Shock Stages Research Articles

Shock metamorphism of ordinary chondrites

A revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed. Six stages of shock (S1 to S6) are defined, based on shock effects in olivine and plagioclase as recognized by thin section microscopy. The characteristic shock effects of each shock stage are: S1 (unshocked)—sharp optical extinction of olivine; S2 (very weakly shocked)—undulatory extinction of olivine; S3 (weakly shocked)—planar fractures in olivine; S4 (moderately shocked)—mosaicism in olivine; S5 (strongly shocked)—isotropization of plagioclase (maskelynite) and planar deformation features in olivine; and S6 (very strongly shocked)—recrystallization of olivine, sometimes combined with phase transformations (ringwoodite and/or phases produced by dissociation reactions). S6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked. In stages S3 to S6, localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure, due to differences in shock impedance. These melting effects are (a) opaque melt veins (shock veins); (b) melt pockets with interconnecting melt veins; (c) melt dikes; and (d) troilite/metal deposits in fractures. Based on a critical evaluation of data from shock recovery experiments, a shock pressure calibration for the six shock stages is proposed, which defines the S1 /S2, S2/ S3, S3/S4, S4/S5, and S5/S6 transitions at < 5, 5–10, 15–20, 30–35, and 45–55 GPa, respectively. Whole-rock melting and formation of impact melt rocks or melt breccias occurs at about 75–90 GPa. The symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite, e.g., H5(S3). We propose this new shock classification and pressure calibration system to replace previous systems, which are out-of-date with respect to the pressure calibration, nominally restricted to L chondrites, and based on incomplete and, in part, illdefined sets of shock effects.We have classified seventy-six ordinary chondrites using the new classification system and conclude the following: 1.1) Shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in H, L, and LL groups. Differences in the frequency distribution of shock stages are relatively minor; e.g., L chondrites appear to have the largest fraction with stages S5 and S6. This suggests that the collisional histories of the H, L, and LL parent bodies were similar.2.2 ) Petrologic type 3 chondrites are deficient in stages S4 to S6 and, with increasing petrologic type, the frequency of stages S4 to S6 increases. We suggest that the more porous and volatile-rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type. Volatiles trapped in pores cause shock-induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites.3.3 ) Stage S3 is the most abundant in nearly all petrologic types.4.4) At shock pressures in excess of about 35 GPa (S5 and S6), 4He and 40Ar are almost completely lost; pressures below 10 GPa (S1 and S2) do not cause noble gas losses.

Composition of the Crystalline Basement and Shock Metamorphism of Crystalline and Sedimentary Target Rocks at the Haughton Impact Crater, Devon Island, Canada

Abstract— About 100 cobble‐sized samples collected from the surface of the central polymict breccia formation of Haughton impact crater, Canada, have been studied microscopically and chemically. Breccia clasts derived from the 1700 m deep Precambian basement consist of 13 rock types which can be grouped into sillimanite‐ and garnet‐bearing gneiss; alkali feldspar‐rich aplitic or biotite‐hornblende‐bearing gneiss; biotite and hornblende gneiss; apatite‐rich biotite and biotite‐hornblende gneiss; calcitediopside gneiss; amphibolite; tonalitic orthogneiss; and basalts. The range of chemical compositions of these rocks is wide: e.g., SiO2 ranges from 40–85 wt.%; Al2O3 from 7–20 wt.%; CaO from 0.01–25 wt.%; or P2Os from &lt;0.01–5 wt.%. Nearly all samples of crystalline rocks are shock metamorphosed up to about 60 GPa. Most conspicuous is the absence of whole‐rock melts and the very rare occurrence of unshocked rocks. The 45 samples examined can be classified into the following shock stages: stage 0 (&lt;5 GPa): 4.5%, stage Ia (10–20 GPa): 9.0%, stage Ib (20–35 GPa): 33%, stage II (35–45 GPa): 29%, stage III (45–55 GPa): 18%, stage III–IV (55–60 GPa): 6.5%. Among Paleozoic sedimentary rock clasts higher degrees of shock than within crystalline rocks were observed such as highly vesiculated, whole‐rock melts of sandstones and shales. Within the northern and eastern sectors of the allochthonous breccia no distinct radial variation of the cobble‐sized lithic clasts regarding abundance, rock type, and degree of shock was observed, with the exception that clasts of shock‐melted sedimentary rocks and of highly shocked basement rocks (stage III–IV) are strongly concentrated near the center of the crater. Based on our field and laboratory investigations we conclude that vaporization and melting due to the Haughton impact affected the lower section of the sedimentary strata from about 900 to 1700 m depth (Eleanor River limestones and dolomites, Lower Ordovician and Cambrian limestones, dolomites, shales, and sandstones). The 60‐GPa shock pressure isobar reached only the uppermost basement rocks so that whole rock melting of the crystalline rocks was not possible.

The Allochthonous Polymict Breccia Layer of the Haughton Impact Crater, Devon Island, Canada

Abstract— The central allochthonous polymict breccia of the Haughton impact structure is up to about 90 m thick and as much as 7.3 km in radial extent. It has been analyzed with respect to modal composition, grain‐size characteristics, and degree of shock metamorphism for the grain‐size ranges 10–∼ 50, 1–10, 0.03–1, and &lt;0.03 mm. The mineralogy of the breccia matrix is dominated by dolomite and calcite, with minor amounts of quartz, other silicate minerals, and rare melt particles. The following lithic clasts have been identified in the 1–10 mm size fraction (averages of vol.% given in parentheses): dolomitic rocks (51), limestones (29), crystalline rocks (10), sandstones and siltstones (3.7), chert (0.7), melt particles (1.9). The mineral clasts (1–0.03 mm) comprise (with decreasing frequency) dolomite, quartz, calcite, feldspar, biotite, amphibole, garnet, opaques, rounded quartz derived from sandstones and accessory minerals. Lithic and mineral clasts display various degrees of shock. Fragments of crystalline rocks are shocked in the 0–60 GPa range; whole rock melts from the crystalline basement are lacking and unshocked rocks are very rare. In contrast, shock‐melted sandstones, shales, and chert were found in most samples. Large clasts of these melt rocks are highly concentrated near the center of the crater. Otherwise, no distinct change of the modal composition with radial range has been observed except that the frequency of limestone clasts increases slightly with radial range. The breccia near the center is more fine‐grained than that beyond about 1 km radius and the sorting parameter increases somewhat with radial range. Except for the high concentration of shock‐melted sedimentary rocks and highly shocked crystalline rocks near the center of the crater, the distribution of shock stages within the lithic clast population is quite uniform throughout the breccia formation. We conclude that the breccia constituents are derived from the lower part of the target stratigraphy (deeper than about 800 m) and that the total depth of excavation at Haughton is in the order of 2000 m. The mixing of sedimentary rocks of the Eleanor River Formation, Lower Ordovician, and Cambrian (∼850 m thickness) with crystalline basement rocks is quite thorough and homogeneous throughout the breccia lens, at least for the analyzed part. This may require an air‐borne mode of emplacement for the upper section of the breccia in analogy to the fall‐back suevite in the Ries crater. A calculation of the excavation (Z‐model) and of the shock pressure attenuation based on reasonable estimates of the energy and crater geometry of the Haughton impact confirms the observed maximum depth of excavation of about 2 km. Shock‐melted crystalline basement rocks, if present at all, must be confined to the very center of the structure below the excavation cavity.

Sequential cardiopulmonary variables of infants and children in septic shock.

Sequential cardiopulmonary variables were analyzed in 32 infants and children with septic shock. Variables were staged by a system based on therapeutic efforts to control blood pressure. There were 14 survivors and 18 nonsurvivors. Systemic circulation variables (MAP, cardiac index [CI], systemic vascular resistance index [SVRI], wedge pressure [WP], left cardiac work index [LCWI]) and pulmonary circulation variables (mean pulmonary artery pressure [MPAP], pulmonary vascular resistance index [PVRI], CVP, right cardiac work index [RCWI]) were similar in survivors and nonsurvivors. Pulmonary variables (intrapulmonary shunt [Qsp/Qt], fraction of inspired oxygen [FIO2], Pao2, PaCO2) revealed significantly more dysfunction in nonsurvivors than survivors during the postresuscitation (PR) and middle (M) shock stages. Even though oxygen delivery was equivalent in survivors and nonsurvivors, nonsurvivors demonstrated decreased oxygen utilization variables (oxygen consumption [Vo2], arteriovenous oxygen content difference [C(a-v)O2], O2 extraction index, core temperature) during the resuscitation (RS) and PR stages.