11:00 AM - 11:15 AM
[SVC29-06] Fragmentation mechanism and ascent times of mantle xenoliths from the Westeifel volcanic field (Germany)
Keywords:Westeifel volcanic field, Mantle xenolith, Fluid inclusion, Microcrack, Hydraulic fracturing
Some maar volcanoes in the Westeifel volcanic field (WEVF) (Germany) are known for their mantle xenoliths [1]. Mantle-derived gases were detected from young (13 ka) Laacher See phonolite center (EEVF) [2]. Recent deep low-frequency earthquakes are believed to reflect fluid migration possibly heralding future volcanic activity [3].
Our study of peridotite xenoliths from a tephra pit exposing Meerfelder maar deposits places new constraints regarding 1) the fracturing and capturing mechanisms of the xenoliths, especially with respect to deep fluids, 2) the ascent rate of the host magma and 3) preheating of the mantle source of the xenoliths.
Many of the mantle xenoliths had (1) a blocky shape surrounded by flat faces, (2) a preferred orientation of microcracks developed parallel to the flat surface along the elongated direction of the xenoliths under a polarizing microscope, and (3) fluid inclusions along the healed cracks. These features strongly suggest that the flat surfaces record the hydraulic fracturing process caused by the fluid at the ascending dike tips. In this study, we further test the working hypothesis of capture depths of the xenoliths and magma ascent rate. Blocky mantle xenoliths may or may not retain their original shape at the time of uptake or are fractured during ascent [4,5]. This is crucial for the interpretation of the time scale derived from the chemical nonequilibrium at the rock surfaces.
The formation pressure of fluid inclusions in microcracks was estimated from the residual pressure of CO2 fluid inclusion [6]. The near-surface inclusions show a maximum pressure close to the petrologic estimate of the original depth of the mantle xenoliths from Meerfelder maar (1.2–1.4 GPa [7]), whereas the fluid inclusions away from the surfaces of xenolith show negative crystalline shapes and much lower formation pressures, indicating that they formed long before the eruption and suffered from diffusive modification of the fluid component. These results suggest that the flat surface preserves the original fracture surface formed by hydraulic fracturing.
The olivine in contact with the melt at the original fracture surface showed no mappable (> 5 micrometers) diffusion in most samples (11 out of 13 samples). The ascending time indicated by olivine Fe-Mg diffusion is 40 hours. Inside the xenoliths, no compositional zoning was observed for Fe-Mg in olivine and pyroxene, while minor zoning profiles were common (11 out of the 13 samples) preserved in Ca in orthopyroxene and rarely (2 out of 13 samples) in Ca in olivine. The maximum CaO concentration in olivine is 0.15-0.18 wt.%, indicating that they had been preheated by the host magma magmas prior to eruption [8]. The preheating time indicated by pyroxene Ca diffusion is 20 to 50 ky. The maar eruptions occurred from a mantle significantly heated over a long period and reached the surface generally within a few days.
[1] Schmincke H-U. 2007.The Quaternary Volcanic Fields of the East and West Eifel (Germany). In: Ritter R, Christensen U (eds) Mantle Plumes – a Multidisciplinary Approach, Springer Heidelberg: 241-322.
[2] Giggenbach WF, Sano Y, Schmincke H-U, 1991, CO2-rich gases from Lakes Nyos and Monoun, Cameroon; Laacher See, Germany; Dieng, Indonesia, and Mt. Gambier, Australia-variations on a common theme. J Volc Geotherm Res 45: 311-323.
[3] Schmincke H-U et al. 2019. Deep low-frequency earthquakes reveal ongoing magmatic recharge beneath Laacher See Volcano (Eifel, Germany). Geophys. J. Int. 216: 2025-2036.
[4] Basu AR. 1980. Jointed blocks of peridotite xenoliths in basalts and mantle dynamics. Nature 284: 612-613.
[5] Wolff JA. 1980. Peridotite xenoliths in basalts and mantle dynamics. Nature 288: 103.
[6] Yamamoto J et al., 2002 Fossil pressures of fluid inclusions in mantle xenoliths exhibiting rheology of mantle minerals: implications for the geobarometry of mantle minerals using micro-Raman spectroscopy. Earth Planet. Sci. Lett. 198: 511-519.
[7] Duda A, Schmincke H-U. 1985. Polybaric differentiation of alkali basaltic magmas: evidence from green-core clinopyroxenes (Eifel, FRG). Contrib. Mineral. Petrol. 91: 340-353
[8] Eiichi T. 1980. Thermal history of lherzolite xenoliths----I. Petrology of lherzolite xenoliths from the Ichinomegata crater, Oga peninsula, northeast Japan. GCA Vol. 44: 1643-1658
Our study of peridotite xenoliths from a tephra pit exposing Meerfelder maar deposits places new constraints regarding 1) the fracturing and capturing mechanisms of the xenoliths, especially with respect to deep fluids, 2) the ascent rate of the host magma and 3) preheating of the mantle source of the xenoliths.
Many of the mantle xenoliths had (1) a blocky shape surrounded by flat faces, (2) a preferred orientation of microcracks developed parallel to the flat surface along the elongated direction of the xenoliths under a polarizing microscope, and (3) fluid inclusions along the healed cracks. These features strongly suggest that the flat surfaces record the hydraulic fracturing process caused by the fluid at the ascending dike tips. In this study, we further test the working hypothesis of capture depths of the xenoliths and magma ascent rate. Blocky mantle xenoliths may or may not retain their original shape at the time of uptake or are fractured during ascent [4,5]. This is crucial for the interpretation of the time scale derived from the chemical nonequilibrium at the rock surfaces.
The formation pressure of fluid inclusions in microcracks was estimated from the residual pressure of CO2 fluid inclusion [6]. The near-surface inclusions show a maximum pressure close to the petrologic estimate of the original depth of the mantle xenoliths from Meerfelder maar (1.2–1.4 GPa [7]), whereas the fluid inclusions away from the surfaces of xenolith show negative crystalline shapes and much lower formation pressures, indicating that they formed long before the eruption and suffered from diffusive modification of the fluid component. These results suggest that the flat surface preserves the original fracture surface formed by hydraulic fracturing.
The olivine in contact with the melt at the original fracture surface showed no mappable (> 5 micrometers) diffusion in most samples (11 out of 13 samples). The ascending time indicated by olivine Fe-Mg diffusion is 40 hours. Inside the xenoliths, no compositional zoning was observed for Fe-Mg in olivine and pyroxene, while minor zoning profiles were common (11 out of the 13 samples) preserved in Ca in orthopyroxene and rarely (2 out of 13 samples) in Ca in olivine. The maximum CaO concentration in olivine is 0.15-0.18 wt.%, indicating that they had been preheated by the host magma magmas prior to eruption [8]. The preheating time indicated by pyroxene Ca diffusion is 20 to 50 ky. The maar eruptions occurred from a mantle significantly heated over a long period and reached the surface generally within a few days.
[1] Schmincke H-U. 2007.The Quaternary Volcanic Fields of the East and West Eifel (Germany). In: Ritter R, Christensen U (eds) Mantle Plumes – a Multidisciplinary Approach, Springer Heidelberg: 241-322.
[2] Giggenbach WF, Sano Y, Schmincke H-U, 1991, CO2-rich gases from Lakes Nyos and Monoun, Cameroon; Laacher See, Germany; Dieng, Indonesia, and Mt. Gambier, Australia-variations on a common theme. J Volc Geotherm Res 45: 311-323.
[3] Schmincke H-U et al. 2019. Deep low-frequency earthquakes reveal ongoing magmatic recharge beneath Laacher See Volcano (Eifel, Germany). Geophys. J. Int. 216: 2025-2036.
[4] Basu AR. 1980. Jointed blocks of peridotite xenoliths in basalts and mantle dynamics. Nature 284: 612-613.
[5] Wolff JA. 1980. Peridotite xenoliths in basalts and mantle dynamics. Nature 288: 103.
[6] Yamamoto J et al., 2002 Fossil pressures of fluid inclusions in mantle xenoliths exhibiting rheology of mantle minerals: implications for the geobarometry of mantle minerals using micro-Raman spectroscopy. Earth Planet. Sci. Lett. 198: 511-519.
[7] Duda A, Schmincke H-U. 1985. Polybaric differentiation of alkali basaltic magmas: evidence from green-core clinopyroxenes (Eifel, FRG). Contrib. Mineral. Petrol. 91: 340-353
[8] Eiichi T. 1980. Thermal history of lherzolite xenoliths----I. Petrology of lherzolite xenoliths from the Ichinomegata crater, Oga peninsula, northeast Japan. GCA Vol. 44: 1643-1658