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Juvenile fragment studies on lapilli tuffs of the Messel maar-diatreme-volcano, Germany: implications for rockmagnetic properties

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Abstract

In 2001 the 433 m deep Messel 2001 bore hole was drilled in the centre of the Messel Pit, 25 km south of Frankfurt (Germany). Interdisciplinary, geoscientific results obtained from this drilling proved the origin of the circular-shaped basin as a maar-diatreme-structure beneath the surface. Recovered deposits consist of sedimentary rocks (0-240 m) and volcaniclastic rocks such as lapilli tuffs (240-373 m) as well as rocks of the underlying diatreme breccia (373-433 m). The lapilli tuffs, as matter of interest here, show little differentiation on a macro- and microscopic scale and appear as unsorted volcaniclastics with dominating juvenile lapilli and accidental clasts in the range of (sub)millimetres to centimetres in diameter. Decimeter-sized blocks of the crystalline basement occur at certain depths, but are comparatively scarce and inconspicuous, concerning the total thickness of the tuffs. Rock magnetic properties measured on core samples of the lapilli tuffs explain the origin of detected downhole magnetic anomalies performed during the drilling project 2001. Thereby, the juvenile fragments as main carrier of ferrimagnetic minerals (titano-magnetites) specify the rock magnetic character of the volcanic material and account for downhole logging data of the magnetic susceptibility (MS) and the natural remanent magnetisation (NRM). Besides similar remanence acquisition properties throughout the lapilli tuffs, differences in the magnetic stability behaviour are realised for the upper and lower half of the tuffs. Thermal magnetic experiments prove the magnetic differences and the acquisition of (partial) thermal remanent magnetisation (pTRM), respectively, and allow estimations of emplacement temperatures 300 ° C for the lower half of the lapilli tuffs. This study deals with image analytical and geochemical investigations on juvenile fragments as implication for the rock magnetic results and provides insights into the heat and magma source of the Messel maar-volcano. The volcaniclastic material can be separated into a relatively hot and geochemically undifferentiated eruption phase as well as a colder, differentiated phase. Thus, we suggest a two-condition eruption stage at the end of the volcanic activity. Furthermore, the chemical composition, size and shape of the juvenile particles account for the temperature evolution and heat conditions during deposition of the Messel tuffs and contribute to the existence of magnetic field anomalies.
Juvenile fragment studies on lapilli tuffs of the Messel
maar-diatreme-volcano, Germany: implications for
rockmagnetic properties
Juvenile fragment studies on lapilli tuffs of the Messel
maar-diatreme-volcano, Germany: implications for
rockmagnetic properties
Thomas Nitzsche
1,2
Helga de Wall
2
Christian Rolf
1
Ulrich Schüssler
3
Gerald Gabriel
1
1
Leibniz Institute for Applied Geosciences, Stilleweg 2, 30655 Hannover, Germany (www.gga-hannover.de)
2
Institute for Geology, Pleicherwall 1, 97070 Würzburg, Germany (www.geologie.uni-wuerzburg.de)
3
Institute for Mineralogy, am Hubland, 97074 Würzburg, Germany (www.mineralogie.uni-wuerzburg.de)
Introduction Rockmagnetic properties
Juvenile fragments
Throughout the lapilli tuffs, fine-grained Fe-oxides bound to the juvenile fragments are
main carrier of ferrimagnetic properties. They have near magnetite composition (Fig 3) and
are disseminated in a juvenile, glassy matrix (Fig. 4). Rock magnetic experiments (Nitzsche
et al.,
in press
) on the lapilli tuffs clarified the origin of the magnetic anomalies and
approved emplacement temperatures >300 °C (section II) and <300 °C (section I) (Fig. 5, 6).
Fig. 3 Ferrites of the lapilli tuffs are
dominated by a near magnetite
composition (Tc: 500-580 °C).
Fig. 2 a) Sketch of the Messel maar-diatreme-structure and the drilled lithozones.
b) Downhole magnetic measurements performed during the drilling project 2001.
Magnetic field anomalies are most pronounced to the lower half of the lapilli tuffs
Fig. 1 Messel Pit genesis is associated to Tertiary
(Quaternary) intraplate volcanism in Germany
image analysis
geochemistry
TEMPERATURE
Due to very difficult petrographical differentiation of the lapilli tuffs on a
macro/microscopic scale (Fig. 7), the volcaniclastic particles have been studied with image
analytical methods in more detail. The accidental clasts do not show a dependency on
magnetic susceptibility (Fig. 8) and NRM intensities, but juvenile fragments do (Fig. 9).
The latter explain the origin of heat source by their amount, size and grade of plastic
deformation.
Conclusions
Rock magnetic and juvenile fragment data suggest a clear subdivision of the
lapilli tuffs into a two-condition eruption phase at the end of volcanic activity.
The volcaniclastic material is separated into a relatively hot, geochemically
undifferentiated and cold, differentiated phase.
The juvenile fragments are mainly identified as primary, despite complex
sedimentation processes (post/syn-eruptive re-sedimentation, subsidence etc.)
occurring in intra-crater settings.
The interdisciplinary analytical studies may explain possible criteria of
diatreme facies and contribute to the understanding of magnetic field anomalies
in volcaniclastic settings.
This work is funded by a grant of the Deutsche Forschungsgemeinschaft (DFG), Germany. We are grateful to
Rüdiger
Schulz
,
Gerald Gabriel
,
Hermann Buness
and
Thomas Wonik
(all GGA Institute) for assistance in this project.
Helene
Brätz
(Institute for Mineralogy, Würzburg) is thanked for measuring rare earth elements.
Nitzsche, T., Rolf, C. and De Wall, H.,
in press
. Origin of magnetic
anomalies in volcaniclastic units of the Messel maar-diatreme (Germany).
Zeitschrift der Deutschen Geowissenschaftlichen Gesellschaft 157, xx-xx.
Reference:
HEAT SOURCE MAGMA SOURCE
The combination of gravity and magnetic
data allows the reconstruction of a 3D
Messel
maar model (Fig. 14).
The fossil-bearing
Messel Pit
(UNESCO World Heritage Site), 25 km south of Frankfurt
(Germany), lies on the Upper Rhine Graben shoulder (Fig. 1) and has a maar-diatreme-
structure beneath its surface (Fig. 2a). In addition to lacustrine sediments (0-200 m), the
research drilling 2001 discovered volcaniclastic units of lapilli tuffs (240-373 m) and the
diatreme breccia (373-433 m). The volcanic material shows a distinct downhole magnetic
anomaly pattern (Fig. 2). Thereby, juvenile fragments attach great importance to the
magnetic signature.
Fig. 4 Fe-oxides are bound to the juvenile fragments and frequently show skeleton-like
structures around Cr-spinels.
Fig. 5 a) Alternating field (AF)
and b) thermal demagnetisation
experiments show differences in
magnetic stability behaviour for
the lapilli tuffs deposited inside
and outside (section I and II)
the anomalies. c) Thermal
heating experiments approve the
differently availed temperatures
of the material.
Fig. 6 The most pronounced temperature effect is given by high
magnetic field and NRM (natural remanent magnetisation)
intensities as well as stable magnetic inclinations.
Fig. 7 Core and thin section photographs of samples from section I and II, reflecting
the onset of the downhole magnetic field anomalies.
Fig. 8 Accidental clast fraction mainly ranges
between 5-35 % and do not show a dependency on
the magnetic susceptibilities, i.e no negative
correlation due to “MS dilution”!.
Averaged major element values of microprobe analysis scans (Fig. 10) separate the
juvenile fraction into two groups (Fig 11, 12). REE elements show a typical trend of
volcanic suits of potassic continental rift zone magmatism (Fig. 13a). The intra-plate
basalts derive from enriched mantle source (Fig. 13b)
Fig. 10 Microprobe analysis scans reflect
different MgO variation diagrams of
juvenile lapilli deposited inside (section II)
and outside (section I) the magnetic
anomalies.
Perspectives
Fig. 14 a) Vertical cross section of the Messel maar
model based on gravity and magnetisation data, b)
results of the measured and modelled potential field
anomalies and c) 3D illustration of the Messel
subsurface.
a
b
c
Frankfurt
Germany
Messel Pit
Fig. 11 Total alkalis (Na2O+K2O wt.%) versus
silica diagram with groupings of the juvenile
material from inside (section II) and outside
(section I) the ∆F-anomalies.
Fig. 12 Harker variation diagrams of SiO2
versus oxides of Mg, Fe, Al, Ti, K, Na and Ca
showing groupings of the juvenile material
deposited inside (section II) and outside (section
I) the ∆F-anomalies.
Fig. 13 a) REE-diagramm of the juvenile
material and the schematised pattern of the
leucitic composition field of volcanic rocks of
the western branch of the East African Rift
(EAR). b) Th/Yb versus Ta/Yb plot showing
the magmatic mantle source character of the
juvenile lapilli.
Fig. 11 Total alkalis (Na2O+K2O wt.%) versus
silica diagram with groupings of the juvenile
material from inside (section II) and outside
(section I) the ∆F-anomalies.
Fig. 11 Total alkalis (Na2O+K2O wt.%) versus
silica diagram with groupings of the juvenile
material from inside (section II) and outside
(section I) the ∆F-anomalies.
Fig. 12 Harker variation diagrams of SiO2
versus oxides of Mg, Fe, Al, Ti, K, Na and Ca
showing groupings of the juvenile material
deposited inside (section II) and outside (section
I) the ∆F-anomalies.
Fig. 13 a) REE-diagramm of the juvenile
material and the schematised pattern of the
leucitic composition field of volcanic rocks of
the western branch of the East African Rift
(EAR). b) Th/Yb versus Ta/Yb plot showing
the magmatic mantle source character of the
juvenile lapilli.
Figure 9 a) Image analytical data shows that the juvenile particle sizes correlate with the susceptibility log with an increasing trend from relatively
low to high values. b) The juvenile proportion (area sums) and the juvenile flattening ratios are most pronounced in section II (red), where high NRM
intensities are due to high depositional temperatures. Low values of image analytical data represents the low depositional temperatures.
<300 °C
>300 °C
Figure 9 a) Image analytical data shows that the juvenile particle sizes correlate with the susceptibility log with an increasing trend from relatively
low to high values. b) The juvenile proportion (area sums) and the juvenile flattening ratios are most pronounced in section II (red), where high NRM
intensities are due to high depositional temperatures. Low values of image analytical data represents the low depositional temperatures.
<300 °C
>300 °C
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