Ignimbrite sequence on Gran Canaria

The Miocene sequence of felsic extrusive rocks of about 1000 m total thickness on Gran Canaria is divided into three units:

1. A lower unit of trachytic rhyolites (lavas, composite flows, ignimbrites) characterized by a phenocryst assemblage of anorthoclase (Or_15–20, wt%), clinopyroxene, hypersthene (amphibole substituted for both in ignimbrites), and Fe/Ti-oxides. The commonest groundmass minerals are anorthoclase and alkali-amphibole, with minor quartz and aegirine.
2. A middle unit of comenditic and pantelleritic ignimbrites characterized by anorthoclase (Or_20–32) and amphibole. Phenocryst minerals restricted to individual flows are Fe/Ti-oxides (several comendites), clinopyroxene, biotite, and sphene. The commonest groundmass minerals are anorthoclase and Tiaegirine, with lesser katophorite, arfvedsonite and quartz.
3. An upper unit of trachvtic and phonolitic ignimbites and lava flows (normative ne rarety exceeding 10%) with nepheline phonolite lava flows becoming increasingly abundant upwards. The ignimbrites have mostly anorthoclase (Or_30-04), and biotite, with rarer Fe/Ti-oxides, hornblende, and clinopyroxene. The commonest groundmass minerals are anorthoclase, aegirine, and alkali-amphiboles, and in some flows nepheline.

The change from Na-rich to K-rich anorthoclase upwards in the sequence supports the conclusion, based on over 50 new stratigraphically controlled chemical analyses that the Na₂O/K₂O-ratio decreases within the sequence. possibly as a result of crystal iractionation processes and this effect is independent of probable loss of Na on post-eruptive crystallization. While hydroxyl-bearing phenocryst minerals are absent from all rocks called lava in the field, they are ubiquitous in the ignimbrites, indicating the importance of P_u2o in the generation of suspension-type cruptions. Compositional gradients must have been particularly pronounced in the small magma chambers that existed beneath Gran Canaria, resulting in a wide range of compositionally zoned or mixed deposits.     

New tie-points for the geomagnetic polarity time scale during the Middle Miocene from the Mogán Group on Gran Canaria and Ocean Drilling Program Leg 157 site 953

A thick sequence of volcaniclastic sediments drilled at site 953 during Ocean Drilling Program (ODP) Leg 157 northeast of Gran Canaria (Canary Islands) contains an almost complete magneto-stratigraphy back to the shield stage of the island 14.8 Ma ago. Onshore, a sequence of reversals has been identified and dated in 19 dominantly peralkaline rhyolitic ignimbrites, one rhyolitic, and one basaltic lava flow of the Mogán Group (13.35-13.95 Ma), which overlie basalt flows of the island's shield stage (>14 Ma). The magneto-stratigraphy of the ignimbrites onshore has been correlated with the marine magneto-stratigraphy at site 953, determined in syn-ignimbritic volcaniclastic turbidites, which were deposited practically synchronously immediately following the entry of the parent pyroclastic flows into the sea around the circumference of the island. The four polarity intervals recorded in the sequence of the Mogán Group ignimbrites correspond to C5ACr, C5ACn, C5ADr and C5ADn. Single crystal ⁴⁰Ar/³⁹Ar-age determinations of the ignimbrites bracketing the polarity changes gave the following ages and uncertainties for the reversals C5AD (t) (13.95ǂ.07 Ma), C5AC(o) (13.89ǂ.08 Ma), and C5AC(t) (13.47ǂ.09 Ma). The newly dated polarity changes fit and refine the Miocene age model proposed in the global polarity time scale.     

Rifting,recurrent landsliding and Miocene structural reorganization on NW-Tenerife (Canary Islands)

We studied mechanisms of structural destabilization of ocean island flanks by considering the linkage between volcano construction and volcano destruction, exemplified by the composite Teno shield volcano on Tenerife (Canary Islands). During growth, Tenerife episodically experienced giant landslides, genetically associated with rifting and preferentially located between two arms of a three-armed rift system. The deeply eroded late Miocene Teno massif allows insights into the rifting processes, the failure mechanisms and related structures. The semicircular geometry of palaeo-scarps and fracture systems, breccia deposits and the local dike swarm reconfigurations delineate two clear landslide scarp regions. Following an earlier collapse of the older Los Gigantes Formation to the north, the rocks around the scarp became fractured and intruded by dikes. Substantial lava infill and enduring dike emplacement increased the load on the weak scarp and forced the flank to creep again, finally resulting in the collapse of the younger Carrizales Formation. Once more, the changing stress field caused deformation of the nearby rocks, a fracture belt formed around the scarp and dikes intruded into new (concentric) directions. The outline, size and direction of the second failed flank of Teno very much resembles the first collapse. We suggest structural clues concerning mechanisms of recurrent volcano flank failure, verifying the concept that volcano flanks that have failed are prone to collapse again with similar dimensions.     

The eruptive center of the late quaternary Laacher see tephra

About 5 km³ of phonolite magma were erupted 11,000 years ago in the Laacher See area in Plinian and Vulcanian eruptions. The eruptive vents for all tephra, in dispute for over 200 years, must have been located entirely within the Laacher See basin, because: (1) All deposits are thickest in the tuffring surrounding the basin, with one exception: pyroclastic flow deposits are thickest 2 to 6 km away from the rim of the basin having ponded in radial valleys. Three Plinian fallout layers show minor secondary thickness maxima 12 to 16 km east of Laacher See. (2) Axes of all depositional fans and single tephra sheets converge in the Laacher See basin. (3) Diameters of both lithic and pumice clasts reach their maxima in outcrops close to Laacher See volcano. (4) Directions of asymmetric ballistic impact sags indicate the Laacher See basin as source area. (5) The dimension of Laacher See crater roughly corresponds to the erupted volume of phonolitic tephra (5.3 km³ magma DRE) and lithic clasts (0.7 km³). (6) Xenolith types are consistent with an eruptive center within the Laacher See basin. (7) The systematic chemical and mineralogical zonation of the tephra deposits — from highly differentiated aphyric phonolite at the base to highly phyric mafic phonolite at the top — strongly suggests eruption from a single compositionally zoned magma column. (8) Several lines of evidence indicate migration of the main eruptive focus from the south to the north during the later part of the eruption, both vents however being confined within the Laacher See basin.None of the four additional eruptive centers previously postulated (Auf Schruf, Meerboden, Frauenkirch, Niedermendig), located up to 7 km south of Laacher See volcano, is supported by empirical or theoretical evidence.

---

Zusammenfassung Mindestens 5 km³ phonolithisches Magma wurde vor ca. 11 000 Jahren im Laacher See Becken durch plinianische und phreatomagmatische Eruptionen gefördert. Die Lage der Eruptionszentren im Laacher See Becken, seit über 200 Jahren umstritten, wird im einzelnen begründet. (1) Alle Schichteinheiten der Laacher See Tephra erreichen ihr Mächtigkeitsmaximum im Tuffring am Beckenrand. Eine Ausnahme bilden pyroklastische Stromablagerungen (Trass) die ihre maximalen Mächtigkeiten in Paläotälern 2 bis 6 km vom Laacher See entfernt erreichen. Drei plinianische Tephralagen zeigen sekundäre Mächtigkeitsmaxima zwischen 12 und 16 km Entfernung vom Laacher See Vulkan. (2) Die Achsen aller Ablagerungsfächer und einzelner Tephralagen laufen im Laacher See Becken zusammen. (3) Die Durchmesser von Bimsklasten und xenolithischen Gesteinsfragmenten nehmen zum Laacher See hin zu. (4) Aus Einschlagsdellen abgeleitete Transportrichtungen ballistischer Blöcke sind Hinweise auf Abschußpunkte im Bereich des Laacher See Bekkens. (5) Die Dimension des Laacher See Kraters steht in Einklang mit dem Gesamtvolumen eruptierten Magmas (5,3 km³) und Nebengesteins (0,7 km³). (6) Xenolithische Gesteinsfragmente in den Tephraablagerungen lassen sich Gesteinskomplexen zuordnen, die vor der Eruption im Bereich des Seebeckens angestanden haben. (7) Die Tephraablagerungen sind systematisch von basalen hochdifferenzierten aphyrischen zu einsprenglingsreicheren mafischen Phonolithen am Top zoniert und sind daher wahrscheinlich von einer einzigen chemisch-mineralogisch zonierten Magmasäule eruptiert worden. (8) Innerhalb des Laacher See Beckens hat sich der Hauptkrater im Verlaufe der Eruption von Süden nach Norden verlagert.Die von früheren Bearbeitern außerhalb des Laacher See Beckens postulierten Eruptionszentren (Auf Schruf, Meerboden, Frauenkirch, Niedermendig), die bis zu 7 km südlich des Laacher Sees liegen, lassen sich weder empirisch noch theoretisch begründen.

---

Résumé Il y a 11 000 ans, un volume d'environ 5 km³ d'un magma phonolitique a été émis dans la région du Laacher See lors de manifestations de types plinien et phréatomagmatique. Les emplacements des centres d'émission, objets de controverse depuis plus de 200 ans, doivent se situer exclusivement dans le lac du Laacher See, pour les raisons suivantes: (1) Tous les dépôts présentent leur épaisseur maximale dans l'anneau de tuf qui encercle le lac, à une exception près: les coulées pyroclastiques («Trass») atteignent leur plus grande épaisseur dans des paléovallées situées à des distances de 2 à 6 km du Laacher See. Trois couches de projections pliniennes montrent des maxima secondaires à 12–16 km à l'est du Laacher See. (2) Les axes de tous les cônes du débris et des nappes isolées de tephra convergent vers le bassin du lac. (3) La dimension des fragments de ponce et de roches augmente en direction du Laacher See. (4) L'orientation des creux asymétriques dûs aux impacts ballistiques désignent le Laacher See comme «zone de lancement». (5) Le volume du cratère du Laacher See correspond approximativement au volume total des produits phonolitiques (5,3 km³) et des fragments lithiques (0,7 km³) éjectés. (6) Les divers types de fragments lithiques peuvent être rapportés aux complexes rocheux qui occupaient le bassin du Laacher See avant les éruptions. (7) La zonation chimique et minéralogique systematique de dépôts de tephra — depuis une phonolite aphyrique fortement différenciée à la base jusqu'à une phonolite mafique largement porphyrique au sommet — suggère fortement une éruption à partir d'une colonne de magma unique de composition zonée. (8) Plusieurs faits indiquent la migration du foyer d'éruption principal du sud vers le nord durant la dernière période d'activité, les deux orifices étant cependant localisés à l'intérieur du bassin du Laacher See.Des travaux antérieurs ont admis quatre centres éruptifs situés hors du bassin du Laacher See, jusqu'à 7 km de celui-ci: «Auf Schruf», «Meerboden», «Frauenkirch», «Niedermendig»; il n'existe aucun argument, théorique ou empirique, en faveur de l'existence de ces centres.

---

, 11 000 , o- 5 ³ . , 200 , : 1) . — Trass —, , 2 6- . 12 16 . 2) , , . 3) . 4) , . 5) (5,3 ³) (0,7 ³). 6) , . 7) ; , . 8) . (Auf Schruf, Meerboden, Frauenkirch, Niedermendig), 7 , , .

---

---

Dedicated to W. von Engelhardt on the occasion of his 75th birthday.     

Enriched,HIMU-type peridotite and depleted recycled pyroxenite in the Canary plume: A mixed-up mantle

The Earth's mantle is chemically and isotopically heterogeneous, and a component of recycled oceanic crust is generally suspected in the convecting mantle [Hofmann and White, 1982. Mantle plumes from ancient oceanic crust. Earth Planet. Sci. Lett. 57, 421–436]. Indeed, the HIMU component (high µ = ²³⁸U/²⁰⁴Pb), one of four isotopically distinct end-members in the Earth's mantle, is generally attributed to relatively old (≥ 1–2 Ga) recycled oceanic crust in the form of eclogite/pyroxenite, e.g. [Zindler and Hart, 1986. Chemical geodynamics. Ann. Rev. Earth Planet. Sci. 14, 493–571]. Although the presence of the recycled component is generally supported by element and isotopic data, little is known about its physical state at mantle depths. Here we show that the concentrations of Ni, Mn and Ca in olivine from the Canarian shield stage lavas, which can be used to assess the physical nature of the source material (peridotite versus olivine-free pyroxenite) [Sobolev et al., 2007. The amount of recycled crust in sources of mantle-derived melts. Science 316, 412–417], correlate strongly with bulk rock Sr, Nd and Pb isotopic ratios. The most important result following from our data is that the enriched, HIMU-type (having higher ²⁰⁶Pb/²⁰⁴Pb than generally found in the other mantle end-members) signature of the Canarian hotspot magmas was not caused by a pyroxenite/eclogite constituent of the plume but appears to have been primarily hosted by peridotite. This implies that the old (older than ~ 1 Ga) ocean crust, which has more evolved radiogenic isotope compositions, was stirred into/reacted with the mantle so that there is not significant eclogite left, whereas younger recycled oceanic crust with depleted MORB isotopic signature (< 1 Ga) can be preserved as eclogite, which when melted can generate reaction pyroxenite.     

Volatile emission during the eruption of Baitoushan Volcano (China/North Korea) ca. 969 AD

3 [magma volume (DRE): 24 ± 5 km³]. The main phase (ca. 95 vol.%) is represented by comenditic tephra deposited dominantly as widespread fallout blankets and proximal ignimbrites. The eruption column is estimated to have reached ca. 25 km and thus entered the stratosphere. A late phase (5 vol.%) is represented by trachyte emplaced chiefly as moderately welded ignimbrites. The comendites contain  ∼ 3, and the trachytes 10–20 vol.% phenocrysts, mainly anorthoclase, hedenbergite, and fayalite. Primary glassy melt inclusions with no signs of leakage were found only in phenocrysts in the comenditic tephra, whereas those in phenocrysts in the trachytes are devitrified. The comendite magma is interpreted to have been generated by fractional crystallization from a trachyte magma represented by melt inclusions in the phenocrysts in the comendite tephra. The mass of volatiles emitted to the atmosphere during the eruption was estimated using the petrologic method. The average H₂O concentration of the comenditic matrix glass is 1.5 wt.% (probably largely secondary) and of the corresponding melt inclusions  ∼ 5.2 wt.%. Melt inclusions in feldspar and quartz present the highest halogen concentrations with a calculated average for chlorine of 4762 ppm and for fluorine of 4294 ppm. The comenditic matrix glasses are represented by a fluorine-rich (3992 ppm F) and fluorine-poor group (2431 ppm F), averaging 3853 ppm for chlorine. Only 20% of all sulfur analyses of the comenditic matrix glasses and melt inclusions are above the detection limit of  ≥ 250 ppm S. The difference between pre- and post-eruptive concentration of H₂O is at least 3.7 ± 0.6 wt.% H₂O taking into consideration re-hydration of the matrix glass and possible leakage of melt inclusions. The difference between pre- and post-eruptive concentrations of the halogens amounts to 909 ± 90 ppm Cl, and 1863 ± 280 ppm and 302 ± 40 ppm F. The difference for S was estimated based on the average of the maximum S concentrations in the melt inclusions (455 ppm S) and the detection limit, resulting in 205 ± 40 ppm S. The calculated mass of volatiles injected into the atmosphere, based on the erupted magma volume and volatile data, is 1796 ± 453 megatons for H₂O, 45 ± 10 megatons for chlorine, 42 ± 11 megatons for fluorine, and 2 ± 0.6 megatons for sulfur. The 969 ± 20 AD eruption of Baitoushan Volcano, one of the largest eruptions of the past 2000 years, is thought to have had a substantial but possibly short-lived effect on climate. Received: 25 July 1998 / Accepted: 8 September 1999     

Mass transfer during sub-seafloor alteration of the upper Troodos crust (Cyprus)

Five major alteration zones in the Extrusive Series and the Sheeted Dike Complex of the Troodos Ophiolite are each characterized by (a) distinct elemental changes compared to the original composition and (b) secondary mineralogy. The upper ca. 300 m of the extrusive crust, the highly oxidatedcold seawater alteration zone (CSA), is strongly enriched in K₂O and depleted in Na₂O. It is followed downwards by alow temperature alteration zone (<170° C) which is most widespread in the Troodos extrusives and where Na₂O and K₂O are enriched, the latter less strongly than in the CSA zone. Three types ofhigh temperature alteration zones (<440° C; HTA I–III) are found in the Sheeted Dike Complex. All are marked by thorough leaching of K₂O, while the behavior of Na₂O (e.g. unchanged in type III) and CaO (depleted in type I, enriched in types II, III) is variable. Mass budgets of elemental changes are quantified by calibration of whole rock analyses via systematic stable element variations of fresh glasses found throughout the extrusive section. The Troodos extrusive crust and upper Sheeted Dike Complex are a major sink for MgO, K₂O, and Na₂O, and a source for CaO; the overall scale of fluxes drastically exceeds estimates based on fresh basalt compositions from present ocean crust.     

Rates of magma ascent and depths of magma reservoirs beneath La Palma (Canary Islands)

Peridotitic mantle xenoliths from historic and prehistoric eruptions on La Palma show many similarities. Prolonged reactions of the xenoliths with their host magmas have been used to place constraints on the magma transport system beneath the island. All xenoliths show crystalline selvages and 0.9–2.6 mm wide diffusion zones in olivine along most of their surface. Diffusion kinetics in olivine, combined with fluid inclusion barometry, document that selvages and diffusion zones formed at crustal levels within 8 to about 100 years. Some xenolith fractures lack selvages and were in contact with the host magma for less than 4 days. A multistage magma ascent is proposed: (i) peridotite wall rock was fragmented and became incorporated into the ascending magma years to decades prior to the eruption; (ii) the xenoliths were rapidly transported to, and deposited in, crustal magma reservoirs, forming selvages and diffusion zones at the xenolith rims; (iii) renewed fragmentation of the xenoliths occurred days to hours prior to eruption, possibly by decompressive strain fracturing during rapid ascent.