{"cells": [{"cell_type": "markdown", "metadata": {"tags": ["module-htg"]}, "source": ["# Sm-Nd Decay\n", "[High-Temperature Geochemistry](module-htg) \n", "```{index} Sm-Nd decay\n", "```"]}, {"cell_type": "code", "execution_count": 3, "metadata": {"tags": ["hide-input"]}, "outputs": [], "source": ["# import relevant modules\n", "\n", "%matplotlib inline\n", "import numpy as np\n", "import matplotlib.pyplot as plt\n", "import pandas as pd\n", "from IPython.display import display\n", "from math import log10, floor"]}, {"cell_type": "code", "execution_count": 4, "metadata": {"tags": ["hide-input"]}, "outputs": [], "source": ["# create our own functions\n", "\n", "# function to round a value to a certain number of significant figures\n", "def round_to_n_sf(value, no_of_significant_figures):\n", " value_rounded = round(value, no_of_significant_figures-1-int(floor(log10(abs(value)))))\n", " if value_rounded == int(value_rounded): \n", " value_rounded = int(value_rounded)\n", " return value_rounded\n", " \n"]}, {"cell_type": "markdown", "metadata": {}, "source": ["## Sm-Nd Decay System\n", "\n", "$${^{147}Sm \\longrightarrow {^{143}Nd} + \\alpha} \\qquad t_{\\frac{1}{2}} = 106\\,Gyr$$\n", "\n", "Both $Sm$ (samarium) and $Nd$ (neodymium) are Rare Earth Elements ($REEs$). In nature, both elements generally occur in dispersed form, with typical concentrations in mantle and crustal rocks of less than $\\sim20\\,ppm$. \n", "\n", "Most $REEs$, including $Sm$ and $Nd$, occur as trivalent ($3+$) ions with ionic radii that decrease systematically with increasing atomic number, so $Sm$ has a smaller ionic radius than $Nd$.\n", "\n", "Both $Nd$ and $Sm$ are moderately incompatible elements, but $Nd$ is slightly more incompatible than $Sm$ during mantle melting because it has a slightly larger ionic radius. \n", "\n", "The $REEs$ are generally considered to be relatively resistant toward mobilization by fluids \u2013 they are fluid-immobile elements. \n", "\n", "## Dating of Terrestrial Rocks\n", "\n", "The continental crust in general and siliceous rocks in particular have low and relatively uniform $Sm/Nd$ ratios (= parent/daughter ratio), so the $Sm$-$Nd$ system is not particularly suitable for dating such rocks. On the other hand, mafic and ultramafic rocks have variable and high $Sm/Nd$ ratios, so they are good candidates for $Sm$-$Nd$ dating. \n", "\n", "Due to the fluid immobility of the $REEs$, $Sm$-$Nd$ ages and initial $Nd$ isotope compositions are not very sensitive to weathering and metamorphism. \n", "\n", "Notice how the $Sm$-$Nd$ system is opposite to the $Rb$-$Sr$ decay system in many ways! \n"]}, {"cell_type": "code", "execution_count": 5, "metadata": {}, "outputs": [], "source": ["# Sm-Nd decay equation - very similar to the Rb-Sr one\n", "# each return depends on what we want to find from the equation\n", "def Sm_Nd_decay_equation(Nd143_Nd144_ratio, initial_Nd143_Nd144_ratio, Sm147_Nd144_ratio, t):\n", " decay_const_Sm = 6.54 * 10**-12 # yr^-1 # decay constant of Sm-147\n", " if Nd143_Nd144_ratio == '?':\n", " return initial_Nd143_Nd144_ratio + Sm147_Nd144_ratio*(np.exp(decay_const_Sm*t)-1)\n", " elif initial_Nd143_Nd144_ratio == '?':\n", " return Nd143_Nd144_ratio - Sm147_Nd144_ratio*(np.exp(decay_const_Sm*t)-1)\n", " elif Sm147_Nd144_ratio == '?':\n", " return (Nd143_Nd144_ratio - initial_Nd143_Nd144_ratio)/(np.exp(decay_const_Sm*t)-1)\n", "\n"]}, {"cell_type": "markdown", "metadata": {}, "source": ["## Problem Set 5\n", "\n", "### Question 1\n", "\n", "A pigeonite basalt ($12039$, $19$) from the Moon yielded the following results:"]}, {"cell_type": "code", "execution_count": 7, "metadata": {}, "outputs": [{"data": {"text/html": ["
Samples | $${^{147}Sm}/{^{144}Nd}$$ | $${^{143}Nd}/{^{144}Nd}$$ |
---|---|---|
Whole rock | \n", "0.209 | \n", "0.513142 | \n", "
Plagioclase | \n", "0.1727 | \n", "0.512365 | \n", "
Pyroxene | \n", "0.2434 | \n", "0.513861 | \n", "