Journal of Petrology - Advance Access

Seismogenic Shear Zones in the Lithospheric Mantle: Ultramafic Pseudotachylytes in the Lanzo Peridotite (Western Alps, NW Italy)

Piccardo, G. B., Ranalli, G., Guarnieri, L. — Fri, 06 Nov 2009 07:34:52 PST

At Mt. Moncuni (Lanzo Massif, Western Alps) plagioclase peridotites and early mid-ocean ridge basalt (MORB) gabbroic dykes are deformed by shear zones containing cataclastic bands and both fault-vein and injection-vein pseudotachylytes, which are crosscut by late MORB porphyritic dykes. Fault-vein pseudotachylytes have thicknesses of the order of 1 mm; injection-vein pseudotachylytes have a typical thickness of 1–10 cm and contain spinifex textures. Structural, petrological and geochemical data show that the pseudotachylytes formed by near-complete melting of the host peridotite, at ambient temperature–pressure conditions (T = 600 ± 100°C, P < 0·5 GPa) close to the brittle–ductile transition of ultramafic rocks, during exhumation of the lithospheric mantle in the early stages of formation of the Ligurian Tethys oceanic basin. Estimates of the average volume fraction of unmelted clasts and of the ambient and liquidus temperature, together with thermophysical parameters, allow the determination of the melting energy per unit volume. Coseismic displacement is not observable at Mt. Moncuni, and consequently the dynamic shear resistance cannot be inferred. We show that commonly proposed relations between fault-vein thickness and displacement are of limited value, given the difficulty in identifying ‘single-event’ pseudotachylytes and the mobility of the melt. However, we also show that dynamic shear resistance can be predicted to decrease sharply if the melt coats the whole fault plane, partly as a consequence of the nonlinear viscosity of silicate melts at high strain rates. The Mt. Moncuni pseudotachylytes are the result of upper mantle seismicity at shallow depth (z < 20 km) over a time period of at most 5 Myr. Estimation of the total seismic energy release and moment (caused by an unspecified number of small to moderate earthquakes) requires an assessment of the total pseudotachylyte volume. This is highly uncertain, with a probable qualitative error margin of ±1 order of magnitude. The inferred values of cumulative seismic energy release and moment are of the order of 10^{15 ± 1} J and 10^{19 ± 1} N m, respectively, resulting in a seismic energy release rate of approximately 10^{8 ± 1} J/a. This value is compatible with present-day seismic rates at extensional plate margins.

Geochemical and Geochronological Constraints on the Nature of the Immediate Basement next to the Mesoarchaean Auriferous Witwatersrand Basin, South Africa

Frimmel, H. E., Zeh, A., Lehrmann, B., Hallbauer, D., Frank, W. — Wed, 04 Nov 2009 22:53:00 PST

A combined petrological, geochemical, and geochronological (Rb–Sr and Sm–Nd whole-rock, U–Pb and Lu–Hf zircon, and Ar–Ar hornblende) study on a section of pre-Witwatersrand basement drilled at the northwestern margin of the Witwatersrand Basin has revealed new insights into the nature and tectonic setting of the likely source area for some of the Mesoarchaean auriferous Witwatersrand sediments. The protoliths of intersected altered granite and hornblende metagabbro are of indistinguishable age (3062 ± 5 Ma) and have very similar geochemical signatures. Trace element characteristics typical of calc-alkaline magmatism and evidence of variable contamination with older crust (subchondritic _Nd and _Hf in zircon) point to an active continental margin setting. The Ar–Ar hornblende ages are within error of the magmatic crystallization age or slightly older. Alteration of presumably primary magmatic hornblende to magnesio-hornblende immediately after gabbro emplacement during late magmatic autometasomatism is suggested. The presence of hydrous melts (>4 wt % H₂O), comparable with fertile Au-bearing magmatic–hydrothermal mineralizing systems in Phanerozoic volcanic arcs, is inferred. Thus, a kind of hinterland is proposed for the Witwatersrand that compares favourably with the tectonic domains that are known to host the majority of post-Archaean gold deposits. Later retrograde hydrothermal alteration at c. 2720 and 2630 Ma led to variable Pb loss in zircon and the resetting of the whole-rock Rb–Sr isotope system whereas the Ar–Ar and Lu–Hf isotope systems in the hornblende and zircon grains, respectively, were not significantly affected. Comparison with published data suggests that these alteration events are the same as those that affected the Witwatersrand Basin fill associated with major early Ventersdorp flood basalt volcanism and possibly a pre-Transvaal thrusting event in response to contractional deformation in the Limpopo Belt.

The Role of Pyroxenites in Formation of Shear Instabilities in the Mantle: Evidence from an Ultramafic Ultramylonite, Twin Sisters Massif, Washington

Toy, V. G., Newman, J., Lamb, W., Tikoff, B. — Wed, 04 Nov 2009 23:50:36 PST

In the Twin Sisters ultramafic massif, NW Washington, an ~0·5 cm thick, isolated ultramafic ultramylonitic shear zone displaces orthopyroxenite and clinopyroxenite dikes, by a minimum of 21 cm. The shear zone exists only adjacent to the orthopyroxenite and clinopyroxenite dikes, with deformation distributed along strike into the wall-rock less than 10 cm from the dikes in the outcrop face. Microstructurally, the shear zone contains domains of different grain sizes and phase proportions. A marginal domain of almost pure olivine displays a mean grain size of ~30–100 µm and an olivine lattice preferred orientation (LPO) indicating that glide occurred on (010)[100] and (010)[001]. A central domain of mixed olivine, pyroxene and amphibole displays a finer grain size, ranging down to ~5 µm. Aligned grain and phase boundaries and weak olivine and pyroxene LPOs indicate that this zone deformed by a grain-size-sensitive deformation mechanism (e.g. grain boundary sliding accompanied by diffusion creep). Geothermometry indicates that shearing occurred at temperatures in the range 650–750°C. We interpret the formation of this ultramylonitic shear zone as a shear instability caused by the presence of compositional heterogeneity. Localization was promoted by a deformation mechanism switch from dislocation creep to grain-size-sensitive creep as a result of syn-deformational grain-size reduction. Mineral compositions indicate that this grain-size reduction was associated with reaction. The fine grain size was stabilized by the presence of multiple phases, particularly pyroxene, within the central shear zone domain. The shear zone did not propagate beyond the vicinity of the pyroxenite dikes because the fine grain sizes necessary for the deformation mechanism switch to occur could not be maintained in the monophase olivine forming the surrounding dunite.

Microstructural and Rheological Evolution of a Mantle Shear Zone

Skemer, P., Warren, J. M., Kelemen, P. B., hirth, G. — Wed, 07 Oct 2009 23:44:21 PDT

We conducted a microstructural study of a high-strain mantle shear zone from the Josephine Peridotite, SW Oregon, USA. The goal of this study is to understand how microstructural evolution at large strains leads to transitions in rheological behavior. The shear zone we investigated exhibits higher strain and greater localization than previously studied shear zones in the Josephine Peridotite. The margins of the shear zone have a homogeneous microstructure, characterized by moderately strong olivine fabrics, fairly weak orthopyroxene fabrics, and grain sizes of 2–3 mm. The highly deformed samples from the center of the shear zone display two distinct microstructural domains—a relatively coarse-grained domain (~550 µm) that contains only olivine and a finer-grained domain (~250 µm) that contains both olivine and orthopyroxene. The coarse-grained domain has a strong E-type olivine lattice-preferred orientation (LPO). Within the fine-grained domain the olivine LPO is also E-type, but significantly weaker. The E-type fabrics are rotated slightly past the shear plane, providing the first field-based confirmation of similar experimental observations. The presence of E-type fabrics, which form in the presence of moderate quantities of water, also highlights the potential importance of water to shear zone evolution. The orthopyroxene in the fine-grained domains has no LPO, suggesting that a transition to grain-size sensitive deformation occurred. The microstructural transition in orthopyroxene may have resulted in a marked weakening of the rock, suggesting that orthopyroxene plays a critical role in shear localization. These samples provide a crucial microstructural link between moderately localized shear zones and highly deformed ultramylonites.

Melt Migration and Intrusion during Exhumation of the Alboran Lithosphere: the Tallante Mantle Xenolith Record (Betic Cordillera, SE Spain)

Rampone, E., Vissers, R. L. M., Poggio, M., Scambelluri, M., Zanetti, A. — Tue, 22 Sep 2009 08:12:15 PDT

Microstructural and in situ mineral chemistry studies on mantle peridotite xenoliths from the Late Neogene alkaline volcanic center of Cabezo Tallante (SE Spain) reveal an exceptional record of a multi-stage history of deformation, recrystallization, melt–rock interaction and melt intrusion tracking the progressive exhumation of this lithospheric mantle sector. Xenoliths include porphyroclastic to equigranular spinel peridotites, impregnated plagioclase peridotites, and composite xenoliths made up of peridotites intruded first by gabbronorite veins and later by amphibole-bearing pyroxenites. The earliest stage involved subsolidus re-equilibration from garnet- to spinel-facies conditions, represented by rounded opx + spinel ± cpx clusters indicative of precursor garnet. The spinel-facies equilibration was followed by development of a porphyroclastic fabric, accentuated in many xenoliths by spinel trails, in response to shear deformation that may be related to the early stages of Neogene extension. Porphyroclastic spinel peridotites subsequently underwent multiple episodes of reactive porous melt percolation documented by crystallization of undeformed olivine replacing pyroxene porphyroclasts, and of undeformed poikilitic orthopyroxene at the expense of both pyroxene porphyroclasts and newly crystallized olivines. The porphyroclastic and melt–rock reaction textures are progressively obliterated by an equigranular structure developed as the result of static, possibly melt-assisted, annealing recrystallization. Clinopyroxenes in equigranular peridotites (i.e. the most equilibrated with the percolating melts) display slight light rare earth element (REE) depletion and almost flat middle to heavy REE spectra (La_N/Yb_N = 0·37–0·62; Sm_N/Yb_N = 0·89–1·23). Computed equilibrium liquids have an enriched tholeiitic affinity, consistent with the sub-alkaline magmatism of the Alboran Domain. Overall, the tectonic and magmatic stages recorded in spinel peridotites from Tallante are remarkably consistent with the evolution documented in the Ronda peridotites of the western Betics. Reactive porous flow and annealing recrystallization were followed by an impregnation event, documented by crystallization of interstitial (plag ± opx ± ol) aggregates in porphyroclastic and equigranular xenoliths; this indicates further exhumation to shallower depths. Diffuse melt percolation was followed by intrusion of melts with distinct chemical affinity. The first event is documented by thin gabbronoritic–noritic veins, showing opx reaction rims against the host peridotite. Comparable gabbronorites were previously ascribed to slab-derived melts. The norite veins are crosscut by centimeter-thick dikelets of amphibole pyroxenite. Geobarometric estimates and the observed crystallization order (ol–cpx–amph–plag) point to 0·7–0·9 GPa for pyroxenite intrusion. Computed melts in equilibrium with clinopyroxene show alkaline affinity, similar to the host Tallante alkali basalts. Textural and geochemical features in the xenoliths thus indicate that the progressive uplift of the Tallante lithospheric mantle was accompanied by interaction with melts of different sources, reflecting the magmatic evolution of the Alboran Domain in response to lithosphere extension and thinning leading to the formation of the Betic–Rif arc.

Stress-driven Melt Segregation in Partially Molten Olivine-rich Rocks Deformed in Torsion

King, D. S. H., Zimmerman, M. E., Kohlstedt, D. L. — Tue, 15 Sep 2009 07:43:29 PDT

Torsion experiments on partially molten aggregates of olivine + chromite + 4 vol. % mid-ocean ridge basalt provide new insights into the interactions between deformation and melt segregation. When samples are sheared, melt segregates into distinct melt-rich bands oriented ~20° antithetic to the macroscopic shear plane. In one series of experiments, samples were deformed at similar shear strain rates (or stresses) to a range of finite shear strains to explore the evolution of melt-rich bands. In another series of experiments, samples were deformed to similar finite shear strains at a range of strain rates to explore the effect of strain rate (or stress) on band spacing and microstructure. We relate variations in strain rate to the compaction length and show that band spacing increases with increasing compaction length. These experiments provide new information on the evolution of melt distribution, the partitioning and localization of strain, and the scaling of experimental results to the Earth's mantle.

Serpentine Mineral Replacements of Natural Olivine and their Seismic Implications: Oceanic Lizardite versus Subduction-Related Antigorite

Boudier, F., Baronnet, A., Mainprice, D. — Mon, 17 Aug 2009 07:35:18 PDT

We report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations. In the first case, mesh-textured lizardite (liz) is developed in a standard 60% serpentinized oceanic harzburgite from the Oman ophiolite where olivine converts to columnar lizardite. The joined columns are perpendicular to the basal plane (001)_liz, corresponding to the pseudofibres observed optically. The plane (001)_liz is locally parallel to the narrow boundary ol–liz; thus column orientations register the interface of serpentinization. The ol–liz relationships are not strictly topotactic, but reflect preferred cracking orientations in olivine, parallel to (010)_ol. In the second case, antigorite (atg) develops in a rare sample of antigorite schist in a kimberlite from Moses Rock (Colorado Plateau), representative of a suprasubduction-zone mantle wedge. High-resolution transmission electron microscopy (HRTEM) images along [010]_atg show domains of very regular modulation with a 43·5 Å wavelength (m = 17, where m is the number of silicate tetrahedra along the wave), with few defects, indicative of HP–HT antigorite, and also heavily kinked regions as fingerprints of strong tectonic shear. TEM imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine: [100]_atg//[010]_ol and <100>_atg//<100>_ol; the planes in contact are (001)_atg//(100)_ol and (001)_atg//(010)_ol, respectively. The [010]_atg//[001]_ol and antigorite lamellae are parallel to the forsterite b-axis. In both cases, the topography of olivine–serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate. In the cases studied, the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite. These results have some relevance to the seismic anisotropy of serpentinized mantle. Anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite. Conversely, the development of antigorite produces a trench-parallel fast S-wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization.

Stress-driven Melt Segregation in Partially Molten Feldspathic Rocks

Kohlstedt, D. L., Zimmerman, M. E., Mackwell, S. J. — Wed, 22 Jul 2009 09:34:58 PDT

High-strain torsion experiments were performed on a series of samples composed of anorthite plus <1 to 12% melt to investigate the formation of melt-rich bands produced by stress-driven melt segregation. Fine-grained (3–4 µm) samples were deformed in the diffusion creep regime at a temperature of 1450 K and a confining pressure of 300 MPa at shear strain rates of 1 x 10^–4 to 16 x 10^–4 s^–1 and shear stresses of 15–150 MPa to shear strains between = 1·9 and 6·6. The dependence of viscosity, , on melt fraction, , for these partially molten aggregates can be expressed as = 2·6 x 10¹² exp (–24 ) Pa s. In each sample, melt-rich bands develop by a shear strain of = 1, forming a population of bands at an angle of 5–15° to the shear plane and 40–30° to the applied maximum principal stress. The spacing between and width of the melt-rich bands increases as melt fraction increases from <0·01 to 0·06, then roughly levels off as melt fraction increases to 0·12. This band spacing, _s, increases linearly with increasing compaction length, _c, according to the relation _s = 0·07 _c when the bulk viscosity is assumed to be twice the shear viscosity. In the Earth, spontaneous stress-driven segregation of fluids is an important mechanism for localizing deformation into shear zones.