Weak B‐Type Olivine Fabric Induced by Fast Compaction of Crystal Mush in a Crustal Magma Reservoir

Abstract

Understanding the deformation mechanisms of olivine has been considered as the crucial factor in tracing the dynamic processes from seismic anisotropy. Due to extensive overprinting of deformation and alteration, natural examples of original magmatic fabric of olivine are scarce. In this study we selected one wehrlite and three dunite samples from a deep borehole in the Poyi mafic-ultramafic intrusion in the Tarim Craton. The fresh samples show adcumulate texture and plastic deformation of dry olivine (≤10 ppm H 2 O). Thermobarometer calculations yield crystallization conditions at 200-400 MPa and >1,124-1,275°C. Olivine developed a B-type fabric characterized by a concentration of [001] axes parallel to the interpreted lineation and that of [010] axes normal to the interpreted foliation. Activation of [001] and [100] dislocations is confirmed by microstructural observations. Simulation of gravity-driven compaction process reveals extremely rapid compaction of kilometer-scale crystal mush in the Poyi magmatic system. During the early compaction with high porosity, crystallographic habit of olivine may produce shape-preferred orientation and the related B-type fabric. In the final stage of compaction and crystallization, the initial shape-preferred orientation was gradually overprinted, whereas the weak B-type fabric has remained with dislocation creep and dislocation-accommodated grain boundary sliding. Calculated seismic velocities of peridotite samples show low P and S wave anisotropies, with the fastest S wave polarization direction normal to the foliation. Hence, a mush-dominated ultramafic intrusion exhibits weak seismic anisotropy, which has potential to better understand seismic anisotropy in the transcrustal magmatic systems.

Metastability and Nondislocation‐Based Deformation Mechanisms of the Flem Eclogite in the Western Gneiss Region, Norway

“…Thus, the weak but obvious CPOs of all major constituent minerals (Grt, Omp, Ol, Opx, and Amp) in the MEC (Figures 8a, 8b, 9a, and 9b) are not the consequences of deformation coeval with eclogitization. The inherence of preexisting CPOs (B‐type like) in Ol can be inferred from the weak SPOs of euhedral Ol grains (Figure 3a) and their weak B‐type like CPOs (Figures 8a and 8b), which were most likely formed by compaction or shearing during the crystallization and accumulation of Ol from magma (Cao et al, 2017; Yao et al, 2019). The good correlations of CPOs between Omp and Amp (Figures 9a and 9b) are strong evidence suggesting that the CPOs of Amp mainly resulted from topotactic replacement of Omp grains (Figure 3d) during retrograde metamorphism (Giuntoli et al, 2018).…”

Tectonics of the Isua Supracrustal Belt 2: Microstructures Reveal Distributed Strain in the Absence of Major Fault Structures

“…Ti-humite and magnesite (Dymek et al, 1988a) may not be discriminating features of subduction zone conditions, as they may be consistent with amphibolite-grade crustal cumulates (Dymek et al, 1988a;Szilas et al, 2015;Yao et al, 2019). Therefore, existing and new petrological information of the Isua supracrustal rocks can be interpreted as preserving quasi-uniform, pre-3.5 Ga amphibolite metamorphism with increasing metasomatic effects toward the northeastern part of the supracrustal belt (Ramírez-Salazar et al, 2021;Webb et al, 2020).…”

“…First, the generation of the two types of basalts and the tonalite protoliths is possible in nonplate tectonic settings that feature extensive melting and recycling of deeply buried hydrous basalts (see next paragraph, also [Johnson et al., 2017; Pearce & Reagan, 2019; Smithies et al., 2007]). Second, B‐type olivine fabrics, depleted mantle‐like geochemistry, and the coexistence of Ti‐humite and magnesite (Dymek et al., 1988a) may not be discriminating features of subduction zone conditions, as they may be consistent with amphibolite‐grade crustal cumulates (Dymek et al., 1988a; Szilas et al., 2015; Yao et al., 2019). Therefore, existing and new petrological information of the Isua supracrustal rocks can be interpreted as preserving quasi‐uniform, pre‐3.5 Ga amphibolite metamorphism with increasing metasomatic effects toward the northeastern part of the supracrustal belt (Ramírez‐Salazar et al., 2021; Webb et al., 2020).…”

Magnetite layer formation in the Bushveld Complex of South Africa

“…The decrease of melt density driven by the crystallization of 2.5 wt.% magnetite from the pore melt is calculated as 58.2 kg•m -3 , which can be considered as the ∆´. At a high porosity, the permeability of crystal mush is simplified as 33,[52][53] :…”

“…Compaction is driven by the large density contrast between solid (~4955 kg•m -3 ) and liquid (~2683 kg•m -3 ) (Methods). Generally, this process initiates with a mechanical reorganization to achieve optimum packing and continues with pressure solution/reprecipitation and possible viscous deformation of cumulate minerals 29,[31][32][33] . The thickness of the MML (<~2.4 m) may not have provided sufficient gravitational load and effective stress to drive plastic deformation of magnetite 23 , and hence we argue that the compaction process was likely to have been achieved mainly through pressure solution and reprecipitation.…”