Abstract
The final ratio equation of an isotopic element in a rock, derived from water/rock formula of McCulloch et al. Earth Planet Sci Lett 46:201-211, 1980, McCulloch et al. J Geophys Res 86:B4 2721-2735, 1981 is used to assess the behavior of diverse suites of rocks towards the alteration effect, and what implications can give about hydrothermal alteration in terms of isotopic compositions. Due to their higher Sr and lower Nd initial ratios than seawater, rocks of metamorphic and sedimentary signatures such as carbonates and Precambrian basement rocks show similar but inverse mixing curves compared with igneous rocks. Sr composition of rocks immediately alters by seawater, while Nd composition keeps unchanged until large volumes of water are added. Although, this can be attributed to the very low Nd concentration in seawater, it indicates that Nd-exchange may only take place under seawater, possibly hydrothermally by circulated seawater, and Nd-concentration of less altered crustal rocks are apparently primary. The isotopic composition and rock mineralogy seem to be the main factors controlling the volume of water required to cause isotopic alteration in rocks. Crustal rocks require higher water volumes due to their relatively low temperature minerals, whereas, mantle peridotites mainly consist of residual olivine minerals that are highly susceptible to alteration and lack of Sr and Nd compositions, and so need less amount of water for metasomatism. This property reduces the limited penetration effect as the mafic affinity increases at depth in the oceanic crust, and enables modified (probably acidified) circulated fluids to maintain ion exchanging and leaching throughout their passageway.
Abstract
معادلة العلاقة النهائية لنظائر العنصر في الصخر والمستنتجة من معادلة الماء\ الصخر ل (1980&1981 (McCulloch et al. ثم استخدامها لتقييم كيفية سلوك انواع مختلفة من الصخور تجاه عملية التغير الماء حراري (Alteration) وماهي النتائج والآثار المستنبطة حول هذا التغير الحرمائي.
نظراً لأن تركيز نظائر Sr و Nd في الصخور المتحولة والرسوبية تكون أساساً مرتفعه ل (Sr ) منخفضة ل(Nd ) عما هو موجود في مياه البحر، فإن هذه الصخور تبدي منحنيات اختلاط بيانيه مشابهه في الشكل ومعاكسة في الاتجاه لما تبديه الصخور النارية.
بشكل عام فإن نسبة (Sr) تتأثر سريعا وبشكل مباشر بفعل مياه البحر، بينما نسبة (Nd) تبقى غير متأثر حتى تضاف كميات كبيره من مياه البحر، وبالرغم من إن هذا الأمر قد يمكن إرجاعه الى قلة تركيز (Nd) الشديدة في مياه البحر، إلا أنه يشير الى أن : تبادل أوتغير تركيز (Nd) في الصخور قد يحدث فقط في مياه البحر عن طريق تغلغل ودوران تيارات البحر الحرمائيه داخل صخور القشره المحيطية. وأن نسبة (Nd) في صخور القشرة الغير متغيره (أو الأقل تغيراً) هي نسبة حقيقية أصيله (وليست معدلة لاحقا).
من تحليل التمثيلات البيانية الناتجة يتبين أن التركيب المعدني يلعب دوراً أساسيا في التحكم بكمية المياه المطلوبة لإحداث تغير مائي حراري في الصخور، حيث أن:
صخور القشرة تحتاج الى كمية كبيرة من المياه كي تتغيّر بسبب احتوائها على معادن ذات درجة انصهار منخفضة ، وبالتالي أصعب تأثراً بالتغير. بينما الصخور الفوق قاعدية (وخاصة البريدوتيت) فأنها تحتوي على معادن ذات درجة انصهار عالية مثل الاوليفين المعروف بشدة حساسيتة وتأثره بالتغير الماء الحراري، كما أنه ذو تركيز منخفض جداً ل Sr و Nd ولذا فأن هذه الصخور تتأثر بكميات أقل من المياه. هذه الخاصية تعمل على تقليل أثر محدودية الشقوق (وبالتالي صعوبة تغلغل واختراق المياه) في صخور القشرة المحيطية الأعمق نظراً لزيادة الصفة القاعدية لهذه صخور بزيادة العمق مما يساهم في استمرارية التبادل الأيوني العنصري للسائل المهجن (وربما المحمض) خلال طريق دورانه من الأسفل الى الأعلى.
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Acknowledgment
The author sincerely appreciates the help of Prof. Gunter Faure (professor emeritus at the Ohio State University, Columbus OH) for providing the main data of calculation and his comments in the initial draft.
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Alaabed, S. Rocks mineralogy controls the metasomatic activities: an indication from water/rock ratio modeling. Arab J Geosci 4, 283–289 (2011). https://doi.org/10.1007/s12517-010-0146-x
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DOI: https://doi.org/10.1007/s12517-010-0146-x