Low-K Basalt

Rock type Low-K basalt K-trachy- basalt Leucite-tephrite Leucitite... 

Apollo 11 brought back two chemically differing groups of basalt high-K and low-K rocks. From age determinations we know that these two rock types solidified more than 200 million years apart. Turner23 found a K-Ar age of 3.55 x 109 yrs for the high-K rocks and 3.82 x 109 yrs for the low-K rocks. 

Tests. These models have chondritic Th/U, subchondritic K/Th, and relatively low contents of refractory components compared to the more extreme cases described above. A typical magma produced from these compositions would have a low-FeO basaltic composition. The mineralogy of the basalt would be quartz normative for the Morgan and Anders bulk composition and consist of plagioclase and high-calcium pyroxene. 

Includes both high- and low-temperature basalt alternation, or K, fixation on deltaic clays. 

Volcanic rocks of the subalkaline series have been further subdivided on the basis of their concentrations of K2O and SiO (Peccerillo and Taylor, 1976). Le Maitre et al. (1989) propose a division of subalkaline rocks into low-K, medium-K and high-K types and suggest that these terms may be used to qualify the names basalt,... 

The samples of high-Ti basalt have comparatively high concentrations of rubidium (72 8 ppm) and potassium (1.58 0.14%) which yield a low average K/Rb ratio of 219 characteristic of sialic rocks of the continental crust. The low-Ti basalt and dolerite have lower concentrations of both rubidium (34 8 ppm) and potassium (0.766 0.196%) which yields a K/Rb ratio of 225 that is characteristic of the Kirkpatrick Basalt and the Ferrar Dolerite of the Transantarctic Mountains. 

The K-Ar dates of basalt and dolerite in Milorgfjella (Heimefrontfjella) reported by Rex (1972) range from 162 to 179 Ma with a mean of 172 6 Ma (Middle Jurassic). However, a pre-Permian dolerite dike in the Proterozoic basement of Milorgfjella yielded a K-Ar date of 455 15 Ma (Ordovician). Two dolerite dikes at Mannefallknausane also formed during the Ordovician magmatic event although one was altered and yielded a low K-Ar date of 258 10 Ma, while the other has a low potassium concentration (0.14%) and yielded a poorly defined date of 564 100 Ma. Fumes et al. (1987) reported anomalously old K-Ar dates in... 

Figure 4. Radiogenic Ca enrichments measured on whole rocks as reported hy Marshall and DePaolo (1989), shown here as 8c, values with 1-sigma uncertainties and plotted versus (a) K/Ca and (h) Enj value. The inferred value for seawater is also plotted in (a). Mid-ocean ridge basalts have no measurable enrichment of Ca/ Ca relative to the initial value for the Earth (151.016), as expected for magma derived from the Earth s mantle. Many granitic rocks, especially those with high K contents and low e j values, have significantly elevated Ec, values.

The interpretation of lithogeochemical data from basaltic structural domes is complicated by lithological changes associated with the transition from basalt to overlying siliciclastic rocks, as well as by the polydeformed nature of the host sequence. Ferroan carbonate alteration is well developed, and low-level Au enrichment extends for a considerable distance away from zones of economic interest. Arsenic and Sb/AI anomalies are restricted to within approximately 10 m of mineralized zones. Sericite alteration is indicated by Na depletion and K enrichment in basalt within 20 to 40 m of mineralized zones. A number of other elements, including Mn, P, S, Zn, Mo, Cu, Se and Ba, are variably enriched within the rocks hosting Au mineralization, but it is not clear whether elevated concentrations of these elements are a product of syn-sedimentary exhalative activity or result from later hydrothermal alteration. 

Age-spectrum diagram and K/Ca plots for lunar basalt sample 10003. The first few (low-temperature) steps give dates that are too young, but for steps between 12-30% release and -90% release, the dates form "plateaus". The plateaus begin at slightly different temperatures for each mineral. The individual temperature steps release gas from sites of different retentivity within the samples. The K/Ca plot provides information about the mineralogy of the sites. After McDougall and Harrison (1988). 

The volcanoes in the Campania Province, Pontine Islands and Vulture are composed of a wide variety of magma types. Silica undersaturated ultrapo-tassic volcanism is restricted to Somma-Vesuvio, whereas mildly undersaturated to oversaturated potassic rocks occur at Campi Flegrei, Ischia, Procida and Ventotene. In the latter two islands, low potassium compositions close to calc-alkaline basalts are found among lavas and lithic ejecta. Pliocene calc-alkaline rocks are found as rhyolites at Ponza and as basalts and basaltic andesites beneath the Campanian Plain. At Vulture, volcanism is highly enriched in both Na and K, a composition that is distinct from any other volcano in the Italian peninsula. 

Shearer C. K., Papike J. J., andLayneG. D. (1996) Deciphering basaltic magmatism on the Moon from the compositional variations in Apollo 15 very low-Ti picritic magmas. Geochim. Cosmochim. Acta 60, 509-528. 

Figure 9,14 Normalized largest fragment mass versus energy density for water ice at low temperature (77K) and 255 K. For the comparison, basalt and pyrophyllite data is also plotted [46], The data at 81 K by Lange and Ahrens [47] shows rather weaker strength than the present data because there might be initial cracks in the target before the shot. (Figure from Kato et al. [35].)...

Many of these reactions are in the direction needed to close the marine mass balances for major ions (Fig. 2.4). The exceptions are that they supply an unnecessary additional siiik for SO4 (CaSO precipitation) and a vast additional source of K+. The additional sink for SO4 does little damage to the marine SO4 mass balance in Fig. 2.4 because its removal affects ordy Ca + and only at the level of about 15% of the Ca + riverine inflow. The hydrothermal source for K+ cannot be rationalized as easily, because there is no adequate sink in the marine environment. Research into the sources and sinks of alkali metals reveals that K+ (and other alkali metals) that are released from basalts at high temperature are reincorporated back into basaltic rock on the sea floor at low temperature. Thus, is recycled in the vicinity of hydrothermal vents. The rates of release and incorporation are uncertain enough to obscure whether the net K+ flux is into or from the ocean in these regions. It is possible that the low-temperature removal of K+ to basalt represents a net sink large enough to accommodate the river inflow. 

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