Occurrence, chemistry, and origin of immiscible silicate glasses in a tholeiitic basalt: A TEM/AEM study

Abstract

The occurrence and chemistry of immiscible silicate glasses in a tholeiite mesostasis from the Umtanum formation, Washington, were investigated with transmission electron microscopy and analytical electron microscopy (TEM/AEM). TEM observation reveals isolated, dark globules (2.1 micron or less in diameter) randomly distributed in a transparent matrix glass interstitial to plagioclase laths. The globules less than 0.3 micron and larger than 0.8 micron fall beyond the linear relationship defined by the 0.3–0.8 micron globules in a plot of the logarithm of number versus size. Large globules (0.7 micron or larger in diameter) range from homogeneous to heterogeneous in optical properties and chemistry. Homogeneous globules are completely glassy, whereas heterogeneous globules contain crystalline domains. AEM analyses show that the globules have high Si, Fe, Ca, and Ti with subordinate Mg, Al, P, S, Cl, K, and Mn, which gives high normative fa , px, il , and ap . The matrix glass consists dominantly of Si with low Al and minor Na and K, yielding a high normative qz, or, ab , and an .