Pierre-Yves Gillot

Geology, geochronology and chemical evolution of the island of Pantelleria

Potassium–argon dating, field relations, geochemical and strontium-isotope compositions are reported for the island of Pantelleria (Strait of Sicily, Italy). These data support the following model for the genesis and evolution through time of the volcanic system: the peralkaline rocks originated from mantle-derived parental magmas; the trachytic magma differentiated in a low pressure magma chamber by crystal–liquid fractionation. This process led to a chemically zoned magma chamber tapped at different levels by successive eruptions. During low-pressure differentiation the 87 Sr/ 86 Sr ratios of some of the most evolved Sr-poor rhyolitic magmas increased from 0.703 up to 0.708 by contamination with crustal material. The chemical variation displayed by the products of each of the defined eruptive cycles in the last 50000 years suggests an open system behaviour of the magma chamber which is episodically refilled by more mafic parent magma, differentiated at high rate and episodically erupted.

The Cassignol technique for potassium—Argon dating, precision and accuracy: Examples from the Late Pleistocene to Recent volcanics from southern Italy

A particular Kue5f8Ar technique, the Cassignol technique, has been developed in order to date Upper Pleistocene and Holocene volcanic rocks. We describe here its principles and its technology. The limit of detectability of the radiogenic Ar portion corresponds to an error of less than 103 a for K-rich minerals and a few 103 a for basalts. The reliability of the results and the validity of the correction for atmospheric contamination have been checked by analysing historical lavas and by comparison with data obtained from radiocarbon and thermoluminescence dating methods. The results demonstrate that the technique is capable of achieving Kue5f8Ar dates as young as 2000 a with a few centuries accuracy. A precision of ± 1.5% is obtained for samples older than 105 a. Moreover, in rocks older than 107 a, the technique permits the accurate dating of minute amounts of pure separated mineral phases. A reconstruction of the recent volcano-tectonic evolution of the Naples area has been carried out. It allows us to establish a model for estimating volcanic hazards in the Phlegrean Fields. The dated terrestrial pyroclastic deposits have been recovered from cores collected in the central Tyrrhenian Sea. The land-sea correlations allow us to refine the chronology of the marine oxygen isotope records for the past 60,000 a.

The eruptive history of Pantelleria (Sicily channel) in the last 50 ka

Six silicic eruptive cycles have been recognized in the last 50 ka at Pantelleria. The products of each cycle exhibit a compositional variation from pantellerite to less peralkaline rhyolite or to trachyte. The relationships between the range of chemical variation, the erupted volume and the time of eruptions, allow us to estimate an average differentiation rate of 5% crystal fractionation per 1000 years and a constant long-term rate of magma discharge of 0.1 km3 per 1000 years. Pressure increase in the magma chamber caused by the addition of new magma, accumulation of highly-differentiated, volatile-rich magma in the roof zone and a concomitant build-up of a vapour phase, is postulated as a possible triggering mechanism for eruptions.

Age of the Laschamp paleomagnetic excursion revisited

Abstract The reverse paleomagnetism of the lava flows of Laschamp and Olby, already discovered by Bonhommet and Babkine, is confirmed. The ages of these flows, measured by 14 C, thermoluminescence and K-Ar dating are respectively 36,000 ± 4000 and 42,000 ± 5000 years. VGPs comparison of the “Laschamp event” with those of the 30,000-year Lake Mungo excursion does not show evidence of coincidence between these two events.

Eruptive history of the Piton de la Fournaise volcano, Reunion Island, Indian Ocean

Abstract In order to establish a general chronology of the volcanic evolution and to determine the temporal succession of the structural units, potassium-argon measurements were made on 15 samples selected as a function of their stratigraphical position on Piton de la Fournaise volcano. The rocks of Reunion Island are essentially oceanic and basaltic lavas of two shield volcanoes: the central, now extinct Piton des Neiges and the more recent, still active, Piton de la Fournaise. Piton de la Fournaise volcano is generally thought to have been developed unconformably on the southeastern flank of the Piton des Neiges volcano. Previous studies have shown four successive phases and three calderas in the construction of Piton de la Fournaise. The subaerial basaltic shield-building lavas of Piton de la Fournaise appear to be older than previously thought: at least 530,000 y. old instead of 360,000 years. In terms of their duration and erupted volumes, the four successive phases are not equivalent. The duration of the first two phases is 240,000 years (from 530,000 to 290,000 y. B.P.) and 155,000 years (from about 220,000 to 65,000 y. B.P.). The duration of the third phase is less than 60,000 years and the fourth phase may actually be an episode of the third. The two volcanoes, Piton des Neiges and Piton de la Fournaise, were active simultaneously for at least 500,000 years. The evolution of Reunion Island appears to be consistent with activity along a developing rift. The evolution of Piton de la Fournaise is mainly linked with the structural development of the shield and to large-scale slumpings due to instability of the slope.

Model for the structural evolution of the volcanoes of Réunion Island

Abstract Reunion Island is an intraplate volcano linked with the hot spot activity that is also responsible for the volcanism of Mauritius and of the Mascarene Plateau. It is composed of two main volcanic complexes, the Piton des Neiges and the still active Piton de la Fournaise. The latter exhibits a special morphostructural evolution characterized by successive nested U-shaped calderas that open seaward. We propose an analysis of this evolution which permits us to deduce a model which can be extended to the general history of the island. From the morphology of the youngest caldera on the one hand and from the K—Ar data on the cliffs corresponding to the successive rims on the other we establish the frequency of landslide collapses and estimate the volumes emitted during each successive phase. This yields evidence of an increasing disequilibrium of the southeastern flank of Piton de la Fournaise. Indeed, through time the frequency of collapses increases and collapse volume decreases. We model the phenomenon with an exponentially smoothed sinusoidal curve, with time on the abscissa and eruptive output on the ordinate. Each maximum on the curve corresponds to the successive slumps and the area of each arch represents the volume erupted during each phase. This can be extrapolated to the earlier volcanic history of Reunion, setting the first collapse at about 1.8 Ma, which affected the main part of the present island, preserving only the Massif de la Montagne at its north-northwestern end. This explains the breccias observed in the deep erosional features of the massif of the Piton des Neiges and takes into account the lack of lavas with dates between 1.8 and 1.1 Ma. Comparison between the curve of emissivity and the total volume estimated for the whole island volcano yields an age of 5 Ma for the beginning of the activity. This age coincides with the end of the main volcanic activity in Mauritius.

Recent volcanic history of pantelleria: A new interpretation

Abstract The island of Pantelleria is located in the Pantelleria Rift between Sicily and Africa. This rift is floored by a 20 km thick continental-type crust that is characterized by horsts and grabens formed by NW-SE tensional faults and NE-SW shear faults. Several faults and fractures that intersect the island follow these regional trends. Prominent volcano-tectonic features on the island include two calderas, the younger being related to the eruption of the green ignimbrite and the uplifted Montagna Grande block. Pantelleria mostly consists of peralkaline silicic rocks with minor trachytes and basalts. Volcanism on the island is characterized by both explosive and effusive activity. Reconstruction of the volcanic history of the island since the eruption of the green ignimbrite is based on geological and geochronological data. The eruption of the green ignimbrite (50,000 y.b.p.) was followed by collapse of the Monastero caldera that has been the site of most of the subsequent acid volcanism. The Montagna Grande volcanic complex (35,000 y.b.p.) fills the central part while the lower pantelleritic lava flows (16,000 y.b.p.) erupted along its rim. Very probably the young Mt. Gelfiser and Khaggiar domes are also located on the caldera rim. The Montagna Grande block rose within the caldera at an estimated average rate between 1.0 and 1.5 cm/y for about 20,000 y. Subsequent pumice cones (9,000 y.b.p.), endogenous domes, and lava flows also erupted inside this caldera. Among the main acid volcanic units younger than the green ignimbrite, only the Mt. Gelkhamar endogenous dome is not linked to this caldera. In the last 50,000 years basaltic activity only occurred in the northern part of the island. The basaltic vents are aligned parallel with the main tensional trends of the rift. At least two phases have been recognized; the older is about 29,000 y.b.p. and the younger is a few thousands of years old. A change occurred in the eruptive style after the eruption of the green ignimbrite and collapse of the Monastero caldera. Explosive activity diminished and extrusion of lava became dominant.

Absolute paleointensities between 60 and 160 kyear BP from Mount Etna (Sicily)

Abstract Paleointensity measurements were conducted on 17 distinct lava flows from Mount Etna (Sicily) in order to obtain records of absolute paleointensities for the period covering the last 60–160 kyear. A complementary aspect of this study was to improve and extend the calibration of relative paleointensities obtained for the same interval from sedimentary sequences based on absolute values of virtual axial dipole moments. Seventy-three samples have been subjected to double heating Thellier-Coe experiments under vacuum and care was taken to perform additional studies dealing with the magnetic mineralogy and to scrutinize the effects of heating upon magnetic minerals. Potassium-argon (Kue5f8Ar) datings provided precise ages in most cases. Absolute paleointensities were obtained with success for ten lavas flows. The succession of the virtual axial geomagnetic dipoles (VADM) oscillate between extrema centered around a mean value of 6.3 ± 2.6 × 10 22 A m 2 . This result is in good agreement with the sedimentary record published for the same period and confirms that there has been an appreciable decrease in geomagnetic field intensity between 80 and 150 kyear.

CHANGES OF THE GEOMAGNETIC FIELD VECTOR OBTAINED FROM LAVA SEQUENCES ON THE ISLAND OF VULCANO (AEOLIAN ISLANDS, SICILY)

Abstract A combined geochronologic ( K Ar ) and palaeomagnetic study has been conducted on five volcanic sections (70 flows) on the island of Vulcano. The sections span the interval from 15 ± 2 to 135 ± 4 ka, with large hiatuses. Rock magnetic investigations indicate that the natural remanent magnetisation (NRM) is carried by titanomagnetites in the four oldest sections, with a contribution from high coercivity magnetic minerals in the youngest section. Palaeomagnetic directions obtained by thermal demagnetisation document a large magnetic excursion, at around 110 ka, with virtual geomagnetic poles (VGPs) that lie over eastern Siberia. These directions may represent a volcanic record of the termination of the Blake event. Only about 15% of the samples (from 19 flows) proved suitable for palaeointensity (Thellier and Thellier) determinations. These measurements indicate that the geomagnetic field has varied at Vulcano between 16 and 53 μT in the explored time interval. When combined with previous results obtained at Mt. Etna for the period from 140 to 60 ka (Tric et al., 1994, Phys. Earth Planet. Inter. , 85: 113–129), these results document a broad low at around 115 ka. The palaeointensity values from Vulcano and Etna are consistent with global values, suggesting the presence of a dominant dipole-field contribution. These values, on the other hand, are significantly smaller than those recently obtained from La Reunion in the southern hemisphere. This suggests that large, long-lived, non-dipolar components of the geomagnetic field may be present at La Reunion in the interval from 130 to 90 ka. These observations are consistent with the fact that the VGP angular dispersion, calculated from non-excursional data, is significantly smaller at Vulcano than at La Reunion.

Mehetia Island, South Pacific: geology and petrology of the emerged part of the Society hot spot

Abstract Mehetia is the youngest island of the Society hot-spot chain, representing the summit of a large submarine volcanic edifice (4035 m in height). Its conical shape results from strombolian-type eruptions. Mehetia comprises two major volcanic formations. The “old edifice” made up of interbedded pyroclastic rocks and lava flows, forms the main part of the island. The base of its stratigraphic pile is dated at 70,000–75,000 yr B.P. by K/Ar. Younger events (⩽ 31,000 yr B.P.) have built up a volcanic cone, with a summit hydromagmatic crater, and emplaced lava flows extending south of the island where they overlie coral reef limestones. Between the two eruptive periods, an erosional phase caused the partial destruction of the old edifice. Six successive stages of construction of the island are described from stratigraphic relations. Mehetia does not show any indication of present-day volcanic or fumarolic activity but should be considered as an active volcano as evidenced by the 1981 volcanoseismic crisis. Mehetia lavas include oceanites, primitive and evolved alkali basalts and basanites together with rare hawaiites and mugearites. Two main petrogenetic processes (i.e. accumulation and crystal fractionation) are inferred from bulk-rock composition and microprobe data on phenocrysts. Oceanites are derived from basalts through accumulation of olivine and clinopyroxene. Major and trace elements trends suggest evolution through crystal fractionation processes involving olivine, clinopyroxene, and Fe-Ti oxides.