Lewis Cliff Research Articles

Experimental partitioning of rare earth elements and strontium: Constraints on petrogenesis and redox conditions during crystallization of Antarctic angrite Lewis Cliff 86010

The partitioning of REE and Sr among anorthite, fassaitic pyroxene, and synthetic melts similar in bulk composition to angrite LEW 86010 was studied experimentally at 1 atm and 1175–1210°C using the percent level doping technique. Most experiments were at an oxygen fugacity 1 log unit above the iron-wüstite buffer, but Eu and Gd partitioning were studied from iron-wüstite to just above quartz-fayalite-magnetite. Pyroxene partition coefficients are correlated with Al content of the pyroxene. Despite the fassaitic nature of the synthetic pyroxenes, partition coefficients are not dramatically different from those for diopside. Pyroxene/melt REE partition coefficients range from .08 for La to .45 for Yb. Plagioclase/ melt coefficients (except for Eu) range from .022 for La to .004 for Yb. D Eu varies by nearly a factor of 2 for pyroxene and nearly a factor of 5 for plagioclase over the four log unit range fO 2 studied. Parent melts calculated by inverting natural pyroxene and anorthite cores from LEW 86010 using partition coefficients from this study are in excellent agreement with one another. This agreement is strong evidence for (1) equilibrium between the natural mineral cores at the time the meteorite crystallized, and (2) lack of subsequent subsolidus diffusive modification of REE abundances in the cores. The overall levels of the computed REE patterns agree well with REE abundances in bulk samples of LEW 86010, supporting the idea that this sample formed through a process approximating closed-system fractional crystallization. The observed variation of D Eu / D Gd with fO 2 for pyroxene and plagioclase was combined with Eu and Gd abundances from mineral cores in LEW 86010 to estimate the oxygen fugacity under which this sample crystallized. Results indicate crystallization at about 1 log unit above iron-wüstite, considerably more oxidizing than conditions under which common basaltic achondrites such as eucrites are thought to have formed.

The natural thermoluminescence of meteorites: 5. Ordinary chondrites at the Allan Hills Ice Fields

Natural thermoluminescence (TL) data have been obtained for 167 ordinary chondrites from the ice fields in the vicinity of the Allan Hills in Victoria Land, Antarctica, in order to investigate their thermal and radiation history, pairing, terrestrial age, and concentration mechanisms. Using fairly conservative criteria (including natural and induced TL, find location, and petrographic data), the 167 meteorite fragments are thought to represent a maximum of 129 separate meteorites. Natural TL values for meteorites from the Main ice field are fairly low (typically 5–30 krad, indicative of terrestrial ages of ∼400 ka), while the Farwestern field shows a spread with many values 30–80 krad, suggestive of < 150‐ka terrestrial ages. There appear to be trends in TL levels within individual ice fields which are suggestive of directions of ice movement at these sites during the period of meteorite concentration. These directions seem to be confirmed by the orientations of elongation preserved in meteorite pairing groups. The proportion of meteorites with very low natural TL levels (<5 krad) at each field is comparable to that observed at the Lewis Cliff site and for modern non‐Antarctic falls and is also similar to the fraction of small perihelia (<0.85 AU) orbits calculated from fireball and fall observations. Induced TL data for meteorites from the Allan Hills confirm trends observed for meteorites collected during the 1977/1978 and 1978/1979 field seasons which show that a select group of H chondrites from the Antarctic experienced a different extraterrestrial thermal history to that of non‐Antarctic H chondrites,

The meteorite collection sites of Antarctica

The meteorite collection sites of Antarctica

Mineralogy, petrology and geochemistry of carbonaceous chondritic clasts in the LEW 85300 polymict eucrite

Abstract— We have performed a detailed petrologic and mineralogic study of two chondritic clasts from the polymict eucrite Lewis Cliff (LEW) 85300, and performed chemical analyses by INAA and RNAA on one of these. Petrologically, the clasts are identical and are composed of dispersed aggregates, chondrules and chondrule fragments supported by matrix. The aggregates and chondrules are composed of olivine (Fo100–45), orthopyroxene (Wo1–2En98–60), plus some diopside. The matrix consists of fine‐grained olivine (Fo60–53), and lesser orthopyroxene and augite. Fine‐grained saponite is common in the matrix. The bulk major element composition of the matrix is identical in both clasts and similar to that of CM, CO and CV chondrites. The bulk composition of the clast studied by INAA and RNAA shows unusual abundance patterns for lithophile, siderophile and chalcophile elements but is basically chondritic. The INAA/RNAA data preclude assignment of the LEW 85300,15 clast to any commonly accepted group of carbonaceous chondrite. The unusual rare earth element abundance pattern may, in part, be due to terrestrial alteration.

The natural thermoluminescence of meteorites 4. Ordinary chondrites at the Lewis Cliff Ice Field

Natural thermoluminescence (TL) measurements have been made on 302 meteorites from the vicinity of the Lewis Cliff in the Beardmore region of Antarctica. The data provide information on terrestrial age and unusual radiation and thermal histories, which, in turn, are helpful in identifying fragments of a single fall and in understanding ice sheet movements and the mechanisms by which meteorite concentration occurs at this site. The present data with data for induced TL, class, find location, hand‐specimen descriptions and mineral composition have enabled 70 of the present samples to be assigned to 27 groups of “paired” meteorites, with between 5 and 2 meteorites in each group. The distribution of meteorites on the ice, the shape of the fields of “paired” meteorites, and trends in the natural TL data indicate that there is a western component to the movement of the ice at this location, as well as the previously supposed movement to the north. This western vector probably explains the concentration of meteorites along the western edge of the ice tongue. Meteorites at the northern end of the tongue (the Lower Ice Tongue), and Meteorite Moraine to the east, have relatively high natural TL, and therefore young terrestrial ages, while those on the upper tongue show a broad range of ages including a great many large ages. Details of the meteorite concentration are different on the Upper and Lower Tongue and it might be that these two parts of the ice sheet are unrelated. These new natural TL data identify several recent falls and several meteorites which probably had unusually small perihelia immediately prior to capture by the earth.

Age and isotopic relationships among the angrites Lewis Cliff 86010 and Angra dos Reis

Results of a wide-ranging isotopic investigation of the unique Antarctican angrite LEW-86010 (LEW) are presented, together with a reassessment of the type angrite Angra dos Reis (ADOR). The principal objectives of this study are to obtain precise radiometric ages, initial Sr isotopic compositions, and to search for the erstwhile presence of the short-lived nuclei 146Sm and 26Al via their daughter products. The isotopic compositions of Sm, U, Ca, and Ti were also measured. This allows a detailed appraisal to be made of the relations between, and the geneology of, these two angrites. LEW proves to be severely contaminated with modern terrestrial Pb, which is shown to result from terrestrial weathering. Nevertheless, concordant Pb-Pb model ages of pyroxene separates were obtained (2σ): 4.55784 ± 52 Ga for LEW and 4.55780 ± 42 Ga for ADOR. Uranium isotopic compositions are normal within error. The inferred initial Pb isotopic composition is within error of primordial Pb, as defined by Cañon Diablo troilite. Because of the extreme U Pb ratios of both angrites, this places an upper limit of ~2 Ma on the time between volatile element-loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates. Initial 87Sr 86Sr ratios were found to be indistinguishable in LEW, ADOR, and the cumulate eucrite Moore County. The derived initial 87Sr 86Sr for all three meteorites is 0.698970 ± 15 (2σ). A 147Sm- 143Nd isochron for LEW was obtained, yielding an age of 4.553 ± 34 Ga with an initial 143Nd 144Nd of 0.506682 ± 49 (2σ). 146Sm 142Nd isotope systematics were also measured. Anomalies on 142Nd arising from extinct 146Sm were clearly resolved, resulting in an initial 146Sm 144Sm of 0.0071 ± 17 with ϵ Nd 142 = −2.57 ± 0.62 at the time of isotopic closure. Both of these Sm-Nd methods imply derivation of LEW from a reservoir with chondritic Sm Nd ratio; they are also quite consistent with the previously reported systematics for ADOR. Overall, the age and isotopic similarities between LEW and ADOR are striking; it suggests almost simultaneous production on the same asteroid, even though recent experimental studies imply that the two are not comagmatic. Calcium and titanium do not exhibit enrichments in the n-rich isotopes, in contrast to CAIs, making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula. No evidence was found for live 26Al in LEW. When combined with the Pb-Pb age and initial 87Sr 86Sr data, this firmly excludes 26Al being an important heat source in the early solar system; heat supplied for, for example, (1) the differentiation of the angrite and eucrite parent bodies, and (2) metamorphism of the chondrite parent bodies must come from some other source. The debate over live vs. extinct 26Al in the earliest solar system remains unresolved by this new data from LEW, however. Both the angrite and eucrite parent bodies have extremely low Rb Sr and high U Pb ratios compared to the solar nebula. Therefore, the common initial 87Sr 86Sr of 0.698970 can be firmly associated with the U-Pb angrite age above, and the formation of both bodies. This enables an absolute chronology of the early solar system to be established. Based upon 87Sr 86Sr differences, the oldest CAIs are a minimum of 11 ± 4 Ma older than the angrites, i.e., 4.569 ± 5 Ga. The age and isotopic constraints are discussed with respect to current collapse, condensation, and accretion timescales calculated for the solar nebula.

Comparative xenology of two angrites

We report the results of Xe analyses in stepwise heating of the angrite meteorite Lewis Cliff 86010 and of a comparative study of Xe component structures in this meteorite and in the first known angrite, Angra dos Reis. Both meteorites are poor in trapped Xe and rich in fission Xe from 244Pu and in spallation Xe produced by cosmic ray irradiation of Ba and the light rare earth elements (REE). Neither meteorite shows evidence for the presence of excess 129Xe derived from 129I, presumably because the volatile element iodine is depleted along with other volatiles in the angrites. Both meteorites show pronounced variations in the composition of spallation Xe attributable to separation of the Ba and REE components. In both meteorites 244Pu-derived fission Xe is correlated with REE spallation, consistent with Pu-REE coherence in igneous partitioning. The Ba spallation Xe composition in Lewis Cliff 86010 differs somewhat from that in Angra dos Reis, primarily in a lower relative yield of the neutron sensitive isotope 131Xe. This suggests a lower neutron flux during cosmic ray exposure for the Lewis Cliff Meteorite, an effect opposite to that suggested by neutron capture effects in Sm. Resolution of these observations appears to require a complex exposure history for at least Angra dos Reis. The 244Pu/REE ratio in Lewis Cliff 86010 is only about half that in Angra dos Reis. This difference probably does not reflect a difference in the time of Xe retention. Unless this difference arose in igneous processes despite the apparent chemical coherence of Pu and Ree in mineral partitioning, it must have characterized the parent magmas and/or precursor materials from which these two meteorites were derived.

Lewis Cliff 85332: A unique carbonaceous chondrite

Abstract— Lewis Cliff 85332 (LEW85332) is a highly unequilibrated (type 3.0–3.1) unique carbonaceous chondrite. It resembles CI and “CR” chondrites in its abundance ratios of refractory lithophiles and refractory siderophiles, but differs significantly from these groups in important ways: relative to CI chondrites, LEW85332 has low abundances of Mn, Se, Zn and most volatile siderophiles; relative to “CR” chondrites, LEW85332 has high abundance ratios of Mn and most volatile siderophiles. Although several petrologic characteristics of LEW85332 resemble those of CO chondrites, LEW85332 differs from this group in having lower abundance ratios of refractory lithophiles and higher abundance ratios of common and volatile siderophiles. Chondrules (mean diameter of 170 μm) are smaller than those in CV and CM chondrites and bigger than those in most CO chondrites. Two melilite‐rich (Åk 22) fluffy type‐A refractory inclusions were observed. Weathering of LEW85332 has resulted in the formation of 6.2 vol.% limonite; 3.9 vol.% metallic Fe‐Ni remains. The inferred original metallic Fe‐Ni abundance (13–15 wt.%) is very high for a carbonaceous chondrite and is most similar to those of Kainsaz and Colony (both CO3).LEW85332 is a breccia: the one thin section we examined contains (a) ≥ 10 primitive carbonaceous chondrite clasts (with both C1 and C2 affinities) that contain magnetite framboids and platelets, (b) two clasts containing numerous 10‐μm‐size clusters of troilite grains, and (c) one clast containing small needles of schreibersite embedded in fine‐grained silicate matrix. The unique nature of LEW85332 underscores the wide diversity of materials produced in the solar nebula.

Rare earth elements in Angra dos Reis and Lewis Cliff 86010, two meteorites with similar but distinct magma evolutions

Rare earth element (REE) and selected trace element abundances were measured, with an ion microprobe, in individual grains of merrillite, fassaite, olivine, kirschsteinite and plagioclase of LEW 86010, the second known angrite, and compared with measurements in merrillite and fassaite of Angra dos Reis (ADOR). The rare merrillite is highly enriched in REE; the REE patterns show steep decreases from La ∼ 3,300 × Cl to Tm ∼ 100 × Cl in LEW 86010, and from La ∼ 1,500 × Cl to Tm ∼ 130 × Cl in ADOR. REE patterns in fassaite of both meteorites are similar although concentrations are higher in ADOR. Whereas ADOR pyroxenes show a high degree of equilibration, the LEW 86010 pyroxenes show much variation in composition. The abundances of the REE, Zr, Al and Ti correlate positively with Fe/Fe + Mg and those of Cr, Sc and V correlate negatively, consistent with an igneous origin in a closed system. Pyroxene and plagioclase cores in LEW 86010 formed in equilibrium with a melt whose composition is close to that of the meteorite bulk composition. The ADOR melt had higher REE concentrations but a lower Fe/Mg ratio than that of LEW 86010. Despite numerous common characteristics, the two meteorites were produced in separate magmatic events involving similar but distinct processes and parent melts. The unusual angrite composition is not due to purely nebular processes.

Shadow of The Indian in the Fiction of John Steinbeck

Journal Article Shadow of The Indian in the Fiction of John Steinbeck Get access Carroll Britch, Carroll Britch Springfield College University of Lowell Search for other works by this author on: Oxford Academic Google Scholar Cliff Lewis Cliff Lewis Springfield College University of Lowell Search for other works by this author on: Oxford Academic Google Scholar MELUS, Volume 11, Issue 2, June 1984, Pages 39–58, https://doi.org/10.2307/467070 Published: 01 June 1984