Metamorphism and aqueous alteration in low petrographic type ordinary chondrites     

Derek W. G. Sears  Andrew D. Morse  Robert Hutchison  R. Kyle Guimon  Lu Jie  C. M. O'D. Alexander  Paul H. Benoit  Ian Wright  Colin Pillinger  Tian Xie  Michael E. Lipschutz 《Meteoritics & planetary science》1995,30(2):169-181

Abstract— In order to investigate the relative importance of dry metamorphism and aqueous alteration in the history of chondrules, chondrules were hand-picked from the Semarkona (petrographic type 3.0), Bishunpur (3.1), Chainpur (3.4), Dhajala (3.8) and Allegan (5) chondrites, and matrix samples were extracted from the first three ordinary chondrites. The thermoluminescence (TL) properties of all the samples were measured, and appropriate subsets of the samples were analyzed by electron-microprobe and radiochemical neutron activation and the water and H-isotopic composition determined. The TL data for chondrules from Semarkona and Bishunpur scatter widely showing no unambiguous trends, although group B1 chondrules tend to have lower sensitivities and lower peak temperatures compared with group A5 chondrules. It is argued that these data reflect the variety of processes accompanying chondrule formation. The chondrules show remarkably uniform contents of the highly labile elements, indicating mineralogical control on abundance and volatile loss from silicates and loss and recondensation of mobile chalcophiles and siderophiles in some cases. Very high D/H values (up to ～8000‰ SMOW) are observed in certain Semarkona chondrules, a confirmation of earlier work. With increasing petrographic type, mean TL sensitivities of the chondrules increase, the spread of values within an individual meteorite decreases, and peak temperatures and peak widths show trends indicating that the TL is mainly produced by feldspar and that dry, thermal metamorphism is the dominant secondary process experienced by the chondrules. The TL sensitivities of matrix samples also increase with petrographic type. Chainpur matrix samples show the same spread of peak temperatures and peak widths as Chainpur chondrules, indicating metamorphism-related changes in the feldspar are responsible for the TL of the matrix. The TL data for the Semarkona and Bishunpur matrix samples provide, at best, only weak evidence for aqueous alteration, but the matrix contains H with approximately terrestrial D/H values, even though it contains much water. Secondary processes (probably aqueous alteration) presumably lowered the D/H of the matrix and certain chondrules. While chondrule properties appear to be governed primarily by formation processes and subsequent metamorphism, the matrix of Semarkona has a more complex history involving aqueous alteration as a meteorite-wide process.  相似文献   

Nitrogen isotope geochemistry of basaltic glasses: implications for mantle degassing and structure?     

R.A. Exley  S.R. Boyd  D.P. Mattey  C.T. Pillinger 《Earth and Planetary Science Letters》1987,81(2-3)

The nitrogen isotope geochemistry of 15 basaltic glasses has been investigated using stepped heating and high sensitivity static vacuum mass spectrometry. At low temperature (< 600°C) the glasses release small amounts of nitrogen with δ¹⁵N_AIR, averaging −0.3‰, suggesting surficial adsorption of atmospheric nitrogen. At high temperature, usually with a maximum at 1000°C, indigenous nitrogen with a concentration ranging from 0.2 to 2.1 ppm is released. The δ¹⁵N values of this high temperature release show a wide range from −4.5‰ to +15.5‰. There is no correlation between N ppm and δ¹⁵N, and the samples apparently form 3 groups with distinctive δ¹⁵N. Six MORB glasses from the Mid-Atlantic Ridge, East Pacific Rise and Juan de Fuca Ridge define a group with δ¹⁵N = +7.5 ± 1.3‰. In contrast two Indian Ocean MORB glasses (Carlsberg Ridge and Gulf of Aden) gave negative δ¹⁵N averaging −3.2‰. Glasses from Loihi Seamount have high δ¹⁵N averaging +14.0 ± 1.0‰. Comparison of the δ¹⁵N data with the mantle models derived from helium and argon isotope studies suggests that the wide range in δ¹⁵N may reflect in part heterogeneities in the mantle related to its degassing history. It is possible, however, that magmatic degassing processes have also affected nitrogen isotopic compositions, and the data cannot yet be unambiguously interpreted in terms of source variations.  相似文献   

Multiple growth events during diamond genesis: an integrated study of carbon and nitrogen isotopes and nitrogen aggregation state in coated stones     

S.R. Boyd  D.P. Mattey  C.T. Pillinger  H.J. Milledge  M. Mendelssohn  M. Seal 《Earth and Planetary Science Letters》1987,86(2-4)

The internal structure and growth history of diamonds have been investigated by the study of carbon isotopes and the abundance, isotope composition and aggregation state of nitrogen. Polished plates of zoned diamonds (“coated stones”) were laser sectioned for infrared mapping and isotope profiling. Nine small cubic and three small octahedral diamonds from Zaïre and two coated stones and five small cubic diamonds specifically from the Mbuji Mayi area of that country were also investigated. The isotope composition and infrared characteristics of the diamonds of cubic habit, including coats, were found to be indistinguishable with δ¹³C in the range −5.9 to −7.5‰ and all the nitrogen being present as the paired structure (IaA) with a δ¹⁵N of between −3.1 and −8.0‰. The diamonds of octahedral habit, including the cores of the coated stones, were more variable: δ¹³C varied from −4.9 to −9.8‰. Infrared spectra of the octahedral diamond exhibited either strong IaA characteristics or a combination of IaA and IaB. Platelets were present in most cases. δ¹⁵N values from octahedral diamonds varied from −5.8 to +13.4‰. The data suggest that the octahedral diamonds represent a variety of growth events separated in time and/or space and that the diamonds resided in the lithosphere for a period sufficient for thermal maturation. In the case of the coated stones the octahedral diamonds were later involved in an event which added a component from a single relatively homogeneous reservoir; cubic habit diamonds were formed at the same time. This latter event is believed to be related to the kimberlite eruption. Comparison of the data with carbon isotope analyses of cubic diamond from other sources suggests that this reservoir could be widespread.  相似文献   

The Danebury Iron Age meteorite—An H5 ordinary chondrite “find” from Hampshire,England     

C. T. Pillinger  J. M. Pillinger  D. Johnson  R. C. Greenwood  A. G. Tindle  A. J. T. Jull  D. H. Allen  B. Cunliffe 《Meteoritics & planetary science》2014,49(6):946-957

What remains of a 30 g sample, first recognized as a meteorite in 1989 during characterization of metalworking debris from Danebury, an Iron Age hillfort, in Hampshire, England, has been classified as an H5 ordinary chondrite. Its arrival on Earth has been dated as 2350 ± 120 yr BP, making it contemporary with the period of maximum human activity at the recovery site. Despite its considerable terrestrial residence age, the interior of the specimen is remarkably fresh with a weathering index of W1/2. There is, however, no evidence of human intervention in its preservation. Its near‐pristine state is explained in terms of its serendipitous burial during the back‐fill of a pit dug into chalk by prehistoric people for the storage of grain. This chance discovery has interesting ramifications for the survival of meteorites in areas having a high pH because of a natural lime content arising as a result of the local geology.  相似文献