Mohammed Ikramuddin

Thermal metamorphism of primitive meteorites—V. Ten trace elements in Tieschitz H3 chondrite heated at 400–1000°C

We determined ten trace elements by neutron activation analysis in Tieschitz (H3) chondrite powder heated in a low-pressure environment (initially ~ 10−5 atm H2) for 1 week at 100°C increments from 400–1000°C. Of these, Co seems unaffected by heating, 20% of Ga is lost only at 1000°C and losses of other elements progress with temperature to extremes of 25% for Se, 75% for Cs and 90–97% for Ag, Bi, In, Te, Tl and Zn. Treating elemental mobilization as kinetically-controlled by diffusion from spherical grains of uniform size, Ag, Cs, In and Se are lost from a single site by a single process while Bi, Te, Tl and Zn are lost from two sites or from one site by different processes at high and low temperatures. Magnitudes of apparent activation energies for loss of the first four elements at all temperatures and the last four at low temperatures are consistent with volume diffusion; at high temperatures Bi, Te, Tl and Zn are lost by a low-energy process, like desorption. We compared trace element abundances, patterns of statistically-significant correlations, factor analysis and two-element correlations between Tieschitz and heated Krymka (L3) and, except for factor analysis, “as-received” H3–6 chondrites. Trends for heated ordinary chondrites are similar though small differences occur; those for Tieschitz and H3–6 chondrites differ markedly indicating that H3–6 chondrites—unlike E3–6 chondrites—probably escaped substantial open-system metamorphism. Sharp contrasts in pictures for E-, L- and H-group chondrites indicate substantial differences in genetic histories.

Thermal metamorphism of primitive meteorites. I - Variation of six trace elements in Allende carbonaceous chondrite heated at 400-1000 C

Determination by neutron activation of 6 trace elements retained in Allende (C3) samples heated at 400–1000°C for 1 week in a low-pressure (initially ~10−5 atm H2) atmosphere reveals loss of small proportions of Ga and Se and large proportions of Bi, In and Tl-Co being unaffected. The retentivity patterns for the 5 volatile elements differ and in no way duplicate a step-function. In contrast to these trace elements, sulfur is initially present in discrete mineral(s) and visually it appears to be released over a narrow temperature range. Elements are lost more easily from powder than from chips but the difference is ≤35 per cent. Above 600°C, the process of loss appears due to process(es) with apparent activation energies of 2 kcal/mole (Bi, Tl), 4 kcal/mole (Se) and 22 kcal/mole (In). Loss of Bi, Se and Tl below 600°C involves higher apparent activation energies. Two-element correlation diagrams involving Bi, In and Tl are consistent with the idea that trends among highly-volatile elements in enstatite chondrites arise from metamorphism.

Thermal metamorphism of primitive meteorites. III - Ten trace elements in Krymka L3 chondrite heated at 400-1000 C

Abstract Ten trace elements were determined by neutron activation analysis in Krymka (L3) chondrite samples heated for 1 week, at 100°C increments, from 400 to 1000°C in a low-pressure environment (initially 10 −5 atm H 2 ). As in other samples studied. Co seems unaffected by heating; ~50% of Cs and Ga are lost only at 1000°C and losses of other elements increase with temperature to extremes of ∼ 25% for Se and 95–99% for Ag, Bi, In, Te, T1 and Zn. Where comparison is possible, ‘open-system’ losses are generally in the order Krymka (L3) > Abee (E4) > Allende (C3). Treating elemental mobilization as representing a kinetic process involving diffusion from spherical grains of uniform size Ag, Bi, In, Te, Tl and Zn are lost from a single host phase or by a single process. This differs from trends shown by many of these elements in Abee and Allende and terrestrial basalt BCR-1. Loss of Tl apparently involves a process with a low activation energy, perhaps desorption. Loss of other elements apparently reflects diffusion-controlled process(es). Trace element contents, patterns of statistically significant interelement relationships, factor analysis and two-element correlation diagrams for unheated and heated Krymka and ‘as-received’ L3-6 chondrites are very different. Thus, significant open-system metamorphism during the genesis of L-group chondrites are not supported by these data. This contrasts sharply with the picture for enstatite chondrites, indicating substantial differences in the origin of various chondritic groups.

Thermal metamorphism of primitive meteorites—II. Ten trace elements in Abee enstatite chondrite heated at 400–1000°C

Abstract We used neutron activation analysis to determine ten trace elements retained in Abee (E4) samples heated at 400–1000°C for 1 week in a low-pressure (initially ~ 10−5atm H2) environment. Eight elements generally are lost progressively with increasing temperature although gas(es) evolved from the samples apparently affect retention of some elements. In the extreme, ‘open-system’ losses are: Se—23%, Cs—40%; Te—87%; Ag, Bi, In, Tl, Zn— ≥93%. Under these conditions Co is not lost; Ga is lost only at 1000°C. At 900°C elements are lost from Abee chips in the same relative order as from Abee powder but the loss is somewhat less facile. Three of the most mobile elements—Bi, In, Tl—are lost more readily from Abee than from Allende (C3), the only other primitive chondrite studied to date. Assuming that elemental loss is a kinetic process involving mobilization from spherical grains, Bi, In, Se, Tl and Zn have different activation energies at high and low temperatures either because each element was originally present in two different sites or each has more than one loss mechanism (diffusion or desorption) in different temperature ranges. Comparison of elemental abundance patterns, patterns of statistically-significant correlations, factor analysis results and two-element correlation diagrams indicate strong similarities between heated Abee and ‘as-received’ enstatite chondrites for mobile elements. These results are consistent with a two-stage evolutionary model for enstatite chondrites involving condensation of cosmochemically fractionated primitive nebular material and subsequent loss of mobile elements from parent material by metamorphism.

Trace elements in primitive meteorites—VI. Abundance patterns of thirteen trace elements and interelement relationships in unequilibrated ordinary chondrites

Abstract Neutron activation analysis was used to determine As, Au, Bi, Cd, Co, Cu, Ga, In, Sb, Se, Te, Tl and Zn in 13 different unequilibrated ordinary chondrites (UOC), i.e. those having chemicallyinhomogeneous silicates. This study together with prior data completes our coverage of this group of 23 primitive chondrites. Four elements are quite variable in UOC (Cd—20 x, In—30 x, Bi—300 x and Tl—1300 x), the others varying by 2–8 x. Three highly-depleted elements—Bi, In and Tl—are richer by 5–35 x in unequilibrated chondrites than in their equilibrated congeners. All 3 elements vary directly in characteristic fashion with disequilibrium parameters for olivine and pyroxene in UOC and generally with petrologic type 3 > 4 > 5 > 6. The data do not provide unambiguous evidence for nebular fractionation of siderophile elements. Examination of statistically-significant interelement relationships among various ordinary chondrite populations involving 34 elements reveals patterns distinct from those of other chondritic groups. These patterns reflect nebular metal-silicate fractionation which preceded or accompanied thermal fractionation. The results point to significant differences in the formation of primitive carbonaceous, enstatite and ordinary chondrites.

Rb-Sr ages of Precambrian dolerite and alkaline dikes, southeast Mysore state, India

Abstract Rb-Sr isochron ages have been determined for two suites of Precambrian dikes in the Bidadi-Harohalli area of southeast Mysore State. Whole-rock samples of unmetamorphosed dolerites yield an age of 2420±246 (2σ) m.y., which is a minimum value for the intruded Peninsular Gneiss and Closepet Granite. The dolerite magma originated in the mantle, as indicated by the initial 87 Sr/ 86 Sr ratio of 0.7012±0.0010 (2σ). A suite of alkaline dikes, also referred to as felsite and feldspar porphyry dikes, has an age of 832±40 (2σ) m.y., which correlates with the intrusion of the Chamundi Hill Granite and the feldspar porphyry dikes near Srirangapatnam. One of the alkaline dikes has a K-Ar age of 810±25 m.y., indicating an absence of subsequent thermal events in the area.

Contents of eleven trace elements in ureilite achondrites

Abstract We determined Ag, Bi, Cd, Co, Cs, Ga, In, Se, Te, Tl and Zn in the 6 ureilite achondrites by neutron activation analysis. All 11 elements are depleted below Cl levels and their characteristic abundance pattern differs substantially from those of chondritic groups. Thus ureilites do not represent a simple mixture of volatile-rich chondrites with achondritic material but perhaps cosmochemically-fractionated achondritic material and a late ‘distillate’ of mobile elements.

Thermal metamorphism of primitive meteorites—IX. On the mechanism of trace element loss from Allende heated up to 1400° C

Abstract We report data for Ag, Bi, Cd, Co, Cs, Ga, In, Se, Te, Tl and Zn determined by neutron activation in Allende samples heated for one week at 100° increments in the 1000–1400°C range in a low pressure (initially 10−5atmH2) environment using an apparatus of novel design. In the extremes, concentrations of these trace elements—initially present at ppm-ppb levels—in unheated material are lowered even farther by factors of 10−4–10−5 over a broad temperature span. Loss of some elements above 1000°C extends trends evident below 1000°C; loss of others is even more extreme. On Arrhenius diagrams some elements exhibit but one apparent activation energy over the entire temperature span of loss while others exhibit 2 or 3, each operative in a particular temperature region. These discontinuities seem related to mineralogic/petrologic alteration and probably reflect differences in diffusion mechanism rather than siting differences. The extension of previous experiments into the temperature regime postulated for chondritic differentiation should lead to a better understanding of the evolution of meteoritic parent bodies.

Effect of thermal metamorphic conditions on mineralogy and trace element retention in the Allende meteorite

HEATING carbonaceous chondrite from the Allende meteorite in a low pressure environment causes visible mineralogical alteration at 700–1,000° C but not at T ≤ 600° C. Samples heated for 29 d at 500° C lose trace elements (Bi, In and Tl) more effectively than those similarly heated for 7 d. At 1,000° C with ∼ 10−5 atm initial pressure of O2, H2 or He these elements, Ga and Se are comparatively more completely lost.