Robert Hutchison

The microstructure of Semarkona and Bishunpur

Abstract The ordinary chondrites, Semarkona and Bishunpur, appear to have experienced in situ hydrous alteration. Here we report the results of the first detailed TEM examination of them, which supports this conclusion. In Semarkona, all but the most magnesian mafic minerals have been destroyed, and primary sulphides, most likely troilite, have been partially altered to phyllosilicates and Ni-rich pyrrhotite. In Bishunpur, alteration is confined to the production of smectite from an amorphous feldspathic material in the chondrule rims and interchondrule matrix. During hydrous alteration of both meteorites, the temperature probably did not exceed 260°C. The equilibrium composition of the gas present during the alteration of Semarkona was not solar but was dominated by H2O. The formation of smectite does not require the presence of liquid water. However, evidence for the redistribution of elements, such as Ca, Zn, and Si, over tens of microns or more implies that there was at least some transient grain-boundary fluid present. The alteration in Semarkona and Bishunpur is far from complete. Thermodynamic modelling suggests that more altered ordinary chondrites would be mineralogically similar to the CM and CI carbonaceous chondrites. Superficially, the mineralogy, mineral chemistry, and bulk chemistry of Semarkona matrix-rims resemble those of some chondritic interplanetary dust particles (CIDPs). The ordinary chondrite parent bodies may, therefore, be a source of some CIDPs.

The Semarkona meteorite: First recorded occurrence of smectite in an ordinary chondrite, and its implications

Abstract Semarkona is an unshocked unequilibrated ordinary chondrite. Much of the interchondrule matrix and the mesostases of some chondrules and clasts have been hydrothermally altered. Analyses of altered materials and opaque matrix are presented. Ca and Fe have been redistributed from primary, anhydrous phases into calcite and Na, Fe smectite, respectively. In Semarkona the process possibly requires open system behavior inconsistent with the conventional view of “metamorphism” of the ordinary chondrites. The low petrologic type previously assigned to Semarkona is the result of retrograde metamorphism, under aqueous conditions, of a higher temperature mineral assemblage. Semarkona, and possibly Bishunpur, should be assigned to petrologic type 2.

Origin and evolution of the Nakhla meteorite inferred from the Sm-Nd and U-Pb systematics and REE, Ba, Sr, Rb and K abundances

Analyses of Sm-Nd and U-Th-Pb systematics, REE, Ba, Sr, Rb and K concentrations were carried out for whole rock and mineral separates from the Nakhla meteorite. The 1.26 ±.07 b.y. Sm-Nd age obtained in this work is in good agreement with those previously obtained by the Rb-Sr and Ar-Ar methods. The high initial ϵNd value of +16 suggests that Nakhla was derived from a light REE-depleted, old planetary mantle source. U-Th-Pb data, after correction for pre-analytical terrestrial Pb contamination assuming an age of 1.26 b.y., suggest that the age of the Nakhla source is ⩽4.33 b.y. The agreement in the age determined by three independent radiometric methods and the high initial ϵNd value strongly suggest that the 1.3 b.y. age dates one thorough igneous event in the parent body which not only reset these isotopic systems but also established the chemical and petrologic characteristics observed for the Nakhla meteorite. n nUsing a three-stage Sm-Nd evolution model in combination with LIL element data and estimated partition coefficients, we have tested partial melting and fractional crystallization models to estimate LIL element abundances in a possible Nakhla source. Our model calculations suggest that partial melting of the light REE-depleted source followed by extensive fractional crystallization (⩾50%) of the partial melt could account for the REE abundances in the Nakhla constituent minerals. The estimated source is depleted in the light REE, Ba, Rb and K and therefore may resemble the MORB source in the earths upper mantle or the upper 60–300 km of the moon. n nThe significantly younger age of Nakhla than the youngest lunar rock; the young differentiation age inferred from the U-Th-Pb data, and the estimated LIL element abundances (including those of K, U and Th) in the source suggest that the Nakhla meteorite may have been derived from a relatively large, well-differentiated planetary body such as Mars.

Origin of chondrule rims and interchondrule matrices in unequilibrated ordinary chondrites

Optical microscopy, electron probe microanalysis, scanning electron microscopy and analytical transmission electron microscopy were used to study the interchondrule matrix and opaque chondrule rims in Bishunpur, Krymka, Semarkona, Chainpur (all LL3), Tieschitz and Sharps (both H3). n nThe size-distribution of grains in matrix in Bishunpur obeys a power law. Rims around chondrules tend to be finer grained than matrix and may be layered. All rims and matrices studied contain clasts of forsteritic olivine and low-Ca pyroxene. In Bishunpur the clasts may be cemented by amorphous “glue”, rich in normative albite, and partly altered to smectite. Rims in Krymka, Sharps, Chainpur and Tieschitz have a densely packed olivine groundmass with a grain-size of about 0.1 μm. In Krymka, groundmass olivine is Fo20–30 and forms interlocking dendrites. Various Fe oxides are present in rims and matrices. Grain-size distribution and chemical data indicate that clastic olivines and pyroxenes are derived from chondrules. From analyses, matrices/rims in the LL-group apparently are enriched in a feldspathic component. Tieschitz rims are slightly enriched, but here the feldspathic component is present as white matrix. Sharps rims are not enriched in a feldspathic component. n nOpaque interchondrule matrices and chondrule rims probably formed from the products of fragmentation of chondrules, partly induced by contraction of protopyroxene on inversion to clinopyroxene. Fragmented silica-rich chondrule mesostases reacted with Fe oxides and Na to form the groundmass of fayalitic olivine and feldspathic “glue”. A low-temperature, nebular or pre-solar, component is limited to 3 vol.% of each meteorite, so equilibrium condensate was not the carrier of volatiles such as Tl and Bi.

The Bovedy meteorite; mineral chemistry and origin of its Ca-rich glass inclusions

Abstract The Bovedy meteorite fell on 25 April 1969 in Northern Ireland; the main mass of 4·94 kg was found at Bovedy (54°57′N, 06°37′W). It is an L3 chondrite with abundant chondrules clearly visible in hand specimen. Bulk chemical analyses are presented, the total Fe content being 22·5%. The olivines are homogeneous (Fa 24 ) but the pyroxenes are not equilibrated (Fs 8–28 ). Brown glass is common within chondrules but a clear glass of composition An 85 is present interstitially in a few orthopyroxene-rich (Fs 17–28 ) chondrules. A bleb, 2 mm across, of clear glass, again of composition An 85 was found in one stone of the meteorite and in the glass five REE (rare earth elements), (La, Sm, Eu, Yb, Lu) were determined. The low REE abundances coupled with a large positive Eu anomaly are characteristic of plagioclase, but the finer details of the pattern suggest that this glass has a closer affinity to the lunar anorthosites than to plagioclases from lunar mare basalts or eucritic meteorites. There is also evidence that the magnitude of the Eu anomaly for plagioclases and anorthosites from extra-terrestrial sources is inversely related to trivalent REE content. The existence of anorthositic material and, as a consequence, a differentiated planetary body prior to the formation of the Bovedy meteorite is suggested.

Chondrules: chemical, mineralogical and isotopic constraints on theories of their origin

Chondritic meteorites have not been involved in planetary melting for the past 4.5 Ga. They contain millimetre-sized subspherical objects, ‘ chondrules ’, dominantly of silicate, but occasionally grading to rare, metal-sulphide chondrules. Some meteorites comprise 70% or more of chondrules by volume. About 30% of chondrules, or less, originated as molten droplets; the remainder formed from solid or plastic rocks. No rock known from the Earth or Moon contains enough glassy spherules to be termed a chondrite, so volcanism and impact on brittle planetary surfaces were not chondrule-forming mechanisms. Chondrites have chemical compositions close to that of the volatile-free Sun, so a nebular origin for chondrules is often favoured. However, the mineralogical and chemical diversity among chondrules indicates that some had planetary origins. Furthermore, chondrules co-existed in space with disrupted fragments of planetary igneous rocks. Some chondrules may be nebular but others are not. Alternatively, all may be planetary. In either case the mechanism (s) of formation is (are) unknown.

Strontium and lead isotopic ratios, heterogeneous accretion of the Earth, and mantle plumes

Abstract If the Earth formed by accretion of volatile-rich material on to a refractory, primitive body with incomplete subsequent mixing, old or deep-seated igneous rocks should exhibit an anti-correlation between radiogenic Pb and radiogenic Sr. The ancient Amitsoq gneiss and some modern oceanic rocks appear to comply with this model. Mantle plumes should not be a source of radiogenic Sr.