ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00667.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00668.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00669.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—Quartz, as a ubiquitous mineral constituent of the Earth's crust, displays the greatest variety of well‐defined residual shock effects among all rock‐forming minerals. It represents an important and most reliable shock barometer and thermometer for terrestrial impact formations. In this paper, the current status of knowledge about the nature, origin, and experimental pressure‐temperature calibration of shock‐induced deformations and phase transformations is reviewed for natural and experimental shock c

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00670.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The Divnoe meteorite is an olivine‐rich primitive achondrite with subchondritic chemistry and mineralogy. It has a granoblastic, coarse‐grained, olivine groundmass (CGL: coarse‐grained lithology) with relatively large pyroxene‐plagioclase poikilitic patches (PP) and small fine‐grained domains of an opaque‐rich lithology (ORL). Both PP and ORL are inhomogeneously distributed and display reaction boundaries with the groundmass. Major silicates, olivine (Fa20–28) and orthopyroxene (Fs20–28Wo0.5–2.5), display systematic differences in composition between CGL and ORL as well as a complicated pattern of variations within CGL. Accessory plagioclase has low K content and displays regular igneous zoning with core compositions An40–45and rims An32–37. The bulk chemical composition of Divnoe is similar to that of olivine‐rich primitive achondrites, except for a depletion of incompatible elements and minor enrichment of refractory siderophiles. Oxygen isotope compositions for whole‐rock and separated minerals from Divnoe fall in a narrow range, with mean δ18O = +4.91, δ17O = +2.24, and Δ17O = −0.26 ± 0.11. The isotopic composition is not within the range of any previously recognized group but is very close to that of the brachinites. To understand the origin of Divnoe lithologies, partial melting and crystallization were modelled using starting compositions equal to that of Divnoe and some chondritic meteorites. It was found that the Divnoe composition could be derived from a chondritic source region by ∼20 wt% partial melting at T ∼ 1300 °C and log(fO2) = IW‐1.8, followed by ∼60 wt% crystallization of the partial melt formed, and removal of the still‐liquid portion of the partial melt. Removal of the last partial melt resulted in depletion of the Divnoe plagioclase in Na and K. In this scenario, CGL represents the residue of partial melting, and PP is a portion of the partial melt that crystallizedin situ. The ORL was formed during the final stages of partial melting by reaction between gaseous sulfur and residual olivine in the source region.A prominent feature of Divnoe is fine μm‐scale chemical variations within olivine grains, related to lamellar structures the olivines displ

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00671.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—Watson, which was found in 1972 in South Australia, contains the largest single silicate rock mass seen in any known iron meteorite. A comprehensive study has been completed on this unusual meteorite: petrography, metallography, analyses of the silicate inclusion (whole rock chemical analysis, INAA, RNAA, noble gases, and oxygen isotope analysis) and mineral compositions (by electron microprobe and ion microprobe). The whole rock has a composition of an H‐chondrite minus the normal H‐group metal and troilite content. The oxygen isotope composition is that of the silicates in the HE iron meteorites and lies along an oxygen isotope fractionation line with the H‐group chondrites. Trace elements in the metal confirm Watson is a new HE iron. Whole rock Watson silicate shows an enrichment in K and P (each ∼2X H‐chondrites). The silicate inclusion has a highly equilibrated igneous (peridotite‐like) texture with olivine largely poikilitic within low‐Ca pyroxene: olivine (Fa20), opx (Fs17Wo3), capx (Fs9Wo41) (with very fine exsolution lamellae), antiperthite feldspar (An1–Or5) with1550 °C. A flat refractory lithophile and flat REE pattern (at ∼1x average H‐chondrites) indicate that melting took place in a relatively closed system. Immiscible metal and sulfide were occluded into the surrounding metal host. Below 1100 °C, the average cooling rate is estimated to have been ∼1000 °C/Ma; Widmanstätten structure formed, any igneous zoning in the silicates was equilibrated, and feldspar and pyroxene exsolution took place. Cooling to below 300 °C was completed by 3.5 Ga B. P. At 8 Ma, a shock event took place causing some severe metal deformation and forming local melt pockets of schreibersite/metal. This event likely caused the release of Watson into interplanetary space. The time of this event, 8Ma, corresponds to the peak frequency of exposure ages of the H‐chondrites. This further confirms the link between HE irons and the H‐chondrites, a relationship already indicated by their common oxygen isotope source.Watson metal structures are very similar to those in Kodaikanal. Watson, Kodaikanal and Netschaëvo form the young group of HE meteorites (ages 3.7 ± 0.2 Ga). They appear to represent steps in a chain of events that must have taken place repeatedly on the HE parent body or bodies from which they came: chondrite engulfed in metal (Netschaëvo); chondrite melted within metal (Watson); and finally melted silicate undergoing strong fractionation with the fractionated material emplaced as globules within metal (Kodaikanal). Watson fills an important gap in understanding the sequence of events that took place in the evolution of the IIE‐H parent body(ies). This association of H‐chondrite with HE metal suggests a surface, or near surface proc

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00672.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—ALH84001, originally classified as a diogenite, is a coarse‐grained, cataclastic, orthopyroxenite meteorite related to the martian (SNC) meteorites. The orthopyroxene is relatively uniform in composition, with a mean composition of Wo3.3En69.4Fs27.3. Minor phases are euhedral to subhedral chromite and interstitial maskelynite, An31.1Ab63.2Or5.7, with accessory augite, Wo42.2En45.1Fs12.7, apatite, pyrite and carbonates, Cc11.5Mg58.0Sd29.4Rd1.1. The pyroxenes and chromites in ALH84001 are similar in composition to these phases in EETA79001 lithology A megacrysts but are more homogeneous. Maskelynite is similar in composition to feldspars in the nakhlites and Chassigny. Two generations of carbonates are present, early (pre‐shock) strongly zoned carbonates and late (post‐shock) carbonates. The high Ca content of both types of carbonates indicates that they were formed at moderately high temperature, possibly ∼700 °C. ALH84001 has a slightly LREE‐depleted pattern with La 0.67x and Lu 1.85x CI abundances and with a negative Eu anomaly (Eu/Sm 0.56x CI). The uniform pyroxene composition is unusual for martian meteorites, and suggests that ALH84001 cooled more slowly than did the shergottites, nakhlites or Chassigny. The nearly monomineralic composition, coarse‐grain size, homogenous orthopyroxene and chromite compositions, the interstitial maskelynite and apatite, and the REE pattern suggest that ALH84001 is a cumulate orthopyroxenite containing minor trapped, intercu

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00673.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—The Yarle Lakes 001 meteorite was a single stone of 913 g found approximately 20 km north of Watson, South Australia, in 1990 October. It consists of olivine (Fa18.7 ± 0.4, n = 30), low‐Ca pyroxene (Fs16.6 ± 0.2Wo12 + 0.4, n = 15). feldspar, high‐Ca pyroxene, metallic Fe‐Ni and troilite. Based on texture and mineral chemistry, Yarle Lakes 001 is classified as a H5 chondrite of shock

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00674.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—Grain size separates of the Cretaceous‐Tertiary (K‐T) fireball layer at marine sites and the entire K‐T boundary interval at nonmarine sites (fireball layer, ejecta layer, coal seam) were analysed for platinum‐group elements (PGE: Ru, Ir, Ft, Pd, Rh) and Au using inductively coupled plasma mass‐spectrometry. X‐ray diffractometry and scanning electron microscopy were performed on each fraction to establish the mineralogy. It was determined that the most abundant minerals in the finest fraction were a smectite‐group mineral at marine K‐T sites and illite‐smectite mixed layer clay at nonmarine K‐T sites. Positive correlation of PGE content and mineralogy indicates that the PGE are presently associated with these clay minerals which were likely formed by alteration of a condensed mafic phase(e.g., olivine or pyroxene). The correlation of clay mineral abundance with PGE content is imperfect because not all of the clay minerals in the fireball layer are derived by alteration of the original condensed ejecta. In addition, undetected PGE host(s), present in trace amounts, ar

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00675.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract—Bison LL6 chondrite is an impact breccia. The meteorite consists of unmolten clasts and accessory melt‐breccia clasts incorporated into a host of the same parent

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1994.tb00676.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY