Formation and destruction of periclase by fluid flow in two contact aureoles
Formation and destruction of periclase by fluid flow in two contact aureoles
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两个接触光环中流体流动形成和破坏方镁石
DOI：
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发表时间：
1997
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通讯作者：
Douglas Rumble III.
中科院分区：
文献类型：
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作者：
J. Ferry;Douglas Rumble III.
由白云石 = 方镁石+方解石+二氧化碳的反应，在苏格兰贝因和蒙大拿州银星光环的陡峭大理石中形成了方镁石。含XCO2的岩石和流体之间的平衡要求反应是由渗透作用驱动的。方镁石后的水镁石假晶产于杜布海希环带的贝因中，或作为白云石的逐层交代，或出现在白云岩与前变质岩脉接触的透镜中。输运理论预测，渗透同时驱动方镁石反应和18O-耗竭前沿，这两个前沿沿流动路径以明显不同的速度移动。水镁石和~(180)O贫化岩石的分布在地表曝光量上是一致的，因此指示向上流动。时间积分通量(Q)为-lt；500mol/cm2，流体来源为岩浆。由于方镁石及其水化等效水镁石不会在逆行反应中改变为白云石，水镁石大理石的暴露准确地反映了峰期变质流体的流动路径。在银星光环中，方镁石之后的水镁石假象仅产于镁/钙比值升高的层状体中。18O亏损的空间格局需要向上的垂直流体流动。推算进动Q ≈ 为10~3-104mol/cm~2，流体来源为岩浆。低镁/钙、贫18O、不含水镁石的岩石构成了一个两难境地，因为方镁石反应前沿应该比同位素蚀变前沿通过它们的距离是≈的18倍。反应织构表明，低镁钙比岩石中的方镁石和水镁石被渗透驱动的逆碳化反应破坏，这一困境得到了解决。逆行Q值为≈10~3~104mol/cm~2。水镁石(方镁石之后)仅保存在方镁石发育较丰富的高镁钙层中。因此，银星的水镁石大理石的几何形状反映了高镁/钙岩层的位置，而不是流体流动的几何形状。在正确解释前行流体流动的矿物学记录之前，必须考虑逆行反应。在这两个光环带中，流体流动、矿物反应和同位素耗竭在构造上受层理和岩性接触控制。
Abstract Periclase formed in steeply dipping marbles from the Beinn an Dubhaich aureole, Scotland, and the Silver Star aureole, Montana, by the reaction dolomite = periclase + calcite + CO2. Equilibrium between rock and fluids with XCO2 < 1 requires that reaction was infiltration-driven. Brucite pseudomorphs after periclase occur in the Beinn an Dubhaich aureole either as bed-by-bed replacement of dolomite or in a lens along the contact between dolomite and a pre-metamorphic dike. Transport theory predicts that infiltration drove both periclase reaction and 18O-depletion fronts which moved at significantly different velocities along the flow path. The distributions of brucite and 18O-depleted rocks are identical in surface exposures, thus indicating upward flow. Time-integrated flux (q) was <500 mol/cm2 and the fluid source was magmatic. Because periclase and its hydrated equivalent brucite are unaltered to dolomite by retrograde reactions, the exposure of brucite marbles accurately images the flow paths of peak metamorphic fluids. In the Silver Star aureole brucite pseudomorphs after periclase exclusively occur in tabular bodies that are beds with elevated Mg/Ca. The spatial pattern of 18O-depletion requires upward vertical fluid flow. Estimated prograde q ≈ 103–104 mol/cm2 and the fluid source was magmatic. Low Mg/Ca, 18O-depleted, brucite-free rocks pose a dilemma because the periclase reaction front should have traveled ≈18 times further through them than the isotope alteration front. The dilemma is resolved by reaction textures that indicate periclase and brucite were destroyed in low Mg/Ca rocks by infiltration-driven retrograde carbonation reactions. Values of retrograde q were ≈103–104 mol/cm2. Brucite (after periclase) was preserved only in high Mg/Ca layers where periclase developed in greater abundance. The geometry of brucite marbles at Silver Star thus reflects the location of high Mg/Ca beds rather than the geometry of fluid flow. Retrograde reactions must be considered before the mineralogical record of prograde fluid flow can correctly be interpreted. In both aureoles fluid flow, mineral reaction, and isotope depletion were structurally controlled by bedding and lithologic contacts.