The transformation of aragonite to calcite in the presence of magnesium: Implications for marine diagenesis
The transformation of aragonite to calcite in the presence of magnesium: Implications for marine diagenesis
复制标题
文石在镁存在的情况下向方解石的转变：对海洋成岩作用的启示
DOI：
10.1016/j.epsl.2021.117166
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发表时间：
2021-09-07
影响因子：
5.3
通讯作者：
Kaczmarek, Stephen E.
中科院分区：
文献类型：
--
作者：
Hashim, Mohammed S.;Kaczmarek, Stephen E.
天然沃茨中的镁在碳酸盐矿物的形成、溶解和成岩作用中起着关键作用。前人的实验表明，镁能强烈抑制方解石的直接沉淀以及文石向方解石的成岩转化。然而，来自自然环境的数据表明，大多数古生代石灰岩中的成岩方解石形成于浅海埋藏区，存在丰富的镁。因此，文石沉积物转化为方解石丰富的石灰岩的成岩条件知之甚少。在这里，我们目前的数据从实验室实验，文石在70摄氏度的含镁溶液中转化为方解石，以调查的流体：固体比（F：S），这在成岩环境中变化很大，镁抑制和方解石中的掺入的影响。我们的数据表明，不仅可以转换的文石方解石发生在解决方案中具有更高的[Mg]比以前所示的可能在实验室实验中，但逐步降低F：S增加文石稳定的方解石的速率。例如，在F：S为0.3 mL/g的实验中，其对应于具有50%孔隙率的封闭系统中的沉积物，当使用相对于文石的初始高欠饱和度时，文石在[Mg] = 30 mM（Mg/Ca = 5.14）的溶液中稳定为方解石，并且当使用低欠饱和度时，文石在[Mg] = 20（Mg/Ca = 5.14）的溶液中稳定为方解石。相比之下，文石不稳定的方解石后，近3000小时，在实验中的F：S的100毫升/克，这是更典型的开放系统，即使在溶液中与[Mg] = 5 mM（Mg/Ca= 5.14），无论欠饱和度。我们的研究结果还表明，镁结合到方解石产品的量线性增加与F：S的增加。总的来说，这些观察结果进一步指出，F：S作为一个重要的因素，在碳酸盐岩成岩作用具有广泛的影响。首先，在高[Mg]和F：S下文石向方解石的转化受到抑制的观察结果意味着方解石沉淀不太可能发生在与海水直接水文接触的海洋成岩环境中。这使得文石溶解成为这些环境中占主导地位的成岩过程，这可能代表了开阔海洋碱度的一个被低估的来源。其次，从富含文石的沉积物的方解石丰富的石灰岩，占主导地位的岩石记录的转化可能是促进减少的F：S和一个封闭的系统在渐进埋藏的发展。(c)2021爱思唯尔有限公司版权所有。
Magnesium (Mg) in natural waters plays a critical role in governing carbonate mineral formation, dissolution, and diagenesis. Previous laboratory experiments show that Mg can strongly inhibit direct calcite precipitation as well as aragonite to calcite diagenetic transformation. Data from natural settings, however, suggest that diagenetic calcite in most Phanerozoic limestones has formed in the shallow marine burial realm in the presence of ample Mg. Thus, the diagenetic conditions under which aragoniterich sediments convert to calcite-rich limestones are poorly understood. Here, we present data from laboratory experiments whereby aragonite is converted to calcite at 70 degrees C in Mg-bearing solutions to investigate the effects of fluid:solid ratio (F:S), which varies greatly across diagenetic environments, on Mg inhibition and incorporation in calcite. Our data show that not only can the transformation of aragonite to calcite occur in solutions with higher [Mg] than previously shown possible in laboratory experiments, but that progressively lower F:S increase the rate at which aragonite stabilizes to calcite. For example, in experiments with an F:S of 0.3 mL/g, which corresponds to sediments in a closed system with 50% porosity, aragonite stabilizes to calcite in solution with [Mg] = 30 mM (Mg/Ca = 5.14) when an initial high degree of undersaturation with respect to aragonite is used and in a solution with [Mg] = 20 (Mg/Ca = 5.14) when a low degree of undersaturation is used. In contrast, aragonite does not stabilize to calcite after nearly 3000 h in experiments with an F:S of 100 mL/g, which is more typical of an open system, even in a solution with [Mg] = 5 mM (Mg/Ca= 5.14) regardless of the degree of undersaturation. Our results also show that the amount of Mg incorporated into calcite products increases linearly with the increase of F:S. Collectively, these observations further point to F:S as an important factor in carbonate diagenesis with broad implications. First, the observations that transformation of aragonite to calcite is inhibited at high [Mg] and F:S imply that calcite precipitation is unlikely to occur in marine diagenetic environments that are in direct hydrologic contact with seawater. This leaves aragonite dissolution as the dominant diagenetic process in these environments, which may represent an underrated source of alkalinity to the open ocean. Second, transformation from aragonite-rich sediments to the calcite-rich limestones that dominate the rock record is likely promoted by a decrease in the F:S and the development of a closed system during progressive burial. (c) 2021 Elsevier B.V. All rights reserved.