Physics – Geophysics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmmr12a..03e&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #MR12A-03
Physics
Geophysics
[1160] Geochronology / Planetary And Lunar Geochronology, [3662] Mineralogy And Petrology / Meteorite Mineralogy And Petrology, [3944] Mineral Physics / Shock Wave Experiments, [4465] Nonlinear Geophysics / Phase Transitions
Scientific paper
Maskelynite [1-2] and the shock-induced melt pockets in shergottites are diagnostic features evidencing a major dynamic event on their parent body, Mars. Several models have been proposed for the origin of maskelynite: shock-induced solid-state vitrification of labradorite [3-4], metastable melting and quenching at high-pressure (high-P) [5], or ductile mobilization [4, 6]. Similarly, the origin and relevance of shock-melt pockets and veins as the main locations of high-P minerals in shergottites are controversial: localized formation by P-temperature (T) spikes in excess of 70-80 GPa [3, 4] or equilibrium assemblages evidencing peak-shock-P in the range of 25-35 GPa are discussed [7-10]. Crystallization ages are also controversial, with peaks at 160-190 Ma [11] and ≥ 4.1 Ga [12]: shock-induced age resetting may have been misinterpreted as igneous ages. We present ample evidence that maskelynite formed by metastable melting of plagioclase and quenching to glass at high-pressures as a result of the sluggishness of its inversion to lingunite. The direct consequence of our findings is the irrelevance of the refractive indices (RIs) of maskelynite as pressure indicators [3-4], since RIs were first established after decompression and quenching of maskelynite at its closure temperature of relaxation. We investigated the phase assemblages in Shergotty, Zagami, DAG 476, SAU 005, NWA 480, and NWA 856. Maskelynite contains the dense silica polymorph seifertite and the very dense monoclinic polymorph [8 -10]. Lingunite, CAS polymorph and Stishovite are present in shock-melt pockets [7-10]. Akimotoite, and silicate titanite were reported in shock-melt veins [13, 14]. The silicate liquids in which these dense minerals crystallized were perfect P-transmitting media, hence, contrary to [3-4], the dense minerals formed in equilibrium. The shock-induced events could be sequentially delineated commencing with the solid-state inversion to seifertite followed by pervasive melting of plagioclase, production of liquidus lingunite and CAS in shock-melt pockets, entrainment and dissolution of seifertite and pervasive crystallization of liquidus Stishovite. High-pressure inventory allows an estimate of the P-T conditions to be in the range of 25-35 GPa and 2300-2500 deg C [7-10]. Pervasive metastable melting of plagioclase and quenching to maskelynite could have caused age resetting recorded at the time of the shock event. Ref.: [1] Tscharmak G. (1872) Sitzber. Akad. Wiss. Wien, Math.-Naturwiss. Kl. Abt. I 65 122. [2] Tscharmak G. (1883) Sitzber. Akad. Wiss. Wien, Math.-Naturwiss. Kl. Abt. I 88 347. [3] Stöffler, D. et al. (1986) GCA 50, 6, 889. [4] Fritz J. et al. (2005) AMR, 18, 116 [5] Chen M. & El Goresy A. (2000) EPSL 179, 498 [6] Yamagushi A. & Sekine T. (2000) EPSL 175, 289 [7] Beck P. et al. (2004) EPSL 219, 1 [8] El Goresy A. et al. (2000) Science, 288, 1632; [9] El Goresy A. et al. (2004) JPCS, 65, 1597 [10] El Goresy A. et al. (2008) EJM, 20, 523 [11] Bogard D. & Park J. (2008) MAPS, 43, 17, 1113 [12] Bouvier; A. et al. EPSL, 240, 221 [13] Langenhorst F. & Poirier J.-P. (2000) EPSL, 176, 259. [14] Langenhorst F. & Poirier J.-P. (2000) 184, 37.
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