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The pluton-volcano connection: Integrating processes, volumes and time scales in magma plumbing systems

Final Report Summary - IMAPS (The pluton-volcano connection: Integrating processes, volumes and time scales in magma plumbing systems)

The iMAPS project “The pluton-volcano connection: Integrating processes, volumes and time scales in magma plumbing systems” was designed to determine the geochemical link between contemporaneous volcanic deposits and its sub-surface intrusives (plutons) with the overarching goal of better understanding volcanic eruptions. Two tilted volcanic calderas in the USA were selected exposing both rock types and thus presenting a rare opportunity to study magmatic processes, volumes and their time scales in complete upper crustal magma plumbing systems. The fellow, with expertise in plutonic rocks, and the scientist in charge, an internationally recognized expert in volcanic rocks and single mineral geochemistry, joined forces to complete this interdisciplinary study and exchange scientific ideas. The aim of the fellowship was to train the fellow in volcanic studies and element and isotope geochemistry in state-of-the-art labs at Durham University as well as scientific independence and project management. It was critical for the fellow to receive training and gain experience in crystal scale geochemical analyses, specifically in micro-Sr isotope techniques developed at Durham University, which was central for the testing of the scientific hypotheses. By moving from the USA to the UK, the fellow also increased her scientific network within the UK and Europe, especially with scientists working in the volcanic field.
The project as proposed included a work program of seven main steps: It started off with field work (step 1) to map and collect samples for petrography, whole rock geochemistry and element mapping (steps 2 to 4). In steps 5 and 6 the goal was to focus on in situ mineral geochemistry and diffusion modelling based on the collected geochemical data. Synthesis of the data and dissemination of the results (step 7) was to occur throughout the tenure of the project. With the exception of step 6 (diffusion modeling), which was replaced by U-Pb zircon geochronology, and step 7, which is still work in progress, all other steps of the project were completed during the time period of the fellowship.
The researcher started the project by establishing crosscutting relationships, estimating volumes of geologic units and collecting samples from three calderas in the USA that have exposed contemporaneous plutonic and volcanic rocks of the same magma plumbing system including the deeply incised Bonanza Caldera in Colorado, the tilted Organ Mountains Caldera in New Mexico and the deeply eroded Minarets Caldera in California. The researcher wanted to be sure that at least one of the calderas had fresh enough rocks to complete the planned geochemical analyses to succeed with the project. Rock samples were prepared and thin sections made from both the Organ Mountains and the Bonanza calderas, which seemed to provide the most promising samples. The researcher conducted careful petrographic observations of both caldera rocks with a polarized microscope to understand textural relationships and added to the compositional and textural information by imaging with color Cathodoluminescence. The samples were then crushed and powdered for a) XRF analyses to get information on whole rock major oxide and some trace elements, b) solution ICPMS (Inductively coupled plasma mass spectrometry) analyses for the full range of trace elements, and c) multi-collector ICPMS for Sr, Nd and Pb isotope analyses. The next step involved collecting crystal scale geochemical information, while focusing on the Organ Mountains caldera only due to funding and time constraints. The fellow first used an electron microprobe to collect major element compositional information across different feldspars, the main mineral in both plutonic and volcanic rocks. This was done qualitatively through element mapping and quantitatively through spot analyses. Selected feldspars were then analyzed for trace element variations across crystal transects with laser ablation ICPMS and for Sr isotope ratios by micromilling different crystal zones of yet another subset of these crystals and analyzing with ID-TIMS (isotope dilution – thermal ionization mass spectrometry). Both whole rock and crystal scale isotope analyses involved running samples through column chemistry, which involved a significant amount of time and training in clean labs. Throughout this project, the researcher studied project relevant literature, received training and gained experience in analytical techniques, high temperature geochemistry and a better understanding of magma processes.
At the beginning of the project the fellow recognized the importance of having high precision ages of the various Organ Mountains rock units that would allow a geochemical correlation, which only makes sense for contemporary volcanic and plutonic units. She thus sought and won funding through the NERC Isotope Geosciences Facilities Steering Committee to do U-Pb zircon geochronology using laser ablation ICPMS and chemical abrasion ID-TIMS techniques in collaboration with two new colleagues (D. Condon, M. Horstwood) at the NERC Isotope Geosciences Laboratory (British Geological Survey). This work is now completed and can be published as is, however, the new collaboration may be continuing.
Although some of the detailed data interpretation, including the publishing of the results, is still ongoing, the main results of this study have been generally suggesting a complementary geochemical relationship between the plutons and the ignimbrites, but no direct geochemical relationship between the plutons and the pre- and post-caldera forming lavas. The new age data revealed that the pre-caldera lavas are by 7 myr older and erupted long before the main Organ Mountains volcanic-plutonic complex formed. In contrast, the post-caldera lavas erupted only ca. 50k to 200k yrs after the pluton crystallized. A comparison of the element geochemistry of lavas and plutons show that there is no obvious equivalent plutonic unit at the exposure level that has complementary or similar composition to the lavas. Isotope data, however, indicate a similar source for both lavas and pluton. We thus conclude that the lavas must have been derived from deeper than 6 km, which is the lowermost part of the plutonic magma chamber exposed at the surface, or directly from the ultimate source that is shared by all magmas.
Whole rock and crystal geochemistry (especially element data) of the ignimbrites and the main pluton, however, are generally complementary in composition suggesting fractionation crystallization as the main process for the compositional variations. Age data suggest that the estimated 500-1,000 km3 large ignimbrites erupted every ca. 200k yrs and that magmatic activity in the main pluton froze immediately after the last ignimbrite erupted. The pluton is compositionally zoned from more evolved compositions at the top to gradually less evolved compositions at the bottom. The upper 1-3 km of the pluton are composed of accumulations of feldspar crystals from which interstitial melt, presumably quite volatile rich, pooled (fractionated) and violently erupted to produce the most evolved and oldest of the three, crystal-poor ignimbrites. Similar to the oldest ignimbrite, the two younger ignimbrites appear to also be more evolved than the plutonic rocks, however, the younger ignimbrites are increasingly more crystal-rich with age and closer in composition to the plutons. This suggests that fractionation may have occurred in slightly deeper parts of the pluton to produce the younger ignimbrites and perhaps fractionation was less efficient to separate interstitial melt from crystals. One added complication is that both younger ignimbrites have higher Sr and lower εNd isotopic ratios compared to the older ignimbrite and the main pluton, indicating that the younger ignimbrites additionally underwent at least 10% of assimilation of old (Precambrian) host rock before they erupted. In the plutonic record, such more crustal like melt compositions are preserved in small Alaskite lenses (melt pockets) at the top of the plutonic body and are interpreted to be not erupted equivalents of these ignimbrites. It is likely that some of the plutonic record in the Organ Mountains that once preserved evidence for different stages of ignimbrite eruption was masked due to subsequent magma replenishment. These results will be presented in three manuscripts currently being prepared for publication in peer-reviewed, high-impact journals.
In summary, based on our study, sub-surface magma systems (plutons) seem to provide a plausible source for large and violent erupting ignimbrites and do not need to be derived from greater depths as appears to be the case for effusive, primitive lavas. Fractionation crystallization in conjunction with assimilation of host rocks and intermittent magma replenishment seems to be the main process responsible for the main compositional variations between ignimbrites and plutons, and is ultimately related to the cause of such large eruptions. This project has helped to recognize the importance of shallow crustal magma processing in producing violent eruptions in large volumes. Added time scale information through geochronology suggests fairly cyclic eruptive behavior every 200k yrs until the magma system shut down, for a reason yet unidentified. If all continental arc magmas behave this way, geophysical and geodetic methods are capable of detecting such shallow crustal magma movement and build up, respectively, which ultimately helps the long term prediction of volcanic eruptions. In contrast, if primitive lavas are generally derived from deeper sources, such eruptions are less likely to get detected.
The fellow benefited from the host’s scientific network interacting with volcanologists and geochemists at Durham, geochronologists from the British Geological Survey and the UK in general. In the two years of the fellowship, the researcher presented results from the project in six national and international conferences (four talks, two posters). She furthermore attended two in-house and four international workshops to broaden her scientific knowledge and increase her interaction with the magma community. The fellow was also an active member of the department and active in scientific outreach: She represented the postdocs in the Athena Swan committee and helped to organize weekly volcanology meetings. She participated in an open-house event at the American Geophysical Union conference teaching the community about volcanism through hands on experiments, visited an elementary school in Durham during meteorite week, involved Durham and American geology undergraduate students in her field and lab research, and helped to lead the 4th yr undergraduate field trip to the SW USA. During her fellowship she was part of a group organizing and leading a Geological Society of America field forum across the Sierra Nevada, California, became senior editor for the publication of the field guide and therein published one first author and two second author papers. The fellow furthermore attended fieldtrips to learn about volcanoes (Tenerife) and UK geology.