Unravelling the development of a spheroidally weathered diorite-gabbro, Santa Margarita Ecological Reserve, Peninsular Ranges, southern California, USA

Abstract

Within Santa Margarita Ecological Reserve (SMER), southern California, an ~108Ma diorite-gabbro pluton has weathered spheroidally resulting in the formation of concentric rindlets, i.e., a rindlet zone, around an unaltered ellipsoidal core of the parent rock. To our knowledge little is known about how spheroidal weathering is produced in Mediterranean climates like that characteristic of portions of the Peninsular Ranges. Hence, we undertook a detailed study aimed at determining the textural and chemical changes associated with the spheroidal weathering of the diorite-gabbro pluton.The parental spheroidally weathered corestone is characterized by a hypidomorphic-granular texture, and consists primarily of plagioclase, amphibole, and biotite, along with minor amounts of pyroxene and quartz. Within the corestone, sericite, and uncommonly, calcite and chlorite, replace plagioclase. The latter mineral also infrequently replaces portions of amphibole and biotite. Such alteration products are derived from sub-solidus deuteric alteration of the pluton.XRD clay mineral analysis of the 13 rindlets indicates the presence of mostly kaolinite and vermiculite, and a far lesser volume of green smectite. Thin section study of rindlet samples suggests that vermiculite is derived from the weathering of biotite, the most extensively altered mineral in the rindlet zone. Such alteration is paralleled by statistically significant losses of K mass across the rindlet zone. In addition, statistically significant losses of Ca and Na mass across the rindlet zone, likely reflect conversion of sericite to kaolinite. In contrast, the absence of statistically significant losses of Mg, Fe, and Mn over most of the rindlet zone, implies that fluids were mostly oxidizing, and that any Mg leached from amphibole was likely fixed within smectite, while leached Fe and Mn precipitated out as oxides or oxyhydroxides. Though calcite was present in the corestone, its absence in the rindlets indicates that fluids were sufficiently acidic to dissolve and remove it.Previous studies have shown that spheroidal weathering occurs when volume expansion produced by a positive ΔV of reaction builds up internal elastic strain energy in the rock. For example, this type of reaction occurs when iron oxidizes within biotite resulting in an expansion of d(001) from 10Å to 10.5Å, or when biotite is transformed into vermiculite leading to an expansion of d(001) from 10Å to 14Å. Data presented here suggest that the conversion of biotite to vermiculite, and the resulting positive ΔV produced the spheroidal fracturing of the corestone at SMER. Hence, these and other data discussed in this paper suggest that the weathering of biotite is the primary driving force in the formation of spheroidally weathered corestone in the Mediterranean climate of the Peninsular Ranges.