Petrography, geochemistry, and origin of deep magmatic cumulates in the Canary Islands – the xenolith record

Abstract

This study investigates xenoliths present in various dykes on Tenerife, the Canary Islands. These xenoliths were studied by means of fieldwork, petrographic analysis, scanning electron microscopy, laser fluorination, and LA-ICP-MS. In conjunction these analyses enable the understanding of the magmatic structures beneath the Anaga massif, with a conceptualisation of the formation of this volcanic stage.
This study identified that the cumulate xenoliths are exclusively pyroxenites and hornblendites, the crystallisation of which is strictly associated with the mineralization of: clinopyroxene, amphibole, apatite, titanite and Fe-Ti ox ides. Significantly, petrographic analysis identified that the hydrous phase of amphibole is in textural equilibrium with clinopyroxene, implying a hydrous initial melt. Clinopyroxene barometry, from SEM compositions, allocates two distinct mantle reservoirs, a deep crystallisation at 30.6 ± 7 km, and a shallow crystallisation at 3 ± 7km.
Zr-thermometry attained the maximum temperature to be 930°C with an uncertainty of 50°C dependant on the activities of Si and Ti. Low δ18O initial melt values, 4.95 ± 0.2 ‰, reflect the conditions of the mantle beneath the Canary Islands. The geochemical analysis determined these to be associated with a deep mantle reservoir of 30.6 ± 7 km, indicating their association with the source. An indication of one source, a HIMU-type mantle plume, with fractionation processes affecting the differentiation of the cumulates acquired. The occurrence of apatite in the xenoliths and locality associations of vesicles in the host rock attribute a volatile association of H2O; CO2, Cl and F. The processes of degassing and decompression propose an intricate relationship between dyke emplacement and flank instability, attributing to the mass wasting event of the Taganana collapse on the north-facing coast of Anaga.

Popular Abstract

Tenerife is the largest, highest, and most central of the Canary Islands. It represents the most diverse rock exposures in relation to the other islands. Ranging from the island-building basalts to the felsic products of the Teide-Pico Viejo stratovolcanoes. The island originated 11.9 million years ago, while the Anaga Massif, which this study is concerned with, represents the last island building stage at 4 million years ago. After the conclusion of this basaltic event, we are introduced to the explosive volcanism, and a sequence of caldera collapses.
This study’s focus lies exclusively on the xenolithic record from the Anaga peninsula due to a shortage of prior research concerning this stage. Xenoliths are foreign pieces of rock confined in a different type of rock. These are brought up from depth during a volcanic eruption. The magma that brings these up becomes the host rock and the xenolithic material is an inclusion that was picked up at a certain depth underneath the volcano. Therefore, these xenoliths may be thought of as records of information about the volcano’s plumbing system. Studies on the minerals found in them reveal valuable data, that can be used to understand the processes that formed this volcano.
The xenoliths in this study reveal a water-bearing mineral, amphibole, as one of the main constituents of this rock. This find was significant as it attributed the source of these rocks to contain water to some extent. To understand this water source I had looked at oxygen isotope signatures in the specific rock-forming minerals of clinopyroxene, hornblende and apatite to relate a significant initial melt value that would
tell us about the conditions of the source itself.
A striking find of this study is the determination of two distinct mantle reservoirs, where the xenoliths in question had crystallized and resided before being picked up. The study finds a shallow reservoir at 3 km and a deep reservoir at a 30km depth. This latter reservoir is studied under a greater emphasis as it related to a crystallization at a mantle depth. Here, the earlier mentioned oxygen isotopes are associated with a depth difference where the deeper xenoliths represent the initial values, whereas the shallow xenoliths are related to some extent of contamination. This study therefore highlights the importance of depth associations of xenolithic material when inferring further geochemical analysis to obtain data relating to a source.

• author: Struzynska, Patrycja
• supervisor: Anders Scherstén; Valentin R. Troll
• organization: Department of Geology
• course: GEOR02 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: cumulate xenoliths, Anaga, petrographic analysis, clinopyroxene barometry, Zr-thermometry, oxygen isotopes, mantle δ18 O values, dyke propagation
• publication/series: Dissertations in Geology at Lund University
• report number: 668
• language: English
• additional info: external supervisor - Valentin R. Troll - Uppsala University
• id: 9136768
• date added to LUP: 2023-09-07 14:24:12
• date last changed: 2023-09-07 14:24:12