment data are therefore considered as less reliable for construction of a secular variation curve, although they may give some qualitative information for the last 25,000 years. Archeological samples provide data for the period, 100–1200 a.d. Their 14 C ages are some times not well constrained, with unrecoverable analytical and statistical procedures, and often ages were only inferred from the stratigraphic position and the paleomagnetic record. The paleomagnetic record of these materials is of high quality with very low internal dispersion, although no magnetic cleaning techniques have been applied. Variations of inclination and declination with time are hampered by large age uncertainties and thus of limited value for the construction of secular variation curves. Paleomagnetic data from an U–Th dated stalagmite provide reasonable directions for most of the last 1200 years, being of lower quality in those parts with very low remanence intensity. The U–Th age determinations were interpreted in terms of a constant growth-rate, which might be only valid as a first-order approximation. Recent volcanic rocks from the Trans-Mexican volcanic belt provide 19 paleomagnetic directions of reasonable to high quality, and often also absolute paleointensities. Ages have been determined from 14 C analysis of charcoal or paleosols found below the lavas, and extend over the last ∼40,000 years. Therefore, volcanic rocks provide the most adequate archive for the construction of a secular variation curve, but much more volcanic structures need to be dated to obtain a semi-continuous secular variation curve. Comparison of the data from different archives is possible mainly for the last 2500 years and despite the aforementioned problems similar variations of declination and inclination are observed. This holds as well when comparing them to data from Holocene volcanic rocks from western North America. © 2002 Elsevier Science B.V. All rights reserved.

Holocene geomagnetic secular variation recorded by volcanic deposits at Mount St. Helens, Washington

A compilation of paleomagnetic data from volcanic deposits of Mount St. Helens is presented in this report. The database is used to determine signature paleomagnetic directions of products from its Holocene eruptive events, to assign sampled units to their proper eruptive period, and to begin the assembly of a much larger database of paleomagnetic directions from Holocene volcanic rocks in western North America. The paleomagnetic results from Mount St. Helens are mostly of high quality, and generally agree with the division of its volcanic deposits into eruptive episodes based on previous geologic mapping and radiocarbon dates. The Muddy River andesite's paleomagnetic direction, however, indicates that it is more likely part of the Pine Creek eruptive period rather than the Castle Creek period. In addition, the Two-Fingers andesite flow is more likely part of the Middle Kalama eruptive period and not part of the Goat Rocks period. The paleomagnetic data from Mount St. Helens and Mount Hood document variation in the geomagnetic field's pole position over the last ~2,500 years. A distinct feature of the new paleosecular variation (PSV) record, similar to the Fish Lake record (Oregon), indicates a sudden change from rapid clockwise movement of the pole about the Earth's spin axis to relatively slow counterclockwise movement at ~800 to 900 years B.P.

Le paléovolcanisme de la bordure occidentale de la boutonnière de Kédougou, Paléoprotérozoïque du sénégal oriental: incidences géotectoniques

The main petrographical and geochemical patterns (major, trace and rare earth elements) of the epimetamorphosed volcanic rocks from the Mako Supergroup (Birimian) from the central and southern part of the western border of the Kedougou-Kenieba Inlier (Palæoproterozoic) are presented. The petrographic patterns indicate a predominance of tholeiites with the middle and upper parts of the lithological sequence containing scarce andesite, rhyodacite and rhyolitic end members of calc-alkaline affinity. The observed geochemical trend is compared with the patterns from other Palæoproterozoic areas of the West African Craton, as well as recent volcanic rocks, whose geotectonic setting is well constrained. The evolution of the Kedougou Inlier corresponds to an ocean island-arc environment, in agreement with previous research on epimetamorphic volcanodetrital and sedimentary formations, and Birimian granitoids. This research underlines the importance of island-arc accretions in crust generation during Palæoproterozoic times in West Africa.

U-series evidence for crustal involvement and magma residence times in the petrogenesis of Parinacota volcano, Chile

In this study, we present Th–U disequilibria as well as radiogenic and trace element data for recent volcanic rocks from the Nevados de Payachata volcano which erupted through ∼70 km of continental crust in the Central Volcanic Zone of the Andes (18°S, 69°W). Both lavas and mineral separates were analyzed by mass spectrometry for 238U–230Th disequilibria. The lavas are characterized either by 230Th enrichment or depletion relative to its parent nuclide 238U. Mineral separates are used to derive U–Th isochron ages and these ages compare favorably with inferred stratigraphic ages or K–Ar ages, although in one case the U–Th age is significantly older than the stratigraphic age. Despite relatively constant Sr, Nd, and Pb isotope ratios, the lavas display inverse trends in 230Th/238U versus Ce/Yb or Ba/Hf diagrams. These trends cannot be interpreted by simple two-component mixing. Rather, there must be three (and perhaps four components) involved in the genesis of the Parinacota lavas. A mantle wedge, a slab fluid, and a lower crustal component can be identified. A sediment component is more difficult to detect as it is difficult to decipher its signature because of the strong crustal influence. The existence of binary arrays can be explained by variable amounts of crustal material. The process of crust–mantle interaction must have been short enough to preserve U–Th disequilibrium (<300 ka).

Recent volcanic rocks from Jan Mayen: Low‐degree melt fractions of enriched northeast Atlantic mantle

Isotopic and trace element analyses of recent alkaline volcanic rocks from Jan Mayen were used to characterize their mantle source. The samples, ranging in composition from ankaramites to trachytes, are isotopically homogeneous with 11 of 12 samples within the following ranges: 87Sr/86Sr 0.7035–0.7036, 143Nd/144Nd: 0.51285–0.51290, 206Pb/204Pb: 18.54–18.76, 207Pb/204Pb 15.45–15.49, and 208Pb/204Pb: 38.19–38.51. Geophysical data indicate that the Jan Mayen Ridge represents a microcontinent rifted off the east Greenland continental margin at 36 Ma. The isotopic homogeneity and poor correlation between the degree of magmatic differentiation and Sr‐Nd isotopic composition show that the recent Jan Mayen magmas escaped significant contamination by Precambrian or Lower Paleozoic continental crust. A pervasive Tertiary to recent magmatic infrastructure may explain the lack of contamination with old crustal material. The Jan Mayen alkali basalts have similar trace element and isotopic composition to other oceanic plume basalts and are dominated by low‐degree melts from an enriched mantle component prevalent in the NE Atlantic. The Icelandic alkali basalts are derived from similar sources but are more diluted with picritic or tholeiitic melt fractions formed by progressive melting at shallower levels. Minimum dilution of the incipient low‐degree melts occurs when the melting column is truncated by a thick lithosphere. The mantle source sampled by the Jan Mayen and other alkaline volcanic systems in the NE Atlantic is strongly enriched in high field strength elements like Nb, has a relatively high μ value, and has 207Pb/206Pb ratios below and 208Pb/206Pb ratios above the Northern Hemisphere Reference Line. Such a Pb isotopic composition fits well with an old mantle domain with a time‐integrated low μ value, enriched by a Paleozoic high μ (HIMU) component. The young HIMU‐component in the NE Atlantic upper mantle could represent recycled oceanic crust entrained in the ancestral Iceland plume and distributed laterally by the ancestral plume head.

Palaeomagnetic study of the Karaman and Karapinar volcanic complexes, central Turkey: neotectonic rotation in the south-central sector of the Anatolian Block

In the Anatolian sector of the Afro–Eurasian collision zone a palaeotectonic collisional phase (Paleocene to Miocene) responsible for emplacement of the Pontide and Tauride orogens has been replaced by a neotectonic phase of continental deformation (Late Miocene/Early Pliocene to Recent). The latter phase appears to have been accommodated mainly by crustal thickening during Late Miocene and Pliocene times, but was succeeded by complex differential rotations of fault blocks during crustal extrusion in Late Pliocene and Quaternary times. In this study we have investigated palaeomagnetism of Miocene–Recent volcanic rocks comprising the western extension of the Central Anatolian Volcanic Province located in the south-central part of the Anatolian Block with the aim of resolving deformations near to the border with the Tauride orogen. Rock magnetic investigations identify low-Ti magnetite assemblages of primary cooling-related origin. These have predominant multidomain structures but significant fractions of single domains are always present; low-temperature alteration is largely absent. The Karaman Volcanic Complex (Late Pliocene) shows a net rotation of −5.7±6.9° not significantly different from the regional field axis during Recent times. The Karapinar Volcanic Field (Brunhes epoch) identifies a larger net rotation of −23.1±12.0° in a restricted sample. The adjoining Karacadağ Volcanic Complex (Late Miocene–Pliocene) and Middle Miocene lavas beneath the Hasandağ Complex define net rotations of −8.1±5.9° and −16.4±8.9° respectively. Analysis of palaeomagnetic results from Late Cretaceous–Recent rock units emplaced in Anatolia during the palaeotectonic and neotectonic regimes shows that rates of rotation have accelerated in post-Pliocene times as crustal thickening has given way to tectonic escape. A near-uniform anticlockwise rotation of 25–35° has characterised much of this block during the most recent phase of deformation and appears to have occurred in common with the Eurasian Plate to the north of the North Anatolian Fault Zone. Whilst this rotation appears to extend south eastwards across the Ecemiş Fault Zone towards the East Anatolian Fault, the present study shows that smaller differential anticlockwise rotations have characterised the south-central region of the block where it has interacted at its southwestern margin with oroclinal bending focussed on the Isparta angle.

Dating recent volcanic rocks through 230Th-238U disequilibrium in accessory minerals: Example of the Puy de Dôme (French Massif Central)

Precise dating of very young (≤10 000 yr) silicic volcanic rocks can be achieved by measurements of 230 Th- 238 U disequilibria in accessory minerals such as apatite and zircon. It is illustrated here by the isochron age of 12 000 ± 1000 yr obtained on a trachyte from the Puy de Dome (French Massif Central). The crystallization age is virtually the same as the eruption age determined by thermoluminescence and 14 C dating. This indicates a short time interval (a few centuries at most) between crystallization and eruption in such small volume silicic magmas.