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Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania 总被引:1,自引:0,他引:1
Jörg Keller Jurgis Klaudius Matthieu Kervyn Gerald G. J. Ernst Hannes B. Mattsson 《Bulletin of Volcanology》2010,72(8):893-912
With a paroxysmal ash eruption on 4 September 2007 and the highly explosive activity continuing in 2008, Oldoinyo Lengai (OL) has dramatically changed its behavior, crater morphology, and magma composition after 25 years of quiet extrusion of fluid natrocarbonatite lava. This explosive activity resembles the explosive phases of 1917, 1940–1941, and 1966–1967, which were characterized by mixed ashes with dominantly nephelinitic and natrocarbonatitic components. Ash and lapilli from the 2007–2008 explosive phase were collected on the slopes of OL as well as on the active cinder cone, which now occupies the entire north crater having buried completely all earlier natrocarbonatite features. The lapilli and ash samples comprise nepheline, wollastonite, combeite, Na-åkermanite, Ti-andradite, resorbed pyroxene and Fe–Ti oxides, and a Na–Ca carbonate phase with high but varying phosphorus contents which is similar, but not identical, to the common gregoryite phenocrysts in natrocarbonatite. Lapilli from the active cone best characterize the erupted material as carbonated combeite–wollastonite–melilite nephelinite. The juvenile components represent a fundamentally new magma composition for OL, containing 25–30 wt.% SiO2, with 7–11 wt.% CO2, high alkalies (Na2O 15–19%, K2O 4–5%), and trace-element signatures reminiscent of natrocarbonatite enrichments. These data define an intermediate composition between natrocarbonatite and nephelinite, with about one third natrocarbonatite and two thirds nephelinite component. The data are consistent with a model in which the carbonated silicate magma has evolved from the common combeite–wollastonite nephelinite (CWN) of OL by enrichment of CO2 and alkalies and is close to the liquid immiscible separation of natrocarbonatite from carbonated nephelinite. Material ejected in April/May 2008 indicates reversion to a more common CWN composition. 相似文献
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Matthieu Kervyn Gerald G. J. Ernst Jörg Keller R. Greg Vaughan Jurgis Klaudius Evelyne Pradal Frederic Belton Hannes B. Mattsson Evelyne Mbede Patric Jacobs 《Bulletin of Volcanology》2010,72(8):913-931
On September 4, 2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Oldoinyo Lengai (OL),
N Tanzania, changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in
March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake
swarm in July 2007. Despite the lack of ground-based monitoring, the evolution in OL eruption dynamics is documented based
on the available field observations, ASTER and MODIS satellite images, and almost-daily photos provided by local pilots. Satellite
data enabled identification of a phase of voluminous lava effusion in the 2 weeks prior to the onset of explosive eruptions.
After the onset, the activity varied from 100 m high ash jets to 2–15 km high violent, steady or unsteady, eruption columns
dispersing ash to 100 km distance. The explosive eruptions built up a ∼400 m wide, ∼75 m high intra-crater pyroclastic cone.
Time series data for eruption column height show distinct peaks at the end of September 2007 and February 2008, the latter
being associated with the first pyroclastic flows to be documented at OL. Chemical analyses of the erupted products, presented
in a companion paper (Keller et al. 2010), show that the 2007–2008 explosive eruptions are associated with an undersaturated carbonated silicate melt. This new phase
of explosive eruptions provides constraints on the factors causing the transition from natrocarbonatite effusive eruptions
to explosive eruptions of carbonated nephelinite magma, observed repetitively in the last 100 years at OL. 相似文献
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Matthieu Kervyn Gerald G. J. Ernst Jurgis Klaudius Jörg Keller François Kervyn Hannes B. Mattsson Frederic Belton Evelyne Mbede Patric Jacobs 《Bulletin of Volcanology》2008,70(9):1069-1086
The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to
April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the
crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed
by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO2). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer
(MODIS) satellite data, which provide the most reliable evidence to constrain the eruption’s onset and variations in activity.
The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2 ± 3.0 × 105 m3. The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion
from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this
unique natrocarbonatite volcano. 相似文献
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Peralkaline silicate lavas at Oldoinyo Lengai, Tanzania 总被引:1,自引:0,他引:1
A detailed study of Oldoinyo Lengai has led to the recognition of two major cone-building stages. An early, predominantly phonolitic stage, Lengai I, forms the southern cone. The recent nephelinitic Lengai II developed following a major sector collapse event over Lengai I. Petrography of Lengai II lavas show that nephelinite is combeite- and wollastonite-bearing. All Oldoinyo Lengai lavas are peralkaline and highly evolved in terms of low Mg#, Ni and Cr values. Within the unique Lengai II combeite–wollastonite–nephelinite (CWN) peralkalinity increases with time to extreme values (Na + K)/Al = 2.36. Mineralogical expression of peralkalinity is the presence of combeite and Na-rich clinopyroxene. In addition, exceptionally high Fe2O3 (up to 10.28 wt.%) in nepheline is an indicator for alumina deficiency. Combeite also shows high Fe3+. Phonolite and CWN of Lengai I and Lengai II show similarly enriched LILE and LREE values and generally parallel patterns in PM normalized and REE plots. 相似文献
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