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 aragonite-rich sediments convert to calcite-rich limestones are poorly understood. Here, we present data from laboratory experiments whereby aragonite is converted to calcite at 70 °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.

天然水中的镁 (Mg) 在控制碳酸盐矿物的形成、溶解和成岩作用中起着关键作用。先前的实验室实验表明，Mg 可以强烈抑制方解石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，大多数显生宙石灰岩中的成岩方解石形成于浅海埋藏区，存在充足的镁。因此，人们对富含文石的沉积物转化为富含方解石的石灰岩的成岩条件知之甚少。在这里，我们提供了来自实验室实验的数据，其中文石在 70 °C 在含镁溶液中研究流体：固体比（F：S）对镁抑制和方解石掺入的影响，该比例在成岩环境中变化很大。我们的数据表明，文石向方解石的转变不仅可以发生在 [Mg] 高于实验室实验中可能显示的溶液中，而且逐渐降低 F:S 会增加文石稳定为方解石的速率。例如，在 F:S 为 0.3 mL/g 的实验中，这对应于具有 50% 孔隙率的封闭系统中的沉积物，当 [Mg] = 30 mM (Mg/Ca = 5.14) 时，文石在溶液中稳定为方解石当使用低不饱和度时，使用相对于文石的初始高度不饱和度并且在 [Mg] = 20 (Mg/Ca = 5.14) 的溶液中。相比之下，在 F:S 为 100 mL/g 的实验中，文石在近 3000 小时后不会稳定为方解石，这在开放系统中更为典型，即使在 [Mg] = 5 mM (Mg/Ca = 5.14) 的溶液中也是如此无论饱和度如何。我们的结果还表明，加入方解石产品中的 Mg 量随着 F:S 的增加而线性增加。总的来说，这些观察进一步表明 F:S 是碳酸盐岩成岩作用的一个重要因素，具有广泛的影响。首先，观察到文石向方解石的转化在高 [Mg] 和 F:S 时受到抑制，这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的主要成岩过程，这可能代表了被低估的公海碱度来源。其次，从富含文石的沉积物到在岩石记录中占主导地位的富含方解石的石灰岩的转变可能是由 F:S 的减少和渐进式埋藏过程中封闭系统的发展所促进的。