Method for Analysis of Trace Elements in Rocks by Inductively Coupled Plasma Mass Spectrometry

Inductively coupled plasma mass spectrometry (ICP-MS) is an instrument for the determination of ultra trace elements and isotope ratios. It consists of a plasma generator, an atomization chamber, a torch tube, a quadrupole mass spectrometer and a fast channel electron multiplier tube (called an ion detector or collector).

1. Its Working Principle is:

The atomizer sends the solution sample to the plasma light source, vaporizes at high temperature, dissociates the ionized gas, and the ions collected by the copper or nickel sampling cone form molecular beam at low vacuum pressure of 133.322pa, then through the 1 ~ 2 mm diameter interceptor plate into the quadrupole mass spectrometer After mass separation by mass filter, it reaches the ion detector. According to the proportional relationship between detector count and concentration, the element content or isotope ratio can be measured.

Its advantages are: low detection limit (ng / ml or lower), small matrix effect, simple spectral line, simultaneous determination of many elements, wide dynamic linear range and rapid determination of isotope ratio.

In geology, it is used to determine the isotopic ratios of trace, trace and ultra trace metal elements, some halogen elements, nonmetal elements and elements in rocks, ores, minerals, inclusions and groundwater.

The total magnetic field intensity (T) at any point on the earth is a vector, which can be divided into seven variables: magnetic declination (D), magnetic inclination (I), horizontal magnetic field intensity (H), East horizontal magnetic field intensity (Y), North horizontal magnetic field intensity (X) and vertical magnetic field intensity (Z). The other three variables can be obtained by knowing the three vectors of X, Y, Z or H, D, Y. By measuring the elements of natural residual magnetic field from the samples, the basic data of paleomagnetism are obtained.Geomagnetic elements (magnetic dip angle, magnetic declination angle) and magnetic pole position change with time. The change time of magnetic pole position is long but not significant. For example, in the past 20 million years (since Miocene), the magnetic pole position of volcanic remanence has always changed around the geographical pole. Since Quaternary, the magnetic pole position of China has been concentrated in the range of 80 ° to 90 ° north latitude and moved around the earth’s axis. The change period of geomagnetic polarity direction is 0.01 ~ 1MA, so the polarity change is more suitable for Quaternary sediment age measurement. It is a basic feature of paleomagnetic history that paleomagnetic polarity alternates in positive and negative directions. Positive polarity (positive magnetization) means that the polarity direction of rock remanence is consistent with that of modern earth, its magnetic dip angle is positive (northern hemisphere), and its magnetic declination is close to zero. Reverse polarity (or reversal of magnetic polarity) means that the polarity direction of rock remanence is opposite to that of the modern earth, its magnetic dip angle is negative, and its magnetic declination angle is close to 180 degrees. In the history of positive and negative changes of geomagnetic polarity of the earth, the time unit with a dominant polarity and a longer duration is generally about 1 MA, which is called polar time (epoch, period). The short period (10000 to 100000 years) of polarity reversal in polarity time is called polarity sub time (event). During the polarity time, there are some short-term events of polarity direction change, which reflects the relationship between the general trend of polarity change and the small change.

2. Application Conditions and Sampling Requirements:

Since the age of the studied geological body is obtained by comparing the paleomagnetic method with the standard paleomagnetic polarity chronology, and the whole history of the earth has magnetic field, this method is not limited by time and can be used for the age study of the whole Quaternary period. Although the method is not limited by time, it is also restricted by the following conditions in practical application: ① The Quaternary strata studied should be sedimentary continuous without sedimentary discontinuity. If there is sedimentary discontinuity, the polarity event may be lost and the correlation result may be wrong;② The stratigraphic section should be relatively thick, and the section too thin is not suitable for paleomagnetic chronology; ③ The grain size of the sediment in the profile should be relatively fine, and clay, silty clay and clayey silt are the best. Gravel and sand are not conducive to the use of this method; ④ The Quaternary sedimentary strata are not affected by the later dike intrusion; ⑤ Paleomagnetic chronology usually needs the assistance of other dating methods.

Paleomagnetic sampling requirements: First, samples were taken from fresh stratigraphic sections. Sampling tools should not be magnetic, and iron tools should not be used, usually, copper tools or plastic products can be used. Secondly, orientational samples must be taken. The occurrence of strata and the upward and northward directions must be marked on the sample box. If sampling is carried out in the core of the borehole, the upward direction shall be indicated and shall not be reversed. Third, take two samples from the same height in each sampling layer for testing and standby. Fourth, usually use 2cm × 2cm × 2cm plastic box sampling, also use cylindrical sampling box, depending on the test instrument. Fifth, in the Quaternary loose layer sampling, firstly clear out a table, draw the north and East directions on the table, and then buckle the sample box on the layer (the straight line on the box is aligned with the north, and the small round hole is placed on the East Side) gently press to take out the sample. Sixth, the vertical spacing of sampling layers should not be more than 1m (or be relaxed as appropriate), and the samples should be continuously sampled at equal intervals in the vertical direction.

The magnetic inclination (I) and magnetic declination (d) were measured by magnetometer or superconducting magnetometer. Based on the first two measurements, especially the polarity column made from the magnetic dip angle, and then compared with the standard polarity chronology, the age of sediments can be inferred indirectly. If a few mammalian fossils or other chronological data are found on the section, the effect will be better. Paleomagnetic methods have been widely used in core research of loess, lacustrine sediments, continental shelf and plain boreholes.

In the application of paleomagnetic methods, it is difficult to compare the measured paleomagnetic polarity column with the standard paleomagnetic polarity chronology. Different scholars often have different results on the same paleomagnetic polarity column. Therefore, in the process of paleomagnetic pole correlation, the determination of the correlation point is the key. In the study of Quaternary paleomagnetism, there are two kinds of paleomagnetic poles: The first is that the end time of the paleomagnetic polarity column is the present, such as the sedimentary section in the ocean and the present lake. It is relatively simple to compare this kind of paleomagnetic pole. If the sediments of the tested section are continuous, the density of the samples is relatively high, and there is no loss of polarity events, then the present (the top of the polarity column) can be determined as the contrast point, which can be compared with the polarity events of the standard paleomagnetic polarity chronology in reverse time. The other situation is more complicated. The obtained paleomagnetic polarity column is incomplete. Only the paleomagnetic polarity column of a certain period does not continue to the present day, and the upper paleomagnetic record is missing, such as the paleomagnetic polarity column of early Pleistocene and Middle Pleistocene. The correlation of this kind of paleomagnetic pole is very complex. The key problem is to determine the correlation point according to fossils, dating data and the general characteristics of paleomagnetic pole. Then, it can be compared with the standard paleomagnetic polarity chronology upward (in the direction of new age) or downward (in the direction of old age).