Joyce C. Brannon

Correlated study of initial 87Sr86Sr and AlMg isotopic systematics and petrologic properties in a suite of refractory inclusions from the Allende meteorite

We have studied a suite of six coarse-grained Ca-Al-rich inclusions (CAIs) from the Allende meteorite, applying in concert detailed petrographic and chemical characterization, ion probe mass spectrometric analyses of the Al-Mg isotopic system to determine the abundance and distribution of 26A1, and thermal emission mass spectrometric analyses of the Rb-Sr system to determine initial 87Sr86Sr ratios. The simplest inclusion is a Type Bl, USNM 3529-Z, that shows evidence of minor alteration and recrystallization since solidification from a melt droplet; e.g., some of the coarse-grained anorthite in the inclusion (especially near the inclusion rim) apparently formed by recrystallization or melting of fine-grained secondary anorthite that had replaced melilite. Four other Type Bs contain evidence of more widespread recrystallization in addition to evidence of local replacement of melilite by coarse anorthite; e.g., melilite zoning is complex and not igneous in origin, and in one inclusion (USNM 3529-21) melilite and pyroxene crystals enclose relicts of an earlier generation of the same phases. The sixth inclusion, USNM 3898, is a Type A whose properties appear to be largely metamorphic in origin. All six inclusions show variations in Mg isotopic compositions indicating that they incorporated 26A1. In 3529-Z the relative abundance of 26A1 (26Al27Al = (4.0 ± 0.1) × 10−5) is close to the “canonical” value (26Al27Al = 5 × 10−5) for CAIs, but both anorthite and melilite show minor departures from a strict isochron relationship between Mg and Al. In particular, the “coarse” anorthite near the inclusion rim has initial 26Al27Al ratios of 1−2 × 10−5, implying formation at least 1.5 million years after the “first-generation” anorthite. The other four Type Bs exhibit larger disturbances of the Al-Mg system that can be understood in terms of local recrystallization and isotopic exchange, primarily between anorthite and melilite. Only 3898 is consistent with a strict Al-Mg isochron relationship, but this CAI lacks primary anorthite, and the small range of Al/Mg among the phases analyzed may preclude resolution of any small isotopic disturbances. The RbSr isotopic system also typically indicates some level of disturbance, some of which must have been relatively recent. Nevertheless, phases characterized by very low Rb/Sr permit precise identification of initial 87Sr86Sr. Our Al-Mg and Sr data are largely consistent with a simple chronological interpretation of both systems; i.e., the 26Al results suggest that all the initial 87Sr86Sr ratios should have the same value and, with one exception, our initial 87Sr86Sr ratios are consistent with a single value; this value is somewhat higher than the primitive value ALL reported by Gray et al. (1973), however. Also with one exception, the very low Rb/Sr CAI data reported by Gray et al. (1973) are consistent with the same value. The exception is the same inclusion in both studies; our analysis of USNM 3898 yields an initial 87Sr86Srhigher than that of the other CAIs, while Gray et al. (1973) obtained an initial 87Sr86Srlower than that of the other CAIs for the same inclusion (labeled as D7, on which ALL is based). Neither result can be readily explained as a chronological aberration, since 3898 has nearly canonical 26Al27Al. The present data are the first which establish a firm association between primitive 26Al27Al and primitive 87Sr86Sr by analysis of both isotopic systems in the same inclusions. The comparison of both isotopic systems, interpreted as simple nebular chronometers, does not reveal any chronological inconsistencies that demand resolution in terms of a grossly heterogeneous distribution of 26Al.

87Sr/86Sr ratios in Permo-Carboniferous sea water from the analyses of well-preserved brachiopod shells

Abstract Sr isotopic analyses of well-preserved portions of Permo-Carboniferous brachiopods distributed globally confirm the general shape of the Sr isotope age curve established by previous workers for this time interval. There is little variation between the Sr isotopic composition of unaltered portions of brachiopods and that of portions of the same shell interpreted to be diagenetically altered (based on cathodoluminescence, elemental, and stable isotopic data). However, the Sr isotopic composition in diagenetically altered micritic matrix adjacent to the shell is more radiogenic. The Sr isotopic composition in the unaltered portions of calcitic megafossils has potential as a stratigraphie tool.

Rare earth geochemistry of fused ophiolitic and alpine lherzolites

Partial fusion hypotheses have been proposed for the origin of lherzolite-harzburgite alpine peridotite associations. Analyzed lherzolites from Othris, Ronda, Lanzo and Beni Bouchera, have light REE depleted to chondritic REE abundances, and clinopyroxenes contain most of the REE relative to depleted olivine and orthopyroxene. Variation in the level of REE enrichment within these lherzolites indicates mantle heterogeneity probably caused by partial melting processes. The Beni Bouchera spinel lherzolite and the Othris plagioclase lherzolite are the best candidates for relatively “undepleted” mantle based on REE studies. Fractional fusion calculations (15–25%) reveal that partial melts have REE characteristics somewhat similar to oceanic tholeiites. Conversely, computed source peridotites from oceanic tholeiites (Schilling, 1975) are similar to the alpine lherzolites reported here. Alpine lherzolites are, however, depleted in trace elements (K, Rb, Sr and Ba, Menzies and Murthy 1976). Since the lherzolites have an undepleted major, minor and REE chemistry close to that of pyrolite, the lost trace element-rich fraction must represent a small degree of melting. It is proposed that alpine lherzolites are residue left after the loss of a nephelinitic/alkalic fraction, ([Ce/Yb]N=2.0–4.01) representing a small degree of partial fusion. This labile fraction may have existed as an intergranular phase or hydrous mineral prior to melting.

Trace element distribution in mineral separates of the Allende inclusions and their genetic implications

Abstract Concentrations of the REE, Sc, Co, Fe, Zn, Ir, Na and Cr were determined by instrumental neutron activation and mass spectrometric isotope dilution analysis for mineral separates of the coarseand fine-grained types (group I and II of Martin and Masons classification) of the Allende inclusions. These data, combined with data on mineral/liquid partition coefficients, oxygen isotope distributions and diffusion calculations, suggest the following: (1) Minerals in the coarse-grained inclusions (group I) crystallized in a closed system with respect to refractory elements. On the other hand, differences in oxygen isotope distributions among minerals preclude a totally molten stage in the history of the inclusion. Group I inclusions were formed by rapid condensation (either to liquid or solid) in a supercooled solar nebula; extrasolar pyroxene and spinel dust were included but not melted in the condensing inclusions, thus preserving their extrasolar oxygen isotope composition. REE were distributed by diffusion during the subsequent heating at subsolidus temperatures; because oxygen diffuses much more slowly at these temperatures, the oxygen isotope anomalies were preserved. (2) The fine-grained (group II) inclusions were also formed by condensation from a super-cooled nebular gas; however, REE-rich clinopyroxene and spinel were formed early and REE-poor sodalite and nepheline were formed later and mechanically mixed with clinopyroxene and spinel to form the inclusions. The REE patterns of the bulk inclusions and the mineral separates are fractionated, indicating that REE abundances in the gaseous phase were already fractionated at the time of condensation of the minerals. (3) Pre-existing Mg isotope anomalies in the coarse-grained inclusions must have been erased during the heating stage thus resetting the 26 Al- 26 Mg chronometer.

Major, minor, and trace element compositions of peridotitic and basaltic komatiites from the precambrian crust of Southern Africa

Major and trace element compositional data are reported for nine mafic and ultramafic rock samples from the Barberton greenstone belt. Rocks from this province are among the oldest fragments of the Earths crust (∼3.5 b.y.). The data are consistent with an oceanic crust related origin for these rocks. The high abundances of Ni in these samples make their origin by fractional crystallization of a primitive magma unlikely but are consistent with their generation by partial melting of an upper mantle source. The basaltic samples from the Komati formation can be related by small degrees of partial melting of a primitive upper mantle source to the peridotitic komatiite which probably derived from much more extensive partial melting of a similar source. REE and especially Ni abundances limit the proportion of olivine that is permitted in the residue.

Strontium isotopic constraints on the origin of ore-forming fluids of the Viburnum Trend, southeast Missouri

Abstract We have measured 87 Sr 86 Sr and Rb and Sr concentrations in several minerals, primarily sulfides, spanning the paragenesis of hydrothermal mineralization in the Viburnum Trend in southeast Missouri. Separate measurements were made for fluid inclusions opened by crushing or thermal decrepitation and for the solids. For comparison, measurements were also made on samples of probable local aquifers, the Bonneterre Formation and the Lamotte Sandstone. For some of the samples, concentrations of K, Ca, Cl, Na, and Mg are also reported. In several cases 87 Sr 86 Sr ratios are different (higher) in the solids than in the fluid inclusions. We have investigated the possibility that either type of sample gives spurious results, e.g., that the fluid inclusions are secondary or contaminated by host dolomite, or that Sr in the solids reflects a detrital rather than an authigenic source. Consideration of mass balance, overall solute chemistry, and examination of non-sulfide dissolution residue, however, suggests that both types of sample reflect primary fluid Sr composition. We thus adopt the working hypothesis that Sr isotopic composition in fluids at the time of hydrothermal mineralization was highly variable. The observed results for the Viburnum Trend do not conform well to expected trends for Mississippi Valley-type (MVT) mineralization based primarily on analyses of gangue carbonates, barite, and fluorite. Fluid inclusion Sr in some of the Viburnum Trend samples is more radiogenic than in the host dolomite but only moderately so; in other samples, notably main-stage octahedral galena, fluid inclusion Sr composition is within the range observed for the host dolomite. In contrast, Sr in some of the sulfides is very radiogenic, much more so than previously reported for MVT minerals, and is very radiogenic early in the paragenesis (pyrite, chalcopyrite, sphalerite), less radiogenic during main-stage ore deposition, and again more radiogenic in later paragenetic stages.

Th-Pb and U-Pb Dating of Ore-Stage Calcite and Paleozoic Fluid Flow

Thorium-232-lead-208 and uranium-238-lead-206 radiometric ages for ore-stage calcite show that Mississippi Valley-type (MVT) ore deposits can form in distinct tectonic settings. An age of 251 ± 11 million years for the Jumbo Mine in Kansas, United States, is in agreement with other ages for MVT deposits in the midcontinent of North America. The similarity of ages of these deposits supports the concept that they formed in response to fluid flow during the late Paleozoic Alleghenian-Ouachita orogeny. An age of 351 ± 15 million years for Twelve Mile Bore and Bloodwood-Kapok deposits in Australia indicates that these MVT ores were deposited in a rifting environment.

Ca Isotope Effects in Orgueil Leachates and the Implications for the Carrier Phases of 54Cr Anomalies

Primitive meteorites contain small 40Ca excesses, in addition to rare anomalies in 48Ca. Refractory inclusions from Vigarano and Allende have larger 40Ca and resolvable 48Ca anomalies. These results imply that Ca isotopic heterogeneities were still present in the early solar system at both the mineral and whole-rock scale. The absence of correlated Ca isotope anomalies in leachates from the CI1 chondrite Orgueil containing large 54Cr anomalies has implications on the origin of the Cr anomalies. 54Cr has to be produced either in massive stars during s-process nucleosynthesis without accompanying 48Ca or in particular zones in the rare Type Ia supernovae. In the latter case, 54Cr has been produced in a zone predominantly enriched in Cr and 54Cr and not mixed with other zones, or 54Cr has been produced together with other neutron-rich nuclides and there has been subsequent decoupling of this material in the star, in the solar system, or in the laboratory.

Temporal benchmarks for modeling Phanerozoic flow of basinal brines and hydrocarbons in the southern Midcontinent based on radiometrically dated calcite

Fluid-inclusion studies of radiometrically dated (U/Pb, Th/Pb) calcite samples document the presence of hydrothermal fluids at ca. 251, 137, 67, and probably 39 Ma within the Tri-State Zn-Pb mining district of Missouri, Kansas, and Oklahoma. Fluid inclusions indicate minimum temperatures and equivalent salinities ranging from highs of 122 °C and 24 wt% NaCl ca. 251 Ma to lows of 52 °C and 0 wt% NaCl ca. 39 Ma. The two earliest calcites contain primary petroleum inclusions, but these are absent in samples younger than ca. 137 Ma. When combined with information on metal-rich shales, paleomagnetism, and modern groundwater flow, our results are most readily explained by periodic influxes of hydrothermal fluids of diverse compositions for >200 m.y. and sporadic migration of hydrocarbons for at least 100 m.y. in the southern Midcontinent.

Major and trace element chemistry of Boulder 1 at Station 2, Apollo 17

Twenty-seven samples of matrix and clast materials from Boulder 1 at Station 2, Apollo 17 have been analyzed for major and trace elements as part of the study of this boulder by Consortium Indomitabile. Both unusual and common types of material have been characterized.Gray and black competent breccia (GCBx and BCBx) and anorthositic breccia (AnBx) have compositions which are common at the Apollo 17 site and were common at the site of boulder formation. Light friable breccias (LFBx) have compositions which are not found at the Apollo 17 site other than in the boulder. Pigeonite basalt is a new type of lunar rock and has characteristics that would be expected of a highland volcanic rock. It is associated with LFBx material, and like LFBx material it is exotic to the Apollo 17 site. Coarse norite is an old primitive rock which is no longer (if ever) found as millimeter fragments at the Apollo 17 site. It was, however, present as millimeter fragments associated with GCBx and BCBx materials at the site and time of boulder formation. Therefore the boulder-forming process combined materials from at least two different localities or vertical strata; at least one of these (LFBx) has not been previously sampled and analyzed.