gtcrustГеодинамика и тектонофизикаGeodynamics & Tectonophysics2078-502XInstitute of the Earth's crust of the Russian Academy of Sciences, Siberian Branch10.5800/GT-2021-12-3-0535gtcrust-1236Research ArticleТЕКТОНОФИЗИКАTECTONOPHYSICSМОДЕЛЬ МЕХАНИЗМА ДЕКОМПРЕССИОННОГО ПЛАВЛЕНИЯ В КОНВЕКТИВНО-НЕУСТОЙЧИВОЙ ТЕРМИЧЕСКОЙ ЛИТОСФЕРЕ (ПЕРВОЕ ПРИБЛИЖЕНИЕ)MODEL OF DECOMPRESSION MELTING MECHANISM IN CONVECTIVE-UNSTABLE THERMAL LITHOSPHERE (FIRST APPROXIMATION)ЛуневБ. В.LunevB. V.

630090, Новосибирск, прт Академика Коптюга, 3

3 Academician Koptyug Ave, Novosibirsk 630090

bobvalmail@mail.ruЛапковскийВ. В.LapkovskyV. V.

630090, Новосибирск, прт Академика Коптюга, 3

3 Academician Koptyug Ave, Novosibirsk 630090

Институт нефтегазовой геологии и геофизики им. А.А. Трофимука СО РАНРоссияTrofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of SciencesRussian Federation202117092021123485498Copyright © Лунев Б.В., Лапковский В.В., 20212021Лунев Б.В., Лапковский В.В.Lunev B.V., Lapkovsky V.V.Данная работа распространяется под лицензией Creative Commons Attribution 4.0.This work is licensed under a Creative Commons Attribution 4.0 License.https://www.gt-crust.ru/jour/article/view/1236

Предложена модель декомпрессионного плавления, сепарации, миграции и замерзания расплава в процессе развития конвективной неустойчивости верхней мантии, позволяющая учесть различие фазовых диаграмм расплава и матрицы и вытекающие особенности поведения расплава, без расчета скорости реакций в многокомпонентной среде, в рамках явного представления о локальном термодинамическом равновесии существующих фаз. Таким образом, дополняется развиваемое нами первое приближение описания процесса конвекции в верхней мантии и формирования крупных эпиконтинентальных осадочных бассейнов, опубликованное ранее.

Вычислительными экспериментами показано, что первичное плавление фертильного вещества верхней мантии происходит интенсивно в узком фронте поднимающегося в восходящем потоке горячего вещества. Далее, вверх от фронта первичного плавления, поднимается деплетированное и частично выплавленное вещество. Дальнейшее плавление деплетированного вещества происходит выше, при меньших давлениях в довольно широком диапазоне глубин (120–77 км). Далее мигрирует расплав уже от двух источников – глубинного, где плавится фертильное вещество, и промежуточного, где плавится вещество деплетированное. Достигнув уровня температур и давлений, соответствующих его солидусу, расплав образует фронт замерзания, примерно такой же узкий, как и фронт первичного плавления. По мере развития восходящего конвективного потока фронт замерзания смещается вверх. В результате под ним формируется довольно толстый (около 40–50 км) слой вещества, насыщенного «базальтовым» компонентом. Важным результатом моделирования является то, что, несмотря на значительные общие объемы выплавляющегося вещества, единовременное содержание расплава в мантии, при осреднении на объемы с линейным размером порядка 1 км, не превышает десятых долей процента. Экстракция базальтовой выплавки, в связи с обеднением мантийного вещества железом, существенно снижает его плотность. При рассчитанных значениях обеднения матрицы базальтовым компонентом 0.1–0.2 дефицит плотности удваивается, по сравнению тепловым расширением вещества. Стало быть, удваивается и число Рэлея, и интенсивность конвекции, что мы и видим в расчетах – после начала плавления конвекция усиливается.

Проведенное опробование модели дает разумную картину, согласующуюся как с известной геолого-геофизической информацией о строении литосферы под развивающимися эпиконтинентальными осадочными бассейнами, так и, в рамках своей детальности, – с результатами моделирования плавления и динамики расплава, полученными путем расчета реакций между компонентами мантийного вещества.

We propose a model of decompression melting, separation, migration and freezing of the melt in the upper mantle during the convective instability process. The model takes into account differences between phase diagrams of the melt and the matrix and the resultant features of the melt’s behavior, without calculating reaction rates in a multicomponent medium. It is constructed under an explicit concept of the local thermodynamic equilibrium of the existing phases. Therefore, we further develop the first approximation of the descriptions of convection in the upper mantle and the formation of large epicontinental sedimentary basins, which have been presented in earlier publications. Our computational experiments show that primary melting of the upper mantle’s fertile material occurs intensively in a narrow frontal part of the ascending hot material flow. Then, the depleted and partially melted material rises farther upward from the front of primary melting. Melting of the depleted material continues at lower pressures in a rather wide range of depths (120–77 km). Further, the migrating melt is supplied by two sources, i.e. a deep-seated one, wherein the fertile material melts, and the medium-depth one, wherein melting of the depleted material takes place. Once the temperature and pressure rates of the melt reach the values corresponding to those of its solidus, a narrow freezing front is formed. Its width is almost similar to the primary melting front. As the ascending convective flow develops, the freezing front shifts upward. As a result, a quite thick (around 40–50 km) basalt-saturated layer occurs above the freezing front. An important observation in our modeling experiments is that, despite a considerably large total volume of the melted material, a one-time melt content in the mantle does not exceed tenths of one percent, when we consider averaging to volumes with a linear size of about 1.0 km. The basalt melt extraction depletes iron in the mantle and significantly reduces the mantle density. Considering the calculated basalt-depletion values for the matrix at 0.1–0.2, the density deficit doubles in comparison to the thermal expansion of the material. Logically, both the Rayleigh number and the intensity of convection also double (and this is confirmed by the calculations), which means that convection is enhanced after the melting start.

Testing of the model shows that it gives a reasonable picture that is consistent with the available geological and geophysical data on the structure of the lithosphere underneath the currently developing epicontinental sedimentary basins. Furthermore, within the limits of its detail, this model is consistent with the results of modeling experiments focused on melting and melting dynamics, which are based on calculations of reactions between components of the mantle material.

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