Age-dating these samples by Sm-Nd and Rb-Sr methods is very challenging because Excluding anomalous phases from the calculation of a Sm-Nd isochron The speed of the LDRIMS measurement allows thousands of samples to be. Feb 19, equation, which is also the basis for radiometric dating techniques, is requirements for Rb-Sr dating over a wider range of geologic conditions .. expanding from this initial event and the velocity of expansion can be. Apr 18, Although we now recognize lots of problems with that calculation, the . We can measure the present ratios of (87Sr/86Sr)t and (87Rb/86Sr)t.
Rb sr dating equation for velocity - Background
While sample return is highly desirable for the range of analytical techniques that can be applied to a specimen, as well as the much greater precision that can be achieved in a geochronology experiment, in situ dating as part of a sample return mission would provide valuable triage data to inform the selection of the best samples for return to Earth.
In situ dating on its own could answer many scientific questions with acceptable precision, and, without many of the costs and risks of a sample return mission, it can be more easily scaled to a campaign of missions to diverse terranes in the inner solar system.
To assess the potential for in situ dating by LARIMS, we have designed and built an instrument for the resonance ionization of Rb and Sr isotopes, so as to employ the radioisotope 87Rb as a chronometer. In this work, we tested our instrument by analyzing the Duluth Gabbro, a compositional analog for lunar basalts enriched in potassium, rare earth elements, and phosphorus, also known as KREEP terranes.
An advantage of the LARIMS technique is that a large number of spot analyses on a rock specimen can be assembled into an isochron. This precludes having to make unverifiable assumptions about the initial abundance of either the parent or daughter nuclide. Strontium abundances in lunar rocks are tens to hundreds of ppm,[e.
To provide useful constraints on the chronology of the Moon, any instrument must be capable of resolving the multiple potential historical hypotheses for the Moon, which span a billion year range of possibilities. The objective of our work was to date a sample of the Duluth Gabbro with Ma precision. Method The method and instrumental approach have been fully described by Anderson et al.
We analyzed our sample of the Duluth Gabbro in much the same way as we analyzed the Martian meteorite Zagami. These multiplicative corrections were then applied to the Duluth Gabbro isotope ratios. Dissolved strontium in the oceans today has a value of 0. Thus, if well-dated, unaltered fossil shells containing strontium from ancient seawater are analyzed, changes in this ratio with time can be observed and applied in reverse to estimate the time when fossils of unknown age were deposited.
Dating simple igneous rocks The rubidium—strontium pair is ideally suited for the isochron dating of igneous rocks. As a liquid rock cools, first one mineral and then another achieves saturation and precipitates, each extracting specific elements in the process.
Strontium is extracted in many minerals that are formed early, whereas rubidium is gradually concentrated in the final liquid phase. In practice, rock samples weighing several kilograms each are collected from a suite of rocks that are believed to have been part of a single homogeneous liquid prior to solidification. The samples are crushed and homogenized to produce a fine representative rock powder from which a fraction of a gram is withdrawn and dissolved in the presence of appropriate isotopic traces, or spikes.
Strontium and rubidium are extracted and loaded into the mass spectrometer, and the values appropriate to the x and y coordinates are calculated from the isotopic ratios measured. Once plotted as R1p i. Using estimates of measurement precision, the crucial question of whether or not scatter outside of measurement error exists is addressed. Such scatter would constitute a geologic component, indicating that one or more of the underlying assumptions has been violated and that the age indicated is probably not valid.
For an isochron to be valid, each sample tested must 1 have had the same initial ratio, 2 have been a closed system over geologic time, and 3 have the same age. Well-preserved, unweathered rocks that crystallized rapidly and have not been subjected to major reheating events are most likely to give valid isochrons. Weathering is a disturbing influence, as is leaching or exchange by hot crustal fluids, since many secondary minerals contain rubidium.
Volcanic rocks are most susceptible to such changes because their minerals are fine-grained and unstable glass may be present.
On the other hand, meteorites that have spent most of their time in the deep freeze of outer space can provide ideal samples. Dating minerals Potassium -bearing minerals including several varieties of mica, are ideal for rubidium—strontium dating as they have abundant parent rubidium and a low abundance of initial strontium. When minerals with a low-rubidium or a high-strontium content are analyzed, the isochron-diagram approach can be used to provide an evaluation of the data.
As discussed above, rubidium—strontium mineral ages need not be identical in a rock with a complex thermal history , so that results may be meaningful in terms of dating the last heating event but not in terms of the actual age of a rock. Dating metamorphic rocks Should a simple igneous body be subjected to an episode of heating or of deformation or of a combination of both, a well-documented special data pattern develops.
With heat, daughter isotopes diffuse out of their host minerals but are incorporated into other minerals in the rock. When the rock again cools, the minerals close and again accumulate daughter products to record the time since the second event. Remarkably, the isotopes remain within the rock sample analyzed, and so a suite of whole rocks can still provide a valid primary age.
This situation is easily visualized on an isochron diagram, where a series of rocks plots on a steep line showing the primary age, but the minerals in each rock plot on a series of parallel lines that indicate the time since the heating event.
If cooling is very slow, the minerals with the lowest blocking temperature, such as biotite mica, will fall below the upper end of the line. The rock itself gives the integrated , more gradual increase. Approaches to this ideal case are commonly observed, but peculiar results are found in situations where the heating is minimal. Epidote, a low-temperature alteration mineral with a very high concentration of radiogenic strontium, has been found in rocks wherein biotite has lost strontium by diffusion.
The rock itself has a much lower ratio, so that it did not take part in this exchange. Although rubidium—strontium dating is not as precise as the uranium—lead method, it was the first to be exploited and has provided much of the prevailing knowledge of Earth history. The procedures of sample preparation , chemical separation, and mass spectrometry are relatively easy to carry out, and datable minerals occur in most rocks.
Precise ages can be obtained on high-level rocks i. The mobility of rubidium in deep-level crustal fluids and melts that can infiltrate other rocks during metamorphism as well as in fluids involved in weathering can complicate the results.
Samarium—neodymium method The radioactive decay of samarium of mass Sm to neodymium of mass Nd has been shown to be capable of providing useful isochron ages for certain geologic materials.
Both parent and daughter belong to the rare-earth element group, which is itself the subject of numerous geologic investigations. All members of this group have similar chemical properties and charge, but differ significantly in size. Because of this, they are selectively removed as different minerals are precipitated from a melt. In the opposite sense, their relative abundance in a melt can indicate the presence of certain residual minerals during partial melting.
Unlike rubidium, which is enriched over strontium in the crust, samarium is relatively enriched with respect to neodymium in the mantle. Consequently, a volcanic rock composed of melted crust would have elevated radiogenic strontium values and depressed radiogenic neodymium values with respect to the mantle.
As a parent—daughter pair, samarium and neodymium are unique in that both have very similar chemical properties, and so loss by diffusion may be reduced. Their low concentrations in surface waters indicates that changes during low-temperature alteration and weathering are less likely.
Their presence in certain minerals in water-deposited gold veins, however, does suggest mobility under certain conditions. In addition, their behaviour under high-temperature metamorphic conditions is as yet poorly documented.
The exploitation of the samarium—neodymium pair for dating only became possible when several technical difficulties were overcome. Procedures to separate these very similar elements and methods of measuring neodymium isotope ratios with uncertainties of only a few parts in , had to be developed.
In theory, the samarium—neodymium method is identical to the rubidium—strontium approach. Both use the isochron method to display and evaluate data. In the case of samarium—neodymium dating, however, the chemical similarity of parent and daughter adds another complication because fractionation during crystallization is extremely limited. This makes the isochrons short and adds further to the necessity for high precision.