LATE EOCENE IMPACT MICROSPHERULES: STRATIGRAPHY,AGE AND GEOCHEMISTRY

Recent discoveries of microtektite and related crystal bearing microspherule layers in deep-sea sediments of the west equatorial Pacific DSDP Sites 292, 315A and 462, off-shore New Jersey in Site 612 and in southern Spain have confirmed the presence of at least three microspherule layers in Late Eocene sediments. Moreover, these discoveries have extended the North American strewn field from the Caribbean and Gulf of Mexico region to the northwest Atlantic, and have established a third strewn field in western equatorial Pacific and Indian Ocean which may extend to the Mediterranean. Stratigraphically the oldest microspherule layer occurs in the planktonic foraminifer Globigerapsis semiinvoluta Zone about 0.5 m.y. prior to the closely spaced crystal bearing microspherule layer and North American microtektite layer in the Globorotalia cerroazulensis Zone. Major element composition of the G. semiinvoluta Zone layer and the crystal bearing microspherule layer overlap, but there is a clear trend towards higher Al₂O₃ and FeO values in SiO₂ equivalent microspherules of the latter layer. The G. semiinvoluta Zone microspherules also contain a higher percentage of non-crystalline spherules (microtektites) than the crystal bearing microspherule layer, but lower than the North American microtektite layer. Excess iridium due to an abrupt increase in supply is associated with the middle crystal bearing microspherule layer and to a lesser extent with the other two layers. But, Ir excess due to concentration as a result of carbonate loss was also observed at two sites (462, 612). The three late Eocene microspherule layers do not precisely coincide with planktonic foraminiferal species extinctions, but a major faunal assemblage change is associated with the G. semiinvoluta Zone layer. Abundant pyrite is present in the North American microtektite layer of DSDP Site 612 suggesting reducing conditions possibly due to a sudden influx of biologic matter (dead bodies) to the ocean floor, and the crystal bearing microspherule layer coincides with five radiolarian extinctions. All three microspherule layers are associated with decreased carbonate possibly due to sudden productivity changes, increased dissolution as a result of sea-level and climate fluctuations, or the impact events.     

LATE EOCENE CRYSTAL-BEARING SPHERULES: TWO LAYERS OR ONE?

Late Eocene crystal-bearing spherules have been found in deep sea cores from the Caribbean Sea, Gulf of Mexico, equatorial Pacific Ocean, and eastern equatorial Indian Ocean. Keller et al. (1987) have suggested that the spherules from the western equatorial Pacific (Site 292, core 38) and eastern Indian Ocean (Site 216) are older (Globigerapsis semiinvoluta Zone) than those from the central equatorial Pacific, Gulf of Mexico, and Caribbean Sea (Globorotalia cerroazulensis Zone). The strongest argument in favor of two layers is the biostratigraphic data; however, published biostratigraphic interpretations are at odds with Keller et al.'s (1987) conclusions. Furthermore, paleomagnetic data for Site 292 seems to contradict Keller et al.'s conclusion that the spherules found in core 36 occur in sediments of the same stratigraphic age as those found in the central equatorial Pacific, Gulf of Mexico, and Caribbean Sea sites. Although the spherules from Sites 216 and 292 (core 38) do have higher average CaO, and lower average Al₂O₃ and FeO contents than the late Eocene spherules from the other sites, there is a great deal of overlap in composition. It is our opinion that the similarities in composition and petrography between the late Eocene crystal-bearing spherules are greater than the differences. Additionally, there seems to be a systematic change in composition and in amount of iridium excess from east to west when all the sites containing the crystal-bearing spherules are considered. We believe, therefore, that it is likely that the late Eocene crystal-bearing spherules all belong to a single event.     

Australasian microtektites and associated impact ejecta in the South China Sea and the Middle Pleistocene supereruption of Toba

Abstract— Australasian microtektites were discovered in Ocean Drilling Program (ODP) Hole 1143A in the central part of the South China Sea. Unmelted ejecta were found associated with the microtektites at this site and with Australasian microtektites in Core SO95–17957–2 and ODP Hole 1144A from the central and northern part of the South China Sea, respectively. A few opaque, irregular, rounded, partly melted particles containing highly fractured mineral inclusions (generally quartz and some K feldspar) and some partially melted mineral grains, in a glassy matrix were also found in the microtektite layer. The unmelted ejecta at all three sites include abundant white, opaque grains consisting of mixtures of quartz, coesite, and stishovite, and abundant rock fragments which also contain coesite and, rarely, stishovite. This is the first time that shock‐metamorphosed rock fragments have been found in the Australasian microtektite layer. The rock fragments have major and trace element contents similar to the Australasian microtektites and tektites, except for higher volatile element contents. Assuming that the Australasian tektites and microtektites were formed from the same target material as the rock fragments, the parent material for the Australasian tektites and microtektites appears to have been a fine‐grained sedimentary deposit. Hole 1144A has the highest abundance of microtektites (number/cm²) of any known Australasian microtektite‐bearing site and may be closer to the source crater than any previously identified Australasian microtektite‐bearing site. A source crater in the vicinity of 22° N and 104° E seems to explain geographic variations in abundance of both the microtektites and the unmelted ejecta the best; however, a region extending NW into southern China and SE into the Gulf of Tonkin explains the geographic variation in abundance of microtektites and unmelted ejecta almost as well. The size of the source crater is estimated to be 43 ± 9 km based on estimated thickness of the ejecta layer at each site and distance from the proposed source. A volcanic ash layer occurs just above the Australasian microtektite layer, which some authors suggest is from a supereruption of the Toba caldera complex. We estimate that deposition of the ash occurred ?800 ka ago and that it is spread over an area of at least 3.7 times 10⁷ km².     

Fragmentation and hydration of tektites and microtektites

Abstract— An examination of data collected over the last 30 years indicates that the percent of glass fragments vs. whole splash forms in the Cenozoic microtektite strewn fields increases towards the source crater (or source region). We propose that this is due to thermal stress produced when tektites and larger microtektites fall into water near the source crater while still relatively hot (>1150 °C). We also find evidence (low major oxide totals, frothing when melted) for hydration of most of the North American tektite fragments and microtektites found in marine sediments. High-temperature mass spectrometry indicates that these tektite fragments and microtektites contain up to 3.8 wt% H₂O. The H₂O-release behavior during the high-temperature mass-spectrometric analysis, plus high CI abundances (0.05 wt%), indicate that the North American tektite fragments and microtektites were hydrated in the marine environment (i.e., the H₂O was not trapped solely on quenching from a melt). The younger Ivory Coast and Australasian microtektites do not exhibit much evidence of hydration (at least not in excess of 0.5 wt% H₂O); this suggests that the degree of hydration increases with age. In addition, we find that some glass spherules (with <65 wt% SiO₂) from the upper Eocene clinopyroxene-bearing spherule layer in the Indian Ocean have palagonitized rims. These spherules appear to have been altered in a similar fashion to the splash form K/T boundary spherules. Thus, our data indicate that tektites and microtektites that generally contain >65 wt% SiO₂ can undergo simple hydration in the marine environment, while impact glasses (with <65 wt% SiO₂) can also undergo palagonitization.     

A first report of microtektites from the shell beds of southwestern Florida

The Plio‐Pleistocene Upper Tamiami Formation (Pinecrest beds) of Florida is well known for its fossiliferous shell beds, but not for its extraterrestrial material. Here we report the first occurrence of tiny (~200 μm in diameter) silica‐rich microspherules from this unit and from the state. This material was analyzed using petrographic and elemental methods using energy dispersive X‐ray spectroscopy (EDS). The majority of microspherules are glassy and translucent in reflected light with some displaying “contact pairs” (equal‐sized micro‐spherules attached to each other). Broken microspherules cleave conchoidally, often with small internal spherical vesicles, but most lack any other evidence of internal features, such as layering. Using the EDS data, the microspherules were compared to volcanic rocks, microtektites, and cosmic spherules (micrometeorites). Based on their physical characteristics and elemental compositions these are likely microtektites or a closely related type of material. The high Na content in the examined material deviates significantly from the abundances usually found in micrometeorites and tektite material; this is enigmatic and requires further study. This material may be derived from a nearby previously unknown impact event; however, more material and sites are required to confirm the source of this material. Because of the focus on molluscan fossils in southwestern Florida shell beds, microtektite material has likely been overlooked in the past, and it is probable that these microspherules are in abundance elsewhere in these units and possibly throughout the region.     

Ocean Drilling Project Hole 689B spherules and upper Eocene microtektite and clinopyroxene-bearing spherule strewn fields

Abstract— Montanari et al. (1993) reported a positive Ir anomaly in the upper Eocene sediments from Ocean Drilling Program Hole 689B on the Maud Rise, Southern Ocean. Vonhof (1998) described microtektites and clinopyroxene-bearing (cpx) spherules associated with the Ir anomaly in Hole 689B and suggested that they belong to the North American and equatorial Pacific cpx strewn fields, respectively. We searched a suite of 27 samples taken through the spherule layer from Hole 689B, and we recovered 386 microtektites and 667 cpx spherules. We studied the petrography of the microtektites and cpx spherules and determined the major element compositions of 31 microtektites and 14 cpx spherules using energy dispersive x-ray analysis. We also determined the minor element compositions of eight microtektites using instrumental neutron activation analysis. We found that the peak abundance of cpx spherules is ～2 cm below the peak abundance of the microtektites (～128.7 m below sea floor), which suggests that the cpx spherule layer may be slightly older (～3–5 ka). The microtektites are mostly spherical and are generally transparent and colorless. They are similar to the North American microtektites in composition, the biggest differences being their generally lower Na₂O and generally higher Zr, Ba, and Ir (up to 0.3 ppb) contents. We agree with Vonhof (1998) that the Hole 689B microtektites probably belong to the North American tektite strewn field. We calculate that the number of microtektites (>125 μm)/cm² at Hole 689B is 52. This number is close to the concentration predicted by extrapolation of the trend of concentration vs. distance from the Chesapeake Bay structure, based on data from other North American microtektite-bearing sites. Thus, the North American strewn field may be at least four times larger than previously mapped. The Hole 689B cpx spherules range from translucent yellow to opaque black, but most are opaque tan to dark brown. They are generally spherical in shape and all are < 125 μm in diameter. Some contain Ni-rich spinels in addition to clinopyroxene microlites. The cpx spherules are petrographically and compositionally similar to cpx spherules previously found in the northwestern Atlantic Ocean, Caribbean Sea, Gulf of Mexico, equatorial Pacific, and eastern Indian Ocean. The abundance and widespread geographic occurrence of these spherules suggest that the strewn field may be global in geographic extent. Assuming a global extent, we estimate that there may be at least 25 billion metric tons of cpx spherules in the strewn field. Based on age, size, and geographic location, we speculate that the 100 km diameter Popigai crater in northern Siberia may be the source of the cpx spherule layer.     

Australasian minitektites discovered in the Indian Ocean

Abstract— Large area sampling with a box core in the Indian Ocean has led to the discovery of minitektites (>1–3.75 mm long) and a tektite fragment (～1.25 mm) occurring with microtektites belonging to the Australasian tektite strewn field. Minitektites and the microtektites are found to have similar major element compositions conforming to the Australasian tektite/microtektite chemistry. Earlier studies based on isotopic evidence, dating, and chemistry had provided evidence of a single large tektite strewn field; however, the physical association of tektites occurring with microtektites has been lacking. The present study provides such an association.     

Australasian microtektites from Antarctica: XAS determination of the Fe oxidation state

The Fe oxidation state and coordination number of 29 impact glass spherules recently recovered from the Transantarctic Mountains (Antarctica) have been determined by X‐ray absorption near edge structure (XANES) spectroscopy. Based on geochemical, isotopic, and fission track data, these spherules are considered as microtektites from the Australasian tektite/microtektite strewn field. Their find location is the farthest so far discovered from the possible source crater region, and their alkali content is the lowest compared with other published data on Australasian microtektite glasses. The Fe³⁺/(Fe²⁺+Fe³⁺) ratio, determined from the analysis of the pre‐edge peak energy position and integrated intensity, is below 0.1 (±0.04) for all the samples, and is comparable to that of most tektites and microtektites from the Australasian strewn field. Also, the pre‐edge peak integrated intensity, which is sensitive to the average Fe coordination geometry, is comparable to that of other Australasian microtektites reported in the literature. The agreement of the Fe oxidation state and coordination number, between the Transantarctic Mountain microtektites (TAM) and the Australasian tektites and microtektites, further confirms the impact origin of these glass spherules and provides an independent suggestion that they represent a major extension southeastward of the Australasian strewn field. The fact that similar redox conditions are observed in tektites and microtektites within the Australasian strewn field regardless of the distance from the source crater area (up to approximately 11000 km) could be an important constraint for better understanding the different processes affecting microtektite formation and transport. The fact that the Fe oxidation state of microtektites does not increase with distance, as in the case of North American microtektites, means that thermal and redox histories of Australasian and TAM microtektites could differ significantly from those of North American microtektites.     

Potassium isotope abundances in Australasian tektites and microtektites

Abstract— We report electron microprobe determinations of the elemental compositions of 11 Australasian layered tektites and 28 Australasian microtektites; and ion microprobe determinations of the ⁴¹K/³⁹K ratios of all 11 tektites and 13 of the microtektites. The elemental compositions agree well with literature values, although the average potassium concentrations measured here for microtektites, 1.1‐1.6 wt%, are lower than published average values, 1.9‐2.9 wt%. The potassium isotope abundances of the Australasian layered tektites vary little. The average value of ^δ41K, 0.02 ± 0.12%₀ (1 s? mean), is indistinguishable from the terrestrial value (= 0 by definition) as represented by our standard, thereby confirming four earlier tektite analyses of Humayun and Koeberl (2004). In agreement with those authors, we conclude that evaporation has significantly altered neither the isotopic nor the elemental composition of Australasian layered tektites for elements less volatile than potassium. Although the average ⁴¹K/³⁹K ratio of the microtektites, 1.1 ± 1.7%₀ (1 s? mean), is also statistically indistinguishable from the value for the standard, the individual ratios vary over a very large range, from ?10.6 ± 1.4%₀ to +13.8 ± 1.5%₀ and at least three of them are significantly different from zero. We interpret these larger variations in terms of the evaporation of isotopically light potassium; condensation of potassium in the vapor plume; partial or complete stirring and quenching of the melts; and the possible uptake of potassium from seawater. That the average ⁴¹K/³⁹K ratio of the microtektites equals the terrestrial value suggests that the microtektite‐forming system was compositionally closed with respect to potassium and less volatile elements. The possibility remains open that ⁴¹K/³⁹K ratios of microtektites vary systematically with location in the strewn field.     

Nd and Sr isotopic evidence for the origin of tektite material from DSDP site 612 off the New Jersey coast

Abstract— Late Eocene tektite material from DSDP site 612 is composed of angular to spherical tektites and microtektites containing abundant vesicles and a few unmelted to partially melted mineral inclusions. The major element compositions of the 612-tektites are generally comparable to those of North American tektites, but the physical features suggest that the DSDP-612 tektites were formed by less severe shock melting. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd compositions of 612-tektites: a) show much wider ranges than the tightly constrained group of North American tektites and microtektites, and b) are significantly different from those of other groups of tektites. The existence of large isotopic variations in tektites from DSDP site 612 requires that they were formed from a chemically and isotopically heterogeneous material in a regime that is distinctive from that of other groups of tektites. T^ND_CHUR and T^Sr_UR model ages of the 612-tektites indicate that they were formed from a crustal source of late Precambrian mean age (800–1000 Ma) which in middle Palaeozoic time (?400 Ma) was further enriched in Rb/Sr during sedimentary processes. These source characteristics suggest that the impact which produced the 612-tektites occurred in rocks of the Appalachian orogeny or sediments derived from this orogenic belt. Potential source materials for both 612-tektites and North American tektites are present on the eastern and southeastern part of the North American continent and its adjacent shelf. The distinct isotopic differences between 612-tektites and North American tektites indicate that the two groups of tektites were either formed by the impact of more than one bolide in the same general area, or by a single impact event that sampled different layers.     

Two layers of Australasian impact ejecta in the Indian Ocean?

Abstract— Only 2 Australasian tektites have been found in the Indian Ocean, and both are associated with surficial sediments. We collected cores from both locations where the tektites have been reported. The microtektites in these cores (and both the tektites, as reported earlier) have chemical compositions within the compositional range previously reported for Australasian tektites and microtektites. In both locations, while the tektites are occurring at the sediment/water interface, the microtektites are found buried in older horizons beneath the seafloor at stratigraphic levels, conforming to the radiometric age of the strewn field. Thus, at first glance, there appear to be 2 layers of Australasian impact ejecta in the Indian Ocean. However, the manganese nodules are associated with the tektites which, although millions of years old, are invariably resting on recent sediments. Therefore, the mechanism that retains nodules at the seafloor also seems to be operative on the tektites, thus leading to this apparent “age paradox” of tektite/microtektite distribution in the Indian Ocean, although they both belong to the same impact event.     

North American tektite debris and impact ejecta from DSDP Site 612

Abstract— A layer of tektite glass and shock-metamorphosed grains found in an upper Eocene section of core 21 from DSDP Site 612 taken on the continental slope off New Jersey may belong to the North American tektite strewn field. However, the Site 612 glasses generally have higher K₂O and lower Na₂O contents for a given SiO₂ content and different Sr and Nd isotopic compositions. In order to better define the layer, a series of samples was taken continuously through the layer at 1 cm intervals. Tektite fragments are in an 8 cm thick layer; microtektites are concentrated in the upper 4 cm, while spherules with “crystalline” textures (microkrystites) are concentrated in the lower half of the layer. Millimeter-size splash forms are mostly in the lower part of the tektite-bearing layer. Rock and mineral grains showing evidence of shock metamorphism are abundant in the upper half of the tektite-bearing layer. Coesite is abundant, and stishovite was found in one rock fragment. The size and abundance of the tektite glass and the abundance of shocked debris indicate that Site 612 is relatively close to the source crater, which may be to the north of Site 612 on the coastal plain or adjacent continental shelf.     

Late Eocene microkrystites and microtektites at Maud Rise (Ocean Drilling Project Hole 689B; Southern Ocean) suggest a global extension of the approximately 35.5 Ma Pacific impact ejecta strewn field

Abstract— Late Eocene microtektites and microkrystites recovered from Ocean Drilling Project Hole 689B at Maud Rise (Southern Ocean) are stratigraphically and geochemically compared to spherules from the North American and Pacific strewn fields, and to devitrified spherules from the Eocene-Oligocene global stratotype section and point section in Massignano, Italy. The ODP 689B microkrystites compare well to the Pacific strewn field microkrystites, which suggests that the geographic extent of the Pacific strewn field was much larger than previously documented. The elemental composition of microtektites of ODP Hole 689B is comparable to tektites of the North American strewn field. Their ⁸⁷Sr/⁸⁶Sr ratio, however, is different. We tentatively interpret this to reflect geochemical heterogeneity within the North American strewn field but can not exclude the option that the chemical discrepancies result from the existence of a third late Eocene impact site.     

Major and trace element compositions of georgiaites: Clues to the source of North American tektites

Abstract— Electron microprobe and laser ablation, inductively‐coupled plasma mass spectrometer analyses of 24 georgiaites show that these tektites are all Si‐rich (79–83 wt% SiO₂) glasses with variable major and trace element abundances (e.g., FeO varies from 2.1 to 3.7 wt%). Glass compositions are similar to but not identical with average upper continental crust. For example, georgiaites are light rare earth element enriched with small negative Eu anomalies (Eu/Eu*=0.73‐0.86) and La‐Th‐Sc systematics are intermediate between that of Archean and post‐Archean continental crust. When the georgiaite data are placed in the context of data for all North American tektites, triangular arrays appear on some oxide‐oxide plots (e.g., FeO‐MgO). Large variations in refractory element abundances and ratios compared to the variation in SiO₂ favors mixing over volatilization as a cause of the compositional variation. If all the tektites formed as a result of a single impact, then triangular arrays in oxide‐oxide variation diagrams require at least three source components. These components include a Si‐rich material, probably a quartz‐rich sand that was predominant in the formation of georgiaites. Two relatively silica‐poor and Fe‐rich components have compositional characteristics similar to shales and greywackes. The La‐Th‐Sc systematics of the georgiaites and most other North American tektites are distinctive and could potentially be used to link the tektites to Eocene sediments at the Chesapeake Bay impact structure.     

New atmospheric pCO2 estimates from palesols during the late Paleocene/early Eocene global warming interval

The late Paleocene to early Eocene was one of the warmest intervals in Earth's history. Superimposed on this long-term warming was an abrupt short-term extreme warm event at or near the Paleocene/Eocene boundary and centered in the higher latitudes. This short-term climate warming was associated with a major benthic foraminiferal extinction and a dramatic 3–4% drop in the ocean's carbon isotopic composition. It has been suggested that the late paleocene/early Eocene global warming was caused by an enhanced greenhouse effect associated with higher levels of atmospheric CO₂ relative to present levels. We present carbon isotopic data from the co-existing paleosols organic matter and carbonates from a terrestrial sequence in the Paris Basin, France that contradict the notion that an increase in atmospheric CO₂ level was the cause of extreme warming for this time interval. Atmospheric pCO₂ estimates for the Late Paleocene/early Eocene estimated from the terrestrial carbon isotopic record spanning the Paleocene/Eocene transition, are indistinguishable from each other and were generally between 300 and 700 ppm.     

The Cuban Tektite Revisited

Abstract— A tektite, probably found in Cuba, was previously classified as belonging to the North American tektite strewn field on the basis of chemistry, age, isotopic, and petrographic characteristics. New major element analyses and trace element analyses show that the sample falls within the range of other North American tektites, and is close to the bediasite compositions. There are, however, some differences to normal georgiaites and bediasites. In a Na₂O/K₂O diagram the sample plots between the two distinct fields formed by georgiaites and bediasites. The rare earth elements and some lithophile trace elements are slightly enriched compared to bediasites, and much higher than in georgiaites. The discovery of tektite fragments from locations at Barbados and a DSDP site off the coast of New Jersey makes it likely that the North American strewn field is larger than previously thought, in agreement with microtektite distributions. Thus it is possible that the “Cuban” tektite really originated from Cuba.     

Experimental and petrological constraints on lunar differentiation from the Apollo 15 green picritic glasses

Abstract— Phase equilibrium experiments on the most magnesian Apollo 15C green picritic glass composition indicate a multiple saturation point with olivine and orthopyroxene at 1520°C and 1.3 GPa (about 260 km depth in the moon). This composition has the highest Mg# of any lunar picritic glass and the shallowest multiple saturation point. Experiments on an Apollo 15A composition indicate a multiple saturation point with olivine and orthopyroxene at 1520°C and 2.2 GPa (about 440 km depth in the moon). The importance of the distinctive compositional trends of the Apollo 15 groups A, B, and C picritic glasses merits the reanalysis of NASA slide 15426,72 with modern electron microprobe techniques. We confirm the compositional trends reported by Delano (1979, 1986) in the major element oxides SiO₂, TiO₂, Al₂O₃, Cr₂O₃, FeO, MnO, MgO, and CaO, and we also obtained data for the trace elements P₂O₅, K₂O, Na₂O, NiO, S, Cu, Cl, Zn, and F. Petrogenetic modeling demonstrates that the Apollo 15 A‐B‐C glass trends could not have been formed by fractional crystallization or any continuous assimilation/fractional crystallization (AFC) process. The B and C glass compositional trends could not have been formed by batch or incremental melting of an olivine + orthopyroxene source or any other homogeneous source, though the A glasses may have been formed by congruent melting over a small pressure range at depth. The B compositional trend is well modeled by starting with an intermediate A composition and assimilating a shallower, melted cumulate, and the C compositional trend is well modeled by a second assimilation event. The assimilation process envisioned is one in which heat and mass transfer were separated in space and time. In an initial intrusive event, a picritic magma crystallized and provided heat to melt magma ocean cumulates. In a later replenishment event, the picritic magma incrementally mixed with the melted cumulate (creating the compositional trends in the green glass data set), ascended to the lunar surface, and erupted as a fire fountain. A barometer created from multiple saturation points provides a depth estimate of other glasses in the A‐B‐C trend and of the depths of assimilation. This barometer demonstrates that the Apollo 15 A‐B‐C trend originated over a depth range of ?460 km to ?260 km within the moon.     

A comparative study of opaque phases in Qingzhen (EH3) and MacAlpine Hills 88136 (EL3): Representatives of EH and EL parent bodies

Abstract— Opaque minerals in the Qingzhen (EH3) and MacAlpine Hills (MAC) 88136 (EL3) enstatite chondrites were studied and compared with other EH and EL chondrites. All opaque minerals usually occur in multi‐sulfide‐metal clasts and nodules in the matrix between chondrules (El Goresy et al., 1988). The higher abundance of opaque minerals, the occurrence of niningerite and various alkali‐sulfides (e.g., caswellsilverite, phases A and B, djerfisherite) are diagnostic criteria for EH chondrites, while alabandite is characteristic for EL chondrites. In addition, EH chondrites are characterized by enrichments of Si in both kamacite and perryite, and alkali elements in sphalerite and chalcopyrite. The Mn contents of daubreelite and sphalerite are lower in EH than in EL chondrites. These are consistent with lower oxygen fugacity and higher H₂S fugacity of EH than EL chondrites. In contrast, the discovery of sphalerite and Zn‐rich daubreelite in MAC 88136 indicates that their absence in EL6 chondrites is probably related to thermal metamorphism in the parent body. Schreibersite microspherules are commonly enclosed in most sulfides in Qingzhen, but are absent in MAC 88136. They were once molten, and probably predated all sulfide host phases. The petrographic setting and chemical compositions of the sulfide hosts of the schreibersite microspherules in EH3 chondrites are consistent with formation by condensation. The earliest sulfide condensates oldhamite and niningerite occupy the interiors of the clasts and nodules, whereas the rims consist of troilite and djerfisherite. In addition, in Qingzhen, some other troilite, djerfisherite and sphalerite assemblages coexist with perryite. They were produced by sulfurization of metallic Fe‐Ni in the nebula. In MAC 88136, sulfurization of Si‐bearing Fe‐Ni metal is less pronounced, and it produced troilite, schreibersite and less abundant perryite. Two kinds of normal zoning and a reverse zoning trends of niningerite, and both normal and reverse zoning of sphalerite were found in clasts and nodules in Qingzhen. The coexistence of normal and reverse zoning profiles in niningerite grains in the same meteorite strongly suggests that they formed before accretion in the parent body, because an asteroidal metamorphic or an impact event in the parent body would have erased these contrasting profiles and destroyed the textural settings. In contrast, alabandite in MAC 88136 shows only normal zoning, with the FeS content decreasing to 9.3 mol% toward troilite, indicating very slow cooling at low temperature.     

Bottle‐green microtektites from the South Tasman Rise: Deep‐sea evidence for an impact event near the Miocene/Pliocene boundary

Abstract— Forty‐eight bottle‐green microtektites (BGMTs) were found in a core sample recovered from Ocean Drilling Program Site 1169, located along the western flank of the South Tasman Rise in the southeastern Indian Ocean. Biostratigraphic evidence loosely constrains the age of the Site 1169 BGMTs to an interval spanning the late‐middle Miocene to earliest Pliocene (12.1–4.6 Ma); incomplete core recovery and a major stratigraphic hiatus prevented a more precise age determination. This broad range of biostratigraphic ages indicates that these microtektites predate the Australasian strewn layer by at least 3.83 Ma, and perhaps by as much as 11.33 Ma. Furthermore, the REE signatures of the Site 1169 BGMTs are incongruent with those of typical Australasian ejecta, indicating that the Site 1169 BGMTs are not part of the larger Australasian strewn field. Among the various australite subgroups, the Site 1169 BGMTs are most similar in age to the HNa/K australites. However, numerous compositional discrepancies indicate that these two ejecta populations are also unrelated; the great distances separating Site 1169 from HNa/K australite‐bearing localities also makes a shared provenance unlikely. Therefore, we conclude that the Site 1169 BGMTs were formed by a late Miocene impact that is distinctly separate from the Australasian and HNa/K australite events, though the location of this impact is unknown.     

STUDIES OF BRAZILIAN METEORITES XIV. MINERALOGY,PETROLOGY, AND CHEMISTRY OF THE CONQUISTA,MINAS GERAIS,CHONDRITE

The Conquista chondrite consists of major olivine, low-Ca pyroxene (both ortho- and twinned clino-), troilite and metallic nickel-iron; minor glassy to microcrystalline material and pigeonite; and accessory chromite, high-Ca clinopyroxene and hydrous ferric oxides that formed by terrestrial weathering of metallic nickel-iron. Results of microscopic, electron microprobe, and bulk chemical studies, particularly the compositions of olivine (Fa_17.2) and low-Ca pyroxene (Fs_15.4); the contents of metallic nickel-iron (18.5%) and total iron (25.83%); and the ratios of Fe°/Fe_total (0.64), Fe°/Ni° (9.59) and Fe_total/SiO₂ (0.69) indicate H-group classification. The pronounced, well-developed chondritic texture; the slight compositional variations in constituent phases; the high Ca contents of pyroxene and the presence of pigeonite, glassy to microcrystalline interstitial material rich in alkalis and SiO₂, and of twinned low-Ca clinopyroxene suggest that Conquista is of petrologic type 4.