Petrofabric and seismic properties of lithospheric mantle xenoliths from the Calatrava volcanic field (Central Spain)

Abstract. Subduction-transform edge propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, northwest Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with lithology. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790–1165 ∘C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second-phase particles. In the coarse-grained peridotites, well-developed olivine crystal-preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010] fiber and the [010] and [001] axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths. The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg no. melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite) and with a low-Mg no. evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt–peridotite reaction – promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing – resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and had a key effect on grain size reduction during the operation of the Tell–Rif STEP fault. Melt–rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.

Download Full-text

Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle

10.5194/se-2019-32 ◽

2019 ◽

Author(s):

Károly Hidas ◽

Carlos J. Garrido ◽

Guillermo Booth-Rea ◽

Claudio Marchesi ◽

Jean-Louis Bodinier ◽

...

Keyword(s):

Grain Size ◽

Lithospheric Mantle ◽

Volcanic Field ◽

Mantle Xenoliths ◽

Coarse Grained ◽

Fault System ◽

Mantle Flow ◽

Second Phase ◽

Fine Grained ◽

Wide Range

Abstract. Subduction-Transform Edge Propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, NW Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with modal variations. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790–1165 °C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second phase particles. In the coarse-grained peridotites, well-developed olivine crystal preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high-temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010]-fiber and the [010]- and [001]-axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths. The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg# melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite), and with a low-Mg# evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt-peridotite reaction – promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing – resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and played a key effect on grain size reduction during the operation of the Rif-Tell STEP fault. Melt-rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.

Download Full-text

Subduction-related metasomatism in the lithospheric mantle beneath the Calatrava volcanic field (central Spain): constraints from lherzolite xenoliths of the Cerro Gordo volcano

International Geology Review ◽

10.1080/00206814.2020.1858453 ◽

2020 ◽

pp. 1-20

Author(s):

Carlos Villaseca ◽

Javier García Serrano ◽

Cecilia Pérez-Soba

Keyword(s):

Lithospheric Mantle ◽

Volcanic Field ◽

Central Spain

Download Full-text

‘Water’ content as a tool to estimate rheological differences in the lithosphere of young extensional basins

10.5194/egusphere-egu2020-19001 ◽

2020 ◽

Author(s):

Nóra Liptai ◽

Thomas P. Lange ◽

Levente Patkó ◽

Márta Berkesi ◽

Csaba Szabó ◽

...

Keyword(s):

Rheological Properties ◽

Upper Mantle ◽

Lithospheric Mantle ◽

Pannonian Basin ◽

Volcanic Field ◽

Mantle Xenoliths ◽

Self Healing ◽

Alkali Basalts ◽

Extensional Basin ◽

Marginal Areas

Nominally anhydrous minerals in the lithospheric mantle, such as olivine and pyroxenes can host a small amount (tens to hundreds of ppm) of structurally bound hydroxyl (&#8216;water&#8217;). Numerous studies pointed out that water has a strong effect on the rheological properties of the lithospheric mantle, such as melting temperature, electrical conductivity, viscosity and seismic wave propagation speed. Water content of mantle xenoliths can thus be used to estimate such rheological properties which can then be compared with geophysical observations.In this study we present effective viscosities and electrical resistivities calculated with the use of &#8216;water&#8217; contents of upper mantle xenoliths from the Carpathian-Pannonian region (CPR). The CPR is a young extensional basin in Central Europe, where intraplate alkali basalts sampled the lithosphere in five areas, including locations from both the central and marginal regions. &#8216;Water&#8217; contents are generally higher in xenoliths from the marginal areas compared with those from the central areas of the CPR, due to significant hydrogen loss during the extension in the Miocene (Patk&#243; et al., 2019). It is demonstrated that due to the different &#8216;water&#8217; contents, the lithospheric mantle in the central areas can be characterized with higher effective viscosity and electrical resistivity, and thus can be considered as more rigid than the marginal areas. This relative rigidity induced by lithospheric thinning may be a general feature of extensional basin systems worldwide, and can be regarded as a &#8216;self-healing&#8217; mechanism of the extending lithosphere.&#160;References:Patk&#243;, L., Liptai, N., Kov&#225;cs, I. J., Aradi, L. E., Xia, Q.-K., Ingrin, J., Mih&#225;ly, J., O&#8217;Reilly, S. Y., Griffin, W. L., Wesztergom, V., Szab&#243;, C., 2019. Extremely low structural hydroxyl contents in upper mantle xenoliths from the N&#243;gr&#225;d-G&#246;m&#246;r Volcanic Field (northern Pannonian Basin): Geodynamic implications and the role of post-eruptive re-equilibration. Chemical Geology, 507, 23-41.

Download Full-text