Shock Melt Veins Research Articles

Transformation textures, mechanisms of formation of high‐pressure minerals in shock melt veins of L6 chondrites, and pressure‐temperature conditions of the shock events

Abstract— The high‐pressure polymorphs of olivine, pyroxene, and plagioclase in or adjacent to shock melt veins (SMVs) in two L6 chondrites (Sahara 98222 and Yamato 74445) were investigated to clarify the related transformation mechanisms and to estimate the pressure‐temperature conditions of the shock events. Wadsleyite and jadeite were identified in Sahara 98222. Wadsleyite, ringwoodite, majorite, akimotoite, jadeite, and lingunite (NaAlSi3O8‐hollandite) were identified in Yamato 74445. Wadsleyite nucleated along the grain boundaries and fractures of original olivine. The nucleation and growth of ringwoodite occurred along the grain boundaries of original olivine, and as intracrystalline ringwoodite lamellae within original olivine. The nucleation and growth of majorite took place along the grain boundaries or fractures in original enstatite. Jadeite‐containing assemblages have complicated textures containing “particle‐like,” “stringer‐like,” and “polycrystalline‐like” phases. Coexistence of lingunite and jadeite‐containing assemblages shows a vein‐like texture. We discuss these transformation mechanisms based on our textural observations and chemical composition analyses. The shock pressure and temperature conditions in the SMVs of these meteorites were also estimated based on the mineral assemblages in the SMVs and in comparison with static high‐pressure experimental results as follows: 13–16 GPa, >1900 °C for Sahara 98222 and 17–24 GPa, >2100 °C for Yamato 74445.

Ultrafast growth of wadsleyite in shock-produced melts and its implications for early solar system impact processes

We observed micrometer-sized grains of wadsleyite, a high-pressure phase of (Mg,Fe)(2)SiO(4,) in the recovery products of a shock experiment. We infer these grains crystallized from shock-generated melt over a time interval of <1 micros, the maximum time over which our experiment reached and sustained pressure sufficient to stabilize this phase. This rapid crystal growth rate (approximately 1 m/s) suggests that, contrary to the conclusions of previous studies of the occurrence of high-pressure phases in shock-melt veins in strongly shocked meteorites, the growth of high-pressure phases from the melt during shock events is not diffusion-controlled. Another process, such as microturbulent transport, must be active in the crystal growth process. This result implies that the times necessary to crystallize the high-pressure phases in shocked meteorites may correspond to shock pressure durations achieved on impacts between objects 1-5 m in diameter and not, as previously inferred, approximately 1-5 km in diameter. These results may also provide another pathway for syntheses, via shock recovery, of some high-value, high-pressure phases.

Evidence for fractional crystallization of wadsleyite and ringwoodite from olivine melts in chondrules entrained in shock-melt veins

Peace River is one of the few shocked members of the L-chondrites clan that contains both high-pressure polymorphs of olivine, ringwoodite and wadsleyite, in diverse textures and settings in fragments entrained in shock-melt veins. Among these settings are complete olivine porphyritic chondrules. We encountered few squeezed and flattened olivine porphyritic chondrules entrained in shock-melt veins of this meteorite with novel textures and composition. The former chemically unzoned (Fa(24-26)) olivine porphyritic crystals are heavily flattened and display a concentric intergrowth with Mg-rich wadsleyite of a very narrow compositional range (Fa(6)-Fa(10)) in the core. Wadsleyite core is surrounded by a Mg-poor and chemically stark zoned ringwoodite (Fa(28)-Fa(38)) belt. The wadsleyite-ringwoodite interface denotes a compositional gap of up to 32 mol % fayalite. A transmission electron microscopy study of focused ion beam slices in both regions indicates that the wadsleyite core and ringwoodite belt consist of granoblastic-like intergrowth of polygonal crystallites of both ringwoodite and wadsleyite, with wadsleyite crystallites dominating in the core and ringwoodite crystallites dominating in the belt. Texture and compositions of both high-pressure polymorphs are strongly suggestive of formation by a fractional crystallization of the olivine melt of a narrow composition (Fa(24-26)), starting with Mg-rich wadsleyite followed by the Mg-poor ringwoodite from a shock-induced melt of olivine composition (Fa(24-26)). Our findings could erase the possibility of the resulting unrealistic time scales of the high-pressure regime reported recently from other shocked L-6 chondrites.

Ferric iron in Al-bearing akimotoite coexisting with iron-nickel metal in a shock-melt vein in an L-6 chondrite

We report evidence for high ferric iron to total iron (Fe 3+ /∑Fe) ratios in Al-bearing akimotoite coexisting with other high-pressure silicates and Fe-Ni metal from shock melt-veins in the Sixiangkou (L-6) chondrite. The measurements were made using electron energy-loss near-edge structure (ELNES) spectroscopy. The results demonstrate that akimotoite in shock-melt veins of this meteorite has high proportions of Fe 3+ , with a Fe 3+ /∑Fe ratio of 0.67(3). In contrast, the coexisting majoritic garnet and ringwoodite, which are the typical Fe-bearing phases in shock veins in this meteorite, are enriched in Fe 2+ rather than Fe 3+ , with Fe 3+ /∑Fe ratios of 0.10(5) and 0.15(5), respectively. We conclude that the higher affinity of Fe 3+ for akimotoite, rather than for the other dense silicate phases, is related strongly to the substitution mechanism of trivalent cations. This mechanism is described as VI(A) Fe 3+ + VI(B) Al 3+ = VI(A) Mg 2+ + VI(B) Si 4+ in the ABO 3 structural formula of MgSiO 3 -ilmenite under high pressures and temperatures, and operates even at a low oxygen fugacity where Fe-Ni metal is stable.

Lithium behavior during cooling of a dry basalt: An ion-microprobe study of the lunar meteorite Northwest Africa 479 (NWA 479)

Northwest Africa 479 (NWA 479) is a lunar meteorite recovered in 2000 from Morocco. This unbrecciated low-Ti basalt is paired with NWA 032. The texture of NWA 032/479 indicates a simple crystallization history and a fast cooling, followed by an impact event. The occurrence of high-pressure polymorphs of olivine (ringwoodite and wadsleyite) in shock-melt veins indicates shock-pressures of at least 20 GPa. Lithium abundances and isotopic compositions were measured by ion microprobe in pyroxene, olivine crystals, and magmatic inclusions. The δ 7Li values in the magmatic inclusions indicate that the NWA 479 parental melt was enriched in 7Li (δ 7Li = +15‰). The behavior of Li depicted by the phenocrysts is complex and is not controlled by their major element compositions. Li abundances and δ 7Li values range respectively from 3.2 to 11.8 μg/g and +2.4 to +15.1‰ in olivine and from 2.8 to 18.4 μg/g and −0.2 to + 16.1‰ in pyroxene phenocrysts. Neither hot desert weathering, closed-system fractional crystallization, involvement of a low-δ 7Li reservoir, degassing of NWA 479 parental melt, nor shock metamorphism correctly explain the Li distribution in the phenocrysts. We propose that the wide range of δ 7Li values displayed by the phenocrysts results from the large diffusivity differences between 6Li and 7Li. It is shown that this difference is able to produce large isotopic heterogeneities in a very short time.

Synchrotron radiation X-Ray diffraction in situ study of fine-grained minerals in shock veins of Suizhou meteorite

The synchrotron radiation X-ray diffraction technique developed for in situ study at high pressure and temperature has also been used to investigate microscopic mineral inclusions in ultra-high-pressure metamorphic rocks and deep-mantle samples. Present study added two more examples of successful utilization of synchrotron radiation X-ray diffraction technique for in situ investigations of fine-grained (0.5―30 μm in size) minerals in very thin shock melt veins of the Suizhou meteorite: (i) X-ray diffraction measurement of extremely small-sized vein matrix minerals, and (ii) identification of the micron-sized new mineral tuite embedded in the vein matrix. It has been revealed that the fine-grained vein matrix consists of well crystallized garnet, kama-cite and troilite, and the powder diffraction pattern consisting of 17 lines with d-values, intensities ( I ), relative intensities ( I / I o ) and Miller indices, as well as the cell parameters for the new mineral tuite has also been successfully obtained. The result of present investigations has enriched the content of dynamic high-pressure mineralogy and that of Earth’s mantle geochemistry.

Tuite, gamma-Ca3(PO4)2: a new mineral from the Suizhou L6 chondrite

Tuite, γ-Ca 3 (PO 4 ) 2 , the high-pressure polymorph of whitlockite, was found in a shock melt vein of the Suizhou chondrite. It occurs as polycrystalline aggregates in association with coarse-grained aggregates of ringwoodite, majorite and NaAlSi 3 O 8 -hollandite. These high-pressure mineral assemblages are enclosed in the fine-grained matrix of the vein that consists of majorite-pyrope ss garnet, metal and troilite. This new mineral is trigonal, R 3 m,a = 5.258 A, c = 18.727 A. The tuite is colourless and transparent with vitreous lustre and white streak; it is uniaxial (+) with high refractive indices (ϵ = 1.706, ω = 1.701) and low birefringence (0.005). Microprobe analyses of tuite yield an empirical formula (Ca 2.51 Mg 0.29 ) 2.80 Na 0.28 (P 1.01 O 4 ) 2 . The simplified formula, Ca 3 (PO 4 ) 2 , is the same as whitlockite outside the veins. Stronger reflections on synchrotron radiation X-ray powder diffraction pattern of tuite are d = 2.628 (100), 2.891 (80), 1.945 (47), 1.730 (25) and 1.567(22). The figures in brackets are relative intensities. Tuite in the shock vein of this meteorite was transformed from whitlockite via a shock-produced solid-state reaction, while the pressure and temperature in the shock vein attained 23 GPa and 2000°C. This new mineral and the mineral name (tuite) have been approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association.

Back-transformation of high-pressure phases in a shock melt vein of an H-chondrite during atmospheric passage: Implications for the survival of high-pressure phases after decompression

We investigated the H6-chondrite Yamato 75267, in which a fusion crust cuts a shock melt vein. The shock vein region, more than 280 μm from the fusion crust, contains high-pressure phases, such as ringwoodite, majorite-pyrope ss garnet and NaAlSi 3O 8 hollandite. However, the shock vein close to the fusion crust entirely consists of the low-pressure polymorphs, olivine, low-Ca pyroxene and plagioclase glass. The boundary between low- and high-pressure phase regions is parallel to the fusion crust. During the atmospheric passage, the peripheral part of the chondrite was melted to form the fusion crust. Our microscopic, laser micro-Raman, electron microprobe investigations and calculations indicate an area up to 300 μm from the fusion crust experienced a temperature of 1400°C after 3 s during the melting of the peripheral part. The high-pressure phases would, at this conditions, quickly transform back to their low-pressure polymorphs. The result obtained here indicates that post-shock temperatures in the interior part of the veins were much lower than 1400°C, thus leading to the survival of high-pressure phases in heavily shocked chondrites.

Shock-related mineralogical features and P-T history of the Suizhou L6 chondrite

The Suizhou meteorite, classified as an L6 chondrite, contains weakly shocked olivine and pyroxene, but almost all the plagioclase in the meteorite was melted and transformed into maskelynite during shock metamorphism. Chromite was heavily fragmented and granulated, and many tiny chromite fragments were incorporated into the molten plagioclase as inclusions. Metal and troilite show no obvious intragranular textures, but many tiny rounded FeNi metal grains were deposited in the intersecting joints of planar fractures in olivine and pyroxene. A few very thin shock melt veins occur in the Suizhou meteorite, which contain abundant high-pressure phases, including coarse-grained ringwoodite, majorite, (Na,Ca)AlSi3O8-hollandite and fine-grained liquidus majorite-pyrope garnet. The shock features of this meteorite match shock stages 3 to 5, while the presence of ringwoodite in Suizhou veins is considered to appear at stage 6. It is estimated that the Suizhou meteorite experienced a shock pressure and shock temperature of up to 22 GPa and 1000°C, respectively. The shearing friction along veins raised the temperature within the veins. Shock-induced pressure and temperature in the shock veins attained 22 GPa and 1900°C. Therefore, the actual shock level of the Suizhou meteorite could correspond to stage 3–4. A longer duration of the shock pressure and temperature regime in the Suizhou meteorite plays an important role in the pervasive melting of plagioclase in the unmelted part of the meteorite, as well as in the formation of abundant high-pressure phases in the very thin shock-melt veins. It appears that maskelynite cannot be used as the sole criteria for evaluating the shock stage of shock-metamorphosed chondrites.

NaAlSi3O8-hollandite and other high-pressure minerals in the shock melt veins of the Suizhou meteorite

The Suizhou L6 chondrite contains a few very thin shock melt veins of 0.02–0.09 mm in width. In spite of small width of the veins, shock-induced high-pressure phases, such as coarse-grained NaAlSi3O8-hollandite, ringwoodite, majorite and fine-grained matrix majorite-pyropess have been discovered in these veins. NaAlSi3O8-hollandite, the high-pressure phase of plagioclase, in Suizhou shock veins occurs as a single phase mineral, no silicate glassy phase, such as albitic glass, was incorporated with it. The presence of above-mentioned high-pressure phases constrains the high pressure (up to 23–24 GPa) and high temperature (up to 1900–2000°C) regime in Suizhou shock veins, and indicates that the duration of high-pressure regime in the veins should be long enough (a few seconds) for phase transformation and crystallization of minerals under pressure. The discovery of the first natural single-phase crystalline NaAlSi3O8-hollandite in Suizhou meteorite is of important significance in understanding the Earth’s mantle geochemistry.

A comparative study of naturally and experimentally shocked chondrites

Samples of the Jilin H5 chondrite were experimentally shock-loaded at the peak pressures of 12, 27, 39, 53, 78, 83, 93, and 133 GPa. The aim of this study is to compare experimentally shock-induced phenomena with those in naturally shocked chondrites and to test the feasibility of experimentally calibrating naturally induced shock phenomena in H- and L-chondrites. Planar fractures, mosaicism, brecciation in olivine and pyroxene, as well as transformation of plagioclase into diaplectic glass were observed in the Jilin samples shocked at pressures lower than 53 GPa. Shock-induced chondritic melts were first obtained at P>78 GPa and more than 60% of the whole-rock melting was achieved at P∼133 GPa, and that shock-induced silicate melt consists of quenched microcrystalline olivine and pyroxene, metal, troilite and vesicular glass. No high-pressure phases were observed in any of the experimentally shocked samples, neither in the deformed nor in the molten regions. Deformation features in Jilin samples shock-loaded below 53 GPa are comparable to those found in H- and L-chondrites. The mineral assemblages in the molten regions in the shocked Jilin samples are also comparable to those encountered in the heavily shocked Yanzhuang (H6) and some Antarctic H-chondrites, but differ considerably from those found in heavily shocked Sixiangkou and many other L6 chondrites. Shock melt veins in L6 chondrites contain high-pressure polymorphs of olivine, pyroxene, plagioclase and high-pressure liquidus phases, whereas shock melt veins in heavily shocked H-chondrites contain mainly low-pressure mineral assemblages. The differences in the mineral constituents of shock melt veins in L- and H-chondrites clearly indicate differences in the shock histories of these meteorites. While crystallization in the shock melt veins in L-chondrites took place at high pressures, crystallization in shock-induced melt in most H-chondrites took place after decompression. It is evident that the thickness and abundance of shock melt veins and size of melt regions is not necessarily a quantitative measure of the degree of shock. The duration of the high-pressure regime, the time of the cooling and the P–T regime during the crystallization path, and the post-shock temperatures are stringent parameters that control the evolution of the shock-induced melt. So, scaling from shock experiments on millimeter-sized samples to natural shock features on kilometer-sized asteroids poses considerable problems in quantifying the P–T conditions during natural shock events on asteroids.

Characteristics in naturally and experimentally shocked chondrites: A clue to P-T conditions of impacted asteroids

The aim of this study is to compare the experimentally shock-induced features with those in naturally shocked chondrites and to test the feasibility of experimentally calibrating naturally induced features in shocked H- and L-chondrites. Samples of the Jilin chondrite (H5) were experimentally shock-loaded at the following peak pressures: 12, 27, 39, 53, 78, 83, 93 and 133 GPa respectively. Chondritic melts were first obtained at P ＞78 GPa and more than 60% melting was achieved at P ～133 GPa. No high-pressure phases were observed in any of the shocked samples, neither in the deformed nor in the molten regions. Textural relations and mineral assemblages of the shocked samples are comparable to those encountered in the heavily shocked H-chondrite Yanzhuang but differ considerably from those found in heavily shocked L6 chondrites. Shock melt veins in L6 chondrites contain high-pressure polymorphs of olivine and pyroxene and high pressure liquidus phases. Scaling from shock experiments on millimeter-sized samples to natural shock features on kilometer-sized asteroids poses considerable problems in quantifying the P-T conditions during natural shock events on asteroids.