Study Notes: Magmatic and Crustal Differentiations of History of Granitic Rocks from Hf-O Isotopes in Zircon

Classification is foundational to petrology; however, classification based on limited understanding can devalue categorization and impede the gathering of meaningful information.
Granites are traditionally categorized into four types based on their source rock origin:
- S-type: Sedimentary source.
- I-type: Igneous source.
- A-type: Anorogenic/Alkaline.
- M-type: Mantle-derived.
I-type granites constitute the majority of the Earth's continental crust.
Historically, the I-type classification was considered too broad to provide specific insights into the diverse conditions of these granitoids. Modern techniques, specifically those pioneered by Kemp et al. in the paper "Magmatic and Crustal Differentiation History of Granitic Rocks from Hf-O Isotopes in Zircon," have allowed for a more granular analysis of their components and sources.

Zircons ( $ZrSiO_4$ ) are the primary subjects for analyzing crustal evolution due to several specific properties:
- High Resilience: They withstand physical and chemical weathering.
- Radiogenic Isotopes: They contain high concentrations of isotopes essential for geochronology, specifically Uranium-Lead ( $U-Pb$ ), Thorium-Lead ( $Th-Pb$ ), and Lutetium-Hafnium ( $Lu-Hf$ ).
- Crystallization Environment: They crystallize in high-silica melts at moderate to high grades.
- Isotopic Retention: They retain magmatic isotope ratios, making them excellent records of the conditions present during their formation.
Hafnium ( $Hf$ ) and Oxygen ( $O$ ) isotopes within zircons are essential for tracing the history of granitic plutonism.

Kemp et al. utilized modern laser ablation techniques to study the origin of I-type granites, building on foundational Hf analysis techniques developed by Griffen et al. ( $2000$ , $2002$ ).
The research examined three specific suites in eastern Australia:
1. Jindabyne plutons.
2. Why Worry suite.
3. Cobargo suite.
The analyzed samples exhibited characteristic I-type features, including an abundance of hornblende and higher concentrations of Calcium ( $Ca$ ), Sodium ( $Na$ ), and Strontium ( $Sr$ ) compared to S-type granites with similar silica content.
Analytical Steps:
1. Dating: Zircons were dated using $U-Pb$ isotope analysis.
2. Oxygen Measurement: $^{18}O$ was measured from the same zircon growth zone using an IMS Ion probe.
3. Hafnium Measurement: Laser ablation was used to measure and analyze $Hf$ ratios.
4. Mapping: $Th-U$ mapping was employed to reveal zoning patterns within the zircons.

$Hf$ Isotopes: Measuring $Hf$ reveals the timing of new crust generation from the mantle. A spectrum of $Hf$ values within a single rock indicates open-system processes where $Hf$ ratios shift toward the melt during precipitation.
Correlation with Neodymium ( $Nd$ ) and Strontium ( $Sr$ ):
- Kemp et al. graphed $Hf$ against $Nd$ , producing results similar to the traditional $Nd-Sr$ plots.
- The "Mantle Array": Originally defined by Paolo and Wasserburg ( $1977$ ) and Zindler et al. ( $1982$ ), this graph plots depleted mantle versus enriched mantle.
- The array appears as a line descending from the top-left to the bottom-right. Kemp et al. inverted the x-axis of their $Hf-Nd$ graph specifically to match this traditional visual model and facilitate easier comparison for readers.
Bulk Earth: This point on the graph serves as a divider to separate depleted mantle signatures from enriched mantle signatures.

$^{18}O$ Isotopes: These are sensitive to low-temperature surface processes such as weathering and sedimentary interactions.
VSMOW Standard: Kemp et al. used Vienna Standard Mean Ocean Water (VSMOW) to determine if a phase was enriched or depleted. Higher VSMOW values indicate interaction with low-temperature water.
Mantle Baseline: Zircons formed in the mantle have a $^{18}O$ ratio of approximately $\sim 5.5$ . Values higher than this indicate the extent of interaction with sedimentary processes.
Supracrustal Components: Defined in the paper as granitoids derived from near-surface sedimentary precursors. The mix of sources is represented by curved lines in the data arrays.

Percentage of Supracrustal Material in Melts:
- Cobargo Suite: $25\%$ precipitated from supracrustal material.
- Jindabyne Plutons: $40\%$ of the melt originated from supracrustal material.
- Why Worry Suite: $60\%$ of the melt originated from supracrustal material.
Zoning Patterns: In all three suites, $Hf$ and $Th/U$ ratios were found to decrease toward the edges/rims of highly zoned zircons. This indicates a reworking of granites over time and the introduction of continental crust-like components during magmatic evolution.
Variation: Some samples extended into mantle-like environments, suggesting that zircon crystallization occurred before the supracrustal material was fully integrated into the melt.

The paper proposes a model for I-type granite formation in eastern Australia based on a hybrid magma system:
1. Generation: Silicic melt is generated and interacts with residual liquids from basalt crystallization.
2. Mixing: This mixture then combines with supracrustal (sedimentary) material.
3. Environment: This occurs in an incremental assemblage starting at a deep crustal hot zone.
Hot Zone Depth: Constraints place this zone at a depth of $< 35$ to $40\,km$ , noted by the distinct lack of a garnet signature.
Crystallization: As the melt rises to higher crustal levels, zircons crystallize, acquiring higher $^{18}O$ and lower $Hf$ values within the hybrid melt.
Conclusion: The presence of continental crust is deemed critical to the process of I-type magmatism.

The study is highlighted for giving substance to modern isotopic techniques and providing a practical application for studying crustal evolution where bulk composition calculations are otherwise difficult.
Impact: The paper has been cited over $1,000$ times.
Case Application: Michelle et al. cited Kemp et al. in their work "Identifying crustal contributions in the Patagonian Chon Aike Silicic Large Igneous Province," using $^{18}O$ and $Hf$ as tracers to constrain crustal and mantle magma sources.
Reviewer Observations:
- The inverted x-axis in Figure 2 was helpful for comparison but could have been more explicitly labeled in the text.
- The compact nature of the paper was effective; despite its short length, it was extremely detailed and avoided extraneous information.
- The figures were vital to the conclusion, although the journal formatting required jumping between pages to reference them during reading.