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This report summaries results of a petrophysical study of different age basalts and andesite basalts from various tectonic environments: island arcs and fold belts - the Kuril-Kamchatka arc (Q, N, P, Cr, J), the Maly Caucasus (Q, N, P, Cr , J), Bulgaria (P), Amour region (Cr), the Crimea (J), Kazakhstan (P, C, D, O, a), the Urals (C, D, S, O), Minusinskaya depression (D), the Altai and Sayan mountains (a), Karelia (Pr1, Ar2); oceans - the Pacific, the Atlantic, the Indian and California Bay (Q); oceanic islands - Iceland (Q, N), Spitsbergen (Q, N), the Canary islands (Q); ancient platforms - flood basalts of Siberian plate (P2-T1) and East-European plate (Timan - D, Karelia - Pr). Totally more than 5500 samples were analyzed.
The following petrophysical properties were determined: density, porosity, ultrasonic velocity, magnetic, thermal and mechanical parameters. All basalts are subdivided in two age groups: (I) N2-Q4 and (II) > N2. Basalts from group (I) are fresh and characterized by vesicular structure. Basalts from group (II) are altered and substituted by secondary minerals and characterized by amygdaloidal structure. The age boundary between two groups is flexible depending of region but approximately is around Pliocene-Miocene. The groups are distinguished by the type and magnitude of correlation between petrophysical properties. Correlation between porosity (density) and ultrasonic velocity is low (or "zero") for basalts from the group (I) and high for basalts from the group (II).
The unexpected phenomenon is observed for ultrasonic velocity in young basalts (Q2-4). Independently on porosity (density) ultrasonic velocity is characterized by extremely low values (2,0-3,5 km/sec). Important to note, that only massive basalts (density > 2,6 g/cm3, porosity < 9%) were selected for the investigation. This phenomenon was observed in basalts from different regions. Sonic velocity values increase greatly in water-saturated samples. To explain this phenomenon we investigated polished sections impregnated with oil in fluorescence microscope. The "shone areas" (micropores) and microcracks network were observed, which were apparently the reasons of low ultrasonic velocity.
An age series collections of basalts from various regions were selected: modern, 50 ka, 100 ka, 200 ka, 300 ka, 400 ka, 600 ka. It is determined that phenomenon of low velocity values is disappeared around 200-300 ka. Ancient basalts are characterized by high velocity values (>4,5 km/sec).
It is concluded that phenomenon of low ultrasonic velocity is directly related to microcracks which are of microns wide. They are formed as a result of thermal stresses due to lava cooling and mineral crystallization. Although the volume of this microcracks is low (not exceed 1,0-1,5%), they affect to ultrasonic velocity greatly. During geological time microcracks are closed due to stress relaxation result in a significant increase of sonic velocity. Financial support of RFBR (07-05-00118-a).
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