Spheroidal Weathering of a 108 Ma Cretaceous Gabbro
Emma Vierra
BS Candidate
Advisor: Dr. Gary H. Girty
May 4th, 2017 – CSL 422, 2 pm
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Abstract
Within the Santa Margarita Ecological Reserve (SMER), southern California, a gabbroic pluton has weathered spheroidally. Spheroidal weathering affects jointed bedrock and results in the formation of concentric layers, also known as rindlets, around an unaltered, ellipsoidal parent rock. Previous weathering studies have shown that spheroidal weathering occurs when a positive ΔV of reaction builds up elastic strain energy in the rock. This type of reaction occurs when iron oxidizes within biotite resulting in an expansion in the d (001) from 10Å to 10.5Å or when biotite is transformed to vermiculite leading to an expansion from 10 Å to 14 Å.
During this study, XRD clay mineral analysis indicates the presence of vermiculite, a result that is consistent with the deep orange to brown colors of biotite within the rindlets. In addition, element mobility patterns indicate that the masses of K and Fe were loss during the conversion of corestone to saprock.
The alteration of biotite to vermiculite and the resulting positive ΔV causes the initial spheroidal fracturing of the corestone. Fluids then enter along the fractures leading to the dissolution of other minerals, such as plagioclase, amphiboles, and pyroxenes. Losses in the masses of Na, Ca, Ti, Fe, and Mg, relative to the corestone, are likely the result of fluid/rock interactions during this phase of spheroidal weathering.
Based on a published model, in the wet climates typical of the tropics, each rind develops approximately every 250 years. Uncritically applying such a model to my study of 13 fracture rinds covering 20 cm, suggests that rindlet formation occurred over a span of about 3,250 years. However, the spheroidally weathered corestone did not develop in a tropical climate, and thus the time span for fracture development may have been greater. Such an interpretation follows from the idea that in a dry climate there are less fluids and oxidation capacities to form the rinds as quickly as in a continuously saturated environment typical of a tropical climate. Finally, the results of my work show that within a dry climate, illuviation of clay particles is irregularly developed because clay-carrying fluids follow complex nonsystematic interconnected sets of cracks, resulting in variable porosities and weathering intensities within rindlets associated with spheroidal weathering.