Pearce Element Ratio Diagrams Research Articles

Lithogeochemistry and Hydrothermal Alteration at the Halfmile Lake South Deep Zone, a Volcanic-Hosted Massive Sulfide Deposit, Bathurst Mining Camp, New Brunswick

The Halfmile Lake South Deep zone, Bathurst Mining Camp, New Brunswick, was discovered by Noranda Inc. (Exploration) as a result of a 3-D seismic survey in 1998 and the subsequent drilling of ten diamond-drill cores. The deposit consists of massive, breccia, and stockwork Pb-Zn-Cu sulfide minerals, and is hosted by a volcano-sedimentary sequence belonging to the overturned Ordovician Tetagouche Group. Epiclastic rocks and interbedded fine-grained felsic pyroclastic rocks dominate the stratigraphic footwall. Locally, crystal-rich felsic tuffs and subordinate epiclastic rocks comprise the immediate stratigraphic hanging wall. The entire stratigraphic sequence was intruded by quartz-feldspar porphyritic intrusions, and cut by intermediate and basic dikes. The volcanic and sedimentary rocks can be discriminated geochemically using trace element ratios such as Zr/TiO2 and Nb/TiO2, despite intensive alteration and cleavage development. These ratios indicate that four protolith volcanic compositions exist: rhyolite, dacite, andesite, and basalt. The aphyric and feldspar- and quartzphyric volcanic rocks are dacitic and rhyolitic in composition; epiclastic rocks have trace element ratios consistent with dacitic compositions. Pearce element ratio diagrams, general element ratio diagrams, and petrographic observations demonstrate that the rhyolitic volcanic rocks exhibit evidence of albite, potassium feldspar, and quartz fractionation, dacitic volcanic rocks exhibit little evidence of any fractionation, and epiclastic sedimentary rocks exhibit evidence of quartz sorting only. Hydrothermal alteration is best represented by the presence of phengitic muscovite and daphnitic chlorite. Minor calcite occurs in the stratigraphic hanging wall, deep in the stratigraphic footwall, and in post-mineralization dikes, and thus is not interpreted to be part of the causative hydrothermal event. Element additions and losses during alteration have been used to determine net alteration reactions, and these have been used to identify alteration parameters that are independent of other forms of alteration and fractionation/sorting, and which can be used to guide exploration. These parameters include: a bulk hydrolysis measure, (2Ca+Na+K–2CO2)/Al; a muscovite alteration measure, K/Al; an albite destruction measure, Na/Al; a chlorite alteration measure, (Fe+Mg–S/2)/Al; a chlorite composition measure, (Fe–S/2)/Mg; a sulfidization measure, S/Ti; and a carbonatization measure, CO2/Ti. With the exception of the carbonatization measure, these alteration parameters define a distinct lateral and vertical zone of intense hydrothermal alteration in the stratigraphic footwall of the deposit, and demonstrate that the epiclastic rocks are predominantly chlorite altered, and the volcanic rocks are predominantly muscovite altered. Within the alteration halo, element additions of Fe and H, and element losses of Na are characteristic. Potassium was added to muscovite-altered rocks, but subsequently removed from chlorite-altered rocks. The results from this study demonstrate that metamorphism and deformation have not significantly obscured hydrothermal alteration signatures.

Petrology of Recent lava flows, Volcano Mountain, Yukon Territory, Canada

Volcano Mountain, Yukon Territory, a Quaternary edifice located near the junction of the Pelly and Yukon Rivers, erupted two Recent nephelinite lava flows between 3000 and 7300 years BP and a third Recent lava flow that is probably younger. The porphyritic nephelinites have olivine phenocrysts, Fo 91-Fo 79, and Ti-rich diopside microphenocrysts. The groundmass consists of olivine, Fo 83-Fo 80), Ti-rich diopside, ulvöspinel-magnetite solid solutions, nepheline, leucite, patches of residual glass and traces of calcite. The nephelinites also carry spinel-lherzolite xenoliths and xenocrystic olivine, Fo 87-Fo 90. The xenocrysts have lower CaO contents than the phenocrysts. Samples from the two older flows have similar Thompson space representations whereas the youngest flow has a distinctly different representation. The Thompson space representations of the Volcano Mountain nephelinites are different from the representations of nephelinites from other centers in the Cordillera. Thompson space representations and Pearce element-ratio diagrams suggest the two older flows are comagmatic whereas the youngest flow is from a separate magma batch. Thermodynamic modeling suggests the magmas that supplied the lava flows originated at pressures near 2.5 GPa, left behind a residue containing olivine, and rose along an isenthalpic path after separating from the source rocks.

Magmatic origins of calc-alkaline intrusions from the Coast Plutonic Complex, southwestern British Columbia

Major element, trace element, and rare earth element data are presented for Permian to Tertiary calc-alkaline plutonic and volcanic rocks along a transect across the southern Coast Plutonic Complex from Vancouver to Anderson Lake. Late Jurassic to Late Cretaceous plutons are divided into two compositional suites based on mineralogy: (1) the hornblende intrusive suite (tonalite, quartz diorite, diorite, and gabbro) characterized by abundant modal hornblende and little or no K-feldspar, and (2) the K-feldspar intrusive suite (mainly granite and granodiorite) containing significant modal K-feldspar and less hornblende. Compositions of hornblende intrusive suite rocks are effectively portrayed on Pearce element-ratio diagrams utilizing axes X1 = [0.8571 Si−0.1429(Fe + Mg) + 1.2857 Ca + 1.8574 K]/Zr and Y1 = 1.1428 Ti + Al + Fe + Mg + Ca + 1.5714 Na + 0.4762 P]/Zr, because the diagram accounts for the stoichiometry of PI ± Hbl ± Bt ± Ep ± Ttn + Ap. Rocks from the K-feldspar intrusive suite are studied on diagrams using the element-ratio pair X2 = [2 Ti + Al + 3.3333 P]/Zr and Y2 = [2 Ca + Na + K]/Zr, which creates a linear trend of compositional variations controlled by the phases PI ± Kfs ± Bt ± Ttn + Ap. Mean intercepts of model trends on the element-ratio diagrams suggest differences among plutons that relate to source-region processes. For example, samples belonging to the hornblende intrusive suite represent a minimum of six batches of magma. Mean intercept values for plutons west of the Owl Lake–Harrison fault zone are significantly higher than those situated east of this structural break. These systematic differences allude to fundamental differences in the nature of Mesozoic magmatism in Wrangellia (and Harrison) terrane compared with that in Cadwallader, Bridge River, and Methow terranes, and probably in the Intermontane superterrane east of the structural break.

Effects of non-conserved denominators on pearce element ratio diagrams

Pearce element ratios (PER's) have “conserved” denominators which have not participated in the material transfer processes that cause chemical variations in rocks. Theoretically, there is no truly conserved element (constituent) which can be used as a PER denominator because in every material transfer process all constituents have non-zero concentrations in the phases that are being transferred. Thus, constituents used as denominators of PERs may have undergone at least a small amount of material transfer. This communication investigates the degree to which a non-conserved PER denominator changes the trend of data produced by a material transfer process from that produced by the same process but plotted on a PER diagram with a truly conserved denominator. An equation is developed that utilizes the partition coefficient as the measure of the degree of involvement of the denominator constituent in the phase undergoing transfer. This equation is examined to determine how the magnitude and direction of a PER diagram data trend change with increasing involvement of the denominator constituent in the transferring phase. A set of plagioclase fractionation examples are presented which use different elements as PER denominators and consider the effects that small amounts of these elements in the plagioclase structure will have on the data trend, as a function of the element partition coefficient between crystal and melt. Results demonstrate that the direction of change in slope of a material transfer data trend is a function of the initial relative magnitudes of the numerator constituents on the PER diagram. Additionally, if the amount of involvement of a PER denominator in a separating phase is very small relative to the amount of the numerator constituents in the separating phase, there is no significant change in the data trend caused by material transfer on a PER diagram. Moreover, if the denominator constituent substitutes for a numerator constituent in the phase undergoing transfer, the intercept of the trend of the data may not converge to zero when there is a large partition coefficient, as would be expected from theory. Thus, statistical tests to determine if a PER denominator is conserved, which evaluate whether the intercept is significantly different from zero, may not be very powerful because a large amount of denominator variation is necessary before the intercept of a data trend is forced through the origin, if at all.

The adaptation of Pearce element ratio diagrams to complex high silica systems

Pearce element ratios (PERs, of Pearce 1968) express geochemical data in a form where variations in absolute compositions of an igneous suite can be evaluated. Generally the denominator value in the ratio is taken as a major element abundance, but it is argued here that Zr provides a more suitable choice. Zr remains incompatible in magmatic systems up to ≈68 wt.% SiO2 because zircon fractionation can be suppressed by high melt temperatures and increased volatile contents. The use of Zr thus permits PER modelling to be extended to much higher levels of silica than previously investigated. However, such systems are more complex than those just involving simple basaltic magmas. Besides fractionation, the processes of magma mixing, combined assimilation and fractional crystallization, and the initial degree of partial melting in the mantle source must also be considered. To distinguish and evaluate these processes a set of example suites are investigated from a complex synextensional calc-alkaline province in the western USA. Samples within most individual suites can be modelled by fractionation, however a significant trend orthogonal to the main fractionation vector is also apparent, and open system processes are inferred. Successful modelling is achieved on an inter-suite basis using diagrams with axis functions of ([4(Ca+Na)+0.5(Fe+Mg)]/Zr versus (Si+Al)/Zr). Potential open system evolution paths between mafic end members and crustal contaminants are also displayed and evaluated on these same diagrams. The encouraging results suggest that such PER diagrams may be employed as a versatile tool for investigating the systematics of related igneous suites over a wide area.

Endmember unmixing of compositional data

Manipulation of the n-endmember mixing equations, appropriate to compositional data, leads to a general method (1) to test hypotheses about endmember compositions and (2) to recover the concentrations of all compositional components in an endmember when the concentrations of n − 1 components are known or measured in that endmember. Additionally, Pearce element-ratio analysis is shown to be a special (and not efficiently posed) two-endmember case of n-endmember unmixing. Information available on Pearce element-ratio diagrams as values of the slopes of lines is contained on Harker diagrams as values of points.

The validity of pearce element ratio analysis in petrology: an example from the Uwekahuna laccolith, Hawaii

Chemical variation diagrams formed from ratios with a common denominator exhibit induced correlations which can obscure the effects of true compositional variations. Nevertheless, important conclusions have been obtained using Pearce element ratio (and isochron) diagrams, even though these diagrams are formed from axes ratios with a common denominator. In this paper, we consider two approaches to mitigate the effects of induced correlation, and in doing so, demonstrate the validity of geochemical analysis using diagrams formed from ratios with a common denominator. First, we propagate analytical error onto Pearce element ratio diagrams to distinguish chemical variations associated with analytical error from chemical variations related to geological processes. Second, we consider an alternative diagram formed from ratios with different denominators which does not exhibit any induced correlation. We demonstrate both theoretically and by example that results produced using this diagram are identical to those using diagrams with a common denominator. These two lines of evidence confirm that Pearce element ratio (and isochron) diagrams are useful and valid tools in the analysis of geochemical variations.

Origins of the 1954–1960 Lavas, Kilauea Volcano, Hawaii: Major element constraints on shallow reservoir magmatic processes

In the 1954–1960 period, Kilauea Volcano, Hawaii, erupted four times: summit eruptions occurred in 1954 (Halemaumau) and 1959 (Kilauea Iki) and flank eruptions occurred in the lower East rift zone in 1955 and 1960. Our analysis demonstrates that the chemical compositions of lavas from each eruption are related solely through fractionation and sorting of the observed phenocryst assemblages. We have used Pearce element ratio diagrams to make the following observations and inferences: 1) The conserved element ratio P/K for each eruption has less variance than is expected from analytical uncertainty. Thus, we cannot reject the hypothesis: Each set of lavas represents a comagmatic suite of compositions. 2) Where each of the four groups of lava analyses is plotted on a Pearce element ratio diagram with axes of Si/K and [0.5(Mg+Fe)+1.5Ca+2.75Na+0.25Al]/K, the data fit a slope of 1.0. Therefore, we cannot reject the hypothesis: The chemical variation in each data set is consistent with sorting of OL±PL±AU. 3) Each of the 4 groups of lavas has the same mean P/K ratio. As a result, we cannot reject the hypothesis: The 1954–1960 lavas originated from a single magma batch or several magmas with identical P/K values. Finally, using the most magnesian glass analysis in the Kilauea data suite (1959: SG‐05), we have modelled the differentiation path with thermodynamic‐based calculations. These calculations predict a liquid line of descent that is consistent with the path inferred from rock and glass analyses. Therefore, the simulations provide an additional test of our hypotheses and corroborate the postulated differentiation model. Additionally, these calculations suggest that differentiation took place at less than 0.3 Gpa and extended over a temperature range of between 100°C and 150°C.

Petrologic hypothesis testing with Pearce element ratio diagrams: derivation of diagram axes

In petrology, Pearce element ratio (PER) diagrams have been used: i) to determine whether members of a rock suite are co-genetic, ii) to identify the minerals involved in differentiation processes, and iii) to evaluate the extent to which those mineral are involved. The axis coefficients of each diagram are chosen such that sorting of minerals or combinations of minerals will generate unique and predictable trends. Unfortunately, selection of the optimal combination of axis coefficients is a difficult task, especially if the system being investigated has a large number of phases or complicated solid solution minerals. Our work has established a formal set of rules and matrix operations which facilitate the determination of PER diagram axes coefficients. This methodology can be used to determine the unit molar vector displacement caused by the addition or subtraction of a specific mineral, given a set of axis coefficients. It can also be used to create PER diagrams on which minerals have predetermined vector displacements. By designating all vector displacements to be parallel, axis coefficients for assemblage test diagrams can be determined to test the following hypothesis: the observed chemical variation is due to the addition (or removal) of a specific set of minerals. Alternatively, by designating all vector displacements to be mutually perpendicular, phase discrimination diagrams can be created which test whether the observed chemical variations require a specific phase to be involved in differentiation. Phase discrimination diagrams also provide a means to estimate the extent of that involvement. This methodology facilitates construction of powerful yet simple PER diagrams which provide an effective means of testing alternative differentiation hypotheses.

PEARCE.PLOT: a Turbo-Pascal program for the analysis of rock compositions with Pearce element ratio diagrams

Pearce element ratios can be used to test specific hypotheses concerning igneous differentiation processes. Pearce element ratio diagrams can be employed to recognize cogenetic rock analyses, the minerals involved in differentiation, and the compositions and proportions of the involved phases. Petrologic hypotheses can be tested by examining the trend of data on a variety of Pearce element ratio diagrams. Hypotheses are rejected on the basis that the data lack correlation, or the best fit line has a zero intercept, or where the slope of the best fit line does not equal the slope predicted by mineral stoichiometry. To facilitate the analysis of petrologic hypotheses with Pearce element ratios we present the following interactive, graphics-supported Turbo-Pascal computer program. The program PEARCE.PLOT reads interactively identified data files and allows the user to select specific samples from data files, assign analytical errors to oxide analyses, and generate and print a variety of Pearce element ratio diagrams. Each plot has options for error representation, labeling of data, and fitting lines to the data. Attributes of the program are illustrated with two data sets from Kilauea volcano, Hawaii. The 1955 and 1960 eruptions of Kilauea are shown to be derived from a single magma which has undergone variable sorting of olivine, plagioclase, and augite. Differentiation of the 1955 lavas and the early 1960 lavas was characterized by subequal amounts of all three (micro-) phenocrysts, whereas the late 1960 eruptions owed their chemical variation mainly to olivine and subordinate amounts of plagioclase and augite. Pearce element ratios illustrate visually and graphically most of the same conclusions reached by conventional mass-balance calculations. However their real strength lies in their ability to analyze extended differentiation trends in terms of mineralogy and mineral proportions. Additionally, they form a rigorous basis for rejecting ill-conceived hypotheses.

The statistics of Pearce element diagrams and the Chayes closure problem

Pearce element ratios are defined as having a constituent in their denominator that is conserved in a system undergoing change. The presence of a conserved element in the denominator simplifies the statistics of such ratios and renders them subject to statistical tests, especially tests of significance of the correlation coefficient between Pearce element ratios. Pearce element ratio diagrams provide unambigous tests of petrologic hypotheses because they are based on the stoichiometry of rock-forming minerals. There are three ways to recognize a conserved element: 1. The petrologic behavior of the element can be used to select conserved ones. They are usually the incompatible elements. 2. The ratio of two conserved elements will be constant in a comagmatic suite. 3. An element ratio diagram that is not constructed with a conserved element in the denominator will have a trend with a near zero intercept. The last two criteria can be tested statistically. The significance of the slope, intercept and correlation coefficient can be tested by estimating the probability of obtaining the observed values from a random population of arrays. This population of arrays must satisfy two criteria: 1. The population must contain at least one array that has the means and variances of the array of analytical data for the rock suite. 2. Arrays with the means and variances of the data must not be so abundant in the population that nearly every array selected at random has the properties of the data. The population of random closed arrays can be obtained from a population of open arrays whose elements are randomly selected from probability distributions. The means and variances of these probability distributions are themselves selected from probability distributions which have means and variances equal to a hypothetical open array that would give the means and variances of the data on closure. This hypothetical open array is called the Chayes array. Alternatively, the population of random closed arrays can be drawn from the compositional space available to rock-forming processes. The minerals comprising the available space can be described with one additive component per mineral phase and a small number of exchange components. This space is called Thompson space. Statistics based on either space lead to the conclusion that Pearce element ratios are statistically valid and that Pearce element diagrams depict the processes that create chemical inhomogeneities in igneous rock suites.