The effects of cooling rate on texture and pyroxene chemistry in DSDP Leg 34 basalt: A microprobe study

A 15.6-m cooling unit from DSDP Leg 34 is investigated to determine the effects of the rate of solidification of a liquid on the resulting textures and mineralogy. A detailed sampling from the top and bottom margins of the cooling unit inward to the center has revealed systematic textural and mineralogical variations that are the result of different cooling rates. Textures change from spherulitic-intersertal in the chilled margins through intergranular to subophitic/ophitic in the interior of the flow. Plagioclase width increases and the length/width ratio decreases with distance from the flow margins (and hence with decreasing cooling rate). Titanium and Al contents are higher in pyroxenes from the chilled margins than in those from the interior regions although there is no systematic change in major element concentrations. Thus, as has been shown by experiments in lunar rock systems [4,33], Ti and Al contents in pyroxene increase with increasing cooling rate. Cooling rates calculated from conductive heat flow equations range from >100°C/hr in the margins of the cooling unit to <0.1°C/hr in the interior (>3 m from the margin). The variations in minor element content of pyroxene due to cooling rate must be taken into account when one attempts to relate coarse-grained gabbros and volcanic rocks to a common parent by comparing their pyroxene compositions.