wAbstract
@In the mountainous Rio Icacos watershed in northeastern Puerto
Rico, quartz diorite bedrock weathers spheroidally, producing
a 0.2-2 m thick zone of partially weathered rock layers (`2.5
cm thickness each) called rindlets, which form concentric layers
around corestones. Spheroidal fracturing has been modeled to occur
when a weathering reaction with a positive ’V of reaction builds
up elastic strain energy. The rates of spheroidal fracturing and
saprolite formation are therefore controlled by the rate of the
weathering reaction.
@Chemical, petrographic, and spectroscopic evidence demonstrates
that biotite oxidation is the most likely fracture-inducing reaction.
This reaction occurs with an expansion in d(001) from 10.0 to
10.5π, forming galtered biotiteh. Progressive biotite oxidation
across the rindlet zone was inferred from thin sections and gradients
in K and Fe(II). Using the gradient in Fe(II) and constraints
based on cosmogenic age dates, we calculated a biotite oxidation
reaction rate of 8.2~10-14 mol biotite m-2
s-1. Biotite oxidation was documented within the bedrock
corestone by synchrotron X-ray microprobe fluorescence imaging
and XANES. X-ray microprobe images of Fe(II) and Fe(III) at 2Κm
resolution revealed that oxidized zones within individual biotite
crystals are the first evidence of alteration of the otherwise
unaltered corestone.
@Fluids entering along fractures lead to the dissolution of plagioclase
within the rindlet zone. Within 7 cm surrounding the rindlet-saprolite
interface, hornblende dissolves to completion at a rate of 6.3~10-13
mol hornblende m-2 s-1: the fastest reported
rate of hornblende weathering in the field. This rate is consistent
with laboratory-derived hornblende dissolution rates. By revealing
the coupling of these mineral weathering reactions to fracturing
and porosity formation we are able to describe the process by
which the quartz diorite bedrock disaggregates and forms saprolite.
In the corestone, biotite oxidation induces spheroidal fracturing,
facilitating the influx of fluids that react with other minerals,
dissolving plagioclase and chlorite, creating additional porosity,
and eventually dissolving hornblende and precipitating secondary
minerals. The thickness of the resultant saprolite is maintained
at steady state by a positive feedback between the denudation
rate and the weathering advance rate driven by the concentration
of pore water O2 at the bedrock-saprolite
interface.x
1. Introduction
@1.1. Spheroidal weathering
@1.2. Field site and sample collection
2. Analytical methods
@2.1. Sample preparation and analysis
@2.2. X-ray microprobe imaging
@2.3. Mass transfer calculations
3. Results
@3.1. Porosity development
@3.2. Chemical mobility
@3.3. Mineralogy
4. Discussion
@4.1. Weathering gradients and reaction stoichiometry
@4.2. Quantification of mineral weathering rates
@4.3. Biotite weathering
@4.4. Plagioclase weathering
@4.5. Hornblende weathering
5. Conclusions
Acknowledgments
References