Unknown
American Geophysical Union
In:
In: Kimberlites, Diatremes, and Diamonds: Their Geology, Petrology, and Geochemistry. , ed. by Boyd, F. R. American Geophysical Union, Boulder, Colorado, USA, pp. 354-363. ISBN 0-87590-212-X
Publication Date:
2020-01-28
Description:
The olivine melilitite diatemes of the Swabian Alb, frequently compared with kimberlite diatremes, are discussed in terms of hydrogeological setting, internal structure and juvenile fraction.
The hydrogeological conditions of the Swabian Alb at the time of diatreme emplacement were characterized by copious amounts of groundwater within the sedimentary cover of the basement. Subsequently to the eruptions groundwater accumulated within the maars of the larger diatremes forming fresh‐water lakes as also happened nearby in the Steinheim and Ries impact craters. The diatremes reveal subsidence structures composed of large wall‐rock blocks, subaerially deposited pyroclastic beds, and well‐bedded reworked pyroclastic debris which accumulated on the floor of the fresh‐water crater lakes. The latter fact implies availability of groundwater at the time the diatremes formed.
The juvenile fraction is developed in the shape of spherical to ovoid nucleated autoliths of ash to lapilli size that are macroscopically nearly devoid of vesicles. The autoliths are interpreted as the product of water vapor explosions which took place when rising olivine melilitite magma contacted groundwater and was fragmented into magma droplets. The droplets were rapidly chilled and thus preserved their shape. Because of the hydrogeological data, the diatreme structure, and the chilled nature of the autoliths a phreatomagmatic origin of the Swabian diatremes is suggested.
Type:
Book chapter
,
NonPeerReviewed
Format:
text
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