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Partially Melted Kyanite Eclogite from Roberts Victor Mine, South Africa
George Switzer and William G. Melson
9 pages, 5 figures, 6 tables
1969 (Date of Issue: 15 April 1969)
Number 1, Smithsonian Contributions to the Earth Sciences
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Three specimens have been studied of the rare kyanite eclogite nodules in kimberlite from the Roberts Victor mine, South Africa. All are essentially the same with the primary assemblage: kyanite, omphacite, garnet, diamond (in one sample), chrome diopside, and rutile. There is also present a fine-grained secondary assemblage that appears in two forms: (1) primary omphacite altered to a mixture of plagioclase, clinopyroxene, and possibly glass; and (2) thin layers along omphacite, kyanite, and garnet grain boundaries. These layers have a clear-cut igneous texture and consist of plagioclase microlites with glass or devitrified glass, or plagioclase microlites and subhedral augite, with or without glass. Hornblende, spinel, and calcite are accessories, and analcite fills vesicles. Corundum and mullite occur at the margins of kyanite grains.

The glass in the secondary assemblage has a composition roughly equivalent to what one might expect if it was derived by incongruent melting of omphacite, followed by partial crystallization. Omphacite at one atmosphere pressure begins to melt at about 1030? C and melting is complete at about 1260? C. At 30 kilobars (O'Hara and Yoder, 1967) melting begins at about 1570? C and is complete at 1600? C. Thus, sudden pressure release of an eclogite at high temperature could cause partial melting of omphacite.

These kyanite eclogites clearly contained an interstitial melt that has been rapidly cooled. Evidence points to this melt having been generated mainly by partial melting of primary omphacite rather than by introduction of an externally derived melt. The partial melting may have occurred in response to one of the following three processes or some combination of them:

  • Increase in temperature at constant pressure.
  • Introduction of water into the eclogite at constant temperature and constant total pressure.
  • Release of pressure at constant temperature.

The third process seems to offer the most reasonable explanation for the partial melting.

Catalog of Chemical Analyses of Rocks from the Intersection of the African, Gulf of Aden, and Red Sea Rift Systems
Paul A. Mohr
7 pages, 392 plates
1970 (Date of Issue: 16 December 1970)
Number 2, Smithsonian Contributions to the Earth Sciences
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A compilation is presented of all published chemical analyses of rocks from the Red Sea, Gulf of Aden, and Ethiopian rift junction area. The chemical analyses are accompanied by further computations, in particular weight-norm and Niggli values, and by brief mineralogical descriptions. A full bibliography and indexes are included.

Minor and Trace Elements in Meteoritic Minerals
Brian Mason and A. L. Graham
17 pages, 1 figure, 17 tables
1970 (Date of Issue: 17 September 1970)
Number 3, Smithsonian Contributions to the Earth Sciences
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Nickel-iron, troilite, olivine, pyroxenes, plagioclase, chromite, and phosphate minerals (chlorapatite and/or merrillite) have been separated from a number of meteorites (Modoc, St. Severin, Winona, Haraiya, Marjalahti, Springwater, Johnstown, Mt. Egerton, Soroti) and analyzed for minor and trace elements with the spark-source mass spectrometer. The elements Ni, Co, Ge, As, Ru, Rh, Pd, Sn, Sb, W, Re, Os, Ir, Pt, and Au are concentrated in nickel-iron: Se and Ag in troilite; Th, U, and the lanthanides in the phosphate minerals and in diopside; Eu, Sr, Ba, Rb, and Cs in plagioclase. Molybdenum and tellurium are concentrated in nickel-iron and troilite. The elements Ti, Sc, V, Cu, Zn, Mn, and Ga are distributed over several coexisting minerals.

Volcanic Eruption at Metis Shoal, Tonga, 1967-1968: Description and Petrology
William G. Melson, Eugene Jarosewich and Charles A. Lundquist
18 pages, 13 figures, 3 tables
1970 (Date of Issue: 16 October 1970)
Number 4, Smithsonian Contributions to the Earth Sciences
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The 1967-1968 eruption of Metis Shoal, Tonga, was evidentially similar to the frequent shallow submarine eruptions of the inner island arc of Tonga. The eruption began about 10 December 1967, and an island eventually emerged; by 19 February 1968, the island had been eroded to beneath wave base. The eruptions were characterized by explosions of steam and ash which hurled bombs a few to several hundred feet into the air. The rocks ejected are pumiceous dacites which, for their silica content, have unusually low alkali contents and rare earth-element contents. The chemical characteristics of the dacite are hard to account for by partial melting of an ocean-ridge basalt parent. The peculiar properties of the dacite appear to characterize other Tongan lavas and support the idea that Tonga is part of a distinct petrographic province.

The Allende, Mexico, Meteorite Shower
Roy S. Clarke, Jr., Eugene Jarosewich, Brian Mason, Joseph Nelen, Manuel Gomez and Jack R. Hyde
53 pages, 36 figures, 6 tables
1971 (Date of Issue: 17 February 1971)
Number 5, Smithsonian Contributions to the Earth Sciences
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The Allende meteorite fell near Parral, Chihuahua, Mexico, between 0105 and 0110 Central Standard Time on Saturday, 8 February 1969. The fireball approached from the south-southwest (S37?W), and broke up in the atmosphere, producing thousands of fusion-crusted meteoritic stones. The smallest individuals were recovered 4 km east of Rancho Polanco (26?432 N, 105?282 W), and the largest near, Rancho El Cairo (27?062 N, 105?122 W), some 50 km to the north-northeast across the Parral-Jimin?z highway. Specimen size increases generally as one moves to the north-northeast within the field, and many large specimens (5-15 kg) were recovered in and around the area enclosed by Pueblito de Allende, San Juan, Rancho Blanco, and Santa Ana. At least two tons of meteoritic stones have been recovered, with crusted individuals ranging in weight from approximately 1 g to one individual of 110 kg. Specimen shapes are mainly fragmental, due to one major disruption of the parent body, followed by minor subsequent fragmentation. Individual stones have primary and secondary fusion crust, and some fresh fracture surfaces due to late-stage breaking. A small percentage of stones shows strong ablative shaping due to oriented flight. The elongate strewnfield possibly exceeds 300 km2 in area, making Allende the largest recorded stony meteorite fall both in its areal extent and in total weight of recovered meteorites. Allende fell near the sites of find of two major iron meteorites, Morito and Chupaderos.

Chemical and mineralogical compositions establish that Allende is a Type III carbonaceous chondrite. Three distinct components can be recognized: finegrained black matrix (<"60%), chondrules (<"30%), and irregular white aggregates (<"10%). The matrix consists almost entirely of iron-rich olivine (average 50% Fe2SiO4), with minor amounts of troilite, pentlandite, and taenite, rendered opaque by dispersed carbonaceous material. Most of the chondrules are magnesium-rich, and consist of olivine (average 9% Fe2SiO4) with minor amounts of clinoenstatite and some glass; a few chondrules are rich in calcium and aluminum, and are made up largely of anorthite, gehlenite, augite, and spinel. The irregular aggregates are also rich in calcium and aluminum, and contain anorthite, gehlenite, augite, spinel, nepheline, grossular, and sodalite (the last two minerals have not previously been recorded from meteorites). Complete chemical analyses have been made of the bulk meteorite, a dark inclusion, the matrix, a chondrule concentrate, two individual chondrules, and a single aggregate.

Distribution of Echinarachnius parma (Lamarck) and Associated Fauna on Sable Island Bank, Southeast Canada
Daniel J. Stanley and Noel P. James
24 pages, 8 figures, 6 plates, 1 table
1971 (Date of Issue: 27 April 1971)
Number 6, Smithsonian Contributions to the Earth Sciences
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A combined bottom photographic and sampling survey of Sable Island Bank southeast of Nova Scotia, Canada, reveals locally high densities (to 180 individual/m2) of the northern sand dollar Echinarachnius parma. Populations of this form are closely related to texture of the sea floor and generally concentrated on moderately sorted fine to medium sand surfaces. Topography and current regime are also correlatable factors; depth, time, salinity, and temperature apparently are not. Sand dollars are second in importance, after current activity, in reworking surficial sediments, and these organisms modify at least a third of the total Bank surface in the study area. Bioturbation is particularly intense in the sector north of Sable Island. Associated epifauna and infauna populations occur in two east-west trending areas on the Bank north and south of Sable Island. Absence of conspicuous fauna, save E. parma, in an east-west zone along the crest of the Bank and near Sable Island results from extremely strong current activity concentrated in this region.

Geology of the Lincoln Area, Lewis and Clark County, Montana
William G. Melson
29 pages, 13 figures, 8 tables
1971 (Date of Issue: 15 October 1971)
Number 7, Smithsonian Contributions to the Earth Sciences
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The Lincoln area (townships 13 and 14 N, ranges 7 and 8 W, about thirty miles northwest of Helena, Montana) is underlain by Pre-Cambrian Belt sedimentary rocks intruded by late Cretaceous (?) granitic stocks with concomitant widespread contact metamorphism and mineralization. The granitic stocks are probably related to the Boulder batholith. The pre-intrusion structure is characterized by high angle faults and broad open folds of Cretaceous age (Laramide). Oligocene (?) volcanic rocks were extruded on an eroded surface of the Belt rocks and granitic stocks. A second period of mineralization followed extrusion of the volcanic rocks.

Fracture cleavage which dips consistently to the southwest as well as the overall structure show that a southeast plunging syncline which marks the north end of the Boulder batholith continues into the Lincoln area. The syncline extends at least twenty miles north of the batholith and dominates the structure over an area of about 350 square miles.

About forty square miles of middle Tertiary volcanic rocks are composed of a lower series of andesitic to latitic flows and an upper series of rhyolitic welded ash flows. The features of the welded ash flows suggest that they were deposited in part by a vesiculating mass of rhyolitic magma (pumice froth flows). The volcanic rocks are presumably about the same age as the Lowland Creek volcanics of the Butte area.

The area and the region several miles to the north are about the northern limit of Boulder batholith activity, Tertiary volcanism, and associated mineral deposits. The superposition of these two periods of igneous activity and their gross similarities imply that they are genetically related.

Gold and silver have been produced from epithermal fissure veins. The scant available data suggests that the veins are vertically zoned. There were probably at least two periods of epithermal mineralization: one during the late stage cooling of the stocks, and a second after extrusion of the lower volcanic series.

Remnants of Tertiary surfaces preserved under the volcanic rocks imply that there have been topographic inversions since the middle Tertiary.

Glacial deposits suggest at least one early period of valley glaciation and later, perhaps recent, periods of restricted mountain glaciation. Rich gold placer deposits, such as in McClellan Gulch, accumulated after the earliest period of valley glaciation.

Late Quaternary Progradation and Sand Spillover on the Outer Continental Margin Off Nova Scotia, Southeast Canada
Daniel J. Stanley, Donald J. P. Swift, Norman Silverberg, Noel P. James and Robert G. Sutton
88 pages, 83 figures, 6 tables
1972 (Date of Issue: 11 April 1972)
Number 8, Smithsonian Contributions to the Earth Sciences
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Three distinct sediment types have prograded seaward from the outer shelf to the slope and rise in the vicinity of Sable Island Bank southeast of Nova Scotia during late Quaternary time. On the slope, the oldest facies recovered in cores is a brown to brick red, irregularly stratified, pebbly-sandy-clayey silt. Locally it is covered by an olive gray, clayey silt with a low sand and pebble content. This more homogenous gray facies displays abundant biogenic structures. A third facies, a thin layer of very fine, gray sand and muddy sand, locally covers brown and olive gray sediments on the slope and upper rise. All three facies contain similar light, heavy, and clay mineral suites.

The regional distribution of these facies has been determined by core traverses normal to the shelf edge, including one passing down the axis of The Gully (largest submarine canyon in the area), and another extending down the dissected slope off Sable Island Bank. The brown, late Pleistocene unit is exposed on the floor of The Gully and on its dissected deep-sea fan; postglacial bottom processes have kept younger sediments from accumulating in these areas. The brown beds also are exposed on the lower slope and rise off Sable Island in areas of slumping or nondeposition. The olive gray facies, late Pleistocene-Holocene in age, occurs primarily on the slope; it is thicker on flanks of slope valleys and thinner or absent on the divides. It is absent on part of the lower slope and upper rise. On the lower rise, tan mud with a coarse fraction rich in Foraminifera and shell debris may be the equivalent of the olive gray slope facies.

These sediments reflect changes in the sedimentary regimen during the post-Wisconsinan transgression. The observed sequence starts with the Wisconsin low stand of the sea when glacial drift, including reddish-brown, fluvioglacial sediments, were deposited over the Nova Scotian Shelf as far as Sable Island Bank. Periglacial outwash spread across the bank and flowed seaward around it. Deposition of the slope and rise brown facies is associated with this period; textural inhomogeneity suggests downslope transport by mass movement. Pebbly lenses resulted, in part, from ice-rafting prevalent during this phase. The contact between brown and the overlying olive gray, clayey silt facies is often abrupt, commonly occurring within several centimeters; this change is correlated with the rise of the late Quaternary sea above the margin of Sable Island Bank.

As the sea transgressed across Sable Island Bank in late glacial time, fines winnowed from fluvioglacial sediment were moved north of the Bank (into the Gully Trough) and seaward onto the slope. Coarse materials no longer reached the slope with former frequency, and the fines were supplied at a markedly lower rate. This decrease in sedimentation rate on the slope coincides with an increase in the organic fraction and bioturbation. Suspended fines were reduced to a gray hue as they passed through the sediment-water interface whose rate of upward growth was now an order of magnitude smaller. The Pleistocene-Holocene boundary of approximately 10,000 years B.P. occurs within the olive gray facies. As sea level attained its near-present position, and the present configuration of bottom currents was established, the lag (modified relict or palimpsest) sands on the Nova Scotian Shelf began a pattern of radial dispersal that may now be observed on Sable Island and associated banks. This bottom current activity has resulted in the development of spillover sands on the upper slope and deposition of thin discontinuous layers (including some turbidites) on the slope and rise and in The Gully Canyon.

Mineral Sciences Investigations, 1969-1971
William G. Melson, editor
94 pages, 34 figures, 34 tables
1972 (Date of Issue: 16 August 1972)
Number 9, Smithsonian Contributions to the Earth Sciences
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Seventeen short contributions from the Smithsonian's Department of Mineral Sciences from 1969 to 1971 are gathered together in this volume. The scientific and technological subjects treated in these contributions include studies of lunar samples from Apollo 12, meteorites, petrology and volcanology, and descriptive mineralogy, as well as of the history and description of one of the Smithsonian Institution's most important recent acquisitions the Carl Bosch Collection of Minerals and Meteorites.

Mineralogy, Mineral-Chemistry, and Composition of the Murchison (C2) Meteorite
Louis H. Fuchs, Edward Olsen and Kenneth J. Jensen
39 pages, 19 figures, 9 tables
1973 (Date of Issue: 14 August 1973)
Number 10, Smithsonian Contributions to the Earth Sciences
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The Murchison meteorite shower, September 28, 1969, occurred in and around Murchison, Victoria, Australia. Chemical and mineralogical analyses established it as a type II carbonaceous chondrite (C2). Murchison consists largely of fine-grained black matrix which has been identified as primarily a mixture of two iron-rich, low-aluminum chamosite polytypes. Contained in the matrix are four main types of inclusions: (1) single crystals and crystal fragments, (2) loosely aggregated clusters of crystals ("white inclusions"), (3) discrete true chondrules, (4) xenolithic fragments of two other meteorite types (mostly a unique kind of C3 chondrite).

The first type of inclusions consists of unzoned and highly zoned olivines, unzoned (disordered and ordered) orthopyroxenes, clinoenstatite, and rare diopside. Prominent minor phases are calcite, chromite, metal (with occasional traces of schreibersite), troilite, pentlandite, and two phases that could not be fully characterized.

The second type of inclusions consists primarily of grains of olivine (Fa 0 to Fa 40), lesser low-Ca pyroxenes, and minor spinel, calcite, whewellite, hibonite, perovskite, chromite, pentlandite, and rare Ca-pyroxene.

The true chondrules consist of olivine, Ca-poor pyroxene, occasional metal, and, in rare instances, one of the poorly characterized phases. The chondrules are not texturally typical of the ordinary chondrites, but resemble more closely those chondrules seen in C3 and C4 chondrites.

The fourth type of inclusion consists mainly of distinct xenolithic fragments of a light blue-gray chondrite type that resembles certain C3 chondrites (like Vigarano), though not in all aspects. These xenolithic fragments consist of disequilbrated olivines and pyroxenes, abundant pentlandite and troilite, and virtually no metal. In addition, a single xenolithic fragment was found of an unknown meteorite type.

Ca- and Al-rich glasses (of varying compositions) are found as blebs, with or without gas bubbles, contained within olivine crystals. The average Ca/Al ratio of these glasses approximates that for all meteoritic matter. They may represent early (nonequilibrium) subcooled condensates from the solar nebula. This nonequilibrium stage was apparently followed by equilibrium condensation through intermediate to low temperatures at which the layer-lattice phases condensed in abundance and incorporated crystals and fragments of the higher temperature phases.

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