Single Clast Research Articles

Four lunar mare meteorites: Crystallization trends of pyroxenes and spinels

Abstract— We studied crystallization trends of pyroxene and spinel in four Antarctic meteorites known to be derived from mare regions of the Moon: Y‐793169 and A‐881757 (YA meteorites) are unbrecciated igneous basalts, EET 87521 is a fragmental breccia, and Y‐793274 is a regolith breccia. All have relatively low bulkrock TiO2 content, and the YA meteorites are uncommonly ancient. Our electron probe microanalysis (EPMA) data indicate that the YA meteorites and the dominant mare components of Y‐793274 and EET 87521 conform to a general trend for Ti‐poor (low‐Ti and very low‐Ti) mare basalts. Their pyroxenes show a strong correlation between Fe/(Fe + Mg) (Fe#) and Ti/(Ti + Cr) (Ti#), both ratios typically increasing from core to rim. These trends presumably reflect local crystallization differentiation of interstitial melt. Previous studies (M. J. Drake and coworkers) have suggested that the detailed configurations of such Fe# vs. Ti# trends may reflect the bulk TiO2 contents of the parent magmas (basalts). As a more systematic approach to this problem, we plot bulk‐rock TiO2 as a function of the Fe# = 0.50 intercept of each rock's pyroxene Fe# vs. Ti# trend. We call this intercept the Fe#‐normalized Ti#. Based on our data for EET 87521, the YA meteorites, and Apollo 12 basalts 12031 and 12064, plus literature data for several other Ti‐poor mare basalts, we find a strong correlation between Fe#‐normalized Ti# and the bulk TiO2 content of the parent basalt. This correlation confirms that fragmental breccia EET 87521 is nearly pure very low‐Ti (VLT) basalt and that the YA meteorites, for which bulk‐rock TiO2 results scatter due to unusually coarse grain size (A‐881757) or scarcity of available sample (Y‐793169), are pieces of an uncommonly Ti‐poor, but not quite VLT, variety of low‐Ti mare basalt. Extrapolating from this correlation, the dominant mare component of regolith breccia Y‐793274 is probably of VLT affinity. Besides the normal mare pyroxene trend of strong correlation between Fe# and Ti#, Y‐793274 includes two additional pyroxene compositional trends, both showing a wide range of Ti# despite relatively constant (and low, by mare standards) Fe#. The most magnesian of these trends consists of a single clast with a mode of orthopyroxene + MgO‐rich ilmenite. These two trends are of uncertain origin. Possibly one or both represents the highland component of this regolith breccia, although, unlike most highland pyroxenes, these appear relatively unaltered by impact brecciation and metamorphism. Compositions of spinels in the coarse‐grained A‐881757 show an extraordinary distribution: chromite and ulvöspinel components vary among grains but are nearly constant within grains. Despite its old age and unusually coarse grain sizes, mineralogical evidence (i.e., heterogeneity within both pyroxene and spinel; typical pyroxene exsolution scale very coarse by mare standards but exceeded by the pyroxenes of EET 87521 and Y‐793274) indicates that A‐881757 was cooled only slightly more slowly than typical mare basalts and may have formed near the center of an uncommonly thick lava flow. Both of the VLT basaltic lunar meteorite breccias, EET 87521 and Y‐793274, are composed dominantly of pyroxenes with exsolution coarser than normal for mare basalts. Possibly VLT basalt flows tend to be systematically thicker, and thus more slowly cooled, than more Ti‐rich flows.

Mineralogy of carbonaceous chondrite clasts in HED achondrites and the Moon

Abstract The majority of the carbonaceous chondrite clasts found in howardites, eucrites and diogenites are CM2 material, a lesser proportion is CR2 material, and other rare types are present. A single clast that was found on the Moon and called the Bench Crater meteorite is apparently shocked CM1 material. The CM2 clasts are matrix supported mixtures of olivine‐pyroxene‐phyllosilicate‐sulfide bearing aggregates, loose olivines and pyroxenes, sulfides, carbonates, and sinuous spinel‐phyllosilicate‐diopside calcium‐aluminum‐rich inclusions (CAIs). Magnetite and metal are rare. Some aggregates have fine‐grained rims of material resembling matrix. The opaque, fine‐grained matrix consists predominantly of serpentine of extremely variable composition and sulfides; tochilinite is occasionally present. The trace element data for one Jodzie clast from this study and the average of similar clasts from Kapoeta support a CM classification; volatiles are depleted relative to CI and enriched relative to CR material.The CR2 clasts are found (in small numbers) in only four howardites: Bholghati, Jodzie, Kapoeta and Y793497. Petrographically, they are matrix‐supported mixtures of olivine aggregates (sometimes containing sulfides), loose olivines, pyrrhotite, pentlandite, low‐Ca pyroxene (minor), hedenbergite (rare), kamacite (rare and only found within olivine), Ca‐carbonates and abundant magnetite framboids and plaquets. Phyllosilicates are fine‐grained and largely confined to matrix; they are mixtures of serpentine and saponite. The matrix of CR2 clasts also contains pyrrhotite, pentlandite, chromite and a significant fraction of poorly‐crystalline material with the same bulk composition as matrix phyllosilicate.There is evidence of heating in a substantial number of clasts, both CM2 and CR2, including: (1) corrugated serpentine flakes, (2) pseudomorphs of anhydrous ferromagnesian material after flaky phyllosilicates, and (3) hedenbergite rims on calcite. While the timing of the hedenbergite rims is debatable, the destruction of phyllosilicates clearly occurred at a late stage, plausibly during impact onto the HED asteroid(s) and Moon, and required peak heating temperatures on the order of 400 °C.We note that in general, CM2 material was the most common carbonaceous chondrite lithology impacting the HED asteroids (with howardites and eucrites taken together), as it is for the Earth today. A total of 61 out of 75 carbonaceous chondrite clasts from HED meteorites belong to the CM clan, petrologic grade 2. This is also supported by published siderophile and volatile element data on howardites, eucrites and diogenites that are taken to indicate that CM‐like materials were the most common impactors on the HED asteroid(s). The ratio of CR/CM clasts in HED asteroids is essentially the same as for modern falls at Earth. This may indicate that the ratio of disaggregated CM2 to CR2 asteroidal material has been approximately constant through the history of the solar system. Finally, our results are also compatible with type‐2 carbonaceous chondrites being equivalent to or from the same source as the material that originally accreted to form the HED asteroid.

A hot Miocene subaqueous scoria-flow deposit in the Shimane Peninsula, SW Japan

Abstract A subaqueous scoria-flow deposit occurs in the Miocene Takashibiyama Formation in SW Japan. Faunal assemblages in associated sediments suggest an outer shelf to upper bathyal environment, with water depth likely between 200 and 1500 m. The deposit, 5–7 m thick, is similar to subaerial scoria flow deposits, containing mainly non- to moderately vesiculated hyalopilitic basaltic andesite lapilli and coarse ash. The essential basaltic andesite clasts are variably vesiculated even within single clasts, and are commonly cracked with curviplanar surfaces, whereas the inner parts are cracked in a pattern similar to perlitic cracks. The clasts are blocky with curviplanar surfaces if non- to poorly vesiculated, and are fibrous with irregular shapes and flattened by compaction if more vesiculated. At one proximal location, fines-depleted, rootless pipes 5–90 cm wide and 1–3 m long occur, and spherical to amoebic vesicles 0.01–0.3 mm in diameter and filled with stilbite, quartz or greenish-brown clay are common in the fine-ash matrix and in siltstone clasts contained in the deposit. The vesicles represent open spaces once supported by gas, and the rootless pipes are interpreted as gas-escape pipes. Magnetization vectors of essential clasts are dispersed not at random but around a single direction, suggesting that the clasts gained thermoremanent magnetism mainly during cooling in the flow or during post-emplacement cooling and then rotated to some extent by differential movement of the flow or by gas-escape in the deposit. The vesicularity, shape and texture of essential clasts suggest a phreatomagmatic eruption, involving magmatic vesiculation and dynamic interaction of magma and ambient water. The perlitic cracks in the inside of the essential clasts and the paleomagnetic vectors of the essential clasts indicate that the flow was initially hot and that the deposit cooled slowly. Internal depositional features change rapidly over only 500 m. Rootless pipes and vesicles in the fine-ash matrix and siltstone clasts are therefore absent at distal locations, indicating that an initially cohesionless debris flow quickly became sufficiently turbulent to mix with ambient seawater.

Melt inclusions and crystal-liquid separation in rhyolitic magma of the Bishop Tuff

Melt inclusions in quartz phenocrysts from a single clast of pumice near the base of the plinian pumice fall of the Bishop Tuff were studied to test ideas concerning separation of melt and crystals in silicic magmas. Ten analyzed inclusions from the pumice clast are of high silica rhyolite composition with very low contents of the highly compatible elements Ba, Sr, and Eu, consistent with extensive fractionation. The concentrations of U, La, Ce, Mg, and Ca of these ten melt inclusions vary considerably as determined by ion microprobe. Petrologic considerations indicate that uranium is an incompatible element with a maximum bulk partition coefficient D of about 0.2 and that the evolution of the uranium content of the melt was controlled by crystallization of the magma. A minimum of 33 wt% perfect fractional crystallization is required to explain the observed range in uranium. However, only 17 wt% crystals occurred in the pumice clast. The greater calculated fraction of crystals requires significant separation of crystals and melt before the eruption of the plinian pumice fall in spite of the fact that crystal mixing (settling, etc.) did not occur in the Bishop magma.

Clastic lawsonite, glaucophane, and jadeitic pyroxene in Franciscan metagraywackes from the Diablo Range, California

Textural relationships and mineral chemistry identify a detrital origin for jadeitic pyroxene and some lawsonite and glaucophane in Franciscan metagraywackes in the Diablo Range. These clasts differ from the neometamorphic (post-depositional) assemblage by having larger grain sizes and deformational features, including bending, cracking, and optical strain polarization. Large strain-polarized jadeitic pyroxenes contain inclusions of strained lawsonite and glaucophane, but none of these minerals has grown across clast/matrix boundaries. By comparison, neometamorphic lawsonite forms small unstrained tablets crossing the margins of large clasts and is associated with quartz + albite + chlorite + white mica in recrystallized metagraywacke matrices. Similarly, authigenic glaucophane forms discrete small prisms or euhedral outgrowths on large clasts of sodic amphibole. Jadeitic clasts in contact with metagraywacke matrices may be deeply embayed and show marginal recrystallization to fine-grained mixtures of albite + chlorite + quartz or internally to pumpellyite + albite. Backscatter electron imaging and microprobe analyses consistently show that the pyroxenes have extremely variable Jd contents. Within a single clast, compositional heterogeneity can be as much as Jd 50 Ac 39 -Aug 11 to Jd 95 Ac 4 Aug 1 . Because their texture, habit, and chemical variability are similar to those of metasomatic jadeites from tectonic inclusions in serpentinite at New Idria, southern Diablo Range, or to those of some jadeite-quartz associations in extensively recrystallized metabasite knockers, such a provenance is inferred for pyroxene clasts in the Franciscan metasediments. In metaconglomerates, pebbles containing pyroxenes (Jd 59-98 ) formed original clasts that were overprinted later by the post-depositional lawsonitic metamorphism. Metabasites at Pacheco Pass have compositionally variable Na-pyroxenes (Jd 30-92 ) which indicate tectonic emplacement of the igneous bodies within enclosing (in some cases, pyroxenefree) metagraywackes. The Diablo Range metagraywackes are weakly developed blueschist-facies assemblages of lawsonite + white mica + albite + chlorite + titanite ± glaucophane ± aragonite ± pumpellyite, which indicate minimum metamorphic conditions of P = 3.5-5.0 ± 0.5 kb at T = 100-200 °C.

886015 Upfreezing process: experiments with a single clast : Anderson, S P Bull Geol Soc Am V100, N4, April 1988, P609–621

886015 Upfreezing process: experiments with a single clast : Anderson, S P Bull Geol Soc Am V100, N4, April 1988, P609–621

The upfreezing process: Experiments with a single clast

Research Article| April 01, 1988 The upfreezing process: Experiments with a single clast SUZANNE PRESTRUD ANDERSON SUZANNE PRESTRUD ANDERSON 1Quaternary Research Center and Department of Geological Sciences, University of Washington, Seattle, Washington 98195 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1988) 100 (4): 609–621. https://doi.org/10.1130/0016-7606(1988)100<0609:TUPEWA>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation SUZANNE PRESTRUD ANDERSON; The upfreezing process: Experiments with a single clast. GSA Bulletin 1988;; 100 (4): 609–621. doi: https://doi.org/10.1130/0016-7606(1988)100<0609:TUPEWA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract No abstract available This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

First samples of acoustic basement recovered from the alpha ridge, Arctic ocean: New constraints for the origin of the ridge

The Alpha Ridge is one of three subparallel trending ridges that cut the Arctic Ocean. It is roughly Late Cretaceous to Eocene in age, and seismic refraction records suggest it comprises a thick sequence of oceanic crust. During the 1983 CESAR expedition 20 similar samples of acoustic basement were dredged from the walls of a major graben of the Alpha Ridge, at one site. These are the only basement samples ever recovered from the ridge and provide the first direct evidence for its nature, composition and possible origin. The basement samples are highly altered pyroclastic rocks composed almost entirely of basaltic volcanic clasts with little matrix. Although the rocks are highly altered, most primary textures and structures are preserved. Most clasts are highly amygdaloidal to scoriaceous, fine grained to glassy, and angular to subround with rare vesicle controlled boundaries. Little reworking is suggested because a single clast type predominates, many of the clasts are subangular, and any amount of reworking would result in destruction of the delicate scoriaceous clasts. Rare clinopyroxene phenocrysts comprise the only unaltered portion of the rocks. They are salitic in composition (Wo 49–53, En 32–41, Fs 11–15), with significant amounts of Ca, Al and Ti. Salitic clinopyroxenes are typical of alkalic basalts. Interpretation of the whole rock geochemistry based on relatively immobile elements, (Nb, Zr, Tio 2, and Y), and chondrite-normalized incompatible trace element and REE patterns indicates that the volcanic rock fragments are of alkalic basalt. Geochemical discriminators suggest a within-plate tectonic setting. Textural evidence suggests that the CESAR basement rocks were sampled from a rapidly emplaced submarine fallout deposit that was erupted at a depth at least less than 800 m and likely less than 200 m. High extrusive rates would have been required to build the ridge up to shallow depth prior to the cessation of volcanism. The alkalic affinity of the rocks strongly suggests that the Alpha ridge was not formed by volcanism at an island arc or a mature spreading centre. It is also unlikely that it formed as a “leaky” fracture zone. Alkalic basalts, however, are commonly associated with various types of oceanic aseismic ridges. It is suggested that the Alpha Ridge is an aseismic ridge that formed due to voluminous hotspot volcanism as spreading began in the Canada Basin. Such hotppot activity may have been responsible for initiating the rifting, breakup, and dispersal that eventually formed the Canada Basin.

The Ventersdorp Contact placer: a gold‐pyrite placer of stream and debris‐flow origins from the Archaean Witwatersrand Basin of South Africa

ABSTRACTFive local varieties of the Ventersdorp Contact placer at the East Driefontein Gold Mine, Carletonville, are distinguished by clast‐type assemblage, reflecting discrete provenances. The placer further comprises six lithofacies which are not restricted to specific provenance‐varieties. The six lithofacies are: massive, matrix‐supported conglomerate (facies Gms): massive, or crudely bedded, clast‐supported conglomerate (facies Gm); channel‐based conglomerate (facies Gt); single clast layers (facies Glag); horizontally stratified sandstone (facies Sh); and trough cross‐stratified sandstone (facies St). Facies Gms represents debris‐flow deposits, whereas the other facies are stream deposits. Lithofacies and provenance‐varieties are complexly interrelated in local environments, which are themselves complexly related to geomorphic elements of a degradation surface. The maximum thickness of the placer is 7 m. The placer was deposited on a hard, i.e. non‐alluvial, bedrock surface, suggesting that it is a pediment mantle. It owes its preservation to a capping of conformably overlying basalts.Economic gold mineralization is present in all the conglomerate facies and all the provenance‐varieties. Gold concentrations are high in gravel‐bar deposits and channel fills but are not restricted to these environments. A major feature of gold concentration is its variability between provenance‐controlled varieties. The gold concentration of a specific primary source is interpreted to be a major control on gold concentration in the final deposit. The origin of the placer as a pediment mantle, and its subsequent preservation by lava‐flows, suggests that there was little potential for selective reworking of heavy minerals. Nevertheless, the Ventersdorp Contact placer on the Carletonville Goldfield is one of the richest gold placers in the Witwatersrand Basin.

Franciscan blueschist-facies metaconglomerate, Diablo Range, California

Certain Franciscan conglomerate units are shown to be metaconglomerate and to exhibit selective blueschist-facies metamorphism of various constituents. Petrographic and x-ray studies were made on pebbles and matrix of 18 Franciscan conglomerate units forming blocks in melanges and lenses in stratiform bodies of graywacke in the vicinities of Mount Hamilton and Pacheco Pass. The conglomerate ranges from little-deformed varieties (textural grade 1) to tectonite with pronounced secondary planar structures (textural grade 2). Significant minerals in conglomerate of textural grade 1 are quartz + albite ± lawsonite ± minor glaucophane + CaCO 3 , and those of textural grade 2 are lawsonite + glaucophane + quartz ± jadeitic pyroxene ± albite ± aragonite. Thus the most intense metamorphism is empirically associated with internal deformation; however, the converse is not invariably true. Lawsonite in granitic clasts favors feldspar, but its abundance varies within single clasts, from clast to clast, and between adjacent conglomerate units; it commonly forms crystallographically oriented prisms in albite and is locally concentrated near the margins of albite crystals. Glaucophane, which is scarce in conglomerate of textural grade 1, is locally concentrated as rinds in some clasts of fine-grained basalt or it forms rare fibrous prisms in granitic clasts. In rocks of textural grade 2, it commonly forms beards on green amphibole in the matrix and in granitic clasts and completely replaces some mafic clasts. Jadeitic pyroxene replaces albite in conglomerate of textural grade 2 and is concentrated in the outer parts of certain clasts. Carbonate, commonly aragonite, shows crosscutting relations with lawsonite, glaucophane, or jadeitic pyroxene. Paragenetic relations between lawsonite and glaucophane are equivocal, but probably both minerals formed earlier than jadeitic pyroxene. Albite is apparently the only feldspar in the metaconglomerate units. The chemistry of granitic clasts is reflected in their metamorphosed mineral assemblages. They are deficient in potassium and enriched in sodium relative to average granite of similar silica content. The mineral assemblages of the metaconglomerate are consistent with those reported from nearby metagraywacke units, although there is generally no structural continuity with the metagraywacke. Petrographic features indicate postdepositional blueschist-facies metamorphism of the conglomerate units and demonstrate the presence of a fluid phase during metamorphism.