Part I. Comparative Petrology of the Apollo 11 Mare Basalts. Part II. The Oxygen Isotope Geochemistry of the Abitibi Greenstone Belt

Citation

Beaty, David Wayne (1980) Part I. Comparative Petrology of the Apollo 11 Mare Basalts. Part II. The Oxygen Isotope Geochemistry of the Abitibi Greenstone Belt. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/EEHY-DG27. https://resolver.caltech.edu/CaltechETD:etd-02232006-092342

Abstract

PART I. Over the past decade a wealth of geochemical and petrological information has been accumulated on the Apollo ll basalts. These data indicate that the 73 thus far identified basalts can be divided into five petrologic groups which must represent at least five separate igneous cooling units. These five igneous bodies range in age from 3.90 b.y. to 3.60 b.y. Photogeologic studies of the landing site indicate that three mare units are present, and that the lunar module set down on the oldest of the three. The exposure age data suggest that the high-K flow(s) is the surficial rock type at the landing area, and is therefore probably the oldest of the three mare units. The three low-K groups of samples are older than (and underlie) the high-K basalts, and were apparently excavated by West Crater. By studying the size frequency distribution and the inferred cooling rates of the individual samples, it is possible to calculate the formation thicknesses within the 30 m-deep West Crater. This suggests that A=9 m, B1=2 m (and may be an ejecta blanket), B2⩾8 m and B3=6 m. Because the Group D samples have not been dated, it is not known whether they lie above or below the high-K unit. They may, however, represent one of the two younger mare units present near the landing site.

PART II. A variety of petrologic, geochemical and geophysical evidence indicates that the modern oceanic crust interacts on a massive scale with seawater. To evaluate whether or not similar processes were taking place in the Archean, the well preserved Abitibi greenstone belt was studied using oxygen isotopes and the petrographic microscope. In thin section, all of the volcanic rocks in the Abitibi area are found to have been subjected to a hydrothermal process of some sort. The original igneous minerals have been largely replaced by secondary hydrous minerals such as chlorite, epidote and actinolite. The metamorphic assemblages range from the prehnite-pumpellyite facies in the core of the Blake River syncline to greenschist facies adjacent to the large Kenoran granitic batholiths. The structural relations are such that the metamorphic grade decreases in a general way with structural height; the lowest temperature rocks are those which are highest in the volcanic pile.

δ¹⁸O is also correlated with structural height. In each of five widely separated traverses (Benoit, Ben Nevis, Noranda, Skead, Timmins), δ¹⁸O increases upwards through the stratigraphic section (typically +6 to +10 from base to top). In the Benoit area δ¹⁸O and structural height also correlate with the silica content of the volcanic rocks. Silica content and the degree of petrographic recrystallization are not correlated, however, whereas δ¹⁸O and the degree of recrystallization are. This indicates that the gradient in SiO₂ is a relic igneous feature and that the gradient in δ¹⁸O is a hydrothermal alteration feature. Additional evidence that these rocks have undergone isotopic exchange comes from the mineral separate data. Relic clinopyroxene in the basalts has δ¹⁸O +5.5, and quartz phenocrysts in the rhyolites are +7.6, indicating that these lavas were not erupted as high-¹⁸O magmas; they have undergone subsolidus enrichments in δ¹⁸O of 0-4 per mil. Using temperatures inferred from the metamorphic assemblages, δ¹⁸O of the fluid responsible for producing these δ¹⁸O shifts can be calculated. Assuming an open system, water flux within 100°C of the final metamorphic temperature, and using the feldspar geothermometer, the hydrothermal fluid has δ¹⁸O = 0 ± 2 and alteration took place under conditions of high water/rock ratio. The oxygen isotopic effects are associated with the prehnite-pumpellyite facies burial metamorphism, which is thought to have taken place during the formation of the volcanic pile. Since the pile formed in a marine environment the only logical source for such large amounts of fluid is seawater itself.

Oxygen isotopic study of the Amulet "A" massive sulfide deposit indicates that it was formed by a fluid with, a similar δ¹⁸O (0.5 ± 1.0). Similar deposits in the Phanerozoic (Cyprus, Kuroko, Gulf of California) are thought to have originated from heated seawater circulating through the oceanic crust. The fact that the Amulet ore fluid is indistinguishable from the inferred Archean seawater indicates that analagous hydrothermal processes were taking place in the Archean. At the Kidd Creek mine, however, the ore-forming solution is thought to have had δ¹⁸O between +6 and +9. This fluid could either have been derived from normal-¹⁸O seawater through evaporation or exchange with high-¹⁸O country rocks, or it could have been some sort of metamorphic fluid. This indicates that massive sulfide ore deposits have formed by more than one mechanism, and that the simple seawater-hydrothermal model may not be generally applicable.

Less extensive data from other greenstone belts throughout the world indicate that like Abitibi, all have undergone ¹⁸O-enrichments relative to primary igneous values. By mass balance, these ¹⁸O-enrichments must have caused a complementary ¹⁸O-depletion in some other oxygen reservoir. That reservoir was apparently not seawater, nor has it been discovered in the geologic record. Because the oxygen isotopic exchange process is self-buffering, the consistent ¹⁸O-enrichments in greenstone belts throughout history suggests that they were not the dominant form of submarine volcanism in the Archean. This, combined with the apparent destruction of the low-¹⁸O reservoir, suggests that seafloor spreading volcanism was also taking place in the early Precambrian. This is consistent with a variety of geological, geophysical, geochemical, isotopic and petrologic data which indicate that greenstone belts resemble modern island arcs in a number of important respects.

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