【PanolaとDavis Runの花崗岩レゴリスについて、長石風化の深さによる変化を、反応速度定数・鉱物表面積・一次/二次透水係数などの変数を用いて、時間-深度コンピュータースプレッドシートモデル（time-depth computer spreadsheet model）により検討】
『Abstract
Differential rates of plagioclase and K-feldspar weathering commonly
observed in bedrock and soil environments are examined in terms
of chemical kinetics and solubility controls and hydrologic permeability.
For the Panola regolith, in the Georgia Piedmont Province of southeastern
United States, petrographic observations, coupled with elemental
balances and 87Sr/86Sr ratios, indicate
that plagioclase is being converted to kaolinite at depths ＞ 6
m in the granitic bedrock.. K-feldspar remains pristine in the
bedrock but subsequently weathers to kaolinite at the overlying
saprolite. In contrast, both plagioclase and K-feldspar remain
stable in granitic bedrocks elsewhere in Piedmont Province, such
as Davis Run, Virginia, where feldspars weather concurrently in
an overlying thick saprolite sequence. Kinetic rate constants,
mineral surface areas, and secondary hydraulic conductivities
are fitted to feldspar losses with depth in the Panola and Davis
Run regoliths using a time-depth computer spreadsheet model.
The primary hydraulic conductivities, describing the rates of
meteoric water penetration into the pristine granites, are assumed
to be equal to the propagation rates of weathering fronts, which,
based on cosmogenic isotope dating, are 7 m/106 yr
for the Panola regolith and 4 m/106 yr for the Davis
Run regolith. Best fits in the calculations indicate that the
kinetic rate constants for plagioclase in both regoliths are factors
of two to three times faster than K-feldspar, which is in agreement
with experimental findings. However, the range for plagioclase
and K-feldspar rates (kr = 1.5 × 10-17
to 2.8 × 10-16 mol m-2 s-1) is
three to four orders of magnitude lower than for that for experimental
feldspar dissolution rates and are among the slowest yet recorded
for natural feldspar weathering. Such slow rates are attributed
to the relatively old geomorphic ages of the Panola and Davis
Run regoliths, implying that mineral surface reactivity decreases
significantly with time.
Differential feldspar weathering in the low-permeability Panola
bedrock environment is more dependent on relative feldspar solubilities
than on differences in kinetic reaction rates. Such weathering
is very sensitive to primary and secondary hydraulic conductivities
(qp and qs), which control
both the fluid volumes passing through the regolith and the thermodynamic
saturation of the feldspars. Bedrock permeability is primarily
intragranular and is created by internal weathering of networks
of interconnected plagioclase phenocrysts. Saprolite permeability
is principally intergranular and is the result of dissolution
of silicate phases during isovolumetric weathering. A secondary
to primary hydraulic conductivity ratio of qs/qp = 150 in the Panola bedrock results in kinetically
controlled plagioclase dissolution but thermodynamically inhibited
K-feldspar reaction. This result is in accord with calculated
chemical saturation states for groundwater sampled in the Panola
Granite. In contrast, greater secondary conductivities in the
Davis Run saprolite, qs/qp
= 800, produces both kinetically controlled plagioclase and K-feldspar
dissolution. Faster plagioclase reaction, leading to bedrock weathering
in the Panola Granite but not at Davis Run, is attributed to a
higher anorthite component of the plagioclase and a wetter and
warmer climate. In addition, the Panola Granite has an abnormally
high content of disseminated calcite, the dissolution of which
precedes the plagioclase weathering front, thus creating additional
secondary permeability.』
『要旨
基盤岩と土壌環境で普通に観察される斜長石とカリ長石の差別的な速度が、化学カイネティックと溶解度コントロールおよび透水係数によって検討されている。米国南東部のジヨージア ピーモント地方のPanolaレゴリスに対して、元素のバランスおよび87Sr/86Sr比と結びついた岩石学的観察から、斜長石は花崗岩質基盤岩中の6m以深でカオリナイトに変換されることが示されている。カリ長石は基盤岩ではもとのままで残るが、その後、上を覆うサプロライトではカオリナイトに風化している。対照的に、バージニアのDavis
Runのような、ピーモント地方の他の場所の花崗岩質基盤岩では、斜長石とカリ長石の両方が安定に残っており、上を覆う厚いサプロライト層においては長石は同時に風化している。カイネティック速度定数、鉱物表面積、および二次透水係数が、時間−深度コンピュータースプレッドシートモデルを用いて、PanolaおよびDavis
Runのレゴリスにおいて深度に関する長石の損失（ロス）に対して適合されている。未風化の花崗岩への天水の浸透の速度を記述する一次透水係数は、風化フロントの伝播速度に等しいと仮定され、それは宇宙線源同位体年代に基づくと、Panolaレゴリスでは
7 m/100万年、Davis Runレゴリスでは 4 m/100万年である。最も適合した計算結果は、両方のレゴリスで斜長石に対するカイネティック速度定数はカリ長石よりも2〜3倍速く、実験結果と一致する。しかし、斜長石とカリ長石の速度の範囲（kr = 1.5 × 10-17 〜 2.8 × 10-16
mol/m2/秒）は、実験による長石溶解速度よりも3〜4桁小さく、天然の長石風化に対して今までに記録されている最も遅いものの中に入る。そのように遅い速度は、PanolaとDavis
Runのレゴリスの地形の年代が比較的古いせいであり、鉱物表面の反応性が時間とともにかなり減少していることを意味している。
透水性の低いPanola基盤岩環境の差別的な長石風化は、カイネティックな反応速度の差よりも、相対的な長石溶解度により依存する。そのような風化は一次および二次透水係数（qp および qs）に非常に敏感であり、これらはレゴリスを通過する流体体積と長石の熱力学的飽和度の両方をコントロールする。基盤岩の透水性は、主に粒子内部にあり、相互に連結した斜長石斑晶の網状組織の内部風化により生じる。サプロライトの透水性は、主に粒子間にあり、等積風化の間に珪酸塩相が溶解する結果である。Panola基盤岩におけるqs/qp = 150 という一次透水係数に対する二次透水係数の比は、カイネティックに斜長石溶解をコントロールするが、熱力学的にカリ長石反応を抑制することになる。この結果は、Panola花崗岩中で採取した地下水に対して計算された飽和状態と一致する。対照的に、Davis
Runサプロライトにおける大きい二次透水係数は、qs/qp
= 800、斜長石とカリ長石の溶解をともにカイネティックにコントロールすることになる。Davis RunではなくPanola花崗岩の基盤岩に風化を導いている斜長石の速い風化は、斜長石のアノーサイト成分が高いことと気候が湿潤で温暖であることが原因である。加えて、Panola花崗岩は、散在する異常に高い方解石成分をもち、その溶解は斜長石の風化フロントより先行し、したがって付加的な二次透水性を生む。』
『1. Introduction
Quantification of silicate weathering rates has important
implications in a diverse range of geochemical issues. Among them
are rates of neutralization of acid precipitation via silicate
hydrolysis reactions (Stauffer, 1990), the rates of release of
macronutrients such as K and Ca in forested catchments (Huntington
et al., 2000), and the linkage between weathering rates and CO2 draw-down and long-term climate change (Berner
and Berner, 1997). Toward addressing these issues, extensive efforts
have been directed at determining reaction mechanisms and rates
of plagioclase and K-feldspar weathering, two of the most common
minerals present in crystalline protoliths (Blum and Stillings,
1995, and references therein).
Despite these efforts, discrepancies remain between laboratory
and field findings. Commonly, estimated field weathering rates
of feldspar are two to four orders of magnitude slower than laboratory
dissolution rates (Brantley, 1992; White et al., 1996). In addition,
laboratory reaction rates for sodic plagioclase are only a factor
of two to three times faster than K-feldspar rates at near-neutral
pHs (Blum and Stillings, 1995 and references therein). These data
contradict commonly observed field situations in which plagioclase
feldspar exhibits more intense weathering features and is significantly
depleted in regoliths relative to K-feldspar (Banfield and Eggleton,
1990; Nesbitt et al., 1997). Possible reasons for differences
in both relative and absolute rates of feldspar weathering include
differences in physical and reactive surface areas and defect
densities (White and Peterson, 1990; Brantley et al., 1999), impacts
of aqueous chemical species and concentrations on reaction affinities
(Burch et al., 1993; Oelkers et al., 1994), and the role of hydrology
in determining mineral surface wetting and solute transport (Swodoba-Colberg
and Drever, 1992; Clow and Drever, 1996).
The present study addresses how differing weathering environments
control natural feldspar weathering rates in granitic regoliths.
The work focuses on well-characterized weathering profiles consisting
of thick soil-saprolite sequences developed on crystalline bedrock
in the Piedmont Province of southeastern United States (Gardner,
1980; Calvert et al., 1980; Pavich, 1986; Cleaves, 1993). These
regoliths are geomorphically old (＞105 yr), are developed
in situ from granitic bedrock, and have undergone isovolumetric
weathering involving significant mass losses. These features make
for long-term steady-state conditions optimal for characterizing
chemical weathering. Detailed elemental and 87Sr/86Sr
distributions and petrologic data are presented for the Panola
regolith in Georgia. Mass balance calculations are used to construct
plagioclase and K-feldspar distributions in the Panola regolith
that are then compared with significantly different feldspar distributions
in other Piedmont regoliths. Feldspar saturation states are determined
on the basis of pore and groundwater chemical compositions. Finally,
a spreadsheet model is presented that uses kinetic rate constants,
surface areas, and primary and secondary fluid conductivities
to explain both absolute and relative differences in plagioclase
and K-feldspar weathering rates.』
2. Field setting and methodologies
3. Results
3.1. Isovolumetric weathering
3.2. Elemental distributions and mobilities
3.3. Mineral distributions and weathering
3.4. Sr and 87Sr/86Sr distributions
3.5. Water compositions and the thermodynamic saturation of feldspars
3.6. Comparison of feldspar weathering in similar granitic regoliths
4. Discussion
4.1. Primary and secondary hydraulic conductivities
4.2. Mass and solubility controls
4.3. Kinetic controls
4.4. Fitting the model to the feldspar weathering in the Panola
regolith
4.5. Fitting the model to the feldspar weathering in the Davis
Run regolith
4.6. Comparing feldspar weathering rates
4.7. Differences in weathering environments
4.8. Generalized weathering regimens
5. Conclusions
Acknowledgments
References