Herman H. Thomas

Petrogenetic implications of some new geochemical data on eclogitic and ultrabasic inclusions

Some eclogitic and ultrabasic inclusions, their separated minerals, host rocks, and related samples have been analyzed by mass-spectrometric isotope dilution for K, Rb, Sr, Ba and rare-earth elements (RE), and by other techniques for selected major elements. The results of these analyses, considered in terms of measured trace-element phenocryst-matrix partition-coefficients, suggest the following trace-element models. Many inclusions are contaminated with at least K, Rb and Ba. Some Kakanui eclogite phases were equilibrated with liquids similar in trace element concentrations to alkali basalt. Some Roberts Victor eclogite and garnet peridotite phases were in equilibrium with kimberlite. Salt Lake Crater eclogite might represent a low-calcium-pyroxene cumulate from alkali basalt. Some peridotites appear to have equilibrated with tholeiitic basalt; but some of these may have been subsequently metamorphosed. Alkali basalt peridotite nodules are not cumulates or residues directly related to their hosts; they might be contaminated metamorphic rocks. Ancillary conclusions are that kimberlite, carbonatite and nepheline-melilite basalt appear to constitute a quantized trace-element group. Rare-earth concentrations are perhaps the best criteria for distinguishing carbonatites from sedimentary carbonates. Roberts Victor omphacite can have high KRb and BaCe. Clinopyroxene/garnet rare-earth ratios tend to be uniform, as, to a lesser extent, do diopside/orthopyroxene rare-earth ratios for coexisting minerals. Mantle rocks appear to have very low Eu2+Eu3+ ratios; this might indicate oxidizing conditions. Alkali basalts approach uniformity in trace-element concentrations. Various degrees of partial fusion of garnet peridotite could account for the whole spectrum of rare-earth concentrations observed for basalts.

Rare-earth and barium abundances in Ivory Coast tektites and rocks from the Bosumtwi Crater area, Ghana

Abstract Abundances of eight rare-earth elements and barium have been determined by isotope dilution in an Ivory Coast tektite composite, an individual Ivory Coast tektite, two impactite glasses and three country-rocks from the Bosumtwi Crater area of Ghana. The rare-earth abundances are lower, in general, than those in previously analyzed tektites from different geographic localities. However, there is some overlap in the data and the differences may not be significant in view of the Limited rare-earth data on tektites. The rare-earth patterns of the meta-sedimentary country-rocks do not exactly match the patterns of the tektites, although there are general similarities. On the other hand, the Bosumtwi Crater impact glasses have rare-earth and barium abundances that are virtually identical to those in the tektites. This identity is viewed as supporting evidence that the Ivory Coast tektites were produced terrestrially by the impact that formed the Bosumtwi Crater.

Luna 16 - Some Li, K, Rb, Sr, Ba, rare-earth, Zr, and Hf concentrations.

Abstract Concentrations of Li, K, Rb, Sr, Na, rare-earths, Zr and Hf have been determined for some Luna 16 core materials by mass-spectrometric isotope-dilution. Two

Satellite and surface geophysical expression of anomalous crustal structure in Kentucky and Tennessee

Abstract An equivalent layer magnetization model obtained from inversion of long-wavelength satellite magnetic anomaly data indicates a very magnetic source region centered in south central Kentucky. The magnetization maximum nearly coincides with a gravity high elongated north-south and extending into Tennessee. Previous refraction profiles suggest that the source of the gravity anomaly is a large mass of rock occupying much of the crustal thickness. The outline of the source delineated by gravity contours is also discernible in aeromagnetic anomaly patterns. Taken together, the geophysical data suggest a large, localized mass of intracrustal rock which is both dense and very magnetic. A simple magnetization/density model is given which accounts for the gravity and long-wavelength aeromagnetic anomalies due to the body. We interpret it as a mafic plutonic complex, and several lines of evidence are consistent with a rift association. The body is, however, clearly related to the inferred position of the Grenville Front. It is bounded on the north by the fault zones of the 38th Parallel Lineament. The inferred mean magnetization (4 A/m) of the body is large, but not inconsistent with values reported by others for deep crustal bodies associated with long-wavelength magnetic anomalies. Such magnetization levels can be achieved with magnetic mineralogies produced by normal oxidation and metamorphic processes and enhanced by viscous build-up, especially in mafic rocks of alkaline character.

Satellite magnetic anomalies and the Middle America Trench

Abstract Quantitative modeling of the Middle America Trench satellite-elevation crustal magnetic anomaly, using seismic epicenter data to constrain the slab geometry and a magnetization (representing induced and viscous contributions) similar to that found for the Aleutian Arc slab, agrees well with reduced-to-pole MAGSAT data. Incorporating minor contributions from crustal sources in Mexico and Central America produces only minor improvement in the agreement between the modeled and observed anomaly, supporting the idea that it is the slab which is the major source body for the Middle America Trench anomaly.

Geochemistry of Apollo 15 Basalt 15555 and Soil 15531

Major and trace element concentrations have been determined by atomic absorption spectrophotometry, colorimetry, and isotope dilution in Apollo 15 mare basalt 15555 from the Hadley Rille area; trace element concentrations have also been determined in plagioclase and pyroxene separates from basalt 15555 and in soil 15531 from the same area. Basalt 15555 most closely resembles in composition the Apollo 12 olivine-rich basalts. The concentrations of lithium, potassium, rubidium, barium, rare-earth elements, and zirconium in basalt 15555 are the lowest, and the negative europium anomaly is the smallest, reported for lunar basalts; this basalt might be the least differentiated material yet returned from the moon. Crystallization and removal of about 6 percent of plagioclase similar to that contained in the basalt would account for the observed europium anomaly; if plagioclase is not on the liquidus of this basalt, a multistage origin is indicated. Mineral data indicate that plagioclase and pyroxene approached quasi-equilibrium. Most of the chemical differences between basalt 15555 and soil 15531 would be accounted for if the soil were a mixture of 88 percent basalt, 6 percent KREEP (a component, identified in other Apollo soils, rich in potassium, rare-earth elements, and phosphorus) and 6 percent plagioclase (anorthosite?).

Oligocene or Younger Thrust Faulting in the Ruby Mountains, Northeastern Nevada

A klippe of unmetamorphosed Devonian carbonate rocks rests on the Harrison Pass intrusive body south of Toyn Creek in the central Ruby Mountains, Elko County, Nevada. This klippe and other klippen of Carboniferous strata—first mapped by R. P. Sharp (1942)—that rest on lower Paleozoic strata are believed to represent a once-continuous thrust sheet that developed after emplacement of the intrusive body. This intrusion of coarse-grained granodiorite to quartz monzonite is exposed over an area of about 45 square miles. Potassium-argon and lead-alpha age determinations on four samples of the intrusive body establish Oligocene or younger age for the thrusting. Potassium-argon age determinations on biotites from the four samples range from 29 to 36 m.y. with a possible analytical error of ±10 percent. Lead-alpha age determinations (all with a possible analytical error of ±10 m.y.) on zircon showed 40 m.y. for three of the samples and 30 m.y. for the fourth. Lead-alpha and potassium-argon dates on a fifth sample collected by R. R. Coats also fall within these ranges. The agreement of the radiometric dates indicates a lack of thermal activity subsequent to the emplacement of the intrusion and establishes a maximum age of Oligocene for the thrust faulting. The present distribution of thermally metamorphosed Paleozoic rocks in the Ruby Mountains seems to preclude the possibility that the thrust plate originated within the Ruby Mountains. Sharp suggested a western source with displacement of from 7 to 10 miles.

Petrologic model of the northern Mississippi Embayment based on satellite magnetic and ground-based geophysical data

Abstract A petrologic model of the northern Mississippi Embayment, derived from gravity, seismic and rift data, is evaluated by converting the model to a magnetization model which is compared with satellite magnetic anomaly models. A magnetization contrast of approximately −0.54 A/m, determined from the petrologic model of the embayment compares favorably to values of −0.62 A/m and −0.45 A/m from a MAGSAT United States Apparent Magnetization Contrast Map and a published POGO magnetization contrast model, respectively. The petrologic model suggests that the magnetic anomaly low associated with the Mississippi Embayment may be largely due to the intrusion under non-oxidizing conditions of low Curie temperature gabbroic material at the base of the crust of the embayment. Near-surface mafic plutons, bordering the Mississippi Valley Graben, appear from aeromagnetic data to have higher magnetizations than the deeper gabbroic material; however, it is impossible to ascertain if this is due to compositional differences or similar material at shallower (lower temperature) depths. These results indicate that variations in the Curie temperatures of intrusions accompanying rifting may account for a large part of the wide range of magnetic anomalies associated with presently inactive rifts with normal heat flow.

Contributions of Cretaceous Quiet Zone natural remanent magnetization to Magsat anomalies in the Southwest Indian Ocean

The Magsat magnetic anomalies over the Southwest Indian Ocean are modeled using a combination of induced plus viscous remanent magnetization (IM/VRM) and natural remanent magnetization (NRM). Two broad, roughly parallel, SW to NE trending triple-peaked positive anomalies dominate the region, one lying south of Africa and the other north of Antarctica. Although these anomaly peaks generally correspond with the Agulhas Plateau/Maud Rise, Mozambique Plateau/Astrid Ridge, and Madagascar Ridge/Conrad Rise conjugate pairs, the IM/VRM contribution from structural characteristics (i.e., crustal thickness) accounts for only about 20% of the anomaly amplitudes. A spatially variable but observationally constrained NRM contribution in Cretaceous Quiet Zone (KQZ) crust is required to account for the location, shape, and amplitude contrast of these anomalies. Many crustal features in the Southwest Indian Ocean near Antarctica have little geophysical data to constrain their structure but do have tectonic conjugates near Africa for which much more geophysical data are generally available. Using geophysical and geological constraints from one member to model the magnetization structure of its conjugate reproduces the observed Magsat reduced-to-pole anomalies over both structures very well. This suggests that no significant alteration in their magnetization structure has occurred since the features split. Models of these conjugate structures show that IM/VRM reproduces the Magsat anomalies associated with non-KQZ crust but that both IM/VRM and a dominant NRM component are required to explain the anomalies associated with KQZ crust.