Petrološke, mineraloške in geokemične značilnosti ter radiometrična datacija magmatske kamnine iz Puerta Angela v Mehiki

Božič, Dominik (author), Dolenec, Matej (mentor), Štrok, Marko (comentor)

Analiziran je bil kos magmatske kamnine, ki je bil najden v okolici Puerta Angela v Mehiki. Preden je bil prežagan, je makroskopsko kazal teksture podobne tistim, ki so značilne za meteorite. Lokacija njegove najdbe je blizu krajev historično povezanih z meteoriti. Na področju preiskave magmatskih kamnin in meteoritov se uporabljajo petrološke, mineraloške, geokemične in radiometrične metode. Podatki so pridobljeni makroskopsko, z optičnim mikroskopom, z SEM-EDS (vrstična elestronska mikroskopija - energijska disperzijska spektroskopija rentgenskih žarkov), XRD (rentgenska difrakcija), ramanovo spektrometrijo, geokemično analizo in datiranjem z uporabo Rb-Sr metode. Radiometrična datacija je postopek, ki je splošno v rabi že več kot pol stoletja, vendar v Sloveniji še ni bil uporabljen do sedaj. Postopek vsebuje drobljenje kamnin, ročno prebiranje, taljenje in kislinski razklop, separacijo Rb od Sr, razredčevanje vzorcev in meritve razmerij izotopov na MC-ICP-MS (multi-kolektorskem masnem separatorju). V postopku je več možnih napak in sicer prisotnost mlajših mineralov, potencialno selektivno razkrajanje in separacija, padanje občutljivosti naprav in nizko število vzorcev. Testni vzorci so pokazali napako 22 Ma, katero uporabimo tudi kot napako meritve starosti PAGZ-1. Na podlagi teh metod je ugotovljeno, da je preiskovana kamnina globočnina, in sicer gabbro-norit, ki jo sestavljajo pirokseni, plagioklazi, biotit rogovača in nekateri sulfidi, sulfati in oksidi, ki so najverjetneje posledica hidro-termalnih procesov. PAGZ-1 nima ne meteoritske skorjice ne regmagliptov, čeprav so vidne vdolbine premera do 2 cm, ki so jim na videz podobne. Starost kamnine smo določili na 201 ± 22 Ma, začetno razmerje 87Sr/86Sr je 0,7042. Zanj je značilna pozitivna Eu anomalija. Primerjanje s kamninami v okolici Puerta Angela in skupinami meteoritov pokaže, da obstajajo petrološke podobnosti s primerki luninih morij, da se PAGZ-1 mineraloško razlikuje od vseh znanih meteoritskih kamnin, da se Eu anomalija razlikuje od tiste v regionalnih kamninah ter da izmerjena starost ni primerljiva z lokalnimi mehiškimi kamninami, prekriva pa se z nekaterimi skupinami meteoritov z Marsa. Na podlagi teh ugotovitev ni mogoče definitivno določiti izvora PAGZ-1, gotovo pa ne sodi v nobeno do sedaj znano skupino.

• Language: Slovenian
• Keywords: Meteorit, gabbro-norit Rb-Sr datiranje, EDS-SEM, MC-ICP-MS, JZ Mehika
• Work type: Bachelor thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2018
• PID: 20.500.12556/RUL-103689
• COBISS.SI-ID: 1424478
• Publication date in RUL: 21.09.2018

Secondary language

• Language: English
• Title: Petrological, mineralogical, geochemical characteristics and radiometric dating of an igneous rock from Puerto Angelo in Mexico

Abstract

A piece of igneous rock found in the surroundings of Puerto Angelo in Mexico (PAGZ-1) has been analyzed. Before it has been sawed it showed some textures similar to those characteristic of a meteorite. Also its location of discovery is close to the places historically tied to meteorites. In the field of igneous rocks and meteorites the methods used to determine in which group the specific specimen belongs are petrological, mineralogical, geochemical and radiometric. Data are acquired macroscopically, with optical microscope, using EDS-SEM (Scanning Electron Microscopy - Energy-dispersive X-ray spectroscopy), XRD (X-ray Diffraction), Raman spectrometry, geochemical analysis and dating using Rb-Sr method. Radiometric dating is a procedure used for more than half of a century, but have not yet been used in Slovenia untill now. Used procedure includes crushing and hand picking the specimen, melting and dissolving it in acid, separation of Rb from Sr, sample dissolution, and measurments of isotope rations on MC-ICP-MS (Multicollector-Inductively Coupled Plasma Mass Spectrometry). This procedure has many potential points where mistakes could happen. These are potential selective disolving and separation, lowering of detectoion trashold and a small amount of measured samples. Test samples showed an uncertanty of 22 Ma, which is also used for PAGZ-1. On basies of these methods we find out, that the analyzed rock is gabbro-norite, composed of pyroxene, plagioclase, biotite, hornblende and some other sulfides, sulfates and oxides, which were most likely consequence of hydro-thermal processes. Neither fusion crust nor regmaglipts were observed, although there are some pits 2 cm in diameter present. The derermined age is 201 ± 22 Ma and the initial 87Sr/86Sr ratio is 0.7042. It has a positive Eu anomaly. The comparison between the rocks in the surroundings of Puerto Angelo and the meteorite groups show that there are certain petrologic similarities with Lunar maria, that mineralogically PAGZ-1 is different to every known meteorite group, that the Eu anomaly is different to those in regional Mexican rock, and the age is not comparable to the local Mexican rocks but it overlaps with some Martian meteorite groups. On the basis of these findings the origin of PAGZ-1 cannot be definitely determined; however it does not belong to any known meteoritg group.

• Keywords: Meteorite, gabbro-norite, Rb-Sr dating, EDS-SEM, MC-ICP-MS, SW Mexico