Domain structure of volcanics' ferromagnetic grains: A case study of the Saatly ultradeep well, Azerbaijan

М агш тш вл асти во ст р1зних магш тних мшерал1в дуж е залеж ать вгд розм1ру зе ­ рен (доменного стану). П рсью породи часто м к тять су м ш рiзних мaгнiтних мгнерaлiв з р iзними розмiрaми зерен (доменних cтaнiв). Як мaтерiaл для визначення до­ менного стану зерен м iнерaлiв використано вулкаш ти з надглибоко! свердловини Caaтлi в iнтервaлi глибин 3540— 8126 м. За даними петрогрaф iчного та електронномiкроcкопiчного aнaлiзiв у верхнш чacтинi р озр iзy (3540— 5500 м) цих поргд зерн а рудних мiнерaлiв предcтaвленi двома генерaцiями: 1) велию зерн а титаномагнети­ ту (бгдш на титан), гомогеннi або зi структурами розпаду; 2) невеликi за розмiром, зазвичай добре огрaновaнi зерна практично стехю метричного магнетиту. Г ом оген н невеликi зерна або ф рагменти розпаду великих зерен (якг дуже змгнеш хiмiчно) мож уть мати властивостг однодоменних частинок; феромaгнiтнi зерн а часто визначають як псевдоодиничний домен. У н иж нш чacтинi розр iзy (5500—8126 м) мультидом енш зерн а титаномагнетиту збiдненi титаном. З глибин 7000 м спостерггаеться також окиснений магнетит. П севдооднодоменних зерен заф ж сован о значно менш е, ш ж у верхнш частиш розрГзу. З глибини 7300 м i ниж че вони повш стю вгдсутш. За результатами дослгджень, ф ером агнетик представлений Гстотно мультидоменними зернам и. Слгд зазначити, щ о критерiй Л о рГ— Фуллера непридатний при 1дентифЬ кацй доменно! структури складного сполучення феромагнГтних зерен. Ключов1 слова: непрямГ магнГтнГ критерй, магнГтний доменний стан, вулканогеннГ породи, надглибока свердловина.

Introduction. For the first time magne tometric researches of volcanogenic rock co re of the super deep Saatly well have been conducted by the staff of department "Geo magnetism" of the Institute of Geology and Geophysics of Azerbaijan National Acade my of Science (ANAS). The necessity in the magnetic researches of a core of the Saatly super deep well has been dictated by ne ed in obtaining the petromagnetic data used at interpretations of results of geophysical activity. The ore minerals' magnetic research together with microprobe analysis of them [Novruzov, Tselmovich, 2018] revealed that magnetic properties of rocks are bound with magnetite and Ti-poor titanomagnetite.
Environmental conditions of generation of ferromagnetic minerals and changes of them leave traces on magnetic domain struc ture [Stackey, Banerjee, 1974]. Application of the rock magnetic method for identifica tion of domain structure of magnetic grains is the purpose of the proposed article. For realization of this purpose we used four mag netic criteria. The criteria are based on cha racteristics of various magnetic parameters of magnetic grains with different magnetic domain structures.
Description of the object. The superde ep well is located near the city of Saatly (l = = 39.91 °N, j = 48.36 °E), Azerbaijan (Fig. 1). The region of the well location (design depth of 15 km) is spaced in the Kur basin placed between the Greater Caucasus and the Les ser Caucasus (the large tectonic construc tions). According to deep structure the Kur basin is subdivided into three depressions: upper Kura basin, middle Kura basin and lower Kura basin. The Saatly superdeep well is located within the middle Kura basin.
According to geophysical data interpre tation the crystalline basement (around the Saatly local gravitational maximum) posses ses a block structure (where there are rocks of persilicic and basic composition at a depth of 8 km). Debatable idea of crustal structu re within the Saatly local gravitational maxi mum was formed in two points of view. Ac cording to the first -the Earth crust con sists of granite and basalt layers, according to the second -the Earth crust is single layer and it is presented by mafic rocks on ly. Features of the Earth crust and the redu ced thickness of a sedimentary cover at Saatly territory formed a basis for drilling of the super deep well.

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Object of our researches are the volcanics located in the depths interval of 3540-8126 m. According to research papers [Ab dullaev, Salakhov, 1983;Abdullaev et al., 1984] the development of the low-tempera ture metamorphism (depth of 3540-7000 m) has been noted in the volcanogenic rocks. But from depths of 7000 m and deeper the development of the low-temperature meta somatism is revealed. The quantities of the studied volcanic rocks were 49. Three dup licate with cubic form (1 cm) were sawn from each sample. The depth interval of 3540-3900 m is cha racterized by plagioclase basalts and is ma inly presented by volcanogenic facies. Ba salts contain the potassium-enriched and so dium-enriched alkalinity.
The depth interval of 4850-5209 m pos sesses much bigger phases of rocks. Except basalts their amfibole-contaning phases and andesites are noted. Along with low-potas sium basalts potassium-enriched groups ha ve been discovered.
In the depth interval of 5209-6100m the double-pyroxene-plagioclase basalts and an desite-basalts prevail. These rocks possess the alkalescence and the strong fracturing.
There are andesites in the depth interval of 6100-6800 m. The microdolerite sills are noted as well.
Upper parts of 6800-8000 m depth inter val are presented by dacites, dacite-tuffs and pumiceous breccia. In the middle horizons and lower ones of the interval dacites are al ternating with plagiorhyolite and andesites.
Technique of works. During the magne tic researches of volcanogenic rocks of the superdeep Saatly well the following magne tic parameters were measured: a natural re manent magnetization (NRM) and magne tic susceptibility (c ). At the laboratory on each duplicate sample (for each one the NRM and the c were measured) saturation isother mal remanent magnetization (SIRM) and iso thermal remanent magnetization (IRM) in the field of 1.5 T have been created. At the la boratory a thermal remanent magnetizati on (TRM) was created on each duplicate of sample by means of cooling from tempera tures above the Curie point (more than 600 °C) in the presence of magnetic field equal to 0.5 • 10-4 T. Partial thermal remanent mag netizations (PTRM) were created from tem peratures 200-500 ° C (mainly from 200 °C) to 20 °C in the field of 0.5 • 10-4 T.
All laboratory measurements were car ried out at the laboratory of "The main geo magnetic field and a petromagnetizm" of the Institute of Physics of Solid Earth of RAS.
Stepwise demagnetization by increasing alternating field was carried out on the RUV semi-automatic installation in the range of fields of 0-80 A/m. The PTRM were ther mally demagnetized by a routine technique [Kobayashi, Fuller, 1967].
Results of the researches. Magnetic re searches of the well volcanogenic rocks sho wed that the measured magnetic parame ters of M rs and M s and TRM and Qn are changing according to depth. Changes of these parameters reflect variations of grain size. Magnetic parameters of NRM, c , SIRM (Mrs), M s and TRM were measured for study of a minerals' magnetic domain state of the Saatly superdeep well. There are three types of different magnetic behavior, depending on the grain size. The defined parameters allowed calculating Qn factor and magni tude of M rs /M s.
For use of magnetothermal criterion the partial thermal remanent magnetizations (PTRM) were created. The PTRM were acquired in the range of temperatures 200-500 °C. For identification of a magnetic domain state of volcanic ferromagnetic grains of the Saatly well the four magnetic criteria were used: factor Qn, M rs /M s, Lauri-Fuller's criterion and thermal magnetic criterion.
Short consideration of identification cri teria of a magnetic domain state had bro ught to light that that any of criteria does not work completely well, even in case of ensembles of "pure" grains. The found struc tural heterogeneities of grains [Novruzov, Tselmovich, 2018] and also grains various by structure and the sizes ( magnetic rese arches of the structure and concentration) make impossible to use only one of the men tioned criteria. Therefore in this article, the most often used in rock magnetism the four criteria are applied. At the same time the judgment of a magnetic domain state was based on coincidence of results of identifi cation by the two or the three criteria.
The results of identification of a magne tic domain state of the ferromagnetic grains are presented in Table. In Fig. 2

MD (PSD)
A

MD (PSD)
A ndesiteb asa lt 6740-6745 2,90 PSD 0,03 MD SD -  the pseudo-single-domain state often beco mes apparent. It demonstrates that rema nent magnetization carriers in this interval of depths are the finest grains. These grains, most probably, belong, to the second gene ration of grains of magnetite and the Ti-poor titanomagnetite determined on the basis of microprobe analysis [Novruzov, Tselmovich, 2018].

MD (PSD)
In the bottom of a section (5500-8126 m) is more often, and in the depth interval 7300-8126 m one and all studied rocks contain only multidomain grains of ferromagnetic minerals.
This result presumably can be bound to that circumstance that the low-temperature metamorphism, especially metasomatism, results in partial or complete collapse (repla Fig. 3. The alternating m agnetic field dem agne tization curves (TRM and SIRM) as Lauri-Fuller's criterion for bottom part of section. cement) of fine grains of the ferromagnetic minerals in the bottom of a section which are contained in rocks.
At the same time artificial "enrichment" of rocks with more coarse magnetic grains is observed. As shown by means of micro probe [Novruzov, Tselmovich, 2018] of ore grains the fine fraction of the deepest rocks is presented, mainly, by fragments of the crushed large grains.
Discussion of results. The done petromagnetic researches showed that in most cases grains of magnetic minerals ( carriers of remanent magnetization) represent an en semble of multidomain particles. In upper part of the section (3540-5500), especially in the depth interval 3540-5000 m, the pse udo-single domain state is often presented. It demonstrates that in this depth interval, except of course (large volume) multidoma in grain generation, fine grains of other gene ration can bear a remanent magnetization.
The results of identification of a domain structure presented in the summary table, in the deeper section (5500 -8126 m) are more often, and in the depth interval 7300-8126 m one and all ferromagnetic minerals are demonstrating only multidomain grains content. This result presumably can be ca used by the fact that in the deeper section the generation (rather) of fine ferromagne tic grains can be partially or entirely destro yed due to occurrence of the low-tempera ture metamorphism (metasomatism). As a re- Fig. 4. Thermal dem agnetization of the PTRM as therm om agnetic criterion of identification of type of dom ain state. suit the share of the ferrimagnetic coarse grains possessing a multidomain structure increases.
As is obvious from the table the Lauri-Fuller's criterion is inapplicable for the com plex ensemble of ferromagnetic grains. Use of this criterion eventuates in the diametri cally opposite data received on the basis of the two or the three other criteria of doma in structure identificationThe thermal mag netic criterion is sensitive to manifestation of single-domain (pseudo-single-domain) and of multidomain ferromagnetic grains of in the complex ensemble of grains.